Assessing wall heat fluxes in the quasi-continuous exhaust regime at ASDEX Upgrade

Abstract The 3D fluid edge code EMC3-EIRENE has been used to simulate three ASDEX Upgrade discharges with the same equilibrium but different upstream collisionality <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mi>ν</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> </mml:mrow> </mml:math> . In this scan, the plasma transits from an ELMy state at low collisionality to being in the quasi-continuous exhaust regime (QCE) at high <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mi>ν</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> </mml:mrow> </mml:math> . Particular attention was devoted to matching the experimental plasma profiles in front of the ion cyclotron resonance heating (ICRH) limiter, aiming to obtain a reliable estimation of the ICRH limiter peak heat flux in the QCE regime discharge. As a result, we observed that while moving from ELMy to QCE, the limiter peak heat flux increases, reaching values in the order of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi>MW</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi mathvariant="normal">m</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> for a limiter-separatrix distance of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mi>cm</mml:mi> </mml:mrow> </mml:mrow> </mml:math> . These values were compared with those at the outer divertor, measured by infrared thermography, revealing that the heat flux perpendicular to the limiter surface can reach up to 90% of the one impinging onto the outer target. Finally, an attempt was made to extend the obtained results towards ITER. This simple analysis suggests that a large wall clearance and a small incidence angle of the magnetic field line, not only on the divertor target but also on ITER’s first wall, might have a crucial role in preventing wall damage during QCE discharges.