Commissioning of Crab RF System

K. Akai
KEKMar. 19, 2007
KEKB Review CommitteeConstruction and start up before beam commissioning•

Construction of two Crab RF stations (FY2004 ~):– Two reused klystrons, conditioned up to 600 kW, have been installed at D11-E andD11-F stations.
– A spare power supply, modified to drive two klystrons, was moved from D2 to D11.
– High power RF system including circulators, dummy loads, wave guides, andcooling system was completed.
– Low-level RF control system as well as operation software has been completed.

•Start up of Crab RF system (Dec. 2006 ~):––––––

System checking with high power RF source was done in Dec. 2006.
Wave guides were connected to crab cavities in Jan. 2007.
Conditioning couplers at room temperature was done in Jan. 2007.
Cavities were cooled down by a rate of -2K/hour. Reached to 4K on Feb. 4.
Adjustment of frequency tuners and alignment of coaxial pipe done on Feb. 5~8.
Final system adjustment and cavity conditioning started on Feb. 9.

Commisioning of Crab RF system (K. Akai)

2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.
Commisioning of Crab RF system (K. Akai)

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Two Toshiba klystrons
Power supply for the klystrons
Circulator and dummy load

Commisioning of Crab RF system (K. Akai)

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

Commisioning of Crab RF system (K. Akai)

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RF power, orbit error and Loaded-Q•

Beam-induced voltage:

Vbr " QL Ib # x!""(Beam current = 2A)
RF Power (kW)#""

–V=2MV (dx=0mm)!

$""
V=1.4MV (dx=1mm)––

V=1.4MV (dx=0mm)%""•QL=1~3 x105 is a good choice.
–"& ' ( )*&%"$#!

& ' ( )*'%"
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.

•For example, for Ib=2A, QL=2E5 and Δ
x=1mm, V br is 0.2 MV.
For stable operation, lower QL isdesired and a large orbit error should
be avoided.
Low QL increases RF power.
RF power of 200 kW is enough forconditioning the cavity up to 2 MV.
Not too sensitive to orbit change:
tolerable to an orbit error of 0.5mm.
The beam orbit will be kept stable byan orbit feedback

Measured at horizontal tests––

QL = 1.34 x 105 (HER)
QL = 2.07 x 105 (LER)

Commisioning of Crab RF system (K. Akai)

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Beam commissioning with crab cavity•Feb. 13~18– Daytime: conditioning crab cavities continued.
– Night time: beam operation with crab cavities detuned.
• Orbit center checked. Local bump made at crab cavities.

•First beam with crab ON (Feb. 19).
– Crab phase was adjusted by observing beam orbit change.
– Crab voltage was calibrated from beam orbit change.
– Tilt of bunches was observed by a streak camera.

••First crab collision (Feb. 21).
Continuous operation with crab ON (Feb. 22~).
– Collision tuning continues– Beam current increase with various fill patterns

Commisioning of Crab RF system (K. Akai)

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Operation summary of crab cavity (HER)
The last 30 days (16 Feb. ~ 17 Mar.)
Vacuum pressure (CCG: volt)

2/22~ Continuous operation w/ crab ON

Beam current (mA)

1389 bunch(3.5 sp.)

Daytime: conditioning
Night: beam w/o crab

Crab voltage (MV)

31 bunch(160 sp.)

51 bunch(98 sp.)51 bunch(49 sp.x8 train)

Commisioning of Crab RF system (K. Akai)

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Operation summary of crab cavity (LER)
The last 30 days (16 Feb. ~ 17 Mar.)
Vacuum pressure (CCG: volt)

2/22~ Continuous operation w/ crab ON

Beam current (mA)

1389 bunch(3.5 sp.)

Daytime: conditioning
Night: beam w/o crab

Crab voltage (MV)

31 bunch(160 sp.)

51 bunch(98 sp.)51 bunch(49 sp.x8 train)

Commisioning of Crab RF system (K. Akai)

9Searching Field Center in Crab Cavity
LER

HER

Inner

••

5kHz

5kHz

40kHz

40kHz

Outer

Inner

Outer

Field center was searched by measuring the crabbing mode amplitude excitedby a beam with the crab cavity detuned. Two measurements with different
detuning frequencies agreed to each other.
A local bump orbit was set to make the beam aligned on the field center.
Commisioning of Crab RF system (K. Akai)

10Adjustment of crab voltage and phase•

Calibration of crab voltage– Usual calibration method by using external Q values of pickup port
measured at vertical test could not be applied, since the coupling waschanged after the vertical test.
– Crab voltage was adjusted from klystron output power and loaded-Q valueof cavities, taking account of power loss from klystron to cavity of 12%(HER) and 5% (LER).

•Comparison with beam measurement– Kick voltage was calculated from beam orbit by changing the crab phase.
– The RF calibration and beam measurement are in very good agreement:• HER 1.0 MV (RF) ⇔ 0.992 MV (beam)• LER 1.0 MV (RF) ⇔ 0.987 MV (beam)•Phase of crab RF stations– Adjusted so as to minimize the COD generated by the crab kick.
– Fine adjustment is done anytime, if needed.
– Phase stability will be discussed later.
Commisioning of Crab RF system (K. Akai)

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LER tuning mechanism problem
•

––

•

Distribution of tuner phase for Vc > 0.3 MV

Problem of LER frequency tuner:
Suffers from something like mechanical backlash.
This results in a large fluctuation of tuner phase,even tuning feedback system is working.
Although some efforts were effective to reduce it, westill have a tuning error of about ±15 degree.

HERConsequences:–––

Cavity phase is successfully stabilized to a required
level by a phase lock loop (next two slides).
Cavity input power inevitably fluctuates to keep thecavity voltage constant.
Some of the LER trips seem associated with thisfluctuation (discussed later).

manualLER

Piezo onLER

LER

Commisioning of Crab RF system (K. Akai)

12Phase stability (histogram of phase detector signal)
HER

LER
0.007

0.046

Linear scale

Log scale

Distribution of cavity phase (cavity feedback loops on)
Commisioning of Crab RF system (K. Akai)

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Phase stability (cavity pick up signal)

Span 3MHz
No significant sidebands seen.

Span 200 kHz
Sideband peaks at 32kHz, 64kHz.
Phase error < ± 0.01 deg (fast)

LER

Span 200 Hz
Sideband peaks
at 32, 37, 46, 50, 100 Hz.
Phase error < ± 0.07 deg (slow)

According to b-b simulation by Ohmi-san, allowed phase error
for N-turn correlation is 0.1×√N (degree).

••Spectrum of pick up signal is consistent with phase detector data.
The measured phase error is much smaller than the allowed values
given by beam-beam simulation.
Commisioning of Crab RF system (K. Akai)

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Bunch fill pattern•

Target fill pattern for the coming medium beam current operation is decided
by commissioning group as:––

Number of bunches is 201.
Bucket spacing is 24 and 25 (repeated for every 49 buckets).
•–

•

––
–

Changed to 51 bunches (98 bucket sp. x 1 train) from the initial 31 bunches (160 bucketsp.) operation.
Next changed to 51 bunches (49 bucket sp. x 8 trains).
Maybe one months later we will gradually increase the number of bunches until 101 (49bucket sp. x 1 train).
Then will gradually increase the number of bunches until 201 (24/25 bucket sp. x 1 train).

This approach has advantage from the view point of HOM resonance.
–•Total beam current will be 300mA (LER) and 200mA (HER).

To smoothly reach this target, we proceed small current operation as:–•

49 bucket spacing enables us to use 2 bunch injection scheme.

No new major HOM resonance appear at each step.

In parallel, we have also checked performance with high total beam current
by storing 1389 bunches (3.5 bucket sp.) up to 200mA.

Commisioning of Crab RF system (K. Akai)

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51 bunches, 98 bucket spacing

51 bunches, 49 bucket spacing x 8 trains

Commisioning of Crab RF system (K. Akai)

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Crab voltage and beam current
HER
Crab Voltage

Horizontal test

1.8 MV

1.93 MV

Conditioning after installation

1.7 MV

1.6 MV→1.3 MV (2/20)→1.0 MV (3/17)

1.3 ~ 1.4 MV

0.9 ~ 1.0 MV

1389 bunches (3.5 bucket sp.)

174 mA

200 mA

31 bunches (160 bucket sp.)

32 mA

40 mA

51 bunches (98 bucket sp.)

47 mA

65 mA

51 bunches (49 bucket sp. x8 trains)

50 mA

75 mA

Operating (most of time)
Beam Current

LER

Commisioning of Crab RF system (K. Akai)

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Measured HOM power and calculation
Measured power from temperature rise ofHOM damper cooling water

HER (51 bunches)•Loss factor @σz=6mm (Y. Morita)–––•

Cavity+ taper = 0.85 V/pC
Damper @LBP = 0.25 V/pC
Total = 1.1 V/pC

Calculated HOM power from Lossfactor is–––Phom = kloss x T0 x I2/Nbkloss =1.1 V/pC, T=10μs
I=50 mA, Nb=51 → kloss= 540 W

Total (LBP+SBP) ~ 500 W @50mA••

Measured power and calculation are in fairly good agreement,
considering poor accuracy of temperature measurement for small
beam current.
Further study will be done with higher beam current.
Commisioning of Crab RF system (K. Akai)

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HOM power for various bunch fill patterns
LER

HER
1389 bunches(3.5 bucket sp.)

HER

LER
51 bunches(49 bucket sp.
x 8 trains)

HER

LER
51 bunches(98 bucket sp.)Commisioning of Crab RF system (K. Akai)

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Lower frequency mode (TM010)•••

TM010 (accelerating mode) is heavily
damped by the coaxial beam pipe.
Nevertheless, it can be resonantly built-up
when hit by one of beam spectrum.
We observed vacuum pressure rise at
coaxial beam pipe for a bucket spacing of
160 or 98, but no significant pressure risefor 49 spacing.
The resonance is avoided for high beam
current operation with a bucket spacing of49 or less.

# bunches (bucket spacing)#201 (24/25 sp.)#101 (49 sp.), also #51 (49 sp. 8 train)#51 (98 sp.)
#31 (160 sp.)none
405.03405.03403.93none
none
410.23407.11410.29415.42415.42415.42413.47TM010 (Q=160)
Commisioning of Crab RF system (K. Akai)

none
none
420.61416.65Frequency (MHz)20

Cavity trips during beam operation•

The number in the table shows beamaborts associated with crab cavity trips.
–––•DateHER

LER

March 424526612732

8129121041

11041212132414221526162

61715

Total in 14 days

2448

Trips / day

1.7

3.4

The average trip rate is 1.7/day for HER
and 3.4/day for LER.
–•

In most cases beam is automatically aborted byinterlocks.
In other cases the beam survives. But the beam
is manually aborted for safety before recovering
the crab cavity.
Down during the recovery process after theevent of abort is not included. Sometimes weneed conditioning to recover it.

Statistics of crab cavity trips
for the last 2 weeks (Mar. 4-17)

In most cases the trip is associated with vacuum
pressure rise and/or quench. It seems that thecavity has accumulated a large amount of gas on
the surface during the initial operation.

So far the trip frequency is acceptable tobeam operation.
–Warming up the cavities to 80 K may further
reduce the trip frequency.

Commisioning of Crab RF system (K. Akai)

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Problems of LER cavity and cures to be taken•

Degradation of crab voltage– Maximum voltage of LER cavity has degraded from 1.6 to 1.3MV, andrecently to 1.0MV.
– HER cavity once experienced similar degradation at the first horizontal test.
It was drastically recovered after warming up to 80 K.
– We hope that the LER performance will also be recovered by warming up to80 K. It needs one and a half day shut down.

•More trips than HER cavity– The warming up to 80 K is also expected to reduce the trip rate.

•Less effective cooling of coaxial pipe
– Due to longer return helium gas pipes the cooling of coaxial pipe is muchless effective compared to HER cavity. It will be improved by making abypass at the return pipes. This can be done while warming up cavities.

•Frequency tuner– As already discussed, tuning phase has a large fluctuation. Although thecavity field is sufficiently stabilized, it is desired to reduce it, if possible.
Reinforce the support rod is planned while the warming up cavities.

Commisioning of Crab RF system (K. Akai)

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Summary•••

Two RF stations for crab cavities have been constructed in D11 intime for the crab cavity installation.
The commissioning operation of crab RF system has beensuccessfully performed.
The crab cavities and RF system have been working satisfactory.
– Cavities with peripheral components as well as RF system have been working,as expected.
– The phase stability well meets the requirement from b-b simulation.
– Trip frequency is acceptable.

•The cavity performance is excellent, except the degradation ofLER cavity voltage.
– In order to recover the LER cavity performance, warming up to 80 K is planned
probably this week.

•We look forward to higher beam current operation.

Commisioning of Crab RF system (K. Akai)

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