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Annual Report 1999

 Awards & Recognition Existing Facilities Plan of research for next year (1999-2000)

DIRECTOR'S NOTE

In pursuance of the national policy recognising plasma physics as one of the thrust areas of the present day research, Government of Assam established the Centre of Plasma Physics in the year 1991 as a research institute to pursue basic research in theoretical as well as experimental plasma physics and its related areas. Centre of Plasma Physics is an autonomous research institute under the Education (Higher) Department of Government of Assam. The Centre started functioning since April, 1991 in a rented house located at Saptaswahid Path, Dispur, Guwahati. The Governing Council of the Centre consists of four eminent scientists of the country with representatives from Institute for Plasma Research, Gandhinagar, Physical Research Laboratory, Ahmedabad, Bhabha Atomic Research Center, Bombay and Saha Institute of Nuclear Physics, Calcutta; top level State Government officers and some local members. The first Chairman of the Governing Council was Professor P.K. Kaw, a world renowned Plasma Scientist who took keen interest for the all-round development of the Centre. After the expiry of its 3 years' term, the Governing Council of the Centre has been reconstituted by the Education Department with Prof. A.C. Das, Dean of Physical Research Laboratory as its chairman. The present Director of the Centre is Prof. S. Bujarbarua, who is a distinguished plasma scientist of the country and recipient of Vikram Sarabhai Research Award' in 1989 and Kamal Kumari National Award' in 1993.

Over the last few years after its formation, the Centre has taken up several theoretical investigations in fundamental plasma processes such as nonlinear phenomena, instabilities, dusty plasma and has set up facilities for conducting basic plasma physics experiments. Recently, from fund available from several Central Government agencies (like Department of Atomic Energy, Department of Science and Technology), the Centre has taken up some experimental programmes in the front line areas of plasma physics, such as dense plasma focus and dusty plasma. During the last seven years the Centre has published more than 50 original research papers in standard international journals. The scientists of the Centre are working in close collaboration with national and international institutes like Institute for Plasma Research, Gandhinagar, Physical Research Laboratory, Ahmedabad, Bhabha Atomic Research Centre, Bombay, Regional Research laboratory, Bhubaneswar, Saha Institute of Nuclear Physics, Calcutta, Kyushu University, Japan, University of Bayreuth, Germany, Culham Laboratory, UK, and Flinders University, Australia, to name a few. The Centre also runs a Ph.D. programme with students registering with Guwahati University. Another component of the academic activity of the Centre consist of holding lecturers and colloquia on plasma physics and other branches of physical sciences. The detail of the work done during the year 1998-99 with research publications, and other achievements are listed here.

 S. Bujarbarua Director Index

GOVERNING COUNCIL

RESEARCH WORK DONE DURING THE PERIOD 1998-99

The principal thrust of theoretical research at the Centre is in the area of plasma maser, nonlinear coherent structures (solitons, double layers), dusty plasma, electron-positron plasma.

According to recent progress in weak plasma turbulence theory, the lowest order mode-mode coupling consists of three processes: the three wave resonance, resonant scattering and nonlinear plasma maser. The plasma maser interaction is a new nonlinear process in turbulent plasma. The process leads to up-conversion of wave energy from low frequency turbulent fields to high frequency waves through nonlinear interaction of waves and particles. Thus the process has potential importance to interpret numerous radiation phenomena in space and laboratory plasmas. The plasma maser effect occurs when nonresonant as well as resonant plasma oscillations are present. The resonant oscillations are those for which the Cherenkov resonance condition is satisfied, while the nonresonant oscillations are those for which the neither the Cherenkov resonance condition nor the nonlinear Landau resonance conditions are satisfied.

A study has been carried out to investigate the amplification of extra-ordinary mode radiation in the presence of kinetic Alfv\'en wave turbulence driven by electron beam [Saikia et al. Pramana-J. Phys. 51, 481 (1998)]. It is shown that plasma maser may be responsible for the amplification of the extra-ordinary mode up-conversion of turbulent energy via wave-particle interaction. This model is applied to explain the radiation mechanism of SKR in extra-ordinary mode.

Recently, numerical simulation of the plasma maser interaction has been carried out and are compared with theoretical predictions. The generation of right-handed polarized waves from whistler mode turbulence by plasma maser interaction has been investigated both numerically and theoretically [Bujarbarua et al. Phys. Plasmas 5, 2244 (1998); Phys. Scr. 59, 62 (1999)]. Simulation using a two dimensional electromagnetic and relativistic particle code show that the high frequency electromagnetic waves with right-handed polarization can be generated by a plasma maser mechanism from low frequency whistler waves excited by electron temperature anisotropy. The numerical results agree quite well with the theoretical findings and verified almost all theoretical predictions. Similar numerical simulation has also been carried out for plasma maser driven by electron beam instability [Nambu et al. Phys. Lett. A 252, 198 (1999); Phys. Plasmas 6, 994 (1999)]. Here simulations show that high frequency electromagnetic waves near $2 \omega_{pe}$, where $\omega_{pe}$ is the electron plasma frequency, with right-handed polarization (R-mode) can be generated by a plasma maser mechanism from electrostatic Langmuir waves excited by an electron beam.

We have also investigated theoretically the plasma maser effect in a magnetized electron-positron plasma of the test Langmuir wave in the presence of Alfven wave turbulence produced by an electron temperature anisotropy (Case I) and also by a positron temperature anisotropy (Case II). As a result of the nonlinear interaction Langmuir waves grow, for both the cases, with the direction of propagation opposite to that of Alfven waves.

In case I, right hand circularly polarized Alfven waves are excited by an electron temperature anisotropy created by a bi-Maxwellian distribution. In case II, left hand circularly polarized Alfven waves are excited by a positron temperature anisotropy created by a bi-Maxwellian positron distribution. Here the positrons are the resonant particles and they play the same role as the electrons do in the case I. All our theoretical results are in good agreement with the simulation results [Eda et al., J. Phys. Soc. Japan, 66, 3470 (1997)], the growth rates, as calculated from theory and simulation results, being of the same order i.e. $10^{-3}$.

We have discussed the formation of double layers solution in a plasma containing electrons, ions and negatively charged massive dust particles. Considering the dust particle motion on a vary slow time scale alongwith the dust charge fluctuations we have shown the existence of both soliton and double layers solution. To get these solutions positive ion concentration should be less than unity.

Our theoretical model is similar to Ma and Liu [Phys. Plasmas, 4, 253 (1997)]. However they have shown the existence of soliton solution in a plasma consists of electrons, ions and positively charged dust particles. They have found that the soliton solution exists in such a plasma provided the positive ion concentration is higher than unity.

Small amplitude shock wave in a dusty plasma

We have studied the existence of small amplitude shock wave profile in a plasma consisting of negatively charged dust particles, warm ion beams and hot isothermal electrons. We have also considered the effect of dust charge fluctuations. Using the reductive perturbation method we obtained a Burgers type equation for the first order perturbed potential. And this equation describes a weak shock wave profile. Finally we obtained a rarefactive type small amplitude shock like solutions. The potential and velocity of the shock wave profile is theoretically calculated. The nature of the shock wave profile is dependent on the nonlinear coefficient of the potential. It will be positive (negative) for positive (negative) value of the potential. Using typical plasma parameters observed in the laboratory we have obtained the shock wave velocity is of the order of $10^5$ cm/sec and the width of the shock is of the order of $10^{-5}$ cm.

The nonlinear properties of solitary wave structures in the presence of non-isothermal ions, electrons and charged dust grains are reported. The modifications in the amplitude and the width of the solitary wave structures due to the inclusion of the effects of reflected ions and charged dust grains are investigated. It has been observed that the presence of dust grains, as well as the reflected ions, changes the amplitude of the solitons. It has been shown that the amplitude of the solitary waves grow more slowly, but with increase of $\alpha$ (the ratio of free to reflected ion temperature), it begins to increase.

Theory of sheath in a multicomponent plasma

We have reported here a simple theoretical analysis of sheath structure in a plasma containing electrons, positively charged potassium ions, negatively charged $C_{60}$ ions and an electron beam which is supposed to be emitted from an electrode. We have analytically calculated the sheath edge potential, floating potential as well as the electric field.

Our model can be applied to decrease the potential difference between an electrode and the plasma. For example, in a RF plasma with asymmetric electrode, the powered electrode is more negatively biased and this causes a damaging effect on the substrate. The injection of electron beam from substrate may compensate the RF voltage.

Plasma sheath interface under gravity effect

Recent upsurge of research activities in dusty plasmas and the observation of Coulomb phase transition (CPT) near the plasma-sheath boundary layer in laboratory produced dusty plasmas have guided plasma physicists in general to look into the behaviour of plasma sheath interface under the gravity effect. An alternate mathematical approach known as three scale analysis has been applied to derive additional information about the gravito-electrostatic plasma sheath formation in a colloidal plasma under external gravity effects. This method was developed by Riemann and applied to a collision sheath problem. Existence of a new intermediate scale under quasi-hydrostatic approximation has found to exist to provide a smooth transitiion between the free fall sheath and main Debye sheath. Thus it is appropriate to term this intermediate zone as the levitatiional kind of transition interfaace in between the free fall sheath and Debye sheath. Likelihood of a nonlinear structure is found to provide a smooth transition.

Stability of a levitational equilibrium under the action of gravito- electrostatic coupling has also been analysed. Recently we reported the existence of a quasi-hydrostatic kind of equilibrium. In this problem we have tried to analyse the stability of such a kind of quasi- levitational kind of equilibrium. This problem deals with the stability behaviour of a quasi-levitational kind of equilibrium which is sopposed to model the intermediate scale of a plasma sheath in colloidal plasma under external gravity effect. Simple hydrodynamical model has been used to discuss about the equilibrium of interest with linear normal mode analysis of the acoustic disturbance. It is found that quasi-levitational kind of an equilibrium is highly unstable to a modified ion acoustic wave with a conditional likelihood of linear driving of the so-called acoustic mode too. The plasma sheath edge of a colloidal plasma undeer external gravity effect should be an active locality to produce linear and nonlinear acoustic turbulence. In consequential role on possible physical mechanism of Coulomb phase transitiion has been conjectured.

Dynamics of Self-consistent Dusty Plasma

A project entitled Dynamics of self-consistent dusty plasma and its effects in space plasmas has been sanctioned by Indian Space Research Organization, Department of Space, Government of India. Work on this project has been started in October, 1996.

Under the project, we have studied the random motion of a particle in a dusty plasma. The charging of the dust grains is basically a Brownian process in which electrons and ions are performing random walk before hitting the grain surface and sticking to it. Also, charging of the grain a unidirectional process in the sense that electrons and ions (with different diffusion rates) keeps sticking to the grains until a substantial fields builds up which repels the electrons and thus a ambipolar diffusion sets near the dust grain.

We have studied the random motion of the plasma particles in a dusty plasma especially highlighting the reflecting or absorbing wall. We have calculated the probability of random flights of electrons and ions in a dusty plasma in terms of diffusion coefficient. We then calculated the current on the surface of a dust particle. This diffusion limit of the charging currents becomes important in plasmas with low ionization rates, where collisional mean free path becomes comparable or smaller that the Debye length. Thus we hope that our theoretical results may be applicable for charging of aerosols in the atmosphere.

Another area of study under the project is the transport properties of dusty plasmas. Knowledge of transport coefficients of a dusty plasma is critical for the understanding of plasma processing, plasma etching etc. Physical processing, e.g. heat transfer, current flow etc. in the presence of charged dust are much more complex than in an ordinary plasma. We have presented a theoretical interpretation of some interesting structures observed in the Saturnian ring system. We have shown that such structures may result from the collisional dynamics of ring particles by taking into account the modifications of transport properties of plasma due to presence of dust grains. We have carried out an analysis to derive diffusion coefficient and Fokker Planck equation for different particle's species by using the well-known methods of plasma physics.

The presence of dust in a plasma has opened to us a window through which we can not only address the problems involved in the physics and chemistry of the solid state and of large molecules but also to study the area where the molecular properties and the bulk behaviour of solids overlap. Dusty plasmas apart from bridging the gap between celestial mechanics and plasma physics has acquired the new status of representing the most general form of space, laboratory and industrial plasmas.

Thus the study of dusty plasmas has gained significant momentum over the years. Investigators have studied the collective effects such as the ability of the charged dust particles to not only alter the properties of various plasma wave modes but also give rise to new modes such as the dust acoustic'' mode and have also studied various wave motions, instabilities etc.

Drift waves has always drawn considerable attention due to their inherent unstable nature. The study of the behaviour of drift waves in a dust-plasma system is important not only due to the presence of such dust-plasma systems in various space and near earth environments but also in industrial as well as laboratory devices especially Tokamak edge regions. The stability of drift wave eigenmodes in a sheared slab geometry with magnetic shear is well established. The stability arises from the shear damping of the drift wave. This is due to the antiwell potential in which energy convects away from the mode rational surface to be absorbed by ion landau damping far from it. However for intensely peaked radial density profiles the possibility of the existence of unstable drift eigenmodes was not ruled out either in the collisional or in the non-collisional case. Recently Rosenberg and Krall [Planet. Space Sci. 42, 889 (1994)] has studied the conditions for the onset of drift instabilities in both the high frequency as well as the low frequency regions in a dusty magnetized plasma. Linear and nonlinear drift waves in the low frequency region in a magnetized dusty plasma were studied by Shukla et al. [J. Geophys. Res. 96A, 21343 (1991)].

However, most of the theories have concentrated on the local theories. We have developed a non-local theory of collisional drift-type waves in a dusty plasma. A non-local theory is essential for the knowledge of the mode structure and hence helps in determining the transport properties.

A non-local theory of low-frequency drift-type waves in a collisional dusty plasma has been developed. This has given rise to a number of striking features . The foremost among them is the existence of a new type of drift wave with a new frequency, a new gyroradius and a new acoustic speed. The behaviour of the new drift wave with variation in the various dust parameters has been investigated. The presence of the negatively charged dust particles in the presence of a sheared magnetic field has been found to have a stabilizing effect on the new drift wave. An increase in the dust density and also in the mass of the dust particles further enhances the stabilizing influence.

Effect of flow profile on drift-type waves in a collisionless dusty plasma

An earlier study on non-local theory of low frequency waves in a collisional dusty plasma has shown the existence of a new type of drift wave with a new frequency, a new gyroradius and a new acoustic speed. It was found that the presence of the negatively charged dust particles has a stabilizing effect on the new drift wave. The present work investigates the effect of a flow profile on the new drift wave in a collisionless dusty plasma. It is found that the velocity curvature term has a strong stabilizing influence which reinforces the stabilizing effect of the sheared magnetic field. The first velocity field derivative plays an insignificant role in the stability analysis. An increase in the dust density and also in the mass of the dust particles also contributes to stability.

Low frequency drift instability in multi-species collisionless plasma

A non-local theory is essential for the understanding of the mode structure and hence corresponding transport in a dusty plasma. An investigation is made to understand the stability criteria for drift waves with positive ion density as well as positively charged dust particles density gradients and in presence of the magnetic shear in a dusty plasma.

A radial eigen value equation is obtained to derive a Weber equation. The growth rate of the drift wave is analyzed. It is found that the presence of heavy ions with multiple charge states decreases the unstable wave number.

Effect of parallel flow shear and curvature in Rayleigh-Taylor instability

Sen et al. [Phys. Plasmas 2, 2705 (1995)] reported that a negative flow curvature in the parallel flow has a stabilizing effect on drift waves and width gets elongated. Velocity shear, on the other hand, plays an insignificant role both in determining the mode stability and its radial width.

Heavy fluid supported by a lighter fluid in the presence of gravitational field leads to Rayleigh-Taylor (RT) instability. The RT instability is one of the several instabilities effecting plasma transport. This instability is also known to be major problem for a wide range of applications, from pulsed power technology to inertial confinement fusion.

In this work the combined effect of parallel flow shear and curvature on the RT instability is investigated. A non-local stability analysis for the RT modes is carried out on the basis of two fluid model. A radial eigen value equation is obtained to derive the Weber equation. It s found that though the parallel flow curvature suppresses the growth of RT instability, the shear does not.

Electron-positron-ion plasma

Recently the studies in different nonlinear coherent structures such as solitons, vortices etc., in the electron-positron-ion plasma, have received a great deal of attention from several scientists.

We have investigated the solitary Alfven waves in electron positron ion plasmas. The Alfven wave is shown to have an exact solitary wave solutions for a small but finite value of $\beta$. It has been shown that the existence regions are different if the value of $\beta$ is changed, however, the change in direction cosine in $x$ simply changes the soliton width.

Double layers in electron-positron-ion plasma

A theoretical model is presented to investigate the double layers, associated with the kinetic Alfven waves, in a magnetized electron-positron-ion plasma with a small but finite value of $\beta$ and small amplitude double layer solutions are obtained. The existence of small amplitude double layers requires that the density of ions is appreciably larger than that of positrons at equilibrium. The properties of the double layers are determined by the ratio between the number densities of positrons and ions at equilibrium, the direction cosines defining the moving frame, as well as the electron to positron temperature ratio.

We have presented a model for investigating DLs, associated with Alfven waves, in a magnetized e-p-i plasma. The solutions admit both compressive and rarefactive DLs. The primary condition for the existence of SADLs is that the positron density should be appreciably smaller than the ion density at equilibrium.

Experimental Work: Low Temperature Plasma

The Centre has developed a moderately equipped laboratory to conduct basic plasma physics experiments. Besides this, funds are also made available to the Centre by several funding agencies for procuring equipment and to carry out some important experiments.

Department of Science and Technology, Govt. of India approved a research project entitled An experiment to measure charge on Dust in Plasma with related optical and Laser diagnostics for a duration of 3 years. The above project has become operative from February, 1997.

The aim of the project is to measure the charge accumulated on the dust grains in the plasma. There have been considerable achievements in the field of theoretical studies on dusty plasma, including the works done at CPP. However, to understand the dust plasma interaction completely, supporting experimental works are very much necessary.

Our experimental arrangement basically contains two chambers connected through a nozzle. In the first chamber, magnetically confined plasma is produced by hot cathode filament with $T_e$ around 5 eV and $n_e$ around $10^8$ per cc. In the second chamber, dust is produced by evaporating metallic silver. The two chambers are maintained around $10^{-4}$ and 10 Torr respectively and dust particles flow to the plasma chamber through the nozzle due to this pressure gradient in the form of a collimated beam. Dust particles collect charge which is predominantly negative because of higher electronic mobility. The dust particles, as they move out of the plasma, pass through deflector plates between which a DC voltage is applied. The field deviates the dust from its original path and this deviation will be measured as the particle will stick to a plate at the extreme end of the chamber designed specially for this. From this deviation, the charge on the dust particle is determined.

The above experimental arrangement has been aasembled and made operational. Preliminary investigations on attaining vacuum and plasma parameters were performed and conducted in pristine plasma and later silver dust was also produced in the newly set-up system. Vacuum nearing $10^{-6}$ mb and plasma density of the order of $10^{13}$ $m^{-3}$ were obtained with temperature nearing 5 eV in a filament plasma. The system is now being fitted with a permanent magnet cage inside the chamber for increasing the electron density. Analysis of the dust formed by evaporation of silver in the oven chamber is being arranged in the R.S.I.C., NEHU, Shillong. The collective results are being used to measure the charge on the dust when they passes through a plasma.

Another work taken up during the year was the designing of a low cost integrated modulator for using in the Plasma Immersion Ion Implanation as an alternative to the Conventional high voltage pulse modulator which share a considerable part of the total cost for PIII equipment. This work was undertaken in the Forchungszentrun Rossendorf ev, Institut fur Ionenstrahlphysik and maerialiforschung Dresden. The low cost modulator consists of anode, cathode and a grid, all immersed in the plasma. A positive high volt electrode of the capacitor bank is connected to the anode which is surrounded by a grid whereas, the other electrode of the capacitor isw connected to the workpiece (cathode) when the grid is negatively biased, the anode is isolated from the plasma. When the grid potential is suited to ground potential, plasma could reach the anode to bring the anode from positive high volt to plasma potential and the cathode to negative high voltage potential. As evident, the performance of the integrated model depends on the maximum of the electron current which can be extracted from the plasma by the anode, efforts are being made to increase it by introducing more grids to accelerate the plasma electrons by biasing a positive voltage. With a single grid, switching of 4 A at 20 kV was possible. The work using a terode system is still being conducted at FZR using specialised electrical feedthrough designs to avoid formation of arcs and to obtain an anode current upto 20 A for use of the prototype into commercial systems.

Experimental Work: High Temperature Plasma

Developed in the early sixties by Mather in USA and FIllipov in USSR, dense plasma focus (DPF) is a combination of elctromagnetic shock tube and Z-pinch with a better stability of pinch plasma column. It is a pulsed plasma device which makes efficient use of self generated magnetic field to compress the plasma to very high density ($\sim$ 10$^{25}$ - 10$^{26}$) and high temperature ($\sim$ 1 - 2 keV) with a life time of about 50-100 nanosec. Besides being a source of hot dense plasma, it also emits neutrons ( when operated in deuterium medium), soft and hard X-rays, energetic ions and relativistic electrons. The DPF has found many applications in different areas, namely neutron activation analysis, X-ray lithography, X-ray microscopy, neutron radiography , pumping source for laser etc. More recently, the application of DPF in the field of material processing motivates the DPF researchers in a significant manner.

The Device

The DPF device, a low energy (2.2 kJ) Mather type, is installed at CPP in collaboration with BARC, Bombay. The device is basically consisted of two cylindrcal coaxial electrode assembly kept inside a vacuum chamber. One end of two elctrode is open while the other end is closed but separated by a coaxial cylindrical glass insulator. The energy to the electrode system is delivered by a 7.1 $\mu$F, 25 kV capacitor with an inbuilt pressurised spark gap switch. The chamber is evacuated to an optimum pressure for focussing action using a rotary pump.

Experimental works

The experimental works carried out using DPF device during the year 1998-99 are as given below:

Surface hardening of High Carbon Steel

After the installation of DPF, the researchers of the institute first attempted for a successful utilisation of energetic ions produced in the DPF for a surface modification of high carbon steel(HCS) sample. The plasma-gen gas chosen was nitrogen to achieve industrially acceptable nitride layer on HCS surface. For this the HCS sample were mounted inside the DPF chamber, axially above the anode ( central electrode), with the help of a movable sample holder. The mounted HCS samples were then irradiated by the energetic nitrogen ions produced in DPF for 12 and 35 times at heights of 4 and 5 cm from the top of the anode. The untreated and iontreated samples were than analysed by X-ray diffraction(XRD), microhardness measurements, optical and scanning electron microscopy (SEM). The XRD analysis showed the formation of Fe-C, (Fe-Cr)$_{2}$N$_{1-x}$, Fe$_2$N, Fe$_2$O$_3$, Fe$_3$O$_4$ FeO in the iontreated samples. The microhardness measurement showed the increment of hardness of the iontreated samples upto three times than that of untreated one. The formation of hard layer is found in the range of 60 to 150 $\mu$ by varying the number of focussing shots at different heights as mentioned above. The microstructure viewed by optical microscopy has showed the surface filled with spheroids and globules in one sample and presence of irregular grains with sparesely distributed hillocks in another one. At higher magnifications when the surfaces were viewed under SEM, the grains of the two samples were looked aliked and the intergranular regions were marked with voids and defects.

From the results obtained from the above said analysis, it can be concluded that the surface morphology of nitrided layer of the iontreated samples is comparable to that of iron nitride deposited by high frequecy plasma discharge and microhardness of the grains is in a range acceptable for industrial applications.

Temporal evolution of soft X-ray emission of DPF using X-ray vacuum photodiode

DPF is an eminent source of various electromagnetic radiations ranging from microwave to X-rays. Various techniques such as semiconductor diodes, surface barrier detectors, X-ray streak camera, framing camera have been used to get the temporal evolution of X-rays from DPF. It is seen that the semiconductor diodes, surface barrier detectors suffers from the saturation problem whereas other high speed detectors like streak and framing cameras have some advantages in time resolution measurement but involves complicated electronics. For this purpose we have used a biplaner vacuum photodiode which is a simple, cost effective and easy to fabricate. The vacuum photodiode basically consist of a mesh (grid) which acts as anode and a circular metal disc behind the mesh to act as a photocathode. We applied -ve potentials to the photocathode while the grid was kept grounded. When X-rays fall on the photocathode, the photocathode releases electrons without any delay whose energies are decided by the wavelength of the incident photons. Due to the biasing voltage present in between photocathode and grid(anode), the ejected electrons are collected by the grid and subsequently a signal from the photocathode can be recorded on a fast response digital oscilloscope across a 50 ohm termination. At present we have obtained this time resolved X-ray signal of DPF using 6 $\mu$ Al filter in front of the vacuum photodiode.

Study of current sheath dynamics of Dense Plasma Focus (DPF) using a Magnetic probe

Magnetic probe is a simple diagnostics used to investigate the structure of current, to calculate the velocity of current sheath, thickness of the current sheath and actual current folowing through it. It consists of a few turns of copper wire wound on a nonmagnetic bobbin, enclosed in an insulating tube.

We had constructed magnetic probes having different turns, winding SWG 46 enameled copper wire on to a small plastic sleeve of about 1 mm diameter. Electrical insulation is made by keeping the coil and the lead wire inside a glass tube of diameter 5 mm. The output of the magnetic probe is fed to oscilloscope using a coaxial cable via a suitable RC integrating circuit. The probe is inserted axially inside the focus chamber and the coil is rotated around its axis such that the magnetic flux will pass through the coil axis resulting in the maximum amplitude signal.

Varying the axial position of the probe and notting the corresponding signal from the oscilloscope we found parabolic structure of the current sheath. Velocity of the current sheath is found to be of the order of 10$^5$ m/s. The current sheath thickness was estimated by multiplying the rise time of the magnetic probe signal with the velocity of current sheath. The calculated thickness is of the order of 10$^{-2}$ m.

Characterisation of ion beam emitted from DPF using a submicrosecond response Faraday Cup (FC)

A low inductance charge collector system capable of BIC mode of operation has been fabricated and tested for measuring the ion energy spectrum of DPF device by time-of-flight analysis. The design features which makes the FC unique is that it can register ion energy of higher kinetic value ($\sim$ MeV) as well as lower kinetic value ($\sim$ keV).

More recently, application of the ion beam of DPF in the field of plasma processing (ion implantation, thermal surface treatment, ion assisted coating, device fabrication, thin film deposition etc.) motivates the DPF research in a significant manner. In order to have a better understanding to the physical and chemical process behind the ion beam interaction with surface, one needs to have a deeper insight to the ion flux and ion energy spectrum produced in this device. In past, very few efforts were made to characterise the nitrogen ions (particularly having low energy) of DPF device. The purpose of the present work is to develope a low inductance fast response FC that detects ion energy with sufficiently high resolution in particular low energy regime.

The FC mainly comprises of a conical graphite cup (inner electrode) that encircled by a conical brass shield (outer electrode). The inner electrode acts as a collector which is 20 mm long having 11$^0$ solid angle. The graphite is chosen as the inner electrode because of its minimal secondary electron emission (SEE) effect. A negative biased voltage is applied to the inner electrode while the outer electrode is grounded. The electrical insulation between electrodes is accomplished by inserting the inner electrode inside a nylon cone. The dimensions of inner and outer electrodes are typically estimated so as to get characteristic impedance of the FC as 50 $\Omega$. The FC entrance pinhole is 0.5 mm in diameter. The inductance and resistance of the FC is measured using LCR Systronics bridge and found to be $\sim$ 1 $\mu$H and 200 m$\Omega$ respectively. The FC operating in the bias ion collector (BIC) mode was employed to characterise nitrogen ions generated in the DPF device.

After a series of preliminary DPF discharges, it was observed that best operating condition (concerning signal-to-noise ratio) in the present investigation corresponded to a biasing voltage -200 V for the FC. Therefore the FC was operated at -200 V throughout the study.

In order to study the ion beam intensity production as a function of filling gas pressure, the DPF device was operated at six different values of pressure (19.74 - 52.63 Pa). In each pressure regime, more than 10 shots were taken because of shot to shot variation in DPF discharges. Five shots with similar voltage signal were considered for each operating pressure and the ion beam intensity corresponding to those five shots were noted. We note that the ion beam intensity initially increases with increase in filling gas pressure and reaches a maximum at a pressure of 32.89 Pa and subsequently decreases. The best operating filling gas pressure for ion beam detection in our case is 32.89 Pa. Therefore, under this condition, further characterisation of ion beams of DPF were carried out. It is to be noted that the FC has registered ions upto a lower kinetic energy threshold of $\sim$ 7 keV which is a value much lower than that obtained by others. From the energy spectrum of ion beam it is seen that ion density is maximum at an energy of 25 keV and minimum at low and high energies. We have found that the ion beam intensity clearly depends on the filling gas pressure.

LIST OF PUBLICATIONS

1. N. Das and K.S. Goswami: Propagation of small amplitude shock wave associated with ion-acoustic like mode in a dusty plasma, Phys. Plasmas 5, 312 (1998).
2. S. Sen and D. Pfirsch: Ponderomotive Modification of drift-ballooning modes, Bulletin of American Physical Society, 42, (1998).
3. S. Sen: Parallel flow and core confinement improvement, Phys. of Plasma, 5, 1000 (1998).
4. S. Bujarbarua, M. Nambu, B.J. Saikia, M. Eda and J.I. Sakai: Numerical simulation and theory of generation of electromagnetic waves in the presence of whistler turbulence, Phys. Plasmas 5, 2244 (1998).
5. S. Sen, P.K. Sharma and D. Bora: Role of flow shear and flow curvature in the suppression of low frequency fluctuations, Phys. Plasmas 5, 2637 (1998).
6. S. Bujarbarua, M. Nambu, B.J. Saikia, M. Eda and J.I. Sakai: Simulation study of nonlinear plasma maser, Phys. Scr. T75, 46 (1998).
7. S. Sen, R.A. Cairns, B. Dasgupta and G. Pantis: On creating transport barrier by radio-frequency waves, Proc. of the Second Asia Pacific Plasma Theory Conference (edited by Y. Tomita, Y. Nakamura and T. Hayashi), NIFS-PROC-38, (1998) p 122.
8. H. Kakati and K.S. Goswami: Solitary Alfv\'en waves in electron-positron plasma, Phys. Plasmas 5, 4229, 1998.
9. S. Sen and R.A. Cairns: Suppression of drift type instabilities by radio frequency waves, Phys. Plasmas 5, 4280 (1998).
10. M. Kakati and K.S. Goswami: Solitary wave structures in presence of nonisothermal ions in a dusty plasma, Phys. Plasmas 5, 4508 (1998).
11. C.B. Dwivedi, K.R. Rajkhowa, S. Bujarbarua and J. Chutia: Effect of external gravity on plasma sheath, J. Plasma Phys. (1998). Communicated.
12. N. Das and K.S. Goswami: Theory of random motion of a particle in dusty plasma, Phys. Plasmas (1998). Communicated.
13. S. Sen and R.A. Cairns: Radio frequency waves and the formation of transport barriers, Phys. Rev. Lett. (1998). Communicated.
14. T.K. Borthakur, A. Sahu, S.R. Mohanty and H. Bhuyan: Possible utilisation of dense plasma focus for surface hardening, Proceeding of the XII National Symposium on Plasma Science \& Technology (1998). Communicated.
15. B.J. Saikia and M. Nambu: Plasma maser instability in magnetized dusty plasma, Phys. Scr. 59 62 (1999).
16. M. Nambu, B.J. Saikia, D. Gyobu and J.I. Sakai: Nonlinear plasma maser driven by electron beam instability, Phys. Lett. A 252, 198 (1999).
17. T.K. Borthakur, A. Shau, S.R. Mohanty, B.B. Nayak and B.S. Acharya: Surface hardening of high carbon steel by plasma focus nitriding, Surface Engg. 15, (1999).
18. M. Nambu, B.J. Saikia, D. Gyobu and J.I. Sakai: Nonlinear plasma maser driven by electron beam instability, Phys. Plasmas 6, 994 (1999).
19. K.R. Rajkhowa, C.B. Dwivedi and S. Bujarbarua: Stability analysis of a model equilibrium for a a gravito-electrostatic sheath in a colloidal plasma under external gravity effect, Pramana-J. Phys. 52 293 (1999).
20. S. Sen and R.A. Cairns: Transport barriers by Alfv\'en waves, Bulletin American Physical Society, 43 (1999).
21. S. Sen, R.A. Cairns and R.G. Storer: Parallel velocity shear driven modes in discharges with reverse magnetic shear, Bulletin American Physical Society, 43 (1999).
22. S. Sen: Radial envelope of toroidal drift waves with general equilibrium profiles, Proc. of the 4th Asia Pacific Plasma Theory Conference, Seoul, Korea (1999).
23. H. Kakati, K.S. Bujarbarua and S. Bujarbarua: Theory of Langmuir wave generation in presence of Alfven wave turbulence in an electron-positron plasma, Phys. Plasmas (1999). In Press.
24. T.K. Borthakur, A. Shau, S.R. Mohanty, B.B. Nayak and B.S. Acharya: Plasma nitriding of high carbon steel using dense plasma focus device, Proceedings of "National Conference on Thermophysical Prop. of Solids and Fluids", March 11 - 13, 1999, Gauhati University. Communicated.
25. H. Bhuyan, S.R. Mohanty and T.K. Borthakur: Development of a submicrosecond response Faraday Cup, Proceedings of "National Conference on Thermophysical Prop. of Solids and Fluids, March 11 - 13, 1999, Gauhati University. Communicated.
26. S. Sen: On Applicability of Ballooning formalism in the presence of parallel flow shear, Phys. Lett. (1999). Communicated.
27. D. Sarmah and S. Sen: Axial flow and Rayleigh-Taylor instability, Phys. Rev. Lett. (1999). Communicated.
28. D. Sarmah, S. Sen and K.S. Goswami: Nonlocal theory of drift-acoustic waves in a dusty plasma, Phys. Plasmas (1999). Communicated.
29. M. Chakraborty and S. Sen: Colissional drift waves in a dusty plasma, Phys. Rev. Lett. (1999). Communicated.
30. M. Chakraborty, S. Sen and B.K. Saikia: Effect of equilibrium flow on low frequency modes in a dusty plasma, Phys. Plasmas (1999). Communicated.
31. N. Das, B.J. Saikia and K.S. Goswami: Theory of Transport Properties of dusty plasmas, Pramana-J. Phys. (1999). Communicated.
32. S. Sen, R.A. Cairns and R.G. Storer: Stability and transport of parallel velocity shear driven modes with negative magnetic shear, Phys. Rev. Lett. (1998). Communicated.
33. M. Kakati and K.S. Goswami: Coherent structures in presence of dust charge fluctuations, Phys. Plasmas (1999). Communicated.
34. S. Sen and G. Pantis: Stability of drift waves in a reverse shear plasma, Phys. Plasmas (1999). Communicated.
35. M.K. Mahanta and K.S. Goswami: Theory of sheath in a multicomponent plasma, Phys. Plasmas, (1999). Communicated.
36. S. Sen and R.A. Cairns: Parallel velocity shear instability, Alfv\'en waves and the formation of the transport barrier, Phys. Rev. Lett. (1999). Communicated.
37. S. Sen, C. Xiao and A. Hirose: On improved confinement in STOR-M by CT injection, Phys. Plasmas (1999). Communicated.
38. H. Kakati and K.S. Goswami: Double layers in a magnetized electron-positron-ion plasma, Phys. Plasmas, 1999. Communicated.
39. S. Sen D. McCarthy: Parallel velocity shear instability in the reverse magnetic shear revisited, Phys. Plasmas (1999). To be Communicated.
40. S. Sen and A. Hirose: Effect of parallel flow on kinetic ballooning modes in the reverse magnetic shear, (1999). (In preparaton).

Conference/Symposium Presentation

• School on PLasma Physics, IASST, Guwahati, April 20-30, 1998.

T.K. Borthakur: Dense plasma focus device
H. Bhuyan: Magnetic probe used in Dense Plasma Focus

• XIII National Symposium on Plasma Science & Technology, Rajkot, December 2-5, 1998.

C.B. Dwivedi, K.R. Rajkhowa and S. Bujarbarua: Three scale analysis of a steady state plasma sheath model under external gravity effect.
K.R. Rajkhowa, C.B. Dwivedi and S. Bujarbarua: Stability analysis of a model equilibrium for a gravito-electrostatic sheath in a colloidal plasma under external gravity effect.
T.K. Borthakur, A. Sahu, S. R. Mohanty, B.B. Nayak and B.S. Acharya: Nitriding of high carbon steel using dense plasma focus.
D. Sarmah, K.S. Goswami and S. Sen: Low frequency drift instability in multi-species collisionless plasma.
D. Sarmah and S. Sen: Effect of parallel flow shear and curvature in Rayleigh-Taylor instability.
M. Chakraborty and S. Sen: Non local theory of low frequency waves in a collsionless dusty plasma
M. Chakraborty, S. Sen and B.K. Saikia: Effect of flow profile on drift-type waves in a collisional dusty plasma
H. Bhuyan, S.R. Mohanty and T.K. Borthakur: Ion beam characterisation of a low energy Dense Plasma Focus

• Regional Conference on Physics Research in the North East, IIT Guwahati, 17 October 1998.
S.R. Mohanty, B.K. Saikia: Experimental research programme at Centre of Plasma Physics
H. Bhuyan, S.R. Mohanty and T.K. Borthakur: Dense Plasma Focus Facility at CPP

• National Conference on Thermophysical Prop. of Solids and Fluids, Gauhati University, March 11-13, 1999.

T.K. Borthakur, A. Shau, S.R. Mohanty, B.B. Nayak and B.S. Acharya: Plasma nitriding of high carbon steel using dense plasma focus device.
H. Bhuyan, S.R. Mohanty and T.K. Borthakur: Development of a submicrosecond response Faraday Cup.

The Centre runs a Ph.D. programme with students registering with Gauhati University. During the last eight years, four students have been awarded Ph.D. degree for their theses.

Colloquia

The Centre regularly organise lecture/colloquia among its members and by invited speakers from various institutes/universities.

Visits/Talks

The Centre's scientists often visit different institutes/universities both within the country and abroad for collaborative research work and to deliver talks/to attend important symposia/conferences to present their research reports.

• S. Sen, Associate Professor
attended the meeting on Current Trends in International Fusion Research: Review and Assessment, Washington, D.C., USA (1999) ( Invited Speaker); IOP Plasma Conference, Pitlochry, UK (1999) ( Invited Speaker); 4th APPTC Conference, Seuol, Korea (1999) ( Invited Speaker);
visited Princeton University, Princeton, USA (1999); Hampton University, Hampton, USA (1999); University of Saskatchewan, Saskatoon, Canada (1999).

• S.R. Mohanty, Asstt. Professor
visited Department of Physics, University of Delhi, Delhi as a TPSC speaker from 11th June to 10th July, 1998 and delivered a lecture on "Surface hardening of high carbon steel using nitrogen ions of Plasma Focus"; Purnima Laboratory, BARC, Mumbai from 16th November to 26th November, 1998; Institute of Material Science, Bhubaneswar from 12th January to 15th January, 1999 and delivered a lecture on "Dense Plasma Focus device, associated phenomena and its application"; BARC, Mumbai from 4th March to 8th March, 1999 to defend the fresh BRNS proposal entitled "Experimental studies of X-ray and ion emission from CPP plasma Focus facility"; Physics Department, University of Delhi, Delhi in AAAPT programme form 1st April to 31 April, 1999.

• B.K. Saikia, Asstt. Professor
visited Institut fur Ionenstrahlphysik und Materialforschungs, Forschungzentrum Rossendorf eV, Dresden, Germany from September 8, 1998 to January 7, 1999; Abdus Salam ICTP, Trieste, Italy during from October 12 to November 6, 1998 and participated in the 5$^th$ College on Microprocessor Based Real Time Systems in Physics; Institute for Plasma Research, Gandhinagar during february-March, 1999.

• T.K. Borthakur, Research Fellow
visited Centre for Advanced Technology, Indore from 13 September to 25 October, 1998.

• K.R. Rajkhowa, Research Fellow
visited Institute for Plasma Research, Gandhinagar, for one month in November, 1998.

• M. Kakati, Research Fellow
visited Laboratory of Vacuum Technique, Bangalore, during June, 1998 and January, 1999; Institute for Plasma Research, Gandhinagar, during February-March, 1999.

Collaboration

The Centre of Plasma Physics has active collaboration with the following Institutes/ Universities:

Bhaba Atomic Research Centre, Bombay; Centre for Advanced Technology, Indore; Institute for Plasma Research, Gandinagar; IPP, Juelich, Germany; IPP, Garching, Germany; Kyushu University, Fukuoka, Japan; Physical Research Laboratory, Ahmedabad; Regional Research Laboratory, Bhubaneswar; Rurh University, Bochum, Germany; Saha Institute of Nuclear Physics, Calcutta; St. Andrews University, UK; Tokyo Metropolitan Institute of Technology, Tokyo; University of Bayreuth, Germany; University of Kyoto, Japan.

AWARDS & RECOGNITION

• S. Sen, Associate Professor, has been awarded EPSRC Professorship Award (1998), UK; JSPS Professorship Award (1999), Japan; Junior Membership Award (1999), Issac Newton Institute for Mathematical Sciences, Cambridge, UK; and Associateship Award (1999-2005), ICTP, Trieste, Italy.
• S.R. Mohanty, presently Assistant Professor, has been awarded Ph.D. degree by the University of Delhi for his thesis entitled "X-ray studies on dense plasma focus and plasma processing".
• M. Kakati, Research Fellow, has been awarded Senior Research Fellowship of the Council of Scientific \& Industrial Research (1999-2001).
• K. R. Rajkhowa, Research Fellow, has been awarded Plasma Science Society of India Fellowship for the year 1999.
• B.J. Saikia, Research Scientist, has been awarded Japan Society for the Promotion of Science post poctoral fellowship for two years.

EXISTING FACILITIES

The Centre is slowly developing a library. It houses technical books, bound volumes of journals, reports etc. in its collection. It also subscribes to several national/international scientific journals.

The library has cooperation with other libraries, particularly with Institute for Plasma Research Library and research groups at Germany and Japan which enables it to borrow materials from them. The Library is equipped with a photocopier.

Computer

CPP has a moderately equipped computer centre consisting of 3 PCs, one laser printer, one dot matrix printer and several software. It is connected to Research and Educational Net (REN) of National Informatics Centre (NIC) with dial-up facility for electronic mail (E-Mail) and to VSNL for internet.

Workshop

The Centre is developing a workshop to help the researchers in their mechanical and electrical jobs in the laboratory. It has acquired a number of machines, electrical tools and electronic gadgets.

PLAN OF RESEARCH FOR NEXT YEAR
(1999-2000)

Theoretical Work

As has been described in Section (\ref{ssec:wtt}), plasma maser effect is one of the lowest order mode-mode coupling process in weak plasma turbulence theory, and it is recognized as a possible mechanism for the up-conversion of energy in space and astrophysical plasmas such as Jovian kilometric radiation, Saturnian kilometric radiation etc. We propose to continue our study on plasma maser effect extending it to inhomogeneous plasma. Most of the space and astrophysical plasmas are basically inhomogeneous, the inhomogeneity may occur both in magnetic field as well as in particle distribution function. It is expected that such inhomogeneity will enhance the growth of the high frequency wave in presence of low frequency fluctuation. We also propose to study the plasma maser interaction in the dusty plasma. The presence of dust enhances the phase velocity of the low frequency electrostatic turbulence, this in turn increases the density of the resonant electrons, which is a favourable situation for plasma maser interaction of a high frequency mode in presence of the turbulence.

Studies on the various collective modes in unmagnetised and magnetised dusty plasma is a very important area of research. With dust as an additional dynamical variable, there will be severe modifications in the collective properties of the dusty plasma. We propose to develop a consistent theoretical model for the charging of the dust particles in presence of external electromagnetic field. The charging of dust particles in magnetised plasma is found to be very difficult. The charge of the dust particles can fluctuate due to turbulence or spatial and temporal variations of plasma parameters. In presence of magnetic field, the motions of the particles are no longer linear, they move along as well as across the field lines. Therefore in this case the usual orbital limited motion theory for the charging of the dust particles is not valid. For strong magnetic field a fully satisfactory theory for the charging has not yet been developed. The study of the charge dynamics of dusty plasma will help us to understand the effect of electromagnetic forces on the dynamics of grains in planetary magnetospheres, rings comets etc.

We also propose to undertake study on transport phenomena in dusty plasma. Knowledge of the transport coefficients of a dusty plasma is critical for the understanding of plasma processing, plasma etching etc. Physical processes like heat transfer, current flow etc. in the presence of charged dust are much more complex than in an ordinary plasma. A study of the transport coefficients will help to understand these phenomena properly.

a) One of the most important classes of plasmas that can be excited nonthermally in laboratories and in which turbulent fluctuation coexists with persistent coherent structures. To our knowledge, the macroscropic properties of such type of systems are not yet known. The main question is that of quasilinear treatment in which coherent part is absent. To understand this highly developed plasma state one should study the stability analysis of the nonlinear coherent structures which will provide information about the normal mode spectrum of the system.

In last few decades, a large number of theoretical models have been put forward for the existence of nonlinear coherent structures associated with different modes in unmagnetized plasma. However we never studied their stabilities. In the proposed plan period we shall try to analyze the linearized eigenvalue problem associated with electrostatic perturbations of localized electrostatic Vlasov equilibria and the evaluation of the spectral operator etc. We hope that for electrostatic coherent wave, a kinetic treatment is more appropriate than the fluid treatment.

In the mid 80's Goswami and Bujarbarua (K.S. Goswami and S. Bujarbarua, Phys. Lett. A, 108, 149, 1985) have shown for the first time that for a ordinary ion acoustic branch the small amplitude double layers can exist in a plasma containing two types of electrons i.e. one hot component and the other cold component along with cold ions. It has been shown that both compressive and rarefactive type of double layers can be possible depending upon the density and temperature ratio of two electrons.

b) The studies of physical processes near the wall i.e. near the boundary between the plasma and a solid surface is very important in many respect. Specifically, sputtering of the wall material and the other modifications in the surface structure caused by the flow of deuterons accelerated in the near wall electric field are very important for tokamak operation. The condition near the solid surface, in particular, the value of the electric field near the wall, substantially affect transition between L- and H- regimes in a hot plasma column.

The main problem is to solve the Poisson's equation as well as to calculate the electron and ion densities present there. Many earlier workers have solved the problem using simplified models. For example, the potential distribution is assumed to be given or the distribution of charges is derived by using certain additional simplifying assumptions. Some workers used models based on the two scale theory (A.V. Nedospasov and M.Z. Toka\'r, Review of Plasma Physics, Moscow: Energoatmizdat, 18, 68,1990) or three (B.Ya. Moizhes and G.E. Pikus, Ed. Thermoemission converters and low temperature plasma, 1973) strongly differing spatial scales - the Debye radius, the ion mean free path, the spatial plasma thickness and so on. Then they joined the solutions obtained for different regions. But this type of simplified models does not ensure correct qualitative results. It is seen that the basic characteristics of the near wall layer, such as spatial dimension of the layer, the value of the electric field surface of the wall and the ion flow to the wall calculated from the simplified models can differ considerably from real values. In the proposed study we shall try to examine the solution of the Poisson's equation by using mechanical analogy. We shall also try to study the phenomena like ion scattering occurring near the boundary between the plasma and the surface, validity of Bohm condition and the relationship between the ion density and the potential and the parameters of the medium.

Positrons are of interest to plasma physics today because they annihilate electrons and having the same mass but opposite charges. Positrons and electrons can be combined to form neutral plasmas with a dynamical symmetry between the charges. It has been shown that the relativistic electron- positron plasma can be produced in the laboratory. The main motivations of studying the plasma physics with positrons are - (i) both charges are highly magnetized and therefore, one can go from a nonneutral plasma to a neutral plasma by varying relative density of electrons and positrons, (ii) it will also be possible to test new theories regarding the confinement of highly magnetized plasma and (iii) to develop methods for accumulation of antimatter. Recent progress in the development of methods for trapping and storing positrons now permits the accumulation of a sufficient number of low temperature positrons to form a plasma.

Recently we have studied the existence of solitary waves in a electron- positron plasma associated with Alfv\'en wave. We have also studied the mode-mode coupling process in electron-positron plasma. Recently we have shown the generation of high frequency Langmuir wave by mode-mode coupling process from low frequency Alfven wave excited by electron and positron temperature anisotropy. In the above studies we have considered only homogeneous plasma in thermal equilibrium. In the proposed plan period we shall try to study the various collective modes and the instabilities in an inhomogeneous electron- positron plasma considering the specific plasma confinement geometry. It is expected that the collective mode depends on the charge excess for the electron-positron plasma much more sensitive than for the electron-ion plasma.

a) The Jovian magnetodisc contains dust particles which have originated from the volcanic activity of the satellite Io. It has been suggested that due to magnetohydrodynamic instabilities such as ballooning instabilities, interchange instabilities under the action of centrifugal force may raise the temperature of the heavy positively charged particle in the Jovian disc to the level of corotational energy. In the proposed plan period we shall try to develop the physical factors affecting the stability of ideal MHD ballooning modes considering a coordinate system corotating with the planet and its magnetic field.

It is well known that the ballooning mode is driven when the ratio between the gravitational acceleration and the density gradient scale length is larger than the square of the Alfven transit frequency along the curved magnetic field lines. Thus the ballooning mode is the electromagnetic interchange mode and appears in the bad curvature regions. The local and non-local properties of ballooning modes have been derived from the magnetohydrodynamic equations when the temperature gradient is absent. However, in a non-uniform plasma with equilibrium density and temperature gradients, the MHD description fails and one has to resort to either the two-fluid or a gyro-kinetic description. In the proposed study we would follow the two-fluid description of the drift-ballooning modes in the absence and presence of the equilibrium flow both rigid and sheared.

b) When the mean free path for collision is much shorter than the characteristic scale length then the flow is hydrodynamics and then it can be determined by the analyzing the fluid equation of motion. On the other hand, when collision cross sections are small then situation is described by the collisional Boltzmann equation. It is interesting to note that this type of equation is a nonzero one because collisions can alter the distribution in phase space of particles. It is therefore extremely difficult to solve. However if the dominant collisional effects arise from small angle scattering then one can use the well known Fokker-Planck equation. Recently we have tried to develop a Fokker-Planck equation for a collisional dusty plasma. In the proposed study we shall try to develop a dusty accretion disc. We hope that the development of such dusty accretion disc should be able to give the idea about the formation of stars, clumps in space etc. We would like to follow the two-fluid description of the drift ballooning modes in the absence and presence of the equilibrium flow both rigid and sheared.

Experimental Work : Low Temperature Plasma

By application of an external variable electric and magnetic field in a linear plasma device, investigations will be made to see whether the flow shear or curvature that is responsible for suppression of fluctuations relating to fusion plasma. The radial electric field profile will be measured from the radial electric field potentials at various radial locations. The data will be compared with the radial profile of the fluctuating $E \times B$ drift originated by the combined electric and magnetic field will be used to verify the theoretical predictions on confinement inprovement. Theoretical, numerical and simulational works are also going to be carried out to investigate the possibility of suppression of fluctuations and micro-instabilities.

A proposal in this regard has been submitted to CSIR, and currently it is being assessed by the agency.

RF plasma discharge will be struck at different pressure and plasma power conditions in different ratios (5-100%)of $N_2$-$H_2$ and $CH_4$-$H_2$ mixtures. Low, medium and high SS alloys would be taken as samples. Formations of different active species like ions, radicals, atoms etc. are going to be detected and monitored by using Optical Emission Spectroscopy. The results are going to be correlated with the modified surface properties of the samples with a view to understanding the chemistry behind the formation of improved quality of the surfaces as a result of the thermochemical diffusion processes such as nitriding or carburising.

The proposal will be submitted to Cross Disciplines in Plasma Sciencs, DST, and will be presented before the Target Group Meeting in May, 1999.

Centre of Plasma Physics has a on going project in dusty plasma, which is being funded by Dept. of Science and Technology, Govt. of India. This experiment has been planned to investigate the interaction of dust particles with a plasma.

In the experimental chamber, a differential pressure is maintained in between the plasma chamber and the dust chamber, where dust particles are produced by joule evaporation of metals. Because of this differential pressure a collimated beam of dust particles is produced, which passes through the plasma and get charged. The charge collected by each grain in the beam will be determined with the help of electrostatic deflectors and a detector plate.

In order to produce a plasma of higher particle density, we will use a multipolar permanent magnet confinement system. This help dust particles get fully charged while passing through finite plasma volume. As more and more dust particles stick to the more massive dust particles, the plasma as a whole gets less negative. We will study this variation of plasma potential as we introduce the dust beam inside the plasma, with the help of simple Langmuir probes and also emissive probes.

We are also planning to develop some diagnostic techniques other than the simple Langmuir probes, for example, Spectroscopic, Polarimetric and laser diagnostics. In the ongoing dusty plasma experiment, we are planning to measure the plasma parameters by spectroscopic methods and compare them with standard Langmuir probe results. With the help of a Monochromator, a Febry-Perrot Interferrometer and CCD cameras, we will measure plasma temperature and density, either by ratio of line intensities emitted by the plasmagen gas or Doppler broadening of those lines.

Polarimetric technique will be used to determine the average size of dust particles in flight. A polarimeter will analyses the polarization status of a laser beam scattered by the dust particles and assuming Mie type of scattering, the sizes of dust particles will be determined. Velocity of the dust particles in the beam will be measured experimentally. Here we will use a pulsed laser and CCD camera and `flash photograph' the dust particles.

In the light of our experimental findings we will analyses whether the plasma permanently traps some dust particles or not and in the possible best case whether the plasma can be really treated as dusty plasma or not. A plasma dispersed with dusts can be termed as a dusty plasma when, the average inter grain distance is less then the Debye length.

For a more elaborate study of this particular field in plasma, we are planning to have a experimental set up for producing a large, extended volume of dusty plasma. So far, out of all devices for producing dusty plasma, the one used by Barkan et. al. at University of Iowa appears to be the best. Of course, in a experiment at IPR, India, Prabhakara et. al. had shown that dusts get trapped in the sheath region if a layer of dusts is exposed to a hot cathode plasma. But this method also suffers from a disadvantage, since the grain density can not be increased in the plasma volume at will. In our dusty plasma device, we will use a rotating dust dispenser similar to Barkan's. This is a mechanical arrangement which continuously disperse plasma with dust particles. Dust will get trapped in the sheath region or in a double layer, produced purposefully for it.

Experimental Work : High Temperature High Density Plasma

As discussed earlier (Section (\ref{ssec:dpf})), the Dense Plasma Focus (DPF) is a rich source for a variety of plasma phenomena including (X-rays soft and hard), neutrons, and particle beams. The following study will be carried out on this device in forthcoming years.

A fresh experimental project entitiled "Experimental studies of X-ray and ion emission from CPP Plasma Focus facility" has been submitted to BRNS, DAE, India in collaboration with Purnima Laboratory, BARC, Mumbai. The total estimated budget for this project is Rs. 30.06 lakhs. The main motivation behind this proposal is to develope the CPP Plasma Focus facility for industrial applications by characterising the hot plasam as well as the emission from plasma. Therefore, in this proposal we will like to have some advanced fast response diagnostics and equipments.

One of our objective is to develope Plasma Focus as an efficient X-ray source so as to utilize it for practical applications such as X-ray lithography and X-ray microscopy. The X-radiation emitted from Plasma Focus will be characterised by using various diagnostics like Diode X-ray Spectrometer, Von Hamos Spectrometer, X-ray pin hole camera and Microchannel plate. The other objective is to develope Plasma Focus as an excellent ion source so as to utilize it for material processing and making new materials. The ion emitted from Plasma Focus will be characterised by Faraday cup and nuclear track detector.

Another interesting component of X-ray studies in DPF is the observation of timeresolved X-ray signals from it with pressure variation at different filter foil thickness in front of the photodiode. After the completion of this work we will try to use a multichannel vacuum photodiode system with different filter foil thickness at each channel. From this study we will be able to know the effect of pressure on X-ray emission from DPF.

Plasma processing is an emergent technology that utilizes unique and exotic properties of plasma to effect physical, chemical and metallurgical changes in the materials to produce either a new material or impart new properties to the conventional materials. This technology has already made inroads into industrial area in advanced countries. But in India it has just made a beginning.

Visualizing the world wide attention in plasma processing and its industrial importance, CPP has recently started some research and development works in that aspect. The researchers of the institute for the first time have utilized their hot high density plasma producing device, namely the Dense Plasma Focus (DPF), for incorporating nitrogen into steel surface which resulted in a significant hardening of steel surface. Thus, their work has introduced a modern state of the art technology in the region of nitriding steel materials.

In coming years our researchers are planning to exploit high density high temperature plasma source as well as a source of rather cold low density plasma for processing. As mentioned in section, DPF device is a rich source of energetic ions and electrons, besides being a source of high density high temperature plasma. The energetic ions and electrons of DPF device are to be suitably utilized for processing works.

Systematic studies of nitriding and carbonitriding of various materials like SS302, Titanium, Aluminium and graphite using the energetic ions of the DPF will be made. For this we will vary the nmber of ion irradiations for the samples from 5 to 35 with an increment of 5 irradiation for each sample respectively. The heights of the samples will also be varied from 4 cm with an increment of 1 cm for each sample upto maximum 10 cm accordingly. The study will be done at different gas medium such as nitrogen, admixture of nitrogen-hydrogen, admixture of nitrogen-hydrogen-argon, ammonia etc. The temperature of samples will be measured by one J-type thermocouple during the work. Now from our Faraday Cup experiment we will be able to know the ion energy spectra at various positions in the axis from the top of the anode. From this we will try to find a corelation of ion energy with nitriding phenomena.

Aluminium nitride (AlN) is another attractive material because of its hardness, high thermal conductivity and excellent insulating properties. Its thin film has the highest SAW velocity of all piezoelectric materials. Hence it is used in high frequency SAW devices and insulating or passivating layers in Ga-As semiconductors. For the deposition of crystalline or transparent AlN films the existing techniques are very cumbersome. We can plan to use the nitrogen ions of DPF device to deposit this film in a much more simpler way. The deposited films thickness, composition, oxidation state, atomic bonding, and surface morphology will be analysed by using XRD, XPS, FTIR and SEM.

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