FIG. 3. Observation of the decay of the derivative of the resonance line at A = 0. The measured time constant is 68±5 sec. the same time constant: 65 ±3 sec.This observation that the spin-lattice relaxation times of the nuclei and the interaction system are equal implies that these two subsystems are strongly coupled to each other.If no such coupling existed, the relaxation time of the interaction system TJ^1 would be of the same order of magnitude, i^ie, 1 as the Zeeman spin-lattice relaxation time r^, which is about 0.5 sec at this temperature. The asymmetry of the Nd line should then have disappeared within a few seconds.Further investigations in other crystals and under different experimental conditions are in progress and will be published in a forthcoming paper. *A. G. Redfield, Phys. Rev. 98, 1787 (1955). 2 B. N. Provotorov, Zh. Eksperim. i Teor. Fiz. 41_, 1582 (1961) [translation: Soviet Phys.-JETP 14, 1126 (1962)]. 3 M. I. Rodak, Fiz. Tverd. Tela 6, 521 (1964) [translation: Soviet Phys.-Solid State 6, 409 (1964)]. 4 V. A. Atsarkin and S. K. Morshnev, Zh. Eksperim. i Teor. Fiz.-Pis'ma Redakt. [translation: JETP Letters 6, 288 (1967)]. 5 T. J. B. Swanenburg, thesis, Kamerlingh Onnes Laboratory, Leiden, The Netherlands, 1967 (unpublished); G. M. van den Heuvel, C. T. C. Heyning, T. J. B. Swanenburg, and N. J. Poulis, Phys. Letters 27A, 38 (1968). 6 W. Th. Wenckebach, T. J. B. Swanenburg, H. Hoogstraate, and N. J. Poulis, Phys. Letters 26A, 203 (1968). 7The shift of the Nd resonance signal which is due to a change of the internal field caused by the polarization of the protons can be estimated to be an order of magnitude smaller than the effect observed here. Moreover, the symmetry of the line should not be affected by this internal field.The effect of electric field strength of the incident wave on the transmission of electromagnetic waves through an anisotropic plasma has been investigated using field strengths of 27.4 to 137 V/m. Near the electron cyclotron frequency increased transmission occurs for the strong field strengths.Most investigations of electromagnetic wave interaction with plasmas are based on a "smallsignal" approach where it is assumed that the electric field strength is too weak to affect appreciably the properties of the plasma. This approach has the advantage of simplicity, since the processes can be described by linear equations. Significant research has also been devoted to the other extreme case of very strong electromagnetic fields which can result in breakdown or in parametric excitation 1 of various wave frequencies. 583

Investigations of strong-field electromagnetic wave interactions with plasmas have been conducted. In the nonresonant frequency regions, the electron velocity distribution function is Maxwellian and considerations based on Maxwellian theory agree with experiment. At resonant and near-resonant frequencies considerations based on an electron velocity distribution which is non-Maxwellian explain the observations if enhanced electron diffusion is absent. However, at near-resonant frequencies, enhanced electron diffusion due to strong fields can be of great significance in reducing the electron density.

The reflection of both right-hand and left-hand circularly polarized plane electromagnetic waves normally incident along the direction of an imposed static magnetic field in an anisotropic plasma, has been investigated. Detailed measurements at a frequency of 9.2 Gc of the intensity and phase of the wave reflected from a helium afterglow plasma over wide ranges of plasma frequency and electron· cyclotron frequency were carried out. The measured reflected power varied from less than 1 percent of the incident power to complete reflection, and large undulations in both intensity and phase of the reflected wave with changing electron density and magnetic field were observed. Comparison with theory based on a simple sharply bounded uniform plasma slab shows good agreement for the general nature of the dependence of the reflected wave on the plasma properties. The exact electron densities corresponding to the maxima and minima and the absolute values of the intensity and phase of the reflected wave are not accurately predicted by this simple theoretical model. In the region of the electron-cyclotron frequency, for an incident right-hand circularly polarized wave, the reflected signal undergoes a reduced variation in phase, and the intensity, after attaining a maximum, is observed to decrease with increasing electron density.

No abstract

An extensive quantitative experimental investigation was conducted on the influence of a magnetic field on the radiation from a horn antenna situated behind a layer or sheath of plasma. The magnetic field direction in the experiments was along the principal direction of radiation of the antenna which is normal to the layer of plasma. The experimental results were compared with theory based on the reciprocity theorem. For the left circularly polarized wave radiation pattern, propagation effects at off‐normal incidence corresponding to the condition for which the ordinary wave mode switches to the extraordinary wave mode as the electron density passes through the critical density are shown to be of importance. For the right circularly polarized wave mode, significant departures in the measured radiation pattern from the theoretical occur in the region of the electron cyclotron frequency and below. Satisfactory correspondence between theory and experiment is found for values of the electron cyclotron frequency that are greater than the wave frequency.