Role of gas breakdown in the charging and discharging of macron clouds

When a pile of dust is dispersed by a blast of air, a charged cloud is formed. The action of gas breakdown due to intense space-charge fields generated on dispersal is shown to explain a surprising invariance in the net charge of the cloud with flow velocity, particle size and material. When the cloud subsequently discharges to a small earthed electrode, the associated energy is of the order of that required to initiate a chemical explosion (this may be of significance in supertankers, where discharges from a charged mist are suspected to be a possible cause of explosions). The collision of particles in an electric field is shown to be an effective charging mechanism during dispersal, and later in the creation of a dust ´plasma´. Electrical oscillations of approximately the calculated `plasma¿ frequency (103¿104 Hz) are observed.