RESEARCH LABORATORY

UCLA BASIC PLASMA PHYSICS RESEARCH LABORATORY

The experiments are performed at UCLA in Knudsen Hall (Rooms 1-107, 1-109, A-137). The main research device, described below, is located Knudsen Hall 1-109. Two smaller machines are housed in Knudsen Hall A-137. The laboratories are supplied with adequate power, cooling water, shielded rooms for computers, construction areas for mechanical devices and electronic circuits. The Department of Physics and Astronomy maintains a professionally staffed machine shop and electronics shop. Faculty, research staff and student offices are in close vicinity to the laboratory.

Discharge plasma devices provide many desirable properties required for basic research: Uniformity, stability, quiescence, low collisionality, widely adjustable parameters, large scale lengths, and accessibility to detailed diagnostics. In discharge devices electrons are emitted from a heated cathode, accelerated, and injected into a low pressure gas which is ionized by electron-neutral impact. In order to produce a large, uniform plasma in an ambient magnetic field a uniformly heated cathode is required. We have pioneered in the development of large oxide-coated cathodes and hold a patent on their construction. Our largest plasma device employs a cathode of one-meter diameter which produces a 2.5 m long, 1 m diameter, magnetized plasma column with a volume of 2 cubic meters. An earlier, smaller version of 50 cm diameter, has been used in the TRW LAMPS device [Ref. 2] and the same cathode, with minor modifications, was earlier used in the UCLA LAPD device [Ref. 3].

These discharge devices produce pulsed plasmas with densities of order 10¹² cm^-3, electron temperatures of several electron Volt, in low pressure rare gases, e.g., Argon at 0.5 mTorr. Our large cathode device operates in a uniform magnetic field of up to 100 G. The cathode requires 40 kW of heating power in steady state. The 50 V, 1000 A discharge supply uses a capacitor bank (1 Farad) for energy storage and a transistor switch for pulsing (1 Hz, 0.5% duty cycle). All discharge parameters and the gas pressure are feedback stabilized so as to produce constant plasma parameters over long periods of time. The pulsed discharge operates continuously until the cathode coating is sputtered off from ion bombardment. Depending on discharge current, the cathode coating lasts between one month and one year. The cathode is recoated with a mixture of barium, strontium, and calcium carbonates, reactivated by slow heating, and ready for operation within about one week. The plasma density is adjustable with discharge current, gas pressure, and decay time in the afterglow. The latter also produces a continuous variation in the electron temperature.

The large cathode is mounted in an aluminium vacuum chamber of 1.5 m diameter, 2.5 m length with double walls and internal water cooling. There are 20 sideports and 10 end ports for diagnostic tools, power and water feedthroughs. High vacuum (10^-6 Torr) is produced with two diffusion pumps (4800 liter/s) and one roughing pump (100 cubic feet/min). Rare gases (Ar, Kr, Ne, He) are introduced via a feedback-stabilized, piezocrystal leakvalve. The vacuum chamber is surrounded by 10 watercooled coils producing a uniform, axial dc magnetic field of up to 100 G, limited by the present size of the power supplies.

Experimental chamber.

The coils can be energized individually so as to vary the magnetic field configuration from a uniform field to a mirror or a cusp. Safety circuits are installed which switch off the cathode in case of loss of vacuum or cooling. Pneumatic valves isolate the chamber from the vacuum pumps in case of accidental leaks. Due to extensive interlocks the machine can operate without human supervision. It runs continuously day and night since the thermalization time does not allow daily turn on and off. In typical week-long data runs the plasma parameters are stable to within 3 digits.

Diagnostic probes are inserted through radial and axial ports. Some of them are equipped with vacuum locks which permit the exchange of probes without loss of vacuum in the main chamber. The probe is retracted into a small cylinder, which is isolated from the chamber by a gate valve and can be separately vented and evacuated. Magnetic measurements are performed with a triple magnetic probe measuring 3 components of time-varying magnetic fields. The probe is shown in the right bottom part of the figure below. The figure displays a directional whislter antenna against the background of the cathode.

Helicity injecting antenna shown with diagnostic magnetic and Langmuir probe.

The telescopic probe is movable in 3 orthogonal coordinates so as to record, from repeated experiments, the vector field B(r, t). The motion is computer controlled, the probe is watercooled, each loop is shielded and coaxially fed into low noise preamplifiers. With averaging techniques, signal below 1 mG can be resolved. The probe is used to obtain the current density in the plasma, J(r, t) = nabla × B/ µ₀. Electron density, temperature, and plasma potential are obtained from a small Langmuir probe also movable in 3 dimensions. A small test electron beam source is used to excite oscillations at the plasma frequency which yield the local density independently. A dipole electric field probe measures E(r, t), and a directional velocity analyzer yields f(v, r, t). Waves and instabilities are investigated with rf probes using interferometry and conditional averaging techniques. Light emission from energetic electrons is detected with a sensitive photomultiplier tube via collimated paths or fiber optics.

References

• Large, Indirectly Heated, Oxide-Coated Cathode for Producing Uniform Plasmas, R.L. Stenzel and W.F. Daley, United States Patent No. 4,216,405 August 5, 1980.
• Whistler wave propagation in a large magnetoplasma, R.L. Stenzel, Phys. Fluids 19, 857 (1976).
• Whistler wave mode conversion to lower hybrid waves at a density striation, J.F. Bamber, W. Gekelman, and J.E. Maggs, Phys. Rev. Lett. 73, 2990 (1994). [Link to original publication.]

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