Relevance of <i>E</i> × <i>B</i> drifts for particle and heat transport in divertors
C.K. Tsui, J.A. Boedo, O. Février, H. Reimerdes, C. Colandrea, S. Gorno, The TCV Team
Abstract
Abstract Radial electric fields up to ∼4 kV m −1 are observed in the boundary between the private flux region (PFR) and the scrape-off layer (SOL) driving E × B drifts between the inner and outer targets at speeds up to 2.8 km s −1 in the Tokamak à configuration variable divertor. The resulting E × B fluxes, located in a narrow region ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi mathvariant="normal">Ψ</mml:mi> </mml:msub> </mml:mrow> <mml:mo><</mml:mo> <mml:mn>0.012</mml:mn> </mml:math> in normalized radius or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> </mml:math> R − R sep <4 mm mapped to the outer midplane) are equivalent to around 20% of the total heat and particle flux to the divertor targets (inner + outer). At the peak E r , the E × B poloidal transport is equivalent to parallel flows with M ∥ ∼ 3. In the snowflake divertor with a second X-point in the outer SOL, the drifts in the PFR-SOL boundary were equivalent to around 30% of the total heat and particle flux to the divertor targets and cover a region ∼50% wider than in the single null ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi mathvariant="normal">Ψ</mml:mi> </mml:msub> </mml:mrow> </mml:math> ∼ 0.018, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> </mml:math> R − R sep ∼ 6 mm). The location of the PFR-SOL boundary drift shifts radially in the E ∥ × B direction when reversing the toroidal field direction. Peaks in density and electron pressure have been identified near the primary X-point along with large gradients in density, temperature, and potential, the latter resulting in a local electric field ∼2.7 kV m −1 which drives a drift (1.9 km s −1 ) upwards towards the closed flux surfaces. Floating potential ( V f ) magnitudes up to 75 V (∼2 kT e ) were measured, indicating that V f and parallel currents should not be neglected when estimating plasma potential.