Regression analyses have been carried out for the international stellarator database which includes 859 discharges from the medium-sized helical devices ATF, CHS, Heiliotron E, W7-A and W7-AS. Recent results from enhanced confinen1ent regime such as H mode and reheat mode are excluded from the database. Optimum fit of all devices is given by the following expression (International Stellarator Scaling 95, ISS95), No dependence of 'tE on the isotropic mass is indicated in the data set. No distinct difference between ECH and NBI can be diagnosed. Because of the different density ranges in the two heating methods, a possible difference might, however, be hidden in the density scaling properties. The density dependence of 'tE also turns to be more complicated than a simple power law. Figure 1 shows a comparison of all data together with ITER L mode database with the ISS95 expression. Although it is crucial to use the appropriate definition of a and -t in the comparison of stellarators and tokamaks, the ISS95 scaling describes tokamak data in L mode very well. In other words, also, the stellarator and the tokamak L mode are of comparable confmement quality.In Fig.1, the data of heliotron/torsatron devices and shearless stellarator have opposite offsets with respect to the ISS95 scaling. It should be noted that data stored in the database are primarily obtained in each standard operation.Operational modes with better confinen1ent are obtained by means of intense wall conditioning and tailoring the magnetic geometry in each device. The ISS95 scaling should be recognized as an L-mode-like scaling. The ISS95 scaling is based on the selection of the iotadependent scaling for heliotron/torsatron confinen1ent. It was tested whether the choice of the radial position at which the -t value is taken influences the results. Regressions using -t at p = 1/3 or 1 do not, however, qualitatively change the results. If the iota-independent scaling is selected, the offsets reduces to a level similar to that when the LHD-scaling expression is used. The next generation experiments LHD and W7-X will allow to distinguished more clearly between the two scaling expression.The predicted operational regime in LHD is also illustrated in Fig. 1, which suggests that the operational regime of LHD will be close to those of the present large tokamaks in L mode.

Optimization studies have been done for the helical axis heliotron configuration. One purpose is to find a configuration suitable for experimental studies of the basic properties of a helical axis heliotron. In the present study, the role of the bumpy field component (toroidal mirror ratio) in MHD stability and neoclassical confinement for this type of configuration is examined. The physical mechanism of the improvement of the neoclassical transport through control of the bumpy field component is clarified. The physics design and current status of the new helical axis heliotron device, Heliotron J, are also described.

Results obtained in the initial experimental phase of Heliotron J are reported. Electron beam mapping of the magnetic surfaces at a reduced DC magnetic field has revealed that the observed surfaces are in basic agreement with the ones calculated on the basis of the measured ambient field around the device. For 53.2 GHz second harmonic ECH hydrogen plasmas, a fairly wide resonance range for breakdown and heating by the TE02 mode has been observed in Heliotron J as compared with that in Heliotron E. With ECH injection powers up to ≈ 400 kW, diamagnetic stored energies up to ≈ 0.7 kJ were obtained without optimized density control.

Two significant problems that need to be solved for any future fusion device are heat removal and particle control. A very promising method to attack these problems in tokamaks and helical devices is the use of a divertor, providing a controlled interaction zone between plasma and wall. By carefully designing a divertor, conditions can be created in front of the divertor targets, which lead to a sufficient reduction of the power load on the targets by strong radiation redistribution. Any solution of course needs to allow for an energy confinement which is at least sufficient for the realization of a fusion reactor. Since energy confinement has been found to be strongly related to edge anomalous transport and edge plasma profiles, the ultimate aim is to find an integral solution which is optimum with respect to exhaust, heat load and energy confinement.Two different types of divertors are presently being investigated in helical devices: the 'helical divertor' and the 'island divertor'. So far divertor concepts have been investigated only in a few helical devices. Theoretical and experimental efforts have mainly concentrated on the suitability of divertor magnetic field structures, while detailed studies of the divertor plasma properties for the two types of divertor configurations have only recently begun. In the course of this exploration, a promising new high-density H-mode (HDH) plasma operational regime has been discovered on the Wendelstein stellarator W7-AS. It benefits from high-energy (up to twice the value of the International Stellarator Scaling ISS95) and low impurity confinement times, complemented by edge radiated power fractions of up to 90% in detached regimes. This allowed quasisteady-state operation for up to 50 energy confinement times and so far was only constrained by machine operability.

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