Fast-Acting Gas Valve

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Abstract
The design and operation of an electrodynamically driven valve, which is employed to deliver rapidly a gas pulse (0.6 cc STP) with a sharp front to the annulus of a coaxial plasmoid source of small aspect ratio (2‐mm annulus at 2‐cm radius), is described. The spring loaded piston holds the plenum closed against pressures up to 2 atm. The piston is driven open by eddy currents induced by discharging a capacitor through a coil surrounding the plenum. A piston velocity of 5 cm/msec is achieved in less than 20 μsec and is maintained for 7 mm of travel. The arrival of a gas front (air) at the annulus was detected in 100 μsec after firing, employing hot‐wire anemometry. The subsequent axial position and velocity of this front in the source annulus were determined as functions of time and position, respectively. It was found that the initial axial velocity of the front exceeded the stagnation sound speed (though not the vacuum escape speed) and remained constant until the displacement thickness from wall friction reached the half‐annulus spacing.

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