Surface modification of tungsten mirrors due to low-energy helium plasma irradiation in the compact PWI simulator APSEDAS

Surface modification of a polycrystalline tungsten (W) mirror due to low-energy helium plasma irradiation has been studied in the compact plasma-wall interaction (PWI) simulator APSEDAS. Many fine irregularities of a few mu m are generated on the W surface at T(s) similar to 1400K due to He bubble formation. On the other hand, a distinctive change in the surface morphology cannot be seen from the SEM micrograph on a W mirror exposed at T(s) < 710 K. In both samples, the optical reflectivity with the incident angle theta = 5 degrees decreases more at shorter wavelength. From in situ measurement of the optical reflectivity with theta = 75 degrees, it is found that the optical reflectivity at lambda similar to 555 nm of the low T(s) exposure increased by similar to 6% and decreased to the original level at a fluence of similar to 3 x 10(24) He(+) m(-2), and then increased again by similar to 6% with the fluence up to similar to 1.6 x 10(26) He(+) m(-2). Calculations using a model of the multiple reflection in the modification region suggest that the thickness of the modification layer of the low T(s) sample exceeds 70 nm.