arXiv subjects

Dmitry Bazyl

Publications and source records attributed to Dmitry Bazyl.

4 recordsLinked to original sources

arXiv2026

Coupling periodic-cell and finite-bunch dynamics for structured photocathodes

Patterning a photocathode with submicrometre features can enhance nonlinear photoemission by concentrating the optical field, but the surface geometry can also increase the transverse momentum spread of the emitted electrons. Resolving nanoscale surface fields across an injector-scale illuminated area within a full rf gun simulation is computationally demanding. To couple these scales, we developed an approach which combines a self-consistent finite flat-cathode calculation with the particle-resolved difference between matched structured and flat periodic calculations. The finite calculation determines the macroscopic bunch evolution and space-charge field. The periodic difference determines the local change caused by the surface. For a finite but relatively small test problem, comparison with a fully resolved finite-array WarpX calculation gives differences of 1.3% in rms energy spread and less than 0.1% in projected normalized emittance. We then apply the method to representative FEL photoinjector parameters with a 100 pC emitted source distributed over more than half a million periods, and track the composed bunch through an L-band rf gun and solenoid. The initial difference between the projected horizontal and vertical emittances becomes much smaller after rf acceleration and solenoid focusing. At 1.52 m downstream of the cathode, the projected emittances are nearly equal and exceed those of the matched flat-cathode reference by less than 2% in both planes. The central-slice emittances exceed the reference values by approximately 3% horizontally and 5% vertically.

physics.acc-ph↗

arXiv2026

Indirect Integration of Longitudinal and Transverse Wake Potentials for Unequal Beam Pipes and Arbitrary Beam Velocity

Indirect integration replaces the long uniform beam-pipe parts of a wakefield calculation by field problems in the pipe cross sections. Earlier ultrarelativistic methods were developed mainly for the longitudinal wake, whereas the transverse wake was usually obtained from the Panofsky--Wenzel theorem. We derive indirect formulas that complete a transverse Lorentz-force integral already accumulated in a time-domain calculation. At $\beta=1$, each semi-infinite tail is found from a Dirichlet Poisson problem driven by $E_z$ and a Neumann Poisson problem driven by $cB_z$, describing the TM and TE contributions, respectively. We obtain both a fixed-time moving-window representation and a fixed-plane time-history representation for equal or unequal input and output pipes. The method is then extended to a rigid bunch moving with constant velocity $0<\beta c<c$. The longitudinal correction satisfies an anisotropic elliptic equation in $(x,y,s)$ and provides an additional source for the transverse TM problem. In a two-port finite-reference convention, the complete fields, including space charge, are integrated directly between two fixed planes, while the semi-infinite tails are calculated after subtraction of the stationary field in each pipe. The resulting Panofsky--Wenzel relation contains the difference of the stationary transverse electric fields at the two ports. Numerical tests for an unequal rectangular step-out at $\beta=1$ and $\beta=0.8$ confirm the transverse indirect integration and the unequal-pipe boundary term.

physics.acc-ph↗

arXiv2025

Multiphysics Analysis of Cryogenically Cooled Photocathode in a CW SRF Injector cavity

The paper evaluates the thermal regime of a cryogenically cooled copper photocathode integrated into a continuous-wave superconducting radio-frequency injector cavity with direct thermal contact. Such a photoinjector layout is being developed at DESY and has recently demonstrated a record-high 50 MV/m axial electric field in radio-frequency tests, marking an important milestone. To address the thermal effect of the picosecond excitation laser, we first develop a two-temperature model to describe the temperature of the emitting surface at cryogenic temperatures and solve it numerically. Subsequently, we present a one-temperature model of the bulk photocathode coupled with an electromagnetic model of the injector cavity. For the current injector design, we predict a negligible impact of the laser on the intrinsic quality factor of the cavity, identifying instead the cryogenic stability of the copper cathode as the primary operational limit. To overcome cooling challenges, we propose an improved configuration of the cathode plug. For the proposed geometry, the multiphysics analysis confirms stable performance at a nominal 2 W laser power, sufficient for 100 pC beams at 1 MHz under optimistic quantum efficiency assumptions. Operation at higher laser loads will benefit from further dedicated cryogenic analysis.

physics.acc-ph↗

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↗