Impact of <i>E</i> × <i>B</i> drift on tungsten impurity transport and analysis of its instantaneous response: a particle Monte Carlo simulation

Abstract Tungsten (W), as the primary plasma-facing material in tokamaks, is incompatible with the core plasma. Even extremely low concentrations of W impurities can contaminate the plasma. Therefore, the transport and core accumulation of W impurities are among the key challenges in the fusion community. In this study, the impurity transport code IMPEDGE is upgraded to include the effect of E × B drift, and then applied to investigate the impact of E × B drift on W impurity transport in single-null configuration on EAST. It is found that E × B drift not only affects the stagnation point location of W impurities but also exhibits an instantaneous response feature—i.e. it governs the impurity motion on a time scale shorter than the collisional relaxation and exceeding the gyro-period. The instantaneous response of E × B drift governs impurity behavior near the divertor targets and significantly affects the core W impurity density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>n</mml:mi> <mml:mo stretchy="false">¯</mml:mo> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>W</mml:mi> <mml:mo>,</mml:mo> <mml:mi>c</mml:mi> <mml:mi>o</mml:mi> <mml:mi>r</mml:mi> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ). By comparing different E × B drift directions, it is found that the region of reversed E × B drift widens with increasing of neon (Ne) injection rate under forward B t , leading to an increase in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>n</mml:mi> <mml:mo stretchy="false">¯</mml:mo> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>W</mml:mi> <mml:mo>,</mml:mo> <mml:mi>c</mml:mi> <mml:mi>o</mml:mi> <mml:mi>r</mml:mi> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . The instantaneous response of E × B drift can induce a deposition phenomenon distinct from prompt redeposition. To evaluate this behavior, this work establishes a simplified theory for analysis. The poloidal drift deposition distance ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>x</mml:mi> <mml:mrow> <mml:mi>c</mml:mi> <mml:mo>,</mml:mo> <mml:mi>p</mml:mi> <mml:mi>o</mml:mi> <mml:mi>l</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) is defined. When <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>x</mml:mi> <mml:mrow> <mml:mi>c</mml:mi> <mml:mo>,</mml:mo> <mml:mi>p</mml:mi> <mml:mi>o</mml:mi> <mml:mi>l</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> exceeds the W ionization mean free path ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mtext>W</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext>mfp</mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ), impurities tend to return to the target due to the effect of E × B drift’s instantaneous response. This deposition behavior is more likely to occur under conditions of high electron temperature ( T e ) and high drift velocity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>v</mml:mi> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:mrow> <mml:mo>×</mml:mo> <mml:mi>B</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ). Simulation results demonstrate that this theory can effectively identify W deposition induced by the instantaneous response of E × B drift. An equivalent force model was introduced in the simulation and compared with the instantaneous effect of E