Recent Status of Crab Cavity in KEKB

Contents•Brief History•First Horizontal Test for HER Crab Cavity•Reforming the various components•Final Horizontal Tests for Both Crab Cavities•Summary

03/19/2007

1Y.YamamotoWe finally succeeded in the first assembly of HER Crab Cavity on Apr/21/2006!
It was moved for the horizontal test on 26/Apr.

experimental site at D10 area2

①

Change of Horizontal Test Area②③④

The pit was very narrow for the Crab Cavity!

3

Various Components in Crab Cavity
CCG @Coaxial Beam PipeIon Pump

CryostatHelium JacketN2 Shield

CCG @Input Coupler
Input Coupler

Tuner RodCCG @Large Beam Pipe
SUS Bellows

HOM Damper

HOM Damper

Gate Valve

Gate Valve
Coaxial Beam Pipe
Notch Filter
Coaxial Beam Pipe Side

Magnetic Shield
CCG @Cryostat
Stub Support
Crab Cavity

Helium Jacket for Coaxial Beam Pipe

Top View

Ion Pump
Large Beam Pipe Side

Pick-up Probe4

The History of the Horizontal Tests
for Both Crab Cavities
Exp.#

PeriodHER#1

Jun/2006

HER#2

Nov/2006

LER#1

Dec/2006

Reform!

During the first horizontal test, it found that the cavity had a problem
which the tuner performance was not good.
Therefore, we needed to reform the mechanical structure of the coaxial beam pipe.
(It must be smoothly driven by the tuner.)

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Result of the First Horizontal Test
for HER Crab Cavity①

Input coupler conditioning
with cavity detuned

90kW
Temporary Warm-Up

1.67MVThe Field was dropped.
1.4MV achieved!

Dramatic recovery!

Although we achieved the cavity voltage of 1.67MV
above the operation one of 1.4MV,
we could not go to the installation to the tunnel,
because the tuner performance was not good andthe coaxial beam pipe was not driven smoothly.
We could not set the cavity frequency
to the operation one of 508.8875MHz,
because the stroke of the bellows was short.
And, we could not set the coaxial beam pipe to the center.
It remained off-center during the first horizontal test.

6cavity quenched!

Result of the First Horizontal Test
for HER Crab Cavity②
The vacuum pressure of the coaxial beam pipe
became higher during the conditioning,as it remained largely off-center at first.
And then, when we set it as center as possible,
the vacuum pressure became lower.

Adjusting the coaxial beam pipe

7Bellows
Material ：Cu⇒ SUS316L
Thickness：0.4mm ⇒ 0.2mm
Stroke：±3mm ⇒ ±10mm
After shaping…

Reform①
The stroke was short,as the copper bellows was rigid.
After the copper plating…

8

Reform②

Movable Base
Base①

Base②

Completion
Combining

9This movable base enables the coaxial beam pipe to drive smoothly.

Room Temperature Baking(HOM Damper, Ion Pump, Cavity, Coaxial Beam Pipe) Tuner Drive Test (Motor & Piezo)
 Input Coupler Conditioning

10

Input Coupler Conditioning
High-Power RF test using Input Coupler only

The achieved power was above 200kW for HER, but we stopped at 100kW for LER.
Because we had no time!
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During Cool-Down & Warm-Up
 Monitoring Frequency and QL
 Monitoring Shrinkage of the Cavity
 Cool-Down Rate Adjustment(for slow cooling)
We were afraid of the vacuum leakage from the cavity!
Therefore, we decided the cool-down rate of 2K/hour.

12Time Profile of Cool-Down
HER

LER

Cavity Temperature

1.8K/hour

2.1K/hour

Cavity VacuumAlthough HER Cavity vacuum became good, LER became higher.
As we were afraid of the vacuum leakage, we did the leak check many times.
After the cavity temperature reached 50K, the vacuum pressure was lower.
13

Correlation between Vacuum and Temperatureduring Cool-Down

Although the behavior of the vacuum pressure was much different
between both cavities during cool-down,
the pressure level around 4K was almost same.

14

Time Profile of Warm-Up
HER

LER

Cavity Temperature

Cavity Vacuum

The vacuum pressure for LER Crab Cavity became higher during warm-up.
After the cavity temperature reached the room temperature,
the pressure level was almost same as before cool-down.
15

Correlation between Vacuum and Temperatureduring Warm-Up

The vacuum pressure was different by 3 orders between both cavities around 50K!
The pressure level of the coaxial beam pipe for LER was always higher at 4K.
We think that this part was the outgas source.
16

Frequency Trend during Cool-Down & Warm-Up
HER

LER

400kHz500kHzWe think that the difference of the change of the frequency between both cavities
is due to the mechanical property of the cavity.
The change of the frequency was 600kHz in the case of the cavity only
without the coaxial beam pipe.
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After the cool-down, we set the frequency to 508.8875MHz by another way.

4K
 Adjustment of the Cavity Frequency
 Adjustment of the Coaxial Beam Pipe
 Tuner Drive Test (Motor & Piezo) Cavity/Coupler/Coaxial Beam Pipe Conditioning QL Measurement (using high-power)
 Tuner Phase Check Tuner Feedback Check Eigen Oscillation Check Q0 Measurement
 Static Loss Measurement
 HOM Measurement (using low-power) Checking the Radiation Level18

During the High-Power Conditioning
LERHER
Temporary Warm-Up

120kW
55kW

The field was not increased!

1.80MV

1.93MV

Dramatic recovery!

19
We decided the temporary warm-up of the HER Crab cavity again, as the field was not increased.

Vacuum Pressure during the Conditioning
HER

LER

20

The pressure level of the coaxial beam pipe for LER was always higher at 4K.

QL Measurement
We usually adopt the result of the decay time method using the transmitted power,
because the readout value of the power meter was fluctuated in the band width method.

HERConditionPin[kW] MethodLER

QL (Jun/2006)

QL (Nov/2006)

QL (Dec/2006)

High power20

Decay time

1.66x105

1.34x105

2.07x105High power

10

Band width

1.59x105

1.64x105

1.86x105

Simulation

HFSS (ver. 9.2)

1.6x105

The difference between two measurements for HER Crab Cavity was above 20%.
We think that this is the systematic error of the alignment in the assembly.
Because the same cavity and the same input coupler were used in these tests.
These measured values are within ±20% compared to the result of the simulation.
21

Q0 Measurement
We used the heater compensation method for Q0 measurement in the horizontal test.
This method requires the fine tuning of the helium level.

LER
HER

In the stage of the vertical test, LER Crab Cavity was much better than HER.
LER Crab Cavity was also better in the horizontal test.
And then we could easily achieve the cavity voltage of 1.93MV.
The cavity performance was comparable to the vertical test.
The static loss of HER and LER Crab Cavity were 32.3W and 25.7W, respectively.
22

Comparison of Tuner Phase between Both Cavities

Although HER Tuner performance had no problem,
LER Tuner performance was not good.
The behavior was unstable and it had the hysteresis.
We were afraid that this behavior prevent the cavity
from operating stably.
And then, we tested the tuner drive of LER Crabso many times by the various ways.
But unfortunately, we didn’t understand the reason.
On 28/Dec, we gave up it and finished the test.
After the beam commissioning,
it found that this behavior was not serious.
Now the both cavities are very stablebeyond our expectations.
We are very lucky!

The expansion of the tail was twice HER.

23Summary The both cavities achieved above the operation voltage of 1.4MV.

(HER : 1.80MV, LER : 1.93MV)
 The temporary warm-up was effective to recover the cavity voltagefor HER Crab Cavity.
 The vacuum pressure of the coaxial beam pipe of LER Crab Cavity
became higher at 4K.
 The measured QL values were consistent within ±20%
compared to the simulation.
 The cavity performance (Q0) in the horizontal test wascomparable to the vertical test.
 The static loss of the both cavities was around 30W.
 We didn’t understand the difference of the tuner performancebetween the both cavities.
 The refrigerator system had no problem in these measurements.
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