Experimental classification of divertor detachment
Avoiding damage of the divertor material by keeping the power load below a certain threshold is a major challenge for the operation of future fusion devices such as ITER. For Tungsten, the foreseen ITER divertor target material, the power load must be kept below 5 MW m^2 in continuous operation. This can in ITER only be achieved with the plasma being detached or partially detached from the divertor. Divertor detachment is characterized by a strong reduction of the ion flux to the target. With a reduction of the temperature, achieved by increasing the main plasma density or by seeding additional impurities, volumetric processes such as charge exchange collisions and recombination become dominant. These processes lead to a strong reduction of the ion flux and plasma pressure in front of the divertor target. Although the single physical mechanisms leading to detachment seem to be understood, it was not yet possible to theoretically simulate detachment correctly with respect to experimental observations. This means that some understanding of this process is still missing. In the detached regime, the region of high electron density is retracted from the target and a knowledge of the electron density distribution in the divertor volume is necessary to understand the detachment process. In this context, a diagnostic determining the electron density in the divertor volume, based on the spectroscopic measurement of the Stark broadening of the Balmer lines, has been installed at ASDEX Upgrade. Initial problems with reflected stray-radiation have been solved and first measurements were successfully compared for consistency with other diagnostics. The detachment process was then investigated with an extensive set of density ramp discharges with different heating powers, fuelling species and magnetic field directions. The density measurements in the divertor volume were combined with all other available divertor diagnostics and a consistent picture of the detachment process was obtained. It was found that detachment is not a continuous evolution but undergoes three different states. During one of these states radiative fluctuations close to the X-point and high densities far away from the separatrix occur. This is a situation which is not described by present day theoretical models. Furthermore, it was shown that the conditions of both the inner and outer divertor are strongly coupled and that the inner divertor even influences the outer divertor. This effect was not shown yet, neither experimentally nor by theoretical simulations. It was further discovered how additional puffing of nitrogen into the divertor, which removes power via radiation, changes the detached divertor conditions and may even change the confined plasma conditions. The effect of an additional magnetic perturbation field on the detachment process has also been investigated. Finally, an unstable situation was found, during which the divertor plasma oscillates between two detachment states back and forth.
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