C-band Structures at SPARC and Overview of C-band Technology at other International Projects D. Alesini (LNF-INFN, Frascati, Italy)

OUTLINE WHY C-BAND FOR ACCELERATORS? C-BAND @ SPARC C-BAND ACCELERATORS OVERVIEW (PSI, SCSS) AND C-BAND TECHNOLOGY NEXT STEP: -STRONG DAMPED C-BAND ACCELERATING STRUCTURES: ELI_NP -C-BAND GUN DESIGN AND FULL C-BAND LINAC

WHY C-BAND FOR ACCELERATORS? We refer to high gradient room temperature pulsed electron LINACS with typical parameters: 10-100 Hz rep. rate, 0.5-5 s RF pulse length, single/multi bunch, 20-100 MV/m average accelerating gradients. S-BAND (2.856 GHz) X-BAND (12 GHz) C-BAND (5.712 GHz) frequency NLC-CLIC projects: 0.5-1 m long sections up to 100 MV/m acc. gradient SLAC LINAC 3 m long sections 3.1 km total accelerator length 960 accelerating structures up to 50 GeV electron energy 20 MV/m average acc. gradient Accelerating gradient (f1/2) Dipole wakefield intensity (f3) Complication in fabrication technology Available commercial components

C-BAND ACCELERATING SYSTEM @ SPARC The energy upgrade of the SPARC photo-injector at LNF-INFN 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 TW and CI, have symmetric axial input couplers and have been optimized to work with a SLED RF input pulse. In the SPARC photoinjector the choice of the C-band for the energy upgrade was dictated by the opportunity to achieve a higher accelerating gradient, enabled by the higher frequency, and to explore a C-band acceleration combined with an S-band injector that, at least from beam dynamics simulations was very promising in terms of achievable beam quality. 2 structures 1.4 m long >35 MV/m acc. Gradient Design and built @ LNF SLED-SKIP RF compression system (IHEP, Beijing) Low gradient S-Band structure 13 MV/m S-Band SLAC-type structure 22 MV/m S-Band gun 120 MV/m

Design of C-Band TW structures for SPARC (D. Alesini, et al, JINST, 8, P05004, 2013) Structure design criteria: -CONSTANT IMPEDANCE (all equal irises) to simplify the fabrication and to reduce the unbalance between the accelerating field at the entrance and at the end of the structure, due to the combination of power dissipation along the structure and SLED pulse profile. -Large irises with elliptical shape to -reduce the peak surface field obtaining at the same time an average accelerating field >35 MV/m with the available power from the klystron; -reduce the filling time of the structure and, consequently, the RF input pulse length thus reducing the breakdown rate; -reduce the dipole wake intensity -increase the pumping speed. -WAVEGUIDE COUPLER design based on “low pulsed heating” couplers for high gradient operation of X Band structures (SLAC). TIARA/FP7 EU funding

Test at high power (@KEK) of the prototype The high-power test started on November 5, 2010 and was completed on December 13, 2010. For almost one month of processing, from November 5 until December 2, more than 108 RF pulses of 200 ns width were sent into the structure with a repetition rate of 50 Hz. For a couple of days the RF pulse length was changed to 300 ns and for one day (November 12) the repetition rate was decreased to 25 Hz. On November 15, SKIP was switched on. After the high power test the structure has been cut in slices for an internal inspection. We have identified the signs of craters and discharges mainly in the first accelerating cell after the input coupler, as expected, because the highest field values are excited at the beginning of CI structures.

FINAL C-BAND STRUCTURES First fabricated C-band structure PARAMETER prototype final structure Frequency 5.712 [GHz] Phase advance per cell 2/3 Number of accel. cells 22 71 Structure length 0.54 [m] 1.4 [m] Group velocity/c 0.0283 Field attenuation 0.206 [1/m] Series impedance (Z) 34.1 [M/m2] Shunt impedance (r) 82.9 [M/m] Filling time 50 [ns] 150 [ns] Surf. peak E field/Acc. field 2.17 Surf. Peak H field@ 35 MV/m 87.2 [kA/m] Pulsed heating @ 35 MV/m <1 oC Av. dissipated power @ 10 Hz 7.6 [W] 59.6 [W] (D. Alesini, et al, JINST 8, P10010, 2013)

Main PROBLEMS ENCOUNTERED IN THE STRUCTURE FABRICATION The main problems were related to the fact that we do not have a vertical >1.5 m long oven for brazing and we had to braze the structures in several steps. Each brazing step is a structure “stress”, requires a full control of the process and introduces unknowns since the success of the brazing cannot be guarantee at 100% even with a long experience. The design, machining and brazing of a new (complicated) structure (such as the SPARC C-band structures), require a strong activity of R&D and prototyping at least in the first phase to investigate all possible criticalities (RF, mechanical). For the SPARC C-band structures, we have fabricated only one small prototype before starting the construction of the first complete structure and this is the reason why we had to re-machine and re-cut the first structure a couple of time. In other words the first structure has been a prototype. At LNF-INFN this experience has been the first experience of a complete in-house design, realization and test of a such long multi-cell TW structure. We gain a lot (a lot) of experience thanks to this work. OK First realization technique New design of the junction, re-machining and brazing

Toshiba ET37202 klystron and solid state modulator by Scandinova C-BAND HIGH POWER STATION @ SPARC Toshiba ET37202 klystron and solid state modulator by Scandinova 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) Test stand for high power test 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

C-BAND STRUCTURES: HIGH POWER TEST BENCH @SPARC From KLY Ceramic windows and power splitter RF pickup FW-RW RF Pickup TRANS Ion pumps C-Band structure RF Loads

C-Band SLED LOW POWER MEASUREMENTS The SKIP SLED has been fabricated in IHEP (Beijing). It has been fixed to the SPARC experimental hall ready for waveguide connection.

HIGH POWER TEST @SPARC: RESULTS (1/2) 1) The RF conditioning has been done in three steps: test of the Klystron system terminated into a loads test of the waveguide system up to the SPARC hall terminated into a load test of the first accelerating structure 2)The high power test on the first C-band structure started on Nov. 2013. We operated at 10 Hz with the nominal pulse width of 165 ns (slightly longer than the filling time of the structure). 3)We progressively increased the power from the klystron (increasing the HV of the modulator) monitoring: the current absorption of the 4 ion pumps (3 connected to the structure and 1 to the waveguide); the RF signals from pickups. Typical event of discharge Control panel RF reflected RF transmitted RF forward

HIGH POWER TEST @SPARC: RESULTS (2/2) 1) The conditioning procedure was semi-automatic and the interlocks on the HV were forced by: operator ion pumps current absorption above a certain threshold (50 A corresponding to a vacuum of 10-7 mbar) including the ion pump absorption directly connected to the KLY output; KLY interlocks (tube vacuum, modulators interlocks); 2) We normally operate at a vacuum level in the structure between 510-10 mbar and 210-9 mbar. 3) The duration of the RF conditioning (not h24) was about 10-15 full days equivalent! We have finally reached:  38 MW input power in the structure (44 MW from the klystron), nominal rep. rate and pulse length.  the corresponding accelerating field was 36 MV/m peak and 32 MV/m average  BDR <10-5 but even less because a correct measurements of the BDR require a long time and we want to test the other structure!  340 KV modulator voltage

C-BAND STRUCTURES FOR Swiss FEL Project 1st construction phase 2013-16 2nd construction phase 2018-19 Linac 3 Linac 1 Injector Linac 2 Athos 0.7-7nm Aramis 0.1-0.7 nm 0.35 GeV 2.0 GeV 3.0 GeV 2.1-5.8 GeV user stations 2.6-3.4 GeV BC1 BC2 Main hardware component: C-band Linac module x 26 Key Parameters: FEL Wavelength: λ = 1 to 7 Å Electron Beam Energy: 5.8 GeV max. Main Linac frequency: 5.712 GHz Bunch charge: 200 pC or 10 pC Bunch length: 25 fs or 0.3 fs (ultrashort case) Core slice emittance (mm·mrad) 0.43 Photon peak brightness* ~1033 Bunch per pulse 2 Bunch spacing 28 ns Total Length: 715 m Courtesy A. Citterio and R. Zennaro (PSI)

PSI C-BAND STRUCTURES: REALIZATION C-band technology: In house development of ultra-precise machined accelerating structure without tuning (short structure program and 2m nominal structure) In house development of the brazing technique for the 2m structure J-type coupler Specifications: Phase adv. 2π/3 Filling Time: 329 ns (th.) vg/c: 3.1% - 1.2% (th.) Iris radius (20°C): 7.238 mm – 5.447 mm Length : 2 m Accelerating gradient 28 MV/m 113 cells, constant gradient Courtesy A. Citterio and R. Zennaro (PSI)

SPring-8 compact SASE source (SCSS) A reduction of the machine size for widespread distribution of XFEL sources is the main idea for the SPring-8 compact SASE source (SCSS). Here, the use of an in-vacuum shorter-period undulator combined with a high gradient C-Band accelerator allows us to realize a compact XFEL machine with a lower-energy accelerator. Although a reduction of the facility scale gives a significant advantage, a new technical challenge was requested for generating and accelerating the extremely high-quality electron beam with a small normalized-slice emittance of less than 1 mm mrad. For this purpose, we proposed to use a thermionic cathode gun, which has a stable emission property and a long lifetime compared to photocathode rf guns. In order to make up for its low emission current, a velocity bunching section, which can push the total bunch compression factor up to a few thousands, was added to the injector. -Nominal accelerating gradient up to 35 MV/m -length 1.8 m -C-Band damped structures in operation

C-BAND TECHNOLOGY The C-Band technology can be considered “commercial”. Several devices are available on the market and their cost (in particular for waveguide components) is even lower than for S-Band structures Klystrons (TOSHIBA) Waveguides standard components RF pulse compression BOC (PSI) RF pulse compression SKIP

NEXT STEP: DAMPED/HIGH GRADIENT/HIGH REP. RATE STRUCTURES FOR ELI_NP In the context of the ELI-NP Research Infrastructure, to be built at Magurele (Bucharest, Romania), an advanced Source of Gamma-ray photons is planned, capable to produce beams of mono-chromatic and high spectral density gamma photons. The Gamma Beam System is based on a Compton back-scattering source. Its main specifications are: photon energy tunable in the range 1-20 MeV, rms bandwidth smaller than 0.5% and spectral density larger than 104 photons/sec.eV, with source spot sizes smaller than 10-30 microns. For this LINAC high gradient/high rep rate and damped structures (for multi-bunch operation) are required to allow an high gamma flux and a compact source. Bunch charge 250 pC Number of bunches 32 Bunch distance 16 ns C-band average accelerating gradient 33 MV/m Norm. emittance 0.2-0.6 mmmrad Bunch length <300 m RF rep Rate 100 Hz

FIRST REQUIREMENT: HOM DAMPED STRUCTURES ELI-NP operates in multi-bunch mode. The passage of electron bunches through accelerating structures excites electromagnetic wakefield. This field can have longitudinal and transverse components and, interacting with subsequent bunches, can affect the longitudinal and the transverse beam dynamics. TRANSVERSE EFFECTS  Cumulative beam break-up (BBU) LONGITUDINAL EFFECTS  beam loading Eacc=average accelerating field Injector S-band Ideal axis Einj=beam energy after injector Booster LINAC (C-band) Normalized Courant Snyder Invariant at the exit of the linac for an initial displacement of all bunches of 500 m

Damping of dipole modes choice Dipoles modes propagate in the waveguide and dissipate into a load CLIC structures X-band, high gradient C-Band structures Spring-8 Advantages 1. Strong damping of all modes above waveguide cut-off 2. Possibility of tuning the cells 3. Good cooling (high rep. rate) Disadvantages Machining: need a 3D milling machine Multipole field components (octupole) but not critical at least for CLIC Advantages Easy machining of cells (turning) 2D geometry: no multipole field components Disadvantages 1. Critical e.m. design: notch filter can reflect also other modes. 2. Not possible to tune the structure 4. Cooling at 100 Hz, long pulse length (?)

DAMPING: HFSS and GdFidL simulations w=13 mm w=7 mm w First dipole mode passband

ACCELERATING STRUCTURES DESIGN The power released by the beam on the dipole modes is dissipated into SiC absorbers. Several different solutions are possible to design the absorber. The final geometry has been optimized to: simplify the realization procedure and the overall cost of the structures. Reduce the transverse size dimensions to allow solenoids positioning PARAMETER VALUE Type TW- quasi CG Frequency (fRF) 5.712 [GHz] Phase advance per cell 2/3 Structure Length 1.8 m (102 cells) Iris aperture (a) 6.8-5.8 mm group velocity (vg/c): 0.034-0.013 Quality factor (Q) 8800 Shunt imp. (r) 67-73 [M/m] RF input power 40 MW EACC_average@ PIN=40 MW 33 MV/m Rep. Rate (frep) 100 Hz Average dissipated power 2.3 [kW] absorber first solution (CLIC-type) absorber

Prototypes realization First prototypes have been realized to verify the feasibility of the machining of the cells and of the brazing process. We are now focalizing in the realization of two prototypes previous the realization of the first complete structure. -The first prototype is a full scale device without precise internal dimensions that we would like to build in order to test the full brazing process, verifying eventual structure deformations and vacuum. -The second prototype is a device with a reduced number of cells that we would like to realize to test the RF properties of the structure at low and high power.

2nd STEP: A FULL C-BAND LINAC All existing (under realization) C-Band LINACS have an S-band injector. A full C-band linac with a C-Band RF gun is the next and definitive step in C-Band photoijnectors. SPARC S-Band injector C-Band Booster PSI SWISSFEL C-BAND LINAC SCSS- SPRING 8

C-BAND RF GUN The designed gun integrate a waveguide coupler that allows: -high efficiency cooling of the accelerating cells -low pulsed heating of the coupler surfaces -arbitrary solenoids position around the accelerating cells and on the beam pipe -100 Hz operation in multi bunch -fabrication of the gun without brazing processes (hard copper->higher gradients) PARAMETERS f [GHz] 5712 Q0 10900 Ecathode @10 MW Pin 200 [MV/m]  2 Filling time  200 [ns] NEXT STEP: $/€/time/people for the first prototype realization

POSSIBLE CONFIGURATION OF A MULTI-BUNCH C-BAND INJECTOR 32 bunches 250 pC 16 ns bunch spacing 42 MW, 100 Hz 7 MW 35 MW 1.1 s Flat top TOSHIBA E37210 (Nominal output power 50 MW) C C L=1.8 m Eacc=31 MV/m E_cathode=170 MV/m 60 MeV beam A. Bacci PARAMETER C-BAND INJECTOR Charge 250 pC Laser pulse length 8.5ps laser spot size 250 m Output energy 95 MeV Output emittance 0.25 mm mrad Output bunch length 800 fs Output energy spread 0.38% Beam loading compensation

CONCLUSIONS C-Band adventure started @ LNF for the SPARC energy upgrade-single bunch operation. The first prototype has been tested at gradients >50 MV/m The two final C-band structures have been designed and fabricated. The first one has been already tested at high power up to 32 MV/m average accelerating gradient. The second one is now under installation. The high power tests will start next December. C-Band accelerators developed all over the world (PSI, SPRING8) have made the C-Band technology “accessible and commercial” in term of waveguide components, RF compressors, RF sources. Damped C-band structures for multi-bunch acceleration with >100 Hz rep. rate have been developed for ELI-NP and are now under construction. They are the next step in C-Band accelerating structure technology and their interest goes above the ELI_NP proposal . The full C-band injector (>100 Hz, Multi-bunch) is now the next and definitive step in this technology. It includes a C-Band RF GUN at gradients >180 MV/m that has been designed (RF+mechanical) but has to be fabricated and tested if we want to go in this direction.

…AND THANK YOU FOR YOUR ATTENTION THANKS TO… V. Lollo, R. Di Raddo, P. Chimenti, R. Boni, M. Ferrario, R. Clementi, M. Bellaveglia, A. Gallo, M.E.Biagini, G. Di Pirro (LNF-INFN) Toshiyasu Higo, Shuji Matsumoto, K. Kakihara (KEK) M. Migliorati, A. Mostacci, V. Spizzo, S. Tocci, L. Palumbo, S. Persichelli, G. Campogiani (La Sapienza) Grudiev, G. De Michele, G. Riddone, Silvia Verdu Andres (CERN) R. Zennaro, A. Citterio (PSI) …AND THANK YOU FOR YOUR ATTENTION