AWAKE Design Report A Proton-Driven Plasma Wakefield Acceleration Experiment at CERN AWAKE Collaboration Abstract The AWAKE Collaboration has been formed in order to demonstrate proton- driven plasma wakefield acceleration for the first time. This technology could lead to future colliders of high energy but of a much reduced length compared to proposed linear accelerators. The SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro-bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV/m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial 3–4 year experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders. CERN-SPSC-2013-013 / SPSC-TDR-003 24/09/2013 1 2 Contents 1 Executive Summary . 5 2 Introduction . 6 3 The Self-Modulation Instability . 7 3.1 Transverse Modulation of a Long Bunch . 7 3.2 Injection and Acceleration of Witness Electrons . 9 3.3 Seeding of the SMI . 10 4 Baseline Design . 11 5 Plasma Sources . 14 5.1 Metal Vapor Plasma Source . 14 5.2 Argon Discharge Plasma Source . 16 5.3 Helicon Plasma Source . 16 5.4 Plasma Density Measurements . 18 6 Particle Beams Diagnostics . 19 6.1 Proton Bunch Diagnostics for SMI . 19 6.2 Coherent Transition Radiation Diagnostics . 20 6.3 Electro-Optical Sampling . 21 6.4 Electron Spectrometer . 22 7 Electron Source . 23 8 The AWAKE Facility at CERN . 24 8.1 Introduction . 24 8.2 Experimental Area . 26 8.3 Proton and Electron Beam Lines . 29 8.4 The SPS Proton Beam . 32 9 Project Planning . 35 9.1 Timeline . 35 9.2 AWAKE Physics Program . 35 10 Summary . 36 3 4 1 Executive Summary New acceleration technology is mandatory for the future of particle physics. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron driver bunch into the plasma. However, the maximum energy gain of accelerated particles in a single plasma stage is limited by the energy of the driver. Proton bunches, being much more energetic, are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale. The objectives of the AWAKE experiment are to understand the physics of the acceleration process, to demonstrate high-gradient acceleration with a proton bunch, and to develop necessary technologies for the long-term perspectives of proton-driven plasma wakefield acceleration. The AWAKE experiment fulfills one of the primary recommendations of the European Strategy for Particle Physics which advocates vigorous R&D in high-gradient accelerating techniques. The AWAKE experiment will use proton bunches for the first time ever to drive plasma wake- fields. A plasma will be used to modulate the long proton bunch into a series of ‘micro-bunches’ which then generate a strong plasma wakefield. Our goal is to accelerate electrons injected into the plasma wakefield to the GeV scale in a few meters of plasma. The evolution of the properties of the proton and electron bunches in the plasma will be studied experimentally using state-of-the-art diagnostic tools and will be compared to predictions from detailed numerical simulations. This information will provide the basis for designing next-generation experiments at CERN. In parallel, we will develop long and uniform vapor, discharge and helicon plasma cells and develop schemes for proton-bunch compression. These are key for the long-term success of proton-driven plasma wakefield acceleration. We will test bunch compression schemes in the CERN accelerators and apply them to the proton bunches used to drive the wakefields. We will also install the different plasma cells in our experimental setup and investigate in detail their characteristics for acceleration. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders. A detailed comparison of two sites for the AWAKE facility has been performed, based on design studies of the proton beam delivery in the SPS, the primary beam lines, the experimental area, civil en- gineering, general services and infrastructures for the facility as well as radiation protection and general safety aspects. The CNGS facility satisfies best the requirements of AWAKE and we therefore propose to carry out the experiment in the CNGS beam line of the SPS. Assuming approval of AWAKE in mid 2013, first protons could be sent to the plasma cell at the end of 2016. Considering four years for the completion of the electron source system, the electron beam will be operational at the end of 2017. Operation and data taking is planned for 3–4 years. Beam is requested for approximately 4 periods of two weeks per year, with bunch repetition rates of approximately 1=30 Hz. Further experimental efforts will be evaluated based on the results from the initial running. The proposed program requires expertise in proton accelerators, electron accelerators, plasma physics, wakefield acceleration, experimental physics, theory and simulations. The AWAKE Collabora- tion, with the strong backing of CERN, has the expertise to fulfill these requirements. 5 2 Introduction Particle accelerators are the fundamental research tools of the high energy physics community for study- ing the basic laws that govern our Universe. Experiments conducted at the LHC will give us new insights into the physical world around us. Complementing this, future lepton colliders should reach the TeV scale. Circular electron colliders are not feasible at these energies; hence future TeV accelerator designs are based on linear colliders. However, as the beam energy increases, the scale and cost of conventional machines become very large. For a linear accelerator, the size and cost depend on the maximum accel- erating gradient in RF cavities. At present, metallic cavities achieve maximum accelerating gradients around 100 MV/m. To reach the TeV scale in a linear accelerator, the length of the machine is therefore tens of kilometers. It is natural to think about how to make future machines more compact, and plasma acceleration is a possible solution. A plasma is a medium consisting of ions and free electrons; therefore, it can sustain very large electric fields (> GV/m) [1,2]. In the last few decades, more than 3 orders of magnitude higher acceleration gradient than in RF cavities have been demonstrated with plasmas in the laboratory [3, 4]. Beam-driven plasma wakefield acceleration experiments performed at SLAC [4] successfully doubled the energies of some of the electrons in the initial 42 GeV beam in less than 1 m of plasma. Generally speaking, a plasma acts as an energy transformer; it transfers the energy from the driver (laser or beam pulse) to the witness bunch that is accelerated. Current proton synchrotrons are capable of producing high energy protons, reaching up to multi TeVs (the LHC), so that a new accelerator frontier would be opened if we could efficiently transfer the energy in a proton bunch to a witness electron bunch. It has been recently proposed to use a high energy proton bunch to drive a plasma wakefield for electron beam acceleration [5]. Numerical simulations have shown [6] that a 1 TeV bunch, with 1011 protons and an rms bunch length of 100 µm as driver could indeed excite a large amplitude plasma wave. Surfing the appropriate phase of the wave, an electron bunch reaches energies over 600 GeV in a single passage through a 450 m long plasma. Recent studies [7, 8] have shown that similar gradients can be reached with a modulated long proton bunch, opening the path for immediate experimental investigations with the existing proton bunches at CERN. The modulation of the proton density on axis results from the transverse focusing and defocusing field along the bunch. For coherent wakefield excitation, this is equivalent to having a series of ultra-short proton bunches with an effective length and period set by the plasma wavelength. The AWAKE experiment will use proton bunches for the first time ever to drive plasma wakefields. The main physics goals of the experiment are: – to study the physics of self-modulation of long proton bunches in plasma as a function of beam and plasma parameters. This includes radial modulation and seeding of the instability. – to probe the longitudinal (accelerating) wakefields with externally injected electrons. This includes measuring their energy spectrum for different injection and plasma parameters. – to study injection dynamics and the production of multi-GeV electron bunches, either from side injection or from on-axis injection (with two plasma cells). This will include using a plasma density step to maintain the wakefields at the GV/m level over meter distances. – to develop long, scalable and uniform plasma cells and develop schemes for the production and acceleration of short bunches of protons for future experiments and accelerators. The results of the experiment will inform future larger-scale R&D experiments on proton-driven plasma wakefield acceleration such as acceleration of electron bunches to ∼ 100 GeV in ∼ 100 m. The results will allow first designs to be made using this technology for future particle physics facilities such as TeV scale ep or e+e− colliders.