Performance of the Pyroreflectometry in Magnetic Fusion Devices for Plasma Wall Interaction monitoring

Abstract

ITER (International Thermonuclear Experimental Reactor) is a large-scale scientific experiment that aims to demonstrate that it is possible to produce commercial energy from fusion reactions. Because of their physical and thermal properties, Beryllium and tungsten have been chosen for Plasma Facing Components to cover the interior surfaces of the vacuum vessel. Such metallic environment raises the issue related to reflecting internal surfaces which can disturb the machine protection system based on Infrared Thermography. In this paper we focus on evaluating the performance of the Pyroreflectometry method in reflective environment. 1-MOTIVATIONS Magnetic FusionFusion is the source of energy in the sun and other stars. A star starts to shine when, under the force of gravity, the matter in its very heart attains sufficiently high densities and temperatures to set off thermonuclear reactions, which then release energy. On Earth, it is possible to reproduce these reactions by confining and maintaining plasma at very high temperature; plasma is the fourth state of the matter where electrons are completely detached from the nucleus. This plasma is confined in a torus-shaped vacuum vessel, also called tokamak, created by magnetic fields, described as magnetic confinement. Although different isotopes of light elements can be paired to achieve fusion, the deuterium-tritium (D-T) reaction has been identified as the most efficient for fusion devices. D + T4 He (3.5MeV) + n (14.1 MeV)6 Li + n 4 He + TIt is on this reaction that research on controlled fusion is conducted. The temperatures required for thermonuclear fusion is greater than a hundred million degrees and the density is very low ~ 10 -5 x the air density. The challenge to perform at steady state operation is to exhaust energy created (14.1MeV from neutrons) and ashes (He) without perturbing the core plasma performance. Such constraints explain the importance of the plasma facing components design as well as the crucial need of reliable temperature monitoring.Using tritium and deuterium as fuel has some interesting consequences, for example there is no possibility of chain reaction, and the fuel has to be fed into the plasma continuously in very small precise amounts, in a similar way to a gas burner. The total amount of fuel confined by the plasma at any moment is only sufficient for a few seconds burn if the supply of fuel is terminated. Deuterium can be found in sea water (33gr/m 3 ) which is sufficient for billions of years. Tritium is a fast-decaying radioelement of hydrogen which occurs only in trace quantities in nature. It can be produced during the fusion reaction, when neutrons escaping the plasma interact with lithium contained in the blanket wall of the tokamak. Lithium is plentiful in the Earth's crust and the known reserves of lithium would last for at least one thousand years.