Materials selection for the in situ mirrors of laser diagnostics in fusion devices

Abstract

When mirrors for the laser scattering diagnostic for large fusion devices need to be inside the vacuum chamber, they are subjected to irradiation by multiple high-energy laser pulses and bombardment by charge exchange atoms. Both of these assaults are known to degrade and eventually damage metal laser mirrors given sufficient time and flux. Our aim in this article is to use current data on these damage mechanisms to make design selections of metal mirror materials for application in fusion device diagnostics. We identify tradeoffs between low sputtering rates and multipulse laser damage resistance in candidate metals. The data for multipulse laser damage are incomplete and extend to a maximum of only 10 4 shots for a few metals. However, there is a clear trend of decreasing laser-damage threshold with increasing number of shots, and damage threshold fluences can fall to 10% of the single-pulse damaging laser fluence. Further experiments up to 10 6 or 10 8 laser shots need to be conducted on the likely mirror candidate metals for use in new plasma devices. We define a figure-of-merit based on current laser damage data and employ it in our analysis. Recent data on the sputtering yield and reflectance degradation of metal mirrors give a different priority ranking of candidate metals. Overall, the preferred material selection depends on the number of laser shots and the number of plasma pulses that the mirror must endure before replacement is allowed. For example, we find that for conditions typical of the LHD ͑10 s plasma pulses with a 10 Hz laser PRF͒, Au, Ag, and Cu are candidate materials if mirrors are replaced after 10 3 plasma pulses; Au and Rh are candidates if the replacement interval is 10 4 pulses; and if the replacement interval is further increased to 10 5 plasma pulses, then Mo is the candidate material. Other materials might also be candidates but the data on them are still insufficient.

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

“…During the whole period of ITER operation there will be approximately 10 4 working discharge pulses of 400 s length, so the total number of laser pulses will reach ~10 8 . As was found in many publications (see survey [81]), with recurring laser pulses the threshold for laser mirror destruction, F N , substantially decreases: the more pulses (N) the mirror has to cope with, the lower should be the energy of each pulse in comparison with the threshold for a single laser pulse, F1. The relationship between the two thresholds essentially depends on the mirror material, as was shown, e.g., in [70] Fig.…”

Assessment of Laser Transmission Mirror Materials for ITER Edge Thomson Scattering Diagnostics

“…The temperature dependence of the optical reflectivity is taken into account based on the method proposed by Ujihara [16]. It is noted that the calculation deduced significant temperature dependence, which is sometimes not consistent with the experiments referenced in [1]. Therefore, we use both the calculated and experimentally obtained reflectivity in the calculation; the differences between them are discussed later.…”

“…3, the three types of experimentally deduced LIDT data, i.e., melting, slip deformation, and significant reflectivity change, summarized in Ref. [1] are also plotted. Here, significant reflectivity change corresponds to an indication of damage directly related to a change in reflectivity; it does not clearly differentiate between the reasons for the degradation in optical properties, such as "slip" or "melting."…”

“…And since in-vessel mirrors are exposed to neutron and gamma-ray radiations, as well as to chargeexchange particles from plasmas, metals are considered the most promising candidate materials [1]. The functions of the in-vessel mirrors can be divided into two different categories: (i) mirrors for collecting the emission from the plasma, and (ii) mirrors for the transmission of the laser beam.…”

“…[1], several materials have been proposed for the in-vessel mirror for laser diagnostics and the figure-ofauthor's e-mail: kajita@ees.nagoya-u.ac.jp a) present address: EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan merit has been presented for the material choice. In the present paper, based on this proposal, the discussion focuses on the material choice for the Thomson scattering diagnostics in ITER, particularly the edge Thomson scattering system.…”