Energy confinement

The basic physics idea behind magnetic confinement fusion is relatively simple: it consists on extracting energy from well-known atomic reactions that take place once the appropriate conditions have been created. These conditions can be summarized by the so-called Lawson Criterion: the triple product nτET (where n is the plasma density, T the plasma temperature and τE the energy confinement time) must be above certain value.

Roughly speaking, the energy confinement time can be defined as the ratio between the energy content of the plasma over the net power input into the device by the external heating systems. Therefore, maximizing the triple product is equivalent to minimizing the input power needed to sustain a given plasma density and temperature. Since, in steady-state, the input power is basically equal to the energy flowing out of the plasma due to transport processes, undertanding and predicting energy transport in magnetic confinement devices is key for achieving fusion-relevant plasma parameters.

Neoclassical transport is caused by the combination of the motion of charged particles in an inhomogeneous magnetic field and particle collisions. In stellarators, the very unfavourable scaling of the neoclassical energy flux with the temperature makes it dominant in the core region in reactor-relevant scenarios. Therefore, predictive transport simulations with neoclassical transport at the core, complemented with simple models for turbulent transport in the edge, are typically used for estimating the confinement time, power load to the walls, needs of heating… of stellarators. This approach has to be supported by a step-by-step systematic validation of the predictions of neoclassical theory with experimental estimates of energy transport.

Examples of such validation studies are:

Other related results can be found in:

  • Tallents, D López-Bruna, J L Velasco, M A Ochando, B Ph Van Milligen, V I Vargas, J J Martinell, D Tafalla, J M Fontdecaba, J Herranz, E Blanco, F Tabarés, T Estrada, I Pastor and the TJ-II Team. Transport analysis in an ECH power scan of TJ-II plasmasPlasma Physics and Controlled Fusion, 56(7):075024, 2014. PDF

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