Energy confinement scaling from the international stellarator database

Abstract

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.

Improved plasma performance on Large Helical Device

“…3 It was raised up to about 2.9 T at the end of the 1999 campaign, which was almost equal to the rated field of 3 T. Hereafter, plasma parameters will be improved by a gradual increase in the heating power, 9 obeying a scaling law, for example, the International Stellarator Scaling 95 ͑ISS95͒, which was derived based on the medium-sized helical devices. 10 However, LHD aims at a remarkable improvement of plasma confinement like the high ͑H͒ mode in tokamaks, 11 which is much better than the present scaling. 10 Therefore, various attempts have started in order to achieve this target and study the related physics.…”

DT Fusion Ignition of LHD-Type Helical Reactor by Joule Heating Associated with Magnetic Axis Shift

“…We assume Z eff = 2 and energy confinement time τ E for the current-carrying helical systems as a hybrid combination of τ IS S 95 [5] and τ IPB98y2 [6] given by…”

Effect of Neoclassical Transport Optimization on Electron Heat Transport in Low-Collisionality LHD Plasmas

“…Figure 3 shows the enhancement factor of the global energy confinement time relative to the ISS95 scaling law [11] as a function of the averaged line density. We can see a clear configuration dependency on the enhancement factors.…”