Theoretical and Experimental Studies of High-Frequency Plasma Structures

The author has studied high-frequency structures at the resonance ωec = ωHF in an inhomogeneous magnetic field. The following points have been developed theoretically: the complete (relativistic and non-linear) analytical description of individual movement, taking into account the fact that the number of revolutions of the particles in the resonance zone is high; study of the self-consistent space charge field, with a view to establishing the conditions necessary for the energy of the ions to be of the order of the energy gained by the electrons in the resonance zone; calculation of the self-consistent HF field in the accelerated plasma, with a view to establishing the conditions for optimum operation of the HF accelerator; the problem of the mirror losses. The experimental results were obtained with various devices constructed for the purpose of verifying the theoretical predictions. With the Circé-Pléiade, where the ionization and acceleration functions have been separated by means of ionization with a Circé device (λ = 3 cm, 300 W), it has been verified the relativistic law of variation of the parallel ion energy as a function of the injected density (existence of a minimum density for the entrainment of ions) and of the applied HF power (TE111 cavity, 3 GHz, 700 W). With the Circé accelerator (λ =3 cm, P = 2.5kW, CW) it has been verified that, for nplasma nc , because of the strong attenuation of the field due to the high index. With the Icare device (λ = 25 cm, PHF = 1 MW for 100 μs), operating with pre-ionization by laser focalization on a solid D2 target with an injection density always higher than the cut-off density, accelerated deuteron currents of more than 20 keV and of about 10 mA/cm2 were obtained. Ion entrainment for a magnetic field twice that necessary for resonance was also verified. Finally, with the Pléiade I device (λ = 100 cm, PHF ∼500 W, CW), the reflection of accelerated plasma jets by a mirror pulse lasting 75 μs was verified. From measurements of the fast neutrals resulting from ion charge exchange (with the residual gas; p ≤ 10-4 mmHg) it was deduced that the density of the ions (directed energy ∼ 1.5 keV) may be increased from 2 x 108 cm-3 (direct beam) to 6 x 109 cm-3 during the mirror pulse. The accumulated density appears to be limited by the charge exchange, and the increased duration of the mirror pulse to 200 μs with p ≲ 10-4 mmHg (unchanged) hardly improves the accumulation ratio. This agrees with the predictions if one takes into account the fact that the ion lifetime is ∼ 10 μs. (author)