Reflection and retention of low energy light ions during bombardment of fusion materials

iHIl m

ELSEVIER

Journal of Nuclear Materials 220-222 (1995) 952-956

Reflection and retention of low energy light ions during bombardment of fusion materials N.N. Koborov a, V.A. Kurnaev % D.V. Levchuk ~, A.A. Pisarev a, V.M. Sotnikov ", O.V. Zabeida a, V.I. Pistunovich b ~' Moscow Engineering Physics Institute, Moscow 115409, Russia b Russian Research centre "Kurchatol' Institute", Moscow 123182, Russia

Abstract Experimental and computational data on light ions reflection and retention for energy below hundreds of eV are presented. Mass spectrometric method is used to measure particle reflection Rr~ and particle retention coefficients for different angles of incidence of D ions on different targets. Strong influence of surface oxidation for Be is observed. Results are compared with the binary collision computer simulations. R N as a function of primary energy reveals the maximum with amplitude and energy depending on the angle of incidence, attractive potential E~ and atomic number of target. Strong isotopic effect in hydrogen backscattering from Be is observed. The regular relief influence on hydrogen ions backscattering from Be and C target was computer simulated and it is shown that considerable increase in R N ( R ~ a X / R N ( O ) = 1.85 for proton normal incidence on Be target) is noticed.

1. Introduction

2. Experimental procedure and results

Low energy particle reflection and retention parameters are of great importance for control of recycling and particle balance in tokamaks. Most of the known experimental data are received for energies E 0 > 50 eV and normal incidence of the beam on the target. There is a progress in computer simulations of fuel particles backseattering from smooth surfaces [1-3] including the low energy region, but the main results were published for normal incidence. Experimental investigations of low energy heavy atoms reflection [4,5] proved that simultaneous interactions with several target atoms became important. The binary collision model is valid for energies as low as few eV [4]. The real surface roughness simulations using regular structures [6-9], or fractal geometry [10] may be considered as a more realistic approach. Unfortunately the real PFC surface structure and composition as well as their influence on backscanering are unknown. Some experimental results and computer simulations concerning the low energy light ions backscattering from fusion materials are presented in this paper.

The high vacuum mass monochromator with decelerating system was used to measure the particle reflection coefficient R N for low energy deuterium ion bombardment of hot targets at different angles of incidence. Angle of incidence varied from 0 to 75 °. Ohmic heating of 10 × 60 × 0.1 mm target strip was used. The temperature was controlled by thermocouple. Massseparated primary ion current ranged from 10 ~ to 10 6 A depending on decelerating energy. Residual deuterium pressure in the target chamber was negligible ( < 10 10 Pa) due to a 4-step differential pumping system. High sensitive 180° mass-spectrometer with permanent magnets was used for D 2 and HD partial pressure control in the target chamber. Two experimental procedures were used for particle reflection coefficient measurements. The first one was based on comparison of the deuterium pressure jump PR after switching on the ion beam with O 2 saturation pressure P~,t (Fig. 1). So, R N could be calculated as: R N = ( PR - P o ) / ( P ~ . t - Po)"

0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-31 15(94)00618-0

(1)

N.N. Koborov et al. /Journal o f Nuclear Materials 220-222 (1995) 952-956 1,2

953

1.0 0.9

l~"t 1

0.8 0.7

0,8

0.6 i

*o = 2.2x1010 D~'cm 2 ~o =5-5xl016 D~cm 2 ®o= 2.2x1017 D; era-2

(

0.5

0,6

0.4 0.3 0.2 0.1 0 0

20

40

0

60

5

1 '0

1 '5

t. s

Time, rain

4()0 6()0 80'0 1000 ;F,K Fig. 2. TDS during linear heating of Be target after preliminary D~ 1.5 keV bombardment with different fluences.

Fig. 1. D 2 partial pressure in the target chamber during D~1.5 keV bombardment of Be target (@o = 30°). T o p r e v e n t t h e d e u t e r i u m t r a p p i n g in the t a r g e t c h a m b e r walls, they w e r e h e a t e d u p to 500 K. A n o t h e r p r o c e d u r e of R N d e t e r m i n a t i o n was b a s e d on the deuterium retention measurements. The amount of c a p t u r e d ions was m e a s u r e d using t h e r m o d e s o r p t i o n spectroscopy ( T D S ) d u r i n g o h m i c h e a t i n g of t h e target after i r r a d i a t i o n (Fig. 2). T h e d e p e n d e n c e of a m o u n t of c a p t u r e d particles ~ as a f u n c t i o n of fluence ~ o , = F(@0) lets to find t h e particle reflection coefficient if t h e recycling p a r a m e t e r R c = D / 2 K (where D is t h e diffusion c o n s t a n t a n d K t h e c o n s t a n t of recomb i n a t i o n ) is k n o w n [11]. F o r low ~ 0 , R N = 1 - l i m ( 0 ~ / 0 @ 0 ) , T o control t h e total a m o u n t of the deut e r i u m a t o m s r e l e a s e d from t h e t a r g e t d u r i n g T D S p r o c e d u r e , partial p r e s s u r e s of D2, H D , D 2 0 a n d H D O molecules w e r e m o n i t o r e d . M e a s u r e d values of R N (as well as calculated o n e s ) for different targets, p r i m a r y energies a n d angles of incidence are shown in T a b l e 1. E x p e r i m e n t a l value for E 0 = 400 e V was meas u r e d by b o t h m e t h o d s m e n t i o n e d above, while for E 0 < 100 e V by T D S m e t h o d . It is necessary to n o t e

t h a t for sliding incidence of low energy d e u t e r i u m ions o n W, the particle r e t e n t i o n coefficient rl was small ('0 <
3. Computer simulation Backscattering data were o b t a i n e d in t h e binary collision a p p r o x i m a t i o n ( B C A ) using two M o n t e Carlo c o m p u t e r programs. T h e c o m p u t e r code S C A T I ' E R 1.1 which is identical to T R I M . S P [13] in the main was

Table 1 Particle reflection coefficients for D bombardment of targets with different primary energy E 0 and angles of incidence O 0 (exp. and cal.) E 0 (eV)

W

Nb

8o = 0 10 30 50 100 400

Be

0o = 60°

8o = 75°

~9o = O° exp.

exp.

cal.

exp.

cal.

exp.

cal.

0.83 0.75 0.7 0.6 0.53

0.84 0.72 0.71 0.65 0.52

0.97 0.95 0.88 0.86 . .

-

0.96 0.98 0.95 0.9 .

0.94 0.95 0.94 0.92 . .

.

.

8o ~ 75°

8o = O*

6)o = 75°

cal,

exp.

cal.

exp.

cal.

exp.

cal.

0.56 0.6 0.58 0.57 .

-

0.85 0.9 0.87 0.84

0.14 5:0.07

0.23 0.19 0.18 0.16 0.125

-

0.54 0.67 0.65 0.63 -

N.N. Koborot' et al. / Journal of Nuclear Materials 220-222 (1995) 952-956

954

It: l{,:2 c'V

Rx

--A---V--

Be' ("

--&

Nh

RN I0

09

O D - - D - - - - [] . . . .

/A~ {}S- i

. V ~

o,7.

X_~__~_ ~

,,i,~A

({-)o

•{}

{}<, 0.5

/

}4-

0.4

{}4- A

~zx~zx

O.

~

I ~. A 4}.2-

A_

_A_ ~ A (-)o

{}o

/

I

0

()

g

~'{,

~'0

A 4He -~Nh, Es=0.42 cV --A.-- D ~ N h , Es=2.0 cV [] D ~ N h , Es=5.0 cV

{1.2 0

0.]

5

O

A--

75" 06

•

A

O

{)7

~ • ~ •

-~-~XxV

o,~. V/?

D-

](',{}

r

r

r

2(1

4{}

60

,

r

'

S{)

I(}{)

E o, cV

Eo, c V

Fig. 3. Particle reflection coefficient R N as a function of primary energy of protons for different targets ( E s = 2 eV) for normal (')0 = 0° and sliding (% = 75 ° incidence (SCATTER code computer simulations).

Fig. 5. R N of D and 4He versus E 0 for Nb target and

used for R N and R E calculations for smooth surface (Table 1, Figs. 3-5). The number of projectile trajectories ranged from 5000 to 10000. The statistical error of presented results did not exceed 0.05. The attractive potential E s for hydrogen atoms is unknown. So we used E ~ = 0 . 5 - 3 eV as a fitting parameter. For D reflection the best agreement was obtained at E~ = 1 eV for W and E~ = 2 eV for Nb and Be. R N reached

different attractive potential E~.

the maximum value at energy E m which for given projectile was a function of angle of incidence O{~, primary energy E 0 and E~ (Figs. 4-6). The computer code [6] based on randomization procedure was used to calculate surface relief influence on R N for normal incidence of hydrogen atoms on Be and C targets. The regular relief was simulated by a roughness of repetitive size and shape with face inclination angle q% varied from 0 (flat surface) to 85 °. 60-

R\ I,{}]

--A-- T

0.9

Em

--i--

$

--v-- H

I)

081

40-

0 7-

v/--~'-----v'~

0.6"t

,if

--

~----~ 2

®o - 75<'

E m , cV

(9o ,O . . . .

75" O

20 -

I I>'~-

o •-~1/t , • i"

~

.-"

" I

A______• I?(-}" = O"

O ~ l

/

-

= 0o

j /

m - -

0

20

40

611 E o,

811

1{)1}

cV

Fig. 4. Particle reflection coefficient for hydrogen isotopes backscattered from Be target for normal and sliding incidence (E~ = 2 eV).

0.5

1,0

1.5

2.0

Es, eV

Fig. 6. Energy E m of maximum value R ~ ''~ as a function of attractive potential E~ for normal and sliding incidence of D ions on Be target.

N.N. Koborov et aL /Journal of Nuclear Materials 220-222 (1995) 952-956

$

RN 0.4-

$

RN 0.5"

I ~o~o~

I jO~-01

j

jO

~

0

0

0.4

i

955

0.3'

\

0.2'

0.3"

0.2!

5

I . . 0 / 0 ~ 0 ~

$ \

O.F

~0~0"-.

.~-~

(a) 0

./'-----.~

5 ~0~0~

0.1 ~O

4 ~ . ~

(b) 3'0 °

,

~0 °

0

90 ~'

0

~0

60

9'O

% % Fig. 7. (a, b) Particle reflection coefficient R N for proton with different energies E 0 as a function of face inclination angle Vsb of regular relief on the target surface: (1) 20 eV, (2) 50 eV, (3) 200 eV, (4) 500 eV, (5) 1000 eV. Arrows indicate R~ ax (a) Be, (b) C.

N o n m o n o t o n i c d e p e n d e n c e of the reflection probability on angle ~ b is established as shown in Fig. 7. The coefficient R N as a function of ~ b rises from RN(0) to some maximum value R ~ a~ at 1/'b = a/t~,ax and then tends to zero as gr b approaches ~r/2. The ratio R ~ / R N ( O ) increases with proton energy and is less for targets with larger atomic number. A universal function was found for d e p e n d e n c e of R ~ / R N ( O ) on

RN(0) for different materials (Fig. 8). This function gave an upper limit of the relief influence on backscattering probability for normal ion incidence. The maximum value R~ax/RN(O)= 1.85 was obtained for Be bombarded with energetic protons ( E = 1000 eV). As for g r ~ x dependence on RN(0), small spread of ~ a x values was observed for RN(0) > 0.3, i.e. for low energy protons.

R~ ~ / RN

o

4. Discussion and conclusion

2

2.0"

1.5-

% ~oo

1.0

0.0,

.......

011

.......

;'0

Rr4

Fig. 8. R~aX/RN v e r s u s R N for proton bombardment of copper (o), graphite (e), beryllium ( • ) : (1) R ~ X / R N = 1 + 0.110(-In RN) t'84, (2) the same for Ar --* Cu(0).

The measurements of Rr~ performed by two methods show that in the low energy region the experimental data are in a good agreement with c o m p u t e r simulation for different angles of incidence. Previous (corrected) experimental data for D bombardment of W target showed that the maximum value R ~ ax for sliding incidence could be close to unity, and the energy E m corresponding to R ~ ax depended on angle of incidence. C o m p u t e r simulations with some attractive potential value E s = 1 eV qualitatively suited the experimental data. So, one can suppose that for low energies, B C A simulations for light ions are qualitatively valid. Results for targets with small atomic number also gave high values of R N > 0.5 for sliding incidence. For heavy targets the difference between normal and glancing incidence was not very large. For Be, the maximum value of R N at O 0 = 75 ° was 3.5 times higher than that at O 0 = 0 ° (Fig. 3). The considerable isotopic effect was

956

N.N. Koborov et al. / Journal of Nuclear Materials 220-222 (1995) 952-956

seen at normal incidence on Be (Fig. 4) R N ( H ) = 1.5RN(D) = 2.5RN(T). The attractive potential value E s influence on R N is dramatic in the most low energy region E 0 < 50 eV for sliding incidence and more pronounced for targets with small atomic numbers (Fig. 6). So it is of great importance to measure the experimental R N values for this targets at low energies. The large difference between R N for hydrogen and helium atoms in the low energy region (as the results of the computer simulations show) can lead to helium enrichment in the fuel "ash" mixture evacuating through the divertor ducts. Calculations showed that for heavy targets, the energy reflection coefficient R E suited the simple approximation R E = (RN)2 with 10% accuracy. For target with small values of M 2 this approximation gave overestimated R E (up to 30%). The results of computer simulations also indicate that for energetic particles (charge exchange neutrals for example) the roughness of the PFC surfaces can be the reason of a noticeable R N increase if to compare with results obtained for smooth surface. But as for particle retention Fig. 7 shows that for angles of slope inclination of ridges 30 ° < ~b < 60° which can exist on the PFC surface due to mechanical treatment, the relief does not considerably influence r/ values. R N and r/ can be affected noticeably only for specific PFC surface transformations characterized by cone and viskers generation (~b > 70o)•

Acknowledgements We wish to acknowledge the technical assistance of Plasma Physics Department of MEPhI staff and students S. Turkuletz; E. Levacheva, N. Trifonov, V.

Vaitonis. This work is supported by the Russian Ministry of Atomic Energy (Contract No 94-3-021-193) and by the International Science Foundation (Grant MJG 000). One of the authors (VAK) thanks International Science Foundation for financial support of his participance at the PSI-11 Conference (grant 1321/1).

References [1] W. Eckstein and H. Verbeek, Nucl. Fusion. 24 (1984) 1250. [2] R. Behrish and W. Eckstein, Ion Backscattering From Solid Surfaces. Physics of Plasma Wall Interactions in Controlled Fusion, eds. D.E. Post and R. Behrisch, (Plenum, 1986) p. 413. [3] W. Eckstein, Nucl. Fusion Suppl. 1 (1991) 17. [4] N.N. Basarbaev, V.V. Evstifiev and N.M. Krylov, Poverchnost' 9 (1988) 140. [5] J.W. Cuthbertson, W.D. Langer and R.W. Motley, J. Nucl. Mater. 196-198 (1992) 113. [6] V.M. Sotnikov, Sov. J. Plasma Phys. 7 (1971) 236. [7] N.N. Koborov, V.A. Kurnaev and V.M. Sotnikov, J. Nucl. Mater. 128&129 (1984) 691. [8] Y. Yamamura, C. Mossner and H. Oechsner, Radiat. Eft. 103 (1987) 25. [9] A.Yu. Pigarov and Yu.L. Iqitchanov, Contrib. Plasma. Phys. 30 (1990) 71. [10] D.N. Ruzic and H.K. Chiu, J.Nucl.Mater. 162-164 (1989) 904. [11] V.V. Bandurko, V.A. Kurnaev and S.K. Zhdanov, Proc. Japan-CIS, Workshop On Interaction Of Fuel Particles With Fusion Materials, Tokyo, 1992, p. 130. [12] V.V. Bandurko and V.A. Kurnaev, Vacuum 44 (1993) 937. [13] W. Eckstein and J.P. Biersack, J. Appl Phys. A 38 (1985) 123. [14] O.S. Oen, M.T. Robinson, Nucl. Instr. and Meth 132 (1976) 647.