Velizar Miltchev

Measurement of the longitudinal phase space at the Photo Injector Test Facility at DESY Zeuthen

A setup for the measurement of the longitudinal phase space at the photo injector test facility at DESY Zeuthen is described. The measurements of the momentum distribution, the length of an electron bunch and of their correlation are discussed. The results of the momentum distribution measurement are shown, a maximum mean momentum of 4.7 MeV/c and a RMS momentum spread of 14 keV/c is observed. The setup for the measurement of the bunch length includes a Cherenkov radiator which is used to convert the electron beam into a photon beam with a wavelength in the visible range. The Cherenkov radiation mechanism is chosen in order to measure the bunch length with good time resolution. A silica aerogel radiator with low refractive index will be used. Geant 4 simulations show that a resolution of 0.12 ps can be reached. The time dependent behaviour and the position of the photon bunch will be measured by a streak camera system. A simultaneous measurement of the bunch length and the momentum spread will provide the full information about the longitudinal phase space. The design considerations of the radiators and their properties are discussed.

Recent Developments at PITZ

The ability to produce high brightness electron beams as required for modern Free Electron Lasers (FELs) has been demonstrated during the first stage of the Photo Injector Test Facility at DESY Zeuthen (PITZ1). The electron source optimization at PITZ1 was successfully completed, resulting in the installation of the PITZ rf gun at the VUV-FEL (DESY, Hamburg). One of the main goals of the second stage of PITZ (PITZ2) is to apply higher gradients in the rf gun cavity in order to obtain smaller beam emittance by faster acceleration of the space charge dominated beams. In order to reach the required gradients a 10 MW klystron has to be installed and the gun cavity has to be conditioned for higher peak power. Another important goal of PITZ2 is a detailed study of the emittance conservation principle by using proper electron beam acceleration with a booster. Further photo injector optimization, including update of the photocathode laser and diagnostic tools, is foreseen as well. Recent progress on the PITZ developments will be reported.

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.

Emission Mechanisms in a Photocathode RF Gun

In photocathode rf guns, emission mechanisms at the photocathode play a crucial role in the overall beam dynamics. An electron beam with a low bunch charge as well as a short laser pulse length allow to study the emission mechanisms with a good phase definition and with a very small influence of the space charge force. This pa per presents experimental and numerical studies toward detailed understanding of the photo emission and secondary emission processes at the Cs2Te cathode.

Dark Current and Multipacting in the Photocathode RF Guns at PITZ

For photocathode rf guns, the amount of dark current depends on the cavity surface as well as on the photocathodes. Smooth conditioning reduces the amount of dark current. Multipacting in the gun cavity changes the surface status of the cathodes and sometimes obstructs the gun operation because of vacuum interlocks. In this paper, dark current and multipacting features of an rf gun are presented including experimental and simulation studies.

Mapping few-femtosecond slices of ultra-relativistic electron bunches

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.

Beam-Based Procedures for RF Guns

A wide range of rf photo injector parameters has to be optimized in order to achieve an electron source performance as required for linac based high gain FELs. Some of the machine parameters can not be precisely controlled by direct measurements, whereas the tolerances on them are extremely tight. Therefore, this should be met with beam-based techniques. Procedures for beam-based alignment (BBA) of the laser on the photocathode as well as solenoid alignment have been developed. They were applied at the Photo Injector Test facility at DESY at Zeuthen (PITZ) and at the photo injector of the VUV-FEL at DESY at Hamburg. The field balance of the accelerating mode in the 1 1/2 cell gun cavity is one of the key beam dynamics issues of the rf gun. Since no direct field measurement in the half and full cell of the cavity is available for the PITZ gun, a beam-based technique to determine the field balance has been proposed. A beam-based rf phase monitoring proce dure has been developed as well.

Design Studies of an S-Band Multipacting Electron Gun

A multipacting electron gun (MEG) is a micro-pulse electron source based on secondary electron emission in a resonant microwave cavity structure for the generation of low emittance electron bunches with high repetition rate. By theoretical simulations a suitable radio-frequency gun design at 3 GHz is established, simultaneously meeting the demands of bunch production and amplification process as well as including the effects of space charge and beam loading for the evolution of a stable beam. In this contribution we show detailed simulation studies of the impact of important design parameters like mechanical dimensions and choice of material on the average output current, which is in the order of mA. For the experimental investigation a test setup is under construction, which may demonstrate the application of MEG’s as a serious alternative or addition to commonly used electron sources like thermionic and photocathodes. INTRODUCTION A well-established way of electron generation for particle accelerators is by utilizing the principle of laser-driven emission from photocathode materials [1]. However, since 1969 there are ongoing investigations on the mulitipacting electron gun concept [2], where electrons are resonantly accelerated towards opposing surfaces for synchronous charge amplification by secondary emission (SE) using a radio frequency (RF) field. One partially transparent surface is used to release electron bunches from the gun cavity. Since the effects of beam loading and space charge forces are heavily affecting important beam properties, but cannot be taken into account analytically with sufficient accuracy, detailed numerical simulations using the ASTRA code [3] were performed.