Journal article

The physics of electron internal transport barriers in the TCV tokamak

Electron internal transport barriers (elTBs) are generated in the TCV tokamak with strong electron cyclotron resonance heating in a variety of conditions, ranging from steady-state fully noninductive scenarios to stationary discharges with a finite inductive component and finally to transient current ramps without current drive. The confinement improvement over L-mode ranges from 3 to 6; the bootstrap current fraction is invariably large and is above 70% in the highest confinement cases, with good current profile alignment permitting the attainment of steady state. Barriers are observed both in the electron temperature and density profiles, with a strong correlation both in location and in steepness. The dominant role of the current profile in the formation and properties of elTBs has been conclusively proven in a TCV experiment exploiting the large current drive efficiency of the Ohmic transformer: small current perturbations accompanied by negligible energy transfer dramatically alter the confinement. The crucial element in the formation of the barrier is the appearance of a central region of negative magnetic shear, with the barrier strength improving with increasingly steep shear. This connection has also been corroborated by transport modelling assisted by gyrofluid simulations. Rational safety-factor (q) values do not appear to play a role in the barrier formation, at least in the q range 1.3-2.3, as evidenced by the smooth dependence of the confinement enhancement on the loop voltage over a broad elTB database. MHD mode activity is however influenced by rational q values and results in a complex, sometimes cyclic, dynamic evolution.

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