Improved confinement with Electron Cyclotron resonance heating of pellet fuelled discharges

The injection of high-speed solid deuterium pellets can improve the global energy confinement (by generating peaked pressure profiles), reduce ion transport to the neoclassical level and suppress the sawtooth activity (generally by modifying the plasma current profile). Electron Cyclotron Resonance Heating has been used to increase the electron temperature in these very high density regimes, and to study the heat flux in the presence of a well-localised heat source.

As the central density is above the cut-off value (ncr=2.4x1020m-3 at 140 GHz), optimum heating conditions are achieved with off-axis resonance.

Time traces for FTU pulse #17839, with pellet injection at t=0.552 s and optimized heating conditions (ECRH deposited just outside the cutoff region).Top frame: central and volume-average density. Second frame: central electron temperature. Third frame:ECRH power. Fourth frame: measured neutron yield (thick solid line); neutron yield calculated with neoclassical and twice neoclassical ion heat flux (thin and dashed lines respectively). Last frame: measured energy confinement time (solid line) and L-mode scaling prediction (dashed line).

Main results:

- Central heating is affected by cutoff problems; as the ECRH power reaches the plasma centre, a strong density pump-out occurs, causes of which are being analysed.

- The cutoff is avoided by placing the resonance at r=a/3; a moderate pump-out is observed in this case.

- Neoclassical ion heat transport and a subsequent improved global confinement is achieved in the optimised conditions.

-The duration of the improved regime is limited by the onset of an internal kink (m/n=1/1) instability that leads to a decrease of the central density. A way to control the evolution of the current profile (with LHCD for instance) or to stabilise the kink mode will be needed to prolong the improved confinement phase.

Experimental Reports