Proceedings of ITC/ISHW2007

A systematic study of impurity ion poloidal rotation andtemperature profiles using CXRS in the TJ-II stellarator
J. M.Carmona, K. J. McCarthy, V. Tribaldos, M. Ochando and TJ-II Team
Laboratorio Nacional de Fusión, EURATOM-CIEMAT, E-28040 Madrid, Spain
A detailed analysis of the calibration methods for the new CXRS-DNBI diagnostic system and a systematic study ofimpurity profiles measured under different plasma conditions have been carried out in TJ-II and presented here. Ion
temperatures profiles tend to be flat in the measured region, showing correlation between ECRH power, averaged electrondensities and the Ti value. Transition of poloidal velocities from ion to electron diamagnetic direction when increasing theline averaged electron density (<ne>) was confirmed, agreeing with previous works. As line averaged density wasconsidered to be the driver-parameter of this transition, shots with the same electron density and different ECRH power
were compared to highlight other contributions as Te. A change in ion temperature and poloidal rotation direction is foundwhen comparing these shots, suggesting that the plasma collision frequency (~neTe-3/2) tends to control the dynamics in
TJ-II, giving a clearer description of the trigger mechanism. First experimental evidence in TJ-II that the shear point for
impurity velocities moves outwards when increasing ne, is also presented.
Keywords: Charge exchange spectroscopy, temperature profiles, poloidal rotation

1. Introduction
Spatially resolved measures of ion temperature andpoloidal rotation are important for the understanding ofplasma dynamics. For this reason, an active
charge-exchange recombination spectroscopy (CXRS)
diagnostic, based around a diagnostic neutral beam
injector (DNBI) and a bidirectional (two verticalopposing views) multi-channel spectroscopic system [1,2], has been set-up on the TJ-II, a four-period [3], lowmagnetic shear, stellarator. It provides 5 ms long pulses,
up to two per discharge, of neutral hydrogen accelerated
to 30 keV with equivalent current of 3.3 equ. A. The
bidirectional diagnostic is designed to measure Doppler
shifts and widths of the C VI line at λ = 529.06 nm in up
to 3 arrays of 12 channels with ~1 cm spatial resolution
across the plasma minor radius. The light dispersion
element is a Holospec spectrograph with three 100 µm
curved entrance slits (these are curved to compensate forits short focal length) and a transmission grating
sandwiched between two BK7 prisms. It provides a
focal-plane dispersion of ~1.15 nm/mm at 529 nm.
Moreover, a narrow bandpass filter (2.0±0.5 nm at
full-width at half-maximum FWHM) centred on 529 nm
prevents spectral overlapping from the multiple fibre
arrays at the image plane. The set-up includes ahigh-efficiency back-illuminated CCD camera and fastmechanical shutter (=4.5 ms time window). With on-chipbinning, multiple spectra can be collected during
discharges (=300 ms). Finally, fibre alignment was
performed by illuminating each fibre bundle with a bright
light source and observing, through an unused viewport,
author’s e-mail: jm.carmona@ciemat.es

the orientation and location of the resultant bright spots
with respect to markings on the inside of the opposing
vacuum flange. In this way, the sightlines throughplasmas can be determined using a cross-sectional
machine drawing and magnetic configuration maps.
Hence, when compared with the neutral beam geometry
the normalized radius, r/a, corresponding to eachbeam/line-of-sight interaction volume is found. Finally,
for the magnetic configuration used in this study,
100-44-64, the viewing system covered the plasma minor
radius from r/a= 0.3 to 0.85.

2. Experimental details
2.1. Diagnostic set-up
The TJ-II is a four field period heliac type stellarator(B(0) = 1.2 T, R = 1.5 m, <a> = 0.22 m) designed toexplore a wide rotational transform range (0.9 = ι(0)/2π =2.2) in low, negative shear configurations (∆ι/ι < 6%).
The experiments reported here were carried out inelectron cyclotron resonance heated (ECRH) plasmas(PECRH= 300 – 500 kW, 53.2 GHz, 2nd harmonic, X-mode
polarization, tdischarge = 250 ms) with hydrogen as theworking gas [3] and lithium coated walls (previouslyboronized). No auxiliary NBI heating was provided. As aresult, central electron densities and temperatures up to~1019 m-3 and ~1.5 keV respectively were achieved. The
strong reduction in impurities concentration (i.e. carbon)
due to the wall-coating results in reduced impurity photon
fluxes thereby requiring efficient detection procedures.
Nevertheless, the use of a high-throughput spectrographand an efficient CCD detector made the goal ofProceedings of ITC/ISHW2007
measuring in such conditions possible.
2.2. Instrument calibration
Special attention has been given to wavelength
calibration and instrumental function determination.
Calibration for each individual fibre was done in order todetect small variations in the alignment of the fibres andin the FWHM of spectral lines. Spectra were captured
between discharges using a retractile pencil-type Neonlamp positioned between the lens and fibre bundle head.
x 1041.5
Fiber data
fitted curve

1.4

Ne I=530.4758 nm

1.3

Ne I=529.8189 nm

Counts1.2
1.1
1

Ne I=528.0085 nm
0.9
0.8
0.7
0.6
460

480500520

540560580

600

620640

PixelsFig.1 Ne I lines used for calibration (close to 529 nm)

Wavelength calibration was performed using asecond order (polynomial) fit to the three nearby Ne I
spectral lines (figure-1). As spectra are recorded
simultaneously for all fibres, fine corrections can be madefor any nonlinear dispersion present in the spectrometeror displacement errors in the entrance slits. Next, theinstrument function was determined from the same
spectra using a Gaussian function fitting. It was found tobe ~7.6 pixels (~1.14 Å) and ~6.3 pixels (~0.95 Å) for themid fibres of the upper and lower arrays respectively(figure-1 in [2]) and to vary along the fibre array.
Figure-2a illustrates the variation in FWHM along
the lower array. Such a variation is inherent to the set-upof this instrument [4]. Part of it may also be due toinaccuracies when manufacturing the slits (a similar
behaviour is observed for the upper array).
9(b)
593.5

X0 (pixels)

FWHM (pixels)

(a)
8.5

87.5

7593

592.5

24

considered preferable to non-robust ones, as the former
are less sensitive to outlier points (points numericallydistant from the rest that can lead to misleading fits).
However, here it was found that robust fits show
significant fibre-to-fibre variations for both FWHMs(figure-2a) and line centres whilst non-robust fits
(figure-2b) are more stable. Indeed, in certain situations
robust methods may be preferable, e.g. for single
Gaussian fits, but they prove to be weak when multiple
Gaussian fits are performed. Hence, the non-robust
method was chosen as the fitting method.

6Fiber

81012592

24681012

Fiber

Fig.2 Variation of (a) instrumental FWHM (b) on fibre
position along the middle fibres for robust (red) andnon-robust (blue) methods. The symbols represent
different frames.

It should be noted that the neon lines fits were madeusing a non-linear least squares method based ontrust-region algorithms. For this, comparisons between
different fitting methods were made (i.e. robust andnon-robust). See figure-2a (FWHM) and figure-2b(central position). In general, robust algorithms are

2.3. Active and passive signal treatment
CXRS is widely employed to obtain spatiallyresolved measurements, although it presents severalchallenges. For instance, contributions to measured
signals from ionized impurities (called the cold
component or passive signal) must be removed since it isa non localized emission. Also plasma conditions shouldremain constant along a discharge, or discharges should
be reproducible (shot to shot technique), so that the active
signal can be obtained by subtraction, i.e. (A+P)-(P) =A.
See figure-3. For the data presented here, both techniques
were employed.
2.4. Data Analysis
Once the instrumental and calibration functions havebeen obtained for each fibre using the neon lamp, fits tothe line of interest (i.e. C VI @ 529.06 nm) can be madeso that Doppler shifts and widths can be estimated
together with errors. Ion temperatures and velocities are
deduced from the usual relations:22 ∆λ fwhm
− ∆λinstrum
Ti = 1.68 ⋅108 A 
2

λ0


(1)
 λ − λ0 v=c λ0 
where A is the mass of the ion [amu], ∆λfwhm themeasured full width at half maximum (FWHM), ∆λinstrum
the instrument FWHM, λ and λ0 are the measured and theunshifted central wavelength of line under consideration
respectively and c the speed of light. For this work, awavelength of λ0=529.06 nm is taken as reference value
in equation (1) (the line centre depends weakly on plasmaconditions).
2.4.1. Fine structure corrections to C VI data
A clear asymmetry was observed about measured
spectra line data. This was interpreted as arising from finestructures in the transitions in this hydrogen-like ion [5,6]. Indeed, there are up to 13 possible lines with different
intensities depending on plasma conditions. Such finestructure give rise to the tail observed on the shortProceedings of ITC/ISHW2007

200I

150T(eV)wavelength side that results in apparent line broadening,and hence overestimated ion temperatures.
Thus, in order to correct for this a multi-Gaussian fit,using relative intensity ratios between fine structurecomponents, was employed here. Finally, after carefulanalysis and consideration, contamination by a nearby O
VI line was excluded as there is a strong oxygen
reduction due to the boron and lithium coated walls.

10050
000.20.4

2000

T CVI=65.4 eV-1Vθ=-2.45 kms

1500

600
400

1000

Counts (active)

1with Pecrh=395 kW and line averaged densities ne=0.42 x
1019 m-3 (dots) and ne=0.90 x 1019 m-3 (triangles) as afunction of normalized plasma radius. Lines are plotted toaid the reader.

200528.87

529.02529.20529.320wavelength (nm)Fig.3 Spectra of subsequent CCD frame at r/a=0.55,
showing CX with C+6. Diamonds, triangles and dots areactive, passive and active+passive signals respectively. A
fit to the active CVI line is also shown.

3. CXRS profiles and discussion
For the study undertaken, hydrogen plasmas withlithium coated walls were made, lasting =250 ms, withline averaged electron densities between 0.4 x 1019 m-3
and 0.9 x 1019 m-3 heated only with ECRH. In order toseparate active from active plus passive C VI signals only
discharges with almost constant electron density andtemperature were used.
3.1. Measurements of ion temperature and poloidal
rotation profiles
First CXRS profiles of measured ion temperatures
for two discharges with 395 kW of ECRH but different
line averaged electron densities (ne) are shown in figure-4.
In these plots, the measured C6+ impurity temperature isfound to be higher for the discharge with lower ne.
Previous studies carried out in the TJ-I tokamak for C4+ions [7] and in the TJ-II for several different ions (C4+ andFe15+, etc…) [8] using passive spectroscopy, reported asimilar behaviour. Then, the anomalous temperatures
were attributed to non-thermal velocity contributions.
It is apparent from figure-4 that ion temperature
profiles are relatively flat and begin to drop for r/a=0.7.
Temperature values are measured between 50 and 150 eV,
in accordance with the weak collisional coupling forthese densities.
For the same shots, poloidal rotation profiles areshown in figure-5. A clear transition from ion to electron
diamagnetic direction can be seen.

For lower line-averaged electron densities (<ne>),C6+ ions appear to rotate poloidally in the ion diamagnetic

direction whereas at higher values this changes towards
the electron diamagnetic direction.
30-1500

Poloidal Velocity (kms )CCD counts

0.8Fig.4. Ion temperature profiles obtained for two discharges

800

2500

0528.72

0.6r/a3000

20Ion
diamagnetic

100-10Electrondiamagnetic

-20
00.2

0.4

0.6

0.8

1r/a

Fig.5. Poloidal rotation profiles of C6+ for shots includedin figure 4. Lines are plotted to aid the reader.

An in depth analysis of this change in rotation has
been performed from a detailed density scan with CXRS
diagnostic. This is supported below.
3.2. Density scan
A density scan (from 0.4 to 0.92 x 1019 m-3) for 395
kW of ECRH has been made and the poloidal velocities
measured are presented. See figure-6. The impurity
poloidal velocities show a change of direction as the line
averaged electron density is increased, from ion toelectron diamagnetic direction (transition from electron toion root). Such behaviour was also reported in the TJ-II
for passive measurements of CV ion temperatures
previously [9] thereby providing us with confidence of
these CXRS measurements.
Note that, the transition point (i.e. where the poloidal
velocity~0) moves towards the edge (from r/a~0.4/0.55 tor/a~0.7/0.8) as <ne> increases from ~0.57 to ~0.77 x 1019
m-3.

Proceedings of ITC/ISHW2007
This is the first experimental evidence for C6+ thatthe shear point of impurity ion rotation moves outwards
in TJ-II with increasing electron density.
r/a=0.4
r/a=0.55
r/a=0.7
r/a=0.8

20
1020

θ

Poloidal Velocity CVI (Km/s)

30scan), or vice versa (density scan) ion temperatures canbe expected to vary, i.e. increasing when (a) Te decreases(fixed <ne>) or (b) <ne> increases (fixed Te).
From this, the impurity rotation profile in the region
accessible, is seen to shift to the electron diamagnetic
direction as the injected PECRH is increased.

0Ion
diamagnetic

10-1

V (kms )-10-30

0.4

0.6

0.8
19average density x10

1-3

m+6Fig.6. Change on C poloidal velocity from ion to electron
diamagnetic direction while increasing line averaged

200IT (eV)

150
100
50

0.6

0.8

1r/a

Fig.7. C6+ temperature profiles for 400 kW (dots) and 500
kW (triangles) of PECRH and the same line averaged
density <ne>=0.6 x 1019 m-3, showing lower temperaturesfor higher PECRH.

In these figures, lower temperatures are found forhigher PECRH at fixed <ne>. The better coupling between
electrons and ions for lower PECRH (lower Te) and thedecreasing impurity confinement times [10] are expected
to cause the increase in temperature in these plasmas
ECRH.
A simple power balance equation can help toexplain the measured profiles. This is given byTbb/ e= ν E (Te − Tb ) − bdt
τE

0.2

0.4

0.6

0.8

1

Fig.8. C6+ rotation profiles for 400 kW (dots) and 500 kW

3.3. Profiles under different PECRH
Ion temperatures and poloidal rotation profiles arecompared for different ECRH conditions. A comparison
between shots with a fixed <ne> and PECRH of 400 kW
and 500 kW are presented. See figure-7 and figure-8.

0.4Electrondiamagnetic

r/aplasma radii r/a. Lines are plotted to aid the reader.

0.2-10-200

electron density. This is plotted for different normalized

000

θ

-20

dTb(2)

where νEb/e~neTe-3/2. Better coupling (higher collisionfrequency νEb/e) between ion and electron species(increasing ne or decreasing Te) give rise to an increase ofion temperature. With fixed <ne> and varying Te (power(triangles) of PECRH and line-averaged electron density<ne>=0.60 x1019 m-3.

4. Summary
A systematic study on the calibration system had
been carried out to characterize system capabilities anddetermine the best data-analysis procedure. Ion
temperature and poloidal rotation profiles were measured
under different plasma conditions. As seen fromexperiments, the change from ion to electron diamagnetic
direction and ion temperatures depends on <ne> and Te,
suggesting that the control-parameter is the plasma
collision frequency νEb/e~neTe-3/2. Experimental evidenceof displacement with electron density of the shear point
for C6+ in ECR heated plasmas is reported for the firsttime in TJ-II.

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