Advanced Laser Diagnostics for Electron Density Measurements

This paper describes innovative laser diagnostics under development at the National Institute for Fusion Science, aiming for the establishment of reliable density measurement techniques in the next step magneticallyconfined fusion devices. There are two approaches, interferometry and polarimetry. A new type of two color laser (47.6/57.2-μm CH3OD) interferometer has been developed and its original function, vibration subtraction,was confirmed in a test stand. The line integrated density measurement at Compact Helical System by using the polarimeter based on Cotton-Mouton effect was demonstrated by the use of a 337-μm HCN laser source

Edgewise Bending Strain in Helical Coils With Geodesic Windings Based on Virial Theorem

Distributions of edgewise bending strain in helical coils with the geodesic winding based on virial theorem are analyzed theoretically and numerically. A force-balanced coil (FBC) is a multipole helical coil combining toroidal field (TF) coils and a solenoid helically wound on a torus. The combination reduces the net electromagnetic force in the direction of the major radius by canceling out the centering force due to the TF coil current and the hoop force due to the solenoid current. The FBC concept was extended using the virial theorem, which shows the theoretical lower limit of stress in the coils and their supporting structure. High-field coils should accordingly have the same averaged principal stresses in all directions, which is named the virial-limit condition. Since FBC winding is modulated to reduce the tilting force, the winding is slightly similar to but different from the shortest geodesic trajectory and has no tensile load. To apply FBC to high-temperature superconducting tapes, the degradation of superconducting properties originating from edgewise bending strain is an important problem. Since the geodesic trajectory is a kind of a straight line on a curved surface and curves only to the normal direction of the surface, it is expected that the tape with geodesic trajectories has a small residual stress. In this paper, we analyze the effect of the winding modulations including the geodesic modulation for the optimization of residual stress in helical windings

Optics Design for Microwave Imaging Reflectometry in LHD

An optics system for microwave imaging reflectometry (MIR) in the Large Helical Device (LHD) was newly developed to optimize the performance of the two-dimensional microwave receiver array. Reflected microwaves from the plasma and the first local oscillator (LO) wave are transmitted to the receiver array via the optics from the front. Finite-difference time-domain (FDTD) calculation was used to design the ellipsoidal or hyperboloidal shapes of the quasi-optical mirrors. It is confirmed that the LO beam in the constructed system covers the receiver antenna aperture area as intended. The S/N ratios of the signals are improved with this optimized optics system from those in the previous system

Force Balance and Stability of Toroidal Helical Coil with Circular Cross Section * )

Stability and force equilibrium of toroidal helical coil with circular cross section were investigated. In this paper, we derived and calculated equations for equilibrium of toroidal helical coil based on an axisymmetric surface current model. The helical coil, which is modulated in such a way that a magnetic surface coincides with the coil surface, can reduce overturning force generated by electromagnetic forces. We analyzed the stability of the toroidally modulated helical coil and confirmed that this modulated configuration is held in equilibrium while it is unstable. This condition means the minimum inductance condition. Moreover, we formulated the relationships between electromagnetic forces and stresses acting on toroidal helical coil with circular cross section, and we compared the stress distributions calculated by the equations with those from finite element method (FEM) analysis