Spark Energy Measurements using an Optical Technique

This paper presents the results of an investigation into light emitted from electrostatic discharges. Measurements of emitted light intensity were made by focussing the image of a spark onto the detector of a photomultiplier and integrating the photomultiplier signal. The apparatus was calibrated by comparing the integrated signal with that released by the discharge of a capacitor through a spark gap. The relationship between spark energy and total emitted light was found to be linear in the energy range 0 - 0.2 J. The technique was used to measure spark energies with a resistor in series with the spark gap. Energies so measured were compared with those calculated by assuming that the resistance of the spark remained constant during the discharge. The results showed that the resistive model was accurate to within 15%. Measurements were also made of light intensity variations and hence energy density distributions along the spark axis. Sparks were scanned to a resolution of 0.5 mm and the total emitted light recorded at each position along the axis. Both symmetric and asymmetric electrode configurations were studied at a variety of spark energies and inter- electrode distances. The results showed a number of important trends. At low mean energy densities and small distances, there was a maximum in emitted light, of up to three times the mean, close to the cathode. As the inter- electrode distance was increased, a second, smaller maximum appeared near the anode. Increasing the mean energy density caused the maxima to move away from the electrodes towards the middle of the spark channel. The paper concludes with a brief description of the implications of these results to ignition by electrostatic discharges.