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

摘要

Summary form only given. Vortex water wall high pressure argon arclamps are used to generate intense light and are capable of operating atradiative efficiencies greater than 50%. These lamps are beingconsidered for use in a pulsed DC mode of operation in the nextgeneration of RTP applications. The transition between the preignitionstate and the arc maintenance state has been observed to consist of aprethermionic electrode phase and a non LTE positive column phase. Theprethermionic electrode phase is important from the standpoint of thepower supply design in that it is responsible for a significant cathodelayer voltage drop. Accurate experimental observations of the cathodespot are impossible because of the inner water wall. Thus a twodimensional finite difference analysis of the electrode heating time hasbeen performed so as to infer an improved understanding of the cathodespot diameter. The cathode is made of tungsten coated copper and iscooled by water internally. The internal argon is maintained at apressure of 7 atmospheres. The axial flow rate of the vortex gas is9.434×10^-4 m²/s. The diameter of the tube is18 mm and thus the axial flow velocity is about 3.7 m/sec. Experimentalresults suggest that the thermonic heating time is in the order of 100ms and thus the area being heated has an average diameter of 1 mm. Fromthis we infer that the diameter of the cathode spot must be on the orderof 1 mm. This diameter agrees with experimental results obtained byobserving the spot size immediately after the arc is extinguished