Progress towards internal transport barriers at high plasma density sustained by pure electron heating and current drive in the FTU tokamak

Strong electron internal transport barriers (ITBs) are obtained in FTU by the combined injection of lower hybrid (LH, up to 1.9 MW) and electron cyclotron (EC, up to 0.8 MW) radio frequency waves. ITBs occur during either the current plateau or the ramp-up phase, and both in full and partial current drive (CD) regimes, up to peak densities ne0>1.2×1020 m−3, relevant to ITER operation. Central electron temperatures Te0>11 keV, at ne0≈0.8×1020 m−3 are sustained longer than 35 confinement times. The ITB extends over a region where a slightly reversed magnetic shear is established by off-axis LHCD and can be as wide as r/a = 0.5. The EC power, instead, is used either to benefit from this improved confinement by heating inside the ITB, or to enhance the peripheral LH power deposition and CD with off-axis resonance. Collisional ion heating is also observed, but thermal equilibrium with the electrons cannot be attained since the e−–i+ equipartition time is always 4–5 times longer than the energy confinement time. The transport analysis performed with both ASTRA and JETTO codes shows a very good relation between the foot of the barrier and the weak/reversed shear region, which in turn depends on the LH deposition profile. The Bohm-gyroBohm model accounts for the electron transport until Te0<6 keV, but is pessimistic at higher temperatures, where often also a reduction in the ion thermal conductivity is observed, provided any magnetohydrodynamic activity is suppressed.