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Basic Experiments

Basic Experiments are performed in the following areas:-

Free Electron Laser ( View )   ( Homepage )

The Free-electron laser or FEL, is slated to be the 4th generation light source and is, in principle, capable of producing immensely powerful, monochromatic radiation in a wide wavelenth range from millimeter waves down to x-rays. Unlike conventional lasers, the essential components of an FEL are accelerated electron beam and a periodic magnetic field called "wiggler". The interaction of the relativistic electron beam with the EM wave(under right resonance conditions) as it propagates through the periodic magnetic field gives rise to the FEL radiation. The FEL program at IPR is aimed at developing a mm wave, high power laser using a Tesla-transformer and pulse forming line type electron beam accelerator. This is aimed at obtaining high powered mm wave radiation with a possible application to tokamak plasma heating. Having obtained FEL radiation in 1998 (first lasing of an FEL experimental laser in India) at a frequency of 86 GHz, current efforts are being undertaken to increase the interaction length in order to obtain higher FEL gain and hence output power.

Helicon Wave Plasma Experiment (view1)     (view2)

In Helicon wave plasma experiment, helicon wave is excited in a cylindrical bounded system using Nagoya 3 type antenna using 8.3Mhz rf source of 1.5kW Power. Density of 10**13cm-3 with temperature of 2 -3eV has been observed. Neutral starvation,where density rises to 1.5x10-13cm-3 for a period of 2 -5 msecs and then falls to 5 x10**12cm-3 in the first stage and again to 10**11cms in second stage, has also been studied. The ongoing experiment aims at single photon counting to study the primary electron accleration.

Basic RF Experiments

Large Volume Plasma Device (LVPD)   (view)

Investigation on the fundamental processes in naturally occurring plasma is important as they are responsible for large release of energy, generation of powerful electromagnetic radiation and acceleration of energetic particles etc. which have consequences to earth bound activities. These processes are highly complex due to the possibility of multiplicity of interactions between fields and flows in the plasma. The result being, study of each phenomenon requires spatially and temporally resolved measurements of field, flow and plasma parameters. As it is not possible to have an exact simulation of these boundary independent phenomena in a normal sized laboratory device, most of the investigations, until recently, were based on direct probing of space plasma. With the advent of space probes, the initial handicap of limited information gained from remote point, ground based measurements in the study of ionosphere, magnetosphere and solar wind was overcome. However, the main shortcoming, which stem from the lack of control over the processes, remains. This is because most of these measurements are carried out at specific times of occurrence of the concerned event, and are not amenable to control of parameter variation. The phenomena occurring are highly variable, and sufficient events can not be sampled to provide adequate data. Further, measurements with space probes do not allow spatial and temporal resolution of the data obtained. A clear need has therefore emerged for a laboratory device with minimum boundary effects, which provides benefits of carrying out controlled experiments. The size of the device should be such that it should accommodate sufficient number of wavelengths of the electromagnetic wave under concern, both along as well as across the ambient magnetic field. The production of dimensionally large, magnetized plasma for the study of electromagnetic phenomena is not easy. As the magnetic field provides directionality to the primary ionizing electrons, there arises a difficult requirement on providing a uniform, large area source of primary electrons. Dissatisfaction of this condition leads to undesirable non uniformity of plasma parameters and generation of large noise, specially during the plasma production. There exists a multiplicity of parameters that needs to be measured with adequate temporal and spatial resolution. Technological solutions to these issues are not simple. Experiments carried out on few devices developed in the past (eg: UCLA, Stenzel's group) have revealed details on certain linear and nonlinear processes of electromagnetic behaviour of the plasma. Many questions related to excitation of nonlinear waves, electromagnetic vortices, EMHD turbulence etc. yet remain unresolved. A Large Volume Plasma Device (LVPD) has been developed in our laboratory, dedicated to investigation of various phenomena related to electromagnetic structure and waves. Some of the major features of this device are: (i) a stable and large area plasma source, (ii) suitable confinement arrangements, (iii) a pulsed power system with turn off time $\sim$ 10 $\mu$ s, (iv) a probe drive system enabling high spatial resolution diagnostic measurements, (v) data acquisition and control system with high sampling rate and large memory storage and (vi) an extended coil system to produce desired magnetic field profiles within the chamber. The LVPD group has been actively involved in carrying out various experiments related electromagnetic structures in EMHD and turbulence. Some of the major experimental activities executed / under way are outlined below: 1) After construction of the large area multifilamentary plasma source, the large volume pulsed plasma produced has been characterized for uniformity and quiescence. Main observations are as follows : a) Plasma density ~ 10^12/cm^3 has been achived in the main glow regime. b) In the afterglow at about 400 µs after the switch off of the pulse, a uniform density has been achieved over axial as well as radial extents of ~ 1m. Te corresponding to this regime is about 2 ev. This is identified as the right parametric regime for excitation of whistler waves. c) A density hollow is observed in the radial profile, about 1000 µs after the pulse is switched off. 2) Whistler waves have been excited using various types of antennae and current waveforms. The experimental dispersion curves show good agreement with theoretical predictions for linear waves. 3) It has been claimed in the previous work that whistler waves are robust against nonlinearities due to their 3D topology. But all these works have been carried out using azimuthally symmetric exciters. In an attempt to change the topology of the whistler wave packet, linear multiwire antenna has been used to excite waves. In the linear regime, the results are found to be similar to earlier observations, leading to force free packets. Another experiment has been performed using large amplitude currents. Results do not show any radial asymmetry in the wave fields (an indication of nolinearity). An experiment has now been initiated in the highly nonlinear regime, where wave fields are comparable to the ambient fields. Experimental data analysis is under way. 4) Studies on EMHD structures in the inertial regime : An experiment has been performed in the hitherto unexplored regime in EMHD, where electron inertia effects start playing a role in the dynamics. All the previous experiments have been performed only in the noninertial regime where the gradient scale lengths are very large compared to the electron skin depth. The structures excited have been observed to transmit only part of their energy on encountering magnetic null points, where it is said that EMHD breaks down. Results from the present experiment have shown that the structures do propagate beyond the null points in the form of a whistler wave. Also, a sinusoidal wave of particular frequency is observed to leak and propagate with the characteristic whistler wave velocity. These results indicate strong nonlinear effects probably arising due to electron inertia. More detailed analysis is underway. 5) Studies on EMHD turbulence: Experiments have been initiated to study diamagnetic behaviour and the associated instabilities in the EMHD regime. Initial observations on the diamagbetic behaviour, and the nature of magnetic field and density fluctuations confirm the deviations from pressure balance equation in this regime. Efforts are now on to study the low to to high beta transition regime.

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