Influence of TJ-II flexibility upon ECRH

The TJ-II is a l=1, m=4 stellarator. One of the main properties of this shearless device is its flexibility: Changing the currents in the circular and helical coils which form its hard core, the value of the vacuum magnetic field will vary and, hence, the properties of the magnetic configuration obtained will be very different. In fact, the rotational transform per period can vary in a wide interval: 0.22<ι(0)/4<0.628. The range of magnetic configurations achievable in this machine is presented in the flexibility diagram. The lines of constant rotational transform are plotted. As we go to the origin following a line, we obtain a configuration with deeper magnetic well and with less average minor radius. The depth of the magnetic well can reach the 6%. The relevant geometrical properties of the plasma are indeed changed. The average minor radius vary in the range of 10<a<25 cm, the plasma position and the indentation of the magnetic surfaces are very different from a magnetic configuration to another. Besides, the resonance position in the plasma, which corresponds to the B=1 T line, changes. The value of the magnetic flux and, so, the electron density and temperature are different. Then, it is clear that any calculation which lays on the plasma geometrical properties, like ECRH computation by ray tracing, should take into account this fact and so, any study should be extended to all the configurations. In this paper, we present the ECRH calculations for two configurations. The first one is the 'reference case', which corresponds to a rotational transform per period ι(0)/4=0.37, an average minor radius a=20cm. The magnetic surfaces have a small indentation. The second case is characterized by its high rotational transform ι(0)/4=0.55, its average minor radius is a=17 cm and its magnetic surfaces have a bigger bigger indentation. (author) 3 refs., 6 figs