Proceedings of ITC/ISHW2007

Use of a High Resolution Overview Spectrometerfor the Visible Range in the TEXTOR Boundary Plasma
Sebastijan Brezinsek, Albrecht Pospieszczyk, Gennadij Sergienko, Phillipe Mertens, Ulrich Samm
Institut für Energieforschung - Plasmaphysik, Forschungszentrum Jülich GmbHEURATOM-Association, Trilateral Euregio Cluster, D-52425 Jülich, Germany
October 22, 2007
Passive spectroscopy is a standard diagnostic to observe the boundary layer of fusion plasmas. This visible
spectroscopy is focused on the measurement of the deuterium recycling flux as well as on the monitoring of
impurity fluxes like e.g. O, He... or W, C... which result from erosion of plasma-facing components. Moreover, thero-vibrational analysis of molecular transitions provide information about the molecular break-up in the plasma.
Spectrometer for the plasma boundary have to fulfil high demands with respect to the spectral, spatial and time
resolution, to the observable wavelength range, to the sensitivity and dynamic range of the detector to observe
and analyse simultaneously the emission of atomic and molecular species present in the observed region. We
present an overview spectrometer system which is able a) to measure at once strong atomic lines (e.g. Dα ) andweak molecular bands (e.g. C2 Swan-band), b) to resolve narrow molecular lines (e.g. D2 Fulcher-α band)
and allow their ro-vibrational analysis, and c) to provide information about the spatial distribution in the plasmaboundary. A full characterisation of the custom-made system in cross-dispersion arrangement with respect toresolving power, simultaneous wavelength coverage, sensitivity etc. is done. Spectra examples (”footprints”)
taken from an injection of C3 H4 into TEXTOR are presented.
Keywords: Plasma-Wall Interaction, Cross-Dispersion Spectrometer, Plasma Boundary Spectroscopy, Fulcher-αband Spectroscopy, Hydrocarbon Break-Up Mechanism

1 Introduction
Present-day fusion devices use graphite and/or tungsten asa plasma-facing material (PFM) at the locations of highest
particle flux such as the divertor target plates. Complex
processes such as physical and, in the case of graphite,chemical sputtering take place at the surface. Material erosion and subsequent deposition leads to the formation of
surface layers, which can -in the presence of two or more
PFMs- contain complex mixed materials. A limited number of in situ diagnostics is currently available to identify
and monitor the different species which are released fromthese PFMs under particle bombardment.
Passive spectroscopy in the visible range is a standard diagnostic to monitor the fluxes of carbon-containing species(C, CD...), which are released from the surface or build
during the molecular break-up [1], other fusion-plasma related impurities (W, O...) as well as the fluxes of recycled
fuel particles (D, D2 ...). To observe and analyse the emission of these species simultaneously from a given observation volume, a spectrometer system has to fulfil high demands with respect to the spectral, spatial and time resolution, to the observable wavelength range, to the sensitivity
and dynamic range of the detector. An optimised systemshould be able a) to measure at once strong atomic lines(e.g. Dα ) and weak molecular bands (e.g. C2 Swan-band),b) to resolve narrow molecular lines (e.g. D2 Fulcher-αauthor’s e-mail: s.brezinsek@fz-juelich.deband) and allow their ro-vibrational analysis, and c) to provide information about the spatial distribution.
We present and characterise a spectrometer in crossdispersion arrangement which can be applied as good compromise and which fulfils most of the needs apart from thespatial distribution from a single measurement. Experiments with injection of C3 H4 were carried out in TEXTOR
and spectra of the break-up products such as CH and C2
were recorded simultaneously to demonstrate the potentialof the device. Moreover the D2 Fulcher-α was observed
during local D2 injection and ro-vibrationally analysed.

2

The cross-dispersion spectrometer –principle function, set-up and technical data

Conventional spectrometers, e.g. in Czerny-Turner aréchelle
grating
entrance
slit

focusing mirror 2
CCD

prism
correction lensfocusing
mirror 1
collimating mirror

Fig. 1 Schematic view of the spectrometer set-up.

rangement, are equipped with only one dispersive element:
grating or prism. Single dispersion systems are restricted

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dispersion due to prism
wavelength

in the simultaneously covered wavelength span to a few
nanometres at a typical resolving power R = λ/∆λ of
20,000 which is mandatory for a ro-vibrational analysis ofmolecular transitions. The most prominent D2 transition infusion boundary plasmas - the Fulcher-α band - is spreadover a wide spectral range (600 nm - 645 nm), thus, several
discharges are necessary to record the full spectrum which
is needed for spectral analysis.
In contrast, the custom-made spectrometer presented here
(MI, model: Mechelle 7500 special; schematic set-up in
fig. 1) consists of two dispersive elements: prism and grating. The system covers a spectral range of more than
300 nm without gap with the requested high spectral resolution. The wide span is achieved though the combined use of (i) a highly dispersive échelle grating (ruling: 31.6 grooves/mm, size 10.0 cm × 3.0 cm, blaze angle 63.5◦ ) for wavelength dispersion in horizontal directionand (ii) an order sorter, a highly dispersive prism (BAF50
glass, reference angle 36◦ ), which separates the orders invertical direction. The prism is used twice in the optical
path (fig. 1) to ensure complete order separation with compact prism size.
Fig. 2a shows the observable spectral orders m projectedon the CCD array, starting from m=153 at the bottom(blue spectral range) to m=79 on the top (red spectral
range), as well as the used spectral range in each orderwhich is applied for the spectrum reconstruction. The
centre wavelength in each order, positioned at the blaze
angle of the grating, is determined by the order constantm × λ[µm] which amounts to 56.783 for this system. The
overlap of different orders is determined by the condition
m=x+0.5m=x+1
m=xwhile five additional pixels on
56.783 = λmin = λmax
each side are used for averaging. Fig. 2b shows the contribution of each order to the standard range between 372 nm(m=152) and 680 nm (m=84). The 92th order is in the next
figures marked in red as guide for the eye. The spectrum ineach order is spread across several pixels in vertical direction. Thus, a summation over 5 to 7 pixels is made to increase the sensitivity of the system and provides a so-called
spectral channel. Additionally, the spectrum reconstruction [2] takes into account the non-equidistant separationof the channels, which are more compressed in the lower
orders, as well as their curvature in the higher orders.
Fig. 2c depicts the linear dispersion as a function of the
wavelength in the standard range. Although the dispersion
varies over the full range, the resulting resolving power,
which was measured by applying the Rayleigh criterium topairs of D2 lines, is within 15% almost constant at 20,000
when the apparatus function is considered. The latter was
estimated by the line width of Hg lines, in particular of HgIat 529 nm to be below 3.1 channel (FWHM) as shown in
the Gaussian fit in fig. 2d.
Further technical data of the Mechelle are: aperture value
f /7, focal length fL = 190 mm, entrance slit 25 µm×75 µm
with SMA905 connector for fibre coupling. Light is coupled to the system by means of 600 m quartz fibre. A 16

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used CCD
area

140
145
order mHg lamp
CCD imagedispersion due to échelle grating
wavelength

a)

b)

150

covered wavelength range
per order
linear dispersion
per channel
induced by prism

induced by gratingc)

Hg I at 546.074 nm
m=104channel height:
5 pixello=546.0745 nm
FWHM=0.0277 nm
FWHM=3.3 channel

4
3

2

1
0

d)

Fig. 2 Spectrum reconstruction: a) 2D image of a Hg line spectrum. The spectral range per order applied for the complete spectrum reconstruction is given by the envelope. b)
Distribution of the wavelength coverage per order. c) The
linear dispersion over the full spectral range. d) A Hg I
line is used to approximate the apparatus function.

bit camera (Andor, model: DV434) with a Peltier-cooled
back-illuminated CCD array, 1024×1024 pixels and 13 µm
×13 µm each pixel, is used as detector. The CCD has abroadband coating optimised for the visible range and provides a quantum efficiency of more than 90% at 500 nm
and −60◦ C cooling temperature. The read-out time for thefull frame amounts to one second at the maximum read-out
frequency of 1 MHz.

3

Spectral and radiometric calibration

The spectral calibration is performed with the aid of a Hg
lamp. The position of seven HgI lines, marked in fig. 2a, iscompared with theoretically calculated positions. The deviation between calculated and measured position is minimised in a least-square fit procedure. The overall position

Proceedings of ITC/ISHW2007
precision lies within two spectral channels and shows almost no drift under controlled temperature conditions. An

dispersion due to prism
wavelength

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4 Spectra examples: hydrocarbon anddeuterium molecules

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order m 150a)b)most constant spectral response above 450 nm. However,the sensitivity drops significantly between 450 nm and 380
nm, which is largely caused by the reduction of the CCD
quantum efficiency of about 50%, and by the decrease of
the glass transmission from 95% to 55% due to 25 mm absorption length in the prism.

Ulbricht
spheredispersion due to échelle grating
wavelength

continuum
spectruminverse sensitivityc)Fig. 3 Radiometric calibration of the cross-dispersion spectrometer. a) 2d image of the system during exposure by a continuum light source. b) Complete reconstructed spectrumof the continuum spectrum. Intensity drops caused by theblaze angle dependence of the grating are considered in
the calibration by a correction function. c) The spectral
response of the system for the full wavelength span.

Ulbricht sphere is used to calibrate the spectrometer system radiometrically. Fig. 3a shows a false-colour 2D image taken from the continuum source; the resulting reconstructed spectrum is depicted in fig. 3b. The spectrum isa convolution of the light source continuum and the sensitivity response of the detection system. Both image andspectrum show a non-uniformity of the measured radiation
within each order. The reduction of the sensitivity of about
40% between the centre and the edges is caused by theblaze angle dependence of the grating. A correction function is introduced to compensate these strong sensitivity
drops in the spectrum. The cross-talk between adjacent orders is measured to be below 10−3 , and any impact on thecalibration can be neglected for the standard range.
The bare inverse sensitivity curve for the standard range
is depicted in fig. 3c. The spectrometer system, which is
optimised for the visible emission range according to the
choice of prism, grating and CCD coating, shows an al-

A series of experiments with the injection of different hydrocarbon species (CH4 , C3 H4 etc.) in similar plasmas has
been started in TEXTOR. The aim of the experiments isa) to check if the injected stable hydrocarbon leaves a”footprint” e.g. with regard to its appearance, intensity
ratio, ro-vibrational population of the break-up products
C2 , CH, CH + etc., and b) to determine the inverse photon
efficiencies and branching ratios which relate the photon
flux of the break-up products to the particle flux of injected species. An example of a ”footprint” spectrum is
depicted in fig. 4a. Spectra of the band heads of the mostimportant transitions, CH Gerö-band and C2 Swan-band,are enlarged. A ro-vibrational analysis and comparison
with spectra of other injected species is outside the scopeof this contribution, but can be found in detail in [3].
The deuterium spectrum of the Fulcher-band transition(3p 3 Πu → 2s 3 Σ+g ) is depicted in fig. 4b. The main diagonal vibrational transitions (v = v0 ) up to v = 5 are indicated. The first main diagonal transition with v = v0 = 0
is enlarged shown and the strongest Q lines (Q1-Q9) aremarked. The rotational population of the excited state
could be fitted according to a Boltzmann distribution witha rotational population temperature T rot of about 1000 K
which is in line with previous observations [4]. A moredetailed analysis of the Fulcher-α band according to themethods described in [4] is topic of a forthcoming paper
which will deal with the interpretation of the deuterium recycling flux and the molecular break-up [5] at the target
plates of the helical divertor in TEXTOR [6].

5 Conclusion and outlook
The cross-dispersion spectrometer presented here is a goodcompromise to study spectroscopically the plasma boundary of fusion devices in the visible range, i.e the fuel recycling and the impurity sources. The system covers at oncethe spectral emission range between 372 nm and 680 nmwith an almost constant resolving power of R=20000 overthe full range. The dynamic range of 16 bit ensures thepossibility of the simultaneous observation of atomic andmolecular lines. The recommended integration time for thefibre-coupled system depends on the signal strength (extrinsic vs. intrinsic), but lays in the second range to ensureand optimised use of the dynamic range. Synergetic effects
can be achieved when – like in TEXTOR – the system isembedded in a set of spectroscopic systems which complement one another [5]. The Mechelle provides the highProceedings of ITC/ISHW2007

Gerö-band

Swan-band

CIII

Ha

CII

CII
CII

CII

Hb
HeI CII

CH

CH

C2

C2
CH+

Ha
(sat.)

C2

CII

CH+

C2

Hg

C2

a)
6

6

Q(0-0)-branch

Q(1-1)-branch

Q(2-2)-branch

4
2

5

0
600.0

4

602.0

604.0

606.0

Q(3-3)-branch3

Q(4-4)-branch
Q(5-5)-branch

2
1

b)

0
600

605

610

615

620

625

630

635

640

Fig. 4 a) C3 H4 injection into the TEXTOR edge plasma through a gas injection module. The emission spectrum in the range between 372
nm and 680 nm recorded gap-free with the cross-dispersion spectrometer during the break-up of the injected hydrocarbon. The
most important observable optical transitions of atomic and molecular break-up products are indicated. The CH Gerö band and theC2 Swan band are highlighted and enlarged in separate figures on top of the ”footprint” spectrum to demonstrate the good spectral
resolution of the molecular bands. The integration time of the system was set to two seconds to ensure the complete coverage of
the C3 H4 injection pulse. b) The complete D2 Fulcher-α band and the Q-lines of the first main diagonal transition. The spectral
range in red indicates the coverage of the observation in the 94th order which is also highlighted in a separate box.

spectral resolution, the large wavelength coverage, and thehigh dynamic range, whereas the other systems with interference filters like photomultipliers or 2D CCD cameras
provide the needed time and spatial resolution.
Experiments at TEXTOR provide ”footprints” of theinjected hydrocarbons C3 H4 and information of D2
molecules in the hot edge plasma. The appearance, thedestruction path, the flux, the ratio, and the ro-vibrational
population of the different species can now be studied in
more detail within a single discharge.
Systems of the next generation of cross-dispersion spectrometers in prism /grating arrangement and equipped withprism, made of low absorption material with high dispersion, are currently developed to ensure a larger sensitivity in the spectral region below 450 nm. Such an opti-

mised system will be used for the observation of the new
first wall and boundary layer at JET after installation of
the new ITER-like wall with W and Be as plasma-facing
material. The installation of an Echelle cross-dispersion
spectrometer system is a also panned for the observationof the boundary layer in W7-X.

References[1] S. Brezinsek et al., Physica Scripta T111, 42 (2004).
[2] P. Lindblom, Anal. Chim. Acta 380, 353 (1999).
[3] S. Brezinsek et al., J. Nuclear Materials 313-316, 967 (2003).
[4] S. Brezinsek et al.,J. Nuclear Materials 363-365, 1119
(2007).
[5] S. Brezinsek et al., Plasma Phys. Control. Fusion 47, 615(2005)[6] M. Lehnen et al., this conference