Measurements Of Multipactor And Dark Current In A 17 Ghz Standing Wave Accelerator Structure

High power testing of a 17 GHz single-cell standing wave disk-loaded waveguide structure was conducted with the intent to study the generation mechanism of the dark current inside a high gradient accelerator, and its connection with the initiation of breakdown. A structure gradient of 70 MV/m was achieved with a pulse length of 100 ns at 17.141 GHz. We measured dark current directly from the high gradient cell through side slits, the behavior of which deviated from the prediction of field emission theories. The amplitude of the dark current showed features of spikes and plateaus at specific gradient levels, instead of increasing monotonically with the gradient. To analyze these features, a particle-in-cell simulation was carried out to calculate the secondary electron emission from the side wall of the accelerator cell initiated by field emission electrons during the pulse. The calculated multipactor and the collision current amplitude were consistent with that of the dark current observed in the experiment. The simulation also showed that the collision current on the structure side wall could reach up to several amps, which could be large enough to give rise to material outgassing and local gas ionization, indicating a possible cause for breakdown.