Analysis of multibunch free electron laser operation

Dohlus

2018

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Superconducting TESLA-type cavities are widely used to accelerate electrons in long bunch trains, such as in high repetition rate free electron lasers. The TESLA cavity is equipped with two higher order mode couplers and a fundamental power coupler (FPC), which break the axial symmetry of the cavity. The passing electrons therefore experience axially asymmetrical coupler kicks, which depend on the transverse beam position at the couplers and the rf phase. The resulting emittance dilution has been studied in detail in the literature. However, the kick induced by the FPC depends explicitly on the ratio of the forward to the backward traveling waves at the coupler, which has received little attention. The intention of this paper is to present the concept of discrete coupler kicks with a novel approach of separating the field disturbances related to the standing wave and a reflection dependent part. Particular attention is directed to the role of the penetration depth of the FPC antenna, which determines the loaded quality factor of the cavity. The developed beam transport model is compared to dedicated experiments at FLASH and European XFEL. Both the observed transverse coupling and detuning related coupler kick variations are in good agreement with the model. Finally, the expected trajectory variations due to coupler kick variations at European XFEL are investigated and results of numerical studies are presented.

Section: B Coupler Kick Variations At Flashmentioning

confidence: 99%

“…The principles of rf induced intrabunch-train trajectory variations have been described in detail in Ref. [42]. At European XFEL, typically 32 cavities with individual operational limits [49] are supplied by one rf power source.…”

Section: Expected Coupler Kick Variations At European Xfelmentioning

confidence: 99%

Detuning related coupler kick variation of a superconducting nine-cell 1.3 GHz cavity

Dohlus

2018

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“…High longitudinal and transverse stability of the beam along the bunch train is essential for multibunch FEL operation. However, intrabunch-train variations of rf parameters as well as structure misalignments induce intrabunch-train trajectory variations which affect the multibunch FEL performance considerably [8]. The accelerating cavities are housed inside 12-m-long cryomodules [9], which restricts the ability for direct alignment measurements.…”

Section: Introductionmentioning

confidence: 99%

Higher-order mode-based cavity misalignment measurements at the free-electron laser FLASH

Baboi

Shi

2017

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At the Free-Electron Laser in Hamburg (FLASH) and the European X-Ray Free-Electron Laser, superconducting TeV-energy superconducting linear accelerator (TESLA)-type cavities are used for the acceleration of electron bunches, generating intense free-electron laser (FEL) beams. A long rf pulse structure allows one to accelerate long bunch trains, which considerably increases the efficiency of the machine. However, intrabunch-train variations of rf parameters and misalignments of rf structures induce significant trajectory variations that may decrease the FEL performance. The accelerating cavities are housed inside cryomodules, which restricts the ability for direct alignment measurements. In order to determine the transverse cavity position, we use a method based on beam-excited dipole modes in the cavities. We have developed an efficient measurement and signal processing routine and present its application to multiple accelerating modules at FLASH. The measured rms cavity offset agrees with the specification of the TESLA modules. For the first time, the tilt of a TESLA cavity inside a cryomodule is measured. The preliminary result agrees well with the ratio between the offset and angle dependence of the dipole mode which we calculated with eigenmode simulations.

“…Within the bunch-train, the low-level-rf system [7,8] is able to restrict the variation of the vector sum of the accelerating gradient of one rf station sufficiently [9]. However, individual cavities have an intrinsic variation of rf parameters within one bunch-train, caused by the effects of beam loading and Lorentz force detuning [10]. Misaligned cavities in combination with variable rf parameters induce intrabunch-train trajectory variations which decrease the multibunch FEL performance significantly [10].…”

Section: Introductionmentioning

confidence: 99%

“…However, individual cavities have an intrinsic variation of rf parameters within one bunch-train, caused by the effects of beam loading and Lorentz force detuning [10]. Misaligned cavities in combination with variable rf parameters induce intrabunch-train trajectory variations which decrease the multibunch FEL performance significantly [10].…”

Section: Introductionmentioning

confidence: 99%

Efficient model for low-energy transverse beam dynamics in a nine-cell 1.3 GHz cavity

Dohlus

Decking

2017

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FLASH and the European XFEL are SASE-FEL user facilities, at which superconducting TESLA cavities are operated in a pulsed mode to accelerate long bunch-trains. Several cavities are powered by one klystron. While the low-level rf system is able to stabilize the vector sum of the accelerating gradient of one rf station sufficiently, the rf parameters of individual cavities vary within the bunch-train. In correlation with misalignments, intrabunch-train trajectory variations are induced. An efficient model is developed to describe the effect at low beam energy, using numerically adjusted transfer matrices and discrete coupler kick coefficients, respectively. Comparison with start-to-end tracking and dedicated experiments at the FLASH injector will be shown. The short computation time of the derived model allows for comprehensive numerical studies on the impact of misalignments and variable rf parameters on the transverse intra-bunchtrain beam stability at the injector module. Results from both, statistical multibunch performance studies and the deduction of misalignments from multibunch experiments are presented.