[en] This publication is the collection of the paper presented at the title conferences on helical system research. The 131 of the presented papers are indexed individually. (J.P.N.)

[en] Recent advances in tokamak physics can be used in the quest to optimize the stellarator concept. In particular the experimental confirmation of strong bootstrap currents suggests the possibility of taking advantage of non-zero current in steady-state stellarators. In addition the observation of neoclassical islands suggests important issues for design optimization, in order to operate in a regime in which neoclassical effects heal stellarator islands. (author)

[en] The programme of W7-AS concentrates mainly on topics related to the optimisation principles of the WENDELSTEIN stellarator line such as improved equilibria with reduced Shafranov shift, neoclassical transport in improved magnetic configurations including the effects of the ambipolar radial electric field, confinement of high energetic particles, radial diffusion of trapped electrons generated by ECR power deposition. But also topics related to stellarator physics, more generally, are intensely treated, such as the radiative limit at high densities, particle transport, GAE-modes, fluctuations and turbulence, the influence of shear on confinement, and the H-mode. Furthermore new diagnostics with respect to long pulse operation of W7-X are developed. Optimum (neoclassical) confinement discharges with energy confinement times (up to 50 ms) of more than 2.5 times larger than predicted by the International Stellarator Scaling, ISS95, with T_i ≤ 1.5 keV are presented as well as discharges with Te ≅ 4 kev showing a strongly positive ECRH driven 'electron root' feature of the radial electric field and high density high power NBI discharges leading to <β> = 1.8%. A 3D Edge code is developed to model edge transport and to optimise energy and particle deposition in a future island divertor. The role of E x B drifts in islands is considered in detail. Heating scenarios with relevance to W7-X are finally addressed which are independent of cut-off densities and do not necessarily require magnetic resonance conditions, such as the O-X-B heating scenario of ECR waves. The prospects of ICRH for long pulse operation are explored. (author)

[en] A new class of stellarators is found that does not rely on quasi-symmetrization to achieve good confinement. These systems depart from canonical stellarators by allowing a small net plasma current. We have developed an optimization procedure with bounce-averaged omnigeneity and other desirable physical properties as target criteria. This method has been applied to show the existence of a compact plasma device having a small aspect ratio A, high β (ratio of thermal energy to magnetic field energy), and low plasma current. The added degree of design flexibility afforded by the plasma current leads to a potentially attractive low A hybrid device which is stable to magnetohydrodynamic (MHD) ballooning modes for <β> <6%. (author)

[en] The particle confinement characteristics of high Ti mode and L-mode on Compact Helical System (CHS) are studied. Comparison between measured electron density profiles and those from the transport model is done by a fitting analysis of transport model. (author)

[en] Experiments about stability operation region (SOR) on HT-7 and high plasma parameters in Ohmic discharge are performed. Two boundaries of low density limit and Murakami density limit are scanned out through about 80 shots. The Murakami limit M_max=2.96x10¹⁹ m^-2/T and Hugill number H=9.5x10¹⁹ m^-2/T are obtained. On the Murakami boundary, the radiation characters are specially analyzed. Coupling of m=1 and m=2 modes leads impurities from boundary to core, then line radiation of those impurities causes energy quenching inside and near q=1 surface and finally disruption occurs. This kind of disruption restricts Murakami density limit. (author)

[en] Electron cyclotron heating (ECH) above the plasma cutoff density with electron Bernstein waves (EBW) was successfully demonstrated at W7-AS stellarator. The EBW's were generated via O-X-B mode conversion from O-waves to X-waves and, finally, to electron Bernstein waves. Clear evidences for both mode conversion steps were detected and resonant absorption with a narrow profile was demonstrated. (author)

[en] A system of modular stellarator coils is proposed for a quasi-axially symmetric tokamak. This coil system is used for investigations of the free-boundary MHD equilibrium of the plasma configuration. It is demonstrated that the deformation and outward shift of the equilibrium and the deterioration of the quasi-axial symmetry due to finite <β> are compensated with good precision by application of a vertical magnetic field. Finally, a design of auxiliary coils for adjusting ι is presented. (author)

[en] A new reduced set of resistive MHD equations is derived by averaging the full MHD equations on specified flux coordinates, which is consistent with 3D equilibria. It is confirmed that the total energy is conserved and the linearized equations for ideal modes are self-adjoint. (author)

[en] This paper presents an overview of the work on Stellarators in Princeton during the first fifteen years. Particular emphasis is given to the pioneering contributions of the late Lyman Spitzer, Jr. The concepts discussed will include equilibrium, stability, ohmic and radiofrequency plasma heating, plasma purity, and the problems associated with creating a full-scale fusion power plant. Brief descriptions are given of the early Princeton Stellarators: Model A, Model B, Model B-2, Model B-3, Models B-64 and B-65, and Model C, and also of the postulated fusion power plant, Model D. (author)