Experimental Study Of Diffuse Arc Plasma And Application On High-intensity Light Source

Arc plasma, which possesses high temperature and high reactivity, is widely used in industrial processes, such as thermal spraying, metallurgy, machinery processing and material preparation. However, arc plasma at atmospheric pressure suffers from small volume, large gradient of plasma parameters and focused power due to its self-contraction effect. This feature owns high brightness and high radiation efficiency in the light source applications, but it is unsuitable for large-scale industrial applications, such as powder preparation, large area surface processing and high-intensity light source. In allusion to the self-contraction effect, this paper investigates the diffuse arc plasma by external conditions control method, and the diffuse arc and constricted arc are employed in research and development for arc plasma generator and high-intensity light source. First, this paper investigates the formation mechanism of diffuse arc plasma and observes the the evolution of rotating arc and cathodic arc root behaviors. Then we design a multi-electrodes plasma generator and obtain large volume diffuse arc plasma and laminar-plasma jet, and the formation of diffuse arc is discussed. At last, we explore the technical feasibility of producing high-intensity light source from this plasma generator and improve the traditional xenon lamp to design a high-intensity light source.The main contents and results are as follows:1. A magnetically rotating arc plasma generator is designed in this paper. The axial gas flow, behaviors of cathodic arc roots, and axial magnetic field are investigated explicitly. The formation mechanism of dispersed arc plasma is discussed. It was found that with arc currents200-400A, the dispersed arc plasma is homogeneous and stable, and arc voltage and its noise are greatly reduced with transition from the constricted arc to the dispersed one. This work confirm that the arc plasma is dispersed by convection between the arc and the ambient gas. With arc currents100-150A, the dispersed arc is derived from that the arc column with spiral structure expands in the radial direction, and connects to each other. Therefore, the dispersed arc owns a long discharge channel, so arc voltage and its noise are rather high.2. Various configurations of cathodic hot spots are observed, including spot/diffuse mode, multi-cathodic spot/root, and diffuse annular spot/root. The plasma parameters near cathode are diagnosed by a water-cooling electric probe and a high speed camera. The interaction between arc roots and arc column is discussed. Results show that with a pure tungsten cathode (5mm diameter), with the magnetic field and arc current increasing, the region of the cathode spot increases gradually, and at last the root translates into diffuse mode, whose highest temperature declined sharply. With a Lanthanum tungsten cathode (10mm diameter), with the increasing of magnetic field, arc currents or/and cathode temperature, the spots amount increases, and finally these spots evolve into a diffuse annular spot. Experiment verifies that the hot spot in this study represents a thermal trace of an arc root, also, a larger arc column is a necessary condition for a non-constricted cathodic arc root.3. A non-transferred DC arc plasma generator with multiple cathodes (3-18) is introduced in this paper for producing a large area homogeneous arc plasma as well as a steady plasma jet at atmosphere pressure. The influences of arc current, plasma gas and axial gas flow on plasma behavior are studied. The electron excitation temperature and electron density of diffuse arc plasma in the chamber are diagnosed by Optical Emission Spectrum. Results show that, as the arc current and helium flux increase, the the arc diffusion is enhanced, but self-contraction occurs at high arc current. The electron excitation temperature and electron density of diffuse arc is much lower than that in contractive arc column under the same current, which means the diffuse arc probably deviates from the LTE state. We infer that the diffusion depends on the region:d(σ/λ)/dT<0(λ,-thermal conductivity, σ-electrical conductivity). Also, atmospheric pressure helium laminar-plasma jet of long length and low noise is generated at gas feeding rate of0.5to2.0Nm3/h and input power of17kW.4. We explore the technical feasibility of producing high-intensity light source with multiple cathodes plasma generator. A high-intensity light source with circle fan system is designed.5. The effect of an axial magnetic field on a xenon short-arc lamp is experimentally investigated. The electric power and radiation power and ascend more than40%and70%respectively, and the radiation efficiency is improved by23%of the best increment at12mT magnetic field. Most of increment of the radiation intensity comes from the arc plasma area near the cathode tip, and partially from the arc column. Also, the influence mechanism of magnetic field to xenon lamp is investigated.6. Base on the traditional xenon lamp structure, a built-in reflector is added, so as to achieve a high-intensity light source. Results show that the irradiance is not homogeneous that the center intensity is5times more than the edge. With the increasing of arc current, gas pressure and electrode distance, both the radiation power and radiation efficiency rise sharply, while the instability of irradiance reduces as the arc current increases and electrode distance decreases.