Quasi-continuous exhaust operational space

Abstract The IPED predictive pedestal code has been used to determine the critical gradients for the onset of 1) separatrix ballooning modes and 2) global peeling-ballooning modes as a function of plasma shaping. This results in a scaling of the onset threshold of separatrix ballooning modes as a function of elongation and triangularity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mtext>edge,crit</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.64</mml:mn> <mml:msup> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mn>2.2</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:mi>δ</mml:mi> <mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mn>0.9</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , while the critical gradient for global peeling-balloonig modes increases as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>≈</mml:mo> </mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mtext>edge,crit</mml:mtext> <mml:mrow> <mml:mn>1.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> . This implies that operational space for a ballooning unstable separatrix and stable peeling-ballooning modes exists at sufficiently high shaping. Applying a collisional broadnening based scaling of the separatrix gradients allows the critical separatrix density, required to drive the separatrix ballooning mode, to be derived from global plasma parameters for any operational scenario on any device. Evaluations for ASDEX Upgrade, JET, and the ITER 15 MA baseline plasma predict critical separatrix densities of 0.3–0.4 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi>GW</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> for QCE access, making the QCE an attractive operational scenario for fusion devices.