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

Abstract

Electron beam pinching is a common physical phenomenon in the working process of high-current electron beam diodes. The radial collapse velocity (<i>V</i>_a) 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 <i>γ</i> 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 <i>V</i>_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 <i>V</i>_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 <i>V</i>_a value (2–4 cm²/ns) of the Gamble II accelerator diode given by the Blaugrund team, the <i>V</i>_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.