The Large Helical Device (LHD) now under construction is a heliotron/torsatron device with a closed divertor system. The edge LHD magnetic structure has been studied in detail. A peculiar feature of the configuration is the existence of edge surface layers, a complicated three dimensional magnetic structure which does not, however, seem to hamper the expected divertor functions. Two divertor operational modes are being considered for the LHD experimenthigh density, cold radiative divertor operation as a safe heat removal scheme and high temperature divertor plasma operation. In the latter operation, a divertor plasma with a temperature of a few keV, generated by efficient pumping, is expected to lead to a significant improvement in core plasma confinement. Conceptual designs of the LHD divertor components are under way.

This paper reports results on the progress in steady-state high-βp ELMy H-mode discharges in JT-60U. A fusion triple product, nD(0)τETi(0), of 3.1 × 1020 m−3 s keV under full non-inductive current drive has been achieved at Ip = 1.8 MA, which extends the record value of the fusion triple product under full non-inductive current drive by 50%. A high-beta plasma with βN ∼ 2.7 has been sustained for 7.4 s (∼60τE), with the duration determined only by the facility limits, such as the capability of the poloidal field coils and the upper limit on the duration of injection of neutral beams. Destabilization of neoclassical tearing modes (NTMs) has been avoided with good reproducibility by tailoring the current and pressure profiles. On the other hand, a real-time NTM stabilization system has been developed where detection of the centre of the magnetic island and optimization of the injection angle of the electron cyclotron wave are done in real time. By applying this system, a 3/2 NTM has been completely stabilized in a high-beta region (βp ∼ 1.2, βN ∼ 1.5), and the beta value and confinement enhancement factor have been improved by the stabilization.

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OVERVIEW OF THE LARGE HELICAL DEVICE PROJECT. The Large Helical Device (LHD) has successfully started running plasma confinement experiments after a long construction period of eight years. During the construction and machine commissioning phases, a variety of milestones were attained in fusion engineering which successfully led to the first operation, and the first plasma was ignited on 31 March 1998. Two experimental campaigns are planned in 1998. In the first campaign, the magnetic flux mapping clearly demonstrated a nested structure of magnetic surfaces. The first plasma experiments were conducted with second harmonic 84 and 82.6 GHz ECH at a heating power input of 0.35 MW. The magnetic field was set at 1.5 T in these campaigns so as to accumulate operational experience with the superconducting coils. In the second campaign, auxiliary heating with NBI at 3 MW has been carried out. Averaged electron densities of up to 6 × 10 19 m-3 , central temperatures ranging from 1.4 IAEA-F1-CN-69/OV1/4 2 to 1.5 keV and stored energies of up to 0.22 MJ have been attained despite the fact that the impurity level has not yet been minimized. The obtained scarling of energy confinement time has been found to be consistent with the ISS95 scaling law with some enhancement.

A formulation of the anisotropic pressure magnetohydrodynamic equilibrium problem for three-dimensional plasmas with imposed nested magnetic surfaces is developed based on a bi-Maxwellian model of the distribution function for the energetic particle species. The hot particle distribution function satisfies the constraint . Large parallel and perpendicular anisotropy factors can be explored within the model through the choice of the hot particle perpendicular to parallel temperature ratio T⊥/T‖. A fixed boundary version of the VMEC code has been adapted to numerically compute three-dimensional anisotropic pressure equilibria. Applications to a 10-field period Heliotron device and a 2-field quasiaxisymmetric stellarator demonstrate that the pressures do not vary significantly around the magnetic surfaces when the total parallel pressure p‖ is larger than its perpendicular counterpart p⊥. For off-axis hot particle deposition with p⊥ > p‖, p⊥ concentrates in the region where the energetic particles are generated. On the other hand, p‖ is distributed roughly uniformly around the flux surfaces in the Heliotron but is localized on the low field side in the quasiaxisymmetric machine. The hot particle density structure correlates more closely with the corresponding perpendicular rather than with parallel pressure. The specific form for the definition of β that best correlates with the Shafranov shift is identified.