The Experimental Study Of The Magnetically Rotating Arc And The Dispersed Arc Plasma

Arc plasma technology is widely used in industry and industrial processes, such as machining, metallurgy, the chemical industry, material preparation and environmental protection. The arc plasma is a highly-concentrated energy source, with small volume and large gradients of temperature and other parameters, due to its strong constriction. The application of arc plasmas is limited by these characteristics, especially in the areas of surface, film and material preparation. The characteristics have adverse effects on the production efficiency, and the uniformity and stability of the product.The magnetically rotating arc plasma can heat the working medium and the chamber uniformly and increase the heat transfer rate, and the electrode erosion can be reduced and the electrode lifetime increased. This article considers the configuration and characteristics, and the mechanism and evolution of the magnetically rotating arc, and the characteristics of the dispersed arc plasma in an arc plasma generator with coaxial electrodes (rod-like cathode and cylindrical anode). The effect on the magnetically rotating arc and the dispersed arc plasma of the axial gas flow, arc current, axial magnetic field (AMF), electrode material and the configuration of the electrode and the generator is also studied. The main results are as follows:As arc current and AMF increasing, the evolution of the arc configuration shows: a spiral arc, partly dispersed parallel arcs with diffusive anode arc root and fully uniform dispersed arc plasma successively. The modes of the cathode arc root are single spot, multiple spots and annulus or fully diffuse on the cathode end face successively as the heat to the cathode by the arc increasing. Specific content as follows:1) The magnetically rotating arc has a spiral structure that is quasi-stable with respect to the AMF. The arc column is in a process of repeated "development and fracture" due to the viscous resistance of the electrodes:the spiral structure would develop with the arc rotating. And then the spiral arc can be destroyed due to breakdown between the arc column and the anode. The arc voltage fluctuates with an approximately-constant frequency. Relatively-low values of the AMF, arc current and axial gas flow promote stability of the spiral arc.2) The constricted arc becomes partly dispersed with a diffuse anode arc root for increasing AMF and arc current. The multiple-anode arc roots and parallel dual-arcs occur for200<I<400A,0.24<I·B<4.8A T, and axial gas flow3Nm3/h in a80mm diameter generator. The multiple-anode arc roots are formed by partial shrinking of the dispersed arc plasma with diffusive arc attachment to the anode. Parallel dual-arcs consist of two partly diffusive arc columns and corresponding anode arc roots. The arc current keeps transferring between parallel arcs. The extinguishment of one of the anode arc roots means the end of the parallel dual-arcs.3) The arc disperses until it fills the whole cross-section of the chamber when the AMF and arc current are increased. The temperature of the arc plasma decreases, while the electric field intensity increases. The arc plasma has a decreasing volt-ampere characteristic. The average voltage increases with the AMF. The configuration of the dispersed arc plasma is disc-like, and mainly located at the side and the end-face of the cathode. The thickness of the arc plasma initially decreases and then increases, and the electron temperature decreases, from the cathode to the anode. The speed and level of the arc dispersion improve with arc current and AMF increasing and the axial gas flow decreasing.4) The configuration of the cathode arc root is mainly relative to the cathode temperature and the dispersion of the arc. The rotating arc keeps heating the cathode and the cathode temperature increases gradually. The configuration of the cathode arc root develops form a single fixed spot to multiple fixed spots. The number of the spots increases with AMF increase. The cathode arc root of the constrictive arc column moves between the spots while the dispersed arc plasma may have multiple cathode arc roots.The experimental results confirm that the dispersion of the magnetically rotating arc is a consequence of the high-speed rotation of the arc due to the AMF, and convection driven by the cold axial gas flow. The plasma is uniform and stable due to its high-speed rotation, and the diffuse anode arc root has a uniform distribution in the azimuthal direction.In the closed generator, the arc may move counter to the Lorentz force induced by the cathode self-magnetic field, means the arc draws back in the axial direction. The greater the AMF, the larger the distance of the arc counter-motion. The mechanism of this phenomenon is thought to be the difference between the pressures in front of and behind the arc in the axial direction, induced by the change of the flow field inside the chamber due to the increased rotation of the arc. The location of the arc is mainly determined by the balance between the pressure difference and the Lorentz force induced by the cathode self-magnetic field.The current density, radius of the cathode arc root and the cathode surface temperature increase and the electric field intensity decreases with increasing arc current (50-100A). The maximum temperature of the cathode surface is in the range3530-3790K. The cathode surface temperature has an approximately exponential distribution:(Tmax:the maximum temperature of the centre of the cathode arc root, r:the distance to the centre of the cathode arc root A(1):48.73-77.32K,H(Ⅰ):0.56-0.44mm). The current density and the electric field intensity have similar distributions. The average current density is5.9-6.9×107A/m2and the average electric field intensity is9.17-7.26×107V/m, depending on the distribution of the cathode surface temperature. The radius of the cathode arc root increases from0.51mm to0.68mm.In the range50-100A arc current and0.01-0.02T AMF, the electron temperature of the plasma near the flat end of a pure tungsten cathode is in the range16100-22500K and the range of the cathode fall voltage is19-23V. The range of the electron temperature of the plasma near the conical end of a pure tungsten cathode is larger, in the range18000-26000K and the cathode fall voltage decreases to16-19V. In both cases, the electron temperature increases with arc current and decreases with increasing AMF, and the cathode fall voltage decreases with increasing arc current and AMF. In particular, the electron temperature decreases significantly to16100K for the diffuse cathode arc root (100A,0.02T). The cathode fall voltage for the diffuse cathode arc root (20.54V) for I-100A, B=0.02T is greater than that for I=50,80A for the same AMF (20.24V,19.5V).