摘要:
纳秒脉冲放电能在大气压下产生高电子能量、高功率密度的低温等离子体,由于经典放电理论无法很好地解释纳秒脉冲放电中的现象,近年来以高能逃逸电子为基础的纳秒脉冲气体放电理论受到广泛关注.纳秒脉冲放电会产生高能逃逸电子,伴随产生X射线,研究X射线的特性可以间接反映高能逃逸电子的特性.本文利用纳秒脉冲电源在大气压下激励空气放电,通过金刚石光导探测器测量放电产生的X射线,研究不同电极间隙、阳极厚度下和空间不同位置测量的X射线特性.实验结果表明,在大气压下纳秒脉冲放电能产生上升沿约1 ns,脉宽约2 ns的X射线脉冲,其产生时间与纳秒脉冲电压峰值对应,经计算探测到的X射线能量约为2.3×10-3 J.当增大电极间隙时,探测到的X射线能量减弱,因为增大电极间隙会减小电场强度和逃逸电子数,从而减少阳极的轫致辐射.电极间距大于50 mm后加速减弱,同时放电模式从弥散过渡到电晕.随着阳极厚度增加,阳极后方和放电腔侧面观察窗测得的X射线能量均有所减弱,在阳极后面探测的X射线能量减弱趋势更加明显,这说明X射线主要产生在阳极内表面,因此增加阳极厚度会使穿透阳极薄膜的X射线能量减少.
Abstract:
Nanosecond-pulse discharge can produce low-temperature plasma with high electron energy and power density in atmospheric air,thus it has been widely used in the fields of biomedical science,surface treatment,chemical deposition,flow control,plasma combustion and gas diode.However,some phenomena in nanosecond-pulse discharge cannot be explained by traditional discharge theories (Townsend theory and streamer theory),thus the mechanism of pulsed gas discharge based on runaway breakdown of high-energy electrons has been proposed.Generally,the generation and propagation of runaway electrons are accompanied by the generation of X-ray.Therefore,the properties of X-ray can indirectly reveal the characteristics of high-energy runaway electrons in nanosecond-pulse discharges.In this paper,in order to explore the characteristics of runaway electrons and the mechanism of nanosecond-pulse discharge,the temporal properties of X-ray in nanosecond-pulse discharge are investigated.A nanosecond power supply VPG-30-200 (with peak voltage 0--200 kV,rising time 1.2-1.6 ns,and full width at half maximum 3-5 ns) is used to produce nanosecond-pulse discharge.The discharge is generated in a tube-to-plane electrode at atmospheric pressure.Effects of the inter-electrode gap,anode thickness and position on the characteristics of X-ray are investigated by measuring the temporal X-ray via a diamond photoconductive device.The experimental results show that X-ray in nanosecond-pulse discharge has a rising time of 1 ns,a pulse width of about 2 ns and a calculated energy of about 2.3 × 10-3 J.The detected X-ray energy decreases with the increase of inter-electrode gap,because the longer discharge gap reduces the electric field and the number of runaway electrons,weakening the bremsstrahlung at the anode.When the inter-electrode gap is 50 mm,the discharge mode is converted from a diffuse into a corona,resulting in a rapid decrease in X-ray energy.Furthermore,both X-ray energies measured behind the anode and on the side of discharge chamber decrease as anode thickness increases.The X-ray energy measured on the side of the discharge chamber is one order of magnitude higher than that measured behind the anode,which is because the anode foil absorbs some X-rays when they cross the foil.In addition,the X-ray energy behind the anode significantly decreases with the increase of the thickness of anode aluminum foil.It indicates that the X-ray in nanosecond-pulse discharge mainly comes from the bremsstrahlung caused by the collision between the high-energy runaway electrons and inner surface of the anode foil.Therefore,increasing the thickness of the anode foil will reduce the X-ray energy across the anode film.
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