Andre Arnold

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

Design of a Stripline Kicker for the ELBE Accelerator

ELBE is a linac based cw electron accelerator serving different secondary beams one at a time. Depending on the user demand the bunch repetition rate may vary from single pulse up to 13 MHz. For the future different end stations should be served simultaneously, hence specific bunch patterns have to be kicked into different beamlines. To use e.g. one bunch out of the bunch train very short kicking durations have to be realized. The variability of the bunch pattern and the frequency resp. switching time are one of the main arguments for a stripline-kicker combined with high voltage (HV)-switches as basic concept. A nearly homogenous field in the kicker has to be realized for uniform deflection of the electron bunch and keep the emittance growth of the bunch as low as possible. Furthermore the fast switching ability of the kicker demands for a fast decay of the HV-pulse resp. its reflections in the structure implying a specific design of the kicker elements. For this reason a design with two tapered active electrodes and two ground fenders was optimized in time and frequency domain with the software package CST. Additionally a first prototype was manufactured for laboratory and first beam-line tests.

Performance and Application Status of the Superconducting Photoinjector at ELBE

A new SRF gun has been commissioned at the ELBE linac. The gun has an improved 3.5-cell cavity and a superconducting solenoid is integrated. Beam parameter measurements have been carried out with a Cu photocathode.

Highlights on Metallic Photocathodes Used in SRF Gun

For the accelerator-based light sources and the electron colliders, the development of photoinjectors has become a key technology. Especially for the superconducting radio frequency cavity based injector (SRF Gun), the searching for better photocathodes is always a principal technical challenge. To use metallic photocathodes for ELBE SRF Gun is the primary choice to prevent cavity contamination. In this contribution, we will report the investigation of Magnesium (Mg) in ELBE SRF gun, including laser cleaning treatment and the measurement on quantum efficiency, Schottky effect, dark current and damage threshold.

Plug Transfer System for GaAs Photocathodes

The transport and exchange technology of Cs2Te photocathodes for the ELBE superconducting rf photoinjector (SRF Gun) has been successfully developed and tested at HZDR. The next goal is to realize the transport of GaAs photocathodes into the SRF Gun, which will need a new transfer system with XHV 10 mbar. The key component of the setup is the transfer chamber and the load-lock system that will be connected to the SRF Gun. In the carrier, four small plugs will be transported, one of them will be put on the cathode-body and inserted into the cavity. The new transport chamber allows the transfer and exchange of plugs between HZDR, HZB and other cooperating institutes. In HZDR this transfer system will also provide a direct connection between the SRF Gun and the GaAs preparation chamber inside the ELBE-accelerator hall. INTRODUCTION The Rossendorf superconducting RF photo injector (SRF Gun), developed within a collaboration of the institutes HZB, DESY, MBI and HZDR, has been put into operation in 2007. It is designed for medium average beam current and operation in CW mode [1]. The superconducting cavity, the main part of SRF gun, consists of three TESLA cells and one optimized halfcell. The Cs2Te photocathode is inserted in the half cell isolated by a 1mm vacuum gap. CATHODE SYSTEM UPDATE The design of the new transfer system for the SRF gun is shown in Fig. 1. Figure 1: View of new transfer system on the cryomodule. The main difference of the new transfer system to that of the present one is, that the moving object is only the plug and not the entire cathode (Fig. 2). • ELBE SRF Gun has been operated with Cs2Te for medium current up to 400μA • for high current operation in the future GaAs(Cs,O) is considered to be combined with the SRF Gun technology. • SRF Gun is supposed to serve as a test bench for GaAs(Cs, O) Cs Te 2 (Fig. 3). • driven by UV light • UV laser shaping complicated • medium current • 10 -10 -10 mbar NEA-GaAs (Cs, O) [2] • high QE in the visible light • laser pulse shaping easier • polarized electron source • critical vacuum requirement GaAs (Cs,O) will be in-situ activated before the transport into SRF gun through a new transfer system. XHV of less than 1×10 mbar is required. Figure 2: Photocathode of the new Transfer system. Figure 3: Arrangement in the coating chamber. Proceedings of SRF2015, Whistler, BC, Canada TUPB010 Projects/Facilities progress A02-Upgrade plans/status ISBN 978-3-95450-178-6 553 C op yr ig ht

RF Performance Results of the 2nd ELBE SRF Gun

An improved SRF gun (ELBE SRF Gun II) has been installed and commissioned at HZDR. This new gun replaced the first SRF gun of the ELBE accelerator which had been in operation since 2007. The new gun has an improved 3.5-cell niobium cavity those SRF performances have been studied first with a copper cathode. After the replacement by our standard Cs2Tecathode we observed a tremendous degradation of the cavity gradient paired with an increase of field emission. In this contribution we will report on our in-situ investigations to find the origin and the reason for the particle contamination that happened during the first cathode transfer.

TWO YEARS EXPERIENCE WITH THE UPGRADED ELBE RF-SYSTEM DRIVEN BY 20 kW SOLID STATE AMPLIFIER BLOCKS (SSPA)

Since January 2012 the Superconducting 1.3 GHz CW Linac ELBE is equipped and in permanent operation with four 20 kW Solid State Amplifier Blocks (SSPA). The project and the design of the new RF system have been described in the papers [1] and. The experience gained within the first two years of operation is encouraging.