Comparison of scrape-off layer profiles in outboard-versus inboard-limited plasmas in Tore Supra

Abstract

In the Tore Supra tokamak, the scrape-off layer profiles of discharges limited either on the inboard or on the outboard side are compared. Inboard-limited discharges are characterized by substantially longer SOL e-folding lengths of ion and electron temperatures and electron density measured near the top of the plasma, providing strong evidence that the ion and electron energy transport across the SOL is enhanced on the outboard side, similar to the particle transport. The parallel heat flux density extrapolated to the last closed flux surface is found to be inversely proportional to its e-folding length in the SOL. The outboard limiters thus receive higher and more peaked heat loads than the inboard limiters. These results are important for the optimization of the plasma start-up scenario and design of the first wall in ITER.PACS 52.25.Fi, 52.25.Xz, 52.35.Ra, 52.55.Fa, 52.70.Ds IntroductionIn tokamaks, the intermittent expulsion of magnetic-field-aligned plasma filaments from the last closed flux surface (LCFS) is responsible for a large fraction of the effective radial transport to the wall [1][2][3][4][5][6]. Filament propagation appears to be governed by a ballooning-type instability in the unfavourable magnetic curvature region on the low field side. As the filaments propagate outward, they also expand freely along the magnetic field lines, driving pressure-driven field-direction-independent parallel flows. Mach probe measurements in many divertor tokamaks (e.g. [7] and references therein) show flow patterns consistent with this model (i.e. stagnant flow poloidally approximately halfway between the outer midplane and the X-point), in which the filaments can be seen as an important particle source in the scrape-off layer (SOL). Similar observations in limiter tokamaks [8][9][10][11][12] prove that enhanced radial particle transport on the outboard side of the plasma is a universal phenomenon, independent of the presence of an X-point divertor. The enhancement of the radial particle transport on the outboard side of the limiter plasmas was confirmed by the 3D fluid turbulence code TOKAM-3D [13]. When the plasma contact point is on the inboard limiters, the filaments propagate freely out to the wall, filling all the available volume outside the LCFS and leading to a broad SOL with nearly flat density profiles. On the other hand, it was demonstrated [12] that outboard modular limiters suppress the radial particle transport when they are near the LCFS, leading to a very thin SOL. It is probable [8] that the limiters act as simple mechanical barriers that intercept the parallel particle flux in the region in which the enhanced radial convection.

Finite ion temperature effects on scrape-off layer turbulence

“…VI and theoretically estimated, also in agreement with experimental observations, 10,11 is smaller than the linear threshold for ITG instability. Therefore, we expect ITG to have a negligible role on SOL turbulence.…”

“…1, the SOL e-folding lengths for T e and T i are shown for different tokamaks, indicating that the electron profile is usually steeper than the ion profile, leading to g e ð¼ L n =L T e Þ > g i ð¼ L n =L T i Þ. The g i value has been measured, e.g., during limited discharges in Tore Supra, 10 and during diverted discharges in JFT-2M 11 resulting in g i < 1 in both cases. The numerical simulation of edge turbulence in the presence of ion temperature dynamics has been the subject of numerous studies.…”

Effect of the limiter position on the scrape-off layer width, radial electric field and intrinsic flows

“…First of all, we observe that the value of L p in the HFS-limited case (red curve in figure 4) is larger than that in the LFS-limited case (blue curve in figure 4), as observed experimentally [6]. The difference is a factor of 2 or less, while factors of 3 and higher have been obtained experimentally [4,7]. Second, in the four configurations considered, the values of L p tend to be larger on the LFS region than on the HFS region, consistent with the ballooning character of the turbulent transport.…”

Ion temperature fluctuations in the ASDEX Upgrade scrape-off layer

“…5 Strong poloidal asymmetry of the radial particle and energy transport was previously observed in the divertor as well as limiter tokamaks (e.g. [3,4,6,[22][23][24] and references therein) and stellarators [25]. These observations indicate that the filaments are expelled into the SOL in the vicinity of the outboard midplane by the ballooning-type instability.…”