Guiding Of Gun Plasma By A Permanent Magnet octopole field

Plasma from a coaxial gun has been guided 1 m by a linear octopole field generated by an array of permanent magnets. The purpose is to separate the plasma from the initial filling gas and fast charge-exchange neutrals. The gun has a 5 cm o.d., 1.25 cm i.d., and 45 cm length. The power supply is a 20 kV, 30 μF low-inductance capacitor band delivering 100-150 kA. The gun is initially filled with 2 std. cm
³
of He by a fast gas valve. The cylindrical ceramic permanent magnets are 22 mm dia. x 25 mm long and have a strength at the poles of 1700 gauss. They are arranged in eight linear arrays which bound a cylindrical volume 11 cm dia. x 120 cm long. Approximately 50% of the surface area is unobstructed to allow the escape of neutrals. The plasma velocity is 2-6 cm/μsec and depends on the delay between the operation of the valve and gun. Calorimetry indicates that 30% of the plasma is delivered to the end of the guide field. The actual fraction is probably higher due to the calorimeter responding to fast neutrals and sputtered metal when near the gun. Optical opacity measurements indicate that the plasma is transparent to the VUV recombination radiation generated when the plasma strikes a small target. Inteferometric measurements are in progress to determine the plasma density.