PressurizedH2rf Cavities in Ionizing Beams and Magnetic Fields

Abstract

A major technological challenge in building a muon cooling channel is operating rf cavities in multitesla external magnetic fields. We report the first proof-of-principle experiment of a high pressure gas-filled rf cavity for use with intense ionizing beams and strong external magnetic fields. rf power consumption by beam-induced plasma is investigated with hydrogen and deuterium gases with pressures between 20 and 100 atm and peak rf gradients between 5 and 50 MV=m. The low pressure case agrees well with an analytical model based on electron and ion mobilities. Varying concentrations of oxygen gas are investigated to remove free electrons from the cavity and reduce the rf power consumption. Measurements of the electron attachment time to oxygen and rate of ion-ion recombination are also made. Additionally, we demonstrate the operation of the gas-filled rf cavity in a solenoidal field of up to 3 T, finding no major magnetic field dependence. All these results indicate that a high pressure gas-filled cavity is a viable technology for muon ionization cooling.

Simulation of plasma loading of high-pressure RF cavities

“…At the HPRF conditions, ternary recombination is dominant [6]. The time evolution of the electron number density is given by the following equation 5) where j = 3, 5, 7, · · · , and n e , n H j + ,Ṅ e , and β j denote the number density of electrons, the number density of H j j hydrogen ion clusters, the production rate of electrons, and the recombination rate of electrons and H j + , respectively.…”

“…dn e /dt = 0). In our model, we use the following fit for the effective recombination rate, applicable to transient processes in the plasma, 6) where X = E/P is the ratio of electric field magnitude to the gas pressure. Values for the parameters a and b were obtained via comparison of HPRF simulations to various experimentally measured quantities, in particular, the plasma loading [8], and extrapolated to the range of parameters used in this work.…”

“…where P, R, C, V max , V (t) are the RF field power, the shunt impedance, the cavity capacitance, the magnitude of the RF voltage, and the instantaneous voltage value, respectively [6]. In the simulations, the total power of plasma loading is computed at each time step as a sum over all computational nodes N of the PIC mesh…”

Gas Filled RF Resonator Hadron Beam Monitor for Intense Neutrino Beam Experiments

“…It is controlled by a gas pressure, RF gradient, and concentration of electronegative dopant in the cavity. Optimization of the RF parameter is done based on the past RF measurements [4,5]. Two operation modes are considered to diagnose the beam quality.…”

Front End for a neutrino factory or muon collider

“…Variations of the Front End using gas-filled rf cavities were developed and found to obtain capture and acceptance approximately equal to vacuum cavity examples, provided the rf gradients are increased to compensate energy loss in the gas [19]. Operation of gas-filled rf has a complication in that beam loading by electrons from beam ionization can drain rf power; this can be moderated by adding a small fraction of electronegative dopant (O 2 ) to the H 2 , to facilitate electron recombination [20,21].…”