Study of Rf Breakdown in Normal Conducting Cryogenic Structure

RF Breakdown experiments on short 11.424 GHz accelerating structures at SLAC have shown that properties of rf breakdown probability are reproducible for structures of the same geometry. At a given rf power and pulse shape, the rf breakdown triggers continuously and independently at a constant average rate. Hypotheses describing the properties of the rf breakdown probabilities involve defects of the metal crystal lattice that move under forces caused by rf electric and magnetic fields. This crystal defect dynamics depends on the temperature of the structure. To study the dependence we designed and built an experimental setup that includes a cryogenically cooled single-cell, standing-wave accelerating structure. This structure will be high-power tested at the SLAC Accelerator Structure Test Area (ASTA). INTRODUCTION We continue experiments directed toward the understanding of the physics of rf breakdown in systems that can be used to accelerate electron beams at ∼ 11.4 GHz [1, 2]. The accelerating structure geometries have apertures, stored energy per cell, and rf pulse duration close to that of the NLC [3, 4] or CLIC [5]. The breakdown rate (breakdown probability) is the main parameter that we use to compare rf breakdown behavior between different structures [6] at a given set of rf pulse parameters (pulse shape and peak power). To date we have tested 39 structures. Most of structures in our tests were fed axially with rf power, through a removable mode launcher [1]. These tests produced a wealth of experimental data which we now use as a reference for new experiments. Experiments with the copper structure described in this paper will study the behavior of rf breakdown while the material properties of copper, such as thermal and electrical conductivity, yield stress, etcetera, change at cryogenic temperatures.