F. Stephan

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.

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.

High Brightness Photo Injectors for Brilliant Light Sources

The large variety of existing and planned light sources sets specific demands on the performance of the corresponding electron injectors which requires specific solutions. In the introductory part of this chapter, a general injector layout and three types of photo emission-based electron sources will be described. In subsequent sections, different designs of electron sources providing a wide range of average electron currents from nA to 100 mA will be discussed. Basics of the experimental optimization of modern electron sources and future options will be presented.

Self-modulation of long electron beams in plasma at PITZ

The Photo Injector Test facility at DESY, Zeuthen site (PITZ), offers the unique possibility to study and demonstrate the self-modulation of long electron bunches in plasma. A set of numerical simulations with the particle-in-cell code OSIRIS has been carried out for a better understanding of the process. Of particular interest is the measurement of the energy modulation induced to the beam itself by means of the generated wakefields in plasma. It will reflect the key properties of the accelerating electric fields such as their magnitude and their phase velocity, both of significant importance in the design of experiments relying on this technique.

RF Commissioning of the Photo Injector Test Facility at DESY Zeuthen

The photo injector test facility at DESY Zeuthen (PITZ) was built to develop, operate and optimize photo injectors for future free electron lasers and linear colliders. First photo electrons were produced in January 2002. An extensive conditioning work on the rf gun has been done in order to achieve high gradients for different pulse lengths and repetition rates. To increase the efficiency and safety aspects of the rf commissioning an Automatic Conditioning Program (ACP) was developed. In addition, a Data Aquisition system (DAQ) which enables a deeper analysis of the commissioning work was realized. The conditioning procedures, the specific diagnostic elements and the achieved results are described. Furthermore, dark current measurements under different conditions are presented.

Observation of the Self-Modulation Instability via Time-Resolved Measurements

Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200  keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

Optimisation of High Transformer Ratio Plasma Wakefield Acceleration at PITZ

The transformer ratio, the ratio between maximum accelerating field and maximum decelerating field in the driving bunch of a plasma wakefield accelerator (PWFA), is one of the key aspects of this acceleration scheme. It not only defines the maximum possible energy gain of the PWFA but it is also connected to the maximum percentage of energy that can be extracted from the driver, which is a limiting factor for the efficiency of the accelerator. Since in linear wakefield theory a transformer ratio of 2 cannot be exceeded with symmetrical drive bunches, any ratio above 2 is considered high. After the first demonstration of high transformer ratio acceleration in a plasma wakefield at PITZ, the photoinjector test facility at DESY, Zeuthen site, limiting aspects of the transformer ratio are under investigation. This includes e.g. the occurrence of bunch instabilities, like the transverse two stream instability, or deviations of the experimentally achieved bunch shapes from the ideal.

Characterization of Self-Modulated Electron Bunches in an Argon Plasma

The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams. In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 10 cm to 10 cm range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing. INTRODUCTION Motivated by the ongoing experiments of the AWAKE collaboration [2] the self-modulation instability [3] is investigated at the electron accelerator PITZ. This effect was demonstrated for the first time by utilizing a lithium heat pipe oven plasma cell [1]. Flat top electron bunches with a FWHM length of about 20 ps and with rise/fall times of <2 ps were generated by impinging similarly shaped photocathode laser pulses [4] onto a Cs2Te photocathode. The bunches were accelerated with an L-band electron gun and a subsequent booster linac to a momentum of 22.3 MeV/c. A gun solenoid and four quadrupole magnets were used to focus these bunches into a heat pipe oven which provided a lithium plasma with densities up to 10 cm. The sharp transition of charge density at the head of the bunch triggers a plasma wake which is seeding the self-modulation instability along the electron bunch. Since the bunch is several plasma wavelengths long this results in a periodical bunch diameter and energy modulation. These modulations were observed on Ce:YAG and LYSO scintillation screens by resolving the temporal charge distribution with an RF deflector and the energy distribution with a dipole spectrometer. Here we describe a follow-up experiment using the same setup with the only difference that the lithium heat pipe oven was replaced with a discharge plasma cell [5]. EXPERIMENTS The setup used for these experiments is depicted in Fig. 1. Argon plasma was generated with a 2.4 kV, 250 A discharge pulse of 2 s length. The timing of the discharge pulse is adjustable with respect to the electron bunch arrival at the plasma cell. Since the plasma is recombining after the discharge pulse has ended, this variable delay translates into a scan of the plasma density which the electron bunch is experiencing. The bunch charge is adjustable by tuning the pulse energy of the photocathode laser, while the focusing of the bunch into the plasma cell can be scanned by changing the drive current of the gun solenoid. Figure 1: Experimental setup. Streaked Bunch For the first set of experiments a removable Ce:YAG screen was inserted to observe the electron bunches which are vertically streaked with an RF deflector [6]. Results of a timing scan are shown in Fig. 2. The bunch charge was 600 pC and the main solenoid current 390 A. The horizontal axis shows the horizontal size of the bunch while the vertical axis is the axis of RF streaking, which is ____________________________________________ * matthias.gross@desy.de Th is is a pr ep ri nt — th e fin al ve rs io n is pu bl ish ed w ith IO P 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-TUPML046 03 Novel Particle Sources and Acceleration Technologies A22 Plasma Wakefield Acceleration TUPML046 1645 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.