L–H power threshold for neutral beam heated plasmas with deuterium, hydrogen, helium and mixed ion species in TCV

Abstract A systematic study of the power threshold for L–H transition has been conducted in TCV. Only results from NB heated plasmas are reported. The dependencies of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>LH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> on plasma current, plasma density and main ion species have been investigated. For deuterium plasmas and at low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">q</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>95</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> , the L–H power threshold dependence on density is well described by the International Tokamak Physics Activities scaling law. Nevertheless at larger <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">q</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>95</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> values (reduced plasma current at constant toroidal magnetic field), the threshold deviates significantly from the scaling law and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>LH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> increases with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">q</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>95</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . As a consequence, a strong correlation of power threshold with transport is reported suggesting that L–H transition is more favorable for L-mode plasmas with already good confinement.