Impact of the SOL and divertor conditions on access to H-mode

Impact of the SOL and divertor conditions on access to H-mode H Meyer1 1CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK E-mail: [email protected] The formation of an edge transport barrier in the so-called H-mode was discovered nearly 30 years ago, but the trigger mechanism still eludes first principle understanding. This leads to large uncertainties in our projections towards future devices such as ITER and DEMO. Advances in diagnostics have recently highlighted the dynamics of the sheared flows generated from either the background [1] or the turbulent driven radial electric fields [2] for the transition from L-mode to H-mode through intermediate phases (limit cycle oscillations or I-phase). In addition the ion heat flux has been identified as a crucial parameter for H-mode access [3]. Whilst these processes in the confined plasma region start to form a consistent picture, the plasma on the open field lines in the scrape-off layer (SOL) is long known to also have a strong impact. In this contribution an overview of the experimental findings of the impact the SOL and the divertor conditions have on H-mode access will be given. Early indications on the importance of SOL comes from the fact that in general H-mode access in diverted configurations is achieved at much lower input power, Pthr, than in limited configurations. Also the well known dependence of Pthr on the ion ∇B-drift direction may point towards the SOL. The earliest direct evidence of the impact of the importance of the SOL and divertor configurations came from JET, where a decrease of Pthr by nearly a factor of 2 was measured in configurations limited by the septum at the X-point. Later studies on JET with the septum removed showed that the same reduction is achieved by simply shifting the magnetic configuration downwards [4]. This sensitive dependence of the magnetic configuration in the divertor has since been observed on many devices such as Alcator C-MOD, COMPASS, DIII-D, EAST, MAST, NSTX and TCV, but so far no consistent picture has emerged. These studies also show an influence of the lower and upper triangularity on H-mode access. Recent studies on JET comparing configurations with both strike points on the vertical targets to configurations with one strike point on the horizontal target [5] suggest that changes to the sheath potential profile could play an important role in altering the near-SOL radial electric field [6]. These changes occur due to the changes in the recycling. The recycling may also play a role in the dependence of Pthr on the wall material. On ASDEX Upgrade and JET a reduction of Pthr about 25%-30% is observed with a W divertor and a metall wall [7,8]. Smaller devices generally benefit from wall conditioning techniques such as boronisation or Li evaporation. The latter has lead to a reduction of Pthr on NSTX by a factor of 2 [9] and is routinely used on EAST to achieve H-mode access. On MAST, NSTX, COMPASS-D and recently on Pegasus also an effect of the fuelling location is observed with easier access with fuelling from the high field side mid-plane. Replacement of gas fuelling with shallow pellets also facilitates the L-H transition on DIII-D and MAST. The changes of the SOL parallel flow direction on Alcator C-MOD correlate with the differences of Pthr observed with opposite ion ∇B-drift direction indicating that a similar SOL flow is needed to access Hmode [10]. On ASDEX Upgrade, MAST and NSTX a change of Pthr in exact double null configuration is observed where the most markable changes on MAST are happening in the high field side SOL and the inner strike points [11]. The focus of this contribution will be on the changes to the local parameters with the changes of Pthr only used as evidence.