The effect of intrinsic tungsten impurity on unmitigated and mitigated major disruptions in ITER

Abstract ITER has recently decided to change the first wall (FW) material from beryllium (Be) to tungsten (W). W being a high- Z impurity behaves differently as compared to Be during normal plasma operations as well as during plasma current disruptions. To study the effects of W impurities during disruptions, especially on radiation fraction, current quench (CQ) times, halo current dynamics etc, unmitigated simulations for various intrinsic W density fractions and mitigated simulations with varying neon (Ne) density fractions have been performed using the Tokamak Simulation Code. The simulations show that, compared to the cases of intrinsic Be impurities in the previous ITER FW design, the CQ times are generally faster with the expected amount of intrinsic W impurities due to the much higher Z value of W, resulting in a higher radiation fraction. For intrinsic W impurity concentrations lower than 0.005% of the background plasma density, the CQ time can be higher than 300 ms and is lower only for higher W concentrations in unmitigated discharges in ITER. In addition, we have studied the effectiveness of mitigation through injection of Ne on a disrupted plasma with a background intrinsic W impurity density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>16</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> m −3 . With injected Ne densities varying from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>5.0</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>19</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> m −3 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1.3</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>20</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> m −3 , the CQ time can be progressively brought down to less than 50 ms with a radiation fraction greater than 80% and a reduction in poloidal halo currents can be achieved.