Kakuyūgō kenkyū

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Some properties of a plasma producd by a giant laser pulse forcused on a surface of a metallic lithium target is investi gated.
Peak laser flux at the target is about 10¹²w/cm².
Measuremenents by double probes yield estimates of electron temperature of 3 eV and electron desity of 10¹⁵cm^-3.
Time-of-flight analysis of double probe signals shows that the plasma expansion velosity corresponds to an ion kinetic energy of approximately 500 eV. Time-resolved photographs reveal that, althogh the plasma appears to expand roughly symmetrivally about the original target posion, a part of the plasme moves away from the target with a normal velosity to the target of about5×10⁶ cm/sec.
When a uniform magnetic field (1000 to 5000G) is applied parallel to the target surface, the plasma is found also to move away from the target, traversing the magnetic lines of force almost as rapidly as it does with no magnetic fild_This PhenOmenon can be explained by the well-known E×B drift model of a plasma. An attempt is made to prove this model by short-circuiting the polarization electric field with a conducting plate introduced near the midplane of the magnetic fipld. The plate is placed with its surface perpendicular to the magnetic field so tnat the polarization electric field nay be short-circuited along the lines of force. The motion of the plasma across the field almost completely disappears, while the motion along tne lines of force becomes quite remarkable under these conditions.

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A method for measuring the diffusion of a plasma produced from a metal of low vapour Pressure with the aid of microchemical analysis is proposed. A pinholed cavity of a suitable material may be used as a complete sink of such a plasma stream : the plasma incoming through the pinhole will finally evaporate onto the inner surface of the cavity. The trace amount of the evaporated plasma is determined by flame photometry, the sensi tivity of which is.exceedinRly.high for alkali metals.
This method is quite useful as a diagnostic technique especially for lithium plasma.
Results are given on the application of this method to the B×B drift motion of a laser-produced lithirlz, plasma across a uniform magnetic field.

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