Coherent Interactions of a Modulated Electron Beam with a Plasma (HXR-4)

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The HX-device, which consists of a high energy molecular ion injector and magnetic mirrors, has been developed and investigated from various angles. In such a mirror-like device, an injected ion beam becomes bunched owing to the negative radial gradient of the magnetic field and interactions of this bunched beam with a cold plasma in the magnetic bottle may cause instabilities. Accordingly, some fundamental experiments are required to research interactions of the modulated beam with the plasma. The amplification of microwave modulation on an electron beam by passing it through a plasma was observed experimentally by G. D. Boyd et al. and by G. S. Kino. In both experiments, strong amplification was found to occur when the plasma frequency was very close to the modulation frequency. It is the purpose of this note to show how the gain of the maximum amplification depends upon the velocity of the electron beam. A schematic diagram of the experimental apparatus is shown in Fig. 1. A hydrogen plasma is produced by the PIG discharge and diffused through a small aperture into a beam-plasma interaction region, where a ring probe coupled to the frequency analyzer is inserted to detect the plasma resonance. Plasma densities of the order of 10^6 cm^<-8> were measured in this region by a Langmuir probe with a background neutral pressure of 5x1O^<-5> mmHg. The apparatus is equipped with an electron gun instead of the ion injector and the electron beam of about 1 mm diameter was modulated by applying a high-frequency sinusoidal voltage to the grid of the gun. The whole system is immersed in the magnetic field to confine the beam and the plasma, and the beam is injected in the direction of the magnetic field. The field strength was 130 Gauss in the interaction region.
1964-07-05

Coherent Interactions of a Modulated Electron Beam with a Plasma (HXR-4)

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