Diagnostic performance of the beam emission

Diagnostic performance of the beam emission spectroscopy system on the National Spherical Torus Experiment D. R. Smith, R. J. Fonck, G. R. McKee, and D. S. Thompson Citation: Rev. Sci. Instrum. 83, 10D502 (2012); doi: 10.1063/1.4728094 View online: http://dx.doi.org/10.1063/1.4728094 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v83/i10 Published by the American Institute of Physics.

Related Articles Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators Phys. Plasmas 19, 063104 (2012) Recent progress of the HCN interferometer on J-TEXT tokamak Rev. Sci. Instrum. 83, 10E303 (2012) Extreme ultraviolet spectroscopy and modeling of Cu on the SSPX Spheromak and laser plasma “Sparky” Rev. Sci. Instrum. 83, 10E101 (2012) A two photon absorption laser induced fluorescence diagnostic for fusion plasmas Rev. Sci. Instrum. 83, 10D701 (2012) A new interferometry-based electron density fluctuation diagnostic on Alcator C-Mod Rev. Sci. Instrum. 83, 10E301 (2012)

Additional information on Rev. Sci. Instrum. Journal Homepage: http://rsi.aip.org Journal Information: http://rsi.aip.org/about/about_the_journal Top downloads: http://rsi.aip.org/features/most_downloaded Information for Authors: http://rsi.aip.org/authors

Downloaded 13 Jun 2012 to 198.35.0.239. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

REVIEW OF SCIENTIFIC INSTRUMENTS 83, 10D502 (2012)

Diagnostic performance of the beam emission spectroscopy system on the National Spherical Torus Experimenta) D. R. Smith,b) R. J. Fonck, G. R. McKee, and D. S. Thompson Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA

(Presented 8 May 2012; received 8 May 2012; accepted 21 May 2012; published online 11 June 2012) The beam emission spectroscopy system on the National Spherical Torus Experiment measures localized density fluctuations on the ion gyroscale. Optical sightlines provide core to edge radial coverage, and the sightlines are aligned to typical pitch angles to maximize cross-field spatial resolution. Sightline images are 2–3 cm, and point spread function calculations indicate image distortion from pitch angle misalignment and atomic state finite lifetimes is minor with a 15% increase in the image size. New generation photodetectors achieve photon noise limited measurements at frequencies up to 400 kHz with refrigerant cooling at −20 ◦ C. Measurements near the pedestal show broadband turbulence up to 100 kHz, and poloidal correlation lengths are about 10 cm. Plasma turbulence signals can be 2–3 orders of magnitude above photon noise and amplifier thermal noise. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4728094] The beam emission spectroscopy (BES) diagnostic system on NSTX (Ref. 1) measures Dα emission (n = 3 → 2, λ0 = 656.1 nm) from deuterium heating beams.2 BES measurements are sensitive to ion gyroscale fluctuations associated with low wavenumber (low-k) turbulence and Alfvén modes. The beam velocity induces a Doppler shift in beam emission, and optical filters isolate beam Dα emission from thermal Dα emission. The intersection of optical sightlines with the neutral beam volume provides spatial localization, but a rigorous assessment of spatial and k-space measurement properties require point spread function calculations.3 BES measurements are sensitive to plasma density fluctuations according to δIDα /IDα = C (δn/n) where C = C(ENB , n, Te , Zeff ) ≈ 1/2.2 The NSTX BES system includes two optical assemblies centered at r/a ≈ 0.45 and 0.85.1 The optical views are pitched upward at 26◦ and 37◦ for alignment with typical NSTX pitch angles to optimize cross-field spatial resolution. As shown in Figure 1, 56 sightlines provide core-to-SOL radial coverage and four discrete poloidal arrays, and sightline images are 2–3 cm at the neutral beam. The measurements are sensitive to√ fluctuations with k⊥ ρ s 1.5 where ρ s ≈ 0.5–1.5 cm (ρs ≡ 2mi Te /qB is the ion sound gyroradius). High throughput optics collect light at f/1.5 with 2.3 mm2 -ster éntendue and image the light onto 9-fiber optical bundles for transmission to photodetectors. Interference filters with 4 nm passbands at 661 nm pass redshifted Dα emission from deuterium heating beams and block thermal Dα emission. At normal incidence, the interference filters pass full energy and 1/2 energy beam components, but 1/3 energy components near Carbon-II lines are attenuated. Tuning the filters to lower wavelength can pass 1/3 energy components and Carbon-II emission. Note that emission spectra from motional Stark efa) Contributed paper, published as part of the Proceedings of the 19th

Topical Conference on High-Temperature Plasma Diagnostics, Monterey, California, May 2012. b) Electronic mail: [email protected].

0034-6748/2012/83(10)/10D502/3/$30.00

fect and fast ion Dα diagnostics indicate the C-II emission is comparable to or less than the neutral beam Dα manifold.1 New generation PIN photodiode detectors and frequencycompensating, wideband preamplifiers provide photon-noise limited measurements at frequencies up to 400 kHz with refrigerant (non-cyrogenic) cooling at −20 ◦ C.1, 4 The data acquisition system samples at 2 MHz (output) with an onboard field programmable gate array (FPGA) post-processor circuit. The FPGA functions as a finite impulse response filter to suppress high frequency thermal noise. The wideband, low noise detectors enable measurements of high frequency Alfvén and energetic particle modes up to about 800 kHz in addition to low-k turbulence at low frequency. The BES poloidal array at R = 140 cm provides multipoint measurements for calculating the spatial and temporal properties of turbulence. Figure 2 shows example BES measurements at two radial locations. Note that BES signals change sharply at NBI steps, and BES auto-power spectra can exceed photodetector dark noise power spectra by 2–3 orders of magnitude. Poloidally separated channels in the R = 140 cm poloidal array (r/a ≈ 0.80–0.95) (see Figure 1) provide measurements of the poloidal correlation length Lc , decorrelation time τ d , and poloidal wavenumber kθ associated with low-k turbulence. Coherence spectra in Figure 3 show reduced coherence with greater poloidal separation, as expected, and cross-phase spectra show greater cross-phase with greater poloidal separation. Autopower spectra (Figure 3(c)) show plasma turbulence signals 2–3 orders of magnitude greater than dark noise and photon noise levels, and filtered data (Figure 3(d)) show turbulent eddies propagating along the poloidal array. The turbulence quantities are derived from time-lag auto- and cross-correlation functions in Figure 3(e). Data windows of 15–40 ms are segmented into 30–150 bins with 512–2048 time points per bin. The time-lag correlation functions for each bin are averaged to obtain the average timelag correlation function and variance for the data window. The cross-correlation envelope (Hilbert transform) at zero

83, 10D502-1

© 2012 American Institute of Physics


Smith et al.

130

140

1

150 Coherence

120

0.8 0.6 0.4 0.2 0

Auto-power spectra (dB [V2/Hz])

15 10 5 0 Z (cm)

Δz = 2 cm Δz = 4 cm Δz = 6 cm

50 Frequency (kHz)

−20

photon noise (ref)

−30 −40

(c)

20

60

−20

20 −40

Signal (V)

0

0 (c) −10 10 (d) 0 −10 −20 5 (e)

1

R=129 cm R=142 cm NBI (au)

5 (f)

0 0

0.5 Time (s)

20 0

40

0 10

0 0

R=142 cm

0 −20

0.5 Time (s) (g)

Autopower (V2/Hz; dB)

Frequency (kHz)

40

(b)

80

APS (V2/Hz; dB)

Frequency (kHz)

100

R=129 cm

60

Neutrons

D−alf [dB] Low−f (G)

(a)

80

534 ms

1

R=129 cm R=142 cm Dark noise

−40 0

50 Frequency (kHz)

100

FIG. 2. BES auto-power spectrograms for measurements at (a) R = 129 cm and (b) 142 cm; (c) low frequency odd-n magnetic fluctuations; (d) thermal Dα emission measurements; (e) neutron measurements; (f) BES time-series data with NBI power (green line); and (g) BES auto-power spectra at 534 ms with photodetector dark noise spectrum.

50 Frequency (kHz)

100

−200 −100 0 100 200 Time lag (µs)

0 −2

(d) 512.7 512.8 512.9 Time (ms)

513

1

kp = 0.16 cm-1

0

2

−4

10 100 Frequency (kHz)

Correlation

Correlation

dark noise

0.5

−0.5

0

4

(e)

100

0

100

Auto-power spectra

1

time-lag decreases with channel separation, and the poloidal correlation length Lc is the corresponding 1/e length as shown in Figure 3. The time-lag of the peak cross-correlation envelope increases with channel separation, and the linear relationship gives the eddy velocity vg . The peak cross-correlation envelope decreases with time-lag, and the decorrelation time τ d is the corresponding 1/e time-lag. The poloidal wavenumber kθ is derived from the eddy size inferred from the eddy velocity and the time-lab between auto-correlation anti-nodes. Finally, note that coherence spectra are above statistical uncertainties at frequencies below about 35 kHz in Figure 3(a),

0.5

0 −10

−50

(b)

1

(a) 0

1

FIG. 1. BES sightlines provide radial coverage from core to scrape-off-layer regions and four discrete poloidal arrays.

1.5 Phase (θ/π)

R (cm) 110

Rev. Sci. Instrum. 83, 10D502 (2012)

Filtered data (au)

10D502-2

Lcp = 9.9 cm

0.8 0.6 0.4 0.2 0

(f) 0

5 10 Channel separation (cm)

FIG. 3. (a) Coherence and (b) cross-phase spectra among four channels in the BES R = 140 cm poloidal array during an ELM-free, MHD quiescent period; (c) auto-power spectra and photon and dark noise for reference channel; (d) filtered data (8–50 kHz) used for calculations in (e) and (f); (e) time lag auto- and cross-correlation functions and envelopes; and (f) correlation envelope at zero time lag vs. channel separation. Black line in (c)–(e) denotes reference channel.

consistent with autopower spectra exceeding noise levels at frequencies below about 35 kHz in Figure 3(c). Backlight fiber images are 2–3 cm at the center of the neutral beam, but accurate spatial and k-space characterization of BES measurements require point spread function (PSF) calculations.3 PSF calculations convolve optical system properties with neutral beam profiles, plasma geometry, and atomic state lifetimes5 that distort the optical image. For tokamak geometry, PSF calculations can ignore small pitch angle corrections and reduce the calculation to 2D without compromising accuracy, but steep ST pitch angles require full 3D PSF calculations. Figure 4 shows fiber bundle images at R = 133, 140, and 143 cm. At R = 140 and 143 cm, excited state lifetimes in the low density edge region dominate image distortion. At R = 133 cm, magnetic field misalignment dominates image distortion. Without excited state lifetime and magnetic field geometry effects, fiber images are 3.75 cm across at fullwidth half-max (FWHM, 50% of peak intensity). With the effects, the FWHM images are in the range 3.6–3.9 cm, so image distortion from atomic excited states and field line geometry is mild. In summary, the BES diagnostic system on NSTX measures density fluctuations on the ion gyroscale to investigate low-k turbulence and Alfvén modes. Sightline images are 2–3 cm at the neutral beam, and point spread function calculations indicate image distortion from field line misalignment and atomic state lifetimes are minor.


Smith et al. 40

optimal pitch angle

8

30

6 20 4 10

2 0 110

120

130

140

Pitch angle (degrees)

13

3

Density (10 /cm )

10

Rev. Sci. Instrum. 83, 10D502 (2012)

0 150

Machine Y Axis (cm)

10D502-3

R=143 cm R=140 cm R=133 cm

160 140 120 NB sources

100

(b)

80 −20

12

5

R=133 cm

10

Z axis (cm)

Misalignment with B (deg)

R (cm)

8 6

R=140 cm

4

0 20 40 60 80 Machine X Axis (cm)

R=133 cm

R=140 cm R=143 cm

This work was supported by (U.S.) Department of Energy (DOE) Grant Nos. DE-FG02-89ER53296, DE-SC0001288, and DE-AC02-09CH11466.

0

2

(c) 0

R=143 cm

−10 0 10 Distance from NB center (cm)

1 D.

(d) −5 −5

New generation photodetectors with refrigerant cooling at −20 ◦ C achieve photon noise limited measurements at frequencies up to 400 kHz with plasma turbulence signals 2–3 orders of magnitude above noise signals. Initial measurements show broadband turbulence near the pedestal top with frequencies up to 100 kHz and poloidal correlation lengths around 10 cm. Two-megahertz sampling and anti-alias filters enable Alfvèn mode measurements at frequencies up to 800 kHz.

0 Y axis (cm)

5

FIG. 4. (a) NSTX density and pitch angle profiles with BES measurements at R = 133, 140, and 143 cm; (b) BES sightlines crossing neutral beam sources, and short cross-lines show imaging planes; (c) sightline misalignment with magnetic field along sightlines; and (d) PSF image contours at 50% (solid) and 10% (dashed) of peak intensity with magnetic field puncture points (diamonds) on successive image planes.

R. Smith, H. Feder, R. Feder, R. J. Fonck, G. Labik, G. R. McKee, N. Schoenbeck, B. C. Stratton, I. Uzun-Kaymak, and G. Winz, Rev. Sci. Instrum. 81, 10D717 (2010). 2 R. J. Fonck, P. A. Duperrex, and S. F. Paul, Rev. Sci. Instrum. 61, 3487 (1990). 3 M. W. Shafer, R. J. Fonck, G. R. McKee, and D. J. Schlossberg, Rev. Sci. Instrum. 77, 10F110 (2006). 4 N. Schoenbeck, S. D. Ellington, R. J. Fonck, K. Jaehnig, G. R. McKee, D. R. Smith, I. Uzun-Kaymak, and G. Winz, Rev. Sci. Instrum. 81, 10D718 (2010). 5 I. H. Hutchinson, Plasma Phys. Controlled Fusion 44, 71 (2002).
