Measuring fill gas pressure in an electrodeless lamp by RF impedance diagnostics

Summary form only given. Filling a small capsule for a light emitting plasma with a rare gas can result in significant uncertainty in the ultimate pressure of the gas in the plasma vessel. This situation is exacerbated when the capsule must be cooled cryogenically while final closure is accomplished by heating the seal region at the same time by means of a hydrogen oxygen torch. The resulting temperature variation within the capsule, in the seal region, and in the discarded quartz fill tube (while the collapse of the tubulation is being carried out during heating to create the hermetic seal), can leave the rare gas in the plasma vessel at a higher pressure than when the capsule plus tubulation were sealed with a long tubulation on a gas fill station. As a result, an independent method for determining the fill pressure on the finished capsule is needed. A procedure has been developed for determining the fill pressure in a capsule by measuring the impedance of residual gas in the linear discard tube. This is done by creating an RF discharge in the gas in the discard tube and measuring the resulting electrical characteristics. Using this procedure along with the basic equation of state and conservation principles, it is possible to determine the rare gas pressure in the capsule. The procedure is demonstrated and used experimentally to reliably establish rare gas pressures in finished capsules.