“强光一号”加速器短γ二极管径向箍缩率计算方法

Methods of calculating radial collapse velocity of short-γ diode field on Qiangguang-Ⅰ accelerator

电子束流箍缩是强流电子束二极管工作过程中广泛存在的物理现象.束流径向箍缩率定义为靶面形成的束斑(环)面积随时间的变化率,是判断二极管的束流箍缩情况和工作特性的重要指标,目前对其的研究方法以光学诊断和针对特定二极管的理论估算为主.在现有研究的基础上,以“强光一号”强箍缩短γ二极管为对象,给出了适用于该“环-板”构型二极管的径向箍缩率理论估算公式,并分别建立了基于粒子模拟和实验测量箍缩中心偏移的箍缩率计算方法,三种方法给出的径向箍缩率值分别为8.43,8.70及7.89 cm^(2)/ns,三者相对偏差<10%.为强流二极管径向箍缩率的研究提供了一种新的思路.

Electron beam pinching is a common physical phenomenon in the working process of high-current electron beam diodes. The radial collapse velocity (Va) of the beam is an important index to determine the beam pinching and the working characteristics of the diode. The current research methods are based on optical diagnosis and theoretical estimation formulas for a specific diode. The radial collapse velocity of Qiangguang-Ⅰ accelerator’ s tight-pinched short γ diode can be obtained by the following three methods in this paper: 1) a theoretical formula, which is used to calculate the radial collapse velocity on the basis of the existing research results, and can very quickly determine the pinching situation because in this case this formula just needs a diode pinching current;2) the method of calculating V_(a), which is established based on particle-in-cell simulation. The simulation model includes the anode ion current, thus can simulate the pinching of electron beam more precisely;3) a method of calculating V_(a), which is given by measuring the pinch center offset and the γ-ray PIN waveform, because the Qiangguang-Ⅰ γ diode is inconvenient for optical diagnosis. The radial collapse velocities obtained by the above three methods are 8.43, 8.70 and 7.89 cm2/ns respectively, and the relative difference among the three methods is < 10%. The third method obtains a slightly smaller value because the ion current assumed in the theory and simulation is H+. The ion current composition in the actual diode is complex, the diffusion speed is slower, then the radial collapse velocity is smaller. Compared with the typical V_(a) value (2-4 cm^(2)/ns) of the Gamble Ⅱ accelerator diode given by the Blaugrund team, the V_(a) value of the short γ diode of the Qiangguang-Ⅰ accelerator is nearly doubled. The diode on Qiangguang-Ⅰ, which works after a plasma opening switch (POS), has a very short rising time (less than 10 ns), and pinches quickly. In contrast, the rising time of the Gamble Ⅱ accelerator diode is about 40 ns, which is different from the working status of the Qiangguang-Ⅰ diode. This paper provides a new way to study the radial collapse velocity of high-current diodes.