Dispenser cathode life model: modified to include a shape factor and refit to more recent life test data

The Dispenser Cathode Life Model (DCLM) was originally published in 1984. More recent life test data have substantiated the basic physics used in the DCLM. Re-evaluation of the model with this newer data alters primarily the numerical parameters but not the basic physics of the model. One modification made to the model is to incorporate the shape factor /spl alpha/ in the emission equation so that accurate descriptions of the cathode activity curves can be made (this is necessary because cathode activity as a function of age was taken along with cathode current Ik at the operating point). The original model was fitted to I/sub k/(t) life test data at the operating point for cathodes running at different temperatures. This revision fits the cathode activity curves (i.e. current vs. temperature) as a function of time and as a function of operating temperature. Cathode current versus temperature is one step closer to the underlying physics and gives a better measure of how the internal parameters, such as work function and knee position and shape, change with time. Knowing these details gives a better measure of how the cathode current at the operating point will change over time.