JET–P(97)18
Observations and Modelling of
Diatomic Molecular Spectra
from JET
G Duxbury1, M F Stamp and H P Summers.
JET Joint Undertaking, Abingdon, Oxfordshire, OX14 3EA, UK.
1
Department of Physics and Applied Physics, University of Strathclyde, Glasgow.
Preprint of a paper to be submitted for publication in
J Physics B: At. Mol. Opt. Phys.
April 1997
“This document is intended for publication in the open literature. It is made
available on the understanding that it may not be further circulated and
extracts may not be published prior to publication of the original, without the
consent of the Publications Officer, JET Joint Undertaking, Abingdon, Oxon,
OX14 3EA, UK”.
“Enquiries about Copyright and reproduction should be addressed to the
Publications Officer, JET Joint Undertaking, Abingdon, Oxon, OX14 3EA”.
Abstract
Spectra in the visible range may be obtained along multiple lines of sight directed at
the axisymmetric poloidal divertor of the JET tokamak. In X-point magnetic field
configurations, the scrape-off-layer plasma contacts the divertor target plates. We
have conducted observations and analysis of diatomic spectra in the divertor plasma
along lines of sight in the strike zone vicinity. We have identified unambiguously
CD, C2 and BeD. Synthetic spectra have been calculated. In addition to identification
of the molecular features and separation of overlapping atomic lines, the comparison
of simulations and observations allow quite sensitive determination of rotational
and vibrational temperatures for the above diatomics in a divertor fusion plasma.
Systematic studies show that Tr and Tν are comparable for BeD but Tν is significantly
less than Tr for CD and C2. The BeD spectrum time history during a pulse (modulated
by sweeping) appears as localised emission from the strike points similar to that of
the atomic BeI visible spectrum lines. On the other hand, the CD bands during a
swept pulse indicate a more distributed source. Thus both the BeD spectrum and
time history suggest physical sputtering of BeD at the strike points. In contrast the
CD spectrum and time history indicate chemical release of higher deuterides followed
by catabolism to CD. This work constitutes the first detailed study of diatomic species
in the JET tokamak.
1.
Introduction to JET divertor observations
Over the last eight years implementation of the axi-symmetric poloidal divertor concept on the
JET machine has let to extensive redesigning of the vessel interior and magnetic field coils to
produce appropriate magnetic null (X-point) positions, connection lengths and divertor target
tiles for the plasma flowing down the scrape-off layer. Experimental campaigns have investigated
the properties of the Mk1 and Mk2A designs. The next stage in this evolution is the Mk2B (gas
box) design due for installation at the end of 1997.
1
a)
(a)
Cryopump
Target plates
b)
J G 9 6.5 1
4/1 2
ic
(b)
,,
,,,,,,
,,
,,
,,,,,,
,,
c
2ii
4/1
MarkII (1996)
.51
96
JG
c)
2.0
Z (m)
1.0
0
JG97.182/1c
–1.0
–2.0
1.0
2.0
3.0
R (m)
4.0
5.0
Figure 1. (a) Schematic poloidal section of the JET torus showing the geometrical arrangement of the coils and
target plates of the JET MK1 divertor. (b) Schematic of the Mk2A divertor. (c) Typical reconstructed poloidal
section of the magnetic flux surfaces showing the last closed flux surface and the strike zones of the scrape-offlayer plasma with the target plates. Viewing lines used in the present study are superimposed.
2
Many diagnostic systems were upgraded and optimised prior to plasma operation with the Mk1
divertor (Breger and Vlases, 1991). As a result of the improved divertor spectroscopy the MK1
divertor campaign has given the opportunity for the first detailed sets of observations of a number
of diatomic molecular species in JET. This paper presents spectral observations of molecular
bands of C2, CD and BeD from the JET divertor in the visible region together with modelled
spectra. CD has been observed in earlier JET campaigns (eg. Behringer, 1990) in limiter spectra.
The present observations reflect typical divertor working conditions. The C2 and BeD
observations are new. The comparison of modelled and observed spectra have allowed us to
infer rotational and vibrational temperatures and so distinguish mechanisms of formation.
Behringer (1991) examined vibrational and rotational temperatures of CH and CD in low
temperature RF excited discharge plasmas and has discussed the catabolic pathways for methane
in the gas phase.
2.
Comparison of observed and synthesised band spectra
2.1. Synthesising the ro-vibronic spectrum of diatomics
The computer program, CALCAT developed at the JET Propulsion Laboratory (JPL) by Pickett
(1991) was used to obtain the energies and transition intensities. Unlike most programs for
calculating diatomic spectra, CALCAT is based on a spherical tensor approach. The primary
intention of the code was the calculation of the microwave spectra of polyatomic molecules,at
JPL. A feature of value for the present work is the easy inclusion of all possible transitions in
open shell diatomics without the need to consider the specific Hönl-London factors for each
system. Resulting transition catalogue files were scaled, using the appropriate temperature
dependence of the partition functions and of the populated rotational states, to generate the
integrated intensities over ranges of vibrational and rotational temperatures. The latter were
transformed with instrumental broadening functions to generate the synthetic spectra for
comparison with the experimental spectra. However its capabilities for other systems was
recognised by Pickett (Pickett - private communication). Some of the extensions are described
in the on-line documentation available
3
Arbitrary units
5
(a)
4
3
2
1
Arbitrary units
0
5100
5120
5160
5140
Wavelength (Angstrom)
(b)
5
(c)
4
3
2
1
0
5100
5120
5140
5160
Wavelength (Angstrom)
5100
5120
5140
5160
Wavelength (Angstrom)
Figure 2. (a) Observed spectrum of the 5100-5170Å region (JET Pulse No. 37925 at 6.7s) showing
3
3
C2 (A Πg - X Πu) . (b) Simulated 0-0 and 1-1 vibrational bands at Tr = 6000K. The 0-0 and 1-1 band
proportions are 0.8:0.3 corresponding to Tn = 3000K. (c) Simulated 0-0 vibrational band at Tr = 6000K.
2.2
C2 (A3Πg - X 3Πu) - Swan System
C2 can be observed in both low and high resolution spectra. At high resolution, the 0-0 band can
be seen to exhibit the characteristic structure of a parallel band with a pronounced head in the P
branch. Owing to the resolution of the spectrometer, the triplet structure of the band cannot be
observed. We used the molecular constants of Prasad and Bernath (1994) in the simulated spectra.
It was possible to estimate the rotational temperature by comparing the relative intensities of the
high N returning P branch with the low N R branch lines. For the JET divertor, this indicates a
lower bound of 6000K for the rotational temperature. Since the 1-1 sequence band is weak, the
molecule appears rotationally hot but vibrationally much cooler. Figure 2a below shows the
experimental spectrum. Figure 2c shows the synthetic 0-0 band. Figure 2b includes the synthetic
1-1 sequence band at 5130Å.
2.3. C D (A2∆ - X 2Π)
C D can also be detected in both low (Stamp and von Hellermann, 1995) and high resolution
spectra. The 0-0 band has the structure associated with a perpendicular band. In the JET spectra,
part of the central Q branch pattern and the P branches can be observed. We used the molecular
4
constants of Herzeberg and Johns (1969) in the simulated spectra. The B value of the v = 2 level
of the A2∆ state was not measured by the authors. The extrapolated values of B for v = 2 from
their expansion produces a large difference in the B values between the A2∆ and X 2Π states,
whereas experimentally the B values are almost the same giving a line-like Q branch. The
experimental values used here have been provided by Fantz (private communication).
Arbitrary units
(a)
6
4
2
0
4300 4320
Arbitrary units
8
(c)
6
4
2
0
4300 4320
4340 4360 4380 4400
Wavelength (Angstrom)
6
(b)
4
2
0
4300 4320
4340 4360 4380 4400
Wavelength (Angstrom)
Arbitrary units
Arbitrary units
8
6
4340 4360 4380 4400
Wavelength (Angstrom)
(d)
4
2
0
4300 4320
4340 4360 4380 4400
Wavelength (Angstrom)
Figure 3. (a) Observed spectrum of the 4300-4400 region (JET Pulse No. 29526 at 13.3s) showing the C D
2
2
(A ∆ - X Π) bands. (b) Simulated 0-0, 1-1 and 2-2 vibrational bands in the proportions 1.0:0.4:0.05. Two such
sets, at rotational temperatures Tr = 7000K and 3500K, are combined in the proprotions 1.2:0.8. (c) As in case
(b) but with the proportions 0.0:2.0. (d) As in case (b) but with the proportions 2.0:0.
The observed spectrum is relatively noisy and a number of atomic lines are present. Even
so, it is evident that peak by peak matching of theoretical and experimental results is not obtained
although a broad correspondence is present. The likely error is in the higher order centrifugal
distortion corrections for the upper electronic state. There has been no recent work on the
electronic spectra of CD (unlike CH) and since the earlier work was at room temperature, some
alteration in these parameters may be anticipated. Thus in the region between 4320Å and 4340Å
it is probable that the constructive superposition of the 0-0 and 1-1 P branches is incorrect. The
critical region for comparison of simulated and experimental spectra is between 4340Å and
4360Å where the high level of the returning P 0-0 branch from the band head implies a high
rotational temperature. Yet, the band head is insufficiently pronounced suggesting a lower
temperature component overlapping the high temperature part. Thus, we believe it impossible
5
to simulate the structure of the P branch using a single rotational temperature. A combination of
a low rotational temperature contribution at ~ 3500K (cf. figure 3c) and a high rotational
temperature contribution at ~ 7000K. (cf. figure 3d) seems necessary. A combination which
approximately represents the experimental spectrum is given in figure 3b. The proportions of
the 1-1 and 2-2 vibrational bands used are certainly lower than thermal at a rotational temperature
of ~ 7000K. The height of the 2-2 line-like feature at 4320Å and the trend of the 1-1 Q branch
at 4310Å are sensitive in this regard.
2.4. BeD (A 2Π - X 2∑)
The green bands of BeD reveal markedly different behaviour from CD and C2. We used the
molecular constants of Colin and de Greef (1975) and Horne and Colin (1972) in the simulated
spectra. The hot 1-1 and 2-2 bands are readily observed. The beating between their slightly
different spaced P branch lines with those of the 0-0 band, and the well defined head in the P
branch give a clear identification both of the carrier of the bands, and of the rotational temperature.
Arbitrary units
6
(a)
5
4
3
2
1
Arbitrary units
0
4
(b)
Tr=Tv=3000K
Tr=Tv=3500K
3
Tr=Tv=4000K
2
Tr=Tv=7000K
1
0
4750 4800 4850 4900 4950 5000 5050 5100 5000
Wavelength (Angstrom)
5050
5100
Figure 4. (a) Observed spectrum of the 4850-5100 region (JET Pulse #35687 at 14.5s) showing BeD
(A2Π - X 2∑). (b) Simulated 0-0, 1-1 and 2-2 bands at Tr = 3500K and Tn = 3500K. The insert shows
the variation of the P branch with temperature.
Rotational and vibrational temperatures appear very similar (Tr ~ Tν ~ 3500K) and are
considerably lower than those recorded with the graphite target. Figure 4a shows the experimental
6
spectrum and figure 4b the superposed synthetic 0-0, 1-1 and 2-2 bands. The agreement between
the two is excellent. The sensitivity to temperature in the wavelength region from 5000Å to
5050Å is marked as shown in the enlargements of figure 4b.
3.
JET time histories and diatomic spectrum formation
In operation with the MK1 divertor, the plasma was normally swept back and forth across the
divertor target region to avoid excessive heating of the target plates. Since the power handling of
the Mk2A divertor showed a substantial improvement on that of the Mk1, rapid sweeping was
no longer necessary. Two types of sweeping are shown in figure 5. The first (figure 5a) is a
single fast sweep followed by a slow sweep and relates to the Mk2A spectral observations of
(a)
last closed
flux surface
MW
3.0
2.0
Input power
(a)
1.0
0.0
m
2.90
(a) Outer strike point position
2.80
2.70
swept outer
strike zone
(b)
line of sight
at 2.773m
12
1.2
MW*10
swept inner
strike zone
0.8
(b)
13
14
15
16
17
secs
Input power
0.4
last closed
flux surface
0.0
m
2.84 (b)
2.80
swept inner
strike zone
swept outer
strike zone
2.76 Outer strike point position
11 12 13 14 15 16 17 18 19
secs
Figure 5. Relative positions of the visible line of sight and the swept outer strike zone of divertor. (a) single fast
then a long slow sweep across the Mk2A graphite target plates - associated with figure 6 below. (b) repeated
fast sweeps across the Mk1 beryllium targets - associated with figure 7 below.
figure 6. The second (figure 5b) is repeated fast sweeps and relates to the Mk1 spectral observations
of figure 7. For the observations of figure 6, the line of sight was at 2.83m. The swept outer
strike zone began inboard of the line of sight moved towards it and back in the fast phase and
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then crossed the line of sight in the slow phase. The line of sight is in the private region after 16
secs. This was an ohmic discharge. For the observations of figure 7, the line of sight was at
2.773m. The outer strike zone approaches the spectroscopic line of sight but does not cross it.
Thus the line of sight just remains in the private region. Neutral beam heating was applied
during the early part of the pulse as shown in figure 5b.
10
13
ph/s/sr/cm 2
In figure 6, the very broad emission for CD and the relatively broad CII and DI emission
with substantial signals between the peaks are of note. These features are to be contrasted with
the narrower BeI emission profile. For DI, the long penetration depth of deuterium (through
resonant charge transfer with D+), as it returns into the plasma, is the cause of its emission
profile width. From the broad emission for CD we conclude that its emission is not localised to
the region of the energetic D+ flux to the target. Thus its source, or that of its precursor, is more
distributed. This distribution probably reflects chemical release via higher hydrides, due to the
general thermal D loading of the graphite target plates. The breadth of the CII profile is anticipated.
The time scale to ionise through the C+ stage allows spreading of the zone of CII emission by
transport of C+ ions along the field lines. It is weakly correlated with the CD signal.
300
200
100
0
4
D-alpha
CD
2
0
20
C II (658nm)
Spectral Line Intensity
10
0
60
40
20
0
C III (465nm)
2
Be I (457nm)
1
0
12
13
14
15
16
17
Time (s)
Figure 6. Time histories of the CD molecular band and atomic lines of the CII, CIII, DI
(Balmer alpha) and BeI spectra during the Mk2A sweep (JET Pulse No.39246).
8
In figure 7, note the narrow time history peaks for the BeD, BeI and BeII lines, and that the
between peak values are close to zero. This is to be contrasted with the broader Dβ and CII
profiles which have large between peak values. This indicates that the spatial emission profiles
of BeD, BeI and BeII are quite narrow and that of CII is broad. For BeI and BeII, we conclude
that physical sputtering is the release mechanism and because of the low ionisation potentials,
neutral and singly ionised beryllium are ionised close to the target plate. The strong correlation
of the BeD and BeI spectra indicate that BeD is released also by the energetic D+ flux to the
target and not via the distributed thermal D loading of the target plates.
6
D-beta
4
ph/s/sr/cm 2
14
10
0
6
BeD
4
2
0
6
Be I (457nm)
4
2
ph/s/sr/cm 2 a.u.
0
10
13
Spectral Line Intensity
2
1.5
1.0
Be II (436nm)
0.5
0.0
1.0
C II (712nm)
0.5
0.0
11
12
13
14
15
16
17
18
19
Time (s)
Figure 7. Time histories of the BeD molecular band and atomic lines of the BeI, BeII, DI
(Balmer beta) and CII spectra during the Mk1 sweep (JET Pulse No.35687).
4.
Discussion and conclusions
The differences between CD and BeD evident in the JET spectra and time histories have their
origin in the different molecular chemistry of beryllium and carbon hydrides. Whereas BeD is
stable, the higher hydrides of Be are unstable. Recent work by Coxon and Colin (1997) has
9
shown that BeD+ should also be stable in the conditions of the JET divertor plasma. Hence the
diatomics, BeD and BeD+ should be the only beryllium hydride molecules present in the plasma
or released into it (Unfortunately, BeD+ is not observable with the spectral range of the visible
spectrometers used for the present study). This contrasts with the expected release of higher
hydrides of CD and especially CD4. The effective (rotational and vibrational) temperature of
BeD is ~ 3500K (0.3eV) and this is to be compared with the typical energies of physically
sputtered particles from plasma contacted surfaces, namely ~ 2 - 4 eV (Roth, 1987). Thus it
seems that the levels of electronic, rotational and vibrational excitation of BeD are those intrinsic
to a BeD molecule on ejection by physical sputtering. Note that the plasma electron temperature
near the divertor target plates is typically 10-30eV (non-detached plasmas) for the JET pulses
considered here. The lower rotational/vibrational temperature of the BeD band emission suggests
that the time scale for vibrational and rotational heating up of the diatomic in the plasma is
longer than that for ionisation and dissociation.
The low rotational temperature part of the CD spectrum, suggested by the spectral
simulation, also indicates the presence of CD molecules physically sputtered into the plasma
with an intrinsic effective rotational/vibrational temperature ~ 3500K. This similarity to BeD is
perhaps expected since the upper and lower level potential curves of the observed BeD and CD
electronic transitions are similar. The second component of the CD spectrum at high rotational
temperature and low vibrational temperature probably has its origin in a different CD population,
namely that from a molecular catabolism. Fantz (1997) has drawn attention to the production of
CD in highly excited rotational states from the break-up of higher hydrides (especially the direct
reaction CD4 + e → CD * + e + other products) in discharge plasmas. For the JET divertor
plasma, it would seem that the latter route to production of the CD band spectra dominates over
direct excitation.
The spectral time histories tend to reinforce these arguments. The point of release (and
emission) for BeD is close to the target strike point. It is at this point that physical sputtering is
localised and so the link between physical sputtering and BeD emission is indicated. A physically
dispersed source of release is likely for CD4 produced by chemical reactions in the graphite
divertor plates. The distributed CD emission region indicates that chemical release of CD4 and
then its catabolism is predominant over sputtering of CD for formation of the CD band emission
observed here.
The C2 spectrum appears to have a catabolic excitation path with the rotational temperature
high and the vibrational temperature low. Evidence from discharge plasmas suggests that both
direct excitation and catabolic pathways to spectral band formation occur for C2 (Fantz, 1997).
10
References
Behringer K 1990 J. Nucl. Mat. 176-177 606
Behringer K 1991 Plasma Phys. Control. Fusion 33 997
Breger P and Vlases G 1991 JET Joint Undertaking Report JET-TN(91)04
Colin R and de Greef D 1975 Can. J. Phys. 53 2142
Coxon J A and Colin R 1997 J. Mol. Spec. - in press
Fantz U 1997 - to be published.
Herzberg G and Johns J W C 1969 Astrphys. J. 158 399
Horne R and Colin R 1972 Bull. Soc. Chim. Belg. 81 93
Pickett H M 1991 J. Mol. Spec. 148 371
Prasad C V V and Bernath P F 1994 Astrophys. J. 426 812
Roth J 1987 J. Nucl. Mat. 145-147 87
Stamp M F and von Hellermann M 1995 Proc. 22nd. Eur. Phys. Soc. Conf. on Controlled Fusion
and Plasma Physics 19C III 89
11