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Helium exhaust in plasmas with strong radiative edge cooling
journalnf nuclear materials
Journal of Nuclear Materials 196-198 (1992) 633-636 North-Holland
Helium exhaust in plasmas with strong radiative edge cooling U. Saturn, J. Boedo a, G. Bertschinger, K.H. Dippel, H. Euringer, K.H. Finken, D. Gray a, D. Hillis b, A. Pospieszczyk, D. Reiter, M. Tokar and B. Unterberg Institut fiir Plasmaphysik, Ass. KFA-EURATOM, Forschungszentrum Jiilich, Germany a Institute of Plasma and Fusion Research, University of California, Los Angeles, CA, USA b Oak Ridge National Laboratory, Oak Ridge, TN, USA
The compatibility of radiative edge cooling by neon injection and He exhaust with the pump limiter ALT-II is studied on TEXTOR. It is demonstrated that in plasmas with strong auxiliary heating (2 MW NBI) and with the highest average electron densities (fie = 5.5 x 1019 m -3) the effective confinement time r~, for He has a minimum. This good pumping performance is maintained even for cases in which up to 90% of the heating power is radiated from the plasma boundary (cold radiative edge). The processes inside the scoops of the pump limiter (neutral particle transport, re-ionization) and the variation of particle confinement in the main plasma dominate this behavior.
1. Introduction The plasma scenario with a "cold radiative edge" has been considered as a possible remedy for the problems of plasma-wall interaction in a tokamak [1,2]. Distributing the power flow onto the wall by radiation reduces the convective heat flux in the scrape-off layer (SOL), thus avoiding dangerous peak loads at the limiter or the divertor plate. Significant radiative edge cooling can only be achieved successfully in a reactor if the following conditions are fulfilled: (a) impurities have to be present, because only line radiation from impurities can maintain a high radiation level, (b) the radiation has to be restricted to the boundary region, (c) fuel dilution in the plasma centre due to impurities must be kept sufficiently low, (d) thermal and MHD stability of the radiative edge is required and (e) compatibility with particle exhaust (ash removal) is necessary [3]. It has been shown in earlier experiments with pure ohmic heating that the light intrinsic impurities C and O essentially fulfill the requirements a-d. Quasi-stationary plasmas with a radiation level 7 = Prad/Pheat close to 100% are possible [4]. For plasmas with strong auxiliary heating additional impurities have to be injected to radiate the additional power. For this purpose neon injection has been applied successfully on TEXTOR [5]. In many cases of a cold radiative boundary in TEXTOR, in particular with a "detached" plasma, in which the ionization zone shrinks to a smaller minor radius, reduced convection in the SOL and a decreasing edge density leads to a deterioration of the particle exhaust efficiency of the pump limiter ALT-II. In view of these difficulties, problems with He
exhaust in connection with intense radiative edge cooling could be expected. That He can be pumped efficiently at a low radiation level (7 = 0.18) with the pump limiter ALT-II has already been demonstrated for plasmas with medium density ( f i e = 3.5 X 1019 m 3) and auxiliary heating by neutral beam injection (NBI) [6]. In this paper we show that the helium exhaust efficiency improves when the line averaged central density fie increases and that this good efficiency can be maintained in the presence of significant radiative edge cooling.
2. Experiment TEXTOR is a medium-sized tokamak with a major radius R = 1.75 m and a minor radius a = 0.46 m. The standard values for the magnetic field B T = 2.25 T and the plasma current Ip = 350 kA have been chosen for this experiment. The toroidal belt limiter ALT-II is designed as a pump limiter using eight turbo pumps [7]. The throat of the limiter collects particles in the SOL in the radial range r = 0.477-0.504 m. All limiter blades are made of graphite and the vessel wall is boronized. The working gas is deuterium. Auxiliary heating with NB co-injection of H (1.7 MW) together with ohmic heating (0.3 MW) provide a total heating power of Ptot = 2 MW. This corresponds to a power density of 0.3 M W / m 3 or 0.1 M W / m 2. 2.1. Radiative edge cooling by neon injection Neon is injected by a gas puffing system with a fast piezo valve. Because the recycling coefficient R of
0022-3115/92/$05.00 9 1992 - Elsevier Science Publishers B.V. All rights reserved
U. Samm et al. / He exhaust in plasmas with edge cooling
634 2
centre
Te >
~6
0.8
0.8
%0.6 >,
0.6
0.4
0.4
0.2
0.2
&
- 6 (0
0 20 minor rodius cm
NBI 0
1 time
2 sec
Fig. 1, Variation of the radiation level (T-scan) from shot to shot by neon injection during auxiliary heating with 1.7 MW of NBI.
neon is unity the amount of gas puffed in accumulates in the plasma if the pumps of A L T - I I are closed. With open pumps the removal time for Ne is about 0.5 s. A feedback control of the neon level is necessary to provide stationary conditions, particularly close to the radiation limit 3' = 1. Such a system has been developed on T E X T O R [5]. The control variable is a Ne VIII line representing well the total radiation. The system allows performance of ,/-scans, e.g. 3, = 0.2-0.98 for Pheat = 2 MW, with a fiat top of > 1 s (see fig. 1). The radiation zone is located in the range r = 0.35-0.46 m. If detachment is avoided, which is clearly the case for 3' ~< 0.9 and strong auxiliary heating, then the neon radiation cools only the plasma boundary. :ire in the centre as well as the energy confinement time ~'E do not change 9 The convective heat flux can be reduced by an order of magnitude. Only the particle confinement time ~-p shows a significant increase, as is manifested in a strong decrease of particle flow at the limiter and a peaking of the n e profiles in the centre (see fig. 2). For 3' = 0.9 the neon concentration in the centre is well below 1%. For radiating 2 M W a neon recycling flux of about 4 • 1020 neon p a r t i c l e s / s is necessary. This flux amounts to about 2 - 5 % of the deuterium flux. In this case the typical values for electron temperature and density at r = 0.46 m are T~---10 eV and n e = 1 x 1019 m -3, in contrast to Te = 50 eV and n e = 1.5 X 1019 m - 3 for T = 0.2.
40
60
Fig. 2. Modification of the profiles of electron density n e and temperature T~ with radiation cooling (thin line with neon injection). ment of the subsequent He decay in the plasma allows deduction of the effective confinement time z ~ ( H e ) according to IHe = I 0 exp( -
t/'r~).
(1)
Two different methods to determine r ~ ( H e ) are applied. (1) A He II line from electron collision excitation at 4686 A is measured to obtain the H e flux at the limiter. (2) The same transition can be observed from charge exchange excitation in combination with NBI and gives a measure of the density of He z+ in the plasma centre, r ~ ( H e ) is determined from the decay of these He II signals. An example is given in fig. 3. The resulting z~'(He)values from the different methods are in excellent agreement. The relation between ~'~'(He) and the confinement time (replacement time) ~-p is given by the recycling coefficient R according to 1"p
r~ = 1 -R"
(2)
The exhaust efficiency is defined as e~xh = 1 - R and represents the probability of a particle at the edge of 6
2
'8
1:6
2
"(3
2.2. He-puff experiments
g ~2
o~
o o
The H e exhaust has been studied for different radiation levels corresponding to the 3,-scan shown in fig. 1. The method of He-removal measurement is similar to that applied in previous experiments [6]. A short H e gas puff of 15-30 ms duration and a maximum recycling flux of about 2 X 10 21 s - t (10% of the D flux) is applied at the beginning of the flat top. The measure-
[3_
0
1
2 time
3
sec
Fig. 3. He gas puff and determination of the effective confinement time ~-~,(He) from the decay of a He II line measured at
the limiter during the flat top of erad and ~ .
635
U. Samm et al. / He exhaust in plasmas with edge cooling 8OO
ms
HELIUM
6OO
// D
~
o/O
m
400
19
,r~e 2OO
=
3.3 4.5
=
5.2
=
5.5
10
-3 m o
"
.
mined by ~-p(He) and eexh. The exhaust efficiency can be expressed as the product of the collection efficiency ecolt ( = probability of a particle at the SOL boundary entering the pump scoop) and the removal efficiency ere m ( = probability of a particle at the scoop entrance being removed from the system by the pumps). The relation can also be expressed in terms of the ion sound speed c s, the H particle density n~ at the scoop entrance, the H e particle density nile in the plenum of the pump and the effective pumping speed Seff (for the local temperature in the plenum). Thus we obtain "rp
0 0
20
40 60 radiation level
80
1007o
Fig. 4. The effective confinement time ~-~(He) as a function of -/ for different line averaged central densities rio. the SOL (r = a) being removed from the plasma by the pumping system.
3. Results The summary of results obtained for r ~ ( H e ) is shown in fig. 4 as a function of the radiation level 3`For three different line averaged electron densities (fie = 4 . 3 / 5 . 2 / 5 . 5 • 1019 m 3) a 3,-scan has been performed. The highest density is close to the density limit. A single value of ~'~ for fie = 3.3 • 1019 m -3 from previous experiments at low 3, is also included. The comparison of ~-~(He) values at low 3' clearly shows an improvement of helium removal with increasing central density, r ~ ( H e ) drops from 450 s to 300 ms as f e changes from 3.3 • 1019 m 3 to 5,5 • 1013 cm 3. A similar improvement, but for deuterium pumping, is described in ref. [8]. The pumping performance deteriorates with increasing radiation, particularly with lower central density and close to the radiation limit 3, = 1. The highest value of r * ( H e ) = 700 ms is found for a condition close to detachment (3` = 0.97). In contrast, for the highest density the effective confinement time is nearly invariant with 3' ( r ~ ( H e ) = 300-350 ms) up to values of 3`=0.9. The data manifest a coincidence of good performance occurring at the highest density: (a) high radiation is obtained with the lowest amount of neon, (b) the effective confinement time for He has a minimum and (c) the global energy confinement is at its maximum.
T~
--
- EcollErern
no~c s A -- Tp - F/He
1 ,
Serf
where A is the area of the scoop entrance. It has to be considered that all parameters may vary with 3'. The particle confinement time of D can be deduced from the m e a s u r e m e n t of the total number of electrons in the plasma and the total recycling flux of deuterium on the limiter "rp(D) = N t o t / [ ' D . The data show a significant increase of rp(D) with 3`. If the plasma is close to detachment this increase can be up to a factor of 3. For the highest central density fie the factor is only about 1.8 when approaching 3, = 0.9. This strong increase of the particle confinement time is not well understood. Similar observations have been made with neon puffing in ohmic discharges on A S D E X [9]. According to a simple confinement model [10], such an increase of particle confinement is expected when the ionization length for neutrals increases, as is the case in a cold edge plasma. The determination of rp(He) is more difficult. We can either assume that ~-p(He) behaves similar to rp(D) or we can estimate ~-p(He) from the recycling flux of He (He II line intensity) and the C X E - p h o t o n emission from the plasma centre. The latter gives an increase of about a factor of 1.4 for the highest density fie" This value agrees with I D model calculations for the boundary plasma. This model calculates self-consistently Te and n e profiles, impurity transport based on a modified Bohm diffusion and impurity radiation [11]. In the following we use this result for ~p. In the high density case ~-~(He) is nearly constant but ~-p(He) is increasing with 3,, thus - according to eq. (3) - the product 8collere m must compensate the variation of rp. The collection efficiency econ is only a function of the S O L thickness An (the influence of Te profile variations is neglected here). Integrating the density profile over the radial coordinates of the scoop entrance (r 1 = 0.477 m, r 2 = 0.504 m, a = 0.46 m) yields Ecol, = exp[ - ( r , - a ) / A n ] - exp[
4. Discussion and conclusion The variation of r ~ ( H e ) can only be understood as a combination of different effects, r ~ ( H e ) is deter-
(3)
~'coll
(r 2 - a)/An].
(4) According to a Bohm-like diffusion, a decrease of radial transport and thus smaller values for An are
636
U. Samm et al. / He exhaust in plasmas with edge cooling
expected with edge cooling. This has been observed, e.g., for density scans and pure ohmic heating [12]. In the particular case of strong heating at very high densities we cannot observe a significant variation of An with y. The values we obtain from measurements with atom beams are about constant (A n --2.5 cm and AT = 2.5 cm measured at the outboard midplane) for the range 3' = 0.2-0.9. Thus ecoH essentially is not changing and the compensation of r e ( H e ) must be due to the removal efficiency e'rem, provided the He transport in the S O L is similar to that of D. The removal efficiency ere m depends on the complicated processes inside the pump scoops. Recycling, reflection probabilities, dissociation and ionization processes and charge exchange determine whether a particle at the neutralizer plate is pumped or flows back out of the scoop. These processes have been modelled with the neutral particle transport code E I R E N E [13] for the A L T - I I geometry. The plasma background has to be prescribed based on experimental data (atom beam and probe measurements). The main result is that with increasing ionization of recycling particles inside the scoop (re-ionization) the probability of particles to escape and flow back into the SOL is increasing. Reionized particles follow the magnetic field lines, whereas neutrals bounce many times inside the scoop. The helium density nile in the plenum of the pump can increase significantly with edge cooling and thus reduced c S. Generally, the removal efficiency improves by reducing ionization inside the scoop. This is the case with radiation cooling and the main cause for compensation of the increasing ~'p. The calculated removal efficiency depends strongly on the plasma parameters Te and n e inside the scoops. For extreme cases (T~ = 35 eV ~ 4 eV and n~ = 5 • 1018 m -3 ~ 0.7 • 1018 m -3) the removal efficiency can vary by nearly a factor of 2. The measurements of n e and Te in the SOL, which are used for the code calculations, give only information about the variations far upstream from the scoops. Internal probes provide single point information inside some scoops. This is not sufficient for a proper description of the plasma parameters, in particular not for the rather strong variations which are expected in the vicinity of the neutralizer plate. However, taking the best estimates about the variations of n e and T~ in the scoop with radiation cooling (average values T~ = 8 eV = 4 eV and n e = 7 • 10 ls m -3 ~ 5 • 1018 m -3) yields an improvement of
ere m for He and for D of about a factor of 1.2. This factor multiplied with the factor 1.15 corresponding to the increase of z ~ ( H e ) just matches the variation of zv(He) (factor 1.4). We conclude that the improvement of the removal efficiency with plasma edge cooling up to 3 ' - - 0 . 9 at high densities is the main reason for maintaining the good He exhaust properties. For plasmas close to detachment at lower densities the rise of confinement z v is too large to be compensated.
Acknowledgements The T E X T O R team is gratefully acknowledged for providing excellent experimental conditions. We wish to thank L. K6nen, A. Kr~imer-Flecken, G. Waidmann, H. Soltwisch and H.R. Koslowski for providing valuable data. O n e of us (M. Tokar) would like to thank the Humboldt Foundation for providing financial support.
References [1] A. Gibson and M.L. Watkins, Control. Fusion Plasma Phys. 1 (1977) 31. [2] K. Lackner et al., Plasma Phys. Control. Fusion 26 (1984) 105. [3] D. Reiter et al., Plasma Phys. Control. Fusion 33 (1991) 1579. [4] U. Samm et al., Plasma Phys. Control. Fusion 29 (1987) 1321. [5] U. Samm et al., Proc. 18th Euro19. Conf. on Plasma Physics and Controlled Fusion, Berlin, 1991, vol. 3, 19. 157. [6] D.L. Hillis et al., Phys. Rev. Lett. 65 (1990) 2382. [7] D.M. Goebel et al., J. Nucl. Mater. 162-164 (1989) 115. [8] D. Gray et al., these Proceedings (PSI-10), J. Nucl. Mater. 196-198 (1992) 1096. [9] M. Bessenrodt-Weber19als et al., Proc. 18th Euro19. Conf. on Plasma Physics and Controlled Fusion, Berlin, 1991, vol. 1, 19. 389. [10] W. Engelhardt and W. Feneberg, J. Nucl. Mater. 76 & 77 (1978) 518. [11] M. Tokar, One dimensional modelling of a tokamak edge plasma under conditions of strong impurity radiation, KFA JilL-report (1992). [12] U. Samm et al., J. Nucl. Mater. 162-164 (1989) 24. [13] D. Reiter, The EIRENE code, KFA Jiil-S19ez report (1992).
Journal of Nuclear Materials 196-198 (1992) 633-636 North-Holland
Helium exhaust in plasmas with strong radiative edge cooling U. Saturn, J. Boedo a, G. Bertschinger, K.H. Dippel, H. Euringer, K.H. Finken, D. Gray a, D. Hillis b, A. Pospieszczyk, D. Reiter, M. Tokar and B. Unterberg Institut fiir Plasmaphysik, Ass. KFA-EURATOM, Forschungszentrum Jiilich, Germany a Institute of Plasma and Fusion Research, University of California, Los Angeles, CA, USA b Oak Ridge National Laboratory, Oak Ridge, TN, USA
The compatibility of radiative edge cooling by neon injection and He exhaust with the pump limiter ALT-II is studied on TEXTOR. It is demonstrated that in plasmas with strong auxiliary heating (2 MW NBI) and with the highest average electron densities (fie = 5.5 x 1019 m -3) the effective confinement time r~, for He has a minimum. This good pumping performance is maintained even for cases in which up to 90% of the heating power is radiated from the plasma boundary (cold radiative edge). The processes inside the scoops of the pump limiter (neutral particle transport, re-ionization) and the variation of particle confinement in the main plasma dominate this behavior.
1. Introduction The plasma scenario with a "cold radiative edge" has been considered as a possible remedy for the problems of plasma-wall interaction in a tokamak [1,2]. Distributing the power flow onto the wall by radiation reduces the convective heat flux in the scrape-off layer (SOL), thus avoiding dangerous peak loads at the limiter or the divertor plate. Significant radiative edge cooling can only be achieved successfully in a reactor if the following conditions are fulfilled: (a) impurities have to be present, because only line radiation from impurities can maintain a high radiation level, (b) the radiation has to be restricted to the boundary region, (c) fuel dilution in the plasma centre due to impurities must be kept sufficiently low, (d) thermal and MHD stability of the radiative edge is required and (e) compatibility with particle exhaust (ash removal) is necessary [3]. It has been shown in earlier experiments with pure ohmic heating that the light intrinsic impurities C and O essentially fulfill the requirements a-d. Quasi-stationary plasmas with a radiation level 7 = Prad/Pheat close to 100% are possible [4]. For plasmas with strong auxiliary heating additional impurities have to be injected to radiate the additional power. For this purpose neon injection has been applied successfully on TEXTOR [5]. In many cases of a cold radiative boundary in TEXTOR, in particular with a "detached" plasma, in which the ionization zone shrinks to a smaller minor radius, reduced convection in the SOL and a decreasing edge density leads to a deterioration of the particle exhaust efficiency of the pump limiter ALT-II. In view of these difficulties, problems with He
exhaust in connection with intense radiative edge cooling could be expected. That He can be pumped efficiently at a low radiation level (7 = 0.18) with the pump limiter ALT-II has already been demonstrated for plasmas with medium density ( f i e = 3.5 X 1019 m 3) and auxiliary heating by neutral beam injection (NBI) [6]. In this paper we show that the helium exhaust efficiency improves when the line averaged central density fie increases and that this good efficiency can be maintained in the presence of significant radiative edge cooling.
2. Experiment TEXTOR is a medium-sized tokamak with a major radius R = 1.75 m and a minor radius a = 0.46 m. The standard values for the magnetic field B T = 2.25 T and the plasma current Ip = 350 kA have been chosen for this experiment. The toroidal belt limiter ALT-II is designed as a pump limiter using eight turbo pumps [7]. The throat of the limiter collects particles in the SOL in the radial range r = 0.477-0.504 m. All limiter blades are made of graphite and the vessel wall is boronized. The working gas is deuterium. Auxiliary heating with NB co-injection of H (1.7 MW) together with ohmic heating (0.3 MW) provide a total heating power of Ptot = 2 MW. This corresponds to a power density of 0.3 M W / m 3 or 0.1 M W / m 2. 2.1. Radiative edge cooling by neon injection Neon is injected by a gas puffing system with a fast piezo valve. Because the recycling coefficient R of
0022-3115/92/$05.00 9 1992 - Elsevier Science Publishers B.V. All rights reserved
U. Samm et al. / He exhaust in plasmas with edge cooling
634 2
centre
Te >
~6
0.8
0.8
%0.6 >,
0.6
0.4
0.4
0.2
0.2
&
- 6 (0
0 20 minor rodius cm
NBI 0
1 time
2 sec
Fig. 1, Variation of the radiation level (T-scan) from shot to shot by neon injection during auxiliary heating with 1.7 MW of NBI.
neon is unity the amount of gas puffed in accumulates in the plasma if the pumps of A L T - I I are closed. With open pumps the removal time for Ne is about 0.5 s. A feedback control of the neon level is necessary to provide stationary conditions, particularly close to the radiation limit 3' = 1. Such a system has been developed on T E X T O R [5]. The control variable is a Ne VIII line representing well the total radiation. The system allows performance of ,/-scans, e.g. 3, = 0.2-0.98 for Pheat = 2 MW, with a fiat top of > 1 s (see fig. 1). The radiation zone is located in the range r = 0.35-0.46 m. If detachment is avoided, which is clearly the case for 3' ~< 0.9 and strong auxiliary heating, then the neon radiation cools only the plasma boundary. :ire in the centre as well as the energy confinement time ~'E do not change 9 The convective heat flux can be reduced by an order of magnitude. Only the particle confinement time ~-p shows a significant increase, as is manifested in a strong decrease of particle flow at the limiter and a peaking of the n e profiles in the centre (see fig. 2). For 3' = 0.9 the neon concentration in the centre is well below 1%. For radiating 2 M W a neon recycling flux of about 4 • 1020 neon p a r t i c l e s / s is necessary. This flux amounts to about 2 - 5 % of the deuterium flux. In this case the typical values for electron temperature and density at r = 0.46 m are T~---10 eV and n e = 1 x 1019 m -3, in contrast to Te = 50 eV and n e = 1.5 X 1019 m - 3 for T = 0.2.
40
60
Fig. 2. Modification of the profiles of electron density n e and temperature T~ with radiation cooling (thin line with neon injection). ment of the subsequent He decay in the plasma allows deduction of the effective confinement time z ~ ( H e ) according to IHe = I 0 exp( -
t/'r~).
(1)
Two different methods to determine r ~ ( H e ) are applied. (1) A He II line from electron collision excitation at 4686 A is measured to obtain the H e flux at the limiter. (2) The same transition can be observed from charge exchange excitation in combination with NBI and gives a measure of the density of He z+ in the plasma centre, r ~ ( H e ) is determined from the decay of these He II signals. An example is given in fig. 3. The resulting z~'(He)values from the different methods are in excellent agreement. The relation between ~'~'(He) and the confinement time (replacement time) ~-p is given by the recycling coefficient R according to 1"p
r~ = 1 -R"
(2)
The exhaust efficiency is defined as e~xh = 1 - R and represents the probability of a particle at the edge of 6
2
'8
1:6
2
"(3
2.2. He-puff experiments
g ~2
o~
o o
The H e exhaust has been studied for different radiation levels corresponding to the 3,-scan shown in fig. 1. The method of He-removal measurement is similar to that applied in previous experiments [6]. A short H e gas puff of 15-30 ms duration and a maximum recycling flux of about 2 X 10 21 s - t (10% of the D flux) is applied at the beginning of the flat top. The measure-
[3_
0
1
2 time
3
sec
Fig. 3. He gas puff and determination of the effective confinement time ~-~,(He) from the decay of a He II line measured at
the limiter during the flat top of erad and ~ .
635
U. Samm et al. / He exhaust in plasmas with edge cooling 8OO
ms
HELIUM
6OO
// D
~
o/O
m
400
19
,r~e 2OO
=
3.3 4.5
=
5.2
=
5.5
10
-3 m o
"
.
mined by ~-p(He) and eexh. The exhaust efficiency can be expressed as the product of the collection efficiency ecolt ( = probability of a particle at the SOL boundary entering the pump scoop) and the removal efficiency ere m ( = probability of a particle at the scoop entrance being removed from the system by the pumps). The relation can also be expressed in terms of the ion sound speed c s, the H particle density n~ at the scoop entrance, the H e particle density nile in the plenum of the pump and the effective pumping speed Seff (for the local temperature in the plenum). Thus we obtain "rp
0 0
20
40 60 radiation level
80
1007o
Fig. 4. The effective confinement time ~-~(He) as a function of -/ for different line averaged central densities rio. the SOL (r = a) being removed from the plasma by the pumping system.
3. Results The summary of results obtained for r ~ ( H e ) is shown in fig. 4 as a function of the radiation level 3`For three different line averaged electron densities (fie = 4 . 3 / 5 . 2 / 5 . 5 • 1019 m 3) a 3,-scan has been performed. The highest density is close to the density limit. A single value of ~'~ for fie = 3.3 • 1019 m -3 from previous experiments at low 3, is also included. The comparison of ~-~(He) values at low 3' clearly shows an improvement of helium removal with increasing central density, r ~ ( H e ) drops from 450 s to 300 ms as f e changes from 3.3 • 1019 m 3 to 5,5 • 1013 cm 3. A similar improvement, but for deuterium pumping, is described in ref. [8]. The pumping performance deteriorates with increasing radiation, particularly with lower central density and close to the radiation limit 3, = 1. The highest value of r * ( H e ) = 700 ms is found for a condition close to detachment (3` = 0.97). In contrast, for the highest density the effective confinement time is nearly invariant with 3' ( r ~ ( H e ) = 300-350 ms) up to values of 3`=0.9. The data manifest a coincidence of good performance occurring at the highest density: (a) high radiation is obtained with the lowest amount of neon, (b) the effective confinement time for He has a minimum and (c) the global energy confinement is at its maximum.
T~
--
- EcollErern
no~c s A -- Tp - F/He
1 ,
Serf
where A is the area of the scoop entrance. It has to be considered that all parameters may vary with 3'. The particle confinement time of D can be deduced from the m e a s u r e m e n t of the total number of electrons in the plasma and the total recycling flux of deuterium on the limiter "rp(D) = N t o t / [ ' D . The data show a significant increase of rp(D) with 3`. If the plasma is close to detachment this increase can be up to a factor of 3. For the highest central density fie the factor is only about 1.8 when approaching 3, = 0.9. This strong increase of the particle confinement time is not well understood. Similar observations have been made with neon puffing in ohmic discharges on A S D E X [9]. According to a simple confinement model [10], such an increase of particle confinement is expected when the ionization length for neutrals increases, as is the case in a cold edge plasma. The determination of rp(He) is more difficult. We can either assume that ~-p(He) behaves similar to rp(D) or we can estimate ~-p(He) from the recycling flux of He (He II line intensity) and the C X E - p h o t o n emission from the plasma centre. The latter gives an increase of about a factor of 1.4 for the highest density fie" This value agrees with I D model calculations for the boundary plasma. This model calculates self-consistently Te and n e profiles, impurity transport based on a modified Bohm diffusion and impurity radiation [11]. In the following we use this result for ~p. In the high density case ~-~(He) is nearly constant but ~-p(He) is increasing with 3,, thus - according to eq. (3) - the product 8collere m must compensate the variation of rp. The collection efficiency econ is only a function of the S O L thickness An (the influence of Te profile variations is neglected here). Integrating the density profile over the radial coordinates of the scoop entrance (r 1 = 0.477 m, r 2 = 0.504 m, a = 0.46 m) yields Ecol, = exp[ - ( r , - a ) / A n ] - exp[
4. Discussion and conclusion The variation of r ~ ( H e ) can only be understood as a combination of different effects, r ~ ( H e ) is deter-
(3)
~'coll
(r 2 - a)/An].
(4) According to a Bohm-like diffusion, a decrease of radial transport and thus smaller values for An are
636
U. Samm et al. / He exhaust in plasmas with edge cooling
expected with edge cooling. This has been observed, e.g., for density scans and pure ohmic heating [12]. In the particular case of strong heating at very high densities we cannot observe a significant variation of An with y. The values we obtain from measurements with atom beams are about constant (A n --2.5 cm and AT = 2.5 cm measured at the outboard midplane) for the range 3' = 0.2-0.9. Thus ecoH essentially is not changing and the compensation of r e ( H e ) must be due to the removal efficiency e'rem, provided the He transport in the S O L is similar to that of D. The removal efficiency ere m depends on the complicated processes inside the pump scoops. Recycling, reflection probabilities, dissociation and ionization processes and charge exchange determine whether a particle at the neutralizer plate is pumped or flows back out of the scoop. These processes have been modelled with the neutral particle transport code E I R E N E [13] for the A L T - I I geometry. The plasma background has to be prescribed based on experimental data (atom beam and probe measurements). The main result is that with increasing ionization of recycling particles inside the scoop (re-ionization) the probability of particles to escape and flow back into the SOL is increasing. Reionized particles follow the magnetic field lines, whereas neutrals bounce many times inside the scoop. The helium density nile in the plenum of the pump can increase significantly with edge cooling and thus reduced c S. Generally, the removal efficiency improves by reducing ionization inside the scoop. This is the case with radiation cooling and the main cause for compensation of the increasing ~'p. The calculated removal efficiency depends strongly on the plasma parameters Te and n e inside the scoops. For extreme cases (T~ = 35 eV ~ 4 eV and n~ = 5 • 1018 m -3 ~ 0.7 • 1018 m -3) the removal efficiency can vary by nearly a factor of 2. The measurements of n e and Te in the SOL, which are used for the code calculations, give only information about the variations far upstream from the scoops. Internal probes provide single point information inside some scoops. This is not sufficient for a proper description of the plasma parameters, in particular not for the rather strong variations which are expected in the vicinity of the neutralizer plate. However, taking the best estimates about the variations of n e and T~ in the scoop with radiation cooling (average values T~ = 8 eV = 4 eV and n e = 7 • 10 ls m -3 ~ 5 • 1018 m -3) yields an improvement of
ere m for He and for D of about a factor of 1.2. This factor multiplied with the factor 1.15 corresponding to the increase of z ~ ( H e ) just matches the variation of zv(He) (factor 1.4). We conclude that the improvement of the removal efficiency with plasma edge cooling up to 3 ' - - 0 . 9 at high densities is the main reason for maintaining the good He exhaust properties. For plasmas close to detachment at lower densities the rise of confinement z v is too large to be compensated.
Acknowledgements The T E X T O R team is gratefully acknowledged for providing excellent experimental conditions. We wish to thank L. K6nen, A. Kr~imer-Flecken, G. Waidmann, H. Soltwisch and H.R. Koslowski for providing valuable data. O n e of us (M. Tokar) would like to thank the Humboldt Foundation for providing financial support.
References [1] A. Gibson and M.L. Watkins, Control. Fusion Plasma Phys. 1 (1977) 31. [2] K. Lackner et al., Plasma Phys. Control. Fusion 26 (1984) 105. [3] D. Reiter et al., Plasma Phys. Control. Fusion 33 (1991) 1579. [4] U. Samm et al., Plasma Phys. Control. Fusion 29 (1987) 1321. [5] U. Samm et al., Proc. 18th Euro19. Conf. on Plasma Physics and Controlled Fusion, Berlin, 1991, vol. 3, 19. 157. [6] D.L. Hillis et al., Phys. Rev. Lett. 65 (1990) 2382. [7] D.M. Goebel et al., J. Nucl. Mater. 162-164 (1989) 115. [8] D. Gray et al., these Proceedings (PSI-10), J. Nucl. Mater. 196-198 (1992) 1096. [9] M. Bessenrodt-Weber19als et al., Proc. 18th Euro19. Conf. on Plasma Physics and Controlled Fusion, Berlin, 1991, vol. 1, 19. 389. [10] W. Engelhardt and W. Feneberg, J. Nucl. Mater. 76 & 77 (1978) 518. [11] M. Tokar, One dimensional modelling of a tokamak edge plasma under conditions of strong impurity radiation, KFA JilL-report (1992). [12] U. Samm et al., J. Nucl. Mater. 162-164 (1989) 24. [13] D. Reiter, The EIRENE code, KFA Jiil-S19ez report (1992).