Abstract
Experiments on the Large Helical Device with the injection of carbon pellets into discharges of low density have demonstrated a significant reduction of the ion heat conduction in the plasma core and an increase in the central ion temperature by a factor of up to 2. These results are interpreted in the framework of a transport model elaborated on the basis of those applied previously to explain the improvement in confinement by impurity seeding into the tokamak devices TEXTOR and JET. The calculations performed reproduce well the strong peaking of the ion temperature profile with increasing carbon density nZ and the consequent drop in the confinement as nZ exceeds a certain critical level. The importance of different elements in the model, such as braking of the main ion rotation by friction with impurity ones and the shape of the density profiles, are investigated. A qualitative assessment of the applicability under fusion reactor conditions, e.g. of much higher plasma density and heating power, is performed.
Highlights
The improvement of the ion energy confinement by seeding of different impurity species, such as nitrogen, neon, argon, have been observed in diverse tokamak devices, e.g., ISX-B [1], ASDEX and ASDEX-U [2, 3], TEXTOR [4], JET [5, 6], DIII-D [7], JT60U [8]
The curvature of magnetic field lines is the main cause of toroidal ion temperature gradient (ITG) modes and one has to expect the development of this instability and accompanying anomalous transport in helical devices
Computations have been performed for the conditions of the Large HelicalDevice (LHD) shot 106455 where the amount of impurity has been varied by injecting two carbon pellets with a certain time interval; in calculations this variation has been mimicked by changing Zef f
Summary
The improvement of the ion energy confinement by seeding of different impurity species, such as nitrogen, neon, argon, have been observed in diverse tokamak devices, e.g., ISX-B [1], ASDEX and ASDEX-U [2, 3], TEXTOR [4], JET [5, 6], DIII-D [7], JT60U [8]. Device (LHD) [9, 10] have demonstrated that this phenomenon is not specific for the tokamak type of magnetic fusion devices: the injection of carbon pellets into the LHD led to a pronounced peaking of the ion temperature profile and strong increase, up to a factor of 2, of the central Ti magnitude. This allows to reduce further the core anomalous transport in discharges where an internal transport barrier is generated by a high enough heating power [11].
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