Control systems for 2K refrigerators at KEK-STF and KEK-cERL

Kota Nakanishi

AFAD2019 WG7 @ IUAC

Cryogenic systems in KEK

STF

cERL

CSC

BelleHe Lequefire for KEKB Superconducting cavity

2K cryogenic systems are worked at STF and cERL at KEK

COI

cERL• Compact Energy Recovery Linac : cERL

• cERL was established as a test machine of ERL.

• The current beam energy is about 18MeV.

(target : e.g. 3GeV)

• The 1.3GHz cavities made of niobium which should be operated at 2K were installed.

Cryomodulefor Injector

Cryomodulefor Main linac

STF• Superconducting RF Test Facility : STF

• STF is a test facility for International Linear Collider.

• STF have the accelerator for beam test and the equipment for surface treatment, vertical test and assembling etc.

• The accelerator have 1.3GHz cavities made of niobium which should be operated at 2K.

Schematic diagram of 2K refrigerator

Liquid He

5K shield

Helium gas (~10K)

To liquefierDewar

Helium gas (room temp)

To liquefier via pumps

4K pot

2K pot

Heat exchanger

Cavity

To pump

To pump

2K pot

cERL

STF

Our situation

• Waiting for the start of the ILC project. (no money!)

• We would like to continue developing and operating 2K refrigerators.

• KEK has two test accelerator facilities that adopt superconducting cavities operated at 2K.

• Helium liquefiers are very old. The control system should be updated.

• 2K refrigerators were designed by ourselves. The control system should be prepared.

→The control systems were developed by ourselves

Status of control room

KEKB cERL STF

He Leq 2K Ref 2K Ref

Local networks for control

PLC PLC PLC

Hitachi EX-8000

FL-net

EPICS

Connection using abandoned optical fiber.

Console of the KEKB refrigerator

Console of the Belle refrigerator

Console of the STF refrigeratorConsole of the cERL refrigerator

Sometimes, local operation is required.

The refrigerator parameters were shared as EPICS records.

Control room for KEKB refrigerator

MLC

He Leq

Central control is a promising way to reduce the operation costs.

Original system was broken.

2K refrigerators were developed at KEK.

Replaced

Old(<100Mbps)

Old(<100Mbps)

Very old(~10Mbps) Very old

(~10Mbps)

EPICS on PLC(The PLC is made by Yokogawa Electric Corporation.)

At that time, Linux CPU became available on PLC.

EPICS is commonly used to control accelerators at KEK.

EPICS can be operated on the Linux CPU.

Our control systems were to be constructed with EPICS running on the PLC.

The PLC systems could purchase with reduced running costs.

STF Liquefier STF 2K refrigerator

cERL (2Units)

STF (4Units (distributed))

Schematic diagram of Cryogenic system for cERL

Y.Kojima

Schematic diagram of Cryogenic system for STF

Y.Kojima

2K Cryogenic system for cERL

2K Cryogenic system for STF

cERL main linac

Cavity jacket(region of 2K)

Cooling pipe for 5K sheld

HOM absorber(region of 80K. Made of ferrite.) Requirment:“Cool slowly!”

It is difficult to cool from room temperature.

Request for control (from RT)

cERL main linac

region # of thermometer Cooling rate Temperature difference

80K 29points 3groups Less than 3K/h Up to 50K in each group

5K 11points 1group Less than 3K/h Up to 50K

2K 8points 2groups Less than 3K/h Up to 50K in each group

These restrictions are applied only above 150K at cERL.In other facilities, the cooling rate is also restricted. However, temperature range and target cooling rate were decided independently.

These data are prepared by cavity group. And they are shared as EPICS records.

Cooling rate control(1)

For example, when liqued nitrogen is supplied to a long tube, it is cooled down from the upstream.

How to monitor the temperature of all points and keep the maximum cooling rate?

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7

tem

per

atu

re[K

]

Elapsed time [h]

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7

Co

olin

g ra

te[K

/h]

Elapsed time [h]

Here, the measurement point of the control target should be switched.

Cooling rate control(2)

• At first, independent PI(D) loops corresponding to all thermometers were prepared and the required minimum valve opening was adopted.

→ An effective control point (bottleneck) is always one point, and in other loops the opening instruction value stays around 100%. Therefore, a large overshoot occurs when the effective control point changes.

𝑉 = 𝐾𝑃 𝑆 − 𝑃 + 𝐾𝐼න0

𝑡

𝑆 − 𝑃 𝑑𝑡

In order to perform stable control, it is necessary to set valid KP and KI value and the integral value should be valid value. To make the integral value appropriate, overshoot is needed.

The over-grown integrated value should be compensated by overshoot.

Cooling rate control(3)

• PI control is represented by (1).

• Since it is processed periodically by computer control, it can be written as (2).

• A recurrence formula is easily obtained(3). Required valve opening can be calculated from measured value and the current valve opening without using the accumulator for integrated value.

• In this way, it is possible to avoid overshooting at loop switching.

𝑉 = 𝐾𝑃 𝑆 − 𝑃 + 𝐾𝐼න0

𝑡

𝑆 − 𝑃 𝑑𝑡

𝑉𝑛 = 𝐾𝑃 𝑆 − 𝑃𝑛 + ∆𝑡𝐾𝐼

𝑖=0

𝑛

𝑆 − 𝑃𝑖

∆𝑉𝑛= −𝐾𝑃 𝑃𝑛 − 𝑃𝑛−1 + ∆𝑡𝐾𝐼 𝑆 − 𝑃𝑛

(1)

(2)

(3)

S:target value, P:measured valueKP KI: Parameters for PI control. They should set for each loops.

𝑉𝑛 = 𝑉𝑛−1 + ∆𝑉𝑛

result80K

2K

5K

The horizontal axis is time, the range is 5 days.The vertical axis is temperature, the range is 0 to 300 K.

Inlet temp

GUI panel for cERL cryogenic system

Child panel

Grandchild panelThis method can be used generally.

Five loops were prepared for almost all valves of the 2K

refrigerator.

Common situation (for example)

• Maintain the liquid level, however flow rate should

be kept below a certain value.

• Maintain the temperature of the equipment, however

the Dewar should not be empty.

• Maintain compressor outlet pressure, however do

not increase suction pressure.

Refrigeration capacity control

• Heat load>refrigeration capacisy→Store the gas in the buffer tank(62m3).

• Heat load<refrigeration capacisy→take the gas from the buffer tank.

• The buffer tank can be operated at 150～1100kPa. (Better from 200~900kPa)

• Since there are upper and lower limits, the heat load and refrigeration capacity must be balanced.

→The capacity was automatically adjusted according to the pressure of the buffer tank.

→The operation becomes easy. We becomes sensitive to the state of refrigerator. When refrigeration capacity is insufficient, it can be regained at night at cERL.

Pressure of the turbine inlet(refrigeration capacity)

Dewar level

Compressor bypass valve opening(Spare capacity of refrigerator)

He flow rate(heat load)

Pressure of buffer tank

Estimation of heat input due to pressure rise

• In order to estimate the cavity Q factor, heat load should be measured.

• When stopping the pump and stopping the supply of liquid helium, is the pressure increase rate proportional to heat load?

0 W 10 W 0 W0 W 20 W 30 W 40 W

kPa

Estimated static heat load is 28.3W

The pressure change is sufficient from 3.1 to 3.5 kPa.

It is only 3min!

I am going to measure RF loss, next time.

summary

• cERL and STF are accelerator facilities with superconducting cavities

operated at 2K.

• Control systems for Helium liquefier and 2K refrigerators for cERL and STF

are developed using EPICS by ourselves. And they works well.

• Functions assumed to be required to operate a large refrigerator are

installed and tested sequentially.