Collecting Si-probes for investigation of deuterium fluxes and energies in the tokamak scrape-off-layer

Journal

1022

of Nuclear

Materials

176 & 177 (1990) 1022-1026 North-Holland

Collecting Si-probes for investigation of deuterium fluxes and energies in the tokamak scrape-off-layer S.A. Grashin and Yu.A. Sokolov Kurchatov Insiitute of Atomic Energy, IUOSCOW, USSR

V.Kh. Alimov. A.E. Gorodetsky, A.P. Zakharov, S.L. Kanashenko, M.A. Lomidze, A.I. Fomichev and V.M. Sharapov Inst.of Physical Chemistry, Academy of Sciences, Moscow, USSR A.F. Burenkov Research Institute of Applied Physical Problems, Minsk, USSR

The method of collecting Si-probes has been further developed and used for determination of deuterium fluxes and energy in the SOL of T-10. SIMS and RHEED were used for probe analysis after exposure. Experiments in which the Si-samples were

exposed to monoenergetic DC-ions (En = 20-1000 eV, Q,= 10’5-10’g cm-*) enabled calibration of the method. In addition, numerical calculations have been performed to simulate the implantation of deuterium with monoenergetic and Maxwellian energy distributions into amorphous silicon. The deuterium energy was estimated from the saturation level deuterium, from radiation defects and also from the implantation depth. The deuterium fluxes were estimated The parallel energy of the D+-ions in the SOL of T-10 is found to be 50-80 eV part of the trapping curve. higher than the measured electron temperature at the same radius. Longitudinal fluxes of (0.5-2.0) X 10’7cm-2

of the trapped from the linear which is much SC’ have been measured. The energy of the deuterium neutrals was measured to be 70-100 eV and their flux in the range (2 - 3) x 1Or6 cm-* S-l.

2. Analysis techniques

1. Intruduction

This paper describes further development of the method of collecting %-probes used previously on TM-4 and T-10 [l] for determination of the fluxes and energy of deuterium ions and neutrals in the SOL-plasma. Computer simulations of the deuterium trapping profiles in silicon were made for low energies together with calibration measurements of the accumulation and trapping of deuterium and radiation defects in Si(lll)-samples. The latter were performed using monoenergetic Dz ions with energy E, = 25-1000 eV and fluence a,, = 10’5-10’g cm-*. Analysis of the Si-probes after exposure was performed by using reflected high energy electron diffraction (RHEED) and by secondary ion mass-spectroscopy (SIMS). 0022-3115/90/$03.50

0 1990 - Elsevier Science Publishers

2.1. Amorphiration The RHEED method allowed us to determine the value of amorphization in the Si(lll)-probes by comparison of the intensity of Bragg reflections from exposed and unexposed samples [2]. For the a-Si/c-Si system a mass thickness of the amorphized layer in the range 0.2-3.0 r&g/cm* was measured, corresponding to a linear thickness of l-15 nm. The intensity of Bragg reflections decreased and the background intensity increased after exposure. The probe surface was covered initially by an amorphous SiO, oxide layer of thickness l-l.5 nm. RHEED measurements showed that the first amorphized layers were formed near the a-SiO,/c-Si

B.V. (North-Holland)

S.A. Grashin et al. / Collection Si-probes for investigation

ENERGY

I5

OF IONS,eV

10'6

o

417

ld'g

10'6 FLUENCL,

D/cm'

Fig. 1. The number of atomic displacements in Si (psi) versus total fluence @o for different incident ion energies, E, (eV). 1: 25, 2: 40; 3: 100 and for T-10: (0). Top: The saturation level versus ion energy. Crosses (+) data from ref. 131.

boundary and then became thicker when the fluence increased. Using the thickness of the layer and the atomic weight we can calculate the number of the displaced atoms (Qsi) in it. To connect the amorphization of the Si-probe surface with the energy and fluence of the incident deuterium, calibration experiments were fulfilled (fig. 1). For fixed ion energy, the Qsi value reaches a saturation level which is independent on fluence. This level increases sharply near the threshold energy for the production of atomic displacement (E, = 80-100 ev). As well as in [3] the amorphization of silicon in our experiments occurred even if the energy of the incident ions was lower than the threshold energy. This can be explained by the presence of the impurities in the incident DC -ion beam or by a decrease of the threshold energy near the SiO,/Si boundary. By plotting the Qsi value for Siprobes after different exposure (l-10 shots) in the tokamak and comparing it with reference curves (fig. 1) one can estimate the energy and fluxes of the incident deuterium. An example of such a comparison is shown in fig. 1 (open squares). In this case, the energy of the incident deuterium is estimated to be 80-100 eV and the fluence for one shot (051.0) X 10” cme2. 2.2. Trapping It should be noted that RHEED gives an upper limit for the deuterium energy and flux due to the increased number of atomic displacements caused by impurities.

of deuterium fluxes

1023

Therefore, in parallel with these structure investigations, the amount of trapped deuterium in the Si probes was measured. Such measurements give an independent estimation of the deuterium energy and fluxes. The deuterium content was determined by SIMS using the signal of negative D--ions sputtered by the argon ion beam. An absolute calibration was achieved by comparison of the area under the SIMS profile, with the total deuterium content determined by thermodesorption. In our calibration experiments using monoenergetic ion beams, the deuterium content in our Si-probes, Q,, reached saturation level (fig. 2). This level increased with the incident ion energy (fig. 2, upper part). At room temperature, both the total content and the profile of trapped deuterium remain unchanged for several months after exposure. Thermodesorption of the deuterium from the exposed samples was observed at temperatures higher then 500 K. Our data for the D-content in Si are in a good agreement with measurements in [4]. Again, by exposing the collecting Si-probes in T-10 at different fluences and comparing the trapping dependence with that from the calibration experiments (fig. 2), we can determine the mean energy using the satura-

ENERCYOFIONS,CV

NUMBER

OFSHOTS

IN

FLUENCE,

J-10

D/cm*

Fig. 2. The total content of deuterium in the Si-probe surfaces as a function of fluence for different energies E, (eV). 1: 40; 2: 80; 3: 500; 4: 1000 and for T-10: (0). Top: The energy dependence of the saturation level.

1024

S.A. Grashin et al. / Collection S-probes for rnvestigation of deuterium fluxes

tion level and estimate the fluence using the linear part of the curve. Such a comparison (open squares on fig. 2) gives a value of the mean deuterium energy 70-80 eV and (0.5-1.0) x 10” cm2 for the fluence during one shot. 2.3. Depth

distributions

A third method for estimation of the deuterium energy is based on SIMS analysis of the depth distribution of deutetium implanted in the Si-probes. Numerical calculations for comparison were performed using the code described in [S] for the implantation of deuterium with monoenergetic and Maxwellian energy distributions into amorphous silicon. Fig. 3 shows that for energies in the range 20-200 eV, the implantation depth is 2-20 nm according to these calculations. The experiments revealed a significant influence of the sputtering argon beam on the profile of secondary D--ions formed during the SIMS analysis (fig. 4). Increasing the argon ion energy leads to an increase in the surface concentration of deuterium, shifts the maximum toward the surface and produces the long “tail” in the depth distribution. The deformation of the trapped atom profile is due to the reemission of deuterium from the

8

16

24

32

Depth, nm Fig. 3. Calculated profiles of deuterium implanted in amorphous silicon for different energies (eV) of incident ions. Monoenergetic ion beam, normal incidence. Fluence Cp, = 10” cmW2.

Depth, nm

-I 4

8Q

12

f6

20

24 Depth, nm

Fig. 4. SIMS-profiles of exposed %-probes (T-10, 2 shots, r = 34 cm) for different energies of the sputtering argon beam EAr (keV). 1: 4; 2: 2; 3: 1. Top: reconstructed profiles of the implanted deuterium.

region of the argon beam penetration and to some deuterium atoms being forced into deeper layers. The first effect have been taken into account in a procedure which allows us to reconstruct the real depth profile of the implanted deuterium from the D- signal intensity (fig. 4, top). One can see that for an argon beam energy EA, = 1 keV, the reconstructed profile of the secondary D--ions is very close to the uncorrected SIMS depth distribution. Using 1 keV argon ions we have a depth resolution during the SIMS analysis of l-2 nm. This value determines a lower limit on the energy of the implanted deuterium which can be estimated from these profile measurements; i.e. 40-50 eV. Comparing the calculated model (for a monoenergetic beam) and experimental (T-10) profiles (fig. 5) one can see that the energy of the D+-ions parallel to the magnetic field in the T-10 scrape-off-layer (r = 36 cm) is about 80 eV. This is in a good agreement with the energy estimated from the accumulation of deuterium and from the production of radiation defects. It is important to note that when the fluence is low (< 3 x 1016 cmW2) and the amorphization of the Si lattice not well pronounced, the deuterium can migrate after losing its energy. It can do so until it is trapped by an intrinsic defect in silicon - the concentration of such defects is usually higher near the SiO,/Si boundary. In this case the profile of implanted deuterium is not

S.A. Grashin et al. / Collection Si-probes for investigation of deuterium fluxes

0

2

4

6

6

1025

IO

Depth, nm Fig. 5. The profiles of trapped deuterium in Si. 1: Monoenergetic incident ion beam (E, = 80eV, 9, = 2.3 x lOI cmm2), 2: calculated depth profile for monoenergetic ions (E, = 80 eV, a,, = 1016 cme2), 3: exposure in T-10 (2 shots, r = 36 cm, i-side), argon beam energy E,,= 1 keV.

matched to the calculated projected range of deuterons. When the fluence is high enough (> 10” cm-‘) deuterium is trapped in amorphous silicon were its mobility is low. Only in this case can the energy of the incident deuterium be reliably deduced from the implantation profiles.

3. Implementation

in T-10

Experiments were performed during reproducible ohmic discharges in T-10 with Bt = 2-2.5 T, ZP = 200 kA, Fi, = (2-2.5) x 1013 cme3, a, = 30-32 cm and for both directions of the toroidal magnetic field. The geometry of experiment is shown on fig. 6. The Si-probes were placed on a special movable holder (both at liner potential) and exposed at different radial positions. It

Table 1 The number of atomic displacements and 5, r = 36 cm, B, t 1 It,)

Fig. 6. Geometry of the experiment in the T-10 chamber. The construction of the collimators for neutral flux measurements is shown.

was possible to measure both the parallel and perpendicular ion fluxes (with open probes) or the neutral fluxes (with collimating probes). Total exposure times ranged from 1, 2, 3 and 10 discharges. Some results of the probe analysis are given in tables 1 and 2 for different probe positions and varying exposure time. From the data one can estimate the fluxes and the energy of deuterium arriving at the probes. For r = 36 cm, the parallel energy is in the range 50-80 eV for both the ion and electron sides (the electron temperature was 7-10 eV at the same radius). SIMS-profiles of the implanted deuterium for different exposure times

Qsi and trapped deuterium atoms Qo for probes registering the toroidal

ion flux (position

Number of shots

Ion side Qsi( x 10” cmv2)

Q,( x 10” cmm2)

Qsi( x lOI cme2)

Q,( x 1015 cmW2)

1 2 3 10

15 20 8 32

16 12 23 10

10 18 7 31

12 20 20 21

Electron side

Table 2 The same as in the table 1 for probes registering the neutral flux (position

11 and 12, r = 34 cm)

Number of shots

Ion side

Electron side

Qsi(x10”cm-2)

Q,( x 10” cm-*)

Qsi( X 10” cmM2)

Qo(

3

18

13

18

34

x

10” cme2)

2

S.A. Grashin et al. / Collection St-probes for tnvestigatton of deutrrrum

1026

jluxes

to monoenergetic DC-ions (ED = 20-1000 eV, a,, = 10’5-10’g cm-‘) have shown that the accumulation of deuterium and radiation defects in the samples reach a saturation level which depends only on the incident ion energy. Thus, it is possible to estimate the energy of the incident deuterium from measurements of the trapped deuterium (by SIMS) and radiation damage (by RHEED) if the fluence is sufficient to cause saturation. It is also possible to determine the incident deuterium fluxes from the linear part of the trapping curves. The deuterium (both ion and neutral) energy can also be estimated from the implantation depth of the deuterium by comparing calculated and calibrated depth profiles. Reconstruction of the actual depth profiles from the secondary ion signal during SIMS allowed us to reduce the lower limit of energy which can be determined from the profile measurements to 40 eV. The parallel energy of D+-ions in the T-10 SOL is measured to be 50-80 eV which is much greater than the electron temperature at the same radius (7 -+ 10 eV). The fluxes parallel to B, (0.5-2.0) X 10’7cm-2 s-’ are close to those measured with Langmuir probes. The energy of D-neutrals is estimated as 70-100 eV at a flux of (2-3) x lOI cme2 s-‘, posed

0

2

4

6

8 Depth,nm

Fig. 7. Profiles of implanted deuterium for different exposure times in T-10 (r = 36 cm). 1: 1 shot, 2: 3 shots, 3: 10 shots.

given in fig. 7 show an average implantation depth of 1.5-2.0 nm from which the energy of the incident deuterium may be estimated as 50-70 eV. The parallel flux was (l-1.5) x 10’7cm-2 s-’ (r = 36 cm) for B, antiparallel to Zr and (0.5-1.0) X 10” cme2 S -’ for reversed toroidal field. The observed increase of the flux when B, is reversed agrees with the increases in ne and T, measured by the Langmuir probes. After 10 shots, both Qsi and Q, are saturated. With B, parallel to I,, a strong asymmetry of the deuterium flux on the ion and electron sides is observed and can be explained by increased plasma transport at the outside midplane [6]. For r = 34 cm, the neutral flux from the plasma (table 2) a,, = (2-3) x lOi cme2 s-’ and the neutral energy is in the range Eg = 70-100 eV. We note that the amount of carbon (the main impurity in T-10) on the probe surface did not exceed l-2 monolayers even after exposure to 10 discharges. Thus, we are confident that impurities deposited on the probes do not greatly influence our estimates of deuterium energy and fluxes.

References [l] S.A. Grashin et al., in: Proc. 13th Europ. Conf. on Controlled Fusion and Plasma Physics, Schliersee, FRG, 1986,

Part 1, p. 423. [2] M.A. Lomidze, [3]

4. Conclusions Measurements with Si collector probes exposed in the tokamak SOL allow estimates of the energy and fluxes of deuterium ions and neutrals. Calibration experiments in which Si-samples are ex-

[4] [5]

[6]

A.E. Gorodetsky and A.P. Zakharov, Poverkhnost 3 (1989) 55 (in Russian). D. Hildebrandt, H. Strusny and R. Groetzschel, Phys. Status Solidi A85 (1984) 35. S.A. Cohen and G.M. McCracken, Report PPPL-1529, Princeton Laboratory (1979). A.F. Burenkov, M.A. Kumakhov, F.F. Komarov and M.M. Temkin, Tables of Ion implantation spatial distribution (Gordon and Breach, New York, London, 1986) p. 465. V.A. Vershkov et al.. J. Nucl. Mater. 145-147 (1987) 611.