Plasma dynamics with second and third harmonic ECRH on TCV tokamak

Intense electron cyclotron resonance heating (ECRH) is employed on the TCV tokamak both in second- and third-harmonic X-mode (X2 and X3). The plasma behaviour under such conditions is driven largely by the electron dynamics, motivating extensive studies on TCV of the heating and relaxation phenomena governing both the thermal and suprathermal electron populations. In particular, the dynamics of suprathermal electrons is intimately tied to the physics of X2 current drive (ECCD). The absorption of X3 radiation has been found to be enhanced by the presence of these high-energy electrons, which can also be generated by the X3 wave itself. Suprathermal electrons are also generated by the strong electric fields created by the magnetic reconnection events accompanying sawtooth crashes. Electrons play a crucial role in high-density plasmas where indirect ion heating can be achieved through ion-electron collisions. Such plasmas can be heated by the X3 system, thanks to its high density cut-off, enabling in particular H-mode studies with strong electron heating. Experiments have been performed by applying X3 heating to an Ohmic H-mode target, resulting in an ELM-free regime with constant density and stored energy, and elevated Dalpha and energy confinement, as well as increased ion temperature and rotation velocity. The toroidal β reached 2.5% (βN∼2) in these plasmas, 70% of the ideal limit. ECRH is also an optimal tool for manipulating the electron distribution function in both physical and velocity space: examples in TCV are the internal transport barriers controlled by current-profile inversion and Fisch-Boozer ECCD, and the energy selectivity of vertically launched X3 waves. Fundamental studies of the energetic electron dynamics have been performed in TCV through periodic, low-duty-cycle bursts of ECRH, with negligible average power injection, and with electron-cyclotron-emission measurements coherently averaged over a long stationary period to improve the statistics. The characteristic times of the dynamical evolution, affected by rf diffusion and collisional scattering and slowing-down in velocity space as well as by cross-field transport in physical space, are clearly revealed by this technique. (author)