Characterization of Alcator C-Mod ICRF Minority Tails via Neutral Particle Detection

Part 1: Design and characterization of neutral particle analyzer

Vincent Tang, John Liptac, Ron Parker, Robert Granetz, and the Alcator C-Mod Team

Oct 28, 200345th APS-DPP

Abstract

• A new tail temperature diagnostic based on detection and pulse-height analysis (PHA) of energetic charged-exchanged (CX) neutral particles has been constructed. It will be used to studyenergetic tails resulting from minority-heated ICRF discharges in Alcator C-MOD. The diagnostic consists of a silicon diode detector viewing a diagnostic neutral beam from a top port. Initial experiments will consist of ICRF heated D(H) plasmas, followed by D(He) plasmas. C-MOD presents unique challenges for on-axis CX diagnostics because of its operating regime. Specifically, its high density makes neutral penetration difficult and result in a large soft x-ray background. The design of the detector and simulations of the performance in C-MOD plasmas are presented.

Introduction

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

• 3 Antennas have provided up to 5 MW of power into the plasma.

• On-axis heating: D(H) at 5.4 T and D(He3) at 8.0T.• Historically, no diagnostic was available to directly measure

minority tails due to the low background neutral density and thelack of a diagnostic neutral beam (DNB). Tail temperature is estimated from bulk electron temperature. Estimates give up to 400keV for the C-Mod D(H) tail energy.

• A DNB is now available for active minority tail diagnostics. It is scheduled to be upgraded in 2004. None of the calculations in this poster uses the upgraded parameters.

DNB Parameters

1See Rowan et al., ‘Development of CXRS on Alcator C-Mod Using a Diagnostic Neutral Beam’, 44th APS, 2002.2See Hutchinson, Marmar and Porkolab, ‘Alcator C-Mod Fusion Research Program 2003-2008 Volume I, Technical Proposal’, April,

2003. See also R. Granetz, ‘A Long-pulse Diagnostic Neutral Beam for Alcator C-Mod’, 45th APS, 2003. 3beam assumed to be one component; the beam is actually four component: (38:18:28:16). The upgrade has a better mix. 41/e parameter. 5for an H-mode Neo=1.5x1020/m3 C-Mod Plasma.

>30%~30%Penetration to Center5

150050Pulse Length (ms)5.45x10151.26x1015Density(#/m3)34.5Radius(cm)4

3.2x1063.1x106Velocity (m/s)84Current (A)5550Energy(keV)3

HydrogenHydrogenType

Upgrade2Current1

Table 1: Parameters of the DNB

A Tail Diagnostic

• Given the importance of the tail in terms of RF heating physics and overall power balance, a direct measurement is useful. Also, such a diagnostic would be useful in studying interactions of the tail ions with TAE and other Alfvén gap modes1.

• Traditionally, either neutral particle analysis2(NPA) or charge-exchange recombination-spectroscopy3(CXRS) is used with a neutral beam. The plasma distribution can be measured after accounting for CX rates, neutral attenuation, and straggling in the detector.

• Because of C-Mod’s dense plasmas, the background bremsstrahlung overwhelms the CXR lines.

• Hence, a compact NPA (CNPA) using a Si-diode has been built and will be tested with D(H) plasmas in November.

1Snipes et al. CO1.009 “Active MHD Spectroscopy of Alfvén Eigenmodes on Alcator C-Mod” 45th APS-DPP, 20032Krasilnikov, A.V., Medley, S.S., Gorelenkov, N.N., et al., Review of Scientific Instruments, 70, No.1, 1999.3Stratton, B.C., Fonck, R.J., et al., Nuclear Fusion, 34, No. 5, 1994.

The C-Mod CNPA

DNB

C-Mod CNPA on F-Port

Detector

0.5m

BNC feedthroughfor detector

Viewing Window

Linear feedthroughfor aperture selection

Viewing Cone

To C-Mod

Detector Specifications and Requirements

• 500µm 25mm2 PIPS Canberra Detector. 500Å thick Au foils are mounted on the detector to attenuate the soft x-rays. The foil attenuates 50keV H neutrals to 41keV+/-4keV.

• eV-products 5091 preamp coupled with either an Ortec 590 shaping amplifier, a 20x 0.5µs width Gaussian shaping amplifier1, or a10x 1µs differential amplifier. Each system will be simulated and tested on the machine to determine the best fit in terms of count rate and energy resolution.

• D-tacq 14bit 40Mhz +/-2.5V Digitizer Card w/1.5s Memory. (A 10Mhz card is currently available for testing)

• To fit a spectrum and define a temperature, we need ~1000 neutral counts in 50ms with ~20keV resolution.

1Liptac et al. FP1.007 “Hard Xray Diagnostic for LHCD on Alcator C-Mod ” 45th APS-DPP, 2003

Potential Challenges

• The absolute number of emitted CX neutrals is not a problem; the DNB injects ~3.2x1018 H atoms/50ms. The concern is potential flooding of the detector due to large x-ray and neutron/gamma background. Estimates show that neutrons and gammas should not cause a problem in terms of count rate because of the thinness of the detector.

• Hence the limiting factors are the neutral to background emissivity, and how the detector system handles the numerous 0-10keV x-rays.

• The best scenario would be a x-ray baseline with low enough fluctuations where the neutrals and other high-energy particles can be clearly seen. The system then only has to count the neutrals and other high-energy particles.

• A more realistic scenario might be a x-ray baseline that fluctuates significantly. The system then has to have enough time resolution (i.e. count rate) to distinguish between neutral particles and the large soft x-ray background.

• A model has been built to simulate the plasmas and the proposed detector systems.

Proposed Detector Systems• Option 1: Traditional Analog Approach with Digitization and Post-shot

PHA.• PHA is traditionally done in real-time. The maximum throughput, determined

by the width of the shaped pulse, is typically around 100k/s . The main limitations are pile-up and the constraints of real-time analysis.

eV-5091preamp

Ortec 590or in-housegaussian shaper

D-tacq 10MhzDigitizer Card

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

Pulse Shapes

Detector

E

C

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. However, this is probably not realistic for pile-ups with more than two peaks.

Proposed Detector Systems (Con’t)• The main concern with using Option 1 with the C-Mod CNPA is the large

soft x-ray pile-up and its effect on PHA of the neutrals.• Option 2: Fast Digitization of Preamp Output and Post-shot PHA• Fundamentally, throughput is limited only by the rise time of the preamp

pulse. The eV-5091 preamp output has a ~90ns rise time. Therefore, in theory, a ~10Mhz throughput could be achieved by analyzing the premapoutput without shaping.

• The issue here then becomes the signal-to-noise ratio. Because real-time analysis is not required, fitting routines could be used to attain the signal-to-noise achieved by shaping without sacrificing the high count rate.

• The proposed C-Mod system has an optional 10x 1µs differential amplifier before digitization. The additional amplification allows for better energy resolution, with the differential time constant selected to prevent railing of the digitizer card. The addition of a differentiator does not affect the general preamp output shape.

• Because of the fast rise time and low voltages involved, a fast high resolution voltage digitizer card is required. Currently, only a 10Mhz 14bit card is available for testing.

• This is the best option if the system has to count both the neutrals and the x-rays.

In contrast to the previous system (Option 1), the peaks and the baselines of each pulse are distinct, as long as the pile-up does not occur during the rise time of each pulse. The trade-off is no noise filtering and the requirement of very fast digitization.

time

Preamp voltage

Ev-5091preamp

D-tacq 40MhzDigitizer Card

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

In-house 1µs Differential Amplifier

(Optional)

E

C

Proposed Detector Systems (Con’t)

Modeling of Plasma and Detector

• Feasibility and performance of the CNPA is tested by coupling models of the plasma, the DNB, and the detector electronics.

• The plasma neutral and x-ray emissivities are calculated, and serve as inputs into the detector system simulation. Each “hit” is randomly chosen based on the emissivity probabilities. Each stage of the proposed detector systems is simulated, resulting in a realistic voltage vs. time output.

• Thus, these calculations estimate the performance of the CNPA and the detector system response.

• Along with gamma and alpha source testing, these simulations should predict the performance of Option 1 with a large x-ray background and decide if it’s sufficient.

Plasma and DNB Model

• For preliminary analysis, the minority distribution is modeled by the pitch-angle averaged Stix distribution. The CX reaction rate is then simply:

• The DNB is assumed to be one component, with the CX cross section given by:

• Beam attenuation is calculated assuming a D-shaped plasma with parabolic temperatures and densities.

• X-ray emissivity is calculated classically, with both the free-free and recombination gaunt factors:

Ryvmx rele

4 wherem

1107 2

25.3

20

=+

=−

ξξ

σ

[ ]srHzmWgge

eVTZnnxvj rrff

Thvie 3

/2/1

2541003.5)( +

= −

−−

vnnxvfvvvvnnvN pbstixbcxbpb σσ∫ =∆∆−−= 33)()(

Additional Plasma Model Details

• Only the on-axis plasma is modeled for x-ray and neutral emissivity. The rest of the viewing cone is ignored.

• Only X-rays from the bulk plasma are considered.• The x-ray attenuation by the gold foil and the detector photon

mean free paths are conservatively not included. The x-ray spectrum is taken to be from 100eV to 100keV.

• The real neutral/x-ray ratio should be within several factors of our estimate, since the center has the highest x-ray emissivity.

• The plasma model simulates a top port 25mm2 detector 2.5m away from the plasma center. The absolute number of photons and neutrals hitting the detector with this étendue is ~1012. These results are scaled to the desired rates for the detector electronic simulations. For the real experiment, we will use anappropriate aperture.

CX Sensitivity to Energy and Pitch Angle

Because the CX cross section maximizes at a relative energy of ~25keV, minority ions with large inboard velocities have higher CX rates. In addition, as expected, distributions with higher average energies will have lower CX rates. The isotropic view gives the reactivity for the hypothetical case of an isotropic συ (e.g. συtot•étendue).

Also, it is clear that the absolute rate of emitted neutrals is not an issue.

Beam and Neutral Transmission

Bulk Plasma X-Ray Emissivity

Effect of Attenuation on the Stix Distribution

Neutral/X-ray Emissivity Ratio

Simulation Results

• Option 1: Traditional Analog Approach with Digitization and Post-shot PHA.

• The simulation results show that even for a x-ray rate of 100Mhz, the neutral PHA is not significantly affected. The x-rays result in a 72.8keV baseline with +/-8.8keV noise. For a desired 1000 neutrals/50ms rate, this indicates a neutral/x-ray ratio of 2x10-4, significantly lower than expected for low density C-mod regimes.

• Thus, at Tbulk=2keV and 1% minority fraction, the operating range extends to on-axis Picrf~20MW/m3 and ni~4x1020/m3. In essence, these results are close to the best case scenario for option 1.

• The x-ray baseline increases linearly with rate, while its deviation increases with the square root of the rate. Both of these results are expected from simple Poisson statistics.

• The estimated resolution for the system is ~20keV(~15keV and <~10keV x-ray noise).

X-ray Background Characteristics

These simulations indicate that the x-ray background is not a show-stopper. Even at a rate of 100Mhz, the x-rays results in just ~+/-10keV noise. A 100Mhz rate with 1000 neutrals/50ms indicates a neutral/x-ray ratio of 2x10-4. Theoretically, this allows the CNPA to operate in fairly dense plasmas, as long as the neutral beam is not significantly attenuated.

Effect of Soft X-rays on Traditional PHA

The effect of the soft x-ray background on the entire neutral PHA process is shown in these series of simulations. The red crosses are neutral particles randomly selected from the calculated neutral emissivity, shown in green. These particles, along with randomly selected x-rays based on the x-ray emissivity, serve as inputs into the detector electronics model. The blue x's, magenta □’s, and black o’s are from a PHA of the model's output voltage, and hence represent the cumulative effects of the x-ray background and the detector electronics.

Conclusions and Summary

• A compact NPA for Alcator C-Mod has been constructed and should be operational in November.

• The large x-ray background in C-Mod results in a baseline with relatively low fluctuations. An analog shaper with digitization and post-shot PHA (Option 1) should be sufficient for the C-Mod NPA.

• If higher time resolution is desired, fast preamp digitization (Option 2) could be used to significantly increase the system throughput.

• Option 2 testing and simulations are on-going.