Control of a large vacuum wave precursor on the sabre voltage adder MITL and extraction ion diode

SABRE is a ten-cavity magnetically insulated voltage adder (6 MV, 300 kA) used to study ion beam production in high voltage extraction applied-B ion diodes. Observations indicate that the machine power initially propagates in a large-amplitude vacuum wave prior to electron emission. This vacuum wave “precursor” has an important impact on the turn-on and impedance history of ion diodes. Typical precursor characteristics are shown using transmission line, diode, and beam current and voltage data and are compared to TWOQUICK PIC-code simulations. Two techniques are under investigation to control the precursor and its effects on diode performance. A plasma opening switch (POS) has been used to erode the precursor. Field enhancing inserts are also planned to decrease the macroscopic fields required for electron emission from the cathode. This will limit the distance over which vacuum and insulated waves separate due to propagation at different velocities. Experimental data from the POS technique and TWOQUICK simulations of the insert technique are presented and discussed