High Precision Sc Cavity Diagnostics with Hom Measurements

Experiments at the FLASH linac at DESY have demonstrated that the Higher Order Modes induced in Superconducting Cavities can be used to provide a variety of beam and cavity diagnostics. The centers of the cavities can be determined from the beam orbit which produces minimum power in the dipole HOM modes. The phase and amplitude of the dipole modes can be used as a high resolution beam position monitor, and the phase of the monopole modes to measure the beam phase relative to the accelerator RF. Beam orbit feedback which minimizes the dipole HOM power in a set of structures has been demonstrated. For most SC accelerators, the existing HOM couplers provide the necessary signals, and the downmix and digitizing electronics are straightforward, similar to those for a conventional BPM. THE DESY FLASH LINAC The experiments described were performed on the FLASH (Free Electron Laser in Hamburg) superconducting linac at DESY [1]. FLASH is also used as a test facility for the International Linear Collider (ILC) and the X-ray Free Electron Laser (XFEL) under the name TESLA Test Facility – Phase 2 (TTF2). FLASH contains 5 accelerating cryo-modules, each composed of 8 cavities, each 1 meter long, and containing 9 cells. The machine operates at a fundamental frequency of 1.3 GHz, with a typical energy between 450 and 700MeV, and approximately 1 nano-Coulomb charge. Each cavity has 2 couplers used to damp the higher order modes (HOM), and cables to bring the HOM power out to room temperature. The typical damped Q of the HOM modes is 10. HIGHER ORDER MODES IN SC ACCELERATOR CAVITIES In addition to the fundamental accelerating mode, Superconducting Cavities (SC) support a spectrum of higher order modes. While HOMs can be a source of a variety of accelerator problems: beam breakup, heating, etc. they can also be used as beam and cavity diagnostics. Mode Coupling to the Beam HOMs can be characterized by their azimuthal dependence as “monopole”, “dipole”, or higher multipole modes. Here we consider the response of these modes to a single electron bunch propagating near the axis of the cavity, and we assume bunches with lengths short compared with the wavelength of the HOM modes.[2] Monopole modes have no first order variation with the beam offset from the axis of the cavity, and are excited with amplitudes proportional to the charge in the bunch and with phase determined by the arrival time of the bunch. Dipole modes occur in doublets with orthogonal polarizations, with a frequency splitting caused by asymmetries due to the cavity couplers, and fabrication imperfections. Dipole modes are excited with an amplitude proportional to bunch charge, and to: • Transverse position relative to the cavity axis • Transverse angle relative to the cavity axis. The strength of this coupling relative to the position coupling is given by a mode “effective length”, on the order of the cavity length. This signal is excited at 90 degrees phase relative to the position signal. • Bunch tilt, with amplitude proportional to the bunch length, and 90 degrees phase relative to the position signal. For the short bunch length in FLASH this signal is not significant.