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We report the direct observations of sub-macropulse beam centroidoscillations correlated with higher order modes (HOMs) which were generated byoff-axis electron beam steering in TESLA-type superconducting RF cavities. Theexperiments were performed at the Fermilab Accelerator Science and Technology(FAST) facility using its unique configuration of a photocathode rf guninjecting beam into two separated 9-cell cavities in series with correctormagnets and beam position monitors (BPMs) located before, between, and afterthem. Oscillations of ~100 kHz in the vertical plane and ~380 kHz in thehorizontal plane with up to 600-{\mu}m amplitudes were observed in a 3-MHzmicropulse 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 basedon HOM detector circuitry targeting the first and second dipole passbands, rfBPM bunch-by-bunch array data, imaging cameras, and a framing camera.Calculations reproduced the oscillation frequencies of the phenomena in thevertical case. In principle, these fundamental results may be scaled tocryomodule configurations of major accelerator facilities.

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