Ultrasonic sensing of a plasmoid

An atmospheric plasmoid that is obtained in a discharge above the water surface has been known since 2000 and studied in several laboratories. The main parameters, in particular, the gas temperature, must be measured for interpretation of this phenomenon. The temperature measurements are complicated, since the plasmoid ascends in air and can be detected by sensors at a relatively short time interval. A method for the ultrasonic sensing of a plasmoid is developed. A scheme of the setup for the ultrasonic sensing is presented and a procedure is described. The speed of sound in a gas is one-to-one related to the gas temperature. The plasmoid temperature is calculated using the speed of ultrasound on the assumption that the plasmoid is formed from water vapor. In accordance with the experimental results, the plasmoid temperature decreases from 2800 to 600-700 K over a lifetime of 500 ms. A decrease in the temperature results from heat emission and mixing with surrounding air.