lep3 rf system: gradient and power considerations andy butterworth be/rf thanks to r. calaga, e....
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LEP3 RF System: gradient and power considerations
Andy Butterworth BE/RFThanks to R. Calaga, E. Ciapala
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Outline• Introduction• RF voltage and limits on cavity gradient• Beam power, input couplers and choice of frequency• Higher order modes• Conclusions
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Choice of RF systemFor a given application, the parameters of a SC RF system depend on a number of factors:• Desired gradient
– beam energy for e- storage ring (SR loss/turn)– available space– available cryogenic cooling capacity (limit gradient, highest
possible Q0)
• Beam power– beam current, synchrotron radiation power– power per input coupler (Pbeam vs. total no. of couplers,
choice of Qext)– available RF power sources (amplifiers, RF distribution)
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LEP3: Collider and injector ringsCollider ring:• 12 GV total RF voltage• High gradient required (space limitation, cost)• High SR power (100 MW)• Reuse of LHC cryogenics plants sufficient?Injector ring:• 9 GV total RF voltage• High gradient as above• Low beam current & SR power (3.5 MW)
TLEP-H• 6 GV total RF voltage• Gradient negotiable (cost, no space limitation…?)• High SR power (100 MW)
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Potential options
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Cryomodule layout
• Approx. cavity length is similar• ILC cryomodule can be used for both frequencies
R. Calaga
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Gradients: 1300 MHz• ILC cavity performance requirements:
– 35 MV/m, Q0 > 0.8 x 1010 vertical test (bare cavity)
– 31.5 MV/m, Q0 > 1.0 x 1010 in cryomodule (mounted)
Test results for eight 1.3 GHz 9-cell TESLA cavities achieving the ILC specification (DESY)
(mounted in cryomodule)
BCP + EP
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Cavity gradient yield (ILC)
J. Ozelis, SRF2011
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High gradient R&D (ILC)
• Ongoing R&D in new techniques– e.g. Large grain niobium
cavitiesLarge-grain 9-cell cavities at DESYD. Reschke et al. SRF2011
• Steady progress in gradients over time (but lots of scatter)
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Gradients: 700 MHz• BNL 5-cell 704 MHz test cavity
(A. Burill, AP Note 376, 2010)
BCP only
LHeC CDR design value for ERL 2.5 x 1010 @ 20MV/m
• R.Rimmer, ADS Workshop, JLab 748 MHz Cavity Test
• First cavities, lots of room for improvement
• Measurement after only BCP surface treatment (no EP cf. TESLA cavities)
BCP only
Courtesy of R. Calaga
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LHC cryogenic plant capacity
Installed refrigeration capacity in the LHC sectors (LHC Design Report)Temperature level High-load sector
(1-2, 4-5, 5-6, 8-1) Low-load sector(2-3, 3-4, 6-7, 7-8)
50-75 K [W] 33000 310004.6-20 K [W] 7700 76004.5 K [W] 300 1501.9 K Lhe [W] 2400 21004 K VLP [W] 430 38020-280 K [g.s-1] 41 27
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1300 MHz 9-cell 700 MHz 5-cell
Prototype BNL LHeC CDR
Gradient [MV/m] 15 20 25 15 20 20Active length [m] 1.038 1.038 1.038 1.06 1.06 1.06Voltage/cavity [MV] 15.57 20.76 25.95 15.9 21.2 21.2Number of cavities 771 579 463 755 567 567R/Q [linac ohms] 1036 1036 1036 570 570 570Q0 [1010] 1.7 1.5 1.3 4 1.4 2.5Heat load per cavity [W] 13.8 27.7 50.0 11.1 56.3 31.5Total heat load [kW] 10.6 16.1 23.2 8.4 31.9 17.9Heat load per sector [kW] 1.3 2.0 2.9 1.0 4.0 2.2
Cryogenic heat loadcf. LHC cryoplant capacity @ 1.9K of 2.4 or 2.1 kW per sector
Heat load per cavity =
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Injector ring• Repeat the above exercise for the injector ring…
Total RF voltage = 9000MV 1300 MHz 9-cell 700 MHz 5-cell
Prototype BNL LHeC CDR
Gradient [MV/m] 15 20 25 15 20 20
Number of cavities 579 434 347 567 425 425
Cryo power per cavity [W] 13.8 27.7 50.0 11.1 56.3 31.5
Total cryo power [kW] 8.0 12.0 17.4 6.3 23.9 13.4
Cryo power per sector [kW] 1.00 1.50 2.17 0.79 2.99 1.68
Together with collider ring 2.32 3.51 5.06 1.83 6.98 3.91
• Cryo capacity not for free for 2-ring design…
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Power required per cavity• Total SR power = 100 kW @ 120 GeV
1300 MHz 9-cell 700 MHz 5-cell
Prototype BNL LHeC CDR
Gradient [MV/m] 15 20 25 15 20 20
Number of cavities 771 579 463 755 567 567
RF power per cavity [kW] 129.7 172.7 216.0 132.5 176.4 176.4Matched Qext 1.8E+06 2.4E+06 3.0E+06 3.3E+06 4.5E+06 4.5E+06
• Do any power couplers exist with these specifications?
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CW input couplers for SC cavities
S. Belomestnykh, Cornell, SRF2007
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Not surprising…
• Physical size and hence power handling decrease with frequency• Thermal design
– cooling of room temperature parts– cryogenic load at 2K
• Multipacting…
R. Calaga
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CW input couplers for ERLs
H. Sakai, KEK, SRF2011
• Injectors: high power, low Qext , low gradient
• Main linacs: low power, high Qext , high gradient
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V. Vescherevitch, ERL’09c
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V. Vescherevitch, ERL’09
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V. Vescherevitch, ERL’09
For main Linac, Qext: 3 x 107
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Injector ring• Assuming a top-up intensity of 7% of collider maximum
Beam S.R. power = 3.5 MW 1300 MHz 9-cell 700 MHz 5-cell
Prototype BNL LHeC CDR
Gradient [MV/m] 15 20 25 15 20 20
Number of cavities 386 290 232 378 284 284
Cryo power per sector [kW] 0.66 1.01 1.45 0.52 2.00 1.12
RF power per cavity [kW] 4.5 6.0 7.6 4.6 6.2 6.2
Matched Qext 5.2E+07 6.9E+07 8.6E+07 9.6E+07 1.3E+08 1.3E+08
• Seems to be within reach of current CW coupler technology
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TLEP-H• Total RF voltage: 6000 MV half as many cavities as LEP3• SR power = 100 MW as for LEP3• Power per cavity 2x that for LEP3
Beam S.R. power = 100 MW 1300 MHz 9-cell 700 MHz 5-cell
Prototype BNL LHeC CDR
Gradient [MV/m] 15 20 25 15 20 20
Number of cavities 386 290 232 378 284 284
Cryo power per cavity [W] 13.8 27.7 50.0 11.1 56.3 31.5
Total cryo power [kW] 5.3 8.0 11.6 4.2 16.0 9.0
RF power per cavity [kW] 259.1 344.8 431.0 264.6 352.1 352.1
Matched Qext 6.7E+06 6.7E+06 6.7E+06 6.4E+06 6.4E+06 6.4E+06
Similar cavity powers as LHeC ring-ring option Solution with shorter cavities or double couplers
cf. LHeC?
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Example: LHeC CDR ring-ring option• 560 MV total RF voltage, 100 mA beam
current, 60 GeV S.R. power losses 43.7 MW
• Consider 5-cell 721 MHz cavities– gradient > 20 MV/m
• 27 cavities would produce the required voltage
• but with 1.6 MW of power per cavity
• Use 2-cell cavities with the same geometry• Use more cavities (112) at lower gradient (11.9 MV/m)
390 kW per cavity• Use 2 input couplers per cavity
195 kW per coupler still high but achievable
beyond reach of current coupler technology!
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Power couplers: conclusionCollider ring:• Currently no input couplers @ 1.3 GHz with sufficient power capacity
(~200 kW)• Some designs for ERL get close but still around 50 kW• Easier with lower frequency (700MHz?)• Consider a dual-coupler design (cf. LHeC)?
Injector ring:• Low power, probably within the capability of current CW coupler
designs
TLEP-H:• With cavities at high gradient, cavity powers are extremely high• look for lower gradients/shorter cavities/multiple couplers cf. LHeC?
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Higher order mode power• Cavity loss factors R. Calaga
For Ib=14mA, Qbunch = 155 nC
• 700MHz: k|| = 2.64 V/pC , PHOM ~ 5.7 kW
• 1.3 GHz: k|| = 8.19 V/pC, PHOM ~ 17.8 kW to remove from the cavity at 2K!
Average PHOM = k||.Qbunch.Ibeam
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HOM damping summary
Project
Beam current [mA]
Average HOM power per cavity [W]
CEBAF 12GeV 0.10 0.05Project X 1 0.06XFEL 5 1SPL 40 22APS SPX 100 2,000BERLinPro 100 150KEK-CERL 100 185Cornell ERL 100 200
eRHIC 300 7,500KEKB 1,400 15,000
Antenna / loop HOM couplers
Beamline HOM loads
Waveguide HOM dampers
RF absorbing materials
After M. Liepe, SRF2011
LEP3 1.3 GHz 14 17,800TLEP-H 700MHz 24 19,700
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IOT & klystron efficiency
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Summary: frequency choice• Advantages 700
MHz• Synergy SPL, ESS,
JLAB, eRHIC• Smaller HOM power• Smaller Heat load• Power couplers
easier• IOT and SSPA
amplifiers available
• Advantages 1300 MHz• Synergy ILC, X FEL‐• Cavity smaller• Larger R/Q• Smaller RF power
(assuming same Qext)• Less Nb material
needed
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Conclusions• Limitations for the collider ring are mainly linked to the high beam power
• 1.3 GHz TESLA/ILC cavities are now a mature technology and have good gradient performance and consistently high Q0 > 1.5 x 1010 @ 20 MV/m
• However, power couplers need > an order of magnitude increase in CW power handling R&D
• 700 MHz cavity developments are in an earlier stage of maturity than TESLA but look promising and may be better suited to high power CW application
R&D needed on input couplers
• High HOM powers to remove from 2K cavity R&D needed on HOM couplers/absorbers
• TLEP-H: low RF voltage but high beam power Lower gradients, more/shorter cavities, multiple power couplers R&D
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LEP2 LHeC LEP3 TLEP-Z TLEP-H TLEP-tbeam energy Eb [GeV] 104.5 60 120 45.5 120 175circumference [km] 26.7 26.7 26.7 80 80 80beam current [mA] 4 100 7.2 1180 24.3 5.4#bunches/beam 4 2808 4 2625 80 12#e−/beam [1012] 2.3 56 4 2000 40.5 9horizontal emittance [nm] 48 5 25 30.8 9.4 20vertical emittance [nm] 0.25 2.5 0.1 0.15 0.05 0.1bending radius [km] 3.1 2.6 2.6 9 9 9partition number Jε 1.1 1.5 1.5 1 1 1momentum comp. αc [10−5] 18.5 8.1 8.1 9 1 1SR power/beam [MW] 11 44 50 50 50 50ΔESR
loss/turn [GeV] 3.41 0.44 6.99 0.04 2.1 9.3VRF,tot [GV] 3.64 0.5 12 2 6 12δmax,RF [%] 0.77 0.66 4.2 4 9.4 4.9ξx/IP 0.025 N/A 0.09 0.12 0.1 0.05ξy/IP 0.065 N/A 0.08 0.12 0.1 0.05fs [kHz] 1.6 0.65 3.91 1.29 0.44 0.43Eacc [MV/m] 7.5 11.9 20 20 20 20eff. RF length [m] 485 42 600 100 300 600fRF [MHz] 352 721 1300 700 700 700δSR
rms [%] 0.22 0.12 0.23 0.06 0.15 0.22σSR
z,rms [cm] 1.61 0.69 0.23 0.19 0.17 0.25L/IP[1032cm−2s−1] 1.25 N/A 107 10335 490 65nγ/collision 0.08 0.16 0.6 0.41 0.5 0.51