New insights into the physics and dynamics of divertor ion current loss during divertor detachment in TCV

Detachment is predicted to be of paramount importance in handling the power exhaust for future fusion devices, such as ITER. However, a direct experimental quantification of the role and spatial profile of the various atomic processes controlling the loss of divertor ion current during detachment has, until now, not been available due to lack of ionisation measurements. The physics of the target ion current loss, a defining and important feature of detachment, is studied in TCV using density ramp, or N2 seeded, L-mode discharges with various plasma currents. Novel spectroscopic analysis techniques utilising the hydrogen Balmer series have been developed to infer the ionisation source magnitude and distribution. This provides, together with the ion sink in the plasma (recombination), electron density and divertor power balance measurements, a detailed picture of particle and power balance along the outer divertor leg. The results for a conventional single-null topology show the divertor ion source tracks the ion target flux both in magnitude and in time: both the ion target current and ion source decrease together at detachment. Surprisingly, the volumetric recombination ion sink - commonly thought to be the primary detachment ion loss mechanism - is only a small (sometimes negligible) portion of the ion current losses. New evidence from TCV shows the driver of the divertor ion source decrease is a decrease in power flowing into the ionisation region combined with an increase in the energy required per ionisation - essentially 'starving' the ionisation region of power. SOLPS modelling of a TCV density ramp discharge with a conventional divertor configuration has reproduced the general characteristics of the ion source reduction. 'Power starvation' of the ionization source, therefore, appears to be central to loss of divertor target ion current. The divertor plasma sink for ions, recombination, is maximised at the highest divertor densities, which are achieved at the highest core densities. Nitrogen seeding enables detachment by 'power starving' the ionisation region at lower core densities and hence lower recombination sink. The ratio between the recombination ion sink and the ion target flux increases when poloidal flux expansion is increased (x-divertor) under constant core conditions. Understanding these changes may be key to understanding the role of magnetic geometry on detachment and possibly the detachment process itself.