Initial Low Recycling Improving Confinement and Current Drive in Advanced Tokamak (AT) and Hybrid Scenarios

To improve stability and confinement in fusion plasmas via sufficient control of the plasma current profile represents a major objective for the progress of thermonuclear fusion research based on the tokamak concept. The lower hybrid current drive (LHCD) would provide a solution, by removing the problematic extrapolation of LHCD to high plasma densities, as demonstrated on FTU. However, since LHCD is not approved yet in ITER and is difficult to foresee its presence in DEMO, the issue remains on how producing a high bootstrap fraction (fbs>50%) in the unfavorable condition of relatively low q95 (< 5). We focus here on the proper (initial) condition of a low particle recycling from the vessel wall, which should be performed before starting the main heating phase, as useful for enhancing the bootstrap current density at large radii of the plasma column. Experiments in ITER-like wall, relevant to hybrid scenario with q95 ~ 4.5 and performed with an initial compression and expansion of plasma volume with the aim of changing the q-profile, exhibited a lower initial recycling, a higher electron temperature of periphery, and a higher normalised b (bN) in H-mode. A similar effect of the initial level of recycling was found in statistics of plasma discharges performed in C-wall, relevant for AT scenario with q95 ~ 5. Modelling of current density profile evolution shows that a larger bootstrap current occurs at large radii in case of lower initial recycling, effect of larger electron temperature produced at large radii. Analysis of microinstability in L-mode phase shows that ETG modes have smaller linear growth rate in case with lower recycling. From stability analysis performed during the high bN phase, a bigger margin of stability minimum shear occurs near the plasma periphery, thanks to current drive improved by the proper initial edge condition.
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