A quasi-isodynamic configuration with good confinement of fast ions at low plasma β
E. Sánchez, J. L. Velasco, I. Calvo, S. Mulas
Abstract
Abstract A new quasi-isodynamic (QI) stellarator configuration optimized for the confinement of energetic ions at low plasma β is obtained. The numerical optimization is carried out using the STELLOPT suite of codes. New proxies to measure closeness to quasi-isodynamicity and quality of fast ion confinement have been included. The new configuration has poloidally closed contours of magnetic field strength, low magnetic shear and a rotational transform profile allowing an island divertor. It shows ideal and ballooning magnetohydrodynamic stability up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>=</mml:mo> <mml:mn>5</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:math> , reduced effective ripple, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> <mml:mo><</mml:mo> <mml:mn>0.5</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:math> in the plasma core. Even at low β , the configuration approximately satisfies the maximum- J property, and the confinement of fast ions is good at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>1.5</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:math> and becomes excellent at reactor values, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>4</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:math> . An evaluation of the D 31 neoclassical mono-energetic coefficient supports the expectation of a reduced bootstrap current for plasmas confined in QI configurations. A set of filamentary coils that preserve the good confinement of fast ions in the core is presented.