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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:
C-band Technology at other International Projects
(LNF-INFN, Frascati, Italy)
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)
0.5-1 m long sections
up to 100 MV/m acc. gradient
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
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.
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
S-Band gun 120 MV/m
(D. Alesini, et al, JINST, 8, P05004, 2013)
Structure design criteria:
-CONSTANT IMPEDANCE (all equal irises) to simplify thefabrication 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 peaksurface fieldobtaining 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).
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 structurehasbeencut in slicesfor an internalinspection. Wehaveidentified the signs of craters and dischargesmainly in the first acceleratingcellafter the input coupler, asexpected, because the highestfieldvalues are excitedat the beginning of CI structures.
First fabricated C-band structure
(D. Alesini, et al, JINST 8, P10010, 2013)
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
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
• 500 W solid state klystron driver supplied by
Toshiba ET37202 klystron and solid state modulator by Scandinova
Test stand for high power test
and power splitter
RF Pickup TRANS
The SKIP SLED has been fabricated in IHEP (Beijing). It has been fixed to the SPARC experimental hall ready for waveguide connection.
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
1) The conditioning procedure was semi-automaticand the interlocks on the HV were forced by:
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 210-9mbar.
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 <10-5but 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
Aramis 0.1-0.7 nm
Main hardware component: C-band Linac module x 26
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)
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
Acceleratinggradient 28 MV/m
113 cells, constantgradient
Courtesy A. Citterio and R. Zennaro (PSI)
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 undulatorcombined 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 emittanceof 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
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
RF pulse compression BOC (PSI)
Waveguides standard components
RF pulse compression SKIP
STRUCTURES FOR ELI_NP
In the context of the ELI-NP ResearchInfrastructure, to be builtatMagurele (Bucharest, Romania), an advanced Source of Gamma-rayphotonsisplanned, capable to produce beams of mono-chromatic and high spectraldensity gamma photons. The Gamma Beam System isbased on a Compton back-scattering source. Itsmainspecifications are: photonenergytunable in the range 1-20 MeV, rmsbandwidthsmallerthan0.5% and spectraldensitylargerthan 104photons/sec.eV, with source spot sizessmallerthan10-30 microns.
For this LINAC high gradient/high rep rate and dampedstructures (for multi-bunchoperation) are requiredto allow an high gamma flux and a compact source.
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
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
Dipoles modes propagate in the waveguide and dissipate
into a load
CLIC structures X-band, high gradient
C-Band structures Spring-8
First dipole mode passband
first solution (CLIC-type)
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.
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.
SCSS- SPRING 8
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)
NEXT STEP: $/€/time/people for the first prototype realization
16 ns bunch spacing
42 MW, 100 Hz
(Nominal output power 50 MW)
Beam loading compensation
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)
ToshiyasuHigo, 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