Effect of Electron-beam Polishing of Electrodes on Hold-off at Pulsed DC and Microwave Electric Fields in Vacuum

Unlike DC and pulsed DC electric fields, microwave fields cause multipactoring when prebreakdown processes become more complicated. Besides field emission, breakdown strength becomes lowered by gas desorption and its ionization. Rare microwave plasma emits charged particles impacting electrode surfaces and causing secondary emission. We used electron-beam surface melting as a technology for finishing the beam-faced surfaces of microwave cavities. The treatment allows one to smooth and to clean surface perfectly. A part of electrodes were treated in the regime of a surface alloying above stainless steel with smooth transition of chemical composition to pure titanium on the surface. Pulsed DC breakdown strength of stainless steel and titanium samples were measured to be the same while microwave breakdown was delayed for titanium surfaces, which allowed a 25% gain in output microwave power due to lengthening radiation pulses.

I. Introduction

High-power microwave (HPM) sources in a pulsed operation regime allows converting electron beam energy into HPM radiation with efficiency up to 70% and producing microwave output power up to 10 GW [1]. One of key problems in progress of HPM sources is the pulse shortening with increase of microwave power. Nature of that is complicated because of many causes are involved but basically all of them converge in several scenarios, namely, a reduction in the accelerating gap due to the expansion of the plasma of the explosive-emission cathode, beam instabilities, and formation of secondary plasma associated with processes inside a slow-wave structure (SWS), including field electron emission, explosive electron emission, secondary emission (multipactor effect), gas desorption and ionization [2]–[6]. Secondary plasma is the most critical cause because it manifests itself faster than gap shortening and beam instabilities [4], [5].