Highly HOM-damped cavities S. Belomestnykh Brookhaven National Laboratory and Stony Brook University March Washington, DC FCC Week 2015

Introduction March 24, 2015 Highly HOM damped cavities2 In this talk I will  give a quick overview of the highly HOM damped cavities developed in the past for high current storage rings and ERLs in the context of the FCC-ee  discuss new challenges presented by future circular colliders  present accelerator R&D efforts on HOM-damped SRF cavities at BNL and JLab and relevance of these efforts to the FCC-ee

Highly HOM-damped cavities for high current storage rings & ERLs March 24, 2015 Highly HOM damped cavities3

HOM damped cavities March 24, 2015 Highly HOM damped cavities4  In 1990’s several heavily HOM-damped SRF cavities were developed for high current storage rings at Cornell (CESR), KEK (KEKB HER), CERN (LHC), SOLEIL. All these cavities were single cell structures to support hundreds mA of beam current. Nowadays these cavities are widely used in light sources around the world.  RF frequencies: MHz; accelerating gradients: 4-9 MV/m; RF power: up to 400 kW; HOM power up to 15 kW per absorber; achieved Q HOM ≈ 10 2 …10 3.  Low (for SRF) gradients: one could even afford to use normal conducting cavities (SLAC B-factory and KEKB LER). Bulk Nb or Nb/Cu technology.  There are three main design types, which use different transmission lines/coupling circuits: beam pipe HOM absorbers (beam pipe = circular waveguide), rectangular waveguide HOM couplers, and lumped-element HOM couplers connected to a coaxial line.  Waveguide couplers have a cut-off frequency and therefore do not need a filter to reject the fundamental RF frequency. Thus they are inherently more broad band.  The cavity beam pipe is enlarged to facilitate propagation of the lowest frequency HOMs toward an absorber, a section of the beam pipe with a layer of microwave absorbing material (ferrite or ceramics).  Coaxial lines can transmit TEM waves (hence no cut- off) and therefore HOM couplers based on these lines need filters to reject the fundamental RF. Lumped- element couplers are more compact than other types.

Example: the beam pipe absorbers on the KEKB SRF cavity March 24, 2015 Highly HOM damped cavities5

Beam pipe absorbers March 24, 2015 Highly HOM damped cavities6  HOM dampers of this type are arguably the most efficient and likely will be required to absorb very high frequency portion of HOM power, which propagates along the beam pipe.  Drawbacks of beam pipe absorbers: i.most absorber materials are brittle, can create particulates that contaminate SRF cavities; ii.parasitic beam-absorber interaction is significant and contributes to the overall HOM power; iii.the main disadvantage for large SRF systems is that they occupy real estate along the beam axis and thus reduce the fill factor; iv.room temperatures absorbers can dissipate kW’s of HOM power, but cryogenically cooled absorbers can dissipate only ~100 W.

Example: SOLEIL HOM couplers March 24, 2015 Highly HOM damped cavities7  Two types of HOM couplers: for dipole and monopole modes. Adapted from the LEP design, but with much higher coupling factor.  Dampers of this type can provide strong damping depending on a particular RF design.  Their main disadvantage is fundamental RF rejection filters, which must be carefully tuned. What the beam sees passing through the SOLEIL module SOLEIL cryomodule

HOM damped cavities for ERLs March 24, 2015 Highly HOM damped cavities8  Developed at Cornell (beam pipe absorbers) and Jefferson Lab (WG couplers).  While waveguide couplers can provide very efficient damping in broad frequency range and don’t compromise the fill factor, they complicate the cavity and cryomodule design. Ampere-class cryomodule: WG absorbers at RT HOM load concept Cornell ERL cavity

Challenges presented by future circular colliders March 24, 2015 Highly HOM damped cavities9

SRF systems of the future energy frontier colliders March 24, 2015 Highly HOM damped cavities10  SRF systems: o low frequency: 400 and 800 MHz for FCC, 650 MHz for CEPC; o strong HOM damping; o need good real estate gradient; o high HOM and RF power.  Should the cavities be akin to single cell storage ring cavities or multi- cell ERL-type cavities?  Do SOMs present a problem? Is damping with FPC sufficient?  How to deal with HOM power propagating through the beam pipes (short bunch length – high frequency part of the spectrum)?  Synergy with future electron-ion colliders MEIC and eRHIC.

March 24, 2015 Highly HOM damped cavities11

Energy frontier circular colliders March 24, 2015 Highly HOM damped cavities12 ParameterFCC-eeFCC-ppCEPCVLLCVLHCLEP2 Beam energy45 to 175 GeV50 TeV120 GeV185 GeV87.5 TeV105 GeV Circumference100 km 50 km233 km 27 km Total SR power100 MW4.8 MW100 MW 2.1 MW18.2 MW RF voltage2.5 to 11 GV> 20 MV6.87 GeV4.66 GV200 MV3.5 GeV Beam current6.6 mA to 1.45 A500 mA16.6 mA12.5 mA68.9 mA6 mA Eacc~10 MV/m at 400 MHz15.5 MV/m8 MV/m 7.5 (9) MV/m 2001 studies HOM-damped single cell (or 2-cell) 400 MHz cavities plus 800 MHz cavities HOM-damped multi-cell cavities State-of-the-art for linacs, but need demonstration on HOM-damped structures. Unlike LEP2 cavities, the CEPC cavities will operate at 2 K Nb/Cu possible

R&D efforts on HOM-damped cavities for EIC’s at BNL and JLab March 24, 2015 Highly HOM damped cavities13

R&D at Jefferson Lab: MEIC ion ring design concept March 24, 2015 Highly HOM damped cavities A beam current, MV/m, up to 110 kW per cavity

JLab high-current cavities March 24, 2015 Highly HOM damped cavities15  Two 1.5 GHz, one 750 MHz prototypes built and tested Results exceed requirements High power RF window demonstrated to > 60 kW CW BBU simulations for 1.5 GHz ERL 1.5 GHz ERL cavities Shape optimization for BBU/HOM power 1.5 GHz ERL cavity 750 MHz ERL cavity 1.5 GHz window Module concept HOM load concept

High-current cavity test results March 24, 2015 Highly HOM damped cavities GHz750 MHz Multipacting seen from low gradient but processed away

R&D at BNL: HOM-damped cavities for eRHIC March 24, 2015 Highly HOM damped cavities17 Electrons for eRHIC experiments are accelerated in a non-scaling FFAG based ERL with 12 passes to reach 15.9 GeV or 16 passes to reach 21.2 GeV. The main linac parameters are:

March 24, 2015 Highly HOM damped cavities18 HOM coupler ports FPC port  A five-cell 704 MHz SRF cavity (BNL3) has been developed at BNL for high current linacs before the eRHIC RF frequency was changed to 422 MHz.  A prototype cavity reached 19.7 MV/m.  Six antenna-type couplers will be attached to the large diameter beam pipes and will provide strong HOM damping while maintaining good fill factor for the linac.  Two HOM filters are currently under consideration: a high pass filter made of lumped elements and a dual-ridge waveguide filter. Frequency703.8 MHz R/Q506.3 Ohm Geometry factor283 Ohm Number of cells5 Flange-to-flange length1.58 m Beam pipe radius0.11 m Q4×10 10 E pk /E acc 2.46 B pk /E acc [mT/MV/m]4.26 mT/(MV/m) Lorentz force detuning0.45 Hz/(MV/m) 2 Loss factor for 2 mm bunch length3.96 V/pC Five-cell cavity with strong HOM damping

2-stage filter HOM coupler March 24, 2015 Highly HOM damped cavities19  Between the two notches, S21 < -65 dB, 1 st HOM is at 0.82 GHz, S21 = -23 dB.  It has good damping at high frequencies.  Work on filter optimization continues. 50 Ω transmission line to room temperature D = 72 mm

Dual-ridge waveguide HOM coupler March 24, 2015 Highly HOM damped cavities20  More compact than rectangular waveguide.  A very broadband coax-to-waveguide transition was developed.  Better transmission than that of the 2-stage coaxial coupler.

March 24, 2015 Highly HOM damped cavities MHz HOM-damped cavity  R&D on the 422 MHz BNL4 cavity is in progress.  The RF design is completed with the cavity shape re-optimized from the scaled 704-MHz BNL3 cavity model: o The first HOM of the BNL4 cavity is 15 MHz further away from the fundamental mode. o The BNL4 cavity’s has higher transverse and longitudinal BBU threshold than the scaled BNL3 cavity. o The HOM power for BNL4 is 10% lower than for the scaled BNL3 cavity.  The 3-cell prototype cavity will be ordered in the near future.

Summary March 24, 2015 Highly HOM damped cavities22  A number of highly HOM-damped cavities was developed around the world in the past for high-current storage rings and ERLs.  There are three main concepts of HOM damping: beam line absorbers, waveguide couplers and lumped-element couplers connected to a coaxial line.  The future colliders (FCC-ee and CEPC) are considering different versions of HOM- damped cavities at RF frequencies of 400 MHz, 650 MHz and 800 MHz.  JLab and BNL are working on their versions of a future electron-ion collider. Such a collider will require HOM-damped SRF accelerating cavities. Requirements to these cavities are very similar to the requirements of future energy frontier HEP circular colliders.  JLab is traditionally developing rectangular waveguide based HOM couplers. They have designs for both single- (952.6 MHz) and multi-cell (750 MHz) structures and obtained good results in vertical cavity tests.  At BNL, 5-cell HOM-damped structures were developed for eRHIC. A prototype 704- MHz BNL3 cavity has reached an accelerating gradient close to 20 MV/m. Two HOM coupler schemes are under consideration: a 2-stage coaxial filter and a dual-ridge waveguide filter. A 422 MHz cavity was designed and a prototype will be ordered in near future.

Acknowledgements March 24, 2015 Highly HOM damped cavities23 CERN: E. Jensen, R. Calaga JLab: R. Rimmer BNL & SBU: Wencan Xu, I. Ben-Zvi, E. Johnson (M.S. 2011), C. Marques (M.S. 2014), …

Thank you! March 24, 2015 Highly HOM damped cavities24