Initial Experimental Results from the Alcator C-Mod Compact Neutral Particle Analyzer

2004 46th APS-DPPV. Tang, R. Parker, J. Liptac, J. Egedal, C. Fiore, R. Granetz, A. Hubbard, J. Irby, Y. Lin, E. Marmar, S. Wukitch, K. Zhurovich (MIT), W. Rowan (FRC).

Nov. 15, 2004

Abstract

• Recent experimental results from the new Compact Neutral Particle Analyzer (CNPA) and a multi-detector upgrade of the diagnostic are presented. The CNPA uses Si diodes to detect charge-exchanged (CX) induced energetic (>20keV) neutral particles for ICRF hydrogen minority tail temperature diagnosis. A diagnostic neutral beam (DNB) provides the neutrals for the CX process. The discussed data involve measurements of the on-axis Hydrogen-minority perpendicular energy spectrum during low density (neo~1020/m3) ICRF D(H) plasmas with ~0.5 to 1.7MW of ICRF. During these discharges, tail temperatures reached >100keV. The energy spectrum is compared with TRANSP and Stix-distribution based calculations, and central ICRF power deposition densities are inferred. The upgrade consists of the installation of three additional detectors which provides the diagnostic with multiple sightlines. Using a new long-pulse DNB, the improved CNPA will give spatial and temporal tailtemperature measurements for moderate (neo~2x1020/m3) densities C-Mod D(H) plasmas.

Introduction and Motivation

• Ion-cyclotron RF(ICRF) minority heating is the main auxiliary heating scheme on Alcator C-Mod.

• In this scheme, the RF power heats the minority species in vPerp, creating an anisotropic energetic minority ion distribution, or tail, which slows down on the bulk plasma.

• 3 Antennas have provided up to 5 MW of on-axis power into D(H) plasmas at 5.4 T and D(He3) plasmas at 8.0T.

• Historically, the C-Mod minority tail temperature is inferred from electron temperature measurements1.

• The new Alcator Compact Neutral Particle Analyzer (CNPA) measures induced charge-exchanged neutrals stemming from this hot ~on-axis ion tail distribution with the aid of a neutral beam. These measurements give the tail distribution directly and are useful for verifying RF physics and heating deposition. The diagnostic is also potentially useful in studying interactions of such tail ions with TAE and other Alfvéngap modes2.

CNPA Introduction

• A one-channel CNPA system utilizing fast digitization techniques and a silicon diode detector with an ~on-axis view (R=65.5cm) of the plasma was installed on C-Mod and tested during the 2004 run campaign.

• Compared with other machines that have successfully implemented Si or diamond diode NPA detectors3,4, Alcator C-Mod provides unique difficulties in terms of signal to noise because of its relatively weak neutral beam5 (i.e. a non-heating beam) and high plasma densities.

• Preliminary analysis of the measured distributions yielded RF power depositions and temperatures qualitatively close to TRANSP# calculations, with noticeable discrepancies.

• The CNPA is upgraded to 4 channels for the upcoming campaign. Augmented with a new long pulse DNB, the multi-array CNPA system will give spatial and temporal tail temperature measurements for moderate (neo~2x1020/m3) densities C-Mod D(H) plasmas.

#J. Onega, M. Evrard, and D. McCune, Trans. Fusion Technology. 33, 181, 1998.

The CNPA Design and Hardware

• Traditional fusion NPAs are based on mass spectroscopy principles. They give good energy resolution and can separate ions with different q/m ratios. However, they are physically very large per channel.

• One compact alternative, employed by the CNPA, is to operate silicon diodes in pulse-height analysis (PHA) mode and detect the neutrals directly. This method can not identify particle type easily but can still give decent resolution. Multiple channels can easily fit on one port. Because they have a direct view of the plasma, these systems are more susceptible to noise.

CNPA Specifications and Features

• 150µm 50mm2 R-Series Ortec Surface Barrier Detector. The detector has a nominal 2300Ǻ Si dead-layer for light tightness. This layer effectively blocks hydrogen atoms less than ~20keV from reaching the detector. However, soft x-rays in the ~1keV range are detected.

• eV-Products 5093 preamp at 158mV/MeV coupled with an Ortec 472 shaping amplifier. The preamp is mounted on a stand and additionally shielded with µ-metal to prevent pickup.

• D-TACQ ACQ216CPCI 14bit 10 to 40Mhz +/-10V Digitizer Card w/3sec memory for storage and post-shot PHA of Ortec 472 voltage data.

• A micrometer is used for aperture selection and collimation.• A filter wheel with a 1µm & 10µm Be filter allows the system to be used as an x-ray

spectrometer.• The system is clamped to the C-Mod cement shield and separated from tokamak

vibrations via steel bellows.

C-Mod CNPA on F-Port Top

DNB

Bellows

Clamp

Filter Wheel

Detector

Preamplifier

MicrometerAperture

Gate Valve

Viewing Cone(~1cm)

To amplifier &digitizer onC-Port

~0.5m

~3.3m

CNPA Electronics• Detector electronics follows an analog approach with digitization and post-shot

PHA.6

• PHA is traditionally done in real-time. The main limitations of such analysis are pulse pile-up and the constraints of real-time analysis. By storing the amplifier voltage and doing post-shot PHA, more complex PHA routines could be employed. Additionally, since the voltage discriminator and bin settings are determined in software, different values could be tailored to each spectrum. Finally, electronic pickup noise can be easily discerned.

eV-5093preamp

Ortec 472 Amplifier at 100xand 0.25µsShaping time

D-tacq10-40MhzDigitizer Card

Storage and in-house software PHA (Post-Shot)

Pulse Shapes

SSB Detector

Energy

Counts

Pile-up Normally, this shaped pulse in a traditional real-time PHA system would be rejected. If the data is saved and analyzed later, a fitting routine could be used to extract the voltage peaks.

Extracting the Minority Distribution

• To extract the on-axis minority ion distribution from the detected spectrum, several factors must be accounted for. Assuming the signal detected is caused only by DNB induced CX neutrals, the minority distribution can be back-calculated from the above chain.

• The straggling estimates are performed with the SRIM code7, while the neutral attenuation calculations uses stopping cross sections from Janev8.

• The calculation gives the minority distribution if the distribution is isotropic; otherwise it results in the RxBtor velocity distribution only, since the detector sees only vPerp

neutrals. • For plasmas with significant x-ray and neutron emissions (both of which can be quite

large for typical C-Mod Plasmas), this background must be excluded from the PHA spectrum before proceeding with the above chain.

PHASpectrum

Account for stragglingin detector dead-layer

Account for plasmaattenuation

Account for CX cross section Minority ion

distributionDNB spectrum on-axis

Determining the Tail Temperature• Because the distribution is non-maxwellian,

we need a model distribution to fit our spectra.

• The Stix distribution9 is a good 1st attempt. It is an isotropic solution of the Fokker-Planck equation for rf-heated resonant ions. Finite aspect ratio and particle effects are not included. In-depth analysis requires a code that accounts for anisotropy and orbit effects, such as Hammett’s FPPRF10.

• The free parameter in the Stix distribution is ξ=Pts/3nminTe, where P is absorbed RF in W/cc and ts is the Spitzer slowing down time.

• A Tavg is easily determined by taking the energy moment of the distribution.

• This Tavg and P can then be compared with the TRANSP calculations.

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −++

+++−

=

⎥⎦

⎤⎢⎣

⎡+

++=

=

=

=

=

=

⎥⎥⎦

⎤

⎢⎢⎣

⎡

++

+−+

+−=

−5.

5.1

5.5.

5.

3/2

5.

2

2/1

2

312tan

631

211ln

612)(

)1(21

)(

32

3

/

)2/(2/

)/()1(

)(1

)1()(ln

xxxxx

xxK

RmmT

E

nTPt

lnlZnR

TmlmvE

EEKRT

TTTRT

Evf

f

f

ff

e

s

eeffff

fff

fff

efef

e

π

ξεξ

ξ

πε

ξ

ξξ

ξ

From Stix, “Waves in Plasmas”, 1992

Operating Results and Experiment Data

• Initial operation of the CNPA in November 2003 resulted in a long shake-down period due to numerous electronic noise pickup issues and burn-out of a detector from radiation damage.

• First successful operation was achieved in Feb 2004. The majority of the useful experimental data was taken in April 2004. The diagnostic, in its current state, is effective with L-mode plasmas with densities below ~1020/m3.

• The following highlights the recent preliminary results from a set of shots involving a low density 0.5-1.7MW power scan.

• The determination and fitting process for one tail distribution, shot 1040415006, or shot 6, is shown in detail, followed by comparisons of all the analyzed shots with TRANSP calculations.

Shot 1040415006 Parameters•These plots show the typical plasma parameters and CNPA raw voltage for the CNPA data presented in the following slides, which involve an RF power scan from 0.5 to 1.7 MW.

•Bo=5.5T, 0.8MA, nh/ne~3.8%

•J-Port Antenna at 78Mhz and 0,π,π,0 phasing.

•The red band indicates the 50ms long DNB period and where most of the CNPA data is collected. The discussed minority tail spectrums are all from this 50ms time span.

(line averaged)

Anatomy of Raw Signal, Shot 1040415006

t=0s t=2s

V=10volts 620keV

Signal during DNB Pulse

Increase signal due to 1MW RF

Noise band: ~0.4V~24keV

Pulse Width:~1.2µs

V=2.2volts

Electronic pickup

Drop out due to preamp saturation (i.e. need a preamp with faster fall time)

•These traces shows in detail the raw CNPA amplifier voltage data from the previous slide before post-shot PHA.

•Clearly, the ability to review the raw voltage data allows for easier troubleshooting than a traditional real-time PHA system.

•10Mhz digitization at 14bit ~35Mb of data per channel for each shot.

Shot 1040415006 PHA Spectrum

0 100 200 300 400 500 600 70010

0

101

102

103

104

Energy(keV)

Cou

nts

per b

in

Before DNBDuring DNBAfter DNB

Total counts during each 50ms period: 793, 2838, 618

Energy discriminator: ~30 keVEnergy bin size: 15 keV

Standard Deviation of Baseline: ~5keV

Shot 1040415006 Minority Spectrum

50 100 150 200 250 300 350 40010

0

101

102

103

104

Energy(KeV)

Cou

nts

per b

in

Raw spectrumBkground subtractionDeadlayer stragglingAttenuation from plasmaCX cross section

•This graph illustrates the effect of each step in calculating the minority distribution from the PHA data.

•The crosses indicate the approximate error (2σ) of the extracted distribution based only on counting statistics.

•The positive slope portion of the spectrum is probably due to particle orbit effects. These are not taken into account by the Stix distribution used to fit the extracted distribution and will be accounted for in a more in-depth analysis.

Minority distribution, normalized to first bin.

Shot 1040415006 Stix Distribution Fit

100 150 200 250 300

102

103

Energy(keV)

A.U

.•The black line indicates the best Stix distribution fit using a weighted (1/σ2) least squares procedure.

•The red crosses are errors (2σ) in the spectrum from counting statistics only.

•A Monte-Carlo analysis was also performed to determine the error in the extracted distribution caused by uncertainties in the plasma, DNB, and CX parameters. The yellow band shows the Stix fits of a thousand such runs.

•Using this model, Tavg=45 ±5.4 keV and RFdep=1.5 ±0.23 W/cc. These bounds include only the errors from the Monte-Carlo analysis.

More Weighted Stix Fits Results

1.27

0.74

2.6

Rfdep (W/cc)(± 15%)

34

57

122

Tavg (keV) (± 12%)

26

14

13

Shot

1.05~8.83.8%

0.58~7.52.3%

1.68~7.54.9%

RF(MW)neo (1019/m3)(± 10%)

nh/ne

(± 15%)

Shot 1040415006 TRANSP Results

•TRANSP minority power balance with sawteeth model on and off during DNB period. •The fits discussed above generally give results which are closer to the sawtooth off case. The following slides plot the fit vs. TRANSP power depositions and temperatures.

TRANSP and Stix RF Deposition Comparison

cm

TRANSP and Stix Tavg Comparison

Operating Results Conclusion

• More work and analysis required. Tail temperatures greater than 100keV are measured.

• The CNPA goal during last campaign was to verify that on-axis neutral particle analysis using compact diodes is possible for C-Mod plasmas. This was accomplished. Background signal which overwhelm the detector is an issue during moderate (2x1020/m3) to high density C-mod plasmas.

• More analysis is required to investigate the effect of rippled trapped particles and other particle orbit effects on the extracted minority distributions. There is clearly a statistically significant rise (or negative temperature) region in the observed spectrum that the Stix distribution can not fit. The problem is compounded by the low absolute number of counts.

• Preliminary Stix fits comparisons with TRANSP match qualitatively for cases with no sawteeth model. However, previous work11 have shown that sawteeth redistribution occurs with ICRF ions.

• More spatial and temporal resolution will help to resolve these issues.

Upgrade: Multi-Array CNPA• Four channels total planned

for next campaign. Three vertical views from F-port Top spanning R=65 to 72cm and one tangential view.

• Best tangential solution to maximize CX cross section: NPA system on inner-wall column in-between F and G Port. Very difficult to engineer, however.

• Workable solution: NPA system in re-entrant tube mounted underneath the Lower Hybrid antenna and fixed to LH rails at C-Port.

DNB beamline

Tangential CNPA Sightline

C-Mod Plasma

Alcator C-Mod Top View

Tangential CNPA

•Utilizes IRD AXSUVHS5 1mm2 x 35µm Sidiode with 1500A Al filter and compact Amptek preamplifier.

•1.5m long sightline to new long-pulse DNB.

•Reentrant tube design with vacuum break at detector.

•Detector is thermally shielded by actively cooled shell. Cold air or nitrogen feed is used.

Cooled thermal shield

Collimator tube1.5” ODReentrant vacuum tube

Preamp location

Vacuum break

Lower Hybrid Antenna

Sightline

Summary and Outlook

• A single channel compact NPA is successfully tested on Alcator C-Mod ICRF D(H) low density plasmas. Background noise that overwhelm the CNPA become significant in moderate to high density plasmas.

• Preliminary analysis using a Stix distribution of the minority spectrum shows temperatures greater than 100keV during shots with ~1.5MW RF. Comparisons with TRANSP calculations shows discrepancies for calculations which employed a sawteeth model.

• More analysis is required to account for orbit effects seen in the measured distributions, which might result in closer agreement with TRANSP.

• A number of hardware improvements, including a new DNB and a multi-array upgrade of the CNPA, should allow for extensive study of minority tail distributions in Alcator C-Mod.

References

1) O’Shea, P., “Measurements of ICRF power deposition and thermal transport with an ECE grating polychromator on the Alcator C-Mod tokamak”, PhD thesis, 1997.

2) Snipes, J. , Invited Session N1.006, “Active and Fast Particle Driven Alfvén Eigenmodes in Alcator C-Mod”, Thursday, Nov. 18. 2004.

3) Krasilnikov, A.V., Medley, S.S., Gorelenkov, N.N., et al. Review of Scientific Instruments, 70, No.1, 1999.

4) Osakabe, M., Yamamoto, T., Takeiri, Y. et al. Review of Scientific Instruments, 72, No.1, 2001

5) R. Granetz, ‘A Long-pulse Diagnostic Neutral Beam for Alcator C-Mod’, 45th APS, 2003. 6) R. O’Connell et al. Rev. Sci Instrum. 74, No. 3, 20017) Ziegler and Biersack, SRIM 2003.20 “The Stopping and Range of Ions in Matter”8) Janev R.K., Boley C.D. and Post D.E. Nuclear Fusion 29, 2125, 1989.9) Stix, T.H., Waves in Plasma, New York: AIP, 2nd ed. 1992.10) Hammett, G., “Fast ion studies of ion cyclotron heating in the PLT tokamak”, PhD thesis,

1986.11) Eriksson, L.-G., Hellsten, T., et al, Nuclear Fusion 29, 87, 1989.

Acknowledgments

• Work supported by USDOE Coop. Agree. No. DE-FC02-99-ER54512.• Thanks to Dr. Rice for the loan of his filter wheel and micrometer aperture, and the

Alcator vacuum shop for their efforts.