High-gradient test at Nextef

Shuji MatsumotoFor KEK Nextef Group

2010.5.3

2010/5/3 144th X-band structure collaboration Meeting, CERN

Contents Nextef, KEK’s X-band 100MW-class structure test stand, started its operation in 2007. Since then, we have been testing a series of CLIC prototype structures here, T18_Disk#2(2008), TD18Quad#5 (2009) and currently we run the facility for TD18_Disk#2.

I am going to report on the testing of 1) TD18_Quad#52) TD18_Disk#2

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T18 parameters for 100MV/m in average gradientRiccardo info on 090202 TD18_Quad 55.5MW 100MV/m no-loaded with Tfill = 32.8ns

Ep/Eacc of TD18_Quad should be similar to that of T18_Disk

Note we applied calibration 62.7MW input →100MV/m for TD18_Disk#2 (Faya Wang).

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TD18_Quad#5

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High power processing and testsof TD18_Quad#5

• TD18_Quad#5 was set in a vacuum chamber and the chamber was installed in Shield-A of Nextef in Aug. 2009.

• Processing and various tests were performed from Sep. to Nov. 2009.

• The input power has been restricted rather soon at about 40MW due to frequent BDs in the accelerating structure.

• Total RFON time is about 1000hrs.2010/5/3 544th X-band structure collaboration

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113ns 293ns

173ns

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Dark current evolutionTD18_Quad#5 Nextef

•Once it went up in the middle of the testing, it went down but did not back to its initial value.•Dark current of Quad structure is larger than that of Disk at least one, probably a few order of magnitude. (Notice that the field Eacc is about half.)

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Dark Current at the very final stage of the test: TD18_Quad#5

FC-UP =76

FC-Mid =50

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Vacuum characteristics• Vacuum total pressure

– Base pressure at <10-6Pa– Typically processing <10-5Pa– Increases every time at few to 5MW range if after RF-OFF

for more than several hours• Mass spectrum

– M=2, 28 and 44 increase with RF-ON, but not M=18• Especially when reaching power limit

– M=2 becomes dominant residual gas after an hour or so run

– M=27 and 28 change in a similar manner as time, indicating hydrocarbon-origin surface contamination

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Summary • RF on hours amounts to nearly 1000hrs.• ACC-related breakdowns counted more than 7000 times.• Power is limited by 19MW (~58MV/m) at 51 ns pulse.• Further increase of field is not expected from the trend

pattern.• Long-pulse operation was tried at 113, 173 and 293nsec.• NEG application was tried to see the effect of hydrogen

contents. The hydrogen pressure decreased about half but the attainable gradient did not changed(improved).

• Q: Does 50microns Chamfer at the iris help to improve the high gradient performance? -> Probably NOT unfortunately.

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KEK’s version: 50 micron chamfer

Made of CuZr without heat treatment.

50 micron rounding: shape with angles and bumps.

Reference planes were formed by milling in a few micron level without re-chucking for shaping cells.

Assembly was done within ten micron level.

1144th X-band structure collaboration Meeting, CERN2010/5/3

Electric field enhancement in a shallow channel with round chamfer

Gap (micron) Bump (micron) Emax / Enominal

0 0 1.39

0 20 1.57

10 20 1.58

Calculation done by T. Abe by CST MS. Waveguide field.

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No.18 Q2-2 to match Q1-1

Yasuo Higashi

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Surface inspection summary

• Optical inspection with long-distance focus microscope

• Still not easy to see the arcing spots• Will go to CERN SEM for full inspection

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Breakdown pulse analysis

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Timing distribution for change>2000

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Breakdown cell distribution >2000

534 events were analyzed out of 1919 INTLK.

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50ns higher target run

Mostly downstream half.

Simply increasing toward output end.

Indication of BD following some field gradient.

TD18_Disk#2

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Test of TD18_Disk2 at NextefDec. 2009: Delivery to KEK made and installation work started.Jan. 2010: Testing of the structure started. Slow conditioning and gradient restricted (about 80MV/m). Frequent small reflection from Left-side output line were observed. Decided to replace whole left output line.Mar. 2010 Restart test with new output line. Reached 100MV/m. Operable with 60MW input (100MV/m), RF pulse width 250ns.Apr. 2010 Measurement of Breakdown rate started.

We are now in the final stage of the testing.

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The Eacc values during 1300~1500 may not be right due to system change.(Some ADCs of data acquisition system were over-ranged .)

Nextef

Lo

ad

ch

an

ge

d

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Changing Load• Acc input power was restricted within about 40MW (but this does not look a “hard limit”), 51ns, operating time was about 1000Hr.• Frequent small reflection signals from the left –side load line are observed at most of the breakdown events.• We have decided to change the whole line in the end of Feb.• Performance of the “new” line had been checked at KT-1 previously.

• The small reflections disappeared after the change. The system looks normal after the change.•We could reach to 100MV/m gradient as well as we could increase the pulse width.

• The “bad” line was tested at KT-1, but the behavior observed in Nextef did not reproduce.

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BDR evaluation planTD18_Disk#2 Nextef

60MW~100MV/m

55MW~95MV/m

50MW~90MV/m

45MW~85MV/m

400 ns

250 ns 1st 1 day7th 1 day

2nd 2days8th 2days 3rd 3 days 4th 5 days

90 ns 5th 2 days 6th 4 days

50 ns

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TD18_Disk_#2 Breakdown rate

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50 100 150 200 25010

-6

10-5

10-4

10-3

Pulse Width (ns)

BD

R (

1/p

uls

e/m

)

@80~140 hr@280~360 hr@700~960 hr

BDR Pulse Width Dependence/NLCTATD18_Disk

Faya Wang

Nextef 250ns 100MV/m

Our data looks consistent nicely with NLCTA one.

Nextef 90ns 100MV/m

10^-3

10^-6

BDR=7e-7 (250ns,100MV/m) for T18 at Nextef

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Breakdown rate measurement 252nsCheck statistic of the data

Run 34 (60MW)80 BD’s BDR [10^-6 /pulse/m]

Run 35+36 (55MW)41 BD’s BDR [10^-6 /pulse/m]

Run 37 (50MW)12 BD’s BDR [10^-6 /pulse/m]

Elapsed time (Hour)

Inst

ant

BD

R(1

0^-6

)

It is naturally expected to be converged to the “final” BDR value, and actually we see this behavior.

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Timing statistics for Rs rise and Tr fall

Rs: 100—350ns for 252nsec pulse Most of Rs change sit around 360+-50nsec

Tr: 150—400ns for 252nsec pulse Most frequent when approaching to the end, from 300 to 370ns. There is a small bump at 170 nsec, at the beginning of pulse.

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Search for the hot cells: identification of the breakdown location

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Some pulse: Run34 #38 and #39

This case was not analyzed reasonably.

Mostly analyzed well as these events.

F Rs

Ra

Tr

FC-UP

FC-Mid I_Rs Q_Rs

Difficulty in automatic analysis is remaining! Should be revised!2010/5/3 2944th X-band structure collaboration

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FC-Mid 51nsRF

10

100

1000

104

105

106

20 30 40 50 60 70 80 90 100

Dark Current FC-MidTD18Disk#2 Nextef

2010.03.0802.2001.2301.1701.1101.06

Pea

k C

urre

nt (nA

)

Eacc(MV/ m)

2010.04.192010.03.19

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FN Plot

0.001

0.01

0.1

1

0.005 0.01 0.015

DarkCurrent FN PlotTD18Disk2/Nextef

01.1101.1701.2302.2003.0803.1904.07 y = 1434.4 * e (-895.53x) R= 0.99809

y = 263.61 * e (-868.12x) R= 0.99532

y = 776.16 * e (-1095.8x) R= 0.98359

y = 2060.9 * e (-1485.6x) R= 0.98228

y = 4280.1 * e (-1705.4x) R= 0.99159

y = 1006 * e (- 1523.5x) R= 0.9975

y = 1576.8 * e (-1629.5x) R= 0.99845

I(nA)/

Es(2

.5)

1/ Es(MV/ m)

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FN Beta History: FC-Mid

0

20

40

60

80

100

0 50 100 150 200 250 300 350

FC-MidNextef/TD18Disk2

FN Beta

FN B

eta

RF pulsesX106

T18@NextefBeta=38

1950hrs

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MidDN

UP

Stil

l goi

ng u

p

Summary of TD18_Disk#2 testing • Finally it reached over than 100MV/m at 50ns width

– But it took much more hours and breakdowns than T18, 5000 BD’s and 1300 hours

• Widening pulse was easily established at 100MV/m level– With only 1000 BD’s and 200 hours

• Dark current was initially reduced but recently has been saturated.

• Breakdown rates were and is being measured. It is 10-4/pulse/m at 100MV/m at 252nsec.

• Pulse shape analysis and phase information are still to be developed.

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Conclusions

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Conclusions: TD18_Quad#5•TD18Quad#5 test was done.

•Attained gradient was as low as some 60MV/m with short pulse width 51ns.•Remarkable outgas activity was seen during the operation. Base pressure of the structure chamber was 10^-6Pa .•Large amount of dark current observed. The beta value of FN plots (for FC-Mid, beta=50) is slightly larger than that of TD18Disk (now about 40).•Follow-up works, such as SEM observations on the surface of the structure, are ongoing.

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Conclusions: TD18_Disk#2•Testing of TD18_Disk#2 structure is at its final stage.•100MV/m with 250ns has been reached.

•BDR has been measured and the values agree with NLCTA data. (Although it looks still decreasing) it may be fair to conclude that BDR of TD18_Disk#2 is greater than that of T18 by about two order of magnitude. •The amount of dark current has been decreasing by a few order of magnitude during the test (over some 2000hrs) and became very similar to that of T18.•The field enhancement factor, beta, has decreased by factor of 2. The final value is some 40.

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