1 - Nuclear Sciences and Applications - IAEA

652 BUDNIKOV et al.
•a I -a
10'
120eV
.320eV
0 1 2
E (keV)
FIG. JO. Charge exchange particle energy spectrum for optimum ion heating.
calculated curve for the LH resonance for f = 600 MHz, N! = 3, Zeff = 2 is
presented in Fig. 9. We see that the ion generation region corresponds qualitatively
to the resonance curve for 600 MHz and not for 920 MHz.
Hence, it is clear that the optimum LH heating conditions consist in the equality
of the central density to the threshold density r^ « n*. Besides this, the following
circumstances should be take into account: (i) this density decreases when we raise
the RF power; (ii) during the RF pulse the density of the plasma increases more
strongly the higher the power, and this results in shifting the region with n* to the
periphery. Both these facts reduce the heating efficiency when the RF power is
raised. The choice of the optimum density was achieved by the maximum enhancement
of the change exchange flux for the energy E = 1 keV, which is important for
the bulk ion heating, and by the minimum flux for E = 3.75 keV at the beginning
of the RF pulse. At B = 2.6 T and Ip = 35 kA, a density of r^ = 1.4 X 10 13 cm" 3
appeared to be optimum: at PRF = 120 kW an ion heating from 120 to 320 eV was
achieved (Fig. 10). By carefully choosing the optimum conditions, we obtained a
directly proportional dependence of ATj on PRF in the range of 30-120 kW.
Thus, our experiments allow us to suggest that LH ion heating in a tokamak
occurs in the following manner: transport of the energy inside the tokamak occurs
at a generator frequency f0 up to a density of n* « (0.3-0;5)nLH, where, as a result
of the parametric decay, two strongly slowed-down waves at frequencies ~0.7f0
and 0.3f0 are generated and interact effectively with the ions (Fig. 11). According
to this model, RF energy is easily transported in the central parts of the tokamak,
as the wave f0 easily reaches the region of n* « nLH; however, the energy cannot
penetrate into the centre with density nLH, as was demonstrated by the ray tracing