Molybdenum limiters for Extrap-T2 upgrade: surface properties and high heat flux testing

A vacuum vessel, a conductive copper shell and plasma-facing components of the Extrap-T2 device, a medium size reversed field pinch, are under major rebuild. The new machine is equipped with an array of 180 molybdenum limiters, which will be exposed to power loads of about 30 MW m−2, but higher loads can not be excluded. Prior to the limiters’ installation in the vessel, they were tested under high heat loads in the JUDITH electron beam facility in order to assess the possible damage to the surface and to the bulk of the material. A test limiter was irradiated with a beam of increasing power density from 15 to 1500 MW m−2. Surface characterization was performed before and after the irradiation using electron and optical microscopy, energy dispersive X-ray spectroscopy, enhanced proton scattering and laser profilometry. Metallography studies were performed for the irradiated areas. The irradiation induced the change in surface morphology, e.g. surface melting and re-crystallization of grains, only following the 10 ms long pulses with the absorbed power density approaching 1500 MW m−2.