Ion impact desorption and hydrogen release

499

Journal of Nuclear Materials 103 & 104 (1981)499-502 North-Holland Publishing Company

ION IMPACT DESORPTION R. Bastasz

AND HYDROGEN

RELEASE*

and L. G. Haggmark

Sandia National Laboratories Livermore, California 94550

Desorption cross sections have been measured for the removal of deuterium from stainless steel by hydrogen ion impact. Using hydroqen ions with energies similar to those in the charqe-exchange neutral flux striking the first-wall in tokamaks, the desorption cross section was determined for substrate temperatures < 170 K as a function of incident particle enerqy (in the range 300-1500 eV) and impact angle (30" to 80" with respect to the surface normal). Calculations of the cross section made with a modified version of the TRIM code are in qood aoreement with the experimental data. With the appropriate cross section information it becomes possible to estimate the release rate of surface hydroaen due to impact desorption exoected durinq reactor operation.

1.

INTRODUCTION

Large fluxes of hydroqenic particles strike the walls in maqnetic confinement fusion devices with consequent implantation, reflection, and desorption of hydrogen isotopes at the wall surfaces. Not only do these processes affect the material properties of the walls rll, but they also influence operating characteristics, such as refuelinq [23, of the device. Recently, the release of adsorbed hydrooen soecies by direct ion impact has been shown to be an efficient process under the conditions expected in fusion reactors C33. In this paoer we examine in qreater detail this process for the case of deuterium adsorbed on stainless steel and compare experimentally determined desorption cross sections with calculations obtained with an extension of the TRIM method. The results are used to estimate the maqnitude of particle impact desorption processes that will occur durinq steady state operatinq conditions in fusion reactors. It is concluded that particle imoact desorption can be a sianificant source of hydrooen recvclinq whenever adsorbed hydroqenic species exist on confinement vessel walls. 2.

EXPERIMENTAL

nesorption cross sections were measured on polished, stainless steel suhstrates (type 316) using a secondary ion mass spectrometer, which has been described previously C4l. In order to avoid interference frown incident ion reflection, deuterium was used as the adsorbate and protium as the incident species. The substrate was cleaned by a combination of Ar+ bombardment and annealing in order to remove any preexisting oxide layers. After 'This work was suoported Department of Energy.

0022-3115/81/0000-0000/$02.75

by the U. S.

0 1981 North-Holland

cooling to < 170 K in order to reduce diffusion from the surface to the bulk, a stream of D,(q) was directed onto the sample until an a;ls&bed deuterium layer formed, Typic lly this required exposures of up to 3 x 10J The presence of adsorbed deuterium Langmuirs. was verified by secondary ion measurements, but the actual surface concentratfon of D(adsl was not determined. Desorption of the adsorbed deuterium was induced by a monoenergetic, mass analyzed beam of H+ (n=2,3) that was focused and rastered over g 0.065 cm2 area on the sample to ensure a uniform flux. During desorption the production rate of D- was continuously monitored and was used as a relative measure of the deuterium desorption rate. Ry simultaneously recording the ion current to the samole the D' signal could be normalized to the primary ion flux so that fluctuations in the primary ion intensity did not affect the mea&rements. The primary ion energy was varied from 333 to 1500 eV and the imoact angle of the beam with the substrate was ’ varied from 30' to 80" with respect to the surface normal. 3.

RESULTS

Cross sections (Od) were obtained from the data by analysis of the secondary ion intensity dependence upon primary ion fluence. In general, the secondary ion signal exhibited inverse exponential behavior as a function of fluence, indicating first-order kinetics with respect tn surface coverage. The cross section for the removal of adsorbed deuterium from stainless s el as found to be in the neiqhborhood of IO-f8 c$ for incident hydrogen i&s 30" from the normal with energies 333 to 1500 eV/atom, as is shown in Figure 1. Measurements were made using either H+ or H+ as the incident ion species [31. Whefi hydragen atom fluences, rather than molecular ion

500

R. Bastasz, L.G. Haggmrk

/ IOII impact desoption

fluences, were used in calculating the cross sections similar values were obtained with H+ and H1 at the same incident energy per of the molecua ?om, as s uminq equipartitioning lar ion kinetic energy to each constituent An increase in the cross section was atom. ohserved with decreasing ion enerqy, rangina from 5 x lo-I7 cm* at 1500 eVlatom to 1.2 x ID-I6 cm2 at 333 @i/atom.

nii

,’0.0

0.5

1.5

1.0

Incident

energy

and h),drogen release

Ni substrate or a combination of Cr, Fe, or Ni would not significantly alter the results. The sputtering yields for D calculated for H :Zi~~~~lo~e~:9t~/~~,~~~~h~~t~i~~~~dt~~ desorption cross section. The only physical ouantity that was varied for the calculations was the surface binding energy E,. TRIM results are shown in Figure i-fo; Es = 0.4 and 0.5 eV. The TRIM results show a similar decrease in the desorption cross section with increasing incident energy as found experimentally, and the Es = 0.4 eV results are more consistent with the measured cross sections at low enerqies. The TRIM results are shown in Figure 2 as a function of incident anqle for 333 eV H usina E, = 0.4 eV. The cross section increases wit6 increasingly oblique impact angles and follows the same trend as the experimental data except at near qlancina anples.

2.0

(k&/H)

Figure 1 : Energy dependence of the desorption cross section for H + D (ads) on stainless TRIM results are steel at 30' incidence. shown for Es of 0.4 and 0.5 eV.

The anqular deoendence of the cross section was studied usino 1 keV H+. Varvins the imoact anole from 30" to 80" witi resoect to the surface normal resulted in a chanqe in the desorotion cross section by a factor of _ 2, with a maximum value of 2.8 x lo-l6 cm2 obThis behavior is illustrated served at 80'. in Finure 2. Also shown in Figures 1 and 2 are results obtained using an extension of the Monte Carlo, ion transport computer program TRIM C51. This extended simulation procedure uses the binary collision approximation to follow the taraet atom trajectories, collision by collision, in addition to those of the incident For purposes of this paper particle C6,71. the computer program has been generalized further to consider a multi-layered, multiThe full details of the ion element target. transport procedures and the extension to sputterins calculations can be found elsewhere In the simulation of the present c5-71. experiments. a laverof D with atomic density N and thickness NL1i3 was "placed" on a suhstrate of Cr. With this thickness and the D atomic density eoual to that of the Cr substrate, a monolayer of D coverage on stainless steel was modeled. Cr was chosen as the substrate since the surface of heat treated stainless steel has been found to be enriched in this element T31. Use of an Fe or

Fiqure 2 : Angular dependence of the desorption Experimental results are shown cross section. (triangles) for H + D (ads.1 on stainless steel at 333 eV. TRIM results (squares) are shown for Es = 0.4 eV.

4.

DISCUSSION

The measured cross sections for desorption of deuterium from stainless steel by energetic hydroqen are rather large, surpassinq a yield, (Yd = udN,e; N, = 2 x IO15 cm-*, the Y!' a sorbate atom density of one monolayer) of D.2 at 333 eV for a fully covered surface te=11. This yield is especially large in comparison to substrate sputtering by h drogen Bohdansky et al. r 81 at similar eneroies. have yeasured a sputtering yield, Y,, of 4.7 x lo- for 330 eV H+ at normal incidence on stainless steel. Taking into account the angular dependence of the sputtering yield (Ys(30"I/ Y,(O')-1.51, the yield ratio of deuterium desorption to substrate sputtering is _ 28. Hence, under these conditions impact desorption is siqnificantly more efficient a

501

R. Bastasz, L. G. Haggmark / Ion impact desorption and hydrogen release

process

than substrate

sputterinq.

The desarption yield depends directly uoon surface coveraoe, so Yd + D as e + 0. Two distinct situations can therefore be identified. First, if there is no replenishment ;; the adsorbate,the desorption.flux, JID, , ~h=Yde: 0 1s incident flux) will decrease as'Fhe"surface becomes depleted. Second when a constant source of adatoms is avaflable steady state conditions nay be reached and JID will remain constant. The first situation is encountered at the beginning of a discharge when qas adsorbed during the interval between discharges is released. If the desorption rate is large in comparison to the readsorption rate, the result is a transient pulse of released qas. The second situation may occur during long duration discharaes when a hioh incident flux of charqe-exchange neutrals saturates the near surface region of the wall and subseauent diffusion of implanted species back to the surface provides continual replacement of adatoms.

210,

20

,

,

be used to2calculate JTD according to JT = 2 kTD e2 N, where kTD = 0.4 exp [-0.98(e?l/kT]. Imposing steady state conditions, we solve for 8 and normalize (normalization factor = qdN,) t.0 obtain:

8 =

- ud$), o
&(dm

1.00

,

10"

10'8

Incident flux (H/cm*

10’2 Incident

10"

(1)

Figure 4 illustrates the dependence of e upon 4 at several temperatures for the case of 333 eV hydroqen bombardment at 30". In general, 8 appears to increase along with + and decreases with increasina temperature.

10’2

10'0

.

lolS

1020

-WC)

Fiqure 4 : Calculated fractional surface coverage of deuterium for s ady state conditions with qd = I.2 X 10wr8 cm2 for several substrate temperatures.

10'6

flux (H/cm*

10’8

1

020

-WC)

Fiqure 3 : Deuterium desorption flux versus incident H flux (333 eV H at 30" incidence) for various fractional deuterium surface coveraqes, 0, on stainless steel.

The desorption flux is shown in Fiaure 3 as a function of incident flux for several surface In order to estimate the desorption coveraaes. flux under reactor operatina conditions it is necessary to know the steady state surface The surface coveraae cannot be coverape. determined due to the current absence of first-wall diagnostic information, but a rou9h estimate of e can be obtained by considering that durinq steady state conditions + = J D + JTD, where JTD is the outgoinq flux attri 6 utable to thermal processes. To evaluate JTD we consider associative thermal desorption. Madix et al. [91 have reviewed rate constant data for desorption of H2 from Ni that can

With an estimate of 8, it is possible to calculate the hydrogen release fraction due to particle impact desorption. The impact desorption release fraction is given by: ‘ID

JIO+JTD

=

'd$

.

ud0+2kTDeNa

At high incident particle fluxes, the impact desorption release fraction predominates, especially at low wall temperatures. This behavior is shown in Figure 5, where the release fraction from 333 eV hydrogen bombardment at 30" is plotted versus + for several wall temperatures. This expression assumes only two pathways for hydrogen release but other pathways may exist, especially under the severe conditions of high flux and temperature, such as direct recombination without equilibration at chenisorption sites [lO,lll. In these circumstances, the release fraction

R. Bastasz, L. G. Haggmark

502

must include an appropriate denominator.

/ Iorr impact desorp tion atzd h~ukqen

5.

flux term in its

r&use

CONCLUSION

Impact desorption will be an efficient hydrogen release mechanism whenever adsorbed hydrogen isotopes exist on confinement vessel walls. Experimental data obtained at substrate temperatures < 170K and calculations indicate that hydrogen impact desorption cross sections are maximized at energies below 500 eV and at oblique impact angles.

10'2

1n’O

1014

Incident

flux

?O'S (H/cm*

10’8

102*

-sea)

Figure 5 : Impact desorption release fracti n !! veisus incident H flux with Ud = 1.2 x 10-l cm for several substrate temperatures.

In a reactor, the particle flux striking the first-wall is more likely to have a Maxwellian rather than a monoenergetic energy distribuAlso, instead of a single angle of tion. incidence. the ansular distribution of the particles-may more closely resemble a cosine Since the desorption cross distribution. section depends on both the energy and impact angle of the incoming particle, these factors will affect the overall desorption efficiency. In Table I, effective desorption cross sections are listed for particles having a Maxwellian energy and cosine angular distributions. The values were obtained with TRIM using E, = 0.4 eV in order to be consfstent with the For these conditions, measured cross sections. the desorption cross section appears to be fairly insensitive to the temperature of the particle distribution, varying by less than a factor of 2 over an order of magnitude ~~~~~~ni~st~~~~~~~~

;; ;;A ~~~~~t~~~

cross

for impact desorption &mains high over a wide range of incident flux temperatures. Table I Hydrogen IID of deuterium from stainless Calculated cross sections.

100 200 333 500 1000

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Cl1 R. Behrisch, J. Nut 1. Mater. 85/86 (19791 1047.

c21 H. C. Howe, J. Nucl * Mater. 93/94 (19801 17.

c31 R. Bastasz, to be published. 141 R. Bastasz, "Secondary Ion Mass Spectrometry; SIMS-II,” edited by A. Benninghoven et al. (Springer-Verlag, 1979) p. 196.

New York,

E51 J. P. Biersack and L. G. Haggmark, Nucl. Instrum. and Methods -174 (19801 257.

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!I71 L. G. Haggmark and J. P. Biersack, this proceedings.

LX81J. Bohdansky,

H. L. Bay, and J. Roth, Proceedings of the 7th International Vacuum Congress and 3rd Internatianal Conference on Solid Surfaces, Vienna (19771 p. 1509.

c91 R. J, Madix, G. Ertl, and K. Christmann,

kT (eV)

steel.

Two situations in which impact desorption is expected to occur in tokamaks are 111 at the beginning of a discharge, when residual gases adsorbed during the interval between discharges will be desorbed in a transient pulse and (21 during the course of a long duration discharge, when return of implanted hydrogen species provides a source of surface hydrogen. Impact desorption will be especially prevalent on wall surfaces that are cool and receive a substantial flux of charge-exchange neutrals. Under these conditions, our results indicate that impact desorption can be an important mechanism for hydrogen release from the first-wall,

( ~O-~~dcm2,D1 1.95 1.56 1.47 1.42 1.27

Chem. Phys. Lett. -62 (19791 38.

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