层流等离子体发生器设计关键技术研究之双弧现象

Study on the Key Design Technology of the Laminar Plasma Torch:Double Arc Phenomenon

分段式结构设计通常被应用于层流等离子体发生器的结构设计中,以避免产生大尺度分流现象,进而避免层流等离子体射流产生脉动。然而,在分段式层流等离子体发生器中,当中间电极长度较长时,经常在该中间电极上产生一个新的阳极弧根和阴极弧根(即双弧现象),导致该电极热负荷严重,造成电极的严重烧蚀和等离子体射流的波动。为了延长层流等离子体发生器中间电极寿命,提高层流等离子体射流稳定性,必须对双弧现象加以抑制。本文基于假设条件,首先建立了层流等离子体发生器正常工作时和产生双弧现象时的分析模型,分析了双弧现象产生机理,并基于最小电压原理,推导出层流等离子体发生器不产生双弧现象对中间电极的尺寸要求;然后,基于前述双弧现象产生原理分析和理论推导,确定了抑制层流等离子体发生器产生双弧现象的措施如下:一是,增加冷气膜击穿电压U_b,使电弧与弧室壁的电势差不足以击穿二者间的冷气膜;改善等离子体发生器冷却通道,优化其对弧室壁的冷却效果,进而降低冷气膜的温度;增大工作气体流量,增强对电弧的压缩,进而增加冷气膜的厚度。二是,限制中间电极轴向长度,降低电弧在对应中间电极区间的电压降,即任意中间电极在轴向的长度均满足最小电压要求。

Segmented anode structure is always used to the structure design of the laminar plasma torch to avoid the large scale shunting,which would result in a pulsation of the laminar plasma jet.However,when the length of the inter-electrode is longer than its limitation,a new anode attachment and a new cathode attachment always appear on the inter-electrode,which is named as double arc phenomenon.The double arc phenomenon leads to a serious heat load of the inter-electrode,resulting in a serious ablation of the inter-electrode and a fluctuation of the plasma jet.In order to prolong the service life of the inter-electrode and improve the stability of the plasma jet,the double arc phenomenon should be avoided.Firstly,based on the assumptions,the analysis models of the laminar plasma torch working with normal condition and double phenomenon were built to analyze the underlying mechanism of the double phenomenon.Besides,according to the principle of minimum voltage,the design requirements of the inter-electrode to avoid the generation of double arc were derived.Then,based on the above analysis,the measures of suppressing the double arc phenomenon were proposed,which included improving the cooling channels of the plasma torch to optimize the cooling effect of arc chamber wall,then decreasing the temperature of the cold layer;increasing the working gas flow rate to enhance the arc compression,which could increase the thickness of the cold layer;limiting the axial length of the inter-electrodes to decrease the corresponding arc voltage drop.