Crab Cavity Operation

K. Akaifor the Crab cavity group and RF group, KEK
Dec. 3, 2007
At the FY2007 KEKB Review Committee

Crab RF system_1•The required RF power to maintain thecrabbing voltage is given as:(for a simple case of β>>1, crab
phase=π/2, and loading angle =0)

We have chosen QL=1~2 x105
for a good compromise.
––This value is suitable for operating
the system with a possible error of
ΔX=1mm, andA high power source of 200 kW issufficient for conditioning the cavityup to 2 MV.

Typical parameters for the crab crossing.

Dependence of RF power on theloaded Q value and a horizontal beam
orbit for a beam current of 2 A.
Crab cavity operation (K. Akai)

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Crab RF system_2•

Two new RF stations, each for one crab cavity, wereconstructed in the D11 building.
– Two reused klystrons that have been tested up to 600 kW were set.
– The high-power system and most of the low-level RF system are similar to
those of the SC accelerating cavity stations.
– Conventional amplitude and phase feedback loops are used to control thecavity field and the klystron output.
– The resonant frequency of the cavity is controlled by the main tuner system
consisting of a motor and piezo element, which moves the coaxial beampipe in the longitudinal direction with respect to the cavity cell.
– The interlock system includes a quench detector, arc sensors at the input
coupler, vacuum pressure gauges, temperature sensors, etc.

Crab cavity operation (K. Akai)

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Crab RF system_3•

Special cares are taken for the coaxial beam pipe.
– A sub tuner is added to align the coax horizontally in order to minimize thecoupling of the crabbing mode between the cell and the coax.
– RF signals monitored at seven pickup ports located in the coax areconnected to the fast interlock system. It protects the ferrite damper at theend of the coax from abnormally large crabbing-mode power that can leakthrough the coax on the occasion of a discharge at the coax or the notch
filter.
(axial view)
Absorbingmaterial

Sub-tuner

Notch filter

Coaxial beam pipe

Absorbingmaterial

inner conductor
Cooling for
inner conductor

Monitor ports"Squashed cell"

Squashed Crab cavity for B-factories(K. Akai et al., Proc. B-factories, SLAC-400 p.181 (1992).)

Crab cavity operation (K. Akai)

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Crab cavities in the KEKB tunnel
HERLER

Crab cavity operation (K. Akai)

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Overview of crab cavity operation
High-current

1500

Low-current
collision tuning

High-current

Physics run
with CrabLow-current
collision tuning

HER beam current (mA)

1000
50002000

LER beam current (mA)

1000
50002.0

Crab voltage (MV)

1.5

Warm up to room temperature

1500

Summer shut down

1.00.5
0.0
Feb.

Mar.

Apr.

MayJun.

Jul.

Aug.

Crab cavity operation (K. Akai)

Sep.

Oct.

Nov.

6

Achieved parameters during beam operation
LER

HERunitBeam current (Crab ON)

1620

850mA

Beam current (Crab detuned)

1700

1350

mA

1.6→1.3→1.1

1.7~1.8MV

0.8~0.95

1.3~1.48

MV

HOM + LOM power

1212kW/cavity

Tuner phase stability (w/piezo)(w/o piezo)± 13± 15±1±3degree
degreeCrab phase stability± 0.1± 0.1

degreeAverage trip rate (before summer)(since October)

1.6
0.4

1.3
3.5

times/daytimes/dayCrab voltage (max)Crab voltage (operation)

Crab cavity operation (K. Akai)

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System Adjustment•

Crabbing voltage– Calibrated from klystron output power and the loaded Q value.
– The measurement to adjust the phase using beam (below) also gives anindependent calibration of the voltage.
– Both are in good agreement within a few percent.

•Crabbing phase– The reference phase is searched to minimize the beam orbit difference
between the crabbing on and off, so that the bunch center is not kicked by
the crabbing voltage.
– In the high-current operation, the phase is shifted by 10 degrees from thereference phase to cure the oscillation observed at high-current crabcollision (will be discussed later).

•Field center in the cavity– Searched by measuring the amplitude of the crabbing mode excited by abeam when the cavity was detuned. A local bump orbit was set to adjust thebeam orbit on the cavity axis.
Crab cavity operation (K. Akai)

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High beam current operation before summer•Beam current increase– The number of bunches are increased, keeping the bunch current constant.

•First trial in April– Not very successful:• Vacuum pressure degraded, and the trip rate increased to an unacceptable level.
• Another problem was temperature rise of the inner conductor of the coaxial beampipe.

– We suspended the trial,• detuned the cavities to scrub the beam pipe chambers using a high-current beam,and then warmed up to room temperature to remove the condensed gas on thesurface.

•Second trial in July
– Successfully performed.
• We stored 1.7 A (LER) and 1.35 A (HER) with Crab detuned.
• We stored 1.35 A (LER) and 0.7 A (HER) with Crab ON at that time.

– The beam current was not limited by the cavity performance.

Crab cavity operation (K. Akai)

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High current: improvements in June

Vacuum pressure in the cavity
before and after the warm up.

Temperature at the inner conductor of thecoax before and after the reinforcement of
the cooling power of the LER cavity.

Crab cavity operation (K. Akai)

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High current: HOM power
Y. Morita••

In each cavity, the HOM power
absorbed by the ferrite dampers
reached to 10 kW (LBP damper)and 2 kW (coax damper).
Comparison with calculation:––•HOM power absorbed by the ferrite dampers
at the large beam pipe (LBP) and the coaxial
beam pipe. The lines are calculated value
from the loss factor for σz = 6 mm.

The power absorbed by the HER
dampers fairly agrees with thecalculation.
In LER, a SiC damper is located at
the downstream of the LBP ferrite
damper, which absorbs 11 kW. The
sum of the power absorbed by theSiC and ferrite dampers agrees withthe calculation.

The operation of the HOM
dampers has been satisfactory
and trouble-free.

Crab cavity operation (K. Akai)

11Parasitic modes (HOM+LOM) spectrum
Y. Morita•Spectrum observation––•

Beam-induced RF modes wereobserved during a machine study with asingle bunch.
Signals from HOM-H port (LBP) mainly
observes the beam spectrum, while
signals from Co-H (coax) observes
beam-induced RF modes around thecoax.

The Q-factor–

Q of these modes were evaluated fromthe width. In particular, the mostdangerous lower-order mode (LOM) is
sufficiently damped to 140, consistent
with calculation and a benchmeasurement.

Crab cavity operation (K. Akai)

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Crabbing voltage•

HER crab cavity– The HER cavity has maintained high crabbing voltage: for most of the time
it was operating at the design voltage of 1.45 MV.
– The voltage was sometimes raised up to 1.7 MV to search for the best
crabbing angle.

•LER crab cavity– The LER cavity degraded from 1.6 MV to 1.3 MV shortly after theinstallation in the tunnel.
– After a heavy quench occurred on March 17, the LER cavity performancefurther degraded from 1.3 MV to 1.0 MV.
– After warm up to 80 K, it slightly recovered to 1.1 MV. However, no further
improvement has been achieved, even after the two times warm up to room
temperature in April and in the summer shut down.
– Fortunately, the necessary crab kick could be maintained by increasing theβx @crab from 40 to 80 m.

Crab cavity operation (K. Akai)

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Trips before summer
HER trips

LER trips
Number of crab cavity trips per day for fourmonth operation. They were operated in theperiod marked by the yellow area, whereas
they were detuned in the period marked bythe blue area. Blue circles show themaximum beam current in a day.

Crab cavity operation (K. Akai)

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Trips since October

Trips of HER crab /dayConditioning for an hourneeded to set the voltage.

1086–

4
20

Nov. 1

Nov. 2912•0.95 MV

86–

42

•0

Oct. 5

Nov. 1

Nov. 29

–

HER

LER

0.5

0.0

Oct. 5

Nov. 1

The LER trip rate was high when the voltage
was raised from 0.875 MV to 0.95 MV for threedays. Conditioning needed three times.
The HER trip rate seemed higher when thevoltage was raised from 1.45 to 1.48 MV.

The effect of accidental warm up of coax

1.5

1.0

The HER cavity had frequent trips at thebeginning of October. Roughly, the trip rate has
gradually decreased.
The average trip rate of HER cavity in the last
20 days is 2.7 times/day. The average trip rateof LER cavity in two months is 0.4 times/day.

Voltage dependence–102.0

Crab voltage (MV)

Trip rate–

1.48 MV

Oct. 5

Trips of LER crab /day

•Refrigerator trouble

12–

The coax part was warmed up to 80 K due torefrigerator trouble (Nov. 7). The HER cavity
had a higher trip rate after this event. The LER
cavity needed conditioning after a trip.
The temperature of the coax was raised in themaintenance conditioning. The next day LER
cavity needed conditioning due to MP at a low
voltage (Nov. 30).

Nov. 29

Crab cavity operation (K. Akai)

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Beam-loading correction•

In the high-current operation, thebeam-loading effect caused by ahorizontal displacement of thebeam orbit at the crab cavity wasobserved.
–•This situation was corrected with alocal bump orbit.
–•

The RF power into the LER cavity
decreased with an increase in the beam
current.

LER orbit was corrected by 0.7 mm inJune. Similarly, the HER beam orbit wascorrected by 0.8 mm in Oct. 15.

Once the local bump orbit is set, theorbit is stably kept by a local orbit
feedback system.

Crab cavity operation (K. Akai)

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Relations between feedbacks for beam•

Vertex feedback––•Horizontal feedback at IP (iBump-H)–•

The orbit in the crab cavity region is monitored and corrected (2~3 sec). It has not beenused since October.

Crab phase–•The orbit in the whole ring is corrected by the CCC system (10~20 sec).

Local orbit feedback at crab–•

Beam-beam kick is detected, and the relative horizontal displacement at the IP is corrected(~1 sec).

Continuous Closed Orbit Correction–•

Data from the Belle detector is used to detect the longitudinal shift of the collision point. It iscorrected by changing the LER RF phase (every 3 minutes).
Whether the LER crab phase needs to follow this change or not depends on the cause ofthe collision point shift. Anyway, in our system the LER crab does not follow this change.

Controlled with respect to the reference RF at Nikko by the low-level RF feedback .

Possible single kick generated by the relative phase shift between the crab
and the accelerating RF is corrected by iBump-H and CCC.
Crab cavity operation (K. Akai)

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Phase stability•••

Spectrum of pick up signal is consistent with phase detector data.
Phase fluctuation faster than 1 kHz is less than ± 0.01°, and slow fluctuationfrom ten to several hundreds of hertz is about ± 0.1°.
They are much less than the allowed phase error obtained from the beam-beamsimulations for the crabbing beams in KEKB.
According to b-b simulation by Ohmi-san, allowed phase error
for N-turn correlation is 0.1 √N (degree).
LER crab phase± 1 deg
HER crab phase

Span 200 kHz
Sideband peaks at 32kHz
and 64kHz.

Span 10 kHz

Span 500 Hz
Sideband peaks
at 32, 37, 46, 50, 100 Hz.

Spectrum around the crabbing mode measured at a pick up port of theLER crab cavity. Beam current was between 450 and 600 mA.
Crab cavity operation (K. Akai)

Phase detector signal. Beam current
was 385mA (HER) and 600 mA (LER).

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Oscillation of high-current crabbing beams•

Beam-beam kick is shaken.

A large-amplitude oscillation wasobserved in high-current crabcrossing operation in June.
– It caused unstable collision, short beam
life time and luminosity degradation.
– Crab amplitude and phase weremodulated at 540 Hz. Horizontal
oscillation of beams was also observedat the same frequency.
– None of the beam orbit feedbacksystems is responsible, since their time
constants are 1 to 20 sec, much slower
than the oscillation.
– The oscillation occurred when the LER
tuning phase migrated to the positiveside. This gave us a hint to understandthe phenomena.

Crab cavity operation (K. Akai)

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A remedy for the oscillation was foundLER tuning offset (degree)•– The oscillation occurred only withhigh-current colliding beams: itnever occurred with a single beam,even at a high current.
– Both beams oscillates coherently.
– The threshold for the oscillation isdependent on the crab phase andtuning phase (see left).

HER tuning offset:
φtun (HER)

oscillation
occurs(HEuntφERn (HutφR)

1=+°)=0nφtu

stable0°0°= -1)
R(HEObservations at a machine study•

Both crab phase (degree)

Dependence on the crab phase and tuning phase.
Beam current was 1150 mA (LER) and 620 mA (HER).

Cause and remedy– We concluded that the oscillation iscaused by beam loading on crab
cavities together with beam-beamforce at the IP (see, next slide).
– We found that it can be avoided byshifting the crabbing phase by +10°
and controlling the tuning offset
angle appropriately.

Crab cavity operation (K. Akai)

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Possible mechanism of the oscillation
Phase control
Kick bunch
Input rf phase

LER

Crab phase
Horizontal displacement at crab and IP

Input rf power

Crab voltage

Level control

Beam-loading on crab cavity
Kick by beam-beam force

Phase control
Input rf phase

Kick bunch

Crab phase

IPHER

Horizontal displacement at crab and IP
Input rf power
Level control

Crab voltage
Beam-loading on crab cavity
Crab cavity operation (K. Akai)

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Frequency tuner problems•LER tuner fluctuation– LER tuner has a large backlash behavior due to a mechanical problem. Asa result, the tuning phase fluctuates by about ± 15°.

•Piezo elements were broken many times.
– LER piezo was broken twice in June.
– HER piezo was broken three times in October.
– Since then, the tuners of both cavities have been operating without thepiezo. HER tuning phase fluctuation is increased from ± 1° to ± 3°.

•Is it OK without piezo?
– The crabbing phase is stably controlled by the low-level feedback system,
even when the tuning phase is largely fluctuated.
– However, the large fluctuation of the tuning phase is unfavorable for theoscillation at the high beam current. (As described, the oscillation thresholdis sensitive to not only the crab phase, but also the tuning phase.)

Crab cavity operation (K. Akai)

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Hardware issues to be solved•

Recover the LER voltage.
– Has not been recovered by warm up and/or any conditioning so far.
– Re-treatment of the cavity before upgrading of KEKB?

•Improve the frequency tuner.
– Large backlash of the LER tuner.
– Using a piezo element is desired for further increase of the beam current.

•HER cavity has higher trip rate after the warm up. It takes
several weeks to reduce the trip rate.
– Because of a higher voltage than LER?

Crab cavity operation (K. Akai)

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Summary_1•
•
•
•
•
•
•

High beam currents of 1.7 A (LER) and 1.35 A (HER) weresuccessfully stored with the crab cavities.
Beam instability caused by the crabbing mode, the LOM, or any
HOM has not been a problem.
HOM power reached 10 kW for the LBP and 2 kW for the coax.
The dampers have not caused any problem.
LER crab voltage has degraded to 1.1 MV. The necessary kickis maintained by increasing the βx @crab.
Trip rate during the physics run is about 3 times/day for the HER
cavity and 0.4 times/day for the LER cavity. Warm up waseffective to reduce the trip rate of LER cavity.
Although the tuning phase has a large fluctuation, the stability ofthe crabbing phase satisfies the requirement.
The oscillation observed with the high-current crabbing beams
was successfully avoided by shifting the crab phase by +10°.
Crab cavity operation (K. Akai)

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Summary_2• KEKB crab cavities have been really working
with high beam currents to conduct physics
run with the crab crossing !!!

Crab cavity operation (K. Akai)

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