Study of Beam Diagnostics with Trapped Modes in Third Harmonic Superconducting Cavities at FLASH

Off-axis beams passing through an accelerating cavity excite dipole modes among other higher order modes (HOMs). These modes have linear dependence on the transverse beam offset from the cavity axis. Therefore they can be used to monitor the beam position within the cavity. The fifth dipole passband of the third harmonic superconducting cavities at FLASH has modes trapped within each cavity and do not propagate through the adjacent beam pipes, while most other cavity modes do. This could enable the beam position measurement in individual cavities. This paper investigates the possibility to use the fifth dipole band for beam alignment in the third harmonic cavity module. Simulations and measurements both with and without beam-excitations are presented. Various analysis methods are used and compared. A good correlation of HOM signals to the beam position is observed.Comment: 2nd International Particle Accelerator Conference (IPAC '11), San Sebastian, Spain, Sep 4-9, 201

ELECTRON BEAM DIAGNOSTICS FOR FLASH II

Abstract Up to now, the FLASH linac serves one SASE undulator. The radiation produced can be guided to one of 5 beamlines. In order to increase the availability of the machine, a second undulator section, called FLASH II, will be built in the next 2 years to generate SASE light. A HHG laser will alternatively serve to produce seeded radiation in the undulators. The electron beam diagnostics in FLASH II has to enable the precise control of the beam position, size, timing, as well as the overlap of the electron beam with the HHG laser. Losses have to be kept under control, and the beam has to terminate safely in the beam dump. In comparison to FLASH, which was designed to run with rather high charge, the dynamic range of the diagnostics has to be between 0.1 to 1 nC, similar to the European XFEL. This paper gives an overview of the diagnostics for FLASH II