Some problems of the material choice for the first mirrors of plasma diagnostics in a fusion reactor

Abstract

We present the results of simulation experiments on the effect of the fusion reactor environment on the optical properties of the first mirrors for spectroscopy and laser diagnostics. We found the greatest effect on the degradation of mirror optical properties was due to charge exchange atoms. These atoms can affect mirror quality in two ways: by sputtering and by redeposition of material sputtered from other inner components of the vacuum chamber. The degradation rates of mirrors made of different metals and subjected to long-term bombardment by ions from hydrogen or deuterium plasmas are compared. From analysis of all data, we concluded that special experiments will have to be conducted in order to make a correct choice of a first mirror material. These tests should include the following: (1) study the role of swelling on mirror surface modification; (2) find the multimillion shot laser-induced-damage threshold of metal mirrors; and (3) develop and test methods to protect the first mirrors from redeposition. In addition, a model should be developed for the charge exchange atom flux and energy distribution that represents real data from operating tokamaks.

Development of surface relief on polycrystalline metals due to sputtering

“…Here, the requirement for mirrors with line-ofsight views of the plasma is that the characteristic scale length of surface roughness developing due to sputtering by charge exchange atoms should be significantly less than the wavelength of the light being analyzed. It had been proposed [4] that the use of thin film mirrors or nanocrystalline mirrors with grain sizes much smaller than the required wavelength may be suitable for such applications. For example, for visible or near-infrared ranges of the spectrum mirrors with crystallite sizes ≤ 70 nm looked like could be acceptable.…”

“…The relief can be characterized with modern microscopy methods as optical interference microscopy, confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The longitudinal size of the developed roughness, i.e., the scale length of the step features, is determined by the grain size, and it would be expected that the scale length for the roughness would be similar to the grain size, as suggested in [4]. However, in experiments with Rh film mirrors [5] it was found that the sputtering of very fine-grain mirrors led to surface roughness with a longitudinal scale noticeably exceeding the grain size.…”

Experimental Simulation of the Behaviour of Diagnostic First Mirrors Fabricated of Different Metals for ITER Conditions

“…Generally, such films have a fine-crystalline structure (≤100 nm). Therefore, it was expected [22] that the scale of microrelief, which appears after long-term sputtering may be appreciably shorter than the light wavelength range for which the mirror is applied.…”

Effect of sputtering on self-damaged recrystallized W mirror specimens

“…Previously such effect was examined for copper and stainless steel mirrors [1,2] irradiated with high energy (1-3 MeV) Cu + and Cr + ions for simulating neutron irradiation. It was found that degradation of these mirrors under long term sputtering with hydrogen plasma occurs with approximately similar rate as that for the mirrors not preliminary bombarded with high energy metal ions.…”