Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

Abstract

In inboard-limited plasmas, the Scrape-Off Layer (SOL) shows two regions: the near SOL, extending a few mm from the Last Closed Flux Surface (LCFS), characterized by a steep gradient of the parallel heat flux radial profile, and a far SOL, typically some cm wide, with flatter heat flux profiles. The physics of the near SOL is investigated in TCV with two series of experiments featuring deuterium and helium plasmas, in which the plasma current, density and elongation have been varied. The parallel heat flux profiles are measured on the limiter by means of infrared thermography. For the first time, the near SOL is reported to disappear for low plasma current or at high density, for values of the SOL collisionality ν * SOL corresponding to a conduction-limited regime. The power in the near SOL ∆P SOL is shown to decrease with the normalized Spitzer resistivity ν as ∆P SOL ∝ ν −1 . The floating potential profiles, measured at the limiter using flush-mounted Langmuir probes (LP), show the presence of non-ambipolar currents, and their relation to the presence of a velocity shear layer is discussed. The shearing rate is shown to strictly correlate with the power in the near SOL ∆P SOL , consistently with a recent theoretical model. Measurements of the near SOL on the Low Field Side (LFS) are performed using a reciprocating Langmuir probe (RP). The near SOL is reported to vanish simultaneously at the LFS and at the limiter. The near and far SOL widths are compared with the predictions from existing theoretical models, to which empirical corrections with resistivity and elongation are proposed.

Filamentary velocity scaling validation in the TCV tokamak

“…Approximately 600 conditional averaged entries exist for these discharges. The discharges were described in [82,83], and an example equilibrium is shown in Figure 3a).…”

Shaping effects on scrape-off layer plasma turbulence: A rigorous validation of three-dimensional simulations against TCV measurements

“…The dashed lines denote an exponential fit between x ¼ 0 and x ¼ 2:5 cm. exponential decay (despite the fact that small deviations are observed for n and j sat for the plasma discharges #54147 and #55391 and that, in general, decay lengths in the SOL are nonexponential, with a steeper profile near the LCFS and smaller gradients in the far SOL 60,63,64 ). Similarly, Figs.…”

Impurity seeding for suppression of the near scrape-off layer heat flux feature in tokamak limited plasmas

“…In Fig. 7d, the time evolution of ∆V f l is plotted in black, and compared with the value ∆V f l = 5 V, for which the power in the near SOL P n has been previously reported to vanish in TCV deuterium plasmas [10]. As the gas injection progresses, the ∆V f l is reduced, reaching values below 5 V, corresponding to a low ω E×B , which according to the model in Ref.…”

“…The procedure used to compute the parallel heat flux radial profiles q || (r u ) is detailed in Ref. [10] and the main steps are summarized in the following. The heat flux deposited on the graphite tiles q dep is evaluated using the THEODOR code [22], and is remapped onto the magnetic coordinates (r u , α), where α is the angle between the magnetic field line and the plane tangent to the tile surface.…”