Stationary magnetic shear reversal experiments in Tore Supra

Abstract

Stable and stationary states with hollow current density profiles have been achieved in Tore Supra with lower hybrid current drive (LHCD) during reduced toroidal magnetic field operation (B t 2 T) and in weak LH absorption regimes. For these plasma conditions, offaxis LH power deposition profiles are obtained in a reproducible manner when the internal LH caustics prevent central absorption of the waves. In the multipass LH wave propagation regime, the validity of the statistical treatment of stochastic wave diffusion is shown both theoretically and experimentally. When a large fraction of the plasma current (above 50%) is non-inductively sustained by the LH waves, the magnetic shear is reversed in the plasma core, i.e. inside a normalized plasma radius of the order of 0.4. The resulting hollow current density profiles have led to an enhancement of the total electron thermal energy content, up to a factor of 1.6 compared with L-mode discharges. The confinement improvement is attributed to a strong reduction of the electron thermal diffusivity in the central reversed shear region, nearly down to its neoclassical level.

Physics of advanced tokamaks

“…An exception to this technique is that used in Tore Supra, where the hollow q profiles are produced by applying lower hybrid current drive (LHCD), which is absorbed and provides current drive off axis (Litaudon 1996a(Litaudon , 1996b. This example illustrates that the NCS regime can be established and maintained with off-axis current drive, consistent with steady state.…”

“…In Tore Supra discharges heated by lower hybrid, with off axis current drive, an enhanced performance regime is observed (Litaudon 1996a(Litaudon , 1996b(Litaudon , 1997. This lower hybrid enhanced performance (LHEP) regime is correlated with reversed q profile or a region of negative central shear.…”

Two-fluid tokamak equilibria with reversed magnetic shear and sheared flow

“…(2)(3)(4)(5)(6)(7)(8) are advantageous in two respects: (i) the momentum equation includes the electric field and therefore the pressure gradient contribution to E can be calculated from this equation; this contribution is missing in the frame of MHD because E is calculated by Ohm's law, E + v × B = 0, and (ii) the current density J is related self-consistently to the fluid species velocities [Eq. (8)].…”

“…Because of symmetry any equilibrium quantity depends solely on the radial distance r; therefore Eqs. (2), (4)[and (9)], (6), and (7) are identically satisfied. Also the flow for both fluid species is incompressible (∇ · v α = 0).…”

“…Long quasi-steady or steady ITB states have been obtained in different tokamaks, e.g ASDEX Upgrate [1,2], JT-60U [3], Tore-Supra [4], and JET [5] where ITBs were maintained for up to 11 s. The ITBs usually are associated with reversed magnetic shear profiles [6], [7] and their main characteristics are steep pressure profiles in the barrier region [8] and radial electric fields associated with sheared flows [9,10]. The mechanism responsible for the formation of ITBs and the underlying physics is not completely understood.…”