Low energy sparks in dielectric liquids

Thermalization is the process during breakdown by which the plasma transitions from having low temperature neutrals to having high temperature. The rate and extent of thermalization are dependent on collisions between energetic electrons and neutrals, the amount of energy added to the system, and energy losses. This process has been studied thoroughly for sparks in air [1]. Recently microsparks in liquids has become a growing area of research and the fundamental physics of this type of discharge are yet unclear [2,3]. The process of spark generation in a liquid medium is thought to transition through several stages: 1) A low density region is generated, 2) localized corona breakdown in reduced field enhanced regions, 3) Streamer formation and propagation leading to complete breakdown 4) Thermalization of the steamers and formed conductive channel both prior to and after breakdown 5) Relaxation of the spark causing it to extinguish due to energy limitations. In order to gain a better understanding of this fundamental process we are exploring the effect of energy input on the degree of thermalization of sparks in dielectric liquids. It is hypothesized that when the total energy supplied for the spark is reduced it will prevent the complete thermalization of the spark resulting in a more non-equilibirum plasma. In order to gauge the temperature of the plasma optical emission spectroscopy is used. The energy of microsparks generated in mineral oil is controlled by varying the capacitance of the external circuit in the 1-10 pF range while the total energy of the spark is estimated from current measurements. Although it is difficult to obtain accurate estimations of temperature optical emission peaks characteristic of thermal plasmas, such as the presence and intensity of the hydrogen alpha peak, are used to establish trends associated with the temperature of the sparks.