New low inductance gas switches for linear transformer drivers

We have developed two new gas switches that are designed to be used with linear transformer drivers. The switches, which can be DC charged to 200 kV and triggered with less than a 2-ns $1\mathrm{\text{−}}\sigma$ jitter, have overall inductances ranging from 69 to 85 nH. When transferring 400 J of energy per shot, the switches have lifetimes in excess of 5000 shots. The two switches are insulated with 130–270 PSIA of air and are submerged in transformer oil during operation. These switches should allow development of linear transformer drivers that are more compact and have higher peak power.

Figure 1

Electrical schematic of a single LTD brick in an accelerator. The rise time of the pulse delivered to the load depends primarily on the inductance of the switch, capacitors, and current busses in the green box. The toroidal magnetic cores block the return current path for the brick. This results in a high-voltage pulse at the load until the cores saturate.

Figure 2

Picture of the LTD-II cavity with 20 bricks stacked around a common load.

Figure 3

Schematic of our single brick test setup. The magnetic cores and coaxial load have been replaced by a simple flowing liquid resistive load. Monitors measure load current and voltage as well as the voltage across the switch.

Figure 4

Picture of the baseline (HCEI) Russian switch, the Kinetech switch, and the L-3 pulse sciences small-diameter switch.

Figure 5

Cross sections of three LTD switches: A: original Kinetech switch with narrow electrode stalks; B: Kinetech 2 switch with low-inductance large diameter electrode stalks; C: Schematic cross section of small-diameter L3-Sandia switch.

Figure 6

Self-breakdown curve for the L-3 small-diameter switch. Note that we actually operated the switch at a significantly higher pressure than our data points.

Figure 7

Self-breakdown curve for the Kinetech-2 switch. Note that the curve changed only slightly after 1700 shots.

Figure 8

Current waveforms for our brick charged to plus and minus 100 kV and discharged into matched loads.

Figure 9

Current waveforms for our bricks with L-3 small-diameter switches and 20, 40, and 80 nF capacitors when discharged into matched loads.

Figure 10

Comparison of voltage waveforms for an LTDR accelerator cavity with 20 nF capacitors and HCEI switches with a waveform for our test brick using 20 nF capacitors and the L-3 small-diameter switch.

Figure 11

Timing sequence of trigger voltage and switch current during triggering of L-3 small-diameter switch.

Figure 12

Plot of delay versus shot number for 5400 shots taken on eight successive days for the Kinetech-2 switch.

Figure 13

Plot of delay versus shot number for 7400 shots taken on nine successive days for the L-3 small-diameter switch.

Figure 14

Delay and jitter of the L-3 small-diameter switch as a function of charge voltage for three different switch pressures.

Figure 15

Comparison of currents into matched loads for LTD-II brick and several improvements of capacitors and switches. The peak current we observed is 70% higher than the peak current in LTD-II.

Figure 16

Impedance of the L3 small-diameter switch with and without a correction for the $\sim 45\mathrm{nH}$ inductance of the switch arc.