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Submacropulse electron-beam dynamics correlated with higher-order modes in Tesla-type superconducting rf cavities

  • Lumpkin, A.
  • Thurman-Keup, R.
  • Edstrom, D.
  • Ruan, J.
  • Eddy, N.
  • Prieto, P.
  • Napoly, O.
  • Carlsten, B.
  • Bishofberger, K.
Publication Date
Jan 01, 2018
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We report the direct observations of submacropulse beam centroid oscillations correlated with higher order modes (HOMs) which were generated by off-axis electron beam steering in TESLA-type superconducting rf cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) facility using its unique configuration of a photocathode rf gun injecting beam into two separated nine-cell cavities in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. Oscillations of ∼100 kHz in the vertical plane and ∼380 kHz in the horizontal plane with up to 600-μm amplitudes were observed in a 3-MHz micropulse repetition rate beam with charges of 100, 300, 500, and 1000 pC/b. However, the effects were much reduced at 100 pC/b. The measurements were based on HOM detector circuitry targeting the first and second dipole passbands, rf BPM bunch-by-bunch array data, imaging cameras, and a framing camera. Calculations reproduced the oscillation frequencies of the phenomena in the vertical case. In principle, these fundamental results may be scaled to cryomodule configurations of major accelerator facilities.

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