Dip test: Rapid cathode activity evaluation of the klystron used in an accelerator

In recent accelerators, radio frequency (RF) is the important technology and the klystron is the most favored RF device. Since a klystron is expensive, and procurement can take a year, proper maintenance work is a recurring theme. In particular, life estimation by measuring the cathode activity of the klystron is important for accelerator operation because after long usage of the klystron, the cathode deteriorates. The standard method used is to measure the cathode emission current as a function of heater voltage (Miram plot), but it takes a few hours to obtain a full curve of emission characteristics for a tube and is hard work for large-scale accelerators. In this paper, a dip test method is described in detail as the rapid cathode activation evaluation in the Beijing Electron-Positron Collider (BEPC II) ring klystron and BEPC injector linear accelerator at the Institute of High Energy Physics, China. This method measures the emission dip from the short interval of a filament power cycle with high voltage remaining on under the high voltage application to the cathode, and evaluates the cathode activity, which relates to the cathode deterioration. In a continuous-wave klystron a 10 s interval for the off-on time, and in pulsed klystron, a 40 s interval is sufficient to perform a dip test. Therefore, there are lots of advantages in using this method due to its rapidity and simplicity.