Two-dimensional edge plasma structures of the l = 2 helical system CHS have been investigated with the use of a neutral lithium beam probe. Two different types of edge magnetic configuration have been compared, namely, an inboard-wall limiter configuration and an ergodic magnetic divertor configuration which is intrinsic to nonaxisymmetric helical devices. The scrape-off plasma in the limiter configuration is essentially one-dimensional and the radial scale length is well described by a simple diffusion model with finite connection length of the magnetic field line. While in the ergodic divertor configuration, edge plasma structure shows up-down asymmetry in the steady state near the outer separatrix region regardless of the symmetric magnetic structure. The asymmetry reverses when the magnetic field direction is reversed. Plasma shift is apparently in the direction of ion B × ∇B drift. The result suggests that plasma equilibrium in the ergodic layer is not determined simply by the magnetic field structure. The effect of plasma flow and drift motions would be important. Such asymmetry in plasma structure may induce nonuniform heat deposition on the divertor plates. Modelling of plasma equilibrium in the ergodic layer is necessary for practical helical divertor design.
Stylus tip reconstruction is imperative in tracing and calibration of micro and nano surface roughness measurement either for surface roughness analyzer or for scanning probe microscopy. This research is to investigate the size effects on stylus tip reconstruction in micro and nano roughness measurement. Aspect ratios within and between tips and gages, such as Tip Aspect Ratio (TAR) of tip width to height, Gage Aspect Ratio (GAR), Height Aspect Ratio (HAR) of tip height to gage height, and Width Aspect Ratio (WAR) have been formulated to develop a stylus tip reconstruction method (STRM) to estimate tip profile from the measured profile image and the traced gage profile. A simulated program has been used to test the developed STRM with different aspect ratios of tips and gages. Experiments have been conducted on a Hommelwerke T4000 surface roughness analyzer with a TKL T100 tip of radius 5 µm and a Veeco Dektak 200 surface roughness analyzer with nominal radius values of stylus tip radius 12.5 µm to measure a traced roughness gage (Mitutoyo Serial No. 0300042) of step height of 10 µm and razor blade in ISO 5436 standard. Experimental results show that the difference of STRM on step gage and razor blade measurement is about 4 % and the developed STRM can be further used to estimate the geometric size effects of tip reconstruction in scanning probe microscopy (SPM).
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