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Pulsed Argon Discharge Plasma Source

Source Design and Plasma Production

The argon discharge plasma source shown here was redesigned and rebuilt from the ground up for the plasma density transition trapping experiment. The mechanism of plasma production, however, remains very similar to earlier versions of the apparatus, see (H. Suk, et. al., Proceedings of PAC 1999, p. 3708). Plasma is produced via an electrical discharge in a diffuse argon gas. The discharge cathode is a 4" disc of Lanthanum Hexaboride (LaB6) which is heated to about 1200 deg C in order to give excellent thermionic emission. A nearby hollow anode is then pulsed (2 msec, 1 Hz) with 200 volts while the space between the anode and cathode is filled with an argon gas at 1 mtorr. The resulting 200 amp discharge produces an argon plasma with a temperature of about 6 eV.

Once produced, the neutral plasma diffuses out through the hollow anode into the area where it will interact with the electron beam. Solenoids provide a weak magnetic field to help confine and guide this flow. The cylindrical plasma column that arrives at the electron beam interaction point is approximately gaussian with a peak density of about 3 x 1013 cm-3 and a width of about 5 cm.

Plasma Column Column Measurement
Plasma Column Column Measurement

Transition Generation

In order to produce plasma density transition trapping, we must first produce a plasma density transition. This is accomplished by partially blocking the flow of the diffusing plasma column with a mesh screen. Some portion of the plasma will still pass through the screen to create a low-density region of plasma on the far side. Under the right conditions, the transition between the high and low-density regions can be very sharp. We have produced and measured density transitions that are shorter than one plasma skin depth using this technique. Transitions shorter than one plasma skin depth are critical to the success of transition trapping.

Illustration of Plasma Screen

Plasma Column Column Measurement
Plasma with Density Screen Transition Measurement


New Vacuum Chamber for the Plasma Density Transition-Trapping Experiment -M. Schneider and M.C. Thompson