Jiang Sun

Experimental study on magnetically insulated transmission line electrode surface evolution process under MA/cm current density

The design of high-current density magnetically insulated transmission line (MITL) is a difficult problem of current large-scale Z-pinch device. In particular, a thorough understanding of the MITL electrode surface evolution process under high current density is lacking. On the “QiangGuang-I” accelerator, the load area possesses a low inductance short-circuit structure with a diameter of 2.85 mm at the cathode, and three reflux columns with a diameter of 3 mm and uniformly distributed circumference at the anode. The length of the high density MITL area is 20 mm. A laser interferometer is used to assess and analyze the state of the MITL cathode and anode gap, and their evolution process under high current density. Experimental results indicate that evident current loss is not observed in the current density area at pulse leading edge, and peak when the surface current density reaches MA/cm. Analysis on electrode surface working conditions indicates that when the current leading edge is at 71.5% of the peak...

Simulation Analysis of a Pulsed Compact FLTD System for Large-Area Hard X-Ray Sources

High-power pulses with edges rising in tens of nanoseconds can be directly produced using a fast linear transformer driver (FLTD). The use of FLTDs facilitates the driving of various high-power electron-beam diodes through vacuum-or magnetically insulated transmission lines (MITLs) and consequently makes radiation simulation sources relatively compact. This paper investigates and optimizes the conformations and parameters of the secondary MITLs, diodes, and bremsstrahlung converter targets through the circuit simulation, particle-in-cell simulation, and Monte Carlo simulation, with a focus on the parameters of a pulse source from a two-level FLTD connected in series. The simulations reveal the influence of voltage on changes in diode with alternating cathode and anode and the relationship between the parameters of the composite target structure, photon/electron transfer efficiency, and radiation energy spectra. The distribution, evenness, and energy fluence of the radiation field in the electron-beam diode driven by the two-level FLTD connected in series are also determined. Results show that the FLTD system shows a significant potential in constructing compact hard X-ray sources.

Design and Simulation Study of MITL for a Multistage FLTD in a Series

The structure and size of secondary magnetically insulated transmission line (MITL) were determined through theoretical calculations based on single-module parameters of a 300-kA/100-ns fast liner transformer driver (FLTD) and a second-order equivalent circuit model. A particle simulation model of secondary MITL for a 10-stage FLTD in a series was also established. The output characteristics and MITL working conditions of the 10-stage FLTD under different loads were discussed. The simulation results showed that the 10-stage FLTD carries an approximately matched electron beam diode. The load peak voltage, load peak current, and peak power are 1.02 MV, 224 kA, and 221 GW, respectively. The leading edge of voltage is 40 ns. However, electron charges undermatching case are distributed in a small area of MITL cathode surface. The proportion of electron charge current in the total current decreases significantly.

Influences of Cell-Driving Sequences on Performances of Magnetically Insulated Induction Voltage Adders

Based on the transmission line code model, influences of cell-driving sequences on both cell feedings and load voltages of a 12-stage magnetically insulated induction voltage adder were analyzed. Simulation results show that the cell-feeding voltages are obviously distorted under nonideal IVA sequences. The distortion degree (such as overshoot voltage and reverse voltage) would be greater as the increment in the parameter n (the amount of cells that are synchronously driven per each group). These phenomena were analyzed in detail and well explained on the basis of transmission line theory. Simulation results also show that the rise time (10%-90%) of load voltages increases as the increment in the parameter n, while the flattop time decreases gradually, which leads to longer rise time and shorter flattop time. But the alterations of the load peak and reverse voltages are negligible. These characteristics indicate that a wide variety of voltage-pulse shape can be formed by varying cell-driving sequences.

Mathematical Derivation of Cell-Driving-Jitter Effects on the Risetime of IVA-Output Voltages

Based on the step function and the probability distribution function, the risetime (10%-90%) increase of induction voltage adder-output voltage pulses, resulting from cell-driving jitters, was analyzed in detail. Using Taylor series expansion, an analytic solution of the risetime increase was obtained for jitters with normal distributions (assuming that the mean is μ, the standard deviation is σ). It is found that the risetime increase of output voltages is a function of the σ, which can be simply expressed as 2.563σ. This analytic solution was directly verified by the numerical calculation results. Analytic solutions of the risetime increase for jitters with three-parameter Weibull distribution are also given. It can be used to approximate several distributions (such as normal distribution, exponential distribution, and Rayleigh distribution). The application limitations of these solutions are discussed and presented, especially the reliable probability. Also, the total risetime of the output voltage is presented by considering the risetime increase resulting from cell-driving jitters.

Experimental Study on the Current Transmission Efficiency for the Transition Structure of Vacuum Transmission Line MITL on Flash-II Accelerator

An experimental platform based on a coaxial cylinder magnetically insulated transmission line (MITL) was established on the basis of Flash-II accelerator. An experimental study was conducted on the current loss characteristics of the transition region between the vacuum transmission line and the MITL under different impedance match conditions by adjusting the loading gap of an e-beam diode. The results showed that under shorted load, the whole vacuum transmission region suffers no current loss. Furthermore, the voltage waveform continuously decreases when approaching the load region. When the load impedance is smaller than the working impedance of the transmission line, the MITL becomes undermatched, and the current transmission efficiency of the transition region reaches 98% without the evident current loss. When the load impedance increases to make MITL overmatched, an MITL operates under a self-limited flow accompanied with huge current loss in the transition region. The current transmission efficiency decreases to 87%, and the current loss process significantly affects the working process of the probe on an anodic differential coil. On the basis of the working mechanism of an MITL and the wave reflection process, a qualitative analysis was conducted, and the experimental results were explained.

Reducing the Axial Spot Size of a Rod-Pinch Diode by a Compound Cathode

The axial spot size of a rod-pinch diode (RPD) has a significant effect on the spatial resolution of the radiographic system, when the RPD is used as an X-ray source for pulsed radiography. The simulation and experiment results indicate that a compound cathode can be used to reduce the axial spot size.