Low Pressure Rare Gas - Mercury Discharge For Striation

Stratified discharge was fisrt discovered by Michael Faraday in 1930s. Research on striations in low pressure gas discharge has witnessed considerable achievements. Nevertheless, most of the works focused on pure rare gas discharge, research on striations in AC discharge of gas mixtures still remains to be done. In the present thesis, investigation on moving striations in low pressure rare gas and mercury discharge is performed, where reasons and properties of this phenomenon are explored. By doing this work, we hope that reasons for this phenomenon can be discovered; properties of low pressure rare gas and mercury discharge can be better understood. Thereby, this phenomenon can be removed from fluorescent lamps in the future.In our experiment, we collect emissive spectra of the lamp under different cold spot temperature. We find that strong moving striations always coincide with substantial ratio of rare gas radiation in total spectrum. Theoretical analyses reveal that moving striations in low pressure rare gas and mercury discharge are incurred due to dominance of stepwise ionization of rare gas atoms in ionization balance of the plasma. Then we perform high-speed imaging to the lamp with an ICCD cameara and find that moving striations only appear on the rising edge of lamp current. On the falling edge, the discharge is uniform. This phenomenon is rarely mentioned in previous works. We couple the ICCD cameara with a monochromator and record emissive atomic lines in the working period of the lamp. Results demonstrate that on the rising edge of lamp current, radiation from both rare gas atoms and mercury atoms is strong; on the falling edge, radiation from both species become weak. Therefore, we can expect that on the rising edge of lamp current, external electrical energy is injected into the plasma, which causes excitation and ionization frequently happen. During this periond, lack of mercury atoms may result in dominance of stepwise ionization of rare gas atoms in ionization balance, which incurs moving striations.In our theoretical investigation, electron response to spatially periodical electric field in low pressure Ar-Hg discharge is examined in the framework of kinetic theory. The simulation is based on non-local stead-state Boltzmann, which is numerically solved with Crank Nicholson scheme. Simulation results depict in detail the distribution of electrons with different energy in spatially periodical electric field and how non-local effect influences the discharge. The EEDF can be divided into two parts:bulb part and tail part, which stand for slow electron and fast electron. These two kinds of electrons have different response to electric field. Slow electrons almost drift along the electric field, while fast electrons tend to collide with and pass their energy to Ar and Hg atoms. Simulation also infers that macroscopic parameters of the plasma such as normalized electron density, electron temperature, excitation frequency and ionization frequency react anomalously to the electric field, which provides the necessary motivation for the propagation of the fluctuation.According to experimental and simulation results, major causes of striations come from stepwise ionization of metastable rare gas atoms dominating ionization balance. This leads to non-local EEDF, which enables amplification and propagation of instabilities. Therefore, the feasible method to eliminate striations is to suppress production of metastable rare gas atoms. This can be realized by increasing discharge current, increasing Hg vapor pressure; and increasing discharge frequency.This thesis bears creative points as follow:1. Provide electrode heating current to the lamp to suppress disturbances in cathode region; then proves that under the experimental condition, striations in low pressure rare gas mercury discharge are not originated from cathode region.2. ICCD high speed imaging reveals that striations only appear on the rising edge of the light signal; on the falling edge, striations disappear.3. Solve the Boltzmann numerically by Crank-Nicholson scheme and simulate electron response to spatially periodic electric field in low pressure Ar-Hg discharge. Motivation of propagation of striations is discussed.