RF system for crab cavity
1.
2.
3.

Beam-loading issues
Tolerance for phase error
Construction status

K. Akai for KEKB-RF group
Mar. 22, 2006
Talk at KEKB review committee

Beam-loading on crab cavity
K. Akai et al, EPAC96, p.2118.

RF Power

Kick voltage
RF System for Crab Cavity (K. Akai)

Beam orbit offset (!x)
2

RF power, orbit error and Loaded-Q•!""(Beam current = 2A)
RF Power (kW)#""!

– For example, for Ib=2A, QL=2E5 and!x=1mm, Vbr is 0.2 MV.
– For stable operation, lower QL isdesired and a large orbit error should
be avoided.
– Low QL increases RF power.

•

QL=1~3 x105 is a good choice.

V=2MV (dx=0mm)$""
V=1.4MV (dx=1mm)
V=1.4MV (dx=0mm)%"""& ' ( )*

&%"

$#!

& ' ( )*'$#!

& ' ()*%"
Loaded-Q(%"

Required RF power as a function of QL.
Vc=1.4 MV for nominal operation, and
Vc=2 MV may be needed for cavity conditioning.
The case of an orbit error of 1mm is also shown.

Beam-induced voltage:
Vbr " QL Ib # x– RF power of 200 kW is enough forconditioning the cavity up to 2MV.
– Not too sensitive to orbit change:
tolerable to an orbit error of 0.5mm.
– The beam orbit will be kept stable byan orbit feedback by Masuzawa-san.

RF System for Crab Cavity (K. Akai)

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CBI due to the crabbing mode
Growth rate•

Instability due to the crabbing mode&(LER 2A)!"

K. Akai– Crab cavity is operated juston resonance.
– Growth rate is smaller than
the radiation damping rateeven at a tuning error of +/30 degrees.

Tuning angle%

&"

$!"

"
Q=2E5

#(with DRFB @G=3)

!"

Q=2E5
radiation damping

!

'&"!"

Q=1E6

"!"

'!""

'&"

"
Detuning frequency (kHz)&"!""Tuning angle (degree)

Growth rate (1/s)!"

Crabbing mode operation•

When parked away fromRF frequency– Should be kept away fromevery half revolution
frequency (horizontal tune ishalf integer).
– Still, growth rate can behigher than the radiation
damping. CBI should be
cured by bunch-by-bunch
feedback.

RF System for Crab Cavity (K. Akai)

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Phase Tolerance• RF phase error (timing error) gives rise to a horizontal
displacement at the IP.
*c"t # A$ x / % cross– Here, "cross is the half crossing angle.
– A is the ratio of allowed offset to horizontal beam size, #x*. The value A
should be determined from! the beam-beam point of view.
KEKB

Super-KEKB

LC

#x*

100µm

70µm

0.24µmA (assumed)

0.05?

~0.05?

~0.2?

"cross+/- 11mrad+/- 15mrad+/- 3.5mrad!t

1.5 ps

0.8 ps0.05 ps

0.27 deg (509MHz)
RF System for Crab Cavity (K. Akai)

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Effect of RF phase jitter on the beambeam performance•– For the correlation time of 10 turns, 5microns is allowed.
– For the correlation time of 1 turn,tolerance is only 1 micron.

•Luminosity

Tolerance is different according tocorrelation time (Ohmi and Tawada).

10 turns
1 turnHow fast is the correlation time inKEKB?
– Possible change, if any, will be slower
than the filling time of accelerating
cavities (2!" turns) or crab cavities (12turns).
– Fast change in several turns has not been
observed except modulation due to abortgap.
– Then, a displacement of 5 microns (phaseerror of 0.27 degree) is allowed.

Beam size
1 turnRF System for Crab Cavity (K. Akai)

10 turns

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Source of phase error and cure
• Slow phase drift due to temperature change, etc.
– CCC (continuous closed orbit correction system) can tell the amount of
single kick caused by the phase error of crab cavities (Koiso-san).
– It can be easily compensated using a low-level phase shifter.

• Fast phase errors– As shown, a displacement of 5 microns is allowed, corresponding to a phaseerror of 0.27 degree.
• Measured phase error in the present RF reference line is about 0.03 degree.
• Accuracy of cavity phase control will be measured in the horizontal test.
• Phase error caused by beam-induced voltage is small at a moderate orbit error.

– No clear problem is seen so far. More information is needed in operation.

• Abnormal phase change due to a trip of crab RF station– The amplitude and phase can not be controlled when a trip occurs.
– The beam must be aborted as fast as possible (can not survive anyway?).

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Transient due to abort gap•

Phase modulation in a bunch train

K. Akai et al, EPAC98, p.1749.

– depends on parameters such as RF
voltage, QL, beam current, gap length.

•Relative displacement is mostlycompensated,– since the same direction in both rings.

•How about residual?
– Residual is +/- 7 µm for head and tailbunches of a train.
– Constant for each bunch. This amountof residual is not disastrous for thecorrelation time of more than ten turns.
– Current ratio may need to be optimized
to minimize the residual.
1 turnRF System for Crab Cavity (K. Akai)

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Construction of crab RF stations• Two new RF stations have been built in D11 for the crabcavities.
– D11-E for the LER crab cavity.
– D11-F for the HER crab cavity (cavity is in D10 tunnel).
– Required power is 200 kW, much lower than the existing stations foraccelerating cavities.

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Layout of D11 klystron gallery

Existing four RF high power stations for SC cavities.

Two RF high power stations have been built for crab cavities.
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Klystron and power supply• Klystrons– Two reused klystrons have been conditioned up to 600 kW at D2 test
stand, enough for crab cavities.
– They have already been installed at D11.

• Power supply for klystrons– A spare power supply for one klystron was modified to drive twoklystrons.
– Moved to D11 and set up completed.
– System check applying HV to the klystrons was successfully done.

• Ready for high power RF operation.

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Two Toshiba klystrons
Power supply for the klystrons

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High power RF system• Construction of major part completed.
– Waveguides on the ground level have been built. Long path (~100m) of
waveguide connection in the tunnel from D11 to D10 is finished.
– 1MW circulators and dummy loads have been installed.
– Vapor cooling system for the klystrons is common to the existing D11 RFstations for SCC: no need for major change.
– Construction is finished except final connection of waveguides to cavities.

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RF System for Crab Cavity (K. Akai)

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Low-Level RF system• RF control hardware– Mostly similar to the low-level system for the SC accelerating cavities.
• Feedback loops, interlocks, monitor and data logging, etc.

– Tuning system• Piezo and stepping motor are used for frequency tuning as SCC.
• Horizontal position of coaxial beam pipe can be adjusted using another tuner
to avoid coupling of the crabbing mode into the HOM damper.

– Installation of RF control modules and cable connection have been done.
– System adjustment is being done now.

• Operation software– EPICS records have been made.
– Modification of application software in progress.

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Low-level RF control at D11-F for the HER crab cavity

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RF system for crab cavity will be similar.
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Z[PIEZOCONTROLLERPIEZO DRIVER

0 to 1600V>??/@ABCDE>??/@ABCDEFGHIJK/@ALMDEQLRSTUVWXYPHASE
DETECTOR
UP&DOWN
pulse

TUNER
CONTROLLER

TUNER DRIVER

5 phase pulses

NOPLMDE!"#$%&\]?

Temperature SW. (*+,)

Mechanical SW(Lower)

GEAR

Frequency tuning(longitudinal)

MOTOR

Mechanical SW(Upper)
Tuner position

Over load?

Load (analog)

Controller

PIEZO LOAD
MONITOR
Motor driverP-./
012345467

UP

LOW

PIEZO

Load cell!"#$%&''()
Load signal

89/:;<=MOTORGEAR

Alignment of coaxial pipe(Kabe-san)
RF System for Crab Cavity (K. Akai)

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Work remaining to do• Performance of the control system for crab cavity will bechecked in the horizontal test.
– Tuning control, feedback loops, quench detector, etc.

• High power RF operation without cavities will start at thebeginning of April.
– HV has already been applied to klystrons.

• The RF system will be commissioned with crab cavities inthe tunnel.

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Summary• RF system for the crab cavities has been investigated tomeet the requirements for KEKB.
• Construction of two new high power RF stations for thecrab cavities is completed. Final adjustment of RF controlsystem will be done in April.

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Appendix 1/4(answer to the question yesterday)•Dipole cut-off frequency in the coax is set as:– crabbing mode < cut-off frequency < any parasitic dipole mode in the cavity•

Parasitic modes are well extracted to HOM damper– All monopole modes including the Lower-Frequency mode (accelerating mode) areheavily damped.
– All dipole modes except the crabbing mode are also heavily damped.
– 1/4 wave length mode have been partly solved so far (Y. Morita-san talk).

•The effect of coax on the crabbing mode
– Provided that the coaxial pipe is aligned perfectly on axis, there exists dipole
coupling only. The crabbing mode attenuates in the coax at -60 dB/m. Thus it isconfined in the cavity cell. In this case no need for the notch filer.
– Misalignment of the coax can give rise to monopole coupling for a part of thestored energy. It propagates in the coax as a TEM mode without attenuation.
– Notch filter rejects the TEM component of the crabbing mode back to cavity cell.
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Appendix 2/4
K. Akai et al, Proc. B-factories, 1992•

Effect of misalignment of the coax–––•

Measurement of 1/3 scale copper cavity–•

A part of stored energy couples as monopole-like to thecoax. It propagates as a TEM wave.
Heat load in the cryostat may increase.
A large power goes to HOM damper at the end of coax.
From the reduction of loaded Q value, external-Q value isevaluated. For the displacement of +/- 1mm (3mm for 1/1scale), Qext is about 106.

Notch filter to solve the problem–––With appropriate tuning, 30dB reduction can be obtained.
Position of the notch filter has been optimized to further
reduce the coupling between coax and cell.
Then Qext is raised to the order of 109 even with a 3mm
displacement. Leaked power to the HOM damper will beless than 100 W. It is tolerable for the damper (OK witheven 1 kW). The increase of heat load in cryostat is also
tolerable with the powerful refrigerator of KEKB.
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Appendix 3/4•K. Akai et al, 1993PACHigh field cold test with 1/3 scale
Niobium cavity with coax.
– The coaxial pipe is attached.
– MP appeared at very low field, probably at thecoax. It was overcome by conditioning for 1hour.
– After the MP zone was passed, the field could
be successfully raised over 1.83 MV kickvoltage. It is above the design kick voltage ofKEKB, 1.4 MV.

•1/1 scale cavity with coax (Hosoyamasan).
– Similar experience and performance wasobtained.

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Summary of Appendix 4/4• With the rejection of notch filter, a displacement of innerconductor of coax by +/- 3mm causes no significant
problem.
• Field in the downstream of coax line and the notch filter issmall, if the coax is well aligned.
• Preliminary cold test showed that MP at the innerconductor could be overcome by conditioning for one
hour.

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