This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years.

EuPRAXIA Conceptual Design Report

The use of C-Band structures for electron acceleration and production of high quality beams has been proposed and adopted in several linac projects all over the world. The two main projects that adopted such type of structures are the Japanese Free Electron Laser (FEL) project in Spring-8 and the SwissFEL project at Paul Scherrer Institute (PSI). Also the energy upgrade of the SPARC photo-injector at LNF-INFN (Italy) from 150 to more than 240 MeV will be done by replacing a low gradient S-Band accelerating structure with two C-band structures. The structures are Traveling Wave (TW) and Constant Impedance (CI), have symmetric axial input couplers and have been optimized to work with a SLED RF input pulse. The paper presents the design criteria of the structures, the realization procedure and the low and high power RF test results on a prototype. The high power tests have been carried out by the Frascati INFN Laboratories in close collaboration with the Japanese Laboratory KEK. Experimental results confirmed the feasibility of the operation of the prototype at 50 MV/m with about 10 6 breakdowns per pulse per meter. Such high gradients have not been reached before in C-Band systems and demonstrated the possibility to use C-band accelerators, if needed, at such high field level. The results of the internal inspection of the structure after the high power test are also presented.

/pdf/the-c-band-accelerating-structures-for-sparc-photoinjector-3bm6sx18eq.pdf

The C-Band accelerating structures for SPARC photoinjector energy upgrade

The gamma beam system of the European Extreme Light Infrastructure--Nuclear Physics project foresees the use of a multibunch train colliding with a high intensity recirculated laser pulse. The linac energy booster is composed of 12 traveling wave $C$-band structures, 1.8 m long with a field phase advance per cell of $2\ensuremath{\pi}/3$ and a repetition rate of 100 Hz. Because of the multibunch operation, the structures have been designed with a dipole higher order mode (HOM) damping system to avoid beam breakup (BBU). They are quasiconstant gradient structures with symmetric input couplers and a very effective damping of the HOMs in each cell based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the cost of the structures. In the paper, after a review of the beam dynamics issues related to the BBU effects, we discuss the electromagnetic and thermomechanic design criteria of the structures. We also illustrate the criteria to compensate the beam loading and the rf measurements that show the effectiveness of the HOM damping.

Design of high gradient, high repetition rate damped C -band rf structures

In this paper we illustrate the tuning procedure we implemented to tune the traveling wave structures designed and constructed for the SPARC photo-injector energy upgrade. The procedure has been derived from that proposed and adopted for the CLIC structures at CERN (J. Shi et al., Tuning of clic accelerating structure prototypes at Cern, Proc. of LINAC 2010, Tsukuba, Japan, 2010). In the first part of the paper we go through the analytical formulas also illustrating and discussing an algorithm to tune the input and output couplers. A detailed description of the measurements we have done before and after the tuning of the SPARC C-band structure prototype is then illustrated.

/pdf/tuning-procedure-for-traveling-wave-structures-and-its-55wf39cqk7.pdf

Tuning procedure for traveling wave structures and its application to the C-Band cavities for SPARC photo injector energy upgrade

C-band RF acceleration is a crucial technology for the compact Free Electron Laser (FEL) facility at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. A project focusing on C-band RF acceleration technology was launched in 2008, based on high-gradient accelerating structures powered by klystron and pulse compressor units. The target accelerating gradient is 40 MV/m or higher. Recently one prototype of C-band RF unit, consisting of a 1.8 m accelerating structure and a klystron with a TE0115 mode pulse compressor, has been tested with high-power and electron beam. Stable operation at 40 MV/m was demonstrated and, 50 MV/m approached by the end of the test. This paper introduces the C-band R&D program at SINAP and presents the experiment results of high-power and beam tests.

Design, fabrication and first beam tests of the C-band RF acceleration unit at SINAP

In the frame of the SPARC-X project, the energy of the FEL accelerator SPARC, in operation at the INFNLNF laboratories, will be upgraded from 180 to 250 MeV by replacing a low gradient S-band travelling wave section with two C-band units, completely designed and developed at the Frascati laboratories. The new system consists of a 50 MW klystron, supplied by a pulsed modulator and a RF pulse compressor to feed 2 high gradient C-band structures. This paper deals with the design of the full system, the C-band R&D activity and study of the related beam dynamics. INTRODUCTION The photoinjector SPARC [1] is an electron linac test facility, operating in the 150÷200 MeV energy range at the Frascati INFN Laboratories. It is specifically designed to generate low emittance, high peak current particle beams to perform SASE at 530 nm and seeded FEL experiments through a series of undulator magnets. The machine was funded by the MIUR (Italian Minister for University and Research) in 2001 and came into operation in 2008. It consists basically of three S-band travelling wave accelerating sections, supplied by two Thales klystrons. One klystron feeds 2 sections out of 3 with a pulse compressor (SLED) so that the input power per section is about 55 MW/0.8μsec. So far, SPARC produced important results in the physics of FEL’s and particle beams [2]. Lasing have been achieved routinely with significant photon amplification factor both at 530 nm and with the 3 rd harmonic seeding process. Important bunch compression experiments have been successfully performed by adjusting the RF field phase of the 1 st accelerating section. Compression factors of about 12 have been obtained [3] though at expense of a growth of the beam emittance, partially compensated by solenoids wrapped around the accelerating sections and a small reduction of the beam energy due to the bunch passage on the voltage zerocrossing. SPARC BEAM ENERGY UPGRADE In order to allow to lase in the UV region and improve the seeding experiment, it has been decided to increase the SPARC beam energy to 250÷260 MeV by replacing the third 3 m. long accelerating section with two, 1.5 meters, C-band structures. We cannot simply add the new sections for lack of space in the machine. The C-band (5712 MHz) allows to get accelerating gradient higher than those achievable with S-band units. RF fields > 35 MV/m have been already achieved [4]. So, with the C-band units in place of the S-band one we can get about 60-70 MeV more. General Layout of the C-band System The new machine layout, after the implementation of the C-band system, will be that shown in Fig. 1. Figure 1: SPARC upgrading with a C-band system. The new C-band power station will consist mainly of: • C-band klystron, manufactured by Toshiba Ltd (JP) • Pulsed HV modulator supplied by ScandiNova (S) • WR187 waveguide system with double resonator pulse compressor. • 500 W solid state klystron driver supplied by MitecTelecom (CDN) The main specifications of the klystron and the modulator are listed in the Tables 1 and 2. Table 1: Toshiba Klystron ET37202 specifications Frequency 5712 MHz Output RF power 50 MW max. RF pulse length 2.5 μsec Pulse rep. rate 50 pps max. Gain 44 dB min Efficiency 40 % min Drive power 300 W The klystron is the same model adopted by the Spring8 X-FEL. The pulsed modulator, selected with a public tender, will be supplied by the Swedish company ScandiNova. It is a fully solid state unit which, instead of the standard LC pulse forming network, employs a set of solid state switches in parallel. The switches, gated by a ______________________________________________ *Work supported by the MIUR, Italian Minister for University and Research. THPEA006 Proceedings of IPAC’10, Kyoto, Japan 3682 07 Accelerator Technology T06 Room Temperature RF trigger signal, produce a medium voltage pulse at the primary of a step-up transformer whose secondary is connected to the klystron cathode. The modulator, including klystron and pulse transformer, is enough compact and well fits in the mid-gallery of the SPARC building. The klystron driver is the CW, class A, 500 W amplifier manufactured by MitecTelecom (CDN). It is a solid state unit, produced for satellite communication purposes, with small pulse-to-pulse phase variation and reduced latency (< 100 nsec) that is required to implement an effective phase feedback loop around the klystron. Table 2: Pulsed modulator specifications Type Fully Solid State Modulator peak power 128 MW Output voltage range 0 ÷ 370 kV Output current range 0 ÷ 350 A Pulse rep. rate 0 ÷ 10 pps Pulse length (top) 2.5 μsec Top flatness < ± 0.5 % Amplitude stability < ± 0.1 % Pulse length FWHM 4 μsec Pulse to Pulse jitter < ± 4 nsec WR187 under vacuum copper waveguides will be used to carry the RF power to the C-band accelerating sections. They are available from the industry together with RF windows, power splitters and directional couplers. The RF losses of the WR187 @ 5712 MHz are 0.03dB/m, therefore the waveguide losses for the 15 m total length are about 10 %. THE C-BAND STRUCTURES We chose to design and develop the C-band travelling wave (TW) accelerating structures at LNF with the collaboration of local firms. Also, to make easier the design and the fabrication we studied a constant impedance structure, instead of a constant gradient one since this is not essential if we use a pulse compressor. Furthermore, the accelerating mode 2 /3 was chosen as it is the most efficient one. The design activity aims to: 1) optimize the single cell profile to minimize the surface electric field and reduce the discharge rate of the structure. 2) study and develop the “beam-pipe” coupling concept 3) study and develop the dual-feed power coupler to minimize the multipole field effects generated by the asymmetric feeding. 4) design a double-cavity pulse compressor along the same lines as the SKIP-type one [5] used at Spring8. Single Cell Optimization The Fig. 2 shows the single cell shape, optimized to reduce the peak surface electric field with an average gradient of 35 MV/m. The optimization was done including the pulse compressor that is with the real RF pulse. The large iris radius (7 mm) allows better pumping speed and higher group velocity. In this way it is possible to shorten the RF pulse length reducing average power and discharge probability. The maximum surface field results 100 MV/m from HFSS© simulations and is considered a safe value for a discharge-free operation. Figure 2: Single cell design. Beam-pipe RF coupler The concept of beam-pipe coupling, in alternative to the cell coupling has been proposed at SLAC for the high gradient X-band cavities [6]. This solution has been applied to SPARC (see Fig. 3). Figure 3: RF Coupler. Its main advantage is that the RF field on the coupling slot edge is weaker than in case of the RF cell coupling. This reduces the risk of discharge and excessive heating of the in-out slots. The RF power is applied symmetrically onto the beam pipe by means of the waveguide splitter depicted in the picture. In this way, the dipole component of the field is completely cancelled in the coupling cell. The C-band prototype A 50 cm long prototype is being manufactured in a Italian firm at the time of this conference. It consists of 20 RF cells that will be brazed in the high vacuum furnace available at LNF. The prototype will be vacuum and low power tested and, after that, high power tested in the KEK C-band test stand (JP). Figure 4 shows a CAD drawing of the prototype. The calculated main parameters of the prototype are listed in Table 3. Proceedings of IPAC’10, Kyoto, Japan THPEA006 07 Accelerator Technology T06 Room Temperature RF 3683 Figure 4: CAD drawing of the C-band prototype. Table 3: C-band prototype main parameters Frequency 5712 MHz Type TW, 2 /3, CI Number of RF cells 20 Iris diameter 14 mm Group velocity/c 0.0278 Field attenuation 0.22/m Shunt impedance per meter 75 M Filling time 42 nsec Peak surface E-field 81 MV/m @ 40 MW Average accelerating field 35 MV/m @ 40 MW Total structure length 500 mm STUDY OF THE BEAM DYNAMICS The beam dynamics study has been addressed by means of particle tracking simulations performed with the help of the TSTEP and ELEGANT codes [7, 8]. A 200 pC beam, generated by a 6 ps long laser pulse, has been tracked with the TSTEP code from the photocathode exit up to the energy of 90 MeV, i.e. after the second S-band section. An RF compression factor of three has been applied obtaining a beam with rms length z 180 μm and rms energy spread 1.1 %. For the c-band particle tracking the ELEGANT code is used where the longitudinal short range wakefield have been calculated from [9]: WL s ( ) Z0c a s ( )exp s 4s0 erfc s 4s0 (1) where a is the iris radius, c is the velocity of light, Z0 =377 , and s0 = g 8 a g /L ( )L 2 (2) with g the cavity length, L the periodicity; it has been taken = 0.465. For the transverse wakefield we have: WX s ( ) = 4Z0cs0 a s ( ) 1 1+ s s0 exp s s0 (3) where the value of so has been modified by K. Bane fitting with numerical results and becomes [9]:

Beam energy upgrade of the frascati fel linac with a c-band rf system

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The C-Band accelerating structures for SPARC photoinjector energy upgrade

Beam energy upgrade of the frascati fel linac with a c-band rf system