Highly HOM-damped cavities S. Belomestnykh Brookhaven National Laboratory and Stony Brook University March 23-27 Washington, DC FCC Week 2015

Highly HOM-damped cavities

S. BelomestnykhBrookhaven National Laboratory and Stony Brook University

March 23-27Washington, DC

FCC Week 2015

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Introduction

March 24, 2015 Highly HOM damped cavities

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

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Highly HOM-damped cavities for high current storage rings & ERLs


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HOM damped cavities


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: 352-500 MHz; accelerating gradients: 4-9 MV/m; RF power: up to 400 kW; HOM power up to 15 kW per absorber; achieved QHOM ≈ 102…103.

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.

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Example: the beam pipe absorbers on the KEKB SRF cavity


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Beam pipe absorbers


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.

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Example: SOLEIL HOM couplers


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

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HOM damped cavities for ERLs


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

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Challenges presented by future circular colliders


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SRF systems of the future energy frontier colliders


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.

11March 24, 2015 Highly HOM damped cavities

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Energy frontier circular colliders


Parameter FCC-ee FCC-pp CEPC VLLC VLHC LEP2

Beam energy 45 to 175 GeV 50 TeV 120 GeV 185 GeV 87.5 TeV 105 GeV

Circumference 100 km 100 km 50 km 233 km 233 km 27 km

Total SR power 100 MW 4.8 MW 100 MW 100 MW 2.1 MW 18.2 MW

RF voltage 2.5 to 11 GV > 20 MV 6.87 GeV 4.66 GV 200 MV 3.5 GeV

Beam current 6.6 mA to 1.45 A 500 mA 16.6 mA 12.5 mA 68.9 mA 6 mA

Eacc ~10 MV/m at 400 MHz 15.5 MV/m 8 MV/m 8 MV/m 7.5 (9) MV/m

2001 studies

HOM-dampedsingle cell (or 2-cell)

400 MHz cavities plus 800 MHz cavities

HOM-dampedmulti-cell cavities

State-of-the-art for linacs, but need demonstrationon HOM-damped structures.

Unlike LEP2 cavities, the CEPC cavities will operate at 2 K

Nb/Cu possible

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R&D efforts on HOM-damped cavities for EIC’s at BNL and JLab


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R&D at Jefferson Lab: MEIC ion ring design concept


• 0.5 A beam current, 7.5-7.8 MV/m, up to 110 kW per cavity

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JLab high-current cavities


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

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High-current cavity test results


1.5 GHz 750 MHz

Multipacting seen from low gradient but processed away

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R&D at BNL:HOM-damped cavities for eRHIC


Linac energy gain 1.32 GeV

Number of passes up to 16

Bunch repetition frequency 9.38 MHz

Average beam current up to 50 mA

RF frequency 422 MHz

Number of SRF cavities 40

Linac fill factor 0.6

Accelerating gradient 18.5 MV/m

Operating temperature 1.9 K

Cavity Q at operating gradient 5×1010

HOM power per cavity 1 kW

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:


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.

Frequency 703.8 MHz

R/Q 506.3 Ohm

Geometry factor 283 Ohm

Number of cells 5

Flange-to-flange length 1.58 m

Beam pipe radius 0.11 m

Q 4×1010

Epk/Eacc 2.46

Bpk/Eacc [mT/MV/m] 4.26 mT/(MV/m)

Lorentz force detuning 0.45 Hz/(MV/m)2

Loss factor for 2 mm bunch length 3.96 V/pC

Five-cell cavity with strong HOM damping

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2-stage filter HOM coupler


Between the two notches, S21 < -65 dB, 1st 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

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Dual-ridge waveguide HOM coupler


More compact than rectangular waveguide. A very broadband coax-to-waveguide transition was developed. Better transmission than that of the 2-stage coaxial coupler.


422 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.

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Summary


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.

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Acknowledgements


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), …

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Thank you!


Documents

Highly HOM-damped cavities S. Belomestnykh Brookhaven National Laboratory and Stony Brook University March 23-27 Washington, DC FCC Week 2015