Turbulent transport and plasma flow in the Reversed Field Pinch

The results of an extensive investigation of electrostatic and magnetic turbulence in the edge region of two Reversed Field Pinch (RFP) experiments EXTRAP-T2R and RFX are reported . In both experiments particle transport is mostly driven by electrostatic turbulence and a highly sheared ExB flow takes place. Recent results have shown that almost 50% of the particle losses is due to coherent structures. These structures have features reminiscent of monopolar or dipolar vortices and their relative population and preferred vorticity is determined by the local ExB shear. It has been demonstrated that the plasma diffusivity D can be separated in two comparable terms: one due to coherent structures and another one due to background turbulence. The ExB shear results to affect both terms, modifying the populations of vortices and the phase of background density and velocity fluctuations . This effect has been proved by several experiments of transport control based on modification of the ExB shear by insertion of active electrodes or by applying transient modifications of the magnetic field. In order to investigate the process responsible for the observed ExB flow profile, the momentum balance has been recently addressed and it has been found that also transport of momentum is anomalous as experimental kinematic viscosity results consistent with anomalous diffusion. Moreover the balance reveals that plasma flow profile is regulated by turbulence and in particular by the electrostatic component of the Reynolds Stress (RS). These results prove the existence of a dynamic interplay between turbulence properties, anomalous transport and mean profiles. All results are discussed highlighting the similarities with other magnetic configurations

Global scaling of the heat transport in fusion plasmas

A global heat flux model based on a fractional derivative of plasma pressure is proposed for the heat transport in fusion plasmas. The degree of the fractional derivative of the heat flux, α, is defined through the power balance analysis of the steady state. The model was used to obtain the experimental values of α for a large database of the Joint European Torus (JET) carbon-wall as well as ITER like-wall plasmas. The fractional degrees of the electron heat flux are found to be α<2, for all the selected pulses in the database, suggesting a deviation from the diffusive paradigm. Moreover, the results show that as the volume integrated input power is increased, the fractional degree of the electron heat flux converges to α∼0.8, indicating a global scaling between the net heating and the pressure profile in the high-power JET plasmas. The model is expected to provide insight into the proper kinetic description for the fusion plasmas and improve the accuracy of the heat transport predictions

Overview of the JET results

Since the installation of an ITER-like wall, the JET programme has focused on the consolidation of ITER design choices and the preparation for ITER operation, with a specific emphasis given to the bulk tungsten melt experiment, which has been crucial for the final decision on the material choice for the day-one tungsten divertor in ITER. Integrated scenarios have been progressed with the re-establishment of long-pulse, high-confinement H-modes by optimizing the magnetic configuration and the use of ICRH to avoid tungsten impurity accumulation. Stationary discharges with detached divertor conditions and small edge localized modes have been demonstrated by nitrogen seeding. The differences in confinement and pedestal behaviour before and after the ITER-like wall installation have been better characterized towards the development of high fusion yield scenarios in DT. Post-mortem analyses of the plasma-facing components have confirmed the previously reported low fuel retention obtained by gas balance and shown that the pattern of deposition within the divertor has changed significantly with respect to the JET carbon wall campaigns due to the absence of thermally activated chemical erosion of beryllium in contrast to carbon. Transport to remote areas is almost absent and two orders of magnitude less material is found in the divertor