Helium refrigerator for SRF

K.Nakanishi, K.Hara, T.Honma, S.Kessoku, H.Nakai, H.Shimizu(refrigerator group)Superconductivity in Accelerator
In the 1980s, superconducting devices for accelerators were put topractical use. Our refrigerator has left a big milestone as follows:• 1985 : Started to operate Superconducting magnet in Tevatron.
• 1988 : Started to operate Superconducting cavities in TRISTAN at KEK.
• 2011 : Stoped to operate Tevatron.

• 2022 : Refrigerator for TRISTAN is still working in SuperKEKB
We are proud of it, but it also means that our refrigerator is very old.

History of our refrigerator
TRISTAN without Superconducting cavities (SCC) 1986~
TRISTAN with SCC

1988~1995

KEKB with SCC

1998~

KEKB with SCC and Crab cavities

2007~2010

SuperKEKB with SCC

2016~

3

Cryogenic system forSuperconducting Cavities.

1988
4kW @ 4.4K
(Design)Compressor(C5,C6) wereadded.

Supercritical
turbine expander(T3) was added.

1989
6.5kW @ 4.4K
(Design)8.1kW @ 4.4K
(Achieved)

TRISTAN (1988~1995)

The heat load at 4.4K was quite large.
Components
Cryostat

Heat loads
22.8 W/cryostat x 16

364.8 W

Transfer Lines (380m)

412.4 W

Cold Valves & Joints

147 W

RF Loss
Total

90 W/cavity x 32

If2880 W

150 W/cavity

3804.2 W

5700 W

Compensation heater power is included

The RF loss was not constant.
Beam Injection (~30W/cavity)
2minites
Top energy operation(90~120W/cavity)

KEKB (1998~2010)
SuperKEKB (2016~)
Components
Cryostat

Heat loads
30 W/cryostat x 8

240 W

Transfer Lines (380m)

412.4 W

Cold Valves & Joints

147 W

RF Loss

100 W/cavity x 8

Total

800 W
~1600 W

Compensation heater power is included• The heat load was smaller than TRISTAN’s.
• The RF loss is stable during beam operation.
• The compensation heaters are even used.
• The refrigerator is powerful enough.

Trouble
The total operation time
TRISTAN: 38,000 hours
KEKB: 62,000 hours
SuperKEKB: 27,000~ hours
Year:
Hour of inhibition/ hour of operation

Upgrading

KEKB

SuperKEKB

2016: 0 / 3343
2017: 0 / 0
2018: 0.1 / 4362
2019: 39.4/ 5334
2020: 4 / 5099
2021: 0 / 5292
2022: 3.7 / 3246

In recent years, the number of troubles has been increasing.
However, by preparing spare parts and/or using alternative functions, the operatingrate of the refrigerator is maintained still high.
(Hour of inhibition is not hour of repair. If it is in operation even during repair, it is included in the operation time.)

Maintenance and Updating• Maintenance are carried out in accordance with
the High Pressure Gas Safety Act.
Required inspection period
Item

Maintenance cycle

Whole system

1 yearCompressor

2 yearPressure gauge

2 year

Thermometer2 yearCold evaporator

3 yearothers1 yearitem

Open inspection cycle

Recovery compressor

10 years or 3000 hours operation

Circulating compressor

10 years or 3000 hours operationothers

exemptionMaintenance and Updating• The cryogenic systems were inspected every year by theprefectural government. The legal inspection is a good guideline formaintenance.
• Check and adjust the actuators of the controlled valves, pressuregauges and thermometers in the inspection cycle. And some onethat can not be adjusted is replaced.
• It is recommended to replace the input / output module of thecontrol system every 5 ~ 7 years. (However, the update cycle isoften extended.)• Compressor open inspections are performed at the manufacturer's
recommended operation hours or at least every 10 years.
• Since the manufacturer has withdrawn from the refrigerator
business, any spare parts are not distributed.→Three spareturbines are kept.

Control system• Initially a control system EX-1000 which made by
Hitachi was adopted in 1988.
• The control system was updated to EX-7000 in 2002.
• EX-8000 was adopted in 2012.

Transfer Line
Transfer Line developed for TRISTANThis transfer line adopted for connection
between the liquid helium dewar and theaccelerator tunnel.
The heat load is about 1W/m.

Transfer Line developed for KEKBThis transfer line adopted for connection
between the liquid helium dewar and theD10 test stand.
The heat load is about 0.05W/m.
Adopted in 2K system(cERL/STF) at KEK.

2K refrigerator
developed at KEKLiquid HeDewar

cERLTo pumpHelium gas (~10K)
To liquefier

2Kpot

5K shield

4K potHelium gas (room temp)

To liquefier via pumps
Heat exchanger

2K potSTFCavity

To pumpAuto Aging program
The aging operation of the Superconducting
cavity in SuperKEKB is to pass through thespecified voltage range six times.
I made the auto aging program.

Cavity group membersimproved the GUI.

Auto Aging program
The aging procedure⓪ RF system turns on by LLRF.
① When the vacuum pressure is lowenough, Vacc rises at maximumrate.
② When the vacuum pressure
approaches the target, Vacc rises
slowly by PI control.
③ When the vacuum pressure exceeds the alarm level, the cavity voltage is lowered insteps and fixed for 2 seconds.
④ Interlock may stop RF.
⑤ Decrease the cavity voltage if the vacuum pressure exceeds the target value evenafter the cavity voltage reaches the target value.
⑥ If the cavity voltage can be maintained at the target value for the specified time, theaging pattern of voltage rise will be completed.

Compensation heater
A member of the Cavity Group told me thatthe high Vacc operation would deplete theliquefied helium, so the following rules
must be followed:① It should pass through quickly.
② High voltage operation should belimited to no more than two cavities atthe same time.
→Controlled by Auto aging program.

(Cavity group gave us the Q-Eacc data.)

D11D
250.0
200.0

Loss[W]

However, Why did the refrigerator detect thatthe aging operation for cavities was done. The
compensation heater doesn’t work well.
We found that the RF loss on the cavity wall was
not properly evaluated.
Since the data was updated, it will be improved
from the next operation.

150.0
100.0
50.0
0.0
0.0

2.0

4.0

6.0

8.0

10.0

Eacc[MV/m]

12.0

14.0

16.0

Summary• In SuperKEKB, superconducting cavities are operated using
a cryogenic system constructed for TRISTAN in 1988.
• The refrigerator is powerful enough to cool thesuperconducting cavities for SuperKEKB.
• Periodic maintenance is done.
• Some spare components are prepared.
• The cryogenic system are very old. However, it is working
very stably.
• We are developing some instrument for current and future
superconducting cavities.

Cool down
ProcedureTemperature controlled helium gas issupplied from the refrigerator to cool
the cavities.
Cooling is started by using helium gas
cooled by liquefied nitrogen.
When the cavity temperature reaches
150 K, the turbines were stated.
When the cavity temperature reaches
40 K, liquid helium was supplied.

• To reduce deformation due to temperature difference and to suppress vacuum leakageand alignment error, all of cavities cooled down slowly (~2.5K/h) until 40K.

TRISTAN (1988~1995)• 32GeV – 32GeV electron-positron collider.
• 8 cryostats were installed. (1988)• 8 cryostats were added. (1989)

16 cryostat
x 2 cavities/cryostat
x 5 cells/cavity=160 cells

KEKB (1998~2010)• 8GeV – 3.5GeV electron-positron collider.
• 8 cryostats for SCC were adopted for HER.
• 2 cryostats for crab cavity were adopted for
HER and LER (1 by 1). (2007~2010)

8 + 2 cells

SuperKEKB (2016~)• 7GeV – 4GeV electron-positron collider.
• 8 cryostats for SCC were adopted for HER.
• Crab cavities were removed

From the viewpoint of the refrigerator,
there is almost no change from KEKB.

Crab Cavity
Two crab cavities wereadopted in KEKB from2007
to 2010The coaxial coupler faced inthe cavity. It should besuperconductor.
To cool it, 5 g/s of liquid Hewas spend. (~100W)

In SuperKEKB, it wasdecided not to be used dueto the strategy change.
They were removed.