• N.R.A. Valles, Z.A. Conway, M. Liepe
  CLASSE, Ithaca, New York

The RF superheating magnetic field of superconducting niobium was measured with a 1.3 GHz re-entrant cavity at several points in the temperature range from 1.9K to 4.2K. This experimental data is used to discriminate between two competing theories for the temperature dependent behavior of the RF superheating field. Measurements were made with <250 μs high power pulses (HPP, ∼1MW). Our test incorporated oscillating superleak transducers to determine the cavity quench locations and characterize changes and the migrations of the quench locations during processing. Using a vertically electropolished cavity, the temperature dependence of the superheating field was found to agree with Ginzburg-Landau predictions to within 10% down to a temperature of 4.2K; whereas prior to this experiment, theory and experiment only agreed at temperatures greater than 6.2K. We also used finite element methods to simulate the internal heating of the cavity, allowing for a more accurate measurement of superheating field as a function of temperature.

• N.R.A. Valles, M. Liepe
  CLASSE, Ithaca, New York

This paper discusses the optimization methods used to design the seven-cell cavities in Cornell's high beam current (100 mA) Energy Recovery Linac. The center cells of the cavity were optimized for high R/Q*G of the fundamental mode to minimize the dynamic cryogenic load, for low electric peak surface fields to minimize the risk of electron field emission, and for increased "cell-to-cell coupling" of the higher order modes to reduce sensitivity to small cell shape errors. The end cells and beam tube sections of the cavity were subsequently optimized to maximize higher order mode damping and thereby increase the beam break up current above the envisioned operating current of 100 mA. The design was then subjected to cell shape perturbations simulating manufacturing variation, and the higher order mode parameter of these imperfect cavities were finally used in beam tracking simulations to determine a realistic estimate for the beam break up current of the ERL main linac.