arXiv subjects

Martin Dohlus

Publications and source records attributed to Martin Dohlus.

9 recordsLinked to original sources

arXiv2021

CW Operation of the European XFEL: SC-Gun Injector Optimization, S2E Calculations and SASE Performance

This note presents optimization results for an injector with a super-conducting gun for the cw operation of the European XFEL, the corresponding start-to-end simulation calculations for the beam transport to the undulators and SASE calculations of the X-ray intensities for the achievable photon energies. The optimizations for an injector with a super-conducting gun are done mainly for an already experimentally shown accelerating gradient of 40 MV/m. Different working points with respect to transverse emittance and bunch length are chosen to see whether the lower transverse emittance of the longer bunches survives the necessary stronger bunch compression to achieve comparable peak currents in the undulators. Also, results for possible further improvement of the gun gradient to 50 MV/m and a modified transverse laser profile (truncated Gaussian) are shown. The S2E calculations are taking into account all relevant collective effects, including a thorough treatment of the impact of the so called Micro-Bunching Instability. For that purpose, tracking with the real number of particles is done, with a resolution in space and time fine enough to calculate instability growth model-free.

physics.acc-ph↗

arXiv2020

Superradiant Cherenkov-Wakefield radiation as THz source for FEL facilities

An electron beam passing through a tube which is lined on the inside with a dielectric layer will radiate energy in the THz range due to the interaction with the boundary. The resonant enhancement of certain frequencies is conditioned by structure parameters as tube radius and permittivity of the dielectric layer. In low loss structures narrow-band radiation is generated which can be coupled out by suitable antennas. For higher frequencies the coupling to the resistive outer metal layer becomes increasingly important. The losses in the outer layer prohibit to reach high frequencies with narrow-band conditions. Instead short broad-band pulses can be generated with still attractive power levels. In the first section of the paper a general theory of the impedance of a two-layer structure is presented and the coupling to the outer resistive layer is discussed. Approximate relations for the radiated energy, power and pulse length for a set of structure parameters are derived and compared to numerical results in the following section. Finally first numerical result of the out-coupling of the radiation by means of a Vlasov antenna and estimates of the achieved beam quality are presented.

physics.acc-ph↗

arXiv2019

Axial representation of external electromagnetic fields for particle tracking in accelerators

We derive a power series representation of an arbitrary electromagnetic field near some axis through the coaxial field components on the axis. The obtained equations are compared with Fourier-Bessel series approach and verified by several examples. It is shown that for each azimuthal mode we need only two real functions on the axis in order to describe the field in a source free region near to it. The representation of dipole mode in a superconducting radio-frequency gun is analyzed.

physics.acc-ph↗

arXiv2018

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

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.

physics.acc-ph↗

arXiv2017

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

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-bunch-train beam stability at the injector module. Results from both, statistical multibunch performance studies and the deduction of misalignments from multibunch experiments are presented.

physics.acc-ph↗

arXiv2010

A Semi-Analytical Modelling of Multistage Bunch Compression with Collective Effects

In this paper we introduce an analytical solution (up to the third order) for a multistage bunch compression and acceleration system without collective effects. The solution for the system with collective effects is found by an iterative procedure based on this analytical result. The developed formalism is applied to the FLASH facility at DESY. Analytical estimations of RF tolerances are given.

physics.acc-ph↗

arXiv2010

High Frequency Impedances in European XFEL

The method of the optical approximation is used to estimate the high frequency impedances of different vacuum chamber transitions of the European XFEL beam line. The approximations of the longitudinal impedances are obtained in terms of simple one-dimensional integrals. The transverse impedances are written in analytical closed form. The analytical results are compared with the results obtained by numerical solution of Maxwell's equations.

physics.acc-ph↗

arXiv2003

Transverse self-interactions within an electron bunch moving in an arc of a circle (generalized)

Transverse self-interactions within a "line" bunch of electrons moving in an arc of a circle have been studied extensively by us from an electrodynamical viewpoint. Here we treat an electron bunch with a given vertical (i.e. perpendicular to the orbital plane) size by a generalization of our previous work to the case of a test particle with vertical displacement interacting with a line bunch. In fact, since a bunch with vertical extent can always be thought of as a superposition of displaced charge lines, all the relevant physical aspects of the problem are included in the study of that simple model. Our generalization results in a physically meaningful and quantitative explanation of the simulations obtained with the code $TraFiC^4$ as well as in successful cross-checking of the code.

physics.acc-ph↗

arXiv2002

Transverse self-fields within an electron bunch moving in an arc of a circle

As a consequence of motions driven by external forces, self-fields (which are different from the static case) originate within an electron bunch. In the case of magnetic external forces acting on an ultrarelativistic beam, the longitudinal self-interactions are responsible for CSR (Coherent Synchrotron Radiation)-related phenomena, which have been studied extensively. On the other hand, transverse self-interactions are present too. At the time being, existing theoretical analysis of transverse self-forces deal with the case of a bunch moving along a circular orbit only, without considering the situation of a bending magnet with a finite length. In this paper we propose an electrodynamical analysis of transverse self-fields which originate, at the position of a test particle, from an ultrarelativistic electron bunch moving in an arc of a circle. The problem will be first addressed within a two-particle system. We then extend our consideration to a line bunch with a stepped density distribution, a situation which can be easily generalized to the case of an arbitrary density distribution. Our approach turns out to be also useful in order to get a better insight in the physics involved in the case of simple circular motion and in order to address the well known issue of the partial compensation of transverse self-force.

physics.acc-ph↗