arXiv subjects

E. Vogel

Publications and source records attributed to E. Vogel.

4 recordsLinked to original sources

arXiv2026

Results of R&D Programmes and LP&CW EXFEL Cryomodule Tests in the Period from 2005 to 2023

In 2005 and 2006, we began to consider the feasibility of long-pulse (LP) and continuous-wave (CW) operation for the E-XFEL. The operation modes considered were assumed to be complementary to the short-pulse operation (SP), with ~1 ms RF pulses and a 10 Hz repetition rate, which at that time had already been chosen and presented in the TDR [1] of the E-XFEL facility. This operation mode originated from the previously proposed linear collider TESLA project [2]. We initiated several R&D programmes in 2005 and 2006 to enable operation modes with a duty factor significantly higher than that of the short-pulse mode, which is still approximately 1%. In this report, we briefly present the initiated R&D programmes and their results, with particular emphasis on the results of the E- XFEL cryomodule tests, which led to minor modifications of the design and subsequently to the implementation of these cryomodules in large-scale X-ray FEL facilities.

physics.acc-ph↗

arXiv2025

Cathodes and Shape Modification of Cavity for DESY Superconducting Photoinjector

Four DESY prototypes of the L-band superconducting RF photoinjector cavity demonstrated on-axis peak gradients above 55MV/m during multiple vertical cryogenic tests. Two of these prototypes,16G09 and 16G10, achieved these gradients with both superconducting and normal-conducting metallic cathodes, fabricated from either high-purity niobium or lower-purity copper. The DESY photoinjector, under development for over two decades as a continuous-wave electron source for FELs, differs from other SRF injectors in that its metallic cathode plug is attached directly to the cavity backplate, exposing the emitting surface to the high electric field within the cavity. This design obviates the need for a choke filter or load-lock system. The initial 1.6-cell cavity geometry was derived from the Low-Loss design developed for the CEBAF 12GeV upgrade, scaled from 1.5GHz to 1.3GHz. This shape was later replaced with the current High Gradient TESLA profile. In this report, we discuss current cathode options and present modifications to the cavity shape aimed at significantly reducing the electric field near the cathode opening

physics.acc-ph↗

arXiv2023

High gradients at SRF photoinjector cavities with low RRR copper cathode plug screwed to the cavity back wall

In recent years we increased the typical maximum peak field on axis gradients obtained in L-band superconducting RF (SRF) photoinjector cavities at vertical tests to around 55 MV/m. This was achieved with niobium cathode plugs directly screwed to the cavity back wall omitting an RF choke filter and a load lock system for cathodes. Copper demonstrated being a suitable cathode material in normal conducting injector cavities used at X-Ray Free Electron Lasers (XFELs) operating with pulsed RF. In this article we present the first experimental confirmation that peak field on axis gradients around 55 MV/m and beyond can be achieved in L-band SRF photoinjector cavities with copper cathode plugs screwed to the cavity back wall. We view this as a major milestone for the development of a high gradient photoinjector operating continuous wave (CW).

physics.acc-ph↗

arXiv1992

The Supercooling of a Nematic Liquid Crystal

We investigate the supercooling of a nematic liquid crystal using fluctuating non-linear hydrodynamic equations. The Martin-Siggia-Rose formalism is used to calculate renormalized transport coefficients to one-loop order. Similar theories for isotropic liquids have shown substantial increases of the viscosities as the liquid is supercooled or compressed due to feedback from the density fluctuations which are freezing. We find similar results here for the longitudinal and various shear viscosities of the nematic. However, the two viscosities associated with the nematic director motion do not grow in any dramatic way; i.e.\ there is no apparent freezing of the director modes within this hydrodynamic formalism. Instead a glassy state of the nematic may arise from a ``random anisotropy" coupling of the director to the frozen density.

cond-mat↗