The LAPD laboratory consists of an experimental device and associated infrastructure which is unique. The plasma source and diagnostics, which are now fully operational, were designed to investigate a broad range of plasma phenomena. In the Winter of 1989 the first full scale experiment on the three dimensional interaction of currents fibers was executed. Since then studies of Alfvn waves and whistler wave propagation in non uniform plasmas were initiated.
The Large Plasma Device (LAPD) is a flexible and low maintenance device for studying a variety of waves and nonlinear effects in fully magnetized plasmas. The plasma column is 50 cm in diameter and 10 meters in length. With 128 radial ports and a dozen end ports the machine has abundant access for probes and antennas.
The vacuum chambers are surrounded by 68 pancake magnets spaced six inches apart. Coil construction, consisting of winding, brazing and epoxying the heavy gauge, Oxygen-free Copper, was carried out by our group at UCLA. Each magnet weighs approximately 1/2 ton. The magnets are driven by seven DC power supplies each rated at 60 Volts, 3000 amps. Each supply powers eight to ten magnets. The magnets are cooled with deionized water from a large (342 GPM) cooling tower, and get their power from a private electrical substation. Both the cooling tower and substation were provided by UCLA in support of this project. The magnetic field can be programmed to be highly uniform along the length of the device or, since the coils are powered by seven independent supplies, there is an option for magnetic mirrors or a number of field profiles.
The vacuum system is centered around two 1500 liter/sec turbo pumps which produce a base pressure of 5x10^-8 Torr. The pumps are magnetically shielded to allow operation close to the high fringing field of the device magnets. The pumps are controlled by a Vacuum Process controller which automatically performs valve sequencing and other protective functions as the system pressure changes. The controller can also shut down the entire device in the event of vacuum failure. The machine is protected by additional circuitry from loss of coolant to the cathode or power supplies, and power outages. Each turbo pump is backed by a 17.7 cfm mechanical pump. Gas may be fed into the system at several locations along its axis to allow for the production of axial density gradients in the plasma. The vacuum conditions and gas introduced to the system are constantly monitored by a quadrupole mass analyzer.
The plasma is produced by a DC discharge driven by an oxide coated cathode. This type of plasma (H, He, Ar, Xe, Ne, or any mixture) has been proven to be quiescent (dn/n = 1 %), can be moderately high in density (n < 6x10^12 /cm^3) and, recently, high power discharges have been utilized to produce very highly ionized plasmas of hydrogen or helium. It is important for research plasmas to be stable over long data runs. In wave experiments, one must be able to launch low amplitude waves and map out their dispersion and ray trajectories before moving on to nonlinear or turbulent experiments. Cathode sources are stable for long periods of time (about 6 weeks ) and produce plasma discharges which are very reproducible from shot to shot.
The LAPD discharge is controlled by a digital clock and transistor switch capable of 4000 amperes of pulsed current. An upgrade of the bank and switch to support 8 kA discharges is underway. The discharge may be controlled by the computers in our data acquisition system. The clock circuits include timing pulses which fan out to trigger additional circuits and the analog to digital converters in the data acquisition system. The plasma is pulsed at one Hz to allow for efficient signal averaging and data processing.
Plasma diagnostics include magnetic pickup loops, electric dipole probes, and Langmuir probes for measurement of B(r,t), E(r,t),n(r,t), Te(r,t), laser induced fluorescence for measurements of the ion distribution function fi(r,v,t) and a 70 GHz interferometer for measurement of chord-averaged plasma density.