Overview of RF techniques in new confinement regimes with modified shear

It has been shown that the core energy confinement in tokamaks can be significantly improved by modifying the plasma current profile. Plasma regimes where the transport is reduced to its neoclassical value in the centre of the plasma within a region called the internal transport, barrier (ITB) can be achieved with a zero or negative magnetic shear. In this paper the role of the radio frequency (RF) methods: electron cyclotron resonance, ion cyclotron resonance and lower hybrid current drive in producing these ITBs is reviewed. Combined techniques where RF waves are used to modify the magnetic shear together with a central heating method (neutral beam injection or RF) are described. Application of RF methods to high fusion yield plasmas in JET with modified shear are presented together with a discussion on the prospects of achieving steady-state high-performance plasmas. RF techniques are already playing an important role in these 'advanced scenarios' and are likely to become indispensable tools for future steady-state tokamak reactors.