A muon collider or Higgs factory requires significant reduction of the six dimensional emittance of the beam prior to acceleration. One method to accomplish this involves building a cooling channel using high pressure gas filled radio frequency cavities. The performance of such a cavity when subjected to an intense particle beam must be investigated before this technology can be validated. To this end, a high pressure gas filled radio frequency (rf) test cell was built and placed in a 400 MeV beam line from the Fermilab linac to study the plasma evolution and its effect on the cavity. Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally, sulfur hexafluoride and dry air were used as dopants to aid in the removal of plasma electrons. Measurements were made using a variety of beam intensities, gas pressures, dopant concentrations, and cavity rf electric fields, both with and without a 3 T external solenoidal magnetic field. Energy dissipation per electron-ion pair, electron-ion recombination rates, ion-ion recombination rates, and electron attachment times to $SF_6$ and $O_2$ were measured.

Pressurized rf cavities in ionizing beams

The accelerating gradient in a RF cavity is limited by many factors such as the surface material properties, RF frequency, the external magnetic field and the gas pressure inside the cavity. In the MuCool Program, RF cavities are studied to understand these basic mechanisms with the aim of improving the maximum stable accelerating gradient of RF cavities in magnetic field. These cavities are being developed for muon ionization cooling channels for a Neutrino Factory or Muon Collider. We report studies using 805 MHz and 201 MHz RF pillbox cavities in the MuCool Test Area (MTA) at Fermilab. New results include data from buttons of different materials mounted in the 805 MHz cavity, a study of the accelerating gradient in the 201 MHz cavity and X-ray background radiation rates from the cavities due to Bremsstrahlung. The 201 MHz cavity has been shown to be stable at 21 MV/m at zero magnetic field, well in excess of its 16 MV/m design gradient. We also discuss results from the 201 MHz cavity operation in magnetic field and introduce the test of E × B effects with the 805 MHz box cavity.

RF Studies at Fermilab MuCool Test Area

A muon collider or Higgs factory requires significant reduction of the six dimensional emittance of the beam prior to acceleration. One method to accomplish this involves building a cooling channel using high pressure gas filled radio frequency cavities. The performance of such a cavity when subjected to an intense particle beam must be investigated before this technology can be validated. To this end, a high pressure gas filled radio frequency (rf) test cell was built and placed in a 400 MeV beam line from the Fermilab linac to study the plasma evolution and its effect on the cavity. Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally, sulfur hexafluoride and dry air were used as dopants to aid in the removal of plasma electrons. Measurements were made using a variety of beam intensities, gas pressures, dopant concentrations, and cavity rf electric fields, both with and without a 3 T external solenoidal magnetic field. Energy dissipation per electron-ion pair, electron-ion recombination rates, ion-ion recombination rates, and electron attachment times to ${\mathrm{SF}}_{6}$ and ${\mathrm{O}}_{2}$ were measured.

/pdf/pressurized-rf-cavities-in-ionizing-beams-rm14of0d4y.pdf

MANX is a 6-dimensional muon ionization-cooling experiment that has been proposed to Fermilab to demonstrate the use of a Helical Cooling Channel (HCC) for future muon colliders and neutrino factories. The HCC for MANX has solenoidal, helical dipole, and helical quadrupole magnetic components which diminish as the beam loses energy as it slow down in a liquid helium absorber inside the magnets. Additional magnets that provide emittance matching between the HCC and upstream and downstream spectrometers are also described as are the results of G4Beamline simulations of the beam cooling behavior of the complete magnet and absorber system.

/pdf/magnets-for-the-manx-6-d-muon-cooling-demonstration-3qdgec3lc5.pdf

Magnets for the MANX 6-D muon cooling demonstration experiment

The MuCool RF Program focuses on the study of normal conducting RF structures operating in high magnetic field for applications in muon ionization cooling for Neutrino Factories and Muon Colliders. This paper will give an overview of the program, which will include a description of the test facility and its capabilities, the current test program, and the status of a cavity that can be rotated in the magnetic field which allows for a more detailed study of the maximum stable operating gradient vs. magnetic field strength and angle.

/pdf/the-mucool-test-area-and-rf-program-2io4w4km0i.pdf

The MuCool Test Area and RF Program

The MuCool Experiment has been continuing to take data with 805 and 201 MHz cavities in the MuCool Test Area. The system uses rf power sources from the Fermilab Linac. Although the experimental program isprimarily aimed at the Muon Ionization Cooling Experiment (MICE), we have been studying the dependence of rf limits on frequency, cavity material, high magnetic fields, gas pressure, coatings, etc. with the general aim of understanding the basic mechanisms involved. The 201 MHz cavity, essentially a prototype for the MICE experiment, was made using cleaning techniques similar to those employed for superconducting cavities and operates at its design field with very little conditioning.

/pdf/recent-rf-results-from-the-mucool-test-area-escholarship-58m6u3xnum.pdf

Recent RF Results from the MuCool Test Area - eScholarship

There is an on going international collaboration studying the feasibility and cost of building a muon collider or neutrino factory [1,2]. An important aspect of this study is the full understanding of ionization cooling of muons by many orders of magnitude for the collider case. An important muon ionization cooling experiment, MICE [3], has been proposed to demonstrate and validate the technology that could be used for cooling. Ionization cooling is accomplished by passing a high-emittance muon beam alternately through regions of low Z material, such as liquid hydrogen, and very high accelerating RF Cavities within a multi-Tesla solenoidal field. To determine the effect of very large solenoidal magnetic fields on the generation of dark current, x-rays and on the breakdown voltage gradients of vacuum RF cavities, a test facility has been established at Fermilab in Lab G. This facility consists of a 12 MW 805 MHz RF station and a large warm bore 5 T solenoidal superconducting magnet containing a pill box type cavity with thin removable window apertures. This system allows dark current and breakdown studies of different window configurations and materials. The results of this study will be presented. The study has shown that the peak achievable more » accelerating gradient is reduced by a factor greater than 2 when solenoidal field of greater than 2 T are applied to the cavity. « less

/pdf/effect-of-high-solenoidal-magnetic-fields-on-breakdown-5clmz5xqg6.pdf

Effect of high solenoidal magnetic fields on breakdown voltages of high vacuum 805 MHz cavities

A goal of the Muon Accelerator Program (MAP) is the development of an ionization cooling channel for muon beams. Ionization cooling channel designs call for the operation of high-gradient, normal-conducting RF cavities in multi-Tesla solenoidal magnetic fields. However, strong magnetic fields have been shown in some cases to limit the maximum achievable gradient in RF cavities. This gradient limit is characterized by RF breakdown and damage to the cavity surface. To study this issue, we have developed an experimental program based on a modular pillbox cavity operating at 805 MHz. The modular cavity design allows for the evaluation of different cavity geometries and materials ‐ such as beryllium ‐ which may ameliorate or circumvent RF breakdown triggers. Modular cavity componentsmayfurthermorebe preparedwith differentsurface treatments, suchas high-temperatureannealingor chemical polishing. This poster presents the design and fabrication status of the modular cavity, as well as plans for the experimental program.

A modular cavity for muon ionization cooling r&d

The frequency of MICE cavities is controlled by pneumatic tuners as their operation is impervious due to large magnetic fields. The mechanical and RF transfer functions of the tuner were simulated in ANSYS. The first of these tuning systems was assembled and tested at Fermilab. The mechanical response and the RF tuning transfer function have been measured and compared with simulation results. Finally the failure of different actuators has been simulated and tested to predict the operational limits of the tuner.

Tuner System Simulation and Tests for the 201-MHz MICE Cavity

Current muon collider schemes call for significant six dimensional (6D) cooling of the muon beam before it is accelerated [1]. One cooling scheme, the Helical Cooling Channel (HCC), employs RF cavities filled with high pressure hydrogen gas [2]. The gas acts both as an energy loss mechanism to allow for ionization cooling [3, 4], and as a buffer in order to prevent RF breakdown [5]. When a beam of particles passes through a HPRF cavity, it will ionize the gas. The amount of plasma generated is dependent on the beam energy, the stopping power of the gas, and the density of the gas. The resulting plasma will gain energy from the RF electric field and transfer it through collisions to the gas. This effect is called plasma loading. An experiment performed at theMuCool Test Area at Fermilab studied the formation of plasma created by a proton beam and its evolution over the course of many 805 MHz RF cycles [6].

Plasma Chemistry in a High Pressure Gas Filled RF Test Cell for use in a Muon Cooling Channel

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