Current-toggled Plasma Opening Switch

Several versions of the current-toggled plasma opening Switch
¹
(CTPOS) were tested on the SuperMite pulser at Sandia National Laboratories. The analysis of the data is now finished, and the switch is well understood. The data indicate that the CTPOS works largely as predicted. The magnetic field of the fast field coil does not penetrate the plasma to any appreciable degree, and the fast field coil current and flux give a good measure of the plasma fill density. The opening current is proportional to the flux in the slow field coil as expected. The SuperMite CTPOS opened to a flow impedance
²
(defined by Z
_flow
= V /[I 2
_{anode upstream}
- I 2
_{cathode downstream}
]
^½
) of 5 to 8 Ω/. The maximum value possible for any opening switch is 20 Ω for the hardware used. A "good" switch would open to 12 to 16 Ω. The problem was caused by plasma being squeezed upstream and downstream of the switch by the fast magnetic field. We have concluded that the fast field coil must be designed very carefully using an MHD code to study the way the plasma is pushed back. The coil needs to be designed to push the plasma behind the anode for a broad range of initial plasma distributions. This is being pursued for the PBFA-II CTPOS.
³