1 - Nuclear Sciences and Applications - IAEA

638 ALLADIO et al.
generation of high energy electron tails. This mechanism is only effective if the
plasma density is lower than a critical value depending on frequency. Above this
value, the power is mainly absorbed by the ionspopulation or by excitation of parametric
decay instabilities at the plasma edge [3£-
In a previous experiment on the Frascati Tokamak (FT), a frequency of
2.45 GHz was used with a very high electron heating efficiency at densities below
5 x 10 13 cm" 3 [4].
As the density limit for wave-electron interaction is known to increase with frequency
[5], an 8 GHz system has been installed on the FT in order to investigate the
possibility of heating a high density plasma with LH waves.
The apparatus consists of four klystrons delivering up to 125 kW, each feeding
a grill composed by 4 x 4 waveguides with an inner dimension of 5 x 35 mm each.
The waveguide phasings used were mainly OOir7T and OTTOTT. The relative spectra are
shown in Fig. 1.
Owing to the loss in the transmission line, the results reported in this paper were
obtained with an RF power coupled to the plasma of less than 220 kW.
The experimental results concerning the coupling properties and the density
limit have already been reported [6] and can be summarized as follows:
(i) The coupling properties of the 8 GHz LH waves to the plasma are very
good. A power density as high as 10 kW-cm" 2 is routinely injected into
the plasma with a reflection coefficient of ~ 15%.
(ii) No evidence of a density limit has been observed in the range of explored
densities (n,. < 3.5 X 10 14 cm" 3 ),
(iii) By comparison with the 2.45 GHz experiments with the same target
plasma the density threshold for the parametric decay instability appears
to increase with frequency,
(iv) No fast ion tails have been detected in all the investigated experimental
conditions.
In this paper, we report on the experimental results concerning plasma heating
and accessibility.
2. PLASMA HEATING
An RF power of ~ 200 kW has been coupled to the plasma at two different densities,
n^ = 3.5 X 10 13 cm" 3 and Re = 10 14 cm" 3 . In both cases, the toroidal field
was 8 T, and the launched spectrum was the fast one, having 80% of its power at
ns < 2.
In these conditions, —25% of the power is accessible to plasma centre in the
low density case and less than 15% in the high density case.
In Fig. 2, we report the electron temperature profile, as measured by sevenpoint
Thomson scattering diagnostics, before and during the application of the RF