Stefan Setzer

First beam measurements at the photo injector test facility at DESY Zeuthen

The Photo Injector Test facility at DESY Zeuthen (PITZ) was built to develop electron sources for the TESLA Test Facility Free Electron Laser and future linear colliders. The main goal is to study the production of minimum transverse emittance beams with short bunch length at medium charge (∼1 nC). The facility includes a 1.5 cell L-band cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and different diagnostics tools. Besides an overview of the facility, its main components and their commissioning, this contribution will concentrate on the first measurements at PITZ with photoelectrons. This will include measurements of the transverse and longitudinal laser profile, charge and quantum efficiency, momentum and momentum spread, transverse electron beam profiles at different locations and first results on transverse emittance.

Detailed numerical studies of space charge effects in an FEL RF gun

The production of short bunches with low emittance is a key issue for the successful operation of an SASE-FEL as proposed by the TESLA collaboration (TESLA Technical design report, DESY 2001-011, Hamburg, 2001). In this paper we present the results of detailed MAFIA TS-2 (CST GmbH, Budinger Strasse 2a, D-64289 Darmstadt) simulations for the FEL RF-gun revealing the main physical effects leading to emittance growth. The simulations prove that the transverse emittance growth can mainly be observed close to the cathode area. This is caused by the non-linear space charge forces acting inside the bunch during the injection process. For the application of an emittance compensation scheme (Nucl. Instr. and Meth. 285 (1989) 313) the slice emittance is of significant importance. Therefore, a wide range of parameters for the photo cathode laser has been investigated in order to find an appropriate operation point.

Experimental characterization of the electron source at the Photo Injector Test facility at DESY Zeuthen

The Photo Injector Test facility at DESY Zeuthen (PITZ) was built in order to study the production of high brightness electron beams, which are substantial for the successful operation of free electron lasers (FEL) and linear colliders. The photoinjector at Zeuthen is based on a 1.5-cell L-band rf cavity with coaxial rf coupler equipped with emittance compensating solenoids, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and various diagnostics tools. The current goal of PITZ is a full characterization of the electron source, which will be installed at the TESLA Test Facility Free Electron Laser (TTF2-FEL) in autumn 2003. In the running periods before the gun is delivered to TTF2-FEL, the rf performance and the beam parameters will be measured in detail. The results presented in this contribution contain the measurements of dark current, driving laser parameters, beam charge, beam size along the facility, transverse emittance, momentum and momentum spread. The electron beam measurements will be presented in comparison with beam dynamics simulations.

Emittance growth in the FEL RF-gun

A high brightness and low emittance is of crucial importance for the SASE-FEL at the TESLA Test Facility. Therefore a photo-RF-gun has been installed as particle source. Numerical simulations with codes like ASTRA and MAFIA show that the space charge dominated processes inside the RF-gun contribute significantly to the emittance. In this paper we present the results of detailed studies with MAFIA TS2 which clarify the effects resulting in emittance growth for space charge dominated beams. It is shown that the resulting emittance can be minimized by changing the laser parameters like pulse length and spot size on the cathode. Additionally we present the concept of slice emittances which allows a more precise prediction of the real transverse emittance achievable with an emittance compensation scheme.

Explicit temporal integrators for low-pass filtering of FDTD signals

The numerical dispersion properties of some higher order explicit marching-on-in-time methods for transient electromagnetic field simulations are investigated. In contrast to the most commonly used leapfrog time integration scheme, these alternative methods exhibit a numerical dissipation for high-frequency components. This property can be interpreted as a low-pass digital filter and exploited to suppress parasitic high-frequency noise (HF noise) in the field signals. The prospective application is the enhancement of the coupled simulation of electromagnetic fields and moving charges.

Simulation of dark currents in a FEL RF-gun

Dark currents caused by field emission are dramatically limiting the lifetime of photocathodes used in RF-guns needed for successful FEL operation. Due to the mismatch of the dark currentelectrons to the transport channel they are also a serious hazard to beam diagnostic systems an may cause severe radiation damage. In the past high dark currents emitted from the laser driven RF-guns have been observed during substantial time of linac operation. For a better understanding of the dark current dynamics the transport through the injector part of the DESY Zeuthen (PITZ) accelerator has been investigated with the help of an appropriate tracking algorithm. Additionally, potential emission spots inside the rf-gun have been located by RF field mapping. Furthermore, the dependency of the dark current on the accelerating field inside the RF-gun as well as on the strength of the focusing solenoid has been studied.

New simulation results for the S-DALINAC electron source

For the operation of its mid-infrared FEL the superconducting electron accelerator S-DALINAC has to produce bunches with a charge of 6 pC and a duration of 2 ps FWHM. In order to achieve these figures a subharmonic injection scheme was added to the 3 GHz cw accelerator. The electron gun consists of a grid controlled thermionic cathode on a 250 kV terminal. It produces electron pulses with a charge of 20 pC and a width of 800 ps FWHM at a repetition rate of 10 MHz. For a reliable simulation of the subsequent parts of the injector (600 MHz chopper/prebuncher section and a superconducting linac consisting of 2- and 5-cell capture cavities and two 20-cell accelerating structures) the beam parameters at its input have to be known as exactly as possible. Since we expect that due to the pulsed operation of the gun, longitudinal space charge effects might be important, the electron gun and the 250 kV electrostatic acceleration have been studied using the 2 1/2 dimensional PIC code MAFIA-TS2 and the tracking code GPT. This calculation takes into account the time structure of the beam and yields results like bunch shape at the gun exit, correlated and uncorrelated energy spread, as well as the transverse emittance. The results of the simulation as well as a comparison with measurements of the transverse emittance and the electron pulse length at the exit of the electrostatic acceleration will be presented in this paper.