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Title

HIGH PRESSURE GAS FILLED RF CAVITY BEAM TEST AT THE FERMILAB MUCOOL TEST AREA

Creator

Freemire, Ben

Date

2013, 2013-05

Description

The high energy physics community is continually looking to push the limits with respect to the energy and luminosity of particle accelerators...
Show moreThe high energy physics community is continually looking to push the limits with respect to the energy and luminosity of particle accelerators. In the realm of leptons, only electron colliders have been built to date. Compared to hadrons, electrons lose a large amount of energy when accelerated in a ring through synchrotron radiation. A solution to this problem is to build long, straight accelerators for electrons, which has been done with great success. With a new generation of lepton colliders being conceived, building longer, more powerful accelerators is not the most enticing option. Muons have been proposed as an alternative particle to electrons. Muons lose less energy to synchrotron radiation and a Muon Collider can provide luminosity within a much smaller energy range than a comparable electron collider. This allows a circular collider to be built with higher attainable energy than any present electron collider. As part of the accelerator, but separate from the collider, it would also be possible to allow the muons to decay to study neutrinos. The possibility of a high energy, high luminosity muon collider and an abundant, precise source of neutrinos is an attractive one. The technological challenges of building a muon accelerator are many and diverse. Because the muon is an unstable particle, a muon beam must be cooled and accelerated to the desired energy within a short amount of time. This requirement places strict requisites on the type of acceleration and focusing that can be used. Muons are generated as tertiary beams with a huge phase space, so strong magnetic fields are required to capture and focus them. Radio frequency (RF) cavities are needed to capture, bunch and accelerate the muons. Unfortunately, traditional vacuum RF cavities have been shown to break down in the magnetic fields necessary for capture and focusing. To successfully operate RF cavities in strong magnetic fields, the idea of filling xv them with a high pressure gas in order to mitigate breakdown was proposed. The gas has the added benefit of providing cooling for the beam. Experiments were successfully performed using different gas species in a test cell cavity placed in a multi-Tesla magnetic field. These encouraging results lead to the necessity of a test closer to actual accelerator conditions, namely sending a beam of particles through the cavity. The electron-ion plasma created in the cavity by the beam absorbs energy and can degrade the accelerating electric field of the cavity. Electrons can recombine with hydrogen ions, however this process is slow compared to the bunch length and spacing. As electrons account for the majority of the energy loss in the cavity, their removal in a short time is highly desirable. The addition of an electronegative dopant gas can greatly decrease the lifetime of an electron in the cavity. The results of two beam tests will be presented. The experimental variables cover a wide range of gas pressure, beam intensity, and cavity electric field. Measurements in pure hydrogen of the power consumption of electrons in the cavity indicate a range of energy loss between 10−18 and 10−16 joules per RF cycle per electron. When hydrogen doped with dry air is used, measurements of the power consumption indicate an energy loss range of 10−20 to 10−18 joules per RF cycle per ion, two orders of magnitude improvement over non-doped measurements. The rate at which electrons recombine with positively charged hydrogen ions ranges from 10−7 to 10−5 cm3 s . The lifetime of electrons in a mixture of hydrogen gas and dry air has been measured from <1 ns, up to 200 ns. Finally, the ion-ion recombination rate falls between 10−8 and 10−7 cm3 s . The results extrapolated to the parameters of a Neutrino Factory and Muon Collider indicate that a high pressure gas filled RF cavity will work in a coolingchannel for either machine. A demonstration experiment is warranted to prove this technology’s validity.
PH.D in Physics, May 2013
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