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    <pubDate>Mon, 10 Dec 2012 11:21:04 +0200</pubDate>
    <lastBuildDate>Mon, 10 Dec 2012 11:21:04 +0200</lastBuildDate>
    <item>
      <title>Experimental classification of divertor detachment</title>
      <link>http://opus4.kobv.de/opus4-ubbayreuth/frontdoor/index/index/docId/942</link>
      <description>Avoiding damage of the divertor material by keeping the power load below a certain threshold is a major challenge for the operation of&#13;
future fusion devices such as ITER. For Tungsten, the foreseen ITER divertor target material, the&#13;
power load must be kept below 5 MW m^2 in continuous operation. This can in ITER only be achieved with the plasma&#13;
being detached or partially detached from the divertor.&#13;
&#13;
Divertor detachment is characterized by a strong reduction of the ion flux to the target. With a reduction of the temperature, &#13;
achieved by increasing the main plasma density or by seeding additional impurities, volumetric processes such as charge exchange collisions &#13;
and recombination become dominant. These processes lead to a strong reduction of the ion flux and plasma pressure in front of the divertor target.&#13;
Although the single physical mechanisms leading to detachment seem to be understood, it was not yet possible to theoretically&#13;
simulate detachment correctly with respect to experimental observations. This means that some understanding of this process is still missing.&#13;
&#13;
In the detached regime,&#13;
the region of high electron density is retracted from the target and a knowledge of the electron density distribution in the divertor&#13;
volume is necessary to understand the detachment process. In this context, a diagnostic determining the electron density in the&#13;
divertor volume, based on the spectroscopic measurement of the&#13;
Stark broadening of the Balmer lines, has been installed at ASDEX Upgrade. Initial problems with reflected stray-radiation have been&#13;
solved and first measurements were successfully compared for consistency with other diagnostics.&#13;
&#13;
The detachment process was then investigated with an extensive set of density ramp discharges with different heating powers,&#13;
fuelling species and magnetic field directions. The density measurements in the divertor volume were combined with all other available&#13;
divertor diagnostics and a consistent picture of the detachment process was obtained. It was found that detachment is not a continuous&#13;
evolution&#13;
but undergoes three different states. During one of these states radiative fluctuations close to the X-point and high densities far away&#13;
from the separatrix occur. This is a situation which is not described by present day theoretical models.&#13;
Furthermore, it was shown that the conditions of both the inner and outer divertor are&#13;
strongly coupled and that the inner divertor even influences the outer divertor. This effect was not shown yet, neither experimentally&#13;
nor by theoretical simulations.&#13;
&#13;
It was further discovered how additional puffing of nitrogen into the divertor, which removes power via radiation, changes the&#13;
detached divertor conditions and may even change the confined plasma conditions. The effect of an additional magnetic perturbation&#13;
field on the detachment process has also been investigated. Finally, an unstable situation was found, during which the divertor plasma&#13;
oscillates between two detachment states back and forth. &#13;
</description>
      <author>Steffen Potzel</author>
      <category>doctoralthesis</category>
      <guid>http://opus4.kobv.de/opus4-ubbayreuth/frontdoor/index/index/docId/942</guid>
      <pubDate>Fri, 12 Oct 2012 11:21:04 +0200</pubDate>
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