EXPERIMENTAL INVESTIGATION OF HIGH TRANSFORMER RATIO PLASMA WAKEFIELD ACCELERATION AT PITZ G. Loisch * , G. Asova, P. Boonpornprasert, J. Good, M. Gross, H. Huck, O. Lishilin, A. Oppelt, Y. Renier, T. Rublack, F. Stephan, DESY Zeuthen, Germany R. Brinkmann, J. Osterhoff, DESY Hamburg, Germany A. Martinez de la Ossa, T. Mehrling, DESY Hamburg and Universität Hamburg, Germany F. Grüner, Center for Free-Electron Laser Science and Universität Hamburg, Germany Abstract Plasma wakefield acceleration (PWFA), the acceleration of particles in a plasma wakefield driven by particle bunches, is one of the most promising candidates for a future compact accelerator technology. A key aspect of this type of acceler- ation is the ratio between the accelerating fields experienced by a witness beam and the decelerating fields experienced by the drive beam, called the transformer ratio. As for lon- gitudinally symmetrical bunches this ratio is limited to 2 by the fundamental theorem of beam-loading in the linear regime, a transformer ratio above this limit is considered high. This can be reached by using a modulated drive bunch or a shaped train of drive bunches. So far, only the latter case has been shown for wakefields in a RF-structure. We show the experimental setup, simulations and first, preliminary results of high transformer ratio acceleration experiments at the Photoinjector Test Facility at DESY in Zeuthen (PITZ). INTRODUCTION Due to the superior accelerating field strength reachable in plasma wakefields, the plasma wakefield accelerator (PWFA) in which a wakefield in a plasma is driven by a relativistic driving particle bunch, has received significant attention throughout recent years. As the driving bunch has to be accelerated by other, complex means (conventional RF-structures, laser driven wakefield, etc.) beforehand, the efficient usage of the driver’s energy is of vital importance in the PWFA. One of the parameters influencing the efficiency is the homogeneity of the deceler- ating field inside of the driving bunch. This homogeneity is also directly connected to the ratio between the maximum accelerating fields behind the drive bunch and the maximum decelerating field inside of the drive bunch [1], the so called transformer ratio. As in linear wakefield theory the trans- former ratio is limited to maximally 2 for (most common) symmetric drive bunches [2], a ratio above 2 is considered high. Such high transformer ratios (HTR) can be reached in a non- linear wakefield or by using shaped drive bunches [1, 3, 4] or trains of drive bunches [5], where the latter was proposed to circumvent driver instabilities, which were found to pre- vent the transport of drive bunches longer than the plasma wavelength [6]. Since the favourable focusing conditions of the ion channel in a nonlinear or quasi-nonlinear wake * gregor.loisch@desy.de have been discovered [7,8], shaping the drive bunch to e.g. a double triangular shape [4] is the most promising way to reach HTR. To demonstrate and investigate such a HTR PWFA, experi- ments have been set up at the Photoinjector Test facility at DESY, Zeuthen site (PITZ) [9,10]. EXPERIMENTAL SETUP A sketch of the PITZ beamline is shown in Fig. 1. Bunches of up to 4 nC are created by a UV-laser pulse from a Cs-Te photocathode and accelerated in the L-band gun and booster cavities to a maximum energy of 25 MeV. The photocathode laser shaping available at PITZ [11] allows the creation of bunch shapes able to drive HTR wakefields directly at the photocathode by splitting a single Gaussian, 1 ps rms laser pulse 13 times in birefringent crystals, forming various shapes of the 14 Gaussian quasi-pulses. The plasma cell is inserted in the high energy section of the accelerator after the cut-disk booster cavity (CDS) and before the transverse deflecting structure (TDS), which allows time resolved measurements of the bunches after beam/plasma- interaction. In combination with the second high energy dispersive section (HEDA2), the longitudinal phase space can be measured. As a plasma source the gas discharge cell shown in Fig. 2 is used. The cell consists of two electrodes at the end of a glass tube, filled with Argon gas. The pressure of 0.2-8 mbar is separated from the accelerator vacuum by thin polymer foils. By applying a high voltage between the electrodes the gas is ionised and conducts a high current pulse (≤ 600 A of several μs length, which heats the plasma and increases ionisation. Plasmas with densities of up to 5 × 10 16 cm -3 at a plasma column length of 100 mm can be created. Changing the delay between plasma ignition and beam arrival time al- lows the user to adjust the plasma density during interaction. SIMULATIONS Simulations of the experiments were conducted with ASTRA [12] until the entrance of the plasma and using PAMASO [13] and HiPACE [14] for simulating the beam-plasma-interaction. To reach the quasi-nonlinear regime, the beam has to be focused tightly into the plasma at comparably low densities of about 10 14 cm -3 . In terms of transformer ratio, a double triangular bunch shape [4] was found to be optimal for TUPIK018 Proceedings of IPAC2017, Copenhagen, Denmark - Pre-Release Snapshot 19-May-2017 10:10 ISBN 978-3-95450-182-3 0 Copyright © 2017 CC-BY-3.0 and by the respective authors - Pre-Release Snapshot 19-May-2017 10:10 03 Novel Particle Sources and Acceleration Techniques A22 Plasma Wakefield Acceleration