Divertor plasma opacity effects

In high recycling and detached plasma regimes the recycling region largely defines the properties of the whole scrape-off layer plasma by setting up the density and temperature profiles. As detachment approaches, the plasma temperature in the vicinity of the divertor targets drops to Te -1 eV and density increases to ne -1020-21 m- 3, and the recycling region becomes increasingly opaque to the hydrogen radiation. Hydrogen radiation trapping influences both power and particle balance, both directly - reducing hydrogen radiation loss and changing plasma ionization/recombination balance inside the recycling region, and indirectly - affecting im-purity radiation, charge-exchange and other important processes through the changes in the edge plasma density and temperature profiles.This paper reviews three-decade-long experimental and simulation efforts of quantifying the effects of hydrogen radiation trapping on the tokamak divertor plasma. It is demonstrated that opacity has a strong impact on the local divertor plasma parameters, changing both the electron and atomic hydrogen densities by a factor of several compared to the transparent plasma limit. Moreover, it is shown that allowing for opacity shifts the operational window of tokamak divertor towards higher separatrix densities and higher impurity content in order to maintain the desired level of power dissipation within the divertor. However, the most important is the effect of the opacity on the spectroscopic diagnostics. It is shown that despite the divertor plasma is virtually always transparent to the Balmer series emission, the resulting signals are affected by the changes in the pop-ulations of the excited states of atomic hydrogen, associated with the Lyman series absorption. As a result, the intensity of individual Balmer series signals can increase by a factor of 4 - 8 and the ratios of the Balmer line signals change by a factor of 2. This result implies that reliable quantitative measurements of plasma parameters with spectroscopic diagnostics at high power, high density tokamaks require opacity strength measurements and corresponding corrections to the rate coefficients used for the analysis of the spectroscopic data.

Automated summary

In high recycling and detached plasma regimes, the recycling region defines the properties of the whole scrape-off layer plasma by setting up the density and temperature profiles. As detachment approaches, the plasma temperature near the divertor targets drops and the density increases, making the recycling region less transparent to hydrogen radiation. This paper reviews the effects of hydrogen radiation trapping on the tokamak divertor plasma and demonstrates its impact on the divertor plasma parameters and spectroscopic diagnostics.