KEKB Review February 10, 2003

RF System for theLongtudinal Instability(-1 Mode Damping at KEKB)KEKB RF GroupShin-ichi YOSHIMOTO
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KEKB Review February 10, 2003

Contents
Instability driven by the accelerating mode
How to cure the instability ?
Is the feedback necessary for KEKB ?
-1 mode damping system at KEKB
Beam test results in LER
Summary

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KEKB Review February 10, 2003

Instability driven by the accelerating mode
Frequency detuning

I0 sin f s Ê R ˆ
Pb tan f sDf =Á ˜ ¥ f rf =2Vc Ë Q ¯
4 pU

(1)

Growth rate of the coupled-bunch instabilities•2eaI0w rev
1+am =( Mm - M + m) Rm - ( Mm - m) Rm } {Â4 pE 0w s m=1
ts(2)

Rm+ = Re Z [( Mm - M + m)w rev + w s ]Rm- = Re Z [( Mm - m)w rev - w s ]3/19

KEKB Review February 10, 2003

How to cure the instability?
Completely different between the PEP-II and KEKBPEP-II
Reduce the impedance seen by the beam using a
combination of feedback loops.
KEKB
Reduce the detuning using high stored energy cavities
Super conducting cavity (high Vc operation)
ARES (Accelerator Resonantly coupled with Energy Storage)
U a Df a + U sDf sUa
Df a
Df p 2 ==Df a =Ua + Us
Ua + Us
104/19(QU a :U s = 1:9)

KEKB Review February 10, 2003

How to cure the instability? (cont'd)
SCC
BMMPILM@RP?LQDMPKCPGLNSRAMSNJCPE?RCT?JTC

JHc

E?RCT?JTCDPCOSCLAWRSLCPFMKB?KNCP
&SBP)

FMKB?KNCP(LBP)L`A?TGRW

GMLNSKN

L0QFGCJB05/190.51m

KEKB Review February 10, 2003

How to cure the instability? (cont'd)
ARES

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AC:

Accelerating Cavity

CC:
SC:

Coupling Cavity
Storage Cavity

HCC:
PMC:

Half-Cell Coupling Cavity
Parasitic Mode Coupler

HWG:
GBP:

HOM Wave-guide
Grooved Beam Pipe

CF:
SS:

Connecting Flange
Supporting StructureKEKB Review February 10, 2003

Is the feedback necessary for KEKB ?
Achieved and design rf-related parameters
LER

HER

Beam Energy [GeV]

3.5

8.0

Beam Current [A]

1.65 (2.6)

1.0 (1.1)

3.41

3.38

508.887508.887

Energy loss/turn [MeV]

1.643.48

Total RF voltage [MV]8 (10)

13 (17.9)

Synchrotron tune

-0.0249-0.0208

Cavity type

ARESARES

SCC

2010 (12)80.4 (0.5)

0.29 (0.5)

1.25 (1.5)

R/Q [Ω]151593Loaded Q (×104)

3.0

3.0

7.0

Momentum compaction (×10-4)
RF frequency [MHz]Number of Cavities
Cavity voltage [MV/cav]

Numbers in ( ) are design values.

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KEKB Review February 10, 2003

Is the feedback necessary for KEKB ? (cont'd)
KEKB
HER: No, LER: Yes (LER current > 2 A)
The -1 mode is not sufficiently suppressed only by
the ARES scheme.
Super KEKB
Yes, of course.
LER current :2.6 A-> 9.4 A
Detuning freq. : 20 kHz -> 71 kHz
Not only the -1 mode but also the -2, -3 mode

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Is the feedback necessary for KEKB ? (cont'd)
Growth rate of the -1 mode in LER (KEKB)

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KEKB Review February 10, 2003

Is the feedback necessary for KEKB ? (cont'd)
Growth rate of the CBI in LER (Super KEKB)

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KEKB Review February 10, 2003

-1 mode damping system at KEKB11/19

KEKB Review February 10, 2003

-1 mode damping system at KEKB (cont'd)
Measured transfer function of the SSB filter

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KEKB Review February 10, 2003

-1 mode damping system at KEKB (cont'd)
Measured transfer function of the digital filter

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KEKB Review February 10, 2003

Beam test results in LER
How to excite the instability ?
･ Intentionally detuning two ARES cavities (one RF station)･ The frequency detuning is about -60 kHz
Experimenral procedure
1. measure the ƒs by the spectrum analyzer.
2. adjust the center frequency of the filter to thesynchrotron upper sideband (-1 mode).
3. adjust the phase using the phase shifter by observing
the spectrum of -1 mode.

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Beam test results in LER (cont'd)Experimental setup (1)15/19

KEKB Review February 10, 2003

Beam test results in LER (cont'd)

SSB Filter

Digital Filter

Experimentalsetup
setup(2)(1)
Experimental
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KEKB Review February 10, 2003

Beam test results in LER (cont'd)-1 mode spectrum with/without feedback

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KEKB Review February 10, 2003

Beam test results in LER (cont'd)Time domain behavior with/without Feedback

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KEKB Review February 10, 2003

Summary
At KEKB, the coupled-bunch instability due to theaccelerating mode of the cavity has been suppressed by thehigh stored energy scheme (ARES and SCC). However, with
the increase of beam current, we need a longitudinal
feedback system to damp this type of instabilities in LER.
The -1 mode damper has been tested using the beam of theKEKB LER, and has proved to be effective in damping thecoupled-bunch instability of the -1 mode. This system has
been incorporated into the RF system and stably operated.
At Super KEKB, we will need to suppress not only the -1
mode but also the -2, -3 mode.

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