HIGH ORDER MODES SURVEY AND MITIGATION OF THE CEBAF

C100 CRYOMODULES*

J. Guo, H. Wang, M. Stirbet, S. Wang

Jefferson Lab, 12000 Jefferson Ave. Newport News, VA 23606, USA

CONCLUSION

We surveyed all the 10 new C100 cryomodules (80 cavities in total) for HOM and SPM modes. We summarized the statistics of modes frequencies and Qs.

In CMTF HOM survey, we identified dipole HOMs with high impedance. When cryomodules are installed in the CEBAF tunnel, waveguide filter brought

most of the high impedance HOMs to satisfy BBU current threshold requirement. One mode needs additional stub tuners to meet the baseline spec in

CEBAF, but is left as is for BBU beam experiment.

C100 cryomudule cavity line up

WEPRI073 IPAC 2014, Dresden, Germany

* Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for U.S. Government purpose.

Cavity 1 HOM1A HOM8B HOM7B HOM6B HOM5B HOM4A HOM3A HOM2A

HOM1B HOM8A HOM7A HOM6A HOM5A HOM4B HOM3B HOM2B

Cavity 6 Cavity 8 Cavity 7 Cavity 5 Cavity 4 Cavity 3 Cavity 2

FPC1 FPC3 FPC6 FPC7 FPC8 FPC5 FPC4 FPC2

C100 cryomodules for CEBAF 12GeV upgrade • 10 new C100 cryomodules, 8 cavities per module, 7-cell low-loss shape cavities

• ~100MV per module nominal (108MV w/ overhead), 1497MHz, 19.2MV/m

• 2 DESY-style HOM couplers on the same side of cavity

• Double the CEBAF energy from 6 GeV to 12 GeV

Beam Breakup (BBU) Instability for C100

Among all the HOM induced instabilities, BBU is the main concern for CEBAF. It’s

caused by transverse kick from dipole RF modes and limits the maximum beam current.

BBU was observed in CEBAF at 40 µA in 2007, due to high impedance HOMs from

one high-gradient type cavity in the “Renascence” prototype cryomodule. Threshold

current is determined by HOM impedance R∙k and beam parameters. For the CEBAF

baseline of 12GeV and ~87.5µA injection current, the impedance threshold is specified

at 2.4×1010Ω/m. The stretched goal will allow CEBAF to operate at 6GeV ~175µA

injection current, with a more stringent impedance budget of 1.0×1010Ω/m.

rdd

thTkmQQRq

pcI

sin)(

12

*

0

Input Coupler HOMA

HOMB

0

2

4

6

8

10

12

2187.5

2187.7

2187.9

2188.1

2188.3

2188.5

2188.7

2188.9

2189.1

2189.3

2189.5

2189.7

2189.9

2190.1

2190.3

2190.5

2190.7

2190.9

2191.1

2191.3

2191.5

2191.7

2191.9

2192.1

H modes

V modes

H fit

V fit

Frequency Distribution of One Dipole Mode in CMTF

RF Frequency (MHz)

Mode

count

(ea/

0.1

MH

z)

Horizontal modes

Simulated frequency: 2189.540MHz

Mean frequency: 2189.664MHz

Standard deviation: 0.686MHz

Max Q: 1.41×105

Max impedance: 4.65×108Ω

Vertical modes

Simulated frequency: 2190.642MHz

Mean frequency: 2190.109MHz

Standard deviation: 0.666MHz

Max Q: 2.15×105

Max impedance: 6.63×108Ω

Frequency and Q statistics for Same Passband (TM010) Modes

Mode Design Mean Standard Dev Min Max Design Mean Standard dev

π 1497.0000 1496.9998 0.0014 1496.9927 1497.0078 3.12E+07 2.77E+07 7.00E+06

6π/7 1495.9081 1495.7510 0.0446 1495.6762 1495.8511 1.68E+07 1.40E+07 3.27E+06

5π/7 1492.8245 1492.5322 0.0978 1492.3332 1492.7317 2.06E+07 1.59E+07 6.31E+06

4π/7 1488.3920 1487.9062 0.1687 1487.5663 1488.2565 2.88E+07 2.22E+07 1.38E+07

3π/7 1483.5045 1482.8054 0.2970 1482.3150 1483.3737 4.77E+07 3.47E+07 1.02E+07

2π/7 1479.1317 1478.1820 0.3715 1477.5521 1478.9709 1.03E+08 6.99E+07 3.20E+07

π/7 1476.1070 1474.9809 0.4215 1474.3147 1475.8021 4.05E+08 1.58E+08 1.09E+08

Frequency(MHz) Q

0

10

20

30

40

50

60

70

80

1474 1476 1478 1480 1482 1484 1486 1488 1490 1492 1494 1496 1498

Design Freq

Mode counts

6π/7

5π/7

4π/7

3π/7

2π/7

π/7 Mod

e co

un

ts (

ea

/100k

Hz)

Frequency Distribution of TM010 Modes (all 80 cavities)

π mode tuned to 1497.000±0.008MHz or scaled to 1497MHz

RF Frequency (MHz)

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

1.0E+11

Dipole Mode Impedance

(Ω/m)

Mode Nomenclature

C100-9 Higher Order Mode Survey at 2K at Cryomodule Test Facility

C100-9 Cavity1

C100-9 cavity2

C100-9 Cavity3

C100-9 Cavity4

C100-9 Cavity5

C100-9 Cavity6

C100-9 Cavity7

C100-9 Cavity8

Baseline impedance budget for 87.5uA injection current, 12GeV, BBU in lower energy pass =2.4×1010×Ω/m

Impedance for stretched goal 175uA injection current, 6GeV, BBU in lower energy pass =1×1010 Ω/m

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

1.0E+11

Dipole Mode Impedance

(Ω/m)

Mode Nomenclature

C100-9 Higher Order Mode Survey at 2K at CEBAF 1L25

C100-9 Cavity1

C100-9 cavity2

C100-9 Cavity3

C100-9 Cavity4

C100-9 Cavity5

C100-9 Cavity6

C100-9 Cavity7

C100-9 Cavity8

Baseline impedance budget for 87.5uA injection current, 12GeV, BBU in lower energy pass =2.4×1010 Ω/m

Impedance for stretched goal 175uA injection current, 6GeV, BBU in lower energy pass =1×1010 Ω/m

Cavity ID and mode

polarization

Frequency

(MHz)

Q Impedance

(Ω/m) CMTF CEBAF

C100-4-3 V 2887.5 2.68×106 9.21×105 5.95×109

C100-4-3 H 2887.8 2.93×106 1.62×106 1.06×1010

C100-6-2 V 2890.8 2.34×106 1.15×106 7.49 ×109

C100-9-3 H 2881.9 1.88×106 7.03×105 4.58×109

C100-9-6 V 2884 2.55×106 5.83×106 3.76×1010

C100-9-6 H 2884.2 6.02×106 2.05×106 1.34×1010

C100-10-2 V 2881.4 1.81×106 9.08×105 5.86×109

Abstract

Ten new C100 cryomodules have been fabricated and installed for the CEBAF 12GeV upgrade in the past few years. The high order modes (HOM) of

these modules need to be controlled to avoid beam breakup (BBU) instability. Over the last few years, we surveyed the HOM for all the 80 cavities of the

C100 modules in both the JLAB Cryomodule Test Facility (CMTF) and the CEBAF tunnel. Additional measures such as waveguide filters were applied to

reduce the quality factor of the out of spec modes. In addition, we also measured the same passband modes of all the cavities. We will present the HOM

survey methodology and results, as well as the same passband modes (SPM). The HOM mitigation measures and their effects will also be discussed.

High Impedance TM111 π/7 modes We found 7 modes (all are TM111 π/7

mode) exceeding the stretched goal

impedance budget during CMTF HOM

survey. 6 of them were brought under the

budget by waveguide filters when installed

in CEBAF. C100-9 Cavity 6 TM111 π/7

vertical mode exceeds baseline budget in

CEBAF, but can be brought down to

2×1010Ω/m (< baseline of 2.4×1010Ω/m)

with stub tuners at the HOM ports. The

cavity is left at this Q now for BBU beam

experiment.

C100 HOM survey methodology

• Survey dipole HOM (TE111, TM110,

TM111),

• 3*7(phase advance) *2(polarization) =

42 modes per cavity

• Measure crosstalk between HOM ports

of two neighboring cavities, need to id

the source cavity of each mode

• Use “Polfit” package to fit Q from S-

matrix traces. NWA direct measurement

only to confirm high Q modes.

• Measure in Cryomudule Test Facility

(CMTF) w/o waveguide filters. In

CEBAF tunnel, waveguide filters

installed on cavity 1, 8, and additional

cavities with high impedance modes

Waveguide HOM filter

• Help to damp those HOMs with

field tilted toward the FPC side.

• Originally designed by Surebeam

Corp (now defunct), manufactured

by CML Engineering Sales, Inc.

with slight modification

• Installed between FPC and klystron

for selected cavities

• Designed for full transmission at

1.5GHz, >10dB absorption at dipole

HOM passbands (1.7-3.0GHz),

>6dB absorption at 3-10GHz.