Measurement of post-disruption runaway electron kinetic energy and pitch angle during final loss instability in DIII-D

Abstract Post-disruption runaway electron (RE) kinetic energy K and pitch angle <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> </mml:mrow> </mml:math> are critical parameters for determining resulting first wall material damage during wall strikes, but are very challenging to measure experimentally. During the final loss instability, confined RE K and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> </mml:mrow> </mml:math> are reconstructed during center-post wall strikes for both high impurity (high-Z) and low impurity (low-Z) plasmas by combining soft x-ray, hard x-ray, synchrotron emission, and total radiated power measurements. Deconfined (wall impacting) RE <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> </mml:mrow> </mml:math> is then reconstructed for these shots by using time-decay analysis of infra-red imaging. Additionally, deconfined RE K and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> </mml:mrow> </mml:math> are reconstructed for a low-Z downward loss shot by analyzing resulting damage to a sacrificial graphite dome limiter. The damage analysis uses multi-step modeling simulating plasma instability, RE loss orbits, energy deposition, and finally material expansion (MARS-F, KORC, GEANT-4, and finally COMSOL). Overall, mean kinetic energies are found to be in the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo fence="false" stretchy="false">⟨</mml:mo> <mml:mi>K</mml:mi> <mml:mo fence="false" stretchy="false">⟩</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> MeV for confined REs. KORC simulations indicate that the final loss instability process does not change individual RE kinetic energy K . Confined RE pitch angles are found to be fairly low initially pre-instability, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo fence="false" stretchy="false">⟨</mml:mo> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> <mml:mo fence="false" stretchy="false">⟩</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>0.1</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> , but appear to increase roughly 2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>×</mml:mo> </mml:mrow> </mml:math> , to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo fence="false" stretchy="false">⟨</mml:mo> <mml:mrow> <mml:mtext>sin</mml:mtext> </mml:mrow> <mml:mi>ϑ</mml:mi> <mml:mo fence="false" stretchy="false">⟩</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>0.3</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:math> for both confined and deconfined REs during instability onset in the low-Z case; this increase is not observed in the high-Z case.