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.

The external 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.

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.

Experimental device

Fig. 1 (a) Side view of the main plasma device. (b) The heart of the device is a 1 m diam cathode which produces a uniform dense discharge plasma. (c) View of the plasma through a glass window. Antennas and a diagnostic magnetic probe are inside the plasma.

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.

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 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.

A mid-sized plasma device has been developed to produce quiescent and strongly magnetized plasmas. The SCAMP device (Source Chamber And Magnetized Plasma) consisted of a weakly magnetized large dc discharge chamber connected to a strongly magnetized linear device for wave experiments. These included lower hybrid wave studies, focused resonance cones and wave turbulence studies. Figure 2 shows (a) a picture of the device, (b) data of converging resonance cones and (c) cross-spectral measurements of current-driven ion sound which shows nearly perpendicular propagating sound waves.

Scamp

Fig. 2. Quiescent magnetized plasma device consisting of a large Source Chamber And a Magnetized Plasma (SCAMP). The device (a,b) has been used for experiments on lower hybrid waves, (b) particle acceleration by focused whistler mode resonance cones and (c) current-driven sound turbulence.

A smaller discharge device has been used for research and teaching. Figure 3 shows the machine of size 0.5 m diam, 2 m length, uses filamentary cathodes in a weak external field. It has been used for research on electron beams, fireballs with double layers, discharges inside a gridded sphere and associated transit time instabilities, sheath ionization effects and magnetron discharges.

Experimental device

Fig. 3. Small plasma machine. It has been used for research and teaching on electron beams (second picture), fireballs with double layers (third picture), source-free discharges inside a gridded sphere (fourth picture), sheath instabilities and magnetron discharges.

The development of large cathodes started at TRW. The objective was to generate large uniform plasmas for whistler wave studies in support of space plasma research. The challenge was to produce uniform cathode emission, large densities, low collisions and stability in an external magnetic field. Figure 4 shows the development from a 15 cm diam cathode to a 50 cm diam cathode in the LAMPS device (LArge Magnetized Plasma Source). It served well for several years to study whistler waves, filamentation instabilities, antenna radiation patterns, electron beam-whistler instabilities, sound turbulence and probe diagnostic developments. The same cathode, with small modifications, was later used in the UCLA LAPD device and in other laboratories.

Experimental device

Fig. 4 LAMPS device, used for early whistler wave experiments, antenna properties whistler wave experiments and current driven sound turbulence studies. The development of large cathodes started with a test model shown in the first picture. It produced a 15 cm diam plasma column in an axial magnetic field. It was scaled up to a 50 cm diam cathode inserted in a 1 m x 2.5 m chamber with external coils, shown in the second picture. A view of the discharge through a side port is shown in the third frame. The machine was operational until the funding ended.

Preceding the LAMPS machine we have also built a large diameter unmagnetized plasma device for laboratory studies of nonlinear effects at the critical layer omega=omegap. This was an important topic in the ionospheric modification program and in the laser-fusion program. The laboratory experiment involved the illumination of a density gradient with a powerful microwave signal to study mode conversion, parametric instabilities, formation of cavities and particle energization.

Experimental device

Fig. 5 The QUIPS (QUiescent Plasma Source) machine. It is a dc discharge with permanent magnet surface confinement, first tested in a small scale device (left picture), then scaled up to a 2mx4m device. It was primarily used to study nonlinear effects at the critical layer, which was of interest in laser-fusion and ionospheric modification.

A 2 m x 4 m space chamber was available. In order to fill it with a dense plasma at a moderate discharge power a small-scale device was built as a proof-of-principle. Figure 5 (first frame) shows a discharge in a bell-jar with permanent magnets surface confinement. The successful small scale device was scaled up to the large chamber (second frame). This was a major construction project as shown in a scrolling picture array.

Ten thousand permanent magnets were mounted on a large frame which could be rolled into the vacuum chamber. The cathode consisted of rings of tungsten filaments on one side of the device which produced an axial density gradient. A microwave dish was mounted on the opposite end to illuminate the plasma gradient toward the critical layer in the middle of the machine. The size of the machine is shown by persons near and inside the open chamber (last two frames). Some of the results of the experiments are described in the research topic Mode Conversion and Nonlinear Effects at the Critical Layer.

The QUIPS device has also been modified for a laboratory experiment on the Farley-Buneman instability. This instability arises when electrons stream across a magnetic field through ions which are unmagnetized due to collisions with neutrals. This instability occurs in the ionosphere due to the electrojet. In the laboratory the electron drift is produced by a radial electric field across an axial magnetic field.

Experimental device

Fig. 6 Farley-Buneman experiment in the QUIPS device. A Helmholtz coil magnetizes an rf produced plasma. A biased ring end plate imposes an electric field. The instability is observed by scattering a 35 GHz microwave signal. Probes are also used for plasma and wave diagnostics.

The magnetic field is produced by a large Helmholtz coil. The plasma is produced by an rf discharge. The radial electric field is produced by a segmented ring end plate where the bias controls the plasma potential in the flux tube of a ring. Figure 6 shows the open device (left frame) and the plasma between source and segmented end plate (middle frame) and a picture of the busy lab experiment (right frame).

References