Effect of ion bombardment on emission characteristics of gyrotron cathodes

Abstract

Summary form only given as follows. One of the important sources of helical electron beam quality deterioration in gyrotrons is inhomogeneous electron emission from thermionic cathodes of magnetron-injection guns. A high vacuum test stand has been manufactured at SPbSPU for diagnostics and ion treatment of large-size gyrotron cathodes. This facility is equipped with a source of potassium ions providing an ion current density on the cathode up to 2times10-6 A/cm2 and an ion energy up to 11 keV. Rotating the cathode, we select cathode surface areas with anomalous emission for their treatment by ion bombardment. Potassium was chosen for these experiments primarily due to the simplicity of an ion source based on surface ionization of its atoms. Besides, our measurements show that potassium atoms deposited on the emitter surface do not practically influence the emission activity because of rapid evaporation of these atoms from the heated cathode surface at operating temperature. Therefore, such atoms can be used for modeling of processes on the cathode surface in gyrotrons being under the action of bombardment by the ions of the residual gases. The experiments with the LaB6 cathode showed the possibility of emission homogeneity improvement for this cathode due to the deactivation of the emissive surface areas with high electron emission resulting from their treatment by potassium ions. Ion bombardment of the W-Ba cathode caused an increase of electron emission from the treated area. The measurements with this cathode were made for wide ranges of temperature up to ~1100degC and of cathode current density up to ~3 A/cm2. The degree of emission increase resulting from ion bombardment depends on treatment duration, ion energy and initial activity of the treated area. A maximum activation enhancement factor of 2.5 was achieved. In the report, we will discuss the causes of cathode emission changes due to ion treatment and the mechanisms of emission homogeneity degradation in gyrotrons connected with the bombardment of the cathode surface by ion flows.