Journal of Nuclear Materials 196-198 (1992) 841-847 North-Holland

juurnalof nuclear

materials

Impurity transport and retention in a gas target divertor: simulation experiments in PISCES-A and modeling results

L. Schmitz, L. Blush, G. Chevalier, R. Lehmer, Y. Hirooka, P. Chia, G. Tynan and R.W. Conn Institute o f Plasma and Fusion Research, UCLA, Los Angeles, CA 90024-1597, USA

Impurity retention in the gaseous divertor regime is investigated in the PISCES-A facility at UCLA. We report measurements and I~D fluid modeling results of impurity transport for typical tokamak divertor plasma parameters (10 x8 _< n~ _< 3 • 10 I'~ m -3, kT~. < 20 eV). The neutral hydrogen density close to the (simulated) divertor target is 1020 _< n 0 _< 3 x 1021 m-3. Gaseous trace impurities (argon, neon) as well as low-Z and high-Z materials sputtering (carbon, tungsten) are studied. It is observed that the impurity retention in a gaseous divertor is substantially improved as compared to conventional divertor operating regimes. The modeling results suggest that the retention of neutral and ionized impurities is mainly due to collisions with hydrogen (deuterium) neutrals and ions streaming towards the divertor target at a velocity of 0.25-0.5c~. A low level of residual impurity transport, observed at high neutral density, is attributed to a plasma flow reversal close to the radial boundary. Sputtering of a tungsten sample by intrinsic impurities has been shown to decrease substantially for target electron temperatures kT~ < 5 eV.

1. Introduction

Control of divertor plate erosion and thermal load- ing are critical issues for the achievement of long pulse or steady-state tokamak operation at reactor-relevant power levels. Typically, the power flow parallel to the magnetic field in the scrape-off layer plasma is ex- pected to reach values of 30-100 M W / m 2 and the power is deposited in a narrow region around the divertor strike point. Radiative or "gas target" diver- tors [1,2] potentially offer an attractive solution to the divertor heat load problem. A key element of this concept is enhanced radiative power loss in the diver- tor area, accomplished by injecting neutral deuterium and /o r impurities closc to the divertor target. As a result, the power flow may be redistributed over a larger surface area and the peak divertor heat load may be reduced substantially. For typical tokamak boundary plasma parameters (1019 ~< n e < 5 x 102~ m-3; kTe, k T i < 300 eV), the electron energy loss is primarily determined by inelastic processes such as electron impact ionization and excitation, and the ion energy loss is determined by resonant charge exchange. Both neutral deuterium and impurity injection have been considered [3,4]. The average energy loss per ionization event is 25-30 eV for deuterium in the relevant parameter regime, and a (spatially averaged) neutral particle density (n0) = 2-4 X 10 2o m -s would be typically required in the divertor plasma stream in order to decrease the target electron temperature to acceptable values around 10 eV. The gas target regime

has been simulated in linear plasma facilities (PISCES- A) [5,6] and elsewhere [7] and a substantial reduction of the electron temperature downstream from the plasma source has been demonstrated. Target electron temperatures of 2-5 eV and correspondingly low sheath potentials have been achieved. Also, the plasma den- sity at the target has been observed to decrease due to radial transport. Recently, a reduced target electron temperature and a broadening of the divertor heat load deposition profile have been observed during di- vertor gas puffing experiments in DIII-D [8].

While the plasma erosion yield of typical divertor armor materials is very low under these conditions, sputtering might still be caused by high energy charge exchange neutrals impacting the walls of the divertor channel. Gas injection also produces a strong particle source in the divertor area, and a practical limit for the neutral density is given by the acceptable plasma den- sity increase in the scrape-off layer. Medium-Z or high-Z impurity seeding in a gas target scenario has the advantage of increasing the radiative energy loss per ionization event, and consequently a lower neutral density is required to reduce the electron temperature to acceptable values. In either gas target scenario, good retention of intrinsic or injected impurities in the di- vertor region is required in order to maintain low radiative losses in the core plasma.

In this paper we report spectroscopic measurements of impurity transport in the gas target regime. The experiments were carried out in the PISCES-A linear plasma device for source plasma densities of 8 x 1017

0022-3115/92/$05.00 �9 1992 - Elsevier Science Publishers B.V. All rights reserved

842 L. Schmitz et al. / Impurity

_< n e _< 1 • 10 t9 m -3 and source electron temperatures kT~ < 20 eV. The ion temperature ( k T i= 2 eV) is found to be close to the Franck-Condon energy. The axial magnetic field is B~ = 0.19 T. Injected gaseous impurities (Ne, Ar) as well as typical divertor armor materials (C, W) are investigated. The axial density distribution of neutral and singly ionized impurities is modeled by solving the 1�89 momentum and continuity equations, including local impurity ionization and ra- dial impurity ion loss.

2. Impurity transport modeling

We are using a 1�89 fluid model to study impurity transport along the magnetic field. For the electron temperature range of our experiment, it is sufficient to consider only neutral and singly ionized impurities, since the densities of Ar III and Ne III are estimated to be less than 5 and 1%, respectively, of the Ar II and Ne II densities (no coronal equilibrium is reached due to strong radial and axial ion transport). We assume that neutral impurities are injected at the divertor target ( z = 0 ) and that only the injected impurity species is present. Also, the main plasma ion tempera- ture gradient (the ion thermal force) is neglected, since the ion temperature k T i = 2 eV in our experiment is determined by the Franck-Condon energy rather than by collisional heating from electrons or ion heat loss along the magnetic field. The electron thermal force is retained. The continuity and momentum equations for

retention in a gas target divertor

neutral and ionized (trace) impurities can then be written:

a

0 Z I 1 i j = • (1)

2 ~-z ( n ' ; m l ( V ~ ~

_ o o n ~ m , v O D A2~ - -ne/~lmlVl (O'ionV e) - �9 a~

o o + n ~ 1 7 6 ) + n l m l u l n ( V n - V t l ' ) , (2)

- - i n + v + - - - (3) 0Z l, I I ) = n e n I 0 ( O ' i o n U e ) n ( D • h2~ a 2 '

2 ~(n;-m,(.;) +n?kT;) h 2

+ n e , f f m,~,O<,,ion~e > + + o = - -n I m lu I D• i

+ r/~-mlPl~i (U i -- U1 ~ ) -{-/'/i~ mil- ' l+n(U n -- U ; - )

OkT~ + en}E~ +~n~- - - (4)

Oz

Here, n ~ u ~ and n( , v~- are the densities and flow velocities of neutral and ionized impurities, respec- tively, kT ~ and kT~ are the temperatures, m I is the mass, and <O'ionV e) is the ionization rate coefficient. /3 = 2.2 (I + 0.52Zeff)/Zef f is the coefficient of the electron thermal force [12,13]. We estimate Zef f = 1.5-3 in our experiment in the presence of trace impurity injection. D• is the impurity ion diffusion coefficient

1.3m Czerny-Turner Spectrometer Fabry-Perot

Etalon

Target [ ~ Plate ~. I

Main - ~ .......... / Magnets

Fast Probe :1:, "".~.i7' ~r (Vertical Chord)

tl tl

" / n l I I I I I I I I

Anode

I I

Source 4"'Magnets

Axial Langmuir

Probe

Secondary Gas Feed

l ~ Cathode

4 Primary B Neutralizer Gas Feed

Baratron Pressure - Tube Measurement

Fig. 1. Experimental setup for the PISCES gas target divertor simulation experiments, showing the optical chords and the probe diagnostics.

L. Schmitz et al. / Impurity retention in a gas target divertor 843

perpendicular to the magnetic field. We take D l to be the sum of the Bohm diffusion coefficient and the classical diffusion coefficient Dcl = k T e ( v ~ +

+ 2 is the impurity ion cyclotron P l n ) / ( m l 0 9 c i ) , where w d frequency [12]. A 0 is the first zero of the Bessel func- tion, and a is the plasma radius. The plasma density and electron energy are obtained as a function of the axial distance z from Langmuir probe measurements. The neutral hydrogen density n o is obtained from absolutely calibrated H~ data. Collisions between im- purity particles and plasma ions (collision frequencies u~ and v{i) as well as between impurities and hydro- gen neutrals (collision frequencies v~ and ~'{n) are included. The rate coefficients for ion-neutral colli- sions are derived including direct interaction (classical "hard sphere" collisions) as well as polarization effects [15].

The plasma flow velocity v i has been measured with Mach probes and reaches values on the order of the ion sound speed c S in the gas target regime. The inclusion of collisions between impurities and hydrogen neutrals is necessary because there is a directional flow of hydrogen neutrals towards the target at some frac- tion of the ion sound speed [16]. The classical "hard sphere" cross sections are used to derive the rate coefficients. The axial electric field E ~ ( z ) is experi- mentally obtained from swept probe characteristics and corresponds, within experimental error, to the value expected from the electron momentum equation [12]

1 e E z ( z ) ne(Z ) ~Z (fte(Z)kTe(Z))

0 - 0.71~z kTe(z ). (5)

Typical values of the electric field range from E z = 10 V / m at low neutral density to E z = 100 V / m at high neutral density.

3. Experiment

The setup for the gas target simulation experiments is shown in fig. 1. The plasma (diameter 0.05 m) is generated in a reflex arc configuration, using a hot cathode made of LaB 6. The axial magnetic field B z is generated by two sets of coils. In the plasma source region, B z = 0.06 T, while downstream in the analysis region, Bz = 0.19 T. The primary hydrogen gas feed is located in the source region. The neutral hydrogen pressure in the plasma source is kept below 1 mTorr. A circular tube (length L = 0.9 m, diameter d = 0.045 m) is located at a distance of 0.55 m downstream from thc source. The simulated divertor target is located at the end of the tube (z = 0). The secondary gas feed (used to inject hydrogen as well as trace impurities) is located close to the center of the target plate. The "neutral-

izer" tube is made of (insulating) anodized aluminum and is electrically floating. The neutral pressure is monitored by baratron gauges located at various axial positions. The plasma density and the electron temper- ature are measured with an axially moveable plane Langmuir probe. A fast scanning probe provides radial profiles of plasma density, floating and space potential (via an emissive filament) at select axial positions. The neutral hydrogen density inside the plasma is deter- mined from absolutely calibrated H a measurements, using the photon yield per ionization event [9-11] and the measured density and electron temperature pro- files. Impurity line intensities are measured at a dis- tance z = 0.43 m upstream from the divertor target using a 1.3 m Czerny-Turner spectrometer with a photomultiplier tube or, in some cases, an OMA sys- tem. Since the transport of neutral and ionized impuri- ties depends on their respective temperature, we have measured the Doppler width of some impurity lines using a Fabry-Perot etalon.

4. Results and discussion

4.1. Argon and neon gas injection

Axial impurity transport is investigated for different levels of hydrogen injection at the target plate. Fig. 2 shows the measured plasma density profiles for some of the conditions. The discharge voltage (U d = 150 V) and current (Id = 30 A), as well as the hydrogen neu- tral pressure in the plasma source region (Po = 7 • 10 -4 Torr) are held approximately constant. Momen- tum transfer from plasma ions to hydrogen neutrals results in the accumulation of neutrals close to the

12

E

10

8

6

�9 <no>=2".2e20 m-3 4

[ ] <no>=2.8e19 m-3

�9 <no>=1.3e19 m-3

2 o <no>=8.5e18 m-3

0 ' , 0.0 0.2 0.4 0.6 0.8

z (m)

Fig. 2. Axial plasma density profiles for different (spatially averaged) neutral hydrogen densities (n o).

844 L. Schmitz et al. / Impurity retention in a gas target divertor

divertor target. This effect is observed even without secondary gas feed and increases the neutral density near the target plate. Hydrogen injection results in increased ionization, raising the plasma density to val- ues of up to 1019 m 3. In separate experiments with higher discharge power, plasma densities of up to 3 • 1019 m -3 have been achieved. Fig. 3 shows the electron temperature for three axial locations as a function of the spatially averaged neutral hydrogen density (n0). The source electron temperature is 12-15 eV with a small percentage of energetic electrons (2- 4% at an energy corresponding to the discharge volt- age). The axial electron heat flux and the electron temperature decrease towards the target. For moder- ate neutral density ((n 0) = 1.3 X 1020 m -3) we achieve a target temperature kTc(O)= 4 eV. As a result, the ionization rate close to the target is substantially re- duced and the plasma density profile is determined by radial and axial particle losses [5]. The radial loss rate is found to be proportional to the neutral density. Consequently the plasma density decreases towards the target at higher neutral density. At the highest gas feed rate (the neutral hydrogen pressure at the target is 80 mTorr) the target electron temperature is reduced to 2.5 eV, and the plasma "detaches" from the target plate due to radiative collapse.

Line densities of injected impurities are measured at a distance z = 0.43 m upstream from the target plate, integrated along a radial chord through the plasma center. The impurity line intensity I ( z ) is re- lated to the impurity concentration nj(r , z):

I ( z ) ~ f,~,(r,~),,o(r,=)<OoxcVo>(=) dr, (6)

where (~xcve) ( z ) is the rate coefficient for electron impact excitation (assumed independent of radius be- cause of negligible radial variation of the electron

-o

c

(7503.87 A) 1

�9 (a)

.01 o ;

Neutral Density n o (10=~ "3)

�9 Measurement [] 1 1/2-0 Model

"~" .I

= c .01

,001

(4806.02 A)

~ ~ @ 4 1 o (b)

e Measured [] 1 1/2-DModel

o ; ~ 3 20 -3 Neutral Denslty n o (10 m )

Fig. 4. (a) Relative Ar I impurity concentration (radially integrated along a central chord) at z = 0.43 m as a function of neutral hydrogen density n o (spatially averaged between z = 0 and z = 0.43 m); (b) relative Ar II impurity concentra-

tion as a function of n 0.

>

r

2o 10" o kTe(z=lm)

�9 kTe (z=0.43m) �9 kTe (z-O)

20 -3 <n > (10 m ) 0

Fig. 3. Electron temperature at three axial positions as a function of (spatially averaged) neutral hydrogen density

<no).

temperature). Excitation rate coefficients are approxi- mated by an analytical expression [17] using the mea- sured electron temperature. The plasma density shows little radial variation except very close to the radial wall, so that Jni(r , z ) dr ~ I(z) /(ne(z)(~excV~)(z)) . Fig. 4a shows the measured relative Ar impurity concentra- tion at z=0 .43 m, derived from the Ar I line at 7503.87 ,~, plotted versus the hydrogen neutral density n o (spatially averaged between z = 0 and z = 0.43 m). Argon is added as a trace impurity at z = 0 with a constant gas feed rate and the argon partial pressure at z = 0.43 m is 10 5 Torr (measured without plasma), as compared to hydrogen pressures P0 > 0.45 mTorr. At higher neutral densities, the impurity retention is im- proved by a factor of 20 as compared to the base case without hydrogen gas injection. The fluid model sug- gests that the impurity retention near the target plate is due to friction with the streaming hydrogen neutrals

L. Schmitz et al. / Impurity retention in a gas target divertor 845

and, to some degree, plasma ions. The modeling re- sults describe the experimental data well except at the highest neutral densities. The discrepancy could be due to two-dimensional effects (for example due to an axial plasma flow reversal at a radius r > 0.02 m, observed from Mach probe data at comparable neutral densities [6]). This effect would lead to increased axial impurity transport. Fig. 4b shows the relative impurity concen- tration of singly ionized Ar (Ar II line at 4806.02 ,~) as a function of the spatially averaged neutral hydrogen density. At the highest neutral density, the ion concen- tration is reduced by a factor of 65 as compared to the case without hydrogen injection. The transport model suggests that impurity entrainment by ion-ion colli- sions (for impurity ions the collisional process with the highest cross section) is very effective even at relatively low plasma density. The electron thermal force coun- teracts the friction force but is considerably smaller (by a factor of more than 5). The electric field is directed towards the target and contributes to impurity reten- tion. The argon ion density is primarily determined by the balance between local ionization of neutral argon and radial loss of argon ions. Ionization is less effective at high neutral hydrogen density due to the reduced electron temperature, while the collisionality and the radial losses are increased. Both effects reduce the argon ion density and contribute to the observed reten- tion. The modeling results again agree well with the experimental data.

Figs. 5a and 5b show a similar comparison for neutral and singly ionized neon. The partial pressure of the injected neon (measured without plasma at z = 0.43 m) is 2 • 10 .5 Torr. Good impurity retention for Ne I is observed for (n 0) >_ 1.5 • 1020 m -3. From the model, collisional impurity entrainment is also ex- pected to be more efficient for Ne I than for Ar I, since the collision cross sections are higher by a factor of about 1.5. Also, the measured Ne I temperature is 0.11-0.15 eV, somewhat lower than the Ar I tempera- ture (0.2-0.25 eV). Both effects contribute to increased impurity screening. The modeling results again diverge from the experimental data at high neutral hydrogen density, suggesting the importance of two-dimensional effects in the experiment. Excited states of Ne II have high threshold energies (30 eV) and the measurements are hampered by extremely low line intensities. Data could be obtained only at low neutral density (high electron temperature). The model predicts very low neon ion concentrations since the ionization rate of neutral neon is lower than that for argon, while ion loss by radial diffusion is calculated to be higher.

4.2. Materials sputtering experiments

In addition to gas injection, we have also studied impurity production from sputtering of tungsten and carbon samples. The samples (diameter 9 • 10 -3 m)

1 0 0

10 "1

(a)

10 -2

10 .3

10 .4

(7173.98 A)

"o

E

2 0 -3 Neutral Dens i ty n (10 m )

0

�9 Measurement

[3 1 1/2-D Model

1 0 0

10"

(3717.2 A)

(b)

v .~ 10":

= �9 Measured

t3 1 1/2-D Model E

1 0 -

1C �9 , . , .

o 1 2 2 0 -3

Neutral Dens i ty n o (10 m )

Fig. 5. (a) Relative Ne I impurity concentration (radially integrated along a central chord) at z = 0.43 m as a function of neutral hydrogen density n o (spatially averaged between z = 0 and z = 0�9 m); (b) relative Ne II impurity concentra-

tion as a function of n 0.

were attached to a probe shaft extending axially into the neutralizer tube. Bias voltages of 100-150 V are applied. The axial profile of the neutral impurity den- sity in front of the sample depends on the particle loss by ionization and the friction forces between impurity neutrals and hydrogen ions and neutrals�9 If the friction forces are small, and redeposition effects on the side walls can be neglected, the neutral impurity density is expected to decay exponentially:

fn,(r,z) d r = fni(r,Zo) dr e x p ( - ( z - z0).~ion) ,

(7) where ni(r ,z 0) is the impurity density at the sample, Aioe = u=/ne(~rionV e) is the ionization mean free path, and the sample is located at z = z 0. In the experiment, the line-integrated impurity density can be derived from the (radially integrated) impurity line emission intensity I ( z ) , if we include a correction factor for the finite impurity emission velocity v I. This correction

846

factor can be expressed as a function of the excitation mean free path Aex r =v~/n~(tr~xev~) for the spectral line under consideration:

f n,( r,z ) d r ~ l ( z ) { n e ( z )(O'~xcUe)

• (1 - exp( - ( z - Zo)/)%x~)) }- 1.

(8)

In our experiment, A~x~>>Aio,, ( z - z o ) . Therefore, 1 - e x p ( - ( z -z0)/Aex r = (z - z 0 ) / A .... and the den- sity decay length can be derived from the measurement of I ( z ) without the exact knowledge of A~,~. The impurity line radiation is observed at a distance z = 0.18 m upstream from the divertor target. Since the optical chord is restricted by a small window mounted on the neutralizer tube, the sample has been moved along z in order to measure the axial distribution of the impu- rity line intensities. For carbon, chemical sputtering is expected to be dominant for the typical sample tem- peratures measured in our experiment (T~=400- 600~ and the CH band at 4314 ,~ is measured [18]. For tungsten, the W I line at 4008 ,~ is used.

The measured axial density decay length A~ for neutral tungsten varies from 6.6 x 10 -3 m at low neu- tral density (n o = 0.8 • 1019 m -3) to 1.4 x 10 -2 m at high neutral density (no= 7 • 1020 m-3). The decay length for CH varies between 9 • 10 -3 and 2.3 • 10 -2 m. The effects of the friction force are obvious if the normalized axial decay lengths Az/Aio e are plotted versus the hydrogen neutral density in front of the sample (fig. 6). The energy distribution of the sput- tered tungsten atoms is peaked around 4.5 eV. The axial decay length is observed to be close to the ioniza- tion mean free path (as expected) at low hydrogen densities. We attribute the decrease of AJAio n at high neutral density to collisions with directional hydrogen neutrals flowing towards the divertor target. For car- bon, the energy of the sputtered CH molecules is only 0.05-0.08 eV, and the effect of collisional friction is readily observed even at low neutral density (A=/Aio ~

L. Schmitz et al. / Impurity retention in a gas target divertor

(w l, 4oos A)

1.80e-2

1.5 o

, .~ ~ 1.0

~t N

o w I (4008 A) 0.5 �9 CH (4314 A)

0 .0 0 2 4 6

20 -3 Neutral Densi ty n o ( 1 0 m )

Fig. 6. Normalized axial density decay length Az/Aio, for neutral tungsten and CH molecules sputtered from a small sample versus the neutral hydrogen density in front of the

sample.

1.20e-2

~ ~

6 . 0 0 e - 3

z - z ~ 0 0 0 3 m 2 ~

0.00e+0 . - - - . . . . . , 4 6 8 1 0 1 2

kT e ( e V )

Fig. 7. Sputtering yield of a tungsten sample (located at z 0 = 0.177 m). Three cases are compared: (a) hydrogen plasma with intrinsic impurities; (b) hydrogen plasma with argon trace injection; (c) hydrogen plasma with argon concentration nor- malized to the value for k T e = 3 eV (sputtering yield shown corresponds to constant argon concentration at the location of

the sample).

�9 H2, Ar added

�9 H2

o H2, 2% Ar

< 1). Breakup processes other than electron impact ionization have been included into the calculation of Aio . for CH [19]. Tungsten and CH ion lines were difficult to detect, probably due to the small sample size and the correspondingly low impurity density.

The W I line intensity is observed to peak at a distance of 3 • 10 -3 m in front of the sample. The relative tungsten concentration n w ( z o ) in front of the sample is related to the sputtering ion flux Fi(z 0) (determined from the ion saturation current to the sample):

n w ( Z 0 ) = Yr,(zo)/Vw. ( 9 )

Here, Y is the sputtering yield, and v w is the emission velocity. Using eqs. (7) and (8), we can evaluate nw(z0) from the measured line intensity. Combining eqs. (7), (8) and (9), we obtain:

Y~ I( z )vw{ ( oxcvo>no( z )ri( Zo)

• (1 - exp( - (z 0 - z0)/Xexc) )

• exp( - ( z - Z0) /Aion)} -1 . ( 10 )

In fig. 7, we show the relative sputtering yield inferred from the W I line intensity, plotted versus electron temperature (high electron temperature corre- sponds to low neutral density, see fig. 3). The ion bombardment energy is U i = 125 V and is much below the sputtering threshold for hydrogen (783 eV). For both a hydrogen plasma and a hydrogen plasma with a small amount of injected argon, the sputtering yield is observed to increase dramatically at an electron tem- perature of about 5 eV. This result is consistent with sputtering of tungsten by HeO-related radical ions, as

L. Schmitz et aL / Impurity retention in a gas target dit'ertor 847

reported by Hirooka et al. [20]. Water vapor is a dominant intrinsic impurity in the PISCES-A vacuum system with a partial pressure of up to 0.3% of the hydrogen background pressure. In the experiments re- ported by Hirooka et al., the sputtering yield from the reaction products H2 O+, OH + is assumed to be close to the oxygen sputtering yield. The sputtering thresh- old for oxygen is 45 eV and therefore is below the ion bombarding energy in our experiment. The concentra- tion of these reaction products is reported to decrease dramatically for electron temperatures below 5 eV, which explains the observed dependence of the sput- tering yield on electron temperature.

In the case of argon injection, sputtering from argon ions is superimposed on the sputtering by intrinsic impurities. The threshold behavior observed at k T e = 5

eV is related to the fact that the argon ion concentra- tion at the sample increases because impurity screen- ing of the argon injected at z = 0 is less effective at higher electron temperatures (corresponding to lower neutral densities, see fig. 4b). If impurity screening is taken into account (as determined from fig. 4b), the sputtering yield extrapolated from our measurements produces a constant offset, as indicated in the figure. The estimated argon ion concentration is about 2% at low electron temperatures.

In summary, we observe that the upstream propaga- tion of impurities in the gas target regime is largely limited by friction forces with hydrogen neutrals and ions. For realistic tokamak gas target divertor parame- ters (ne(0) = 3 • 1020 m 3, no(0 ) ___ 1021 m 3, kTe(O) =

kTi(0) = 2-10 eV), the retention of impurity neutrals close to the divertor target is expected to improve by a factor of about 10 (as compared to a conventional divertor plasma with comparable plasma density) due to collisions between impurity neutrals and directional hydrogen neutrals. The impurity ion density is strongly coupled to the impurity neutral density and is primarily determined by the local balance between impurity ion- ization and radial ion loss. Furthermore, the strong axial electric field directed towards the target aids impurity ion retention and counteracts the electron thermal force. For typical tokamak divertor parame- ters, the ion thermal force could overcome the com- bined effects of the friction forces and the electric field only if k T i >> kT~ close to the divertor target. Im- proved impurity retention has been observed for gaseous impurities as well as for carbon and tungsten. Tungsten seems attractive as a divertor armor material in the gas target regime because of the absence of chemical sputtering and the high threshold energy for

physical sputtering by hydrogen or deuterium. Physical sputtering from intrinsic impurities like oxygen or wa- ter vapor is found to decrease dramatically at low electron temperature (kT~ < 5 eV).

Acknowledgements

The authors wish to thank T. Sketchley, L. Chousal, G. Gunner , and M. Srinivasan for their technical assis- tance during the experiments. This work is supported by the US Depar tment of Energy under grant no. DE-FG03-86ER-52134.

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