Experimental Investigation of Plasma Production by Irradiating Solid Hydrogen Foils with an Intense Pulse Laser

Abstract

Foils of solid hydrogen were irradiated with a giant pulse ruby laser. The formation and ex-pansion of the plasma produced this way was investigated by time-resolved absorption measure-ments, by streak and framing photography, by holographic interferometry and by time-off-flight and charge collection measurements. The measurements yield a fairly complete picture of this process: After the initial breakdown in the solid a plasma is produced at the surface of the foil facing the laser. Owing to the high pressure of the plasma the solid hydrogen is compressed by a shock wave. This allows the layer in which the plasma is produced to penetrate into the interior of the foil with a velocity of 3 × 10⁶ cm/sec. Thus under the action of the focused beam foils up to 1 mm thickness are pierced by the light before the end of the pulse. The plasma cloud, ejected towards the laser, contains 4 × 10¹⁶ ions with a mean kinetic energy of 200 eV this corresponding to about half of the absorbed energy. The observations are compared with the well known results obtained in a plane, one-dimensional geometry. If allowance is made for the influence of the focusing of the laser beam on the structure of the shock front, the observations are readily understood.