In partial fulfillment of the requirements for the degree of Doctor of Philosophy from Michigan State University.
Computer simulations and experimental evidence indicate that beam collimators produce wake fields in the form of dipole and quadrupole waveguide modes which can propagate tens of meters from their source before depositing energy at remote locations. Simulations confirm that coupling through narrow slots into bellows cavities occurs for beam pipe modes.
Two proposals are set forth to mitigate wake field effects. The first proposal is to reduce the quality factor of resonant structures with a water cooled dielectric lossy material. Electromagnetic energy coupling into resonant structures can be isolated and safely dissipated. Prototype devices have been built and have been shown to reduce resistive heating in large pumping chambers coupled to the beam chamber. Designs and simulations which incorporate such techiques into bellows devices are presented.
The second proposal incorporates novel devices introduced in the accelerator vacuum chamber which selectively traps dipole and quadrupole propagating wake fields before they can couple into sensitive beam line components without introducing impedance to the beam. Scattering parameter analysis is used to tailor device response to specific modes. Dangerous modes are extracted from the beam chamber, trapped and dissipated in a water cooled lossy material. Modes which represent an impedance to the beam are not affected. After design optimization, production and installation, beam line devices registered a factor of two decrease in electromagnetic heating in the vicinity of the installed device.
Methods for reducing harmful wake field effects resulting from intense charge densities have been demonstrated. This enables the higher bunch currents and shorter bunch lengths required to achieve productive B-factory operations at the highest possible luminosity.
S. Weathersby (2007)
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