Self -organization in cathode boundary layer discharges.

摘要

Cathode boundary layer (CBL) discharge, which has been developed as a UV light source, operates in a direct current between a planar cathode and a ring-shape anode that are separated by a dielectric with an opening of the same diameter as the anode. The nonthermal CBL discharges operate in a medium pressure range down to 30 Torr, emitting excimer radiation when operated with noble gases. The radiant excimer emittance at 172 nm in xenon reaches 1.7 W/cm2, and a maximum excimer efficiency of 6% has been obtained. The high excimer radiant emittance, in addition to low cost and simple geometry compared to other UV sources, makes CBL discharges an excellent choice for deep UV lamps and a candidate for integrated flat UV panels (Moselhy et al. 2004). It has been found that CBL discharges spontaneously give rise to regularly arranged filaments, i.e., self-organization, at a low current, e.g., less than 0.2 mA at 75 Torr (Schoenbach et al. 2004). In this thesis, the self-organization of direct current xenon discharges in the CBL configuration and parallel-plate geometry have been studied for a pressure range from 30 to 140 Torr and currents from 20 muA to 1 mA. Comprehensive examinations have been performed to investigate the behavior of those filaments by the use of optical, electrical, and spectral measurements. Side-on and end-on observations of the discharges have provided information on axial structure and distance of the filaments from the cathode fall. The electrical measurement has recorded a discrete I-V characteristic associated with the change of the numbers of the filaments. The spectral measurement provides scaling information on the relative population of high-lying states (1s 4, 1s5, and 2p6) of excited xenon atoms. Moreover, temperature measurement has revealed that the thermal electron emission from the cathode surface is negligible for the formation of filaments. The reactor geometry with parallel-plate electrodes analogously gives self-organization. The gas species, the cathode material, and the reactor geometry are varied to facilitate the understanding of the CBL xenon discharges and the self-organization. When krypton is used instead of xenon, rather homogeneous plasma far from organized pattern formation is observed with decreasing current. Of the tested aluminum, copper, and tungsten cathodes, the aluminum cathode achieved higher excimer intensity at 250 Torr than that of the molybdenum cathode by a factor of two. The diameter of the plasma reactor was reduced to 300 mum, and it gave rise to a single filament, illuminating with an enhanced excimer power density of 500 mW/cm2 at 62 Torr. Three mechanisms of these self-organizations are given and discussed in this thesis. The first mechanism explains that the axial electric field can initiate instability. This instability is caused by N-shaped negative differential conductivity (NNDC) in the vicinity of negative glow, which is attributed to electron-electron collisions. Positive feedback of the current density and the electric field due to the NNDC causes fluctuation to develop. Another positive feedback effect of the gas temperature is that the first Townsend coefficient can become dominant as, the current density increases. The filaments are assumed to undergo Coulomb force from the positively charged cathode fall channels and positive space charges on the surface of the surrounding dielectric spacer. The calculations, based on these assumptions on the Coulombic interactions, showed good agreement with experimental data. The second mechanism for the pattern formation is presented as to the development of Turing instability. The discussion is based on the idea that the regular arrangement of filaments is merely a result of general phenomena such as diffusion, ionization, or drift. A reaction-diffusion process with respect to the variation of local current density j and voltage v is the physical effect in the discharge. The numerical calculation was performed and obtained was a 2-D patterned structure exhibiting resemblance to the observed pattern. The transition time to self-organization was calculated to be 6.2 ms. A discussion of the dynamics of the fluctuations and the qualitative explanation for the pattern formation is presented. Benilov's argument (2007) is introduced as the third mechanism. The standpoint of this discussion shows an analogy to that of the second mechanism. The potential distribution behaves as the standing wave described by the Helmholtz equation in the discharge area. In conclusion, applicability of these three mechanisms to self-organization is discussed and compared.