Numerical evaluation of the cathode heating time during the post ignition and prethermionic electrode emission phase

Summary form only given. Vortex water wall high pressure argon arc lamps are used to generate intense light and are capable of operating at radiative efficiencies greater than 50%. These lamps are being considered for use in a pulsed DC mode of operation in the next generation of RTP applications. The transition between the preignition state and the arc maintenance state has been observed to consist of a prethermionic electrode phase and a non LTE positive column phase. The prethermionic electrode phase is important from the standpoint of the power supply design in that it is responsible for a significant cathode layer voltage drop. Accurate experimental observations of the cathode spot are impossible because of the inner water wall. Thus a two dimensional finite difference analysis of the electrode heating time has been performed so as to infer an improved understanding of the cathode spot diameter. The cathode is made of tungsten coated copper and is cooled by water internally. The internal argon is maintained at a pressure of 7 atmospheres. The axial flow rate of the vortex gas is 9.434/spl times/10/sup -4/ m/sup 2//s. The diameter of the tube is 18 mm and thus the axial flow velocity is about 3.7 m/sec. Experimental results suggest that the thermonic heating time is in the order of 100 ms and thus the area being heated has an average diameter of 1 mm. From this we infer that the diameter of the cathode spot must be on the order of 1 mm. This diameter agrees with experimental results obtained by observing the spot size immediately after the arc is extinguished.