Beam Position Diagnostics with Higher Order Modes in Third Harmonic Superconducting Accelerating Cavities

Vol. XVI, Editorial Series on Accelerator Science (R.S. Romaniuk, J.-P. Koutchouk - Editors) 
 
For more information see page eucard-old.web.cern.ch/eucard-old/activities/communication/booklets/
 
Słowa kluczowe: nadprzewodzące wnęki akceleratorowe, mody wysokiego rzędu, trzecia harmoniczna, wnęki rezonansowe dla 3,9 GHz, diagnostyka pozycji wiązki, wnęki rezonansowe dla 1,3 GHz
 
Keywords: beam position diagnostics, higher order modes, third harmonic, superconducting accelerating cavities, SRF cavities for 3,9 GHz, SRF cavities for 1,3 GHz, partial beam measurements
 
Abstract:

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.
 
At the FLASH facility in DESY, 1.3 GHz (known as TESLA) and 3.9 GHz (third harmonic) cavities are installed. Wake-field in 3.9 GHz cavities are significantly larger than in the 1.3 GHz cavities. It is therefore important to mitigate the adverse effects of HOMs to the beam by aligning the beam on the electric axis of the cavities. This alignment requires an accurate beam position diagnostics inside the 3.9 GHz cavities. It is this aspect that is focused on in this thesis. Although the principle of beam diagnostics with HOM has been demonstrated on 1.3 GHz cavities, the realization in 3.9 GHz cavities is considerably more challenging. This is due to the dense HOM spectrum and the coupling of most HOMs amongst the four cavities in the third harmonic cryo-module.