BNL, Upton, Long Island, New York
Funding: This work supported by the U.S. Department of Energy, contract no. DE-AC02-98CH10886.
Beam cooling for a future neutrino factory or muon collider requires high gradient rf cavities in the presence of strong magnetic fields. Experimental measurements suggested that the maximum accelerating gradient drops as the axial magnetic field increases. Little is known about the explicit dependence of the gradient on the strength of the magnetic field. The experimental observation of dark currents arising from local regions with enhanced surface field intensities under external magnetic fields however, suggests a new possible mechanism of breakdown based on electron field emission. A model of magnetic field breakdown is proposed. We illustrate that the field emitted electrons are focused by the external fields into small spots on the other side of the cavity and estimate the energy density they deliver to the wall. We show that this energy increases with the magnetic field, and this may lead to melting of the cavity surface. The influence of local fields at the emitter side is discussed and the extent to which space-charge affects this process is investigated. Results of our model are compared with recent experimental data from the 201 MHz and 805 MHz cavities.
BNL, Upton, Long Island, New York
UMD, College Park, Maryland
JLAB, Newport News, Virginia
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
In this paper we report on a proof of principle experiment for demonstrating the possibility of reconstructing the time resolved-phase-space distribution of a space-charge dominated beam by a tomographic technique which provides us with far more information than a time-sliced emittance. We emphasize that this work describes and demonstrates a new methodology which can be applicable to any beam pulse using imaging methods with the appropriate time resolution for the pulse duration. The combination of a high precision tomographic diagnostic with fast imaging screens and a gated camera are used to produce phase space maps of two beams: one with a parabolic current profile and another with a short perturbation atop a rectangular pulse. The correlations between longitudinal and transverse phase spaces are apparent and their impact on the dynamics is discussed.
JLAB, Newport News, Virginia
UMD, College Park, Maryland
BNL, Upton, Long Island, New York
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
Longitudinal space-charge waves can introduce energy perturbations into charge particle beams and degrade the beam quality, which is critical to many modern applications of particle accelerators. Although many longitudinal phenomena arising from small perturbations can be explained by a one-dimensional cold fluid theory, nonlinear behavior of space-charge waves observed in experiments has not been well understood. In this paper, we summarize our recent investigation by means of more detailed measurements and self-consistent simulations. Combining the numerical capability of a PIC code, WARP, with the detailed initial conditions measured by our newly developed time resolved 6-D phase space mapping technique, we are able to construct a self consistent model for studying the complex physics of longitudinal dynamics of space-charge dominated beams. Results from simulation studies suggest that the unexplained nonlinear behavior of space-charge waves may be due to transverse mismatch or misalignment of beams.
UMD, College Park, Maryland
BNL, Upton, Long Island, New York
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
Beam matching is critical for avoiding envelope mismatch oscillations that can lead to emittance growth and halo formation, especially if the beam has significant space charge. The University of Maryland Electron Ring (UMER) is a research storage ring that is designed for scaled studies that are applicable to many larger machines. Using 10 keV electron beams at relatively high current (0.6 100 mA), space charge forces are relatively strong. Matching of the UMER beam is rendered difficult by the space charge, the crowdedness of the lattice, and especially the unique injection scheme where an offset oversized quadrupole is shared between the ring and the injector. In this paper we discuss several schemes for optimizing the matching at injection, both analytical and beam-based, which we test using particle-in-cell simulations with the code, WARP. Comparison to UMER experimental data is provided where available.