T. Kamps

Status and perspectives of superconducting radio-frequency gun development for BERLinPro

As part of the BERLinPro study, HZB is developing an SRF photoelectron injector. The R&D will be carried out in three stages, the first of which is currently being installed at HZBs HoBiCaT facility. It consists of an SRF cavity with SC solenoid, electron beam diagnosics and drivelaser systems.

Electron beam diagnostics for a superconducting radio frequency photoelectron injector

A superconducting radio frequency (SRF) photoelectron injector is currently under construction by a collaboration of BESSY, DESY, FZD, and MBI. The project aims at the design and setup of a continuous-wave SRF injector including a diagnostics beamline for the ELBE free electron laser (FEL) and to address R&D issues on low emittance injectors for future light sources such as the BESSY FEL. Of critical importance for the injector performance is the control of the electron beam parameters. For this reason a compact diagnostics beamline is under development, serving a multitude of operation settings. In this paper the layout and the rationale of the diagnostics beamline are described. Furthermore detailed information on specific components is given, together with results from laboratory tests and data taking.

Operation of the superconducting RF photo gun at ELBE

As the first superconducting RF photo-injector (SRF gun) in practical operation, the SRF gun has been successfully connected to the superconducting linac ELBE at Forschungzentrum Dresden-Rossendorf. The injection with this new gun will improve the beam quality for the users of the radiation source. The SRF gun contains a 3½ cell superconducting accelerating cavity with a frequency of 1.3 GHz. The design is for use of normal conducting photocathodes. At present, caesium telluride photocathodes are applied which are illuminated by an ultraviolet laser beam. The kinetic energy of the produced electron beam is 3 MeV which belongs to a peak electric field of 16 MV/m in the cavity. The maximum bunch charge which is obtained and measured in a Faraday cup is about 400 pC (20 μA average current at a repetition rate of 50 kHz). The SRF gun injector is connected to the ELBE accelerator via a dogleg with two 45° deflection magnets. This connection beam line was commissioned in January 2010. A first beam injection into the ELBE accelerator has been carried out with a bunch charge of 120 pC (6 μA at 50 kHz). Detailed measurements showed that beam loss occurred in the dogleg above 60 pC due to the correlated energy spread. In order to find the optimal operation conditions, energy spread was measured in dependence of bunch charge, laser phase and further gun parameters. The Cs2Te photocathode shows an excellent life time. It is in the gun since May 2010 with about 300 h beam time and about 7 C extracted charge. In the present cavity, the limit for the acceleration gradient is field emission due to some defect on the cavity surface and problems during cleaning. Therefore a modified 3½ niobium cavity has been fabricated, which will increase the RF gradient in the gun and thus improve the beam parameters further.

Rossendorf SRF‐Gun Cavity Characteristics

At the Forschungszentrum Dresden‐Rossendorf the development and the setup of the 2nd superconducting radio frequency photo electron injector (SRF‐Photo‐Gun) is finished. This new injector is placed next to the existing thermionic gun of the superconducting linear accelerator ELBE. A connection between the accelerator and the SRF‐Gun will provide improved beam parameters for the users at the second half of 2009. At the moment the commissioning is fully under way. We will report on important results concerning cavity commissioning like measurements of: Q vs. E, microphonics, Lorentz detuning, tuner parameters, pressure sensibility and in‐situ fundamental mode field distribution calculated from measured pass band.

The new superconducting RF photoinjector at the ELBE linac

Most of the proposed electron accelerator projects for future free electron lasers, energy recovery linacs, or 4th generation light sources require electron beams with an unprecedented combination of high-brightness, low emittance and high average current. For that reason existing electron injectors must be considerably improved or new injector concepts developed. One very promising approach represents the superconducting radio frequency photoinjector (SRF gun). This injector type combines the advantages of a conventional photoelectron injector with that of superconducting acceleration, i.e. the very low RF losses and simple continuous wave operation. A SRF gun was developed and installed at Forschungszentrum Dresden-Rossendorf for operation at the ELBE superconducting linear accelerator. In November 2007 the first beam was produced. First commissioning results have been collected. Besides an improvement of beam quality and parameter range the SRF gun serves as a test bench for further development, evaluation and optimization since it is the first injector of its type which is operating at an accelerator worldwide

The spectrometer for the GunLab experiment

The spectrometer for the GunLab experiment is described. This spectrometer incorporates a dipole magnet, a fluorescent screen, and a CCD camera and is designed to measure the momentum of electron beams in the range of 1–10 MeV/c with a resolution of 0.1%. If a transversely deflecting RF cavity is installed in front of the dipole magnet, one may investigate the longitudinal phase portrait of a beam. The spectrometer is distinctive in that a Hall sensor is placed in the magnetic field of the dipole magnet. This sensor allows one to accurately measure the magnetic field and, consequently, the momentum of an electron beam.

SRF Gun Development for Energy Recovery Linac Applications

The SRF gun development programme for the bERLinPro compact ERL pushes the brightness and average current limits by tackling challenges related to electron beam dynamics, operation of high QE photocathodes and suppression of dark current.

Influence of oxides on the field emission of molybdenum

Molybdenum is widely used in fundamental research and industry, for example as substrate for photocathodes or electrode material in DC photoelectron guns. Usually, Mo is heated in situ to several hundred degrees to achieve an oxygen free surface for the photocathode deposition or to reduce the surface outgassing rate in the electron gun. Since enhanced field emission (EFE) is often observed there, we have investigated the influence of oxides on the EFE of Mo by means of a field emission scanning microscope (FESM) and x-ray photoelectron spectroscopy (XPS).