Improvements in synchronization of the pbfa-II accelerator with laser-triggered gas switches

PBFA II is a 36-module ion accelerator built by Sandia National Laboratories for inertial confinement fusion studies./sup 1/ In the water-filled, pulse-forming section of the accelerator, each module is fitted with a 5.0-MV, SF/sub 6/-filled gas switch located between an intermediate storage capacitor and the first pulse forming line (Line 1). The intermediate storage capacitor is charged to 4.8-5.0 MV in approximately 950 ns by a Marx generator located in the oil section of the machine. The gas switch is required to close on command and transfer the stored capacitor energy to Line 1, a coaxial transmission line of 100 ns two-way electrical length. The switches are triggered by a single 3.0-J KrF laser located under the accelerator; a complex beam-splitting/distribution system is used to deliver 20-40 mJ, 35 ns FWHM beamlets to the individual switches. In order to properly drive the experimental load on PBFA II, equal-amplitude pulses must be produced by each pulse-forming line with a module-to-module first-to-last timing difference (spread) of less than 20 ns. The gas switch, the last command-triggered point in the module, is the major determinant of total machine synchrony. To compensate for the additional (<3 ns) rms jitter of three sets of self-breaking water switches downstream of the gas switches, first-to-last timing spread of the 36 gas switches must be less than 15 ns. At the last Pulsed Power Conference, we reported on prototype experiments in which four Rimfire switches were modified and tested, side-by-side, with four of the original PBFA II switches./sup 2/ In these tests, the, modified switches exhibited an rms jitter of less than 2 ns, and low prefire rate. The data taken on the modified switches were used in predicting a total 36-switch spread of less than 15 ns, and were the basis for proceeding with a complete retrofit of the PBFA II switch set. This retrofit was completed in the Fall of 1987. Since the retrofit, several hundred machine shots have been taken at voltages ranging from 3.5 MV to 5.0 Mv. Over this range of voltage, the rms jitter of each switch has been measured as less than 2 ns. The average firing times of the switches, however, is not identical; analysis of individual switch data reveals that each switch has an offset (either early or late) that is consistent shot-to-shot. These offsets range from 5.5 ns early to 4.0 ns late. The mechanical design of the laser optical system allows these offsets to be "tuned out" by adjusting the laser pathlengths of individual switches. Such "tuning" now results in 36-module switch spreads of less than 10 ns, and module spreads downstream of the water switches of less than 15 ns.