Collisionality scaling in Tore Supra: detailed energy confinement analysis, turbulence measurements and gyrokinetic modelling

A collisionality scaling experiment associating a confinement analysis, turbulence measurements across the whole plasma and gyrokinetic modelling is reported. In Tore Supra L-mode plasmas, mid-radius dimensionless collisionality nu* has been varied performing a four-points scan from similar or equal to 0.1 to similar or equal to 0.7. The normalized confinement time exhibits a dependence with respect to collisionality: B tau(E) proportional to nu*(-0.3 +/- 0.3) which is strongly modified when accounting for the confinement dependence on the normalized Larmor radius, rho*, and normalized pressure, beta, since one obtains B tau(E) proportional to nu*(0.0 +/- 0.7). This weak dependence is consistent with ITER L mode scaling laws and dedicated experiments elsewhere (Luce 2008 Plasma Phys. Control. Fusion 50 043001). The global analysis is confirmed by normalized effective heat transport coefficients which do not vary outside their error bars in a limited radial range of reliability. The analysis is completed by density fluctuation delta n(e)/n(e) measurements across the whole plasmas. For normalized radius r/a < 0.7, delta n(e)/n(e) does not depart from its error bars and the radial wave-vector spectra are not modified. These observations are well reproduced by non-linear gyrokinetic simulations, where, despite high nu* values, no zonal flow damping mechanism is at play. At the plasma edge (r/a > 0.7), the lowest poloidal wave-vector measured by the Doppler reflectometer exhibits a decrease in delta n(e)/n(e) with increasing nu*, while the other turbulence measurements remain unaffected.