Energy and particle transport

To confine a plasma means to produce a discharge with a hot core and a cold edge. Such a configuration can be maintained only if the plasma has the capability of retaining heat in the central part of the plasma column. Such a capability can be quantified in terms of the so-called energy confinement time which measures the characteristic decay time of the plasma energy if the heating system is swiched-off.

Experimentally, a minimum value for the tokamak confinement is obtained in the so-called low-confinement regime (L-mode). However, most of the tokamak devices which operate in the presence of a magnetic separatrix, exhibit a transition to the so called high-confinement regime (H-mode), characterized by the formation of a transport barrier at the edge of the discharge. The ITER parameters have been chosen in order to operate in the this regime. Even though the L-mode confinement plays a minor role in a reactor, its understanding may increase the degree of confidence in the knowledge of tokamak physics.

The energy confinement is the result of turbulent processes occurring in the plasma, and their correct description is still a subject of active studies. Therefore, in order to describe the confinement in L- and H-mode an empirical approach is used: the experimental data of the existing tokamaks have been used to determine empirical scaling laws

for the confinement time in the various regimes. At present, the scaling law which better describes the existing L-mode data is the ITER89-P scaling, obtained using the data of auxiliary heated tokamaks. The comparison with the results of tokamaks with ohmic heating only is particularly interesting since it allows an independent validation of the scaling law. FTU has provided a significant contribution to the validation of such empirical scaling law since it can cover a range in magnetic field which is not covered by other tokamaks, as shown in the figure, and which includes the magnetic field value of ITER.

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