Experimental Characterization of the Electric Sheath at Divertor Surfaces Using Micro-Engineered Targets on DiMES at DIII-D*

We report an experimental determination of the characteristic magnetic pre-sheath (MPS) width, L_MPS, in the divertor of the DIII-D tokamak by performing detailed measurements of the azimuthal and polar ion impact angles using micro-engineered targets. Analytical and numerical studies reported the wide MPS formation for several ion gyro radii, which dominates the sheath potential structure rather than the classical Debye sheath, when the grazing magnetic field (< 5º) enters into the divertor surface under conventional Tokamak and ITER-like plasma parameters [1]. A validation of the theoretical sheath models supports its applicability to ITER and pilot plant divertors. The divertor sheath width is defined by an approximated MPS potential Φ ∝ exp(-2z/L_MPS), where z is the distance from the surface. The sheath potential critically controls the ion trajectory of low-Z species (D, T, He, and C), as well as the prompt re-deposition of high-Z species. Hence, L_MPS is a critical parameter to successfully predict plasma-materials interactions. We exposed specially engineered micro-trenches (30×30×2-4 µm) to L-mode D plasmas via DiMES. Deposition patterns of C impurities resulting from the D ion shadowing effect on the trench floors were measured by energy-dispersive X-ray spectroscopy and compared with a gross erosion calculated by the Monte Carlo micro-patterning and roughness code. The C deposition profiles showed that the erosion was maximized for the azimuthal direction of φ = -40° (referenced to B_T) and polar angle of θ = 80°. The erosion simulation using input of D ion angle distributions calculated for the assumption k = 3 reproduced the experimental C deposition profiles. This result verified a kinetic modeling result, k ~ 3 [1]. [1] D. Coulette, G. Manfredi, Plasma Phys. Control. Fusion 58 (2016) 025008