Analysis of Laser Induced Plasma in High Pressure SF6 Gas

Abstract

Summary form only given. The Laser Triggered Switch Program at Sandia National Laboratories is an intensive development study to optimize and improve the laser triggered gas switch (LTGS) for the Z-Refurbishment (ZR) project. The laser triggered gas switch is the final command-triggered switch in the machine, and reliability and performance of the switch is crucial. A modified LTGS trigger section with optical viewing windows perpendicular to laser propagation is used to analyze a laser induced plasma spark in SF6 gas in order to quantity parameters such as spark length and plasma temperature. The laser spark is created through a focusing lens by the fourth-harmonic (266nm) of a 5ns FWHM pulsed Nd: YAG laser with 30mJ maximum energy output. Several diagnostic methods are used to analyze the laser spark. Visible spark length measurements are made using a lens system mounted to a CCD camera at gas pressures ranging from sub-atmosphere to 4 atmospheres. Differing f-number lenses are compared to determine optimal visible spark length for a given gas pressure. Spark length is used as an indicator of the ability of a switch to trigger at a given gas pressure and charge voltage. Typically, the visible spark length must be at least 30% of the electrode gap spacing to produce acceptable switch run-time and jitter. Experiments have shown that for switch operating pressures near 4 atmospheres, a dramatic increase in spark length (~12mm to 24mm) is noted between f/9.8 and f/14.8 lenses (500mm and 750mm respectively) while the increase in spark length slows markedly (~24mm to 30.5mm) with an increase in lens focal length to f/19.7 (lm).