Assessment for erosion of and impurity deposition on first mirrors in a fusion reactor

Abstract

Hot atoms with chaotically directed velocities are generated by charge-exchange with plasma ions of neutrals recycling from the vessel wall of a fusion reactor. Some of them flee into openings in the vessel made for ducts guiding to diagnostic installations; in particular, to first mirrors for optical observations. On the one hand, hot atoms, hitting the mirror directly, can erode its surface. On the other hand, impurity species, released from the walls of the vessel and of the diagnostic duct, migrate to the mirror and can be deposited there. Both the erosion of and impurity deposition on the mirror decline its reflection properties. Models elaborated to describe processes above are outlined, including a 2D kinetic description for neutral species in the vicinity of the duct opening, an assessment for the erosion of the duct walls and mirror surface by hot atoms, estimates for influxes of the wall material into the duct and a consideration of the migration of impurity atoms along the duct. Calculations are done for the conditions predicted for a fusion reactor like DEMO. The rates for erosion of and impurity deposition on first mirrors of Mo are assessed versus input parameters such as the duct radius, the distance from the opening to the mirror, the density ng of the working gas in the duct, the probabilities for impurity sticking to the duct wall and mirror surface. It is demonstrated that, by ng exceeding a level of m−3, the mirror sputtering can be reduced to the target level of 1 nm per full power year. Moreover, for long enough ducts the erosion rate of impurities deposited onto the mirror exceeds their deposition one and no formation of impurity precipitations on the mirror surface has to be expected.

Preliminary study of a visible, high spatial resolution spectrometer for DEMO divertor survey

“…Exhaust plasma divertor control represents one of the most important challenges for DEMO D&C. The implementation of a robust high-resolution spectroscopic system for parallel divertor plasma observation has been introduced and a preliminary optical design concept based on plane and toroidal mirrors proposed, aimed to measure the plasma radiation emission in the NUV/VIS range, following the evolution of the enhanced high Balmer line intensities and Stark broadening as indicators of high density plasma (plasma detachment) [1,2]. The system presented for the observation of the whole divertor region is a combination of three parallel imaging optical systems with demagnification factor around ≈ 20 in poloidal orientation, protected from neutrons within labyrinth/shielded paths (ducts) with a gas target density n G = 3e19m −3 [8]. The location of the mirrors has been parameterized to simplify and optimize the space required.…”

“…In view of the unprecedented high levels of neutron and gamma fluxes, and fluences expected, together with high energy charge-exchange (c-x) atoms penetrating into diagnostic ports in DEMO [8], the location of any optical component considering the geometrical constrains at close proximity to the plasma will have a strong impact in terms of durability and imaging quality, determining its applicability for DEMO control. The focus of this paper is on the preliminary optical design of a high-resolution visible (VIS) spectrometer system for divertor plasma detachment detection via measurements of Balmer strong intensity emission lines and Stark broadening from hydrogen isotopes.…”

Comparison of electron temperature and density measured by helium line intensity ratio and Thomson scattering methods in ECH spherical tokamak plasma

“…In tokamak fusion DEMO reactors under conceptual design [1], the erosion of diagnostic first mirrors by impingements of high-energy charge exchange atoms [2] is believed to become a significant issue accompanying the increase in discharge * Author to whom any correspondence should be addressed. duration [3,4].…”

Three-dimensional simulation of neutral transport in gases and weakly ionized plasmas

“…Capacitively coupled RF discharge has unique combination of comparative hardware simplicity and ability to remove both conductive and dielectric deposits [6]. The baseline cleaning scenario is a cleaning discharge in noble gases where the physical sputtering [7] is the main mechanism for the diagnostic mirror cleaning [8][9][10][11][12]. One of the most important aspect of the RF plasma cleaning, usually left beyond the scope of detail considerations, is transport and re-deposition of the sputtered materials affecting dramatically the cleaning efficiency and homogeneity.…”

“…Recent assessments of erosion and impurity deposition in remote and shaded areas of fusion reactor performed in [13] and [14] have some limitations due to simplified neutralneutral collision models. Simulation presented in [10] assumes negligibly low neutral density and are basically concentrated on surface processes such as cascade sputtering/re-deposition without collisions with neutral gas. While this assumption is valid for transport in upper and equatorial ports during main plasma operation, there are situations (e.g.…”