Submacropulse Electron-Beam Dynamics Correlated With Higher-Order Modes In Tesla-Type Superconducting RF Cavities

TL;DR

This study observes sub-macropulse beam oscillations in TESLA-type superconducting RF cavities caused by higher order modes, revealing correlations with beam steering and providing insights for accelerator design.

Contribution

It provides the first direct measurements of beam oscillations linked to HOMs in superconducting cavities, with experimental data and calculations supporting the phenomena.

Findings

Oscillations at ~100 kHz and ~380 kHz observed in beam with up to 600 μm amplitude.

Oscillations reduced at lower charge (100 pC/b).

Calculations match vertical oscillation frequencies.

Abstract

We report the direct observations of sub-macropulse 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 9-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-{\mu}m 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.