Plasma Physics

The interaction of two Maxwellian distributions of particles will differ from the interaction of the two particles at their respective Maxwell averages if the strength of the interaction varies significantly with energy over the widths of the distributions. This effect is considered here in particular for fusion (both D-T and D-D) for the mean energy of the reacting particles as a function of temperature and for the spectrum of the neutrons produced.

A drift kinetic equation is derived which contains higher order effects. The upper bound appropriate to the drift ordering is only imposed so that the result is quite general and can be reduced to previous drift equations. The differential geometry of the magnetic field enters only trivially and a single recursion suffices to obtain the desired result.

The present understanding (May 1983) of current drive by lower hybrid waves in Tokamak plasmas is reviewed with particular stress on the theory. First, the "classical" model of Fisch and its variants are discussed in light of recent experimental data. It is argued that these simple models do not account for most of the features of the results obtained. Next, effects of runaways on current generation are considered. It is shown that runaways, resulting from a residual ohmic electric field, may play an important role. Further, the mechanism of runaway-current sustainment, as proposed by LIU et al. (1982b) is re-examined. It is shown that, for a realistic range of parameters, this mechanism does not allow a significant current to be sustained. Finally, some results of a recently-developed numerical code, which include the evolving electron distribution function, ray tracing and transport, are briefly noted. It is concluded that none of the existing theories can be used to interpret satisfactorily all the experimental observations.

Stimulated Brillouin scattering and stimulated Raman scattering are important instabilities in the large underdense plasma encountered in inertial confinement fusion targets. Recent experimental observations are reviewed, with emphasis on the novel features which are observed as lasers become more powerful.

Processes for radiative power loss in optically thin high temperature plasmas due to electronic transitions in the field of an impurity ion are summarised. The theoretical calculation of the net power arising from these various radiative loss mechanisms is reviewed and a theory for the frequency distribution of radiated power presented. The description of radiative power loss from elements in dynamic states of ionisation is then addressed. Formal generalisation of current collisional-radiative theory is made to accomodate low metastable levels consistently. This is proposed as the basis for the most reliable power loss calculations. The state of practical calculations of the important collisional-dielectronic recombination and ionisation coefficients is briefly reviewed and suggestions made for the practical implementation of the metastable generalisation. Illustrative examples are given of power loss coefficients and functions, together with important intermediate quantities.

Three approaches to the problem of EUV and soft X-ray lasers are being pursued experimentally at the NOVETTE and OMEGA laser facilities. Photoionization pumping of neon gas may lead to a self- terminating population inversion in Ne II at 27 eV, and experiments planned at OMEGA are planned to observe amplified emission. Line radiation may be used to drive an inversion in helium-like or hydrogen- like fluorine and neon at energies between 53.6 and 151 eV. We have carried out an exploratory experimental sequence on NOVETTE designed to test integral laser targets. Experience gained points the way towards a second generation target design and future coincident spectroscopy and integral laser experiments. A third approach involves collisional excitation of 3p leading to a 3p-3s inversion near 68 eV. The scheme was tested at NOVETTE, and a null result obtained, which may be explained by beam bending caused by large index of refraction gradients. Phase correction is proposed as an improvement for the scheme.

We have studied the effects of a colder electron species on the propagation of fully nonlinear acoustic waves in a magneto-plasma with cold ions. Numerical analysis of the Sagdeev potential shows that in contrast to the unmagnetized case, where both potential hump and potential dip are possible, in a magnetized plasma only potential hump is found to exist.