Fire at Nextef
Tetsuo ABE
For Nextef teamThe 23rd KEKB Accelerator Review Committee Meeting2019-07-09

Missions of Nextef (New X-band Test Facility)
 X-band high-gradient accelerating structure collaborative development
under agreement between CERN and KEK (ICA-JP-0103)
 Development of efficient and cost-effective high-gradient normal
conducting accelerating structures as a US-Japan cooperative program(mainly with SLAC)Nextef: New X-band Test Facility (11.4 GHz)for testing Normal-Conducting High-Gradient Accelerating Structures
Bunker size: A > B(Since 2007)(Basic study usingsingle-cell structures)

In Shield-B

Shield-A will be
converted to S-band
test stand after 2019.

Operation time: ~4,000 hours / year

(Development ofmulti-cell prototypestructures)

In Shield-A

WhereisNextef ?

X-band Prototype Structures Tested at Nextef / Shield-A
T18 Quad  TD18T24TD24TD24R05TD24R05 T24THUTD24R05 Deflector TD24R05  TD26CC  DCS  T24-K1 (terminated by the fire)2009

T18_Disk_#2
2009

2015

2016

Deflector (SINAP)

2014-2015

TD24R05_K1

T24THU_#1 (Tsinghua)
2019
 KEK-madeT24-K1
2016-2017
 Meet the CLIC specification (< 3 x 10-7 breakdowns/pulse/m) for Eacc = 100 2013-2014
MV/m2018
2018

TD18_Quad

TD24R05_K2

2010

2011

TD18_Disk_#2

T24_Disk_#3

TD26CC_K1

SLAC-DCS

2011-2012

TD24_Disk_#4

TD24R05_#4
2012

TD24R05_#2

Fire on April 3rd, 2019

Before the fire

After the fireBABurnt modulator

Photo before the fireOperation History in April 3rd, 2019

Continuous Operation Started for High-Power Testing of a Newly
Installed Accelerating Structure(Final target RF power of this test: ~40 MW)

Routine inspection ofsite by Linac operator

Linac staff
passed-byRF power [MW]0.6
0.4
0.20.0Conditioning
startedDaytime manual operation

To 50Hz
operation

Evening automatic
operation
Automatic power
control to have thevacuum pressure
below a specified
thresholdFire

Just before the fire, History of the operation and INTLK messages
Continuous operation

Building fire alarm

3-minute operation

CCG

No particular until 21:38

 INTLK reset(21:38)

Charging
failure 2

Breaker offCharging
failure 1


INTLK
StatusCode(nobody in the site of Nextef)

 INTLK reset

SMOKE and CUBICLE TEMP activated(for the first time)Operation Panel with the final INTLK messages
At 21:55 on April 3rd, 2019

LV-ON
HV-OFFTRG-OFF
RF-OFF

Operation panel
manually opened andchecked ten minutes
later than the building
fire alarm.



SMOKETEMP

X-band Modulator

Nextef modulator configuration(PFN: Pulse Forming Network)

Inverter power supply unit

 PFN2 with 24 capacitors(12 capacitors used)

INV1
INV2

 PFN1 with 24 capacitors(12 capacitors used)
Thyratron2

Thyratron1

 Two klystrons A pulse transformer(1:24)

Modulator Drawing(PFN: Pulse Forming Network)

FansThyratron for PFN2Ceiling

PFN2

B

B

PFN1
Thyratron for PFN1

Top view (excluding the ceiling)

B-B cross-section
Made by NISSIN(NKD-06900)

INTLK sensors in the cubicle of the X-band modulator
Smoke sensor
located at the center of the ceiling

Temperature sensor (Threshold: 40 degC)
located at the edge of the ceiling

Long-term operation of the modulator

10,000 hours

2008

Replaced all plasticcase capacitors

Es (charging voltage)

Es=42kV (spec.)

Vk (cathode voltage)
Ik (cathode current)
35,000 hours at 50 Hz

2019
45,000 hours

The Nextef modulator had been operated below the spec.

What is the cause of the fire?
 No deterioration of the modulator performance observed The most likely cause is puncture of the plastic-case capacitors (although
there is no direct evidence).
 Fire occurred at other facilities using modulators with plastic-case capacitors.
 At Nextef, such puncture occurred twice.
PFN250 mm

Puncture of plastic-case capacitors in the Nextef modulator

2010-02-21

Oil splash

No fireParameters of the capacitors Safety-oriented
ItemUnitNextef(PFN2)

ATF(after improvements(*))

Injector Linac(S-band)

Sizemm110 x 140 x 250

150 x 130 x 380∅162 x 260

Volumecm33850

7410

5359

Material

Plastic

Plastic

Ceramics

Punctureprotection

None

None

Equipped

Material of electrode

AlAlAlMaterial of dielectric

Polypropylene

Polypropylene

Polypropylene + lowdensity paper

Type of insulating oil

Plant oil(flash point: 230 degC)

Plant oil(flash point: 230 degC)

Synthetic oil(flash point: 148 degC)

Case

Weightkgf

4.7~1010.0

Capacitance

nF

30.5

46

15.5

Charging voltage in op.

kV38444320 to 222521# of elements
Electrode gapµm363647

Potential gradient in op.

V/µm

52.8

48.9

43.6

Repetition rate in op.

Hz50

350

Designhour

3.9x105

1.0x105

1.5x105

Estimatehour――

2.8x1015

hour~35,000> 10,000(No failure)> 150,000(No failure)

Lifetime

Time to failurePuncture protection mechanism in the capacitors for Injector Linac(Cut model)

250 mm

Mechanically weak area
Capacitor elements

Ceramic case

Acuate Bellows

Ceramic-case capacitor at Injector LinacComparison

Plastic-case capacitor at Nextef=in capacitance250 mmSafe+Price :~3,000 USD~500 USD
Cost-effective x”

(*) Significant improvements made for plastic-case capacitors
used at ATF
 The winding method to form a capacitor from films was improved to reduce theappearance of wrinkles and make the electrode gap uniform.
 The potential gradient in operation was reduced to make the lifetime longer.
 75  48.9 V/µm
 lifetime ∝ (potential gradient)n (n >> 1)

 The fabrication process was improved to suppress foreign matter inclusion intothe insulting oil.

 Foreign matter inclusion was observed in the previous versions of plastic-case capacitors.
 Foreign matter inclusion is known to induce partial discharge.

 Various sizes were reviewed and re-optimized.
 etc.

In the Nextef modulator, we used the previous versions of plastic-case capacitors
fabricated before the abovementioned improvements were made.

Recovery

All the devices, equipments and instruments have been removed
for cleaning the experimental hall. (Taken at the end of Jun., 2019)(Taken in April., 2019)

BABA

Nextef system improvements under consideration
1.
2.
3.
4.
5.

No plastic case, use metallic or ceramic case for capacitors.
Reduce potential gradient in capacitors
Routine check of capacitors on size, capacitance, loss tangent, etc.
Install monitoring the modulator by TV cameras
The smoke detection in the cubicle of the modulator should be integrated in the fire
alarm system of the building.

6.
7.
8.
9.

If smoke is detected, stop cooling fans attached on the modulator ceiling.
Fire extinguisher should be installed inside the modulator, and activated automatically.
Location and sensitivity of smoke sensors should be optimized.
Temperature sensors inside the modulator should be optimized.

Nextef recovery protocol proposal after modulator-related INTLK activation
1. Check INTLK messages, recorded pulse shapes, and videos recorded byTV cameras
2. Input check marks to make sure all of the above items are checked
3. Temporally reset by physical switch located near the modulator• Remote reset is then impossible(**).
• By this temporal reset, we can resume the operation, and proceed to the next step.

4. Confirm normal operation• Locally check the situation by seeing the operation for half or a minute in a close
distance from the modulator• Then we can push the main reset button• Unless this main reset button is pushed within a defined period (e.g. 60 s), HV isforced to be OFF.
(**) At ATF, remote reset of modulator-related INTLK has been impossible from the beginning.

Summary Nextef is a high-gradient test facility for normal-conducting accelerating
structures.
 Operated since 2008, basically 24-hours, ~4,000 hours/year
 Achieves enough performance of X-band prototype structures to meet the CLICspecification.

 Fire in the modulator on April 3rd, 2019

 Severe damage to only Nextef but also Injector Linac
 The most likely cause is puncture of the plastic-case capacitors.
 Recovery process underway
 More safe capacitors will be used for the next Nextef modulator.
 Routine check of capacitors on size, capacitance, loss tangent, etc.

 Nextef control system will be reconstructed in a safety-oriented manner.
 Monitoring and INTLK will be stepped up.