1. Comment for bubble formation in mirrors by He bombardment (Action Item: 09a213)
Presented by T. Sugie (ITER-IT)
N. Yoshida (Kyushu University, Japan)
S. Takamura (Nagoya University, Japan)
Tenth Meeting of the ITPA Topical Group on Diagnostics
Moscow, 10 – 14 April 2006

2. At the 9th ITPA Meeting (Deaejeon, Oct., 2005)
Prof. Yoshida:
Effects of He Bombardments on Surface Morphology and Optical Reflectivity
Prof. Takamura:
Effects of Helium Irradiation on Tungsten Optical Mirror at Moderate Surface
Temperature
It was shown that bubble formation in the mirrors were observed by helium irradiation and the damage is much serious than that of hydrogen isotopes.
T. Sugie for Dr. Kukushkin:
Estimation of the particle fluxes at the diagnostic mirror locations for ITER
It was required to make the effect clear in more ITER relevant conditions.
< Action Item >
09a213: Yoshida/Takamura/Sugie to quantitatively address bubble formation in mirrors by He bombardment in more ITER relevant conditions (i.e. using fluences at relevant mirror locations rather than first wall fluences).

3. Positions for Particle Flux Calculation
and the He-Flux vs Average energy
At the mirror position:
- Averaged energy of helium atom is very low (<1 eV)
- Flux: 1017 – 1021 m-2s-1 7 2 3 12 4 10 1 13
5, 11 8
6, 9
14, 15 (in dome of divertor) He
at mirror
at first wall
from Dr. Kukushkin’s calculation

4. Expected He-Flux and Fluence (ITER life is assumed 1x107 s)
from Dr. Kukushkin’s calculation
Position 7
Flux
(m-2s-1)
Fluence
(m-2)
Average
energy (eV) He
1E+16
1E+23 10
He+
3E+16
3E+23 50
He2+
1E+18
1E+25 80
Position 8 (solid angle: 0.1 sr)
Flux
(m-2s-1)
Fluence
(m-2)
Average energy (eV) He
4E+18
4E+25
0.5
Position 12
Flux
(m-2s-1)
Fluence
(m-2)
Average
energy (eV) He
2E+16
2E+23 3
He+
1E+16
1E+23 50
He2+
7E+17
7E+24
Position 14,15
(solid angle:
0.02 sr)
Flux
(m-2s-1)
Fluence
(m-2)
Average
energy (eV) He
1E+21
1E+28
0.1
Position 13 (solid angle: 0.15 sr)
Flux
(m-2s-1)
Fluence
(m-2)
Average energy (eV) He
1E+18
1E+25 1

5. Recent Experimental Result and Discussion
< from irradiation experiment >
From the helium irradiation experiments on the W-mirror, it is expected that some degradation (bubble formation) of the mirror surface will occur if the particle energy is larger than 5 eV and the fluence is larger than 1022 m-2. (by Prof. Takamura)
Bubble formation was observed even at the irradiation condition of the energy of 250 eV and the fluence of 51020 m-2 for W-mirror. (by Prof. Yoshida)
< from Calculation >
Expected energy and fluence of helium particles at the mirror:
- Averaged energy: <1 eV, Fluence: 1024 – 1028 m-2
< Discussion >
It seems that no bubble formation of the mirror surface is expected in the ITER because the particle averaged energy is smaller than 5 eV.
However, it is important to consider the energy distribution of the helium particles.
If the fluence of the particles with the energy > 5 eV is larger than 1022 m-2, bubbles will be formed.
More study is needed.