Formation process of tungsten nanostructure by the exposure to helium plasma under fusion relevant plasma conditions

Abstract

Helium irradiation on tungsten changes the surface morphology dramatically by forming a nanometre-sized fibreform structure which could bring about serious problems for fusion reactors. From the experimental results in liner divertor simulators, it is revealed that the incident ion energy and surface temperature are key parameters for the formation of the structure. It is shown that the tungsten nanostructure is easily formed when the temperature is in the range 1000–2000 K, and the incident ion energy is higher than 20 eV. Furthermore, on the basis of the helium irradiation experiments performed in the divertor simulator NAGDIS-I, the initial formation process of the nanostructure is revealed. It is shown that the nanostructure formation is related to pinholes appearing on the bulk part of the material, and then, the rough structure develops to a much finer nanostructure. The nanostructure was also observed on the molybdenum surface that was exposed to the helium plasma. It increases interest in the possibility that nanostructure formation by helium irradiation is a common phenomenon that occurs on various metals.

Tungsten erosion under combined hydrogen/helium high heat flux loading

“…Synergistic effects of simultaneous irradiation of tungsten surfaces with He particles and high heat flux were observed for instance in [1,2]. The formation of surface and near-surface features with typical dimensions in the range of nanometres to micrometres under irradiation with He or H/He mixtures has been reported for tungsten from a number of plasma or high heat flux experiments [3,4,5]. The resulting morphology after irradiation with a H/He mixture is distinctly different form the morphology after irradiation with the pure species.…”

“…In the following we investigate two of these possible causes. The most obvious is the formation of a distorted surface layer, as shown in several experiments (see references [1][2][3][4][5][6][7][8][9][10][11]), which is investigated as a function of fluence and temperature in section 3.2.1. The second is the dependence of the sputtering yield on the crystal orientation.…”

Pulsation Effects of Incident Ion Energy on W Fuzz Growth

“…An issue to be investigated further is whether such morphology changes will actually occur on plasma facing materials in fusion devices. It is known that the growth requires the surface temperature range of 1000 -2000 K and the incident ion energy, E i , of greater than 20 -30 eV [3]. Thus, detached plasmas, in which the temperature is lower than several eV [4], are thought to have an inhibiting effect on the growth of the fuzzy structure, because E i could be lower than the threshold energy.…”

Plasma Device at Nru «mpei» for Testing of Refractory Metals and Creation of Highly Porous Materials of New Generation

“…Такая структура пуха имеет ряд положитель-ных качеств, таких как пониженный коэффициент распыления и большое тепловое излучение, прак-тически как у абсолютно чёрного тела, снижаю-щее температуру поверхности при тепловых на-грузках и повышающее стойкость поверхности к тепловым нагрузкам [8]. Поверхность со структу-рой пуха обладает большой удельной площадью, что имеет большое значение для адсорбции газов и катализа.…”