Experimental study on plasma heat flow to plasma-facing materials

jou.rO,.,.dof ELSEVIER

Journal of Nuclear Materials 223 (1995) 286-293

fluunr mlmgnls

Experimental study on plasma heat flow to plasma-facing materials S. Masuzaki

a,*, N.

Ohno

b

S. Takamura b

a Department of Electrical Engineering, School of Engineering, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan b Department of Energy Engineering and Science, School of Engineering, Nagoya University,Nagoya 464-01, Japan Received 15 June 1994; accepted 25 November 1994

Abstract

The plasma heat flow to plasma-facing materials in a well-defined simulated divertor plasma system is investigated experimentally in order to specify the influences of the ion reflection, surface recombination and change in electron energy distribution function on it. A simulated divertor plate (tungsten or carbon), located normally to the magnetic field, was irradiated by hydrogen and helium plasmas. The reduction of the plasma heat flow caused by the ion reflection is clearly shown, to depend on a kind of materials. For the case of tungsten target, the ion energy reflection coefficients obtained experimentally is quantitatively in good agreement with those estimated by the empirical formula.

1. Introduction

The plasma heat flow to the divertor plate is one of the most crucial factors for the erosion of plasma-facing materials and for the associated impurity contamination of core plasma in fusion devices. In particular, carbon materials currently used as plasma-facing component in large confinement devices have two unique erosion processes related to their temperature: radiation-enhanced sublimation ( > 1000°C) and chemical sputtering ( ~ 600°C). Therefore, a quantitative estimation of the plasma heat flow has been recognized to be very important issue. The plasma heat flow to a facing component, q ( W / m 2) is written as follows [1]: q = y(lis/e)kTe,

(1)

where Iis/e ( i o n s / m 2) is the particle flux, k is Boltzmann's constant, T~ (K) is the electron temperature and 3' is the so called energy transmission factor described based on a simple sheath theory. For the case

* Corresponding author.

of electrically floated wall in the hydrogen plasma, a value of 7 to 8 is employed. However, it has been found in the experiment that, the y value deviates from that predicted by the simple theory for various reasons, for example, secondary electron emission from the material surface [2], and some atomic processes in magnetic presheath regions for grazing incidence of magnetic lines to the divertor plate [3]. In this work, the plasma heat flow to some kinds of target plates is studied, stressing the following two effects due to both ion reflection at the material surface and the high energy electron component in the plasma. An ion reflection is an interesting phenomena expected to reduce, and therefore broaden the divertor heat load. It is also concerned with particle recycling in the edge layer of fusion devices. The ion reflection coefficient depends on the combination of incident ion species and target materials as well as the incident ion energy. It has been studied by using ion beam in the experiment and computer simulations using a Monte Carlo method [4,5]. However, few experimental data have been obtained in the plasma environment and, in

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S. Masuzaki et al. /Journal of Nuclear Materials 223 (1995) 286-293 the range of relatively low incident ion energy, below several hundred eV, which is really important for plasma-surface interactions in fusion devices [4,5]. In this energy region, the data obtained by computer simulations show that the ion particle and energy reflection coefficients are several tens of percents [4,5], so that they must substantially affect the plasma heat flow. The ion particle reflection coefficient of hydrogen ions has been estimated by spectroscopy in the TEXT O R tokamak [6]. However, we don't have any investigations so far on the effects of ion energy reflection effects in the plasma environment. The high energy electron component in the electron energy distribution is produced by additional R F heating in tokamaks [7]. With even a small fraction of high energy non-thermal electrons, the sheath formation is modified very much fi'om the case of Maxwellian distribution [8], so that the plasma heat flow through the sheath is also influenced greatly. The plasma heat flows to simulated divertor target plates have been experimentally measured in a linear plasma device NAGI-)IS-I [9,10], which has a well-defined plasma-target plate system, and in which the complexity encountered in the tokamak configuration is excluded. Here, tungsten and carbon are chosen as the target materials, which are considered to be candidates for the divertor plate in ITER (International Thermonuclear Experimental Reactor). The plasma heat flows were measured systematically for not only electrically floated but also biased target. The latter corresponds to the divertor or limiter biasing experiment in tokamaks and helical systems for the sake of better plasma confinement by controlling the edge plasma [11]. In the next section, the theory of plasma heat flow including the above important factors is described. The experimental setup is shown in Section 3. In Section 4, the comparison of experimental data with theory, and the estimation of ion energy reflection coefficients are discussed. A summary is given in Section 5.

287

thermal velocity, and ~bps is the potential drop through the presheath region. The normalized plasma heat flow, 8, defined by

6 = q/nscCskT e = q/{ (Iis/e) kTe},

(3)

is used in this paper, where /is the ion saturation current density. The 6 value corresponds to the sheath energy transmission factor, if the electron velocity distribution is Maxwellian. It is well known that 6 -- 7 for the electrically floated divertor plate for T¢ = Ti in hydrogen plasmas. When the effects of ion energy reflection and surface recombination are considered q and 8 are modified as follows [1]: q(~b) = nseCs{ ( 2 k T i - e~b)(1 - Rie ) + e~br} 1 _ + -~n~eCe2kT e exp {e(¢-~bpS)}kTe ,

(4)

xexp/e(& -~bp,)},

(5)

kTe

,

where Rie is the ion energy reflection coefficient and e~br is the recombination energy. Rie depends on species of incident ions, angle of incidence, their energies, and the target material [4,5]. Fig. 1 shows the incident energy dependence of Rie for the case of hydrogen and helium ion incident normally to tungsten

~J

i0

I

I

I

I

O

q~ 0

.o

2. Theory The plasma heat flow q obtained by the simple sheath theory is described by [1]

He + ->tungsten ..... H e * _ _ > c a r b o n

q( fb) = nseCs( 2kT i -e~b) +lnse~.~2k]% exp{ e( d ? - 4 ~ ) ) kT¢

10 -I

~J r..) ~) ,---) q~ O)

t:n

- - H

+- - > t u n g s t e n H+--- >carbon

'

(2)

where 4, is the target potential relative to plasma potential, ns¢ is the plasma density at the electrostatic sheath edge, C s = {k(Te + Ti)/mi} °s is the ion sound velocity, Te and Ti are the electron and ion temperature, respectively, t ~ = {8kTJ'xm¢} °'s is the electron

i0 -~ i0

I 150 Incident

I 200 Ion

i

I

250

300

350

Energy[eV]

Fig. 1. The ion energy reflection coefficients for hydrogen and helium ion incident to carbon and tungsten target as functions of ion incident energy derived from the empirical formula [5,12].

S. Masuzaki et al. /Journal of Nuclear Materials 223 (1995) 286-293

288

LaB6 HOT

~FAST SCANNING I [ ~ LANGMUIR PROBE []~V WATER COOLED II,~ll TARGET PLATE(SUS304)

IONIZATION~ GAU'iE

ANODE

m PLASMA GI:'NI(RATOR

]

lm

,~

RF HI:ATING ANNTI:NA

GAUGE

PLASMA TRANSPORT & S URFA CE INTERA CTIf IN TI:ST REGION

I

PLASMA PARAMETERS Electron l)ensitv o.SxlOlVm ~(H/He, steady ~tate) Eh,ctron Temperatnre 5-15eV h~n Temperalure ¢3)eV Plasma Diameter 12cm Cohmm Length 2.5m Magnetic Field l). 16T

DIAGNOSTICS .VISIBLE LIGHT SPECTROSCOPY .VISIBLE LIGHT CT .LANGMUIR PROBE INFRARED THERMOMETER .QMA

Fig. 2. Schematic drawing of linear plasma device NAGDIS-I.

and carbon target, calculated by the empirical formula [5,12]

respectively, the heat flow expression is modified as follows,

Rie = ( 0 . 7 0 5 / f ) / [ 1 + (•/0.047) °'597 + (a/0.619)1"5],

q(d#) = nseC s { (2kT i - e~b)(1 - Rie ) + e~br}

where f is the scaling factor for the backscattering coefficients and • is the reduced energy, respectively. It is clearly shown that the incident ions are reflected back to the plasma as the neutrals with several tens percents of original incident energies. Rie for tungsten is several times larger than the value for carbon. When there are two electron components in a plasma, having cold and hot temperatures, T= and T~n,

+ l n s e [ ( 1 - a)Cec2kTee exp{ e(~b-)c_Te7 ~bps) }

+aCeh2kreh exp( e( d) - ~ps) kTeh )],

(6)

where the population ratio of hot component to total number of electrons is a, and Cec and C~h are the

Target Bias Power Supply

) l) Fig. 3. Experimental setup for measurement of the plasma heat flow to tungsten or carbon (isotropic graphite IG-430U, Toyo Tanso Co., Ltd.) target plate.

S. Masuzaki et aL /Journal of Nuclear Materials 223 (1995) 286-293 thermal velocities of cold and hot electrons, respectively. The modified sound velocity C,* is defined by the effective electron temperature, T,n = { ( 1 - a)/T,c + a/Teh} -1, as follows: C~* = {k(Ti + Teff)/mi} 0"5 [8]. If Ti is much smaller than Te~, t~ is defined using T¢c as below: q

0.25

I e-

0 .2

--

289 I

>C,

W

I

target

carbon

I

turlgsten

target

target

/ 1

3.3xi0

~

"

i°I

29e

1.6x1011I

--

o

32/

(9

N

0

ns¢ Csc k T~

(1 -~)/3

m0.05

-For

--

et~r

2/-~-]-~ r

-a)exp[

•V t(

e(~-~bPs)

0 I I I i 600 800 i000 1200 1400 1600 Infrared Radiation Intensity I ir [A.U. ]

}

exp{e(6 - 6 s) /3kT~¢

}]"

(7)

%

In this equation, Cs~ and/3 are defined as (kTec/mi)°'5 and Teh/Tec, respectively.

3. Experimental setup

The experiment has been carried out in the linear plasma device N A G D I S - I [9,10], 0.18 m in diameter and 2.5 m in length as shown in Fig. 2. The steady state hydrogen and helium plasmas are produced by PIG discharge under the conditions of magnetic field B = 0.12 T, the pressure PH2 = 1 mTorr and Pile -- 2 mTorr, respectively. Fig. 3 shows the schematic drawing of the experimental setup. The simulated divertor target plate with the area of 4 cm :z is located in the plasma centre and about 2 m distant away from the plasma source. The target surface is set normally to the magnetic field line. A 1 mm O tan,mlum wire, covered by ceramic tube, supports the target plate and plays the role of heat isolator. The plasma heat flow to tungsten or carbon (isotropic graphite IG-430U, Toyo Tanso Co., Ltd.) target plate is measured by change in infrared radiation ( ~ 1 ~m) from the target plates detected by an infrared thermometer. To avoid a stray light from the hot cathode of the plasma source, the thermometer observes the target side opposite to the plasma source. The calibration was done in advance by an electron beam irradiation to the target plate. Target heat load, Qe, can be calculated by using electron acceleration voltage, Va, and electron beam current into the target, Ib, such as Q~ = Va x I b. The calibration curves are shown in Fig. 4 for tungsten and carbon target plates. Electron density, temperature and space potential were measured by the fast scanning Langmuir probe

Fig. 4. Calibration curves between the heat load to the targets and the changes of infrared radiation with the wavelength of 1 I~m detected by infrared thermometer which is proportional to the target temperature. The heat loads were generated by an electron beam injection.

located 0.2 m away from the target plate. In the present series of experiments, the range of electron density and temperature are 1 x 1017 < n e < 1.2 z 1018 m -3 and 4 < Te < 16 eV, respectively. In NAGDIS-I, the ion temperature can be considered to be neglected, and the incident ion energy is determined by the potential drop of electrostatic sheath in front of the target plate. The target plate can be biased by using an external voltage source. The bias voltage makes the change of incident ion energy, and it corresponds to the situation of the divertor biasing experiment in tokamaks [11]. Fig. 5 shows the reduced energy over the range of the incident energy up to 350 eV, which

i0 °

10-i

~3

10 -2

L

U t°

~)

- -

1 0 -3 : me

hydrogen helium

t

-~

-h y d r o g e n

..... helium 10- 4

plasma-->C plasma-->C plasma-->W plasma--

>W

, , , , I , , , , I , , , , I , , , , 0 Incident

0.i Ion

0.2

0.3

Energy

E °

0.4 [keV]

Fig. 5. The incident ion energy range employed in the present experiment, and the corresponding reduced energy e.

S. Masuzaki et at/Journal of Nuclear Materials 223 (1995) 286-293

290

was used for estimation of the reflection data. Here, the reduced energy is defined as [4,5] M1

32.55E 0

M 1 + M~2 ZaZ2(Zl/2 + z l / 2 ) 2/3'

(8)

where E 0 (keV) is the ion incident energy, M1, M2, Z1, Z 2 are nuclear mass and charge of incident particle "1" and target material "2", respectively.

4. Results and discussion

The plasma heat flows were investigated for two kinds of target material, tungsten and carbon, and two

o

different plasmas, hydrogen and helium, respectively. To make clear the effect of ion reflection, the target potential corresponding to ion incident energy was changed. The normalized plasma heat flow, 3, was obtained from Eq. (3) using the experimental data of plasma heat flow q, target's ion saturation current /is, and electron temperature Te. In Figs. 6a and 6b, the values of t~ determined experimentally are plotted as a function of normalized sheath potential, eeh/kTe, for different combinations of ion species and target materials. The solid thin lines in Fig. 6 are the theoretical curves from Eq. (3), simple sheath theory, with assumptions as below: (A) Ti = 0, (B) 4, = (target potential) (plasma space potential), (C) the potential drop of presheath is 0.5kTe/e. The dashed lines come from Eq. (5) with no ion reflection, that is, Rie = 0, and ion surface recombination energies, e~br, are 13.6 eV for hydrogen and 24.6 eV for helium, respectively. Typical electron temperatures T~ = 15 eV for hydrogen plasma and 10 eV for helium plasma are substituted into Eq.

(5). o N

4.1. Ion saturation region

o Z

Normalized

Sheath

Potential

60

o

50

40

3O N

;~2o

0 -70 -60 -50 -40 -30 Normalized Sheath

-20 -i0 0 Potential

Fig. 6. The normalized plasma heat flows from experiment and theory. Thin solid curves are obtained from Eq. (3),

simple sheath theory, that does not include the effects of ion reflection and surface recombination. Dashed lines come from Eq. (5) with Rie = 0 and typical electron temperatures Te = 15 eV for hydrogen plasma and 10 eV for helium plasma are substituted, respectively. The hatched regions are the ion saturation region, in which ion contribution to the heat flow dominate. The experimental data points are fitted by the bold solid lines. (a) Hydrogen plasma--, carbon or tungsten, (b) helium plasma ~ carbon or tungsten.

In Figs. 6a and 6b, we call the hatched regions as the ion saturation region, where the current into target is the ion saturation current. In this region, the electron heat flow can be neglected. Fig. 6 shows that experimental data points are located below the theoretical curve without considering ion reflection, and the plasma heat flows to the tungsten target are about 70% of that to the carbon target. It is very reasonable that the observed reduction of heat flow comes from the effect of ion reflection since Fig. 1 shows the ion energy reflection coefficient of tungsten is several times larger than that of carbon. We can see from Eq. (5) that ~ changes linearly with the slope of (1 - Rie) as a function of normalized sheath potential, if Rie is unchanged over the energy range concerned, and Ti << Te. In fact as shown in Fig. 1 the ion reflection coefficient does not change much in these energy ranges, and can be considered as a constant. Therefore, we can derive Rie values from Fig. 6. Experimental plasma heat flow data are fitted as straight lines in ion saturation region. From their slopes, Rie's are obtained as follows;' for hydrogen plasma irradiation, Rie = 0.46 (tungsten), 0.18 (carbon), and for helium plasma irradiation, Rie = 0.43 (tungsten), 0.19 (carbon). The ion reflection coefficient is known to be influenced by the ion incident angle O, especially, when O is larger than about 30° [13]. 0 can be determined by the ratio of velocity components, parallel (,) and perpendicular ( l ) to the magnetic field lines just on the target as O = t a n - l ( V ± / v i i ) . Now, we consider the case that Ti = 10 eV = Te. The ion kinetic energy is assumed to contribute only to V±, and VII is given as VII= ¢Cs2 + 2eda/mi. If the target is floated,

S. Masuzaki et al. /Journal of Nuclear Materials 223 (1995) 286-293 that is, 4)= - 4 0 V for He plasma, then the ratio of ions whose incident angles are larger than 30° is about 30%. If 4)= - 1 0 0 V, the ratio is less than 5%. As mentioned before, Ti is much lower than T~ in NAGDIS-I. Therefore the number of ions which have large incident angle can be neglected in this region, so ions can be considered to incident almost normally to the target surface. For hydrogen plasma, above result can be adopted. In the case of tungsten target, these Rie's agree with the data from empirical formula. On the other hand, in the carbon target case, experimental values of Rie are by a factor of 2 larger than that of empirical formula. The most probable reason considered for this difference is the surface contamination of carbon target by small amount of metal impurities. Fig. 7 is a result of RBS analysis for another carbon (AX650K) target exposed hydrogen plasma for 5 h [14]. It shows that SUS (Cr, Fe, Ni), Mo, La and Ta which are thought to come from the plasma source(hot cathode), plasma endplate, and vacuum vessel wall by ion sputtering. The reflected ions go back into plasma as neutral particles which have the energies of large fraction of the sheath potential for large Rie , and the plasma heating associated by them is expected. However, the electron density and temperature were relatively low in this experiment, and almost all such energetic neutral particles escape to vacuum vessel wall without enough interactions with plasma particles. So we could not find any plasma heating by energetic neutral particles. The influence of ion surface recombination is below 10% of total heat flow and can be negligible in this region.

4.2. Low voltage region with electron contribution Fig. 8 shows the detail of Fig. 6b for the region of - 2 0 < eqb/kT~ < O. From Eq. (4), the minimum heat

×10 4

31 I I T l l l 1.5MeV He + --> Graphite(AX650K) r exposedto hydrogenplasmain NAGDIS-I

t

NO

1

0

i00

200 300 Channel

/

I Mo La Ta

400

00

Fig. 7. A RBS spectrum of 1.5 MeV He + from graphite (AX650K) exposed to 80 eV hydrogen plasma in NAGDIS-I at 1000°C up to a fluence of 1.8x 102 ions/cm2 [14].

50

103

291

I

I

I

I

I

/

I

o

11

,-4 b~

Ii

~ i0

.8eV

G) 3O ~I:

i0 o

(I)

" -,-I o

i

i

2o -<°i o

I

lo

_

-20

/ I

,o

•

y / "

I

I

I

I

-16

-12

-8

-4

Normalized

Sheath

0

Potential

Fig. 8. The plasma heat flows in the range of potentials where the electron heat flow is large. The solid circles show the experimentally determined ~ values. The bold curve comes from Eq. (7) including hot electron component, where a = 0.14 and fl = 2.5 are determined by the Langmuir probe characteristics. The thin solid and the dash curves come from simple theory and Eq. (5), respectively. The insertion is an example of the single probe characteristics clearly showing the presence of the hot electron component. In this figure, Rie is assumed to be 0.55.

flow is obtained near the floating potential, that is, normalized sheath potential e ~ / k T e --- - 3 for hydrogen plasma, - 4 for helium plasma. However, as shown in Fig. 8, we have the minimum at e ¢ / k T e ~ - 8 for helium plasma, well deeper than above value of - 4 . The plasma heat flow at the target potential close to the plasma potential is well above the value predicted by Eq. (5) shown by the thin solid line for Rie = 0 and by the dashed line for Ri¢ = 0.55. The latter Rie value is reasonable in this relatively low incident ion energy region. A typical Langmuir probe characteristic for such plasma is shown in insertion of Fig. 8. We should note that the plasma consists of two components with different electron temperatures. In the present case, the temperature of hot electrons is about 2.5 times as large as the one for cold component, and the population ratio a is about 0.14. In this range of target potentials, the electron heat flow dominates so that the plasma heat flow is sensitive to the electron velocity distribution function. It is noted that the appearance of hot electrons is not related to the setup of targets, rather it depends on the experimental conditions of plasma generator, such as the gas pressure, the surface condition of hot cathode, etc. The target plate temperature was around 800°C, and the influence of hydrogen retention in the target is considered to be negligible [15].

S. Masuzaki et al. /Journal of Nuclear Materials 223 (1995) 286-293

292

As is shown in Fig. 6, the influence of ion surface recombination is several tens percents of total heat flow in this region. In a low temperature, for example, in a gaseous divertor, surface recombination can play a main part of divertor plate heat load.

4.3. Other factors to be considered to contribute the plasma heat flow Other factors to be considered to contribute to the plasma heat flow are estimated below: (a) Hydrogen molecular formation at the target surface: A neutralization occurs associated with ion impact on the target, but most of them reflected as atoms in our experimental condition [4,5]. The rate of particles remaining in the target is ( 1 - R i n ) times the incident particle flux, where Rin is the ion particle reflection coefficient. The energy released with hydrogen molecular formation is 4.5 eV. However, Rin is so large in the incident ion energy range concerned with this experiment [4,5], and the heat load associated with hydrogen molecular formation can be negligible. (b) Secondary and / o r thermal electron emission from the target surface: Secondary a n d / o r thermal electrons emitted from the target surface take away the energy corresponding to the work function of target material (a few eV). In this experiment, the electron temperature is low, 4-16 eV, and the target surface temperature is below 1600°C (for tungsten) or 1000°C (for carbon) so that the emission of secondary a n d / o r thermal electrons can be negligible. (c) Target surface roughness:Surface roughness may influence the ion reflection [16]. Though in this experiment, well polished targets have been used, targets surfaces must be roughened by plasma irradiation. This effect cannot be distinguished. (d) Hydrogen retention in graphite surface layers: Hydrogen retention in graphite surface layers may modify surface characteristics, that is, ion reflection [17], secondary electron emission [18]. In this experiment, the carbon target temperature is around 800°C, so the amount of retained hydrogen is small [15], and the effect of them is considered to be negligible.

(e) The excited states of reflected neutral particles: The lowest energy for excitation of helium atom is about 20 eV, that is comparable with the recombination energy. In extreme thinking, if all reflected neutral particles are excited, then the contribution of the surface recombination is cancelled. In this experiment, the effect could not be distinguished.

5. Summary The plasma (hydrogen and helium) heat flows to tungsten and carbon targets, from which the energy

reflection coefficients were different, were measured respectively in the linear device NAGDIS-I. Experimental data were compared with theoretical values including the effects of ion reflection, surface recombination and deviation from Maxwellian energy distribution of electrons, and they agreed well with each other. Ion energy reflection coefficients were determined experimentally in the deeply biased region. The coefficients obtained in the experiments agree with the values derived by the empirical formula for the case of the tungsten target. On the other hand, the carbon target gives the coefficients by a factor of 2 greater than that obtained by the empirical formula. The reason for this difference is not understood fully, however, a surface contamination by metal impurity might be one of the strong candidates which explain the larger experimental results. The large effect of the deviation of electron distribution function from Maxwellian to the plasma heat flow was also demonstrated experimentally.

Acknowledgements We would like to thank Dr. Y. Yamamura (Okayama University of Science), Dr. Y. Uesgi (Nagoya University) for valuable discussions, and Mr. M. Takagi, Mr. S. Fukao for their technical and experimental assistance. This work was supported by the Grant-in-Aid of Science Research from Japan Ministry of Education, Science and Culture (JSPS Fellowship No. 0332) and by the Grant-in-Aid for Cooperative Research (No. 04302059).

References [1] P.C. Stangeby, in Physics of Plasma-Wall Interactions in Controlled Fusion, eds. D.E. Post and R. Behrisch (Plenum, New York, 1984) p. 41. [2] H. Kimura, H. Maeda, N. Ueda, M. Seki, H. Kawamura, S. Yamamoto, M. Nagami, K. Odajima, S. Sengoku and Y. Shimomura, Nucl. Fusion. 18 (1978) 1195. [3] A.H. Futch, G.F. Matthews, D. Buchenauer, D.N. Hill, R.A. Jong and G.D. Porter, J. Nucl. Mater. 196-198 (1992) 860. [4] W. Eckstein and H. Verbeek, in Data Compendium for Plasma-Surface Interactions, Nucl. Fusion, special issue (IAEA, Vienna, 1984) p. 12. [5] W. Eel
S. Masuzaki et al. /Journal of Nuclear Materials 223 (1995) 286-293 [9] S. Masuzaki, N. Ohno, M. Takagi and S. Takamura, Trans. IEE Japan 112-A (1992) 915, in Japanese. [10] S. Masuzaki, H. Konno, N. Ohno and S. Takamura, Proc. 20th Europ. Conf. on Controlled Fusion and Plasma Physics, Lisboa, 199'3, vol. II, p. 743. [11] T.E. Stringer, Nucl. Fusion 33 (1993) 1249. [12] R. Ito, T. Tabata, N. Itoh, K. Morita, T. Kato and H. Tawara, Data on the Backscattering Coefficients of Light Ions from Solids (A Revision), Rep. IPPJ-AM41, Institute of Plasma Physics, Nagoya University (1985). [13] T. Tabata, R. Ito, Y. Itikawa, N. Itoh, K. Morita and H. Tawara, Dependence of the Backscattering Coefficients of Light Ions upon Angle of Incidence, Rep. IPPJ-AM-34, Institute of Plasma Physics, Nagoya University (1984).

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[14] K. Morita, K. Mori, S. Ikeda and S. Takamura, Erosion of metal-carbide deposited graphite induced by high heat flux plasma irradiation at high temperatures, Proc. 2nd Research Coordination Meeting on Plasma Interaction Induced Erosion of Plasma-Facing Materials (IAEA, Vienna, 1993). [15] W. Moler, J. Nucl. Mater. 162-164 (1989) 138. [16] N.K. Koborov, V.A. Kurnaev and V.M. Sotnikov, J. Nucl. Mater. 128&129 (1984) 691. [17] R. Aratari, W. Eckstein, Nucl. Instr. and Meth. B 42 (1989) 11. [18] K. Shiraishi, N. Ohno and S. Takamura, J. Plasma Fusion Res. 69 (1993) 1371.