Optimization of Helical Plasma Confinement

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Suppression of Pfirsch-Schluter (P-S) current has been investigated in a conventional helical system as one of helical-plasma-optimization procedures.P-S current causes a deformation of magnetic surfaces which results in the deterioration of the energy confinement.Strong inward shift of magnetic axis shift by a vertical field is effective to suppress the P-S current for an 1=2 stellarator with no shear and for an 1=3 heliotron/torsatron with no rotational transform at axis.For an 1=2 heliotron/torsatron with shear and central rotational transform, it is necessary to elongate the magnetic surface vertically by controlling the quadrupole field in addition to the inward shift by controlling the dipole field.New scenario of stellarator optimization, which was based on the 2-dimensional helical system, was shown.This is more general than the W7-X strategy, but is limited near the magnetic axis.In the 2-D configuration, the particle confinement is dramatically improved compared with a conven … More tional helical system which is of 3-D geometry.This idea helps to promote the quasi-toroidal symmetric stellarator which is now being considered as a new optimized helical system.Heavy ion beam probe is successfully operated in CHS,for the first time in helical machines, to measure the electrostatic potential of the plasma.Here, heavy ion beam trajectory is controlled by octopole deflectors both at the inlet and outlet of the beam.The control method of beam trajectory was compared with that of the russian type and there were some comments from russian side to improve our system.Some of them are for reducing UV light effect which causes emission of secondary electrons resulting in the operation limit on the electron density.Electron heating of ICRF in CHS was compared with L-2 results.In CHS reflection at the high field side R cut-off is newly introduced.ICRF wave physics in real machines is in progress to explain experimental results.Future collaboration on stellarator optimization, transport code in the optimized configuration, diamagnetic measurement, electron distribution function in ECH plasma, modular coil design, has been discussed. Less