Investigation of the feasibility of sensing transient velocity by means of gaseous ionization

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Virginia Polytechnic Institute


Sensitivity Sip is defined as the ratio of the fractional change of gas ionization current to the fractional change of gas pressure. A maximum obtainable sensitivity was calculated, Sip]max = -0.309 A/B V, where A, B are constants for a given gas within a particular range of field-to-pressure ratio. Experiments were conducted to investigate the sensitivity of air and krypton under different combinations of voltage, electrode separation and primary electron source strength. A curie of tritium placed on the central portion of either electrode produced a constant initial emission current and a high sensitivity. Sensitivity could be increased by using a stronger current source, higher voltage and a larger electrode separation. Observed values of Sip for high voltage at larger separation are greater than those expected even when breakdown occurs at a (pd) quite larger than the one for Sip]max. The fractional pressure change, Δp/p, is independent of the initial pressure, p, and is greater for a heavier gas subject to the same acceleration, a fact which suggests the use of a heavier gas to obtain a higher fractional current change for the same sensitivity. Krypton behaves in a similar way as air in the system. Operating current level may be higher than 10⁻⁵ ampere if stronger source is used. There is a limit in the separation for a given source configuration. Further increase in Sip and current level may be achieved through better design of the configuration and location of the current source between the electrodes. The fractional current change, Δi/i, depends more on Δp rather than on the initial operating pressure, p.

From the experiment performed, it appears that a gas system will perform satisfactorily as a transient velocity gauge.