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EXPERIMENTS have been conducted to focus pulsed optical radiation on to the surface of the Moon and to detect the echoes.A ruby optical maser radiating pulses of approximat,ely 50 joules energy, 0•5-msec. duration, at 6934 A.was used as the source. The transmitting optical system included a Cassegrainian telescope of 12-in. diameter. The echoes were received on a Cassegrainian telescope of 48-in. diameter, passed through an interference filter of 7 A. band-width and were detected with a photomultiplier tube of spectral response type S-20, cooled to liquid nitrogen temperature. The field of view of the receiving telescope was 0•2 milliradians. The photoelectron count obtained in a 0•5-msec. interval at the expected time-delay was compared with tho counts obtained in 0•5-msec. intervals where no echoes would be expected and where the only relevant contributions to the count were those due to noise, that is, to Earthlight and scattered light (photoelectric dark current was negligible).

Plasma electron heating was accomplished by exciting plasma oscillations with a pulsed, 10-kV, 1-A electron beam drifting in a longitudinal magnetic field of 100 to 1000 Oe. The oscillatory energy gained by plasma electrons leads to an increase in their density and mean kinetic energy. The resulting plasma, which is called the ``beam-plasma discharge,'' is an rf discharge in which a beam-plasma interaction generates the rf fields. At the beginning of a beam pulse, beam electrons generate a plasma by ionization of the background gas. Within a few microseconds excited plasma electrons become the dominant ionization source. This paper describes the transient rf oscillations that appear in the first few microseconds of the beam pulse. From a detailed study of this transient it was concluded that a nonconvective instability initiates the oscillations, which first appear at the electron cyclotron frequency, and that a convective instability at the plasma frequency generates the oscillations that sustain the discharge.

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