Higher Order Mode-based Beam And Cavity Diagnostics In 1.3 GHz Superconducting Cavities At FLASH

FLASH(Free Electron Laser in Hamburg)is a linac based SASE(Self-Amplified Spontaneous Emission)FEL(Free Electron Laser)facility generating high quality EUV(Extreme Ultra-Violet)to soft X-ray radiation pulses.TESLA(TeV Energy Superconducting Linear Accelerator)1.3 GHz cavities are used for the acceleration of the electron bunches.Higher Order Modes(HOMs)are excited when electron bunches pass through the accelerating cavities,and may induce emittance growth and deteriorate beam quality.Thus these HOMs need to be damped by a careful cavity design and by two specially designed couplers.On the other hand,HOMs can deliver useful information about the beam and the cavity.HOMs can be classified into monopole,dipole,quadruple modes and so on based on their azimuthal symmetry.The dipole modes have a linear dependence on the transverse beam offset.By concentrating on a specific dipole mode signal,it was previously demonstrated that one can determine the transverse beam position in each cavity.We developed a new method based on fitting the signal waveforms for the beam position measurement.As a benefit of the new method,the TESLA cavities proved to be able to work as HOMBPMs(HOM-based Beam Position Monitors),delivering consistent results over several months.The resolution of the involved HOMBPMs is determined,with a result of better than 10?m RMS.By aligning the beam on the cavity axis,the damage to the beam performance caused by HOMs can be also reduced.However the eight cavities installed in each cryo-module are themselves misaligned with respect to the module axis.Two of the most relevant misalignments are cavity offset and tilt.Due to the offset dependence of the beam excited dipole modes,the cavity offset misalignment can be determined during the HOMBPM calibration.However,the cavity tilt has so far proved to be more difficult to measure,because of the weak angular dependence.By carefully targeting the beam through the middle of a cavity,the strong offset contribution to the dipole mode excitation can be reduced.An efficient data analysis routine also based on the fitting method enabled us then to obtain the cavity tilt.For the first time,the tilts of all cavities inside a cryo-module have been measured.Beside using dipole modes for beam or cavity diagnostics,the second monopole band is utilized to monitor the beam phase with respect to the accelerating RF field in each cavity,which can not be measured directly by the phase monitors at FLASH.The RMS resolution of the HOM-based beam phase measurement is about 0.2°with a scope-based experimental setup.The resolution can be improved by limiting the noise level with dedicated electronics under design.Phase drifts between the HOM phase and the probe phase have been observed by monitoring the beam phase during a period of one month.The main achievements of this thesis are summarized into three aspects:(1)The new method based on signal fitting was developed which proved to stabilize the HOMBPM system over almost one year.Also it determines the polarization axes and cavity offset misalignment.(2)The cavity tilt was measured for the first time in all cavities within one accelerating module.(3)The beam phase was monitored over a long time and the phase drifts between the beam phase and the accelerating RF phase could be observed.