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Investigations of hydrogen recycling and impurity release in front of a limiter by laser-induced fluorescence
231
Journal of Nuclear Materials 121 (1984) 231-236 North-Holland, Amsterdam
INVESTIGATIONS OF HYDROGEN RECYCLING AND IMPURITY RELEASE IN FRONT OF A LIMITER BY LASER-INDUCED FLUORESCENCE
G. REINHOLD,
J. ~ACK~ANN
and J. UHLENBUSCH
Physikalisches Institut II, Universitiit Dbseldorf; D - 4000 Diseldorf, Fed. Rep. Germany
Particle densities of neutral hydrogen and chromium are measured in the vicinity of a stainless steel toroidal limiter by resonance fluorescence at H, and Cr I (429 nm) wavelengths. The results allow conclusions about particle c&inement, recycling and impu~ty release processes, and the properties of the plasma edge region.
1. Introduction
In order to minimize the release of impurities from limiters in a tokamak, a relatively cool plasma boundary is desired so that a low ion temperature and low sheath potential may reduce sputtering yields as well as unipolar arcing [l]. The need of energy removal from the boundary plasma will therefore require correspondingly high particle fluxes, a situation where recycling dominates the hydrogen particle balance [Z]. The experiments described here were done at the comparatively small tokamak UNITOR (R = 0.3 m, cmm3 f kTe -200 a=O.l m, B,=l.7 T, n, -2x10” eV, I,., = 50 kA; see ref. [3]) to investigate the interaction of a toroidal limiter with the plasma under high-recycling conditions and with ion energies in the range of sputtering thresholds (lo-50 ev). This tokamak is characterized by a stainless-steel vacuum chamber with rectangular cross-section (25 cm x 20 cm, see fig. l), the outer side walls acting as toroidal limiters. Hydrogen recycling was studied by H, resonant scattering. The basic experiment has been described elsewhere [4,5]. Here we report results of more detailed studies of the limiter region. Laser induced fluorescence was used in addition to detect neutral chromium as a representative of the released metals (Cr, Fe, Ni).
2. Mepsun’ng technique Fig. 1 shows the set-up of the laser fluorescence experiment, as used for chromium detection. The nitrogen laser pumped dye laser produces a pulse of ap-
proximately 5 ns duration and 0.1 nm line width (FWHM). In case of H, scattering the ~~gement is similar 141,using a flashlamp pumped dye laser instead. The scattering volume has a diameter of approximately 3 mm for the chromium experiment and 10 mm for the H, fluoresence measurements. The fluorescence pulse is recorded on a storage scope. A delayed monitor signal is addition~ly recorded for simultaneous calibration purposes. The point of observation can be scanned both vertically and horixontally over the entire torus cross section. Absolute calibration is achieved by Rayleigb scattering in nitrogen. The H, transition is saturated by the laser and an equilibrium of the level populations is maintained during the largest part of the laser pulse duration (10e6 s). By measuring the increase of spontaneous emitted H, radiation, it is possible to determine the initial population density n!jr of the first excited level. This evaluation was performed by means of a collisional-radiative model 14-61, in which changes of the populations of all other levels except 2 and 3 are neglected during the laser pulse. A comparison with a more rigorous treatment [7] shows that the model used here gives correct results if the electron density does not exceed 2 X 1013 cme3. In case of the Cr I- transition, the fluorescence signal is directly correlated to the ground-state population density nC’. Since the laser pulse is shorter (5 ns) than the radiative lifetime of the upper level (32 ns [Xl), the transient hehaviour of the fluorescencesignal has to be considered. A detailed analysis of the measured signals shows that collisional depopulation of the upper level is negligible. Determination of nCr is therefore possible using a simple two-level model described in ref. [9].
0022-3115/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
G. Reinhold et al. / Investigations of hydrogen recycling and impurity release
232
f
L3 IF LL
us
UN7
Y -----____ i
I
detoy line
,/-S-’ i
I
G+-__yy___,___t_i Wmonitor
I dm p2
L2
p
DI
Ll dY.
lam
++G+-
n
Fig. 1. Experimental set-up for Cr I resoance fluorescence.
ny ( 108cme3) 2
t = 4 ms x = 8.5 cm p. =
ls
1
5
0.
-l&
Yo.
i
i y
;
ii.
Is.
km1
Fig. 2. Vertical distribution of excited hydrogen in front of the limiter.
Z2 x ltimbat
G. Reinhold et al. / Investigations
of hydrogen reqcling
Fig. 2 shows vertical distributions of excited hydrogen measured 15 mm inside the major radius of the outer torus wall. In one of the two cases shown, the laser observation port was closed by samples of stainless steel to reduce the break in the toroidal limiter. Only horizontal slits of 5 mm height were left for observation. A substantial slits of 5 mm height were left for observation. A substantial increase of the neutral hydrogen density in front of the limiter was observed in this case, indicating that interaction with the limiter surface provides the main source of neutral hydrogen. The processes responsible for the production of neutral hydrogen are assumed to be reflection of protons and dissociation of H, molecules released from the surface (2,101. There is no limiter effect at the inner torus wall, as can be seen from fig. 3, where horizontal distributions of excited hydrogen are plotted. A relatively constant amount of excited hydrogen is found at positions far away from the limiter. For given plasma parameters, the excited hydrogen density is proportional to the flux Q,, of neutrals entering the plasma at the limiter (no pulsed gas feed). By comparing the measurements with the
ny ( ~O*~ITI-~ 1 t = L ms y=Ocm
Fig.
3.
-5.
0.
x (cm1 Horizontal distribution of excited hydrogen.
5.
233
results of Monte-Carlo calculations, this flw was evaluated to be Qn = 1021 s-l (at t = 4 ms) corresponding to a flux density of 10’s cme2 s-i at the limiter [5]. Due to the high recycling occurring under these conditions, the value Qn is close to the flux Q,, of protons and fast neutrals onto the walls. The value of Qn determined by this procedure should be correct to a factor of 2. The effectiveness of recycling processes can also be estimated by considering the characteristic time constants of the particle balance. The total number of protons inside the plasma is NP = fi,V with the plasma volume V and the average electron density g, (assuming n, = nP). The particle confinement time rr, may be defined as or = N,/Q,. The average lifetime of hydrogen particles undergoing recycling processes is in = NP( QP - en)-‘. Since the discharges under consideration have been performed without pulsed gas feed, we may also write rn = Ti,(dnJdr)-‘, taking into account that QP - QH describes the net loss of plasma particles deposited into the wall. Measurements of Qn (see above) and of the electron density (see fig. 6) yield or = 1 ms and in = 10 ms. The overall recycling coefficient defined as Qt./Q, = (rn - T~)/T~ is 0.9 in this case.
3. Hydrogen recycliig
-lo.
and impurity release
lo.
234
G. Reinhold et al. / Investigations of hydrogen recycling and impurity release
4.Chromiumrelease
10 .
Fig. 4 shows the measured vertical distribution of ground-state chromium at a distance of 7 mm in front of the limiter. The values indicate that the toroidal limiter area in contact with the plasma has a vertical diameter of at least 6 cm. Radial density profiles of neutral chromium are plotted in fig. 5. The spatial density decrease is caused by ionization and poloidal spreading. Neglecting the latter at a distance from the limiter much smaller than the height of the limiter zone, the following one-dimensional continuity equation holds: $&(ncru)=n,Si(T.)V
(1)
where ncr denotes the chromium ground-state density at a distanckx from the torus axis, u denotes the local drift velocity of chromium atoms and Si is the ionization rate coefficient, which depends on electron temperature. Eq. (1) allows determination of T, if ne and u are known. In our case, only rough estimates are possible. If sputtering is the release process, the drift velocity u will be typitally 3 X lo5 cm/s [11,12]. Inserting electron densities determined from microwave iterferometry and ionization coefficients according to ref. [13], the profiles of fig. 5 require electron temperatures between 5 and 20 eV for the plasma edge region with no significant variation during the discharge period. 1
3
\\\\ pP Y
g
- -_---_
z 62 c “c
-___.
8.
3 5 01
2.
f
“.
of ground-state
d i/
0.
+
t=6ms
a6.
L. 2. n--P/
d
a6.
1i16ms
L-
7 J
wall -
6.5 9 x km1
95
2. +-n--P 7.5
B
Fig. 5. Horizontal distribution different times after ignition.
x = 9.3 cm t =Gms po= 8.L x lOAmbar
\
I
Fig. 4. Vertical distribution
!
*
wall with observation slits
3
p
t=Lms
L.
1
-3
/ P
6.
‘\
-6
I =2.5ms
e-
--_
0
P
6-
6
chromium in front of the limiter.
of ground-state
chromium
at
G. Reinhold et al. _/ ~n~sti~~ti#
of hydrogen recyciiag and impurity release
Fig. 6 illustrates the time dependence of the groundstate chromium density in front of the limiter. The central electron density obtained from microwave interferometry is plotted versus time as a measure of the proton flux onto the limiter. Since the time dependences of these two qu~titi~ are quite different, we conclude, that proton and neutral hydrogen sputtering are not the dominant mechanisms producing chromium atoms. This conclusion is supported by comparing the flux densities ofprotonshittingthelimiter(~10’8cm-2s-’at?-6 ms, [4]) and of chromium atoms entering the plasma (= 2 X 1Or5 cmv2 s-l at t = 6 ms). If we further take into account the amount of iron which should be additionally sputtered, the assuption of proton sputtering requires a yield of 10e2, which can hardly be achieved with proton energies of at most 60 eV [12,14]. This is the largest possible energy achievable by acceleration with a 3kT, sheath potential. For neutral hydr~en similar arguments apply, because the neutral particle temperature is below 50 eV and the outflux is even smaller than for protons. In this energy range, sputtering by light and heavy impurities is much more efficient, because the threshold energies are much lower than for hydrogen and sputte~ng yields are up to 1000 times larger [12,14]. Impurity sputtering is therefore dominant at an impurity concentration of the order of 1% at the plasma boundary (see also ref. [l]).
tb
t (msl
235
In the following we shall discuss a simple model for the global impurity particle balance. ‘Ihe impurity species are divided into two groups representing light and heavy particles (C, N, 0 and Cr, Fe, Ni respectively). We assume that light impurities are desorbed at a rate proportional to the total flux of hydrogen ions and atoms QP onto the walls, and that metals are sputtered by both light and heavy impurity ions. for the total numbers of light and metal impurity particles inside the plasma, designated as NL and NM, the following rate equations apply:
d4. ---~ dt
dN, dt
-
2 +&,Q, ,
NL = SLM- +(SMM- l)?. 7L
ri, and 7M denote overall impurity ~n~nement times for the light and metal impurity ions. The release processes are described by a coefficient S,, for desorption of light impurities, the sputtering yield S,, for light ion sputtering of metals and the self-sputtering yield S,, of the metals. The desorption coefficient, S nL, may in general depend on time, because it is proportional to the amount of adsorbed light impurities. As a first approach we assume S,, to be constant and consider an exponentially decaying hydrogen flux
20
Fig. 6. Time dependence of ground-state chromium density in front of the limiter and central electron density.
236
G. Reinhold et al. / Investigations
of hydrogen recycling and impurity release
QP = Q, exp( - t/~~) with 7” = 10 ms (see section 3). The solution for N,_ reads:
(2) The measured neutral chromium density (fig. 6) is proportional to the released metal flux:
nc=v -s
LM
%+s,,%. TL
If self-sputtering is negligible (S,, GZl), the neutral chromium density at the plasma edge is proportional to the amount of light impurities NL. In principle, the time dependence of NL in eq. (2) can explain the observed behaviour, if a light ion confinement time of 7L = 4 ms is assumed. If self-sputtering is included, the situation becomes more complicated because a third time constant ~~/(l - S,,) is involved, but the solution for nCrv in eq. (3) will still show a maximum and an asymptotic decay with the largest time constant, as long as the self-sputtering yield S,, is less than one. For our conditions rH should be the largest time constant, so that we interpret the final decay of the measured chromium density as being proportional to the hydrogen flux onto the walls. In a more general view, one may conclude that the impurity concentrations and fluxes will reach an equilibrium with respect to the plasma density after an initial phase of the order of the impurity confinement times, if the hydrogen flux varies on a larger time scale. Though arc tracks have been found on the limiter surface, unipolar arcing seems not to provide a major source for neutral chromium, because the observed time dependence could hardly be explained, taking into account that T, is relatively constant at the plasma edge. Unipolar arcs may however produce ionized metal atoms, which are not detected by the described diagnostics but may contribute to self-sputtering.
5. Conclusions Laser induced fluorescence has been used for local detection of neutral hydrogen and neutral chromium in
order to study the influence of plasma wall processes on the particle balance of hydrogen and impurities. As the results show, the release processes are localized on a toroidal limiter zone at the outer torus wall. High recycling of hydrogen particles takes place in this region. The time dependence of the neutral chromium density indicates that sputtering by neutral hydrogen and protons is not dominant. The assumption of light ion sputtering can explain the observed time behaviour. Self-sputtering may additionally contribute to the metal release. As a further application, the spatial decay of the chromium density in front of the limiter leads to an estimation of the plasma edge electron temperature.
References (11 S. Sengoku and H. Ohtsuka, J. Nucl. Mater. 93&94 (1980) 75. [2] F. Waelbroeck, P. Wienhold and J. Winter, J. Nucl. Mater. lllBi112 (1982) 185. 131R. Flohr, C. Gillet, J. Hackmann, G. Reinhold, G. Ritter, J. Uhlenbusch and S. Zakaria, Physica 104C (1981) 423. 141 G. Reinhold, J. Hackmann and J. Uhlenbusch, J. Plasma Phys. 28 (1982) 281. PI J. Hackmann, C. Gillet, G. Reinhold, G. Ritter and J. Uhlenbusch, J. Nucl. Mater. lllhll2 (1982) 221. 161 G.T. Radzdobarin, V.V. Semenov, L.V. Sokolova, I.P. Folomkin, VS. Burakov, P. Ya. Misakov, P.A. Naumenkov and S.V. Nechaev, Nucl. Fusion 19 (1979) 1439. [71 P. Gohil, G. Kolbe, M.J. Forrest, D.D. Burgess and B.Z. Hu, J. Phys. D (Appl. Phys.) 16 (1983) 333. PI S.M. Younger, J.R. Fuhr, G.A. Martin and W.L. Wiese, J. Phys. Chem. Ref. Data 7 (1978) 495. 191 P. Bogen and E. Hintz, Comments Plasma Phys. Cont. Fusion 4 (1978) 15. PO1 C. Gillet, J. Hackmann and J. Uhlenbusch, Computer Phys. Commun. 24 (1981) 301. 1111 H.L. Bay, B. Schweer, P. Bogen and E. Hintz, J. Nucl. Mater. 111&112 (1982) 732. WI R. Behrisch, G. Maderlechner and B.M.U. Scherzer, Appl. Phys. 18 (1979) 391. I131 W. Lotz, Report IPP l/76 (1968). v41 J. Bohdansky, J. Nucl. Mater. 93&94 (1980) 44.
Journal of Nuclear Materials 121 (1984) 231-236 North-Holland, Amsterdam
INVESTIGATIONS OF HYDROGEN RECYCLING AND IMPURITY RELEASE IN FRONT OF A LIMITER BY LASER-INDUCED FLUORESCENCE
G. REINHOLD,
J. ~ACK~ANN
and J. UHLENBUSCH
Physikalisches Institut II, Universitiit Dbseldorf; D - 4000 Diseldorf, Fed. Rep. Germany
Particle densities of neutral hydrogen and chromium are measured in the vicinity of a stainless steel toroidal limiter by resonance fluorescence at H, and Cr I (429 nm) wavelengths. The results allow conclusions about particle c&inement, recycling and impu~ty release processes, and the properties of the plasma edge region.
1. Introduction
In order to minimize the release of impurities from limiters in a tokamak, a relatively cool plasma boundary is desired so that a low ion temperature and low sheath potential may reduce sputtering yields as well as unipolar arcing [l]. The need of energy removal from the boundary plasma will therefore require correspondingly high particle fluxes, a situation where recycling dominates the hydrogen particle balance [Z]. The experiments described here were done at the comparatively small tokamak UNITOR (R = 0.3 m, cmm3 f kTe -200 a=O.l m, B,=l.7 T, n, -2x10” eV, I,., = 50 kA; see ref. [3]) to investigate the interaction of a toroidal limiter with the plasma under high-recycling conditions and with ion energies in the range of sputtering thresholds (lo-50 ev). This tokamak is characterized by a stainless-steel vacuum chamber with rectangular cross-section (25 cm x 20 cm, see fig. l), the outer side walls acting as toroidal limiters. Hydrogen recycling was studied by H, resonant scattering. The basic experiment has been described elsewhere [4,5]. Here we report results of more detailed studies of the limiter region. Laser induced fluorescence was used in addition to detect neutral chromium as a representative of the released metals (Cr, Fe, Ni).
2. Mepsun’ng technique Fig. 1 shows the set-up of the laser fluorescence experiment, as used for chromium detection. The nitrogen laser pumped dye laser produces a pulse of ap-
proximately 5 ns duration and 0.1 nm line width (FWHM). In case of H, scattering the ~~gement is similar 141,using a flashlamp pumped dye laser instead. The scattering volume has a diameter of approximately 3 mm for the chromium experiment and 10 mm for the H, fluoresence measurements. The fluorescence pulse is recorded on a storage scope. A delayed monitor signal is addition~ly recorded for simultaneous calibration purposes. The point of observation can be scanned both vertically and horixontally over the entire torus cross section. Absolute calibration is achieved by Rayleigb scattering in nitrogen. The H, transition is saturated by the laser and an equilibrium of the level populations is maintained during the largest part of the laser pulse duration (10e6 s). By measuring the increase of spontaneous emitted H, radiation, it is possible to determine the initial population density n!jr of the first excited level. This evaluation was performed by means of a collisional-radiative model 14-61, in which changes of the populations of all other levels except 2 and 3 are neglected during the laser pulse. A comparison with a more rigorous treatment [7] shows that the model used here gives correct results if the electron density does not exceed 2 X 1013 cme3. In case of the Cr I- transition, the fluorescence signal is directly correlated to the ground-state population density nC’. Since the laser pulse is shorter (5 ns) than the radiative lifetime of the upper level (32 ns [Xl), the transient hehaviour of the fluorescencesignal has to be considered. A detailed analysis of the measured signals shows that collisional depopulation of the upper level is negligible. Determination of nCr is therefore possible using a simple two-level model described in ref. [9].
0022-3115/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
G. Reinhold et al. / Investigations of hydrogen recycling and impurity release
232
f
L3 IF LL
us
UN7
Y -----____ i
I
detoy line
,/-S-’ i
I
G+-__yy___,___t_i Wmonitor
I dm p2
L2
p
DI
Ll dY.
lam
++G+-
n
Fig. 1. Experimental set-up for Cr I resoance fluorescence.
ny ( 108cme3) 2
t = 4 ms x = 8.5 cm p. =
ls
1
5
0.
-l&
Yo.
i
i y
;
ii.
Is.
km1
Fig. 2. Vertical distribution of excited hydrogen in front of the limiter.
Z2 x ltimbat
G. Reinhold et al. / Investigations
of hydrogen reqcling
Fig. 2 shows vertical distributions of excited hydrogen measured 15 mm inside the major radius of the outer torus wall. In one of the two cases shown, the laser observation port was closed by samples of stainless steel to reduce the break in the toroidal limiter. Only horizontal slits of 5 mm height were left for observation. A substantial slits of 5 mm height were left for observation. A substantial increase of the neutral hydrogen density in front of the limiter was observed in this case, indicating that interaction with the limiter surface provides the main source of neutral hydrogen. The processes responsible for the production of neutral hydrogen are assumed to be reflection of protons and dissociation of H, molecules released from the surface (2,101. There is no limiter effect at the inner torus wall, as can be seen from fig. 3, where horizontal distributions of excited hydrogen are plotted. A relatively constant amount of excited hydrogen is found at positions far away from the limiter. For given plasma parameters, the excited hydrogen density is proportional to the flux Q,, of neutrals entering the plasma at the limiter (no pulsed gas feed). By comparing the measurements with the
ny ( ~O*~ITI-~ 1 t = L ms y=Ocm
Fig.
3.
-5.
0.
x (cm1 Horizontal distribution of excited hydrogen.
5.
233
results of Monte-Carlo calculations, this flw was evaluated to be Qn = 1021 s-l (at t = 4 ms) corresponding to a flux density of 10’s cme2 s-i at the limiter [5]. Due to the high recycling occurring under these conditions, the value Qn is close to the flux Q,, of protons and fast neutrals onto the walls. The value of Qn determined by this procedure should be correct to a factor of 2. The effectiveness of recycling processes can also be estimated by considering the characteristic time constants of the particle balance. The total number of protons inside the plasma is NP = fi,V with the plasma volume V and the average electron density g, (assuming n, = nP). The particle confinement time rr, may be defined as or = N,/Q,. The average lifetime of hydrogen particles undergoing recycling processes is in = NP( QP - en)-‘. Since the discharges under consideration have been performed without pulsed gas feed, we may also write rn = Ti,(dnJdr)-‘, taking into account that QP - QH describes the net loss of plasma particles deposited into the wall. Measurements of Qn (see above) and of the electron density (see fig. 6) yield or = 1 ms and in = 10 ms. The overall recycling coefficient defined as Qt./Q, = (rn - T~)/T~ is 0.9 in this case.
3. Hydrogen recycliig
-lo.
and impurity release
lo.
234
G. Reinhold et al. / Investigations of hydrogen recycling and impurity release
4.Chromiumrelease
10 .
Fig. 4 shows the measured vertical distribution of ground-state chromium at a distance of 7 mm in front of the limiter. The values indicate that the toroidal limiter area in contact with the plasma has a vertical diameter of at least 6 cm. Radial density profiles of neutral chromium are plotted in fig. 5. The spatial density decrease is caused by ionization and poloidal spreading. Neglecting the latter at a distance from the limiter much smaller than the height of the limiter zone, the following one-dimensional continuity equation holds: $&(ncru)=n,Si(T.)V
(1)
where ncr denotes the chromium ground-state density at a distanckx from the torus axis, u denotes the local drift velocity of chromium atoms and Si is the ionization rate coefficient, which depends on electron temperature. Eq. (1) allows determination of T, if ne and u are known. In our case, only rough estimates are possible. If sputtering is the release process, the drift velocity u will be typitally 3 X lo5 cm/s [11,12]. Inserting electron densities determined from microwave iterferometry and ionization coefficients according to ref. [13], the profiles of fig. 5 require electron temperatures between 5 and 20 eV for the plasma edge region with no significant variation during the discharge period. 1
3
\\\\ pP Y
g
- -_---_
z 62 c “c
-___.
8.
3 5 01
2.
f
“.
of ground-state
d i/
0.
+
t=6ms
a6.
L. 2. n--P/
d
a6.
1i16ms
L-
7 J
wall -
6.5 9 x km1
95
2. +-n--P 7.5
B
Fig. 5. Horizontal distribution different times after ignition.
x = 9.3 cm t =Gms po= 8.L x lOAmbar
\
I
Fig. 4. Vertical distribution
!
*
wall with observation slits
3
p
t=Lms
L.
1
-3
/ P
6.
‘\
-6
I =2.5ms
e-
--_
0
P
6-
6
chromium in front of the limiter.
of ground-state
chromium
at
G. Reinhold et al. _/ ~n~sti~~ti#
of hydrogen recyciiag and impurity release
Fig. 6 illustrates the time dependence of the groundstate chromium density in front of the limiter. The central electron density obtained from microwave interferometry is plotted versus time as a measure of the proton flux onto the limiter. Since the time dependences of these two qu~titi~ are quite different, we conclude, that proton and neutral hydrogen sputtering are not the dominant mechanisms producing chromium atoms. This conclusion is supported by comparing the flux densities ofprotonshittingthelimiter(~10’8cm-2s-’at?-6 ms, [4]) and of chromium atoms entering the plasma (= 2 X 1Or5 cmv2 s-l at t = 6 ms). If we further take into account the amount of iron which should be additionally sputtered, the assuption of proton sputtering requires a yield of 10e2, which can hardly be achieved with proton energies of at most 60 eV [12,14]. This is the largest possible energy achievable by acceleration with a 3kT, sheath potential. For neutral hydr~en similar arguments apply, because the neutral particle temperature is below 50 eV and the outflux is even smaller than for protons. In this energy range, sputtering by light and heavy impurities is much more efficient, because the threshold energies are much lower than for hydrogen and sputte~ng yields are up to 1000 times larger [12,14]. Impurity sputtering is therefore dominant at an impurity concentration of the order of 1% at the plasma boundary (see also ref. [l]).
tb
t (msl
235
In the following we shall discuss a simple model for the global impurity particle balance. ‘Ihe impurity species are divided into two groups representing light and heavy particles (C, N, 0 and Cr, Fe, Ni respectively). We assume that light impurities are desorbed at a rate proportional to the total flux of hydrogen ions and atoms QP onto the walls, and that metals are sputtered by both light and heavy impurity ions. for the total numbers of light and metal impurity particles inside the plasma, designated as NL and NM, the following rate equations apply:
d4. ---~ dt
dN, dt
-
2 +&,Q, ,
NL = SLM- +(SMM- l)?. 7L
ri, and 7M denote overall impurity ~n~nement times for the light and metal impurity ions. The release processes are described by a coefficient S,, for desorption of light impurities, the sputtering yield S,, for light ion sputtering of metals and the self-sputtering yield S,, of the metals. The desorption coefficient, S nL, may in general depend on time, because it is proportional to the amount of adsorbed light impurities. As a first approach we assume S,, to be constant and consider an exponentially decaying hydrogen flux
20
Fig. 6. Time dependence of ground-state chromium density in front of the limiter and central electron density.
236
G. Reinhold et al. / Investigations
of hydrogen recycling and impurity release
QP = Q, exp( - t/~~) with 7” = 10 ms (see section 3). The solution for N,_ reads:
(2) The measured neutral chromium density (fig. 6) is proportional to the released metal flux:
nc=v -s
LM
%+s,,%. TL
If self-sputtering is negligible (S,, GZl), the neutral chromium density at the plasma edge is proportional to the amount of light impurities NL. In principle, the time dependence of NL in eq. (2) can explain the observed behaviour, if a light ion confinement time of 7L = 4 ms is assumed. If self-sputtering is included, the situation becomes more complicated because a third time constant ~~/(l - S,,) is involved, but the solution for nCrv in eq. (3) will still show a maximum and an asymptotic decay with the largest time constant, as long as the self-sputtering yield S,, is less than one. For our conditions rH should be the largest time constant, so that we interpret the final decay of the measured chromium density as being proportional to the hydrogen flux onto the walls. In a more general view, one may conclude that the impurity concentrations and fluxes will reach an equilibrium with respect to the plasma density after an initial phase of the order of the impurity confinement times, if the hydrogen flux varies on a larger time scale. Though arc tracks have been found on the limiter surface, unipolar arcing seems not to provide a major source for neutral chromium, because the observed time dependence could hardly be explained, taking into account that T, is relatively constant at the plasma edge. Unipolar arcs may however produce ionized metal atoms, which are not detected by the described diagnostics but may contribute to self-sputtering.
5. Conclusions Laser induced fluorescence has been used for local detection of neutral hydrogen and neutral chromium in
order to study the influence of plasma wall processes on the particle balance of hydrogen and impurities. As the results show, the release processes are localized on a toroidal limiter zone at the outer torus wall. High recycling of hydrogen particles takes place in this region. The time dependence of the neutral chromium density indicates that sputtering by neutral hydrogen and protons is not dominant. The assumption of light ion sputtering can explain the observed time behaviour. Self-sputtering may additionally contribute to the metal release. As a further application, the spatial decay of the chromium density in front of the limiter leads to an estimation of the plasma edge electron temperature.
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