Olaf Hensler

High precision SC cavity alignment measurements with higher order modes

Experiments at the FLASH linac at DESY have demonstrated that the higher order modes (HOMs) 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. For most superconducting accelerators, the existing HOM couplers provide the necessary signals, and the downmix and digitizing electronics are straightforward, similar to those for a conventional BPM.

Electronics and Algorithms for HOM Based Beam Diagnostics

The signals from the Higher Order Mode (HOM) ports on superconducting cavities can be used as beam position monitors and to do survey structure alignment. A HOM-based diagnostic system has been installed to instrument both couplers on each of the 40 cryogenic accelerating structures in the DESY TTF2 Linac. The electronics uses a single stage down conversion form the 1.7 GHz HOM spectral line to a 20MHz IF which has been digitized. The electronics is based on low cost surface mount components suitable for large scale production. The analysis of the HOM data is based on Singular Value Decomposition. The response of the OM modes is calibrated using conventional BPMs.

High Level Software for the Commissioning of the European XFEL

The European X-Ray Free-Electron Laser (XFEL) will generate extremely short and intense X-ray flashes from the electron beam of a 2.1 km long superconducting linear accelerator. The commissioning and operation of the accelerator relies heavily on high level software for the automatization of measurements and procedures. The paper gives an overview of the ongoing work and highlights some new measurement techniques.

Test and first application of artificial sapphire sensors

Samples of monocrystalline sapphire sensors of 10×10 mm2 area and 0.5 mm thickness with the initial charge collection efficiency of about 2% have been studied in electron beams. The response as a function of the absorbed dose revealed excellent radiation hardness. The signal size dropped smoothly with the absorbed dose approaching 30% of the signal of non-irradiated sensors after the dose of 12 MGy. Four sapphire sensors have been installed, together with four pCVD diamond sensors, as a beam halo monitor of the FLASH facility. A report on the commissioning and operation experience is given.