Fermilab, Batavia, USA
Muons, Inc, Batavia, USA
Fermilab, Batavia, USA
Handong Global University, Pohang, Republic of Korea
Politecnico di Torino, Torino, Italy
Muons, Inc, Batavia, USA
IIT, Chicago, Illinois, USA
Breakdown plasma in a high-pressure hydrogen gas filled RF cavity has been studied from a time domain spectroscopic light analysis. The observed breakdown plasma temperature and density reached 21,000 K and 1020 cm-3, respectively. The electron recombination rate has been evaluated from the decay of plasma density in various gas pressures. The recombination mechanism in dense plasma will be discussed. Finally, the similarity and difference of the breakdown processes between the high-pressure hydrogen gas filled RF cavity and a vacuum RF one will be discussed.
IIT, Chicago, Illinois, USA
Handong Global University, Pohang, Republic of Korea
Politecnico di Torino, Torino, Italy
Fermilab, Batavia, USA
Muons, Inc, Batavia, USA
A high pressure hydrogen gas filled RF cavity was subjected to an intense proton beam to study the evolution of the beam induced plasma inside the cavity. The electron recombination rate with the dense ionized hydrogen plasma has been measured under varying conditions. Recombination rates as high as 10-7 cm3/s have been recorded. This technique shows promise in the R&D program for a muon accelerator. The use of hydrogen, both as a way to prevent breakdown in an RF cavity and as a mechanism for cooling a beam of muons, will be discussed.
Fermilab, Batavia, USA
Handong Global University, Pohang, Republic of Korea
Politecnico di Torino, Torino, Italy
IIT, Chicago, Illinois, USA
Muons, Inc, Batavia, USA
Muon Colliders and Neutrino Factories call for R&D for a high-gradient RF system capable of operating in a high magnetic field. Adding a high pressure gas inside an RF cavity (HPRF) prevents cavity breakdown, allowing higher gradients in a magnetic field. A high-energy beam passing through an HPRF cavity ionizes the gas, producing plasma. Plasma electrons absorb cavity’s energy, reducing the energy available for beam acceleration. Doping cavity gas with electronegative gas (gas that tends to attract and bond electrons) reduces the number of plasma electrons. The experiments were carried out at the MuCool Test Area (MTA) facility at Fermilab. Different concentrations of an electronegative gas SF6 were added to hydrogen gas. The results of room-temperature tests showing a great reduction in power drop in the cavity will be presented. However, a realistic cavity would operate at liquid nitrogen temperature, where SF6 freezes. Thus, a search for a better electronegative gas candidate is underway; we plan to test oxygen-doping next.
Fermilab, Batavia, USA
Muons, Inc, Batavia, USA
Fermilab, Batavia, USA
The Helical Cooling Channel (HCC), a novel technique for six-dimensional (6D) ionization cooling of muon beams, has shown considerable promise based on analytic and simulation studies. However, the implementation of this revolutionary method of muon cooling requires new techniques for the integration of hydrogen-pressurized, high-power RF cavities into the low-temperature superconducting magnets of the HCC. We present the progress toward a conceptual design for the integration of 805 MHz RF cavities into a 10 T Nb3Sn based HCC test section. We include discussions on the pressure and thermal barriers needed within the cryostat to maintain operation of the magnet at 4.2 K while operating the RF and energy absorber at a higher temperature. Additionally, we include progress on the Nb3Sn helical solenoid design
Muons, Inc, Batavia, USA
JLAB, Newport News, Virginia, USA
Fermilab, Batavia, USA
Muons, Inc, Batavia, USA
Fermilab, Batavia, USA
A high-luminosity Muon Collider requires bunch recombination for optimal luminosity. In this paper, we take advantage of the large slip factor in a helical transport channel (HTC) to coalesce bunches of muons into a single one over a shorter distance than can be achieved over a straight channel. The coalescing subsystem that is designed to merge 9 bunches has a horizontal length of ~105m and is able to achieve efficiencies of 99.7%, 98.4%, and 94.2% for 9, 11, and 13 bunches, respectively, where each bunch has emittances expected at the end of an HCC. Simplified designs incorporating fill factors for RF cavities of ~25% and ~50% obtained efficiencies of 96%, 94-95%, and 90-91% for 9, 11, and 13 bunches, respectively. The efficiencies above do not include decay losses, which would be ~8% for muons with kinetic energy of 200 MeV.
Fermilab, Batavia, USA
Handong Global University, Pohang, Republic of Korea
Politecnico di Torino, Torino, Italy
IIT, Chicago, Illinois, USA
The recent High Pressure RF (HPRF) cavity experiment at the MuCool Test Area (MTA) used a 400 MeV Linac proton beam to study the beam loading effect. When the energetic proton beam passes through the cavity, it ionizes the inside gas and produces electrons. These electrons consume RF power inside the cavity. The number of electrons produced per cm inside the cavity (at 950 psi Hydrogen gas) per incident proton is 1200. The measurement of beam position and profile are necessary. The MTA is a flammable gas (Hydrogen) hazard zone, so we have developed a passive beam diagnostic instrument using a Chromox-6 scintillation screen and CCD camera. This paper presents quantitative information about beam position and beam profile. A neutral density filter was used to avoid saturation of the CCD camera. Image data is filtered and fitted with a Gaussian function to compute the beam size. The beam profile obtained from the scintillation screen will be compared with a multi-wire beam profile.
Fermilab, Batavia, USA
Handong Global University, Pohang, Republic of Korea
IIT, Chicago, Illinois, USA
Muons, Inc, Batavia, USA
One of the greatest challenges in constructing a Muon Collider is cooling the hot muons into a focused beam after their production. Because the beam must be cooled quickly before the muons decay, compact cooling designs require high gradient cavities inside strong magnetic fields. Unfortunately, due to focused field emission, an external magnetic field degrades the performance of the cavity below what is required for a muon collider. High-pressure gas inside the cavity has been proposed to both mitigate this effect, as well as serve as an absorber for transversely cooling the muon beam. A prototype of a high pressure gas-filled cavity is currently being studied at the Muon Test Area at Fermilab. The experimental setup as well as several measurements of the physics and performance of the apparatus while operating in a 400-MeV proton beam will be discussed.
Voss Scientific, Albuquerque, New Mexico, USA
LBNL, Berkeley, California, USA
Muons, Inc, Batavia, USA
Fermilab, Batavia, USA
Recent studies have shown that high gradients can be achieved quickly in high-pressure gas-filled cavities without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual vacuum and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of radiofrequency and surface preparation. A series of experiments at 805 MHz using hydrogen fill pressures up to 0.01 g/cm3 of H2 have demonstrated high electric field gradients and scaling with the DC Paschen law limit, up to ~30 MV/m, depending on the choice of electrode material. For higher field stresses, the breakdown characteristics deviate from the Paschen law scaling. Fully-kinetic 0D collisional particle-in-cell (PIC) simulations give breakdown characteristics in H2 and H2/SF6 mixtures in good agreement with the 805 MHz experimental results below this field stress threshold. The impact of these results on gas-filled RF accelerating cavity design will be discussed.
Muons, Inc, Batavia, USA
LANL, Los Alamos, New Mexico, USA
Fermilab, Batavia, USA
IIT, Chicago, Illinois, USA
An 805 MHz RF pillbox cavity has been designed and constructed to investigate potential muon beam acceleration and cooling techniques for a Muon Collider or Neutrino Factory. The cavity can operate in vacuum or under pressure to 100 atmospheres, at room temperature or in a liquid nitrogen bath at 77 K. The cavity has been designed for easy assembly and disassembly with bolted construction using aluminum seals. To perform vacuum and high pressure breakdown studies of materials and geometries most suitable for the collider or factory, the surfaces of the end walls of the cavity can be replaced with different materials such as copper, aluminum, beryllium, or molybdenum, and with different geometries such as shaped windows or grid structures. The cavity has been designed to fit inside the 5-Tesla solenoid in the MuCool Test Area at Fermilab. In this paper we present the vacuum conditioning results and discuss plans for testing in a 5-Tesla magnetic field. Additionally, we discuss the testing plan for beryllium (a material research has shown to be ideal for the collider or factory) end walls.