Comparison of kinetic and cold fluid models of three-wave mixing and Thomson scattering

Abstract

The use of the cold fluid (CF) model for describing scattering from thermal fluctuations was initially suggested by Sitenko (1967) and has since been widespread. This use of the CF model was shown to be inappropriate by Bindslev (ibid., vol. 35, p. 1615 (1993)) where the low-temperature kinetic (LTK) model was proposed instead. Sitenko in his present comment (ibid., vol. 37, p. 163 (1994)) appears to reject the findings of Bindslev but only deals with scattering from cold collective fluctuations, not thermal fluctuations. Here we confirm the findings by Bindslev that the CF model is inappropriate for describing the bilinear interaction of thermal waves and, in particular, scattering from thermal fluctuations. We note that although both the LTK and the CF models describe scattering in collisionless plasmas and both models involve only fields and zeroth- and first-order moments of distribution functions, they are nonetheless fundamentally different: within the limits of the collisionless plasma model, the LTK model makes no assumptions about the nature of the interacting waves and fluctuations. The CF model, on the other hand, has only been shown to be correct when the cold fluid relations apply to both interacting waves and thus is only guaranteed to correctly describe the interaction of cold collective electron oscillations in a collisionless plasma. We demonstrate that the use of the CF model in present microwave scattering experiments on fusion plasmas can lead to significant errors, and finally note that there is no computational advantage in using the CF model rather than the LTK model.