Conventional plasma, by definition, is a quasi-neutral collection of mutually interacting electrons and ions having a collective effect. In contrast, there is an exotic world of plasmas, which are ‘nonneutral’ having only one species. Such plasmas are therefore an exception to the definition. Pure electron plasmas are perhaps the most popular, although ion plasmas, positron plasmas and anti-proton plasmas are also possible. They all have certain peculiarities significantly different from their neutral cousins but are called plasmas because they too, like their neutral counterparts, exhibit rich collective phenomena. For more information on the underlying basic physics, one can have a look at the web pages, say, nonneutral plasma group at Physics Department, University of California, San Diego, U.S.A.
In order to carry out experiments on pure electron plasmas (the simplest possible nonneutral plasma to create and confine), an experimental system SMARTEX – C with some unique features shown below, has been setup in Institute for Plasma Research, Gandhinagar, India.
The SMall Aspect Ratio Toroidal Experimental System with C – shaped trap (SMARTEX – C) mimics the inject-hold-dump mechanism of cylindrical traps in a toroidal geometry. The experimental set-up shown in Figure 1 is the modified version of a device that earlier used to confine toroidal electron plasma. The inner radius of the vacuum chamber is 2.5 cm. A second inner wall placed concentric with the vacuum wall has a radius of 5 cm and acts as the inner wall for the plasma. The outer radius is 22 cm. This yields a ‘low’ aspect ratio of 1.6. The chamber is 32 cm in height. It is maintained at ~10-9 mbar (ultra-high vacuum conditions). The toroidal symmetry is broken at one location (0°) by an Stainless Steel plate. A tungsten filament comprising of a single circular loop of diameter 10 cm is placed on a poloidal cross-section on one side of the separator plate. The filament emits electrons by thermionic emission when a current of approximately 18 A is passed through it. It is biased to 300 volts negative with respect to ground. A grid (injector-grid) placed in front of the filament is biased initially to -370 volts, 70 volts more negative than the filament. Another grid (collector-grid) placed behind the filament in the poloidal cross-section, is also biased to -370 volts. The toroidal symmetry is thus broken. The injector and collector grids are placed at 30° and 345° toroidal locations respectively.
Figure 1: Schematic Layout of SMARTEX – C
A toroidal magnetic field is established in the chamber by pulsing a current through a 28-turn coil. The pulsed magnetic field has an overall duration of about 100 ms and a flat top of roughly 30 ms. As the B field reaches its flattop, the injector-grid is pulsed to 0 volts (+300 volts with respect to filament) for 60 µsec. Electrons are thereby injected along the field lines. Thereafter the grid reverts back to -370 volts, ensuring no further fuelling. The injected electrons are now trapped toroidally between the negatively biased injector-grid and collector-grid. Radial confinement is achieved by the toroidal magnetic field. SMARTEX-C has increased the confinement time of pure toroidal electron plasmas from several hundred µs to tens of milliseconds. Several investigations of toroidal modes, vortex evolution and instabilities have been carried out in the device. Figure 2 shows the actual experimental system with toroidal field coils and other associated subsystems
Figure 2: SMARTEX- C with toroidal magnetic field coils (red cables wound over the torus)
The indicative figure drawn below shows a typical variation of Toroidal Magnetic Field BT with time (plotted in blue) as well as the signal acquired by one of the capacitive probes (plotted in red):