Lower hybrid ion heating in the FT-2 tokamak

Abstract

The paper generalizes the results of plasma heating experiments in the FT-2 tokamak. It is shown that the generation of fast ions is associated not with the region of linear transformation, nLH, but with the parametric decay region, nd ≃ (0.3-0.5)nLH. In the narrow zone nd, strongly slowed down waves were observed (by a method relying on enhanced scattering of microwave radiation) at a frequency of about 0.7f0 (where f0 is the frequency of the heating radiation); these waves had a narrow spectrum with a width of a few megahertz. As the plasma density increases, the decay region is shifted towards the edge, but it remains ‘attached’, for all practical purposes, to one local density. This process helps to explain the central heating of the main ion component, the generation of fast ions at the edge, and the switching off of the interaction between the lower hybrid wave and the electrons, which stops current drive.

The behaviour of fast ions in tokamak experiments

“…The spatial location of the fast ion tail seems to depend upon the density profile (Table 11). On FT-2, passive charge exchange profiles suggested that the tail moves from the edge to the centre with increasing density [172]. Both edge and central profiles have been observed on other tokamaks, but the density dependence was not reported.…”

“…Without beam injection, many experiments have noted a direct correspondence between parametric decay of the fundamental LH wave and tail formation [158,166,167,171,172,241,2421. In these experiments, the tail develops at a lower density than that predicted for direct ion heating by LH waves (that obey the cold plasma dispersion relation).…”

The behaviour of fast ions in tokamak experiments

“…15). This threshold energy is typically ten times larger than the central ion temperature (Table II), scales with Ti0 [172] and is independent of the RF power [160, 1721. Comparison of different parallel wavelength spectra in WEGA found a lower value of Emin and a more intense tail for higher values of 7211 [164]. The critical density for tail formation depends only weakly upon toroidal field.…”

“…The perpendicular wavenumber kl predicted by the cold plasma dispersion relation was too small for direct interaction between the LH and the beam ions. Without beam injection, many experiments have noted a direct correspondence between parametric decay of the fundamental LH wave and tail formation [158,166,167,171,172,241,2421. In these experiments, the tail develops at a lower density than that predicted for direct ion heating by LH waves (that obey the cold plasma dispersion relation).…”

“…In these experiments, the tail develops at a lower density than that predicted for direct ion heating by LH waves (that obey the cold plasma dispersion relation). The most detailed study was performed in the small tokamak FT-2 [172]. In this device, the fundamental wave decayed into a daughter LH wave with w N 0.7Wo in a narrow region in the plasma [172].…”