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Title
Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak
(invited).
Permalink
https://escholarship.org/uc/item/1xv3z17g
Journal
The Review of scientific instruments, 83(10)
ISSN
0034-6748
Authors
Grierson, BA
Burrell, KH
Chrystal, C
et al.
Publication Date
2012-10-01
DOI
10.1063/1.4739239
Copyright Information
This work is made available under the terms of a Creative Commons Attribution License,
availalbe at https://creativecommons.org/licenses/by/4.0/
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University of California
REVIEW OF SCIENTIFIC INSTRUMENTS 83, 10D529 (2012)
Active spectroscopic measurements of the bulk deuterium properties
in the DIII-D tokamak (invited)
a)
B. A. Grierson,
1,b)
K. H. Burrell,
2
C. Chrystal,
3
R. J. Groebner,
2
D. H. Kaplan,
2
W. W. Heidbrink,
4
J. M. Muñoz Burgos,
5
N. A. Pablant,
1
W. M. Solomon,
1
and M. A. Van Zeeland
2
1
Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
2
General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3
University of California San Diego, La Jolla, California 92093, USA
4
University of California Irvine, Irvine, California 92697, USA
5
Oak Ridge Institute for Science Education, Oak Ridge, Tennessee 37831-0117, USA
(Presented 7 May 2012; received 4 May 2012; accepted 9 July 2012; published online 1 August
2012)
The neutral-beam induced D
α
emission spectrum contains a wealth of information such as deu-
terium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density,
and local magnetic field strength magnitude |B| from the Stark-split beam emission spectrum, and
fast-ion D
α
emission (FIDA) proportional to the beam-injected fast ion density. A comprehen-
sive spectral fitting routine which accounts for all photoemission processes is employed for the
spectral analysis. Interpretation of the measurements to determine physically relevant plasma pa-
rameters is assisted by the use of an optimized viewing geometry and forward modeling of the
emission spectra using a Monte-Carlo 3D simulation code. © 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4739239]
I. INTRODUCTION
One popular means of measuring the ion temperature,
density, and toroidal rotational velocity in deuterium neutral
beam heated tokamak discharges with deuterium fueling is
active charge-exchange recombination (CER) spectroscopy.
This technique exploits the charge-exchange reaction between
a high energy neutral donor atom from neutral beam injection
(NBI) and a thermal impurity ion species in the plasma.
1–3
Spectroscopic measurements represent an “apparent” tem-
perature and line-of-sight velocity that is different from the
true temperature and velocity of the emitting ion. The appar-
ent values from the impurity species must be corrected for
energy-dependent atomic cross-section distortions
4,5
in order
to infer the true impurity temperature, velocity, and density.
Impurities are chosen for measurement due to the relative sim-
plicity of the emission spectrum, and the thermal deuterium
properties are inferred from t he impurity measurements.
Previous direct measurements of the bulk deuterium
properties from D
α
spectra
6–8
exposed a number of issues re-
lating to the technique. Some of these issues are as follows:
(i) bright emission originating from plasma edge neutrals,
(ii) halo emission from multiple charge-exchange reactions,
9
(iii) cross-section distortions due to the energy dependence
of the charge-exchange cross section, (iv) finite lifetime of
excited states, and (v) overlapping features in the spectral
a)
Invited paper, published as part of the Proceedings of the 19th Topical Con-
ference on High-Temperature Plasma Diagnostics, Monterey, California,
May 2012.
b)
Author to whom correspondence should be addressed. Electronic mail:
bgriers@pppl.gov.
range, all contributing to uncertainties in the interpretation
of the spectral fits. Underpinning this uncertainty is a failure
to quantify the beam emission intensity and thermal charge-
exchange intensity required for accurate interpretation of the
measurement. Uncertainty (i) is significantly reduced with
a large plasma-wall gap and no neutral fueling during the
measurement, and operation in ELM-free H-mode. Uncer-
tainty (ii) is reduced by full 3D simulation of the halo spa-
tial diffusion and emission, and comparisons with direct halo
measurements. Uncertainties (iii), (iv) are reduced by time-
dependent collisional radiative modeling of non-equilibrium
excited neutral emission, and uncertainty (v) is reduced by
timeslice subtraction of high-speed measurements and a com-
prehensive spectral fitting model incorporating all known
emission features described in Ref. 10. Advances in CCD
technology have also increased the signal to noise ratio and
quality of the measured spectral emission. The technique is
supplemented by detailed knowledge of the content of impu-
rity ions and fast ions from NBI in the plasma and a mature
suite of complimentary diagnostics for integrated modeling.
This article describes advances on the measurement and
modeling of the active D
α
emission spectrum for the purpose
of direct measurement of the thermal deuterium properties of
temperature, density and toroidal rotational velocity, detailing
the basis for results presented in Refs. 10 and 11. Measure-
ments are made using modulated NBI to discriminate the core
localized “active” emission from the passive edge emission
by timeslice subtraction. Simultaneous measurements from
both co-current and counter-current NBI provide a strong con-
straint on the true rotational velocity. A full three-dimensional
time-dependent collisional radiative model
12
is employed for
complete and detailed forward modeling of all active photo-
emission processes.
0034-6748/2012/83(10)/10D529/6/$30.00 © 2012 American Institute of Physics83, 10D529-1
10D529-2 Grierson
et al.
Rev. Sci. Instrum. 83, 10D529 (2012)
Core
Tangential
Core
Tangential
Edge
Tangential
Core/Edge
Vertical
Main-Ion
I
p
B
T
Co-I
p
NBI
Counter-I
p
NBI
330RT
30LT
210RT
330LT
Co-I
p
NBI
FIG. 1. Plan view of neutral beams and CER sightlines. [Adapted from
and reprinted with permission from B.A. Grierson, et al., Phys. Plasmas 19,
056107 (2012). Copyright 2012 American Institute of Physics.]
II. EXPERIMENTAL CONFIGURATION AND
DIAGNOSTIC SPECIFICATIONS
Active charge-exchange recombination (CER) spec-
troscopy on DIII-D (Ref. 13) uses fiber optic viewchords of
six of the eight neutral beamlines, displayed in Fig. 1.The
main-ion CER diagnostic covers 1.70–2.19 m of the co- and
counter-current NBI, with 8 sight lines on each beam (16 to-
tal viewchords). Main-ion CER and carbon CER viewchords
exist at matched radii pairs.
Four McPherson model 207 2/3-m scannable spectrom-
eters with custom asymmetric coma correction are used for
spectral dispersion with 1200 g/mm ruled gratings. Each spec-
trometer accepts four fibers. Two Sarnoff SRI Avanti-768
CCD cameras are mounted “face-to-face” horizontally inside
each spectrometer body with a 90
◦
silver coated prism mirror
between the cameras, reflecting the fiber images from the fo-
cusing mirror both up and down in pairs. The cameras have
a 768 × 256 pixel configuration with 18 μm square pixels.
At 6561.03 Å the reciprocal dispersion is 0.18 Å/pixel. The
system is typically run with 2.5 ms integration, although high
speed acquisition at 100 μs is possible.
Timeslice subtraction is used to extract the active emis-
sion localized to the region of the plasma where the viewchord
and neutral beam cross. An example of spectra demonstrating
the technique of timeslice subtraction is displayed in Fig. 2.
III. NEUTRAL BEAM
It is critical to accurately describe the NBI geometry,
shape, power and injected current fractions in order to inter-
pret photoemission intensities, and to guarantee consistency
amongst neutral beam models. Current implementations of
neutral beam models are used for three primary purposes:
(i) computing profiles of carbon density from the impurity
CER system using a “pencil” beam model, (ii) NUBEAM
(Refs. 14 and 15) Monte-Carlo method called from TRANSP
(Ref. 16) analysis for computing the injected neutral density,
attenuation, beam-ion birth profiles, buildup of beam ion
density, and pitch-energy resolved distribution function,
200 400 600
Channel Number
0
100
200
300
400
Intensity
–4
–2
0
2
4
6
Weighted Residual
Thermal D
α
Carbon CII (1,2)
Cold D
α
Full BES
Half BES
Third BES
Spectral Data
Complete Spectral Fit
6500 6520 6540 6560 6580
Wavelength (Å)
0
5
10
15
20
25
30
146595.02130
Chord: M02
Beam: 30LT
t
integ
: 2.5 ms
Beam Off
Beam On
Active
Carbon CII
Radiance (10
16
ph/s-m
2
-sR-A)
FIG. 2. Timeslice subtraction to obtain active emission (inset). Complete
spectral fit for active spectrum incorporating all known emission features.
and (iii) FIDAsim (Ref. 12) 3D Monte-Carlo simulations of
beam neutral injection, attenuation, halo spatial diffusion,
and the resulting photoemission from beam excited states,
direct charge-exchange, halo emission and fast-ion charge
exchange. Neutral beam descriptions (Gaussian height,
width, and divergence) are obtained from in-vessel laser
alignment and fast-camera imaging.
17
A. Beam into gas
Injection of a neutral beam into a low pressure gas-filled
tokamak reveals the characteristics of the neutral beam
source without measurable attenuation or Stark-splitting of
the emission lines. Beam-into-gas measurements without
and with toroidal field have three primary functions for
main-ion CER: (i) determination of the shape of the neutral
beam emission spectrum for a “beam profile” used for Stark
analysis,
18–20
(ii) evaluation of the accuracy of the spatial
calibration through Doppler line shifts of all viewchords for
a single beam neutral injector voltage,
21
and Stark splitting
of the beam emission for beam into gas with toroidal field,
and (iii) evaluation of the dynamic and steady-state mix of
fractional current injected for the full, half and third energy
components extracted from the beam ion source.
22
Photo-
emission from thermal deuterium charge exchange is more
sensitive to the fractional density of full, half and third energy
components because the cross-section peaks at lower relative
velocity than carbon, which is sensitive to the full energy.
An example of the dynamic change in beam neutral density
over a heating beam “blip,” typically used for diagnostic
purposes, is displayed in Fig. 3 for NBI pulses into deuterium
neutral gas at 81 kV. Evolution of the fractional neutral beam
current for each energy components is highly reproducible,
and analytic formulas for this evolution are used in modeling.
IV. DIRECT CHARGE-EXCHANGE AND
HALO EMISSION
Thermal D
α
emission is produced from two processes
with fundamentally different origins. Direct charge exchange
10D529-3 Grierson
et al.
Rev. Sci. Instrum. 83, 10D529 (2012)
144854 Main-Ion M01 and 30LT
1800 2000 2200 2400 2600
Time (ms)
0
1
2
3
4
5
Beam Current (AU) and F,H,T
Beam Emission Brightness (x10
16
)
Full Photons
Half Photons
Third Photons
Beam Current
FIG. 3. High speed measurement of the beam emission from the three neutral
beam energy components, full (black), half (red), and third (blue).
[DCX, Eq. (1)] is the process of charge exchange from a beam
neutral D
0
b
(n) in some principal atomic number n toathermal
ion D
th
(n
) followed by photoemission. The direct charge ex-
change process is evaluated at the beam neutral velocity less
the plasma rotational velocity with a proper Maxwellian av-
eraged rate. Therefore, the DCX emission displays the cross-
section distortions creating an “apparent velocity” which is
different from the true rotational velocity,
4,5
D
0
b
(n) + D
+
th
→ D
+
b
+ D
0
th
(n
)DCX, (1)
D
0
th
(n) + D
+
th
→ D
+
th
+ D
0
th
(n
)Halo. (2)
The product of the direct charge emission is a thermal
neutral D
0
th
(n
) which possesses a finite lifetime. This first
thermal neutral, which travels ballistically, is now available
for a second charge-exchange reaction with the thermal ions,
if it is not ionized first. Mean-free path for a ballistic thermal
neutral is determined by the probability of ionization by elec-
trons, ions and impurities. When a second charge exchange
reaction occurs, the first halo neutral is formed [Eq. (2)]. Be-
cause the charge capture can occur in any gyro-phase, the halo
eventually occupies a larger physical volume than the beam
through random-walk. The interaction relative velocity of the
halo formation is thermal-thermal, hence the halo does not
suffer from the cross-section distortions associated with the
highly directed beam neutrals. Halo emission intensity is de-
termined by the density of halo neutrals and the fractional oc-
cupation of the n = 3 level, which is largely determined by
the thermal deuterium density, electron density and tempera-
ture. The ratio of direct charge-exchange emission intensity to
halo emission intensity is displayed graphically in Fig. 4 for a
mid-radius tangentially viewing main-ion CER viewchord.
It is clear from Fig. 4 that in low temperature discharges,
increasing the plasma density increased the halo emission in-
tensity, seen by the approximately 10% ratio of DCX/halo
emission. In contrast, low density plasmas with appreciable
ion temperatures are dominated by DCX emission. Figure 4
also indicates that the two processes are the same order of
magnitude for a large range of plasma conditions.
The combination of direct charge-exchange and halo
emission occurs as a single feature in the D
α
emission spec-
trum. Modeling with FIDAsim shows that the Doppler shift
from the cross-section distortion is not sufficiently shifted
–1.50
–0.75
0.00
0.75
1 2 3 4 5 6 7
1
3
5
7
Ratio DCX/Halo Brightness
1.50
T
i
(keV)
n
D
(10
19
m
–3
)
Log
10
FIG. 4. FIDAsim lookup table of the ratio of DCX to halo emission bright-
ness as a function of ion temperature and deuterium ion density.
from the halo emission to be un-ambiguously fit as two sepa-
rate Gaussians. Therefore, the interpretation of the measured
Doppler shift “apparent velocity” depends strongly on the ra-
tio of the emission intensity of one process compared to the
other. In the DCX limit, there are strong cross-section dis-
tortions. In the halo limit, the apparent velocities are very
near the true velocities, but the spatial resolution is compro-
mised. It is this uncertainty which has confounded previous
attempts to accurately determine main-ion parameters from
active main-ion spectroscopy in deuterium plasmas. Uncer-
tainty in the DCX/ Halo ratio is overcome in the present study
by two methods: (i) direct measurement of the halo emission
and (ii) views of both co- and counter-current NBI.
A. Direct measurement of halo emission
Halo emission exists in a spatial volume larger than the
neutral beam. Core vertical viewchords displayed in Fig. 1
are oriented underneath one of the neutral beams, with an-
other beam adjacent. When the perpendicular beam is fired
into gas, no measurable beam emission is detected from the
core vertical views. Thus any thermal D
α
emission detected
when this neutral beam is injected originates from the spa-
tially larger halo. This viewing configuration permits a di-
rect measurement of the neutral halo. An experiment was per-
formed whereby the central vertical viewchords were tuned
to D
α
, and the neutral beam was modulated in time to enable
timeslice subtraction.
Figure 5 displays the midplane values of halo, full energy
beam, DCX, and FIDA emission intensities, as well as the
locations of the vertically viewing CER chords. It can be seen
by comparing Fig. 5(a) and Figs. 5(b) and 5(c) that the halo
has expanded to a volume larger than either the beam, or the
DCX, which enables the halo measurement. FIDA emission
is nonzero over the vertical sightlines, however the intensity
of the FIDA emission is lower than the halo emission by well
over an order of magnitude.
Active D
α
emission was fit for the discharge and one such
time at the highest plasma density is displayed in Fig. 6(a).Pa-
rameters of interested are noted in figure. Also included is the
halo emission predicted by FIDAsim. Although there is sig-
nificant scatter in the measured photoemission intensity and
profile shape of the halo emission intensity are reasonably
well captured, however FIDAsim under-predicts the amount
of halo emission. Halo emission intensity profiles are largely
determined by the neutral beam attenuation, in keeping with
10D529-4 Grierson
et al.
Rev. Sci. Instrum. 83, 10D529 (2012)
Halo Photons
–1.6 –1.4 –1.2 –1.0 –0.8 –0.6
0.5
1.0
1.5
2.0
0.00
1.25
2.50
Centerpost
Vessel Wall
Separatrix
Full
Energy
Beam
–1.4 –1.0 –0.6
0.5
1.0
1.5
2.0
0.00
2.75
5.50
DCX FIDA
Midplane u (m)
Midplane v (m)
(a)
(10
16
ph/s-m
2
-sR)
(b)
(d)(c)
–1.4 –1.0 –0.6
–1.4 –1.0 –0.6
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.00
2.75
5.50
0.000
0.175
0.350
FIG. 5. Midplane quantities of of photoemission computed from FIDAsim
in DIII-D discharge 145241 at 02400 ms. Displayed are (a) halo, (b) beam,
(c) DCX, and (d) FIDA emission intensity.
simple considerations that the halo density is determined by
the number of beam neutrals present to begin the multi-step
halo creation and diffusion process, which is l argely a func-
tion of the steep beam attenuation in plasma with a relatively
flat density profile. An illustration of the spatial smearing ef-
fect on measured ion temperature incurred by vertically inte-
grating through a volume which is much taller than it is wide
is displayed in Fig. 6(b). The measured reduction in appar-
ent ion temperature is consistent with the halo effect, not an
actual reduction in the deuterium temperature. It is notewor-
thy that the tangential main-ion CER system does not suffer
from such gross halo distortions, because the neutral beams
are much narrower than they are tall. In principle, the mea-
sured halo emission contains the thermal deuterium poloidal
velocity profile unaffected by the complexities of gyro-orbit
cross-section corrections;
23
however, the spatial accuracy is
compromised.
B. Combined cross section and halo effects
In order to accurately deduce the true velocity of an
emitting ion species from the “apparent” velocity determined
by the Doppler line shift obtained from a spectral fit, de-
tailed knowledge of the viewing geometry, neutral beam
characteristics and emission rate coefficient must be taken
into account.
4,5
An alternate technique to determine the
true rotational velocity is possible by having complimentary
views of neutral beams which inject into the plasma in
opposing directions.
24
Synthetic pure plasma profiles of
electron and deuteron density and temperature were created
and forward modeled in FIDAsim for creating a lookup table
of corrections. One such profile is presented in Figs. 7(a)–7(c)
for central density of n
e
= 2.5 × 10
19
(m
−3
), zero toroidal
rotation, and a relatively high central ion temperature of
8 keV to demonstrate conditions where the cross-section dis-
0.0
0.2
0.4
0.6
0.8
1.0
Halo Br. (10
18
ph/s-m
2
-sR)
145241.02400 ± 50 ms
〈
n
e
〉
= 6.35 (10
19
m
–3
)
Measured
FIDAsim
1.7 1.8 1.9 2.0 2.1
R (m)
0.0
1.0
2.0
3.0
Carbon
Measured
FIDAsim
(b)
(a)
T
i
(keV)
T
e
= 1.9 keV
T
i
= 2.2 keV, Z
eff
= 3.0
V
tor
= 200 km/s
FIG. 6. Radial profiles of halo emission intensity and apparent temperature
which is reduced due to spatial smearing.
tortion will be significant. Figures 7(a)–7(c) present the input
ion temperature profile and the temperature profile obtained
from fitting the combination of halo and DCX photoemission
spectra that have been added together; Fig. 7(b) contains the
amount of photoemission from DCX and the halo which is
predicted; Fig. 7(c) contains the input velocity (zero) and
apparent toroidal rotational velocities from single-Gaussian
FIG. 7. FIDAsim predicted apparent velocities in a non-rotating plasma. (a)
Input and synthetic diagnostic ion temperature profiles. (b) FIDAsim pre-
dicted DCX and halo brightness. (c) FIDAsim predicted apparent velocities
for a non-rotating plasma, as well as co- and counter-view correction. (c)-(i)
Displays synthetic spectrum for co-view and (c-ii) displays synthetic spec-
trum for counter-view.
![FIG. 1. Plan view of neutral beams and CER sightlines. [Adapted from and reprinted with permission from B.A. Grierson, et al., Phys. Plasmas 19, 056107 (2012). Copyright 2012 American Institute of Physics.]](/figures/fig-1-plan-view-of-neutral-beams-and-cer-sightlines-adapted-2l2qg3ps.webp)
