Gregor Loisch

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 200u2009u2009keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

Generation of quasi continuous-wave electron beams in an L-band normal conducting pulsed RF injector for laboratory astrophysics experiments

Abstract We report on an approach to produce quasi continuous-wave (cw) electron beams with an average beam current of milliamperes and a mean beam energy of a few MeV in a pulsed RF injector. Potential applications are in the planned laboratory astrophysics programs at DESY. The beam generation is based on field emission from a specially designed metallic field emitter. A quasi cw beam profile is formed over subsequent RF cycles at the resonance frequency of the gun cavity. This is realized by debunching in a cut disk structure accelerating cavity (booster) downstream of the gun. The peak and average beam currents can be tuned in beam dynamics simulations by adjusting operation conditions of the booster cavity. Optimization of the transverse beam size at specific positions (e.g., entrance of the plasma experiment) is performed by applying magnetic focusing fields provided by solenoids along the beam line. In this paper, the design of a microtip field emitter is introduced and characterized in electromagnetic field simulations in the gun cavity. A series of particle tracking simulations are conducted for multi-parametric optimization of the parameters of the produced quasi cw electron beams. The obtained results will be presented and discussed. In addition, measurements of the parasitic field emission (PFE) current (dark current) in the PITZ gun will be exemplarily shown to distinguish its order of magnitude from the produced beam current by the designed field emitter.

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 uf0bb10 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 uf06ds 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.