On the non-stiffness of edge transport in L-modes

Transport analyses using first principle turbulence codes as well as 1½-D transport codes often study transport properties between the plasma axis and a normalized radius around 0.8. In this region, heat transport shows significant stiffness properties leading to R/LTe values that are relatively independent of auxiliary input power. In this work, we study experimentally in the TCV tokamak, the transport properties of the edge region, close to the last closed flux surface, namely between ρV=0.8 and 1 (ρψ≥0.9). It is shown that electron transport is not stiff in this region and extremely high R/LTe values can be attained even for L-mode confinement. These results bring a new perspective to several "accepted" understandings. In particular a specific study related to the Ip scaling of ohmic and ECH L-mode discharges shows that the strong Ip scaling is in fact related in a large part to this non-stiff edge region. The Te scale length is shown to be proportional to Ip in the edge region and constant (independent of Ip) in the core region. The relation with L-H transition and the I-modes will also be discussed, since it shows that the edge gradient can be continuously increased, by increasing the input power, even in the L-mode phase. It is also proposed that the pedestal width is related to the width over which the transport is non-stiff, and can be studied in detail already in L-mode. The present study also explains the large improved confinemet obtain with negative triangularity and a new model is proposed including non-stiff edge local transport which can recover the experimental observations. This work was supported in part by the Swiss National Science Foundation.