HIGH UV (LAMBDA-GREATER-THAN-OR-EQUAL-TO 2200-A), VISIBLE, AND NEAR-INFRARED (LAMBDA-LESS-THAN-OR-EQUAL-TO 0.8-MU-M) TRANSMISSIVE LIQUID COOLANT FOR HIGH-POWER MICROWAVE (2.45 GHZ) PLASMA TUBES

Commercially available dimethyl polysiloxane is shown to be an excellent liquid coolant for the containing walls of high power plasmas (greater-than-or-equal-to 300 W/cm3) excited with 2.45-GHz microwaves. This stable, nontoxic, and nonflammable fluid has very low microwave absorption (tan-delta = epsilon"/epsilon' = 3.5 x 10(-4) at this frequency. Fortuitously, it also has excellent ultraviolet, visible, and near infrared (lambda less-than-or-equal-to 0.8-mu-m) transmission. The UV transmission over 1 cm shows 2% transmission at 2000 angstrom, a 30% transmission at 2200 angstrom, 80% at 2400 angstrom, and 100% for lambda > 2500 angstrom. These features plus its good physical (viscosity, thermal conductivity, specific heat capacity, heat of vaporization, and density) and electrical (resistivity and breakdown) properties make it very unique as a liquid coolant of UV emitting microwave plasma lamps. The clear coolant's infrared absorption for lambda greater-than-or-equal-to 1.0-mu-m reduces the radiative (heat) emission from the plasma. In addition, dimethyl polysiloxane has a very wide operating temperature range, - 73-degree-C less-than-or-equal-to T less-than-or-equal-to 260-degree-C. Particular procedures required to assure its excellent ultraviolet transmission are described along with different geometrical combinations of liquid cooled plasma lamps and reflector housings. This liquid may also be used to cool microwave excited laser tubes.