Large Plasma Device

From Wikipedia, the free encyclopedia

The Large Plasma Device during an experiment.

The Large Plasma Device is an experimental physics device at UCLA. It is designed as a general-purpose laboratory for experimental plasma physics research. The current version of the device began operation in 2001. The modern LaPD is operated as a national user facility, which means that half the research time on the device is open to scientists at other institutions.

1 Machine overview
- 1.1 Plasma parameters
- 1.2 Plasma source
2 Diagnostic access and probes
3 External links

[edit] Machine overview

Photo taken through an end port with the plasma off, showing the thermionic cathode.

The LaPD is a pulsed-discharge device operated at a high (1 Hz) repetition rate, producing a strongly magnetized background plasma which is physically large enough to support Alfvén waves. Plasma is produced from a cathode-anode discharge at one end of a 20-meter long, 1 meter diameter cylindrical vacuum vessel (diagram). The resulting plasma column is roughly 16.5 meters long and 60 cm in diameter. The background magnetic field, produced by a series of large electromagnets surrounding the chamber, can be varied from 400 gauss to 2.5 kilogauss (40 to 250 mT).

[edit] Plasma parameters

Because the LaPD is a general-purpose research device, the plasma parameters are carefully selected to make diagnostics simple without the problems associated with hotter (e.g. fusion-level) plasmas, while still providing a useful environment in which to do research.

Density: n = 1-4 $\times$ 10¹² cm^-3
Temperature: T_e = 6eV, T_i = 1eV
Background field: B = 400-2500 gauss (40–250 mT)

In principle the plasma may be generated from any gas. Gases typically used are helium, argon, nitrogen, neon and (for short periods of time, as it destroys the oxide coating on the cathode) hydrogen.

At these parameters, the ion Larmor radius is a few millimeters, and the Debye length is tens of micrometres. Importantly, it also implies that the Alfvén wavelength is a few meters, and in fact shear Alfvén waves are routinely observed in LaPD. This is the main reason for the 20-meter length of the device.

[edit] Plasma source

Plasma is produced via discharge from a barium-oxide coated cathode, which emits electrons via thermionic emission. The cathode itself is made from a thin nickel sheet, uniformly heated to roughly 900 °C. The circuit is closed by a molybdenum mesh anode a short distance away. Typical discharge currents are in the range of 3-8 kiloamperes at 60-90 volts, supplied by a custom-designed transistor switch backed by a 4-farad capacitor bank.

The plasma is pulsed at 1 Hz, and is typically on for 10-20 milliseconds at a time. The use of an oxide-cathode plasma source, along with a well-designed transistor switch for the discharge, allows for a plasma environment which is extremely reproducible shot-to-shot.

One interesting aspect of the LaPD plasma source is its ability to act as an "Alfvén Maser", a source of large-amplitude, coherent shear Alfvén waves. The resonant cavity is formed by the highly reflective nickel cathode and the semitransparent grid anode. Since the source is located at the end of the solenoid which generates the main LaPD background field, there is a gradient in the magnetic field within the cavity. As shear waves do not propagate above the ion cyclotron frequency, the practical effect of this is to act as a filter on the modes which may be excited. Maser activity occurs spontaneously at certain combinations of magnetic field strength and discharge current, and in practice may be activated (or avoided) by the machine operator.

[edit] Diagnostic access and probes

The main LaPD diagnostic is the movable probe. The relatively low electron temperature makes probe construction straightforward and does not require the use of exotic materials. Probes currently used at the facility include magnetic field probes, Langmuir probes, Mach probes (to measure flow) and many others. Standard probe design also allows external users to bring their own diagnostics with them, if they desire. Each probe is inserted through its own vacuum interlock, which allows probes to be added and removed while the device is in operation.

A 1 Hz rep-rate, coupled with the high reproducibility of the background plasma, allows the rapid collection of enormous datasets. An experiment on LaPD is typically designed to be repeated once per second, for as many hours or days as is necessary to assemble a complete set of observations. This makes it possible to diagnose experiments using a small number of movable probes, in contrast to the large probe arrays used in many other devices.

The entire length of the device is fitted with "ball joints," vacuum-tight angular couplings (invented by a LaPD staff member) which allow probes to be inserted and rotated, both vertically and horizontally. In practice, these are used in conjunction with computer-controlled motorized probe drives to sample "planes" (vertical cross-sections) of the background plasma with whatever probe is desired. Since the only limitation on the amount of data to be taken (number of points in the plane) is the amount of time spent recording shots at 1 Hz, it is possible to assemble large volumetric datasets consisting of many planes at different axial locations.

Visualizations composed from such volumetric measurements can be seen at the LaPD gallery.

Including the ball joints, there are a total of 450 access ports on the machine, many of which are fitted with windows for optical or microwave observation. Other diagnostics at LaPD include a microwave interferometer, fast (3 ns) photography and laser-induced fluorescence.