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

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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%

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

Hellert

Dohlus

2018

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“…In [9], a periodic Poisson model for beam dynamics simulations is developed and applied for the simulation of laser modulated beams. Another efficient model proposed for low energy beam dynamics in nine-cell 1.3 GHz cavities is described in [10]. A procedure is found in [4] for analytically choosing machine working points in the parameter space of the longitudinal beam dynamics.…”

Section: Introductionmentioning

confidence: 99%

Beam dynamics of realistic bunches at the injector section of the European X-ray Free-Electron Laser

Chen

Zagorodnov

Dohlus

2020

We consider a 3D electron bunch at the cathode, modelled via photoemission, with realistic (asymmetric) spatial and temporal distributions for accelerator beam dynamics studies at the European X-ray Free-Electron Laser (European XFEL). A series of measurements are performed for low energy beams in the European XFEL injector. Using modeled 3D electron bunches, beam dynamics simulations, enabled by a three-dimensional (3D) space-charge solver with valid image-charge calculation on the photocathode plane, have shown improved agreements on measured beam properties (e.g., charge, bunch length, and shape) over a large range of variable machine parameters (e.g., cathode drive laser pulse energy, rf gun phase) for injector operation. In addition, the beam dynamics close to the cathode in a strongly spacecharge dominated regime is studied in the rf gun. In light of the choice of a suitable machine working point along a so-called emission curve, the impact of the space-charge effect on the longitudinal length and shape of the produced electron bunch is analyzed in comparison to the measurements downstream the injector exit.

“…Because of the low beam energy, and thus beam sensitivity to off-axis fields, the injector module (ACC1) has the largest impact on the intrabunch-train trajectory variation, and therefore knowledge on its cavity alignment is the first priority. Dedicated studies to derive accelerating structure misalignments from multibunch rf and beam position measurements [11] at the injector module have not been conclusive.…”

Section: Introductionmentioning

confidence: 99%

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

Hellert

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.