LEP3 RF System: gradient and power considerations Andy Butterworth BE/RF Thanks 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 system For 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 Q 0 ) Beam power – beam current, synchrotron radiation power – power per input coupler (P beam vs. total no. of couplers, choice of Q ext ) – available RF power sources (amplifiers, RF distribution)

LEP3: Collider and injector rings Collider 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, Q 0 > 0.8 x vertical test (bare cavity) – 31.5 MV/m, Q 0 > 1.0 x 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 cavities Large-grain 9-cell cavities at DESY D. 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 10 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) K [W] K [W] K [W] K Lhe [W] K VLP [W] K [g.s-1]4127

1300 MHz 9-cell700 MHz 5-cell Prototype BNLLHeC CDR Gradient [MV/m] Active length [m] Voltage/cavity [MV] Number of cavities R/Q [linac ohms] Q 0 [10 10 ] Heat load per cavity [W] Total heat load [kW] Heat load per sector [kW] Cryogenic heat load cf. LHC cryoplant 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 = 9000MV1300 MHz 9-cell700 MHz 5-cell Prototype BNLLHeC CDR Gradient [MV/m] Number of cavities Cryo power per cavity [W] Total cryo power [kW] Cryo power per sector [kW] Together with collider ring Cryo capacity not for free for 2-ring design…

Power required per cavity Total SR power = GeV 1300 MHz 9-cell700 MHz 5-cell Prototype BNLLHeC CDR Gradient [MV/m] Number of cavities RF power per cavity [kW] Matched Q ext 1.8E+062.4E+063.0E+063.3E+064.5E+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 Q ext, low gradient Main linacs: low power, high Q ext, high gradient

V. Vescherevitch, ERL’09c

V. Vescherevitch, ERL’09

For main Linac, Q ext : 3 x 10 7

Injector ring Assuming a top-up intensity of 7% of collider maximum Beam S.R. power = 3.5 MW1300 MHz 9-cell700 MHz 5-cell Prototype BNLLHeC CDR Gradient [MV/m] Number of cavities Cryo power per sector [kW] RF power per cavity [kW] Matched Q ext 5.2E+076.9E+078.6E+079.6E+071.3E+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 MW1300 MHz 9-cell700 MHz 5-cell Prototype BNLLHeC CDR Gradient [MV/m] Number of cavities Cryo power per cavity [W] Total cryo power [kW] RF power per cavity [kW] Matched Q ext 6.7E E+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: conclusion Collider ring: Currently no input 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 I b =14mA, Q bunch = 155 nC 700MHz: k || = 2.64 V/pC, P HOM ~ 5.7 kW 1.3 GHz: k || = 8.19 V/pC, P HOM ~ 17.8 kW  to remove from the cavity at 2K! Average P HOM = k ||.Q bunch.I beam

HOM damping summary Antenna / loop HOM couplers Beamline HOM loads Waveguide HOM dampers RF absorbing materials After M. Liepe, SRF2011 LEP3 1.3 GHz 14 17,800 TLEP-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 Q 0 > 1.5 x 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

LEP2LHeCLEP3TLEP-ZTLEP-HTLEP-t beam energy E b [GeV] circumference [km] beam current [mA] #bunches/beam #e − /beam [10 12 ] horizontal emittance [nm] vertical emittance [nm] bending radius [km] partition number J ε momentum comp. α c [10 −5 ] SR power/beam [MW] ΔE SR loss /turn [GeV] V RF,tot [GV] δ max,RF [%] ξ x /IP0.025N/A ξ y /IP0.065N/A f s [kHz] E acc [MV/m] eff. RF length [m] f RF [MHz] δ SR rms [%] σ SR z,rms [cm] L/IP[10 32 cm −2 s −1 ]1.25N/A n γ /collision