First Test of the X-ray Spectrometer

K. L. Wong, A. L. Roquemore

Electronics: J. Dong, J. WertenbakerSoftware: G. Zimmer

NSTX Physics MeetingLSB318, PPPLAug 3, 2009

NSTXNSTX Supported by

College W&MColorado Sch MinesColumbia UCompXGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

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Purpose of the X-ray spectrometer

• Microtearing modes produce stochastic B, can explain e in

some NSTX discharge

• The old PHA system (from TFTR) saw very few photons

above 10 keV , much less than one would expect in a Te(0) ~

3 - 4 keV plasma - due to stochastic B ?

• The purpose of the spectrometer is to study electrons in

some NSTX discharges in 5 - 25 keV energy range

- a cheap and quick method to obtain a vital piece of info’

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X-123 spectrometer (Amptek)Ref: Y. Shi et al., RSI (2004) - KETEK detectors

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Specifications

• < 0.001 counts/s from 2 -150 keV, our range: 5 - 25 keV . Need tangential port ?

Von Goeler et al. RSI (1994):

Bremsstrahlung radiation pattern

• 2 stage Peltier cooling - ∆T~80oC

• Thermal noise ~ T ∆ f, S/N ~ Area /∆f.T

~ same as old (TFTR) PHA

• Thickness: 0.5 mm vs 3-4 mm, less sensitive to neutrons, limit not known,

NOT INTENDED FOR high performance plasmas

(1.e14 n/s)

• Shielding: Von Goeler’s PDX X-ray camera has S/N ~ 1 at 1.e13 n/s with 4” lead + 8” polyethelene

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Hardware/Software Modifications for NSTX Application

• Hardware: Add gate input - +4 volts to take data (from 404/412 timing module)

• Software: Temporal resolution - write software to read & store up to 10 blocks of data/shot

• Amptek spec: time gap = DT = ti - tsi-1 ~ 12 ms,

we measured DT ~ 14 - 15 ms to retrieve data + overhead(additional dead time)

• For reliable operation, need DT ~ 100 ms > 50 ms

- decided on 6 blocks of data, 100 ms each

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Sequence of Events

Mon july 6: finish wiring after 5 pm

Tues july 7: software not ready, test with manual trigger (RF plasma), add shielding in evening

Wed july 8: test Labview software - see no counts, trigger did not work - no gate Vin

Fix pulse transformer LEMO cable after 5pm

Thurs july 9: first try with 404/412 trigger - see very few counts -

Saturated by photons ? Alignment problem ?

Fri july 10: try again with smaller aperture & thicker foil, still no good.

Wrong parameter settings ? call AMPTEK: random bits.

Should read parameter settings with ADMCA

Mon july 13: Thomson complaint - lead; reconfigure software

Tues july 14: read spectrometer parameter with ADMCA, read data with Labview,

ADMCA crashed, power reboot in test cell after 5 pm.

Read data from wrong buffer ?

Wed july 15: control spectrometer with ADMCA, read last data files and compare with

Labview data - they agree - software usable. Optimize DT ( 1 pm )

July 14-15: vary apertures & foils, no obvious effect, noise creeping up …

Only high power beam shot - No useful data.

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Spectrum from manual trigger - no temporal resolution

HHFW only - triggered a few sec before shot

- detector noise + photon signal

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Technical problems encountered

• Hard ware: - no trigger input - LEMO cable problem

• Soft ware: - late start - wrong file loaded or wrong readout data ?- Amptek & Labview clash - read from correct buffer ?

• It was Wed July 15 when software became usable- change Al foil & aperture - no obvious effect - only high beam power shots : neutrons ( ~ 2.e14 n/s)

• Detector noise creeping up …

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Labview Software has 3 parts

1. Load a file (from Amptek) to set spectrometer parameters

2. Read the parameter settings and store in a file - needed for data analysis

3. Clear buffer, read data file in buffer at specified times and transfer data to PC disk and store them

Temporary solution: cut out 1 & 2, just use part 3

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Parameter settings

ADMCA (Amptek) Labview

__________________________________________________ PUR on PUR 1

RTD off RTD 10 ?

gate input active low gate input 2

course gain 106 coarse gain 3

fine gain 1.0 fine gain 8192

1. Wrong parameter file loaded ?

2. Can we read parameter file with ADMCA instead of Labview ?

3. Read from wrong buffer ( 1 = A or B? )

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Spectrometer data [ time->channel # ]

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Estimation of Neutron Counts

(A) Detector area = 25 mm2, R ~ 4 m, 2.e14 n/s

Neutron rate at detector: 2.4e7/s

About 1% of neutrons hitting detector produce a current pulse

2.4e5 counts/s saturation

Note: (a). Recommended max. count rate is 1.2e5/s

(b). Scattered neutrons & ’s not yet included (i.e. perfect shielding

assumed)

(B) Ken Hill: PILATUS pixel sees up to 320 counts/s, no shielding

pixel area: 0.172x0.172 mm2, thickness = 0.3 mm , can count up to 1 MHz

0.6% neutrons produce current pulse

Amptek detector (w/o shielding) should see : 3168 x 320 = 1.e6 counts/s

Shielding cuts it down by 4 - reasonable agreement.

------ Neutrons are NOT the only problem ------

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Detector noise problem: bad cooler ?- still under warranty

In-situ calibration with 55Fe: 2 peaks at 5.9 keV and 6.49 keV (7/22/2009)Resolution for #2049 was 228 eV (10/06/2008)

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What did we learn ?

• Use the not-yet-perfected software to do NSTX experiments

- just use the clock cycle to sort out problems, never asked a single plasma shot

• X-ray PHA not possible in 2.e14 n/s (not our intent, never been done anywhere)

– Highly unlikely in high e13 n/s (do-able for Pilatus at 1 MHz count rate)

– Possible in low e13 n/s

• Can we use this spectrometer for microtearing mode experiment ?

- Yes. The experiment has two parts:

1. HHFW with beam blips - no problem

2. Beam heated plasma - needs modification

To avoid neutron saturation, just try to inject 1 D-source into a 3.5 kG, 500 kA,

helium target plasma before consider other exotic means:

– H-beam, (b) He plasma, (c) Just look at afterglow, (d) High ne with HHFW

• Conclusion: 1. Spectrometer should work if we fix noise problem, move to bottom port

and add more shielding (on concrete floor), improve alignment

2. Do we still need this when PILATUS becomes operational ?