Field Emission (rf breakdown)
Field emission is believed to be a trigger of rf breakdown. The research of field emission under high electric field has been performed at the Argonne Cathode Test-stand (ACT) at AWA.
Evolution of field enhancement factor and emission area has been observed during rf conditioning, which implies continuous surface modification by rf field. Strong dependence of field emission on stored energy has been discovered, which suggests the influence of emission-induced local higher order modes excitation. Overlapping of most strong field emitters with rf breakdown locations has been revealed on well-conditioned surface. In-situ observation of field emitter evolution during rf conditioning and stable operation will be performed in the near future.
Structure performance under high gradient with a short pulse length is of particular interest in AWA’s two-beam acceleration approach.
With a high charge bunch train, 300 MW and 10 ns rf power has been obtained by an X-band low-impedance metallic power extractor. The rf power has been used to drive a high-impedance accelerator and 150 MeV/m acceleration has been experimentally demonstrated. Statistical study of short pulse rf breakdown driven by the same power extractor is planned in the near future by using single-cell accelerating structures with different rf designs.