Experimental demonstration of dark current mitigation by an over-inserted plug in a normal conducting very-high-frequency gun

The room temperature continuous wave (cw) very-high-frequency (VHF) gun is one of the candidates for the electron gun of the high-repetition-rate free-electron lasers (FELs). The VHF gun operates with a cathode gradient of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mo>∼</a:mo><a:mn>20</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MV</a:mi><a:mo>/</a:mo><a:mi mathvariant="normal">m</a:mi></a:mrow></a:math> and an accelerating voltage of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mo>∼</d:mo><d:mn>750</d:mn><d:mtext> </d:mtext><d:mtext> </d:mtext><d:mtext>kV</d:mtext></d:math>. The gun dark current emission leads to beam loss along the FEL machine, therefore, it is a critical parameter for the performance of the cw gun. In this paper, we present a systematic study of the dark current reduction of the VHF gun, including cathode region optimizations, dark current tracking simulations, and measurements. Over-inserted cathode plugs were tested in two VHF guns of different acceleration gap sizes, and both demonstrated significant dark current reduction ratios of more than two orders of magnitude.