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Beam Position Diagnostics with Higher Order Modes in Third Harmonic Superconducting Accelerating Cavities
J.-P. Koutchouk (red. nauk.)
R.S. Romaniuk (red. nauk.)
Vol. XVI, Editorial Series on Accelerator Science (R.S. Romaniuk, J.-P. Koutchouk - Editors)
Higher order modes (HOM) are electromagnetic resonant fields. They can be excited by an electron beam entering an accelerating cavity, and constitute a component of the wake-field. This wake-field has the potential to dilute the beam quality and, in the worst case, result in a beam-break-up instability.
It is therefore important to ensure that these fields are well suppressed by extracting energy through special couplers. In addition, the effect of the transverse wake-field can be reduced by aligning the beam on the cavity axis. This is due to their strength depending on the transverse offset of the excitation beam. For suitably small offsets the dominant components of the transverse wake-field are dipole modes, with a linear dependence on the transverse offset of the excitation bunch. This fact enables the transverse beam position inside the cavity to be determined by measuring the dipole modes extracted from the couplers, similar to a cavity beam position monitor (BPM), but requires no additional vacuum instrumentation.