C-band Structures at SPARC and Overview of C-band Technology at other International Projects

D. Alesini(LNF-INFN, Frascati, Italy)

OUTLINE

1. WHY C-BAND FOR ACCELERATORS?

2. C-BAND @ SPARC

3. C-BAND ACCELERATORS OVERVIEW (PSI, SCSS) AND C-BAND TECHNOLOGY

4. 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)

SLAC LINAC3 m long sections 3.1 km total accelerator length960 accelerating structuresup to 50 GeV electron energy20 MV/m average acc. gradient

NLC-CLIC projects:0.5-1 m long sectionsup to 100 MV/m acc. gradient

Accelerating gradient (f1/2)

Dipole wakefield intensity (f3)

Complication in fabrication technology

Available commercial components

C-BAND (5.712 GHz)frequency

C-BAND ACCELERATING SYSTEM @ SPARCThe 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.

S-Band gun 120 MV/mS-Band SLAC-type structure 22 MV/m

Low gradient S-Band structure 13 MV/m

2 structures1.4 m long>35 MV/m acc. GradientDesign and built @ LNF

SLED-SKIP RF compression system (IHEP, Beijing)

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).

DESIGN OF C-BAND TW STRUCTURES FOR SPARC(D. Alesini, et al, JINST, 8, P05004, 2013)

TIARA/FP7

EU funding

TEST AT HIGH POWER (@KEK) OF THE PROTOTYPEThe 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 STRUCTURESFirst fabricated C-band structure PARAMETER prototype final structure

Frequency 5.712 [GHz]Phase advance per cell 2/3Number of accel. cells 22 71

Structure length 0.54 [m] 1.4 [m]Group velocity/c 0.0283Field 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.

First realization technique New design of the junction, re-machining and brazing OK

Toshiba ET37202 klystron and solid state modulator by Scandinova

Test stand for high power test

C-BAND HIGH POWER STATION @ SPARC

Frequency 5712 MHzOutput RF power 50 MW (max).RF pulse length 2.5 μsecPulse rep. rate 50 pps max.Gain 44 dB minEfficiency 40 % minDrive power 300 W

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 resonatorpulse compressor.• 500 W solid state klystron driver supplied byMitecTelecom (CDN)

C-BAND STRUCTURES: HIGH POWER TEST BENCH @SPARC

RF pickupFW-RW

C-Band structure

RF Loads

RF Pickup TRANS

From KLY

Ion pumps

Ceramic windowsand power splitter

C-BAND SLED LOW POWER MEASUREMENTSThe 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 forward

RF transmitted

RF reflected

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 510-10 mbar and 210-9 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

HIGH POWER TEST @SPARC: RESULTS (2/2)

1st construction phase2013-16 2nd construction phase

2018-19

Linac 3Linac 1Injector Linac 2

Athos 0.7-7nm

Aramis 0.1-0.7 nm0.35 GeV 2.0 GeV 3.0 GeV 2.1-5.8 GeV

user stations2.6-3.4 GeVBC1 BC2

Key Parameters:FEL Wavelength: λ = 1 to 7 ÅElectron Beam Energy: 5.8 GeV max.Main Linac frequency: 5.712 GHzBunch charge: 200 pC or 10 pCBunch length: 25 fs or 0.3 fs (ultrashort case)Core slice emittance (mm·mrad) 0.43Photon peak brightness* ~1033

Bunch per pulse 2Bunch spacing 28 nsTotal Length: 715 m

Main hardware component: C-band Linac module x 26

C-BAND STRUCTURES FOR SWISS FEL PROJECT

Courtesy A. Citterio and R. Zennaro (PSI)

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

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

PSI C-BAND STRUCTURES: REALIZATION

Courtesy A. Citterio and R. Zennaro (PSI)

J-type coupler

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 TECHNOLOGYThe 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)

RF pulse compression BOC (PSI)

RF pulse compression SKIP

Waveguides standard components

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 mmmrad

Bunch length <300 m

RF rep Rate 100 Hz

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.

NEXT STEP: DAMPED/HIGH GRADIENT/HIGH REP. RATE STRUCTURES FOR ELI_NP

FIRST REQUIREMENT: HOM DAMPED STRUCTURES

TRANSVERSE EFFECTS Cumulative beam break-up (BBU)

LONGITUDINAL EFFECTS beam loading

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.

Injector S-band

Booster LINAC (C-band)

Ideal axis

Eacc=average accelerating field

Einj=beam energy after injector

Normalized Courant Snyder Invariant at the exit of the linac for an initial displacement of all bunches of 500 m

Advantages

1. Strong damping of all modes above waveguide cut-off2. Possibility of tuning the cells3. Good cooling (high rep. rate)

Disadvantages

1. Machining: need a 3D milling machine2. Multipole field components (octupole)

but not critical at least for CLIC

Advantages

1. Easy machining of cells (turning)2. 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 structure4. Cooling at 100 Hz, long pulse length (?)

DAMPING OF DIPOLE MODES CHOICE

Dipoles modes propagate in the waveguide and dissipateinto a load

CLIC structures X-band, high gradientC-Band structures Spring-8

DAMPING: HFSS AND GDFIDL SIMULATIONS

w

w=7 mmw=13 mm

First dipole mode passband

ACCELERATING STRUCTURES DESIGNThe 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

first solution (CLIC-type)

absorber

absorber

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/mRep. Rate (frep) 100 Hz

Average dissipated power 2.3 [kW]

PROTOTYPES REALIZATIONFirst 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.

C-BAND LINAC

2nd STEP: A FULL C-BAND LINAC

S-Band injector C-Band Booster

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

PSI SWISSFEL

SCSS- SPRING 8

C-BAND RF GUNThe 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

42 MW, 100 Hz

7 MW 35 MW TOSHIBAE37210(Nominal output power 50 MW)

E_cathode=170 MV/m

1.1 sFlat top

CCL=1.8 mEacc=31 MV/m

32 bunches250 pC16 ns bunch spacing

60 MeVbeam

adjustable, Gun C-band solenoid, N=34, ++++

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A. BacciPARAMETER C-BAND INJECTOR

Charge 250 pCLaser pulse length 8.5ps

laser spot size 250 mOutput energy 95 MeV

Output emittance 0.25 mm mradOutput bunch length 800 fs

Output energy spread 0.38%

CONCLUSIONS

1. C-Band adventure started @ LNF for the SPARC energy upgrade-single bunch operation.

2. The first prototype has been tested at gradients >50 MV/m

3. 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.

4. 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.

5. 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 .

6. 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.

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)

A. Grudiev, G. De Michele, G. Riddone, Silvia Verdu Andres (CERN)

R. Zennaro, A. Citterio (PSI)

THANKS TO…

…AND THANK YOU FOR YOUR ATTENTION