Interpretation of divertor ion temperature measurements from an attached towards a detached regime

Abstract We present simultaneous profiles of the poloidal distribution of electron temperature and density ( T e , n e ), the poloidal emissivity distributions of a range of spectral lines together with the corresponding ion temperatures, T i , in the TCV divertor plasma. These measurements were performed over divertor plasma conditions evolving from strongly attached towards a detached regime. The poloidal n e and T e distributions were measured by Divertor Thomson Scattering (DTS) diagnostic and by exploiting TCV’s flexibility to align divertor magnetic configurations to the DTS diagnostic laser path. T i are inferred from Doppler broadening of C II, C III, and He II ions transitions measured by the high-resolution Divertor Spectroscopy System (DSS). The measured T i represents an emissivity-weighted ion temperature along the DSS lines of sight. To study the electron/ion thermalisation, we compare T i of each ion with a corresponding emission-weighted electron temperature, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:msubsup> <mml:mstyle scriptlevel="0"/> <mml:mi>e</mml:mi> <mml:mo>∗</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> . We find that T i of C + is lower than its corresponding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:msubsup> <mml:mstyle scriptlevel="0"/> <mml:mi>e</mml:mi> <mml:mo>∗</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> for all plasma parameters except for near-detachment conditions in the vicinity of the target. Conversely, T i of C 2+ and He + are approximately equal to their corresponding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:msubsup> <mml:mstyle scriptlevel="0"/> <mml:mi>e</mml:mi> <mml:mo>∗</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> , except near the target in attached conditions. To explain these observations, we present a simple model of T i evolution that includes the thermalisation between plasma species, and the ionisation state evolution with a source of low temperature neutrals. In the framework of this model we also show that T i of C 2+ and He + closely follow T i of ionized deuterium D + in all our plasma parameters and therefore may be used as an indirect measurement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:msub> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi>i</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">D</mml:mi> </mml:mrow> <mml:mo>+</mml:mo> </mml:msup> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . The results here clearly show the need for a collisional-radiative model that includes local neutral density of deuterium and impurities, to understand T i of various ion species, and their emission intensities in the divertor plasma.