Proceedings of IPAC2016, Busan, Korea WEPMY024 A SPECTROMETER FOR PROTON DRIVEN PLASMA ACCELERATED ELECTRONS AT AWAKE - RECENT DEVELOPMENTS∗ Lawrence Charles Deacon, Simon Jolly, Fearghus Keeble, UCL, London Aurélie Goldblatt, Stefano Mazzoni, Alexey Petrenko, CERN, Geneva Bartolomej Biskup, CERN, Geneva; Czech Technical University, Prague 6 Matthew Wing, UCL, London; DESY, Hamburg; University of Hamburg, Hamburg Abstract SPECTROMETER DESIGN The AWAKE experiment is to be constructed at the CERN Neutrinos to Gran Sasso facility (CNGS). This will be the first experiment to demonstrate proton-driven plasma wake- field acceleration. The 400 GeV proton beam from the CERN SPS will excite a wakefield in a plasma cell several meters in length. To probe the plasma wakefield, electrons of 10–20 MeV will be injected into the wakefield follow- ing the head of the proton beam. Simulations indicate that electrons will be accelerated to GeV energies by the plasma wakefield. The AWAKE spectrometer is intended to measure both the peak energy and energy spread of these accelerated electrons. Results of beam tests of the scintillator screen Figure 1: A 3D CAD image of the spectrometer system output are presented, along with tests of the resolution of annotated with distances along the z direction from the exit the proposed optical system. The results are used together of the plasma cell to the magnetic centers of magnets, and with a BDSIM simulation of the spectrometer system to pre- the center of the scintillator screen. dict the spectrometer performance for a range of possible accelerated electron distributions. INTRODUCTION RESOLUTION Proton bunches are the most promising drivers of wake- Optical System fields to accelerate electrons to the TeV energy scale in a The resolution of the energy spectrometer will ultimateley single stage. An experimental program at CERN — the depend on the resolution of the optical system imaging the AWAKE experiment [1, 2] — has been launched to study in screen. Due to the radiation environment, the intensified detail the important physical processes and to demonstrate CCD camera (Andor iStar 340T) will need to be located 17 proton-driven plasma wakefield acceleration. m away in an adjacent tunnel. The light will be reflected AWAKE will be the first proton-driven plasma wakefield to the camera using a series of mirrors. In order to collect experiment world-wide and is currently being installed in as much light as possible whilst maintaining good spatial the CERN Neutrinos to Gran Sasso facility [3], with data resolution, a large diameter, 400 mm focal length, f#/2.8 lens taking with the proton beam phase scheduled to take place has been selected (Nikon 400 mm f/2.8 FL ED VR). This lens summer 2016 [4]. An electron witness beam will be injected was tested by imaging various targets back-lit with green with into the plasma to observe the effects of the proton-driven various line widths, and the modulation transfer function plasma wakefield: plasma simulations indicate electrons was calculated. Mirrors of varying quality were tested. Tests will be accelerated to GeV energies [5]. In order to measure were carried out with the screens at various distances from the energy spectrum of the witness electrons, a magnetic the camera and at various positions within the image plane spectrometer will be installed downstream of the exit of the of the camera. From 17 m away the smallest resolvable plasma cell. The design of the spectrometer was outlined bar width was 1 mm and a bar width of 0.5 mm was not in [6] and in [7] an updated spectrometer design along with resolvable. This result was independent of whether or not a estimated energy resolution for various quadrupole and mag- mirror was used, the type of mirror used, and the transverse net settings was presented. In this paper the resolution of the position of the target. This indicates that a low quality mirror system and minimum detectable bunch charge under various could be used, the resolution is good across the image plane, beam conditions are discussed. and the indicated resolution of the optical system of is ∼ σ = . ∗ 1 0 mm. The resolution of the system finally installed This work was supported in parts by: EU FP7 EuCARD-2, Grant Agree- at AWAKE will depend on the optical transfer line finally ment 312453 (WP13, ANAC2); and STFC, United Kingdom. M. Wing acknowledges the support of DESY, Hamburg and the Alexander von installed and tests are planned during the commissioning Humboldt Stiftung. stages. 03 Alternative Particle Sources and Acceleration Techniques ISBN 978-3-95450-147-2 A22 Plasma Wakefield Acceleration 2605 Copyright © 2016 CC-BY-3.0 and by the respective authors WEPMY024 Proceedings of IPAC2016, Busan, Korea Screen Table 1: Predicted beam parameters for the accelerated elec- tron beam at the plasma cell exit calculated from the simu- Studies are currently ongoing to optimize the vacuum lated phase space distribution (figure 2) chamber window thickness. The point spread function of the screen will depend on the finally chosen window/screen σx [μm] 327.1 ± 0.6 thickness. Studies are ongoing. For the purposes of this σx [mrad] 1.048 ± 0.002 study we assume the line spread function of the screen will [μm] 0.34280±0.0009 be negligible compared to the optical system. Emittance Beam Line The beam line down stream of the plasma The resolution of the spectrometer will depend on the cell will consist of the components listed in table 2. The 7% beam size of the accelerated electrons at the screen which in offset between the strengths of the two quadrupoles causes turn will depend on the beam parameters and the magnetic the horizontal and vertical foci to coincide at the screen [7]. beam line components which will be used to focus the beam. Table 2: Beam Line Components Downstream of Plasma Beam Parameters The accelerated electron beam has Cell. l is magnetic length. k is the magnet focusing strength. been simulated in plasma simulations using LCODE [8, 9]. The phase space distribution of the accelerating beam is non- Name Type l [m] k [m−1] Gaussian with long tails. It also appears to be composed qf0 quad. 0.31 -4.2900 of a number of distinct phase space ellipses with different d1 drift 0.185 orientations. In this study we approximate the overall phase qd0 quad. 0.31 3.9897 ellipse using the overall RMS position and angular distri- d0 drift ∼3 m (energy dependent) butions. The resulting beam parameters at the exit of the plasma cell are give in table 1. The parameters were calcu- lated by assuming twiss parameter α = 0, corresponding to Based on the above parameters, a transfer matrix was an unrotated phase ellipse, which is true for the central part calculated analytically using the thick quadrupole transfer of the phase space (figure 3). Emittance was estimated from matrices [10] to transfer the beam from the upstream face of the RMS area of the phase ellipse i.e. = σx σx . qd0 to the screen. As the quadrupole strengths and the length of d0 vary as a function of energy, these were left as functions of energy in the transfer matrix. ld0(E) was determined 20 104 from a tracking simulation using BDSIM [11–14]. The quad 15 focusing strengths are simply scaled linearly with energy. 10 3 x' [mrad] 10 As the position on the screen is a function of energy, it was 5 possible to derive a function (beam size function) describing 0 2 10 beam size (and therefore energy resolution) as a function of -5 position (and therefore energy) using the energy dependent -10 10 transfer matrix and the estimated beam parameters. -15 -20 1 Overall Resolution -5 -4 -3 -2 -1 0 1 2 3 4 5 x [mm] The resolution due to the emittance, when combined with the resolution of the optical system of 1.0 mm by adding Figure 2: A sample of the simulated phase space distribution the beam size due to emittance and the optical resolution in of the witness electron beam. quadrature, is plotted in figure 4. The result for the nominal emittance in plotted, together with emittances 10 and 100 times smaller/larger. The resolution in the experiment could be estimated by measuring the vertical beam size on the 5 screen and assuming a circular beam. 4 3 10 3 x' [mrad] 2 MINIMUM DETECTABLE CHARGE 1 102 DENSITY AND SCREEN LINEARITY 0 -1 Method -2 10 -3 Beam line tests were carried out to determine the mini- -4 mum detectable charge density of the system. A 5.5 MeV -5 1 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 electron beam was used from the PHIN beam line [15]. A x [mm] 850 μm thick terbium-doped gadolinium oxysulphide scin- tillator screen was placed after a 0.2 mm aluminum window Figure 3: Central region of a sample of figure 2 (rebinned) at the end of the beam line. The electrons passed through ISBN 978-3-95450-147-2 03 Alternative Particle Sources and Acceleration Techniques Copyright ©2606 2016 CC-BY-3.0 and by the respective authors A22 Plasma Wakefield Acceleration Proceedings of IPAC2016, Busan, Korea WEPMY024 with a diameter of 143 mm has an acceptance 8 times larger 10 ∈ = 3.5e-09 m ∈ /E = 3.5e-08 m than the small lens. E ∈ = 3.5e-07 m σ ∈ = 3.5e-06 m ∈ = 3.5e-05 m 1 EMITTANCE MEASUREMENT 10-1 It may be possible to use the electron spectrometer im- age to provide an emittance measurement in a single shot. 10-2 This will be useful to study the quality of the beam that is accelerated at AWAKE.