Large Plasma Device

The LAPD is a linear 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 barium oxide (BaO) cathode-anode discharge at one end of a 20-meter long, 1 meter diameter cylindrical vacuum vessel (diagram).

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).

The main source of plasma within the LAPD is produced via discharge from the barium oxide (BaO) coated cathode, which emits electrons via thermionic emission.

The cathode is located near the end of the LAPD and is made from a thin nickel sheet, uniformly heated to roughly 900 °C.

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.

A secondary cathode source made of lanthanum hexaboride (LaB6) was developed in 2010[7] to provide a hotter and denser plasma when required.

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.

A 1 Hz rep-rate, coupled with the high reproducibility of the background plasma, allows the rapid collection of enormous datasets.

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.

Including the ball joints, there are a total of 450 access ports on the machine, some of which are fitted with windows for optical or microwave observation.