Speaker
Description
Stellarators intrinsically avoid pulse-length limitations of the magnetic confinement, since the rotational transform is generated by external coils rather than by a plasma current. We report on the development of discharge scenarios that enable long-term stabilization of high-performance plasmas in the optimized stellarator W7-X. For the first time, dimensionless parameters of reactor relevance are achieved within stellarator plasmas sustained for up to 43 s. The attained fusion triple products are comparable to those of JET at similar pulse lengths but at about one third of the plasma volume.
The reported scenarios exploit repetitive pellet fueling combined with second-harmonic electron-cyclotron O-mode heating, which together reliably raise the ion temperature beyond the ion-temperature clamping limit and sustain a quasi-stationary operational cycle at high density and plasma energy. Surprisingly, these plasmas exhibit indications of central fueling. In order to stay within technical heat-load limits, targeted impurity seeding within this PULSE regime (Pellet-Utilized Long-pulse Steady-state Enabled) mitigates heat loads without accumulation of the seeding species.
The PULSE-regime proves robust: irregularities in pellet size reduce plasma performance, but recovery is observed once nominal fueling is restored. In first experiments, central ion temperatures exceeding 2 keV at central densities above $1.5\times 10^{20}$ m$^{−3}$ were obtained, reaching central beta values up to 4% while approaching confinement as expected form the ISS04 scaling ($f_{ren} \approx 0.8 \ldots 1.1$).
While first experiments (reported here) were limited by technical availabilities of components, the PULSE-regime appears to be controllable by adapting the pellet mass, velocity and frequency as well as heating actuators. Therefore, the approach is suggested to be further assessed as a potential scenario for stellarator reactor operation.