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Atom energy distributions from Hα lineshape measurements during gas puff experiments in DITE
482
Journal
of Nuclear
Materials
162-164
North-Holland,
(1989) 482-488 Amsterdam
ATOM ENERGY DISTRIBUTIONS FROM H, LINESHAPE MEASUREMENTS DURING GAS PUFF EXPERIMENTS IN DITE S.J. FIELDING I, P.C. JOHNSON ‘, M.J. FORREST ‘, D. GUILHEM 2 and G.F. MATTHEWS ’ ’ Wham Laboratory (Euratom/UKAEA Fusion Association), Abingdon, Oxon, OX14 3DB, United ’ Association
Key words:
Euratom -CEA,
DITE,
DRFC,
atom energy
CEN Cadarache,
spectra,
H,
linewidths,
13108 Saint-Paul-Lez,
methane,
Kingdom Durance, Cedex, France
gas puff
H, lineshapes have been measured for Ha, D, and CH, gas puff into DITE from the tip of an instrumented probe limiter. Neutral hydrogen (deuterium) atom energy .spectra have been derived directly from these and are discussed. H, lineshapes from hydrogen recycling at a graphite surface are presented and discussed in terms of the gas puff data. No evidence of methane production is observed.
1. Introduction In most tokamak plasmas the density is sustained by an influx of hydrogen (deuterium) at the plasma edge from recycling at the limiter surfaces, usually supplemented by a gas feed. This particle influx not only refuels the plasma but can play a dominant role in the gross plasma behaviour, e.g. the low recycling requirement for H-mode [l]. Hence a knowledge of the detailed processes by which the particle influx is transported and absorbed into the plasma is most important. Measurement of the absolute H, line emission is a standard technique for estimating the H atom ionisation rate, while analysis of the H, lineshape can provide information on the energy distribution of the hydrogen atom influx and, from this, the atom production mechanisms [2-41. Observations of asymmetric H, lineshapes have been recently reported from TEXTOR [4] and JET [5], when viewing recycling from the limiter region. Other features of the H, lineshape on TEXTOR have been attributed to methane being the source of the hydrogen atoms rather than direct hydrogen recycling. In order to clarify these and other observations we have performed a series of experiments on DITE involving the puffing of hydrogen, deuterium or methane gases into the tokamak plasma from the tip of a small limiter probe. High resolution H, line shape measurements have been made of the gas. puff region and compared with the line shapes from direct recycling. Previous methods of analysis have relied on a computa0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
tional simulation of the process of recycling and gas puff, using available published data for dissociation and ionisation rate coefficients and product atom energy distributions (where available). The resultant modelled lineshapes are then compared with the experiment. In contrast to this method we directly interpret the lineshape, after deconvolving instrumental and geometric effects, in terms of a hydrogen atom energy distribution function. In a sense this is forced upon us by the lack of data on the break up kinetics of the methane molecule. However, by adopting this procedure we not only obtain a characteristic signature for the particular molecule, but also a direct measure of the most important parameter determining the ionisation source function in the plasma - the energy distribution function. Notwithstanding this we have also compared the experimental data with predictions from a 1D code modelling the molecular and atomic hydrogen transport and excitation.
2. Experiment Fig 1 shows a schematic of the experimental system for gas puff observations. A small 70 mm diameter graphite probe limiter [6] is installed on one of the DITE upper ports on a drive system which allows the limiter to be positioned at minor radii between 280 mm (the wall) and 180 mm (main limiter radius 240 mm, 90” toroidally away from probe). Gas can be pulsed B.V.
483
S.J. Fielding et al. / H, lineshape measurements in DITE
Fig. 1. Schematic of probe limiter gas puff experimental system on DITE.
through an aperture in the limiter tip using a fast acting piezo electric control valve. The limiter probe is fitted with two Langmuir probes at different radial positions providing information on local T, and n, values during gas puffing. H, emission is observed via the bottom port using an unfocussed 1 mm diameter 30 m long quartz fibre optic coupled to a monochromator. Note that the optic axis of the light collection system is at an angle of 36’ to the radial direction. Because of the strong toroidal magnetic field perpendicular to the plane of observation the H, line is split into three Zeeman components. The two (I components were eliminated by use of a suitably orientated polariser and all measurements refer to the unshifted single P component. The monochromator is a 1 m Monospek *, with an echelle grating of ruling frequency 316 lines/mm used in 6th order, dispersion 2.48 A/mm. A blocking filter of 40 A bandwidth is used to eliminate unwanted light from other orders. The Monospek is fitted with a PAR OMA * * diode array (1024 pixels, 40 pixels/mm, dis-
* Model 1000, Rank Precision Industries,
Westward Industrial Estate, Ramsgate Road, Margate, Kent. ** Model 1420, Princeton Applied Research, P.O. Box 25665, Princeton, NJ 08540, USA.
persion 0.062 A/pixel) interfaced with a PDPll data acquisition system and has a measured instrumental width of less than 0.2 A (3 pixels). The monochromator wavelength scale was calibrated in between tokamak shots by a reference H, lamp to an accuracy of - 0.01 A. Drifts of up to 0.3 A/h were observed due to thermal effects. For the gas puffing experiment H,, D, or CH, gas was admitted into a series of standard DITE discharges (Bo = 2 T, Zr = 100 kA, T plasma 500 ms, T gas puff 100 ms, helium working gas to minimise recycling H,). Lineshape data were taken in 40 time slices of 16 ms duration during the plasma discharge and residual background recycling, measured before gas puff, was subtracted off. Measurements have been made also of the recycling from the probe limiter graphite in a similar series of experiments in hydrogen plasmas.
3. Data analysis The measured H, line convolution of a number function, the geometry of the source, the effect of molecular processes. The
shape is determined by the of effects: the instrument observation, the geometry of plasma profiles, atomic and instrument function is taken
S.J. Fieldmg et al. / H, lineshape measuremenfs in DITE
484
Fig. 2. H, and D, lineshapes, deconvolved from instrument function, from gas puff of H,, CH, and D, at two different radii into helium plasmas. The T, values refer to measured temperatures at the probe limiter front surface. The vertical line near line center indicates the wavelength of the reference H, (D-1 lamp.
as the measured shape of the 6328 A He-Ne laser line. This is then deconvolved from all the H, lineshape measurements by a maximum entropy procedure [7]. Fig. 2 shows several H, (D,) lineshapes, deconvolved from the instrument function, from the gas puff experiments. The lineshapes are clearly asymmetric with a slight but significant blue shift of the peak. These two features can be explained in terms of geometric effects. We approximate the gas puff to that of an influx of molecules through an aperture in a plane surface. The molecules enter the plasma in some unspecified spatial flow pattern with sonic velocities at approximately room temperature. They are dissociated or ionised within 5-10 mm, estimated from the measured plasma profiles, defining a small volume atom source. We assume that the dissociated neutral atoms are emitted isotropically from within this source with some energy distribution. The mean free path (mfp) of atoms at typical dissociation energies of several eV is - 150 mm, defining an
Ha radiating volume much larger than the source volume. Because of the close proximity of the atom source to the plane surface (limiter) atoms with outward velocity components will strike the limiter and either be absorbed or reflected back into the plasma, albeit at reduced energy. We therefore approximate the situation to a point source at a plane surface emitting atoms isotropically with energy distribution f(E), but with the complete absence of any particles with velocity components directed away from the plasma. The simulation code HSLAB, described later, largely confirms this picture. Consider now electron impact excitation of the atoms to produce Ha: radiation. The resulting lineshape is dependent on the direction of observation, due to the Doppler effect. Fig. 3 shows schematically how the analytically evaluated lineshape changes from fully symmetric about h,, the usual line-centre wavelength, when viewed parallel to the surface, to a complete absence of light of wavelength longer than h, when viewed at normal incidence. At the intermediate angle of 36 O. corresponding to the experimental arrangement, the line shape is asymmetric with a blue shift to the peak, as observed experimentally. However, it can be shown by intuitive argument, or by analytic integration in phase space, that the lineshape at normal incidence can be recovered from that at any intermediate angle, for any
E F
gas puff
Fig. 3. Schematic of effect of viewing angle on line shape. Dissociation of gas puff molecules is assumed to give rise to an isotropic distribution of atoms, but with an absence of any particles moving to the right. The assumed atom energy distribution, for illustrative purposes only, is gaussian. 8 = 36 O, as in the experimental system on DITE.
S.J. Fielding et al. / If, lineshape measurements in DITE
485
where I-, is the gas puff rate. Eq. (2) can be inverted [9] to give f(E(Ah)) a dl(Ah)/dh, i.e. the energy distribution function is proportional to the gradient of the H, lineshape. We use this procedure to derive atom energy distributions from the geometry-corrected lineshapes of fig. 4. ..
..
”
4. Results and discussion
7.
-
1
,*a .
02
R>IO.”
“2 +
PIXEL
HO
PIXEL
I
Fig. 5 shows the resulting atom energy distributions. Before attempting to explain the gross features of the energy spectra, and because of the overall similarity of the results, we must check that the approximations used in the analysis procedure have not lead to large scale distortions. Ha emission from the molecular influx can also arise from the dissociation process itself when an atom is left in the n = 3 state (cascade from higher states is negligible). These excited atoms decay by photon emission on a microsecond timescale and hence are
NO
Fig. 4. L&shape data of fig. 2 corrected for viewing geometry.
arbitrary f(E), by the process of reflecting the red shift component about h, and adding it to the blue component. We use this procedure to deconvolve the viewing geometry effects from the DITE data. Fig. 4 shows the symmetric line shapes resulting from performing this operation on the data of fig. 2. The H, line intensity, at wavelength Ah from line centre, assuming the isotropic source of atoms described above and for normal incidence viewing can be written as:
where no is the local atom density, rze the electron density, (UV) the excitation rate coefficient for H, emission and E = mc2(AX)*/2X2. The spatial integral is over the plasma volume defined by the neutral atom mfp. For temperatures above 10 eV the number of H, photons emitted per ionisation [8] is approximately constant (to within 10%) and eq. (1) can be simplified to:
(2)
Fig. 5. Atom energy distributions f(E) derived for H,, CH, and Dz gas puff into helium plasmas. T, values refer to measured temperatures at a probe limiter front surface. Cpen circle data points in the upper left hand trace correspond to analysis of the blue side of the H, line.
486
S.J. Fielding et ol. / Ii, ~i~es~~~emeasurmzents in DITE
fully spatially isotropic. The Ha: fineshape from such a distribution of atoms is symmetrical and independent of viewing angle and the data analysis process of reflection about X0 would lead to a factor of two greater weighting of this component in the derived energy spectrum. Computational modelling (see later) suggests that dissociative excitation gives rise to only - 10% of the H, intensity and the distortion associated with the analysis would be expected to be of this order or less, depending on the energy distribution of the dissociation excited atoms. As a check for this we have analysed only the blue side of the line shapes, without the h, reflection. This correctly treats the fully isotropic dissociative excitation component but underweights the low energy fraction of the electron impact excited atoms (by ignoring the He contribution from some fraction of the atoms with large velocity components parallel to the limiter surface). The results from the analysis of one of the cases is shown in fig. 5 and can be seen to give similar features to the previous analysis, other than at very low energies. We can therefore be confident that the main features of the energy spectra are not artefacts of the analysis. Fig. 5 shows two sets of data for each of the three gases corresponding to gas puff into a cold plasma, limiter probe radius rp = 250 mm, r, - 15 eV and gas puff into a hot plasma, rP = 220 mm, T, > 50 eV. All 6 cases show a strong peak at - 0.3 eV, with a marked additional peak at - 3 eV for H, and D, puff into a cold plasma. No significant amount of atoms with energies greater than 8 eV is found in any of the cases. The one distinguishing characteristic of the methane energy distribution is the smooth decay from the low energy peak with no shoulder or feature in the 3 eV region. The energy spectra from H, and D? are somewhat surprising in terms of the relatively large number of low energy atoms. For the T, regime of the DITE experiments there are two principal breakup reactions. dissociation into two ground state atoms, H, + H(ls) + H(ls), and ionisation H, + Hz followed by dissociation Hz 4 H(ls) -t Hf (see reviews [lo-121. Both reactions give rise to so-called ‘fast’ atoms. For molecular dissociation the calculated energy distribution extends between 2 and 6.5 eV, peaking, at 3 eV. For molecular ion dissociation the neutral atom energy is sensitively dependent on the state of vibrational excitation of the II;. Using calculations of Franck-Condon factors for ionisation from the ground vibrational state H,( u = 0) which suggest a range of molecular ion vibrational energies H;(U) u = 0 to 8 with significant population, Janev et al. [II] compute a mean dissociated atom
energy of 4.3 eV. So-called slow atoms, E’ - 0.3 eV, are produced by dissociative excitation, where the molecule is first raised to an excited bound molecular state. The dissociation process leaves one of the atoms in an excited state and the available kinetic energy is much smaller in this case. Dissociation to n = 3 gives rise to direct emission of He but the reaction of principal importance appears to be H, + H(2s) + H(ls). The neutral atoms produced in this reaction have energies from 0 to 1.4 eV peaking at 0.3 eV. Fast atoms can also be formed from dissociative excitation via doubly excited states but not in significant numbers. The molecular ionisation process leads to negligible production of slow atoms. We have modelled the hydrogen gas puff using the 1D simulation code HSLAB [3], which solves the molecule and atom rate and transport equations for measured plasma profiles and has been modified to include the calculation of the line shape from the velocity distribution of the H atoms. For the simulation of data in helium plasmas charge exchange is neglected. Fig. 6 shows the predicted H, lineshapes corresponding to hydrogen gas puff into the two plasma conditions. The lineshapes are very asymmet~c, largely confirnling the simple model shown schematically in fig. 3. The different contributions to the lineshape from the fast and slow atom groups are resolved, that from fast atoms clearly dominating the lineshape. In this respect, there is a lack of agreement between the computed lineshapes and the experimental data. One factor contributing to the discrepancy could be the presence of the nearby graphite surface of the probe tip which will reflect particles with high efficiency but with energy loss [13]. This will result in a relative increase in slow atom component but is unlikely to explain the dominance of the 0.3 eV feature. Work is in hand to include this effect in HSLAB, which, in the simulations shown in fig. 6. treats the limiter surface as an absorber. At 15 eV the cross sections for molecular dissociation and ionisation are approximately equal, and published rate coefficients for dissociative excitation suggest that 12% of the product atoms will be in the slow group. If in the measured energy spectra we designate all atoms with energies less than 2 eV as ‘slow’, all those in excess of 2 eV as ‘fast’, the data of fig. 5 gives the slow group fraction as 50%. At 50 eV molecular ionisation dominates over dissociation to H(ls) but dissociative excitation to H( n = 2) reaches a peak and rate coefficient data suggests that 10% of the atoms will be in the slow group, compared with a measured value of 60%. Of further significance are the absolute levels of the H, signals which, experimentally, are in the ratio I
S.J. Fielding et al. / H, lineshape measurements in DITE
Te I
15e.V
total - - fast
Te -
total - - fast
50eV
s-m-
slow
----
slow
+*++
expt
+*++
apt
I-0.5i-l
Fig. 6. Results
from 1D simulation
code HSLAB
of H, lineshape resulting from gas puff of H, data is overlaid for comparison.
(T, = 15 ev) to 1.5 (T, = 50 ev). The gas puff rate is the same for both cases, the H, photon efficiency constant to 10% and since dissociation results in 2 atoms, molecular ionisation only 1, rate coefficient calculations suggest a ratio of 1 (T, = 15 eV) to 0.7 (c = 50 ev). Both the energy distribution and absolute H, experimental data tend to suggest that the published rate coefficients for dissociative excitation are too small. However, the two discrepancies cannot be removed by a simple temperature independent multiplication factor. Experimentally the D, energy spectra are very similar to H, with similar ratios of fast to slow atoms. Turning to the methane data, we note that the energy spectra lack the 3 eV feature of H, (D,). There are several channels for the breakup of the methane molecule: ionisation, dissociative ionisation and dissociation [14] and data is lacking on the reaction kinetics. The only published measurements of the product energies are from dissociative excitation to n = 3 [15] where values from 2 to 4 eV were observed for electron impact energies of 20 to 300 eV, respectively. This reaction is not the principal contributor in our measurements, since it gives rise to a symmetric line shape which is clearly not observed. At this stage we do not feel competent to comment further on the details of the results but we will use the energy spectrum, with its absence of any 3 eV shoulder, as a strong signature of methane. We have used the probe limiter, without gas puff, in hydrogen plasmas and measured the H, line shape associated with hydrogen recycling at a graphite surface.
into helium
plasma.
Experimental
Fig. 7 shows the experimental data for two limiter positions corresponding again to limiter front edge c values of - 15 eV and 50 eV. In both cases a strong 3 eV feature is present in the energy spectra, evidence that H, rather than CH, is determining the lineshape. We conclude therefore that methane production is not dominating the recycling in the plasma conditions
m.w, w
Fig. 7. H, line shapes and derived energy spectra from recycling of hydrogen plasma at the probe limiter graphite surface for edge electron temperatures of 15 and 50 eV.
488
S.J. Fielding et al. / H, lineshape measurements in DITE
studied. This agrees with the experiments of Pitcher [16] who, using a similar probe limiter in conjunction with carbon emission spectroscopy estimated that methane production by chemical sputtering was insignificant. Fig. 7 shows that the H, lineshape at c - 15 eV is considerably broader than at 50 eV, an effect similar to that noted by Samm [4]. We find that this broadening is mainly on the red side of X, and corresponds to a near symmetric lineshape. At these low edge temperatures recycling at the probe limiter tip will be reduced and the H, signals may be determined by wall influxes, both close to the viewing window and around the probe limiter. This would tend to give rise to a symmetric lineshape. The H, signals for this case are quite weak and the additional peaks at higher energies seem to be noise related artefacts. The0 underlying broad energy feature, up to 50 pixels (0.8 A, 100 eV) on either side of the line is due to charge exchange of H atoms on plasma ions.
5. Conclusions
We have made high resolution lineshape measurements of H, from Hz, CH, and D, gas puff into DITE plasmas. The line shape asymmetry is determined by geometric effects. Energy spectra are derived from the deconvolved lineshapes and for H, (Dz) show a considerably larger fraction of low energy atoms than suggested by published cross sections and from comparison
with the simulation code HSLAB. The energy spectra from CH, lack the 3 eV feature of H, and analysis of H, from hydrogen recycling at a graphite surface gives no evidence of methane production.
References
VI F. Wagner et al., J. Nucl. Mater. 121 (1984) 103. PI D.H. McNeil1 and J. Kim, Phys. Rev. Lett. A25 (1982) 2152. 131 D. Guilhem, These de Docteur, L’Universite Paul Sabatier de Toulouse (Sciences) (1985). [41 U. Samm et al, Plasma Phys. and Contr. Fusion 29 (1987) 1321. [51 M.J. Forrest, private communication (1987). [61 C.S. Pitcher et al., J. Nucl. Mater. 145-147 (1987) 539. [71 SF. Gull and G.J. Daniell, Nature 272 (1978) 686. PI L.C. Johnson and E. Hinnov, J. Quant. Spectrosc. Radiat. Transf. 13 (1973) 333. [91 T. Ogawa and M. Higo, Chem. Phys. Lett. 65 (1979) 610. DOI F.J. De Heer, Phys. Scripta 23 (1981) 170. IllI R.K. Janev et al., Springer Series in Atoms and Plasmas, Vol. 4, (Springer Verlag, Heidelberg, 1987). WI M.F.A. Harrison, in: Physics of Plasma-Wall Interactions in Controlled Fusion, Eds D.E. Post and R. Behrish, NATO ASI Series (Plenum, New York, 1986) P 281. D31 W. Eckstein and J.P. Biersack, Appl. Phys. A38 (1985) 123. 1141W.D. Langer, Nucl. Fusion 22 (1982) 751. [I51 K. Ito et al., Chem. Phys. 21 (1977) 203. WI C.S. Pitcher, Nucl. Fusion 26 (1986) 1641.
Journal
of Nuclear
Materials
162-164
North-Holland,
(1989) 482-488 Amsterdam
ATOM ENERGY DISTRIBUTIONS FROM H, LINESHAPE MEASUREMENTS DURING GAS PUFF EXPERIMENTS IN DITE S.J. FIELDING I, P.C. JOHNSON ‘, M.J. FORREST ‘, D. GUILHEM 2 and G.F. MATTHEWS ’ ’ Wham Laboratory (Euratom/UKAEA Fusion Association), Abingdon, Oxon, OX14 3DB, United ’ Association
Key words:
Euratom -CEA,
DITE,
DRFC,
atom energy
CEN Cadarache,
spectra,
H,
linewidths,
13108 Saint-Paul-Lez,
methane,
Kingdom Durance, Cedex, France
gas puff
H, lineshapes have been measured for Ha, D, and CH, gas puff into DITE from the tip of an instrumented probe limiter. Neutral hydrogen (deuterium) atom energy .spectra have been derived directly from these and are discussed. H, lineshapes from hydrogen recycling at a graphite surface are presented and discussed in terms of the gas puff data. No evidence of methane production is observed.
1. Introduction In most tokamak plasmas the density is sustained by an influx of hydrogen (deuterium) at the plasma edge from recycling at the limiter surfaces, usually supplemented by a gas feed. This particle influx not only refuels the plasma but can play a dominant role in the gross plasma behaviour, e.g. the low recycling requirement for H-mode [l]. Hence a knowledge of the detailed processes by which the particle influx is transported and absorbed into the plasma is most important. Measurement of the absolute H, line emission is a standard technique for estimating the H atom ionisation rate, while analysis of the H, lineshape can provide information on the energy distribution of the hydrogen atom influx and, from this, the atom production mechanisms [2-41. Observations of asymmetric H, lineshapes have been recently reported from TEXTOR [4] and JET [5], when viewing recycling from the limiter region. Other features of the H, lineshape on TEXTOR have been attributed to methane being the source of the hydrogen atoms rather than direct hydrogen recycling. In order to clarify these and other observations we have performed a series of experiments on DITE involving the puffing of hydrogen, deuterium or methane gases into the tokamak plasma from the tip of a small limiter probe. High resolution H, line shape measurements have been made of the gas. puff region and compared with the line shapes from direct recycling. Previous methods of analysis have relied on a computa0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
tional simulation of the process of recycling and gas puff, using available published data for dissociation and ionisation rate coefficients and product atom energy distributions (where available). The resultant modelled lineshapes are then compared with the experiment. In contrast to this method we directly interpret the lineshape, after deconvolving instrumental and geometric effects, in terms of a hydrogen atom energy distribution function. In a sense this is forced upon us by the lack of data on the break up kinetics of the methane molecule. However, by adopting this procedure we not only obtain a characteristic signature for the particular molecule, but also a direct measure of the most important parameter determining the ionisation source function in the plasma - the energy distribution function. Notwithstanding this we have also compared the experimental data with predictions from a 1D code modelling the molecular and atomic hydrogen transport and excitation.
2. Experiment Fig 1 shows a schematic of the experimental system for gas puff observations. A small 70 mm diameter graphite probe limiter [6] is installed on one of the DITE upper ports on a drive system which allows the limiter to be positioned at minor radii between 280 mm (the wall) and 180 mm (main limiter radius 240 mm, 90” toroidally away from probe). Gas can be pulsed B.V.
483
S.J. Fielding et al. / H, lineshape measurements in DITE
Fig. 1. Schematic of probe limiter gas puff experimental system on DITE.
through an aperture in the limiter tip using a fast acting piezo electric control valve. The limiter probe is fitted with two Langmuir probes at different radial positions providing information on local T, and n, values during gas puffing. H, emission is observed via the bottom port using an unfocussed 1 mm diameter 30 m long quartz fibre optic coupled to a monochromator. Note that the optic axis of the light collection system is at an angle of 36’ to the radial direction. Because of the strong toroidal magnetic field perpendicular to the plane of observation the H, line is split into three Zeeman components. The two (I components were eliminated by use of a suitably orientated polariser and all measurements refer to the unshifted single P component. The monochromator is a 1 m Monospek *, with an echelle grating of ruling frequency 316 lines/mm used in 6th order, dispersion 2.48 A/mm. A blocking filter of 40 A bandwidth is used to eliminate unwanted light from other orders. The Monospek is fitted with a PAR OMA * * diode array (1024 pixels, 40 pixels/mm, dis-
* Model 1000, Rank Precision Industries,
Westward Industrial Estate, Ramsgate Road, Margate, Kent. ** Model 1420, Princeton Applied Research, P.O. Box 25665, Princeton, NJ 08540, USA.
persion 0.062 A/pixel) interfaced with a PDPll data acquisition system and has a measured instrumental width of less than 0.2 A (3 pixels). The monochromator wavelength scale was calibrated in between tokamak shots by a reference H, lamp to an accuracy of - 0.01 A. Drifts of up to 0.3 A/h were observed due to thermal effects. For the gas puffing experiment H,, D, or CH, gas was admitted into a series of standard DITE discharges (Bo = 2 T, Zr = 100 kA, T plasma 500 ms, T gas puff 100 ms, helium working gas to minimise recycling H,). Lineshape data were taken in 40 time slices of 16 ms duration during the plasma discharge and residual background recycling, measured before gas puff, was subtracted off. Measurements have been made also of the recycling from the probe limiter graphite in a similar series of experiments in hydrogen plasmas.
3. Data analysis The measured H, line convolution of a number function, the geometry of the source, the effect of molecular processes. The
shape is determined by the of effects: the instrument observation, the geometry of plasma profiles, atomic and instrument function is taken
S.J. Fieldmg et al. / H, lineshape measuremenfs in DITE
484
Fig. 2. H, and D, lineshapes, deconvolved from instrument function, from gas puff of H,, CH, and D, at two different radii into helium plasmas. The T, values refer to measured temperatures at the probe limiter front surface. The vertical line near line center indicates the wavelength of the reference H, (D-1 lamp.
as the measured shape of the 6328 A He-Ne laser line. This is then deconvolved from all the H, lineshape measurements by a maximum entropy procedure [7]. Fig. 2 shows several H, (D,) lineshapes, deconvolved from the instrument function, from the gas puff experiments. The lineshapes are clearly asymmetric with a slight but significant blue shift of the peak. These two features can be explained in terms of geometric effects. We approximate the gas puff to that of an influx of molecules through an aperture in a plane surface. The molecules enter the plasma in some unspecified spatial flow pattern with sonic velocities at approximately room temperature. They are dissociated or ionised within 5-10 mm, estimated from the measured plasma profiles, defining a small volume atom source. We assume that the dissociated neutral atoms are emitted isotropically from within this source with some energy distribution. The mean free path (mfp) of atoms at typical dissociation energies of several eV is - 150 mm, defining an
Ha radiating volume much larger than the source volume. Because of the close proximity of the atom source to the plane surface (limiter) atoms with outward velocity components will strike the limiter and either be absorbed or reflected back into the plasma, albeit at reduced energy. We therefore approximate the situation to a point source at a plane surface emitting atoms isotropically with energy distribution f(E), but with the complete absence of any particles with velocity components directed away from the plasma. The simulation code HSLAB, described later, largely confirms this picture. Consider now electron impact excitation of the atoms to produce Ha: radiation. The resulting lineshape is dependent on the direction of observation, due to the Doppler effect. Fig. 3 shows schematically how the analytically evaluated lineshape changes from fully symmetric about h,, the usual line-centre wavelength, when viewed parallel to the surface, to a complete absence of light of wavelength longer than h, when viewed at normal incidence. At the intermediate angle of 36 O. corresponding to the experimental arrangement, the line shape is asymmetric with a blue shift to the peak, as observed experimentally. However, it can be shown by intuitive argument, or by analytic integration in phase space, that the lineshape at normal incidence can be recovered from that at any intermediate angle, for any
E F
gas puff
Fig. 3. Schematic of effect of viewing angle on line shape. Dissociation of gas puff molecules is assumed to give rise to an isotropic distribution of atoms, but with an absence of any particles moving to the right. The assumed atom energy distribution, for illustrative purposes only, is gaussian. 8 = 36 O, as in the experimental system on DITE.
S.J. Fielding et al. / If, lineshape measurements in DITE
485
where I-, is the gas puff rate. Eq. (2) can be inverted [9] to give f(E(Ah)) a dl(Ah)/dh, i.e. the energy distribution function is proportional to the gradient of the H, lineshape. We use this procedure to derive atom energy distributions from the geometry-corrected lineshapes of fig. 4. ..
..
”
4. Results and discussion
7.
-
1
,*a .
02
R>IO.”
“2 +
PIXEL
HO
PIXEL
I
Fig. 5 shows the resulting atom energy distributions. Before attempting to explain the gross features of the energy spectra, and because of the overall similarity of the results, we must check that the approximations used in the analysis procedure have not lead to large scale distortions. Ha emission from the molecular influx can also arise from the dissociation process itself when an atom is left in the n = 3 state (cascade from higher states is negligible). These excited atoms decay by photon emission on a microsecond timescale and hence are
NO
Fig. 4. L&shape data of fig. 2 corrected for viewing geometry.
arbitrary f(E), by the process of reflecting the red shift component about h, and adding it to the blue component. We use this procedure to deconvolve the viewing geometry effects from the DITE data. Fig. 4 shows the symmetric line shapes resulting from performing this operation on the data of fig. 2. The H, line intensity, at wavelength Ah from line centre, assuming the isotropic source of atoms described above and for normal incidence viewing can be written as:
where no is the local atom density, rze the electron density, (UV) the excitation rate coefficient for H, emission and E = mc2(AX)*/2X2. The spatial integral is over the plasma volume defined by the neutral atom mfp. For temperatures above 10 eV the number of H, photons emitted per ionisation [8] is approximately constant (to within 10%) and eq. (1) can be simplified to:
(2)
Fig. 5. Atom energy distributions f(E) derived for H,, CH, and Dz gas puff into helium plasmas. T, values refer to measured temperatures at a probe limiter front surface. Cpen circle data points in the upper left hand trace correspond to analysis of the blue side of the H, line.
486
S.J. Fielding et ol. / Ii, ~i~es~~~emeasurmzents in DITE
fully spatially isotropic. The Ha: fineshape from such a distribution of atoms is symmetrical and independent of viewing angle and the data analysis process of reflection about X0 would lead to a factor of two greater weighting of this component in the derived energy spectrum. Computational modelling (see later) suggests that dissociative excitation gives rise to only - 10% of the H, intensity and the distortion associated with the analysis would be expected to be of this order or less, depending on the energy distribution of the dissociation excited atoms. As a check for this we have analysed only the blue side of the line shapes, without the h, reflection. This correctly treats the fully isotropic dissociative excitation component but underweights the low energy fraction of the electron impact excited atoms (by ignoring the He contribution from some fraction of the atoms with large velocity components parallel to the limiter surface). The results from the analysis of one of the cases is shown in fig. 5 and can be seen to give similar features to the previous analysis, other than at very low energies. We can therefore be confident that the main features of the energy spectra are not artefacts of the analysis. Fig. 5 shows two sets of data for each of the three gases corresponding to gas puff into a cold plasma, limiter probe radius rp = 250 mm, r, - 15 eV and gas puff into a hot plasma, rP = 220 mm, T, > 50 eV. All 6 cases show a strong peak at - 0.3 eV, with a marked additional peak at - 3 eV for H, and D, puff into a cold plasma. No significant amount of atoms with energies greater than 8 eV is found in any of the cases. The one distinguishing characteristic of the methane energy distribution is the smooth decay from the low energy peak with no shoulder or feature in the 3 eV region. The energy spectra from H, and D? are somewhat surprising in terms of the relatively large number of low energy atoms. For the T, regime of the DITE experiments there are two principal breakup reactions. dissociation into two ground state atoms, H, + H(ls) + H(ls), and ionisation H, + Hz followed by dissociation Hz 4 H(ls) -t Hf (see reviews [lo-121. Both reactions give rise to so-called ‘fast’ atoms. For molecular dissociation the calculated energy distribution extends between 2 and 6.5 eV, peaking, at 3 eV. For molecular ion dissociation the neutral atom energy is sensitively dependent on the state of vibrational excitation of the II;. Using calculations of Franck-Condon factors for ionisation from the ground vibrational state H,( u = 0) which suggest a range of molecular ion vibrational energies H;(U) u = 0 to 8 with significant population, Janev et al. [II] compute a mean dissociated atom
energy of 4.3 eV. So-called slow atoms, E’ - 0.3 eV, are produced by dissociative excitation, where the molecule is first raised to an excited bound molecular state. The dissociation process leaves one of the atoms in an excited state and the available kinetic energy is much smaller in this case. Dissociation to n = 3 gives rise to direct emission of He but the reaction of principal importance appears to be H, + H(2s) + H(ls). The neutral atoms produced in this reaction have energies from 0 to 1.4 eV peaking at 0.3 eV. Fast atoms can also be formed from dissociative excitation via doubly excited states but not in significant numbers. The molecular ionisation process leads to negligible production of slow atoms. We have modelled the hydrogen gas puff using the 1D simulation code HSLAB [3], which solves the molecule and atom rate and transport equations for measured plasma profiles and has been modified to include the calculation of the line shape from the velocity distribution of the H atoms. For the simulation of data in helium plasmas charge exchange is neglected. Fig. 6 shows the predicted H, lineshapes corresponding to hydrogen gas puff into the two plasma conditions. The lineshapes are very asymmet~c, largely confirnling the simple model shown schematically in fig. 3. The different contributions to the lineshape from the fast and slow atom groups are resolved, that from fast atoms clearly dominating the lineshape. In this respect, there is a lack of agreement between the computed lineshapes and the experimental data. One factor contributing to the discrepancy could be the presence of the nearby graphite surface of the probe tip which will reflect particles with high efficiency but with energy loss [13]. This will result in a relative increase in slow atom component but is unlikely to explain the dominance of the 0.3 eV feature. Work is in hand to include this effect in HSLAB, which, in the simulations shown in fig. 6. treats the limiter surface as an absorber. At 15 eV the cross sections for molecular dissociation and ionisation are approximately equal, and published rate coefficients for dissociative excitation suggest that 12% of the product atoms will be in the slow group. If in the measured energy spectra we designate all atoms with energies less than 2 eV as ‘slow’, all those in excess of 2 eV as ‘fast’, the data of fig. 5 gives the slow group fraction as 50%. At 50 eV molecular ionisation dominates over dissociation to H(ls) but dissociative excitation to H( n = 2) reaches a peak and rate coefficient data suggests that 10% of the atoms will be in the slow group, compared with a measured value of 60%. Of further significance are the absolute levels of the H, signals which, experimentally, are in the ratio I
S.J. Fielding et al. / H, lineshape measurements in DITE
Te I
15e.V
total - - fast
Te -
total - - fast
50eV
s-m-
slow
----
slow
+*++
expt
+*++
apt
I-0.5i-l
Fig. 6. Results
from 1D simulation
code HSLAB
of H, lineshape resulting from gas puff of H, data is overlaid for comparison.
(T, = 15 ev) to 1.5 (T, = 50 ev). The gas puff rate is the same for both cases, the H, photon efficiency constant to 10% and since dissociation results in 2 atoms, molecular ionisation only 1, rate coefficient calculations suggest a ratio of 1 (T, = 15 eV) to 0.7 (c = 50 ev). Both the energy distribution and absolute H, experimental data tend to suggest that the published rate coefficients for dissociative excitation are too small. However, the two discrepancies cannot be removed by a simple temperature independent multiplication factor. Experimentally the D, energy spectra are very similar to H, with similar ratios of fast to slow atoms. Turning to the methane data, we note that the energy spectra lack the 3 eV feature of H, (D,). There are several channels for the breakup of the methane molecule: ionisation, dissociative ionisation and dissociation [14] and data is lacking on the reaction kinetics. The only published measurements of the product energies are from dissociative excitation to n = 3 [15] where values from 2 to 4 eV were observed for electron impact energies of 20 to 300 eV, respectively. This reaction is not the principal contributor in our measurements, since it gives rise to a symmetric line shape which is clearly not observed. At this stage we do not feel competent to comment further on the details of the results but we will use the energy spectrum, with its absence of any 3 eV shoulder, as a strong signature of methane. We have used the probe limiter, without gas puff, in hydrogen plasmas and measured the H, line shape associated with hydrogen recycling at a graphite surface.
into helium
plasma.
Experimental
Fig. 7 shows the experimental data for two limiter positions corresponding again to limiter front edge c values of - 15 eV and 50 eV. In both cases a strong 3 eV feature is present in the energy spectra, evidence that H, rather than CH, is determining the lineshape. We conclude therefore that methane production is not dominating the recycling in the plasma conditions
m.w, w
Fig. 7. H, line shapes and derived energy spectra from recycling of hydrogen plasma at the probe limiter graphite surface for edge electron temperatures of 15 and 50 eV.
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S.J. Fielding et al. / H, lineshape measurements in DITE
studied. This agrees with the experiments of Pitcher [16] who, using a similar probe limiter in conjunction with carbon emission spectroscopy estimated that methane production by chemical sputtering was insignificant. Fig. 7 shows that the H, lineshape at c - 15 eV is considerably broader than at 50 eV, an effect similar to that noted by Samm [4]. We find that this broadening is mainly on the red side of X, and corresponds to a near symmetric lineshape. At these low edge temperatures recycling at the probe limiter tip will be reduced and the H, signals may be determined by wall influxes, both close to the viewing window and around the probe limiter. This would tend to give rise to a symmetric lineshape. The H, signals for this case are quite weak and the additional peaks at higher energies seem to be noise related artefacts. The0 underlying broad energy feature, up to 50 pixels (0.8 A, 100 eV) on either side of the line is due to charge exchange of H atoms on plasma ions.
5. Conclusions
We have made high resolution lineshape measurements of H, from Hz, CH, and D, gas puff into DITE plasmas. The line shape asymmetry is determined by geometric effects. Energy spectra are derived from the deconvolved lineshapes and for H, (Dz) show a considerably larger fraction of low energy atoms than suggested by published cross sections and from comparison
with the simulation code HSLAB. The energy spectra from CH, lack the 3 eV feature of H, and analysis of H, from hydrogen recycling at a graphite surface gives no evidence of methane production.
References
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