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Title:Experimental studies of multipass beam breakup and energy recovery using the CEBAF injector LINAC
Author(s):Sereno, Nicholas S.R.
Doctoral Committee Chair(s):Cardman, Lawrence S.
Department / Program:Physics
Subject(s):beam breakup (BBU)
superconducting linacs
higher-order modes
multipass beam breakup
energy recovery
cebaf infector linac
Abstract:Beam breakup (BBU) instabilities in superconducting linacs are a significant issue due to the potentially high Q of the cavity higher-order modes (HOMs ). The CEBAF accelerator, which employs five pass recirculation through two superconducting linacs to accelerate high CW current (up to 200 pA), poses unique instability problems. An experimental investigation of multipass BBU at CEBAF has been completed using a single recirculation through the CEBAF injector linac. This recirculator is calculated to be more sensitive to the instability than the full CEBAF accelerator. Successful recirculation through the injector of a beam with greater than 200 pA average current from an injection energy of 5.6 MeV to a final energy of 80.1 MeV indicates that BBU should not be a problem in the full CEBAF accelerator. In addition, we were able recover all the energy from a 30 pA 42.8 MeV first pass beam by recirculating it out of phase and decelerating it to a final energy equal to the injection energy (5.6 MeV). The recirculator constructed in the CEBAF injector was designed so that the recirculation leg optics could be changed in an attempt to induce multipass BBU. The various optical settings were modeled using the computer code DIMAD and the resulting recirculation transfer matrices were used in the multipass BBU code TDBBU. Threshold currents for the onset of multipass BBU were calculated using TDBBU for each optical setting. The computed threshold currents were found to be at least an order of magnitude larger than the maximum average current available from the CEBAF injector ("' 200 pA). The primary experimental investigation of multipass BBU consisted of a series of RF measurements using a stripline kicker to deflect the beam to excite cavity HOMs and a superconducting cavity as a pickup device to detect HOM signals. The lll (normalized) amplitude of the cavity HOM signal was measured as a function of the average CW beam current for each optical setting. These data were analyzed in terms of a simplified model of a recirculating linac. Based on an analysis of the simple model, a lower bound on the threshold current was determined from the data for each optical setting. The lower bound on the threshold current inferred from the data for the majority of recirculator optical settings was beyond the currents available from the CEBAF injector. The RF experiment combined with the results of the TDBBU calculations indicate that the full CEBAF superconducting linac should not be limited by multipass BBU for CW beam currents well above the maximum design current of 200 f1A. In addition, a single pass measurement was performed using a stripline pickup (identical to the kicker device) to detect the HOM deflection of the beam. In this experiment, the actual deflection of the electron beam due to a HOM was observed using the pickup. The data obtained in this experiment were used to estimate the value of the shunt impedance R/Q of the HOM detected.
Issue Date:1994
Genre:Dissertation / Thesis
Rights Information:1994 Nicholas S. R. Sereno
Date Available in IDEALS:2011-04-26
Identifier in Online Catalog:3644323

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