J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Observations of Hα emission profiles in Aditya tokamak
Santanu Banerjee, J. Ghosh, R. Manchanda, R. Dey*, N. Ramasubramanian, M.B. Chowdhuri, Ketan
M. Patel, Vinay Kumar, P. Vasu, P. K. Chattopadhyay, P.K. Atrey and Aditya Team
Institute for Plasma Research, Bhat, Ghandhinagar 382 428, India* Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, D-85748, Garching, Germany(Received: 30 October 2009 / Accepted: 15 January 2010)

Emissions from the hydrogen Balmer alpha (λ = 656.28 nm) has been recorded for a large number of plasma
discharges in the Aditya tokamak using a 1m Czerny-Turner spectrometer equipped with an 1800 grooves/mmreflection grating [5]. Eight simultaneous vertically collimated line-of-sights, using individual lens – fiber
combination from a top port of the tokamak view a poloidal cross-section of the plasma. The line-of-sights can bemoved along the major radius to obtain emissions from different major-radial positions on a shot–to–shot basis.
Abel-like matrix inversion has been performed to obtain radial profile of volume emissivities from thesechord-integrated intensities. Considerable Hα emission is observed in the bulk plasma indicating a considerable
neutral penetration. Further, a second peak in the Hα radial profile has been observed at ρ(r/a) ~ +/- 0.3 -- +/- 0.5 inmajority of discharges irrespective of the plasma column position. This observation suggests a considerable
accumulation of neutrals in the region of ρ(r/a) ~ +/- 0.3 -- +/- 0.5. CV to CIII line ratio variations at the same
location also suggest a substantial presence of neutrals explained by the charge-exchange, involving collisions
between H-like carbon ions and neutral hydrogen atoms.
Keywords: tokamak, Abel-like matrix inversion, Balmer alpha, neutral transport.

In section 3 an overview of the calibration of thespectroscopic instruments has been given. Finally, in
section 4 the obtained results are discussed.

1. Introduction

Neutral atom transport properties play an importantrole in tokamak plasma confinement [1]. Therefore, it isnecessary to measure the neutral atom density profile in atokamak. Direct measurements of the neutral (hydrogen)density n0, in tokamak plasmas are almost lacking.
Except for few attempts with LIF [2], often indirect
information via charge exchange spectroscopy (CX) ofsensible intensity ratios of impurity (carbon) ions is used
[3,4].
Several investigations of spectral line ratios in both
VUV and X-ray regions of the spectra often required
substantial presence of neutral atoms in the hotter central
regions of the tokamak to resolve the observed anomalies.
In this paper we measured the neutral atom density
profile by monitoring the hydrogen Balmer alpha (λ =656.28 nm) emission from the Aditya tokamak. This
radiation results from the collisional excitation of aneutral hydrogen atom from the device wall or limiter
surface enters into the main plasma. It is generally
assumed that the neutral hydrogen atoms can penetrate to
the hot central plasma by diffusion from the edges
through successive charge transfers with the working gas
ions [1].
Section 2 briefly describes the experimental setup.

2. Experimental setup

Up to eight simultaneous chords or lines-of-sight(LOS) can be set up from a top port window on Aditya
along a poloidal diameter from the inboard to outboard
edge. The 1 mm core diameter of fibers and collimating
lenses (diameter = 11 mm, focal length = 19 mm) yield aspatial resolution of 25 mm at the mid-plane (increasing to42 mm at the bottom edge of the plasma). Figure 1A shows aschematic of the viewing geometry. The back ends of these(∼20 m long) fibers couple to eight 400 m core diameter
input fibers on the spectrograph entrance slit with acenter-to-center separation of 700 m. The slit height canaccommodate up to nine of these 400 m core fibers. The
ninth fiber is used as a reference track for recording
spectrum from a neon lamp during discharges [5].
The Multi-track Spetrometer system consists of a 1.0
m long Czerny–Turner spectrometer fitted with a 2D CCD(charge-coupled device) camera with 1024 × 256 pixels (1pixel = 26 × 26 m).
A normal incidence spectrometer was used to record
the spectrum in the wavelength region 110 – 300 nm,
which views plasma from a radial port. The view chordauthor’s e-mail: sbanerje@ipr.res.in29©2010 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchS. Banerjee et al., Observations of Ha Emission Profiles in Aditya Tokamak

can be chosen vertically from the midplane to 20 cm
above the midplane, by tilting a silver mirror in thetransmission path as shown in the figure 1B. The detector
attached to the exit slit of the spectrometer is an
Intensified Charge Coupled Device (ICCD) camera.

6

Chord Integrated Brightness (Photons Sr.

-1-2-1cm s )13x 10
6.5

Optical Fiber Bundle to Spectrograph

LENSa = 25 cm
33 cm

5.5
5
4.5
4A3.53-1-0.5

00.5

11210-1-3-1

Emissivity (Photons Sr. cm s )ρ (r/a)AB

1110B-1-0.5

00.5

1

ρ (r/a)13

2.2

Figure1: Schematic of the viewing geometries ofMulti-track Spectrometer (A) and Normal
Incidence Spectrometer (B) systems respectively

x 102-3

Electron density (cm )1.8

3. Calibration

We first established a relative calibration among allnine tracks. Light from a fluorescent lamp was diffusedover an area of 50cm×10cm using a milky white plastic
diffuser plate in transmission. The uniformity of thediffused light over this area was ensured using the same
fiber across the length and the width of the region. Then
all of the nine input fibers were illuminated with thisextended uniform source and the nine tracks of spectra
were recorded at different wavelengths from 350 to 800
nm. The spectrum consisted of both mercury lines and abackground continuum. Relative calibration factors do
not show any wavelength dependence over the entire
visible range. A statistical variation of less than 1% foreach frame was observed by acquiring a number of thesenine track frames at a single wavelength. Further, A
standard tungsten lamp (NIST, USA) was used tocalibrate the fiber–spectrometer–detector system forabsolute intensity measurements. One of the ninechannels was calibrated using the lamp. The calibration
was transferred to the other fibers using the relative
calibrations obtained above [5].

1.6
1.4
1.2
10.8

C

0.6
0.4
-1-0.500.5

1ρ (r/a)300

DElectron temperature (eV)

250

200

150

100

50

0-1-0.50ρ (r/a)

30

0.5

1

S. Banerjee et al., Observations of Ha Emission Profiles in Aditya Tokamak

observed spectra are fitted with Gaussian profiles using
the least squares fitting routines and the brightness iscalculated for each chord by integrating the Gaussian
curve used in the fit. The standard errors in the amplitude
from a Gaussian fit are calculated assuming a normal
distribution of detector noise, which depends mainly on
the signal-to-noise ratio in the brightness observed.
Uncertainty in the fitting procedure is less than 1%. Theerror propagation in the inversion was calculated
considering that σb2 = [M2] σa2, where b and a are vectors
operated by matrix [M], b = [M] a, and σa and σb are thestandard deviations of vectors a and b. The error in [M] isconsidered negligible. Furthermore, the statistical errors
present in the relative and absolute calibration of thecollection optics are incorporated. Owing to the excellent
signal to noise ratio for Hα, the uncertainty in therecorded chord integrated brightness is less than 5%,
inclusive of all the contributing factors. For an averagenoise in the brightness of 5%, uncertainty introduced byinversion in the obtained emissivities is less than 15%.
Neutral atom density profile is now calculated form theemission profile along with the electron density andtemperature profiles and the electron impact excitation
and recombination rate coefficients from ADAS [10].
Electron density profile and central electron temperature
are obtained experimentally from the microwave
interferometry and soft x-ray diagnostics respectively. It
has been found that the neutral atom density decreases up
to 1.81 × 108 at ρ (r/a) = 0.4 from its value of 9.04 ×
1010 near the limiter location and then peaks up again to avalue of 2.5 × 109. Simple calculation for neutral
penetration shows that the neutral density should decay
exponentially with minor radius. These results arerepeated in many shots and the dip in the neutral density
profile seems to be real as it is quite above theexperimental error bars.
Furthermore, investigation of line ratio of CV (C4+), theHe like carbon lines are identified at wavelengths 227.1 and
227.7 nm (two of the lines are merged), to CIII (C2+), Be likecarbon seen at 229.7 nm, also shows an anomaly at thelocation ρ (r/a) = 0.3 which suggests a substantial presenceof neutral at that location for charge exchange to take place
and resolving the anomaly (figure 4, top panel). Since
neutral beam has not been used in Aditya, charge exchange
involving slow collisions between the recycling hydrogenneutrals and the CVI (H-like Carbon ions) are of interest as
shown in equation (1).

Figure 2: From A to D: Chord integrated and absolutecalibrated brightness profile of Hydrogen Balmer
α emission, Emissivity profile of the sameobtained from Abel-like matrix inversion of thechord integrated signal, Electron density andtemperature profiles in one of the characteristic
discharges in Aditya tokamak.

4. Results and Discussion-3

Neutral Density (cm )The Aditya tokamak [6] is an air-core machine with
BT = 0.75 T. The plasma with major radius R = 0.75 m,minor radius a = 0.25 m is limited by a poloidal graphite
limiter of circular cross-section. For the shot data used here,
the typical discharge parameters were: plasma current, IP =60– 70 kA, average electron density, ne = 1×1013 cm−3 andplasma discharge duration = 100 – 120 ms.

1010910-1-0.5

00.51

ρ (r/a)Figure 3: Neutral hydrogen density profile obtained fromthe Hα emissivity profile, electron density andtemperature profiles and atomic reaction rate
coefficients from ADAS
From the multi-track chord-integrated measurements,
recorded with a 200 m entrance slit width of thespectrometer (Figure 2A) radial emission profiles (Figure2B) are obtained using an Abel-like matrix inversion
technique [7-9]. The plasma volume is partitioned intoradial zones with constant emission, Ej, constanttemperature and constant velocity, with the radial extent
of each zone being the coverage of a particular chord.
The brightness, Bi, of a chord i is given by Bi = Lij E j ,where Lij is the path length of the ith cord through the jthzone. The emission, Ej, can be obtained by inverting theabove equation. The inversion algorithm was tested bygenerating synthetic chord-line integrated profiles ofemission. Different amounts of Gaussian white noise has
been incorporated in the simulated brightness profile toascertain the sensitivity of the inversion algorithm. TheC 5+ + H → C 4+ + H + + ∆E(1)

However, at the location where the neutral density goes
down there is no anomaly observed in spite of presence of31

S. Banerjee et al., Observations of Ha Emission Profiles in Aditya Tokamak

References

CVI ions as shown in figure 4, bottom panel, calculated
from STRAHL code [11,12]. If the neutral density remains
large at that point, which is not the case (figure 3) then weshould have observed the anomaly there too. This has beenreported earlier in similar tokamaks [4]. Hence, the neutral
density profile observed seems to be correct with a second
peak coming at ρ = 0.3. The reason for the observed increasein the neutral density and thereby enhancing charge
exchange with CVI at ρ = 0.3 is still not clear and further
investigation and analysis are underway to understand theaccumulation of neutrals at ρ = 0.3.

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Ratio (C V / C III)

1086
4
200.0

0.2

ρ

0.6

0.8

1.0

B

Gm-3

2 – CIII
3 – CIV
4 – CV
5 – CVI

0.4

45

32Figure 4: 4A shows the ratio of CV to CIII as a functionof ρ (r/a) measured with the normal incidence
spectrometer; 4B shows the density distribution
for different charge states of Carbon as estimatedfrom the STRAHL impurity transport code.

5. Acknowledgement
The authors would like to thank R. Jha, D. Raju, Rajiv
Goswami and Nirav Joshi and for many useful discussions.

32