LEP3 RF System: gradient and power considerations

Andy Butterworth BE/RFThanks to R. Calaga, E. Ciapala

Outline• Introduction• RF voltage and limits on cavity gradient• Beam power, input couplers and choice of frequency• Higher order modes• Conclusions

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)

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)

Potential options

Cryomodule layout

• Approx. cavity length is similar• ILC cryomodule can be used for both frequencies

R. Calaga

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

Cavity gradient yield (ILC)

J. Ozelis, SRF2011

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)

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

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

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 =

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…

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?

CW input couplers for SC cavities

S. Belomestnykh, Cornell, SRF2007

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

CW input couplers for ERLs

H. Sakai, KEK, SRF2011

• Injectors: high power, low Qext , low gradient

• Main linacs: low power, high Qext , high gradient

V. Vescherevitch, ERL’09c

V. Vescherevitch, ERL’09

V. Vescherevitch, ERL’09

For main Linac, Qext: 3 x 107

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

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?

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!

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?

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

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

IOT & klystron efficiency

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

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

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