Possible nonlinear effects in electron-cyclotron plasma production and heating by microwave radiation are analyzed. They include nonlinear regimes of electron acceleration in strong microwave fields under cyclotron resonance conditions, nonstationary regimes of microwave beam self-focusing, and some parametric processes. Presented results may be of importance for predictions of plasma response to electron-cyclotron resonance (ECR) heating or preionization by powerful radiation.

When hydrogen is diffused into calcium fluoride crystals which contain a rare earth impurity and a relatively low concentration of oxygen two types of RE3--H-complexes of lower than axial symmetry are formed. These centres have been detected by measurements of the infrared absorption .from the localized vibrations of the paired H-(or D-) ions. In crystals containing Ce3+ ions, a new EPR spectrum has been observed which correlates with one of these centres. Models for the centres are discussed. and it is suggested that one may be a simple,modification of the trigonal (T,) centre R E 3 -0 2 -F ; , where one of the Fions is replaced by an Hion.

Absorption of microwave energy at electron-cyclotron frequency is a promising method of heating plasma to thermonuclear temperatures in large toroidal devices in which the energy confinement time exceeds the characteristic time of energy exchange between the electron and ion components of the plasma. The smallness of the wavelength compared with the dimensions of the system makes it possible to use quasioptical waveguides which are easy to match with the plasma. The microwave power, injected in the form of an ordinary wave from the outer part of the torus in the direction of increase of the toroidal component of the magnetic field, is efficiently absorbed if the dimensions of the tokamak are greater than or of the order of those of the T-10 device and the initial temperature of the plasma is greater than 1 keV. In the quasi-continuous heating regime and with reasonable microwave beam cross-sections non-linear effects have no appreciable effect on the cyclotron heating efficiency.

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A plasma-resonance field-ionization instability of uniform gas breakdown produced by intense laser fields via tunneling ionization of atoms is studied theoretically and by computer simulation. The field amplitude and produced plasma are found to be unstable relative to spatial modulation in the direction of electric field with the spatial period shorter than the wavelength. In a dense gas the process, at the nonlinear stage of instability, becomes explosive and leads to the formation of thin resonance layers and sharp peaks of the field amplitude.[S0031-9007 (97)02954-2] PACS numbers: 52.40.Nk, 52.50.JmIt is well known that a powerful electromagnetic wave beam in a medium with positive (focusing) nonlinearity is subject to filamentation instability with a characteristic transverse scale large compared to the wavelength. This instability was predicted about thirty years ago based on the paraxial approximation for the scalar wave field [1]. Its manifestations were repeatedly observed in experiments with powerful laser and microwave pulses (see, for example, [2,3] and references therein).A less known fact is the existence of "vector" smallscale instability of the wave in a transparent medium with a "defocusing" (ionization-type) nonlinearity. This instability was originally described based on the vector wave equation [4], and studied theoretically and experimentally [5-7] as one of a wider class of ionizationfield (or electrodynamic) instabilities of high-frequency and microwave discharges in gases. Further, we will use the term "plasma-resonance ionization (PRI) instability," revealing the underlying physical mechanism of its generation. The PRI instability results in the filamentation of the wave and produced plasma with density gradients parallel to the wave electric field and with the spatial period shorter than the wavelength. So, it may be considered as an ionization analog of the known modulation instability of the field in a collisionless plasma with positive (ponderomotive-force-induced) nonlinearity. However, unlike the latter, it evolves not only in a narrow plasma resonance region (near the critical-density surface) but covers the entire transmission medium and affects significantly its electrodynamic characteristics.The PRI instability, probably, has not yet been observed in experiments with powerful ionizing laser pulses, since the majority of the experiments realized the electronimpact (avalanche-type) mechanism of gas ionization; the characteristic time of instability for this mechanism turns out to be longer than the time of the avalanche itself or the time of gas heating. However, advances in the generation of powerful laser pulses with field amplitudes comparable to atomic fields have stimulated interest in studies on the dynamics of the laser breakdown determined by opticalfield-induced (tunneling) ionization of gas atoms [8][9][10][11][12][13][14].The growth rate of the PRI instability with this ionization mechanism, as we find below, can be high enough so that, at the final stage of the breakdo...

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The self-action features of wave packets propagating in a two-dimensional system of equidistantly arranged fibers are studied analytically and numerically on the basis of the discrete nonlinear Schrödinger equation. Self-consistent equations for the characteristic scales of a Gaussian wave packet are derived on the basis of the variational approach, which are proved numerically for powers P < 10Pcr exceeding slightly the critical one for self-focusing. At higher powers, the wave beams become filamented, and their amplitude is limited due to nonlinear breaking of the interaction between neighbor light-guides. This make impossible to collect a powerful wave beam into the single light-guide. The variational analysis show the possibility of adiabatic self-compression of soliton-like laser pulses in the process of their three-dimensional self-focusing to the central light-guide. However, the further increase of the field amplitude during self-compression leads to the longitudinal modulation instability development and formation of a set of light bullets in the central fiber. In the regime of hollow wave beams, filamentation instability becomes predominant. As a result, it becomes possible to form a set of light bullets in optical fibers located on the ring.