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Abstract: Neutrino Factories and Muon Colliders- cooling lattices require both highgradient rf and strong focusing solenoids. Experiments have shown that theremay be serious problems operating rf in the required magnetic fields. The useof high pressure gas to avoid these problems is discussed, including possibleloss problems from electron and ion production by the passage of an ionizingbeam. It is also noted that high pressure gas cannot be used in later stages ofcooling for a muon collider. Experimental observations using vacuum rf cavitiesin magnetic fields are discussed, current published models of breakdown withand without magnetic fields are summarized, and some of their predictionscompared with observations.A new theory of magnetic field dependent breakdown is presented. It isproposed that electrons emitted by field emission on asperities on one side ofa cavity are focused by the magnetic field to the other side where they meltthe cavity surface in small spots. Metal is then electrostatically drawn fromthe molten spots, becomes vaporized and ionized by field emission from theremaining damage and cause breakdown. The theory is fitted to existing 805 MHzdata and predictions are made for performance at 201 MHz. The model predictsbreakdown gradients significantly below those specified for either theInternational Scoping Study ISS Neutrino Factory or a Muon Collider.Possible solutions to these problems are discussed, including designs for`magnetically insulated rf- in which the cavity walls are designed to beparallel to a chosen magnetic field contour line and consequently damage fromfield emission is suppressed. An experimental program to study these problemsand their possible solution is outlined.

Author: Robert B. Palmer, Richard C. Fernow, Juan C. Gallardo, Diktys Stratakis, Derun Li

Source: https://arxiv.org/


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