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Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (O.Motojima et al., Nucl. Fusion 47 (2007. The beta value reached 5 % and a high beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes also have been extended. The central density has exceeded 1.0×10 21 m -3 due to the formation of an Internal Diffusion Barrier (IDB). The ion temperature has reached 6.8 keV at the density of 2×10 19 m -3 , which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

The voltage-temperature characteristic curve (V-T curve) observed in the large-current Nb 3 Sn CIC conductor, which was used in the ITER CS Insert, showed more gradual take-off toward normal state than that of an individual strand composing the conductor. More gradual take-off corresponds to a reduction in so-called "n-value", and measured n-values of the strand and conductor of the CS Insert were 30 and 7, respectively. This reduction cannot be explained by a tensile strain of the conductor caused by a hoop deformation which is uniform along the conductor length. Investigation is therefore required to clarify the strain states of each strand, especially those caused by a transverse electromagnetic force acting on each strand. In a CIC conductor, since strands are twisted to form a cable, each strand is mechanically supported by nearby strands at an interval (typically 5 mm) set by the twist pitch. Between two supporting points, the strand is free to move under the transverse force and a cyclic deformation may occur along the strand length. This deformation will produce nonuniform bending strain along the strand. In order to verify the above consideration and to quantitatively evaluate the effect of this deformation, we prepared an apparatus to simulate this cyclic deformation by artificially applying a transverse load on the strand and its V-T characteristic was measured. When the strand received the transverse force of 10 3 10 4 N m (which is expected value for a strand of the CS Insert operated at 13 T, 46 kA), n-value reduced to less than 15 from the original value of 30, which agreed to the phenomena observed in the CS Insert. This indicates that the transverse force acting on each strand causes the reduction in n-value of the CIC conductor.

The world's largest pulsed superconducting coil was successfully tested by charging up to 13 T and 46 kA with a stored energy of 640 MJ. The ITER central solenoid (CS) model coil and CS insert coil were developed and fabricated through an international collaboration, and their cooldown and charging tests were successfully carried out by international test and operation teams. In pulsed charging tests, where the original goal was 0.4 T/s up to 13 T, the CS model coil and the CS insert coil achieved ramp rates to 13 T of 0.6 T/s and 1.2 T/s, respectively. In addition, the CS insert coil was charged and discharged 10 003 times in the 13 T background field of the CS model coil and no degradation of the operational temperature margin directly coming from this cyclic operation was observed. These test results fulfilled all the goals of CS model coil development by confirming the validity of the engineering design and demonstrating that the ITER coils can now be constructed with confidence.