Peitian Cong

Simulation Analysis of Transmission-Line Impedance Transformers With the Gaussian, Exponential, and Linear Impedance Profiles for Pulsed-Power Accelerators

Based on the transmission-line code, a 1-D circuit model for the transmission-line impedance transformer with an arbitrary impedance profile is developed, and the simulation results are compared with those of the existing literatures. By using this model, the power efficiencies and pulse compression ratios of the transformers (with Gaussian, exponential, and linear impedance profiles) were quantified as functions of Ψ (the ratio of the output impedance to the input impedance of the transformers) and Γ (the ratio of the pulsewidth to the one-way transit time of the transformers). The simulation results indicate that the exponential transformers have the maximum power efficiencies and pulse compression ratios. However, in the limit h <; 0.05 (where h is the Gaussian parameter), the Gaussian transformers almost have the same power efficiencies and pulse compression ratios as the exponential ones. Finally, the performances of a radial transformer line in smoothing the arrival time variations in the forward-going power pulses were studied. The simulation results show that, for the cases considered, the rise time (0-1.0) of the output pulse is prolonged by about half of the maximum difference in the time at which the pulses are launched, and the reduction of the peak output power is less than 2% when the maximum difference is 30 ns.

Low voltage pulse injection test of a single-stage 1 MV prototype induction voltage adder cell

Low voltage pulse injection tests have been done on a 1 MV prototype induction cell. The tests mainly aim at measuring azimuthal uniformity of feed currents and evaluating cell electrical parameters. Tests are conducted under two cases that the cell is fed by a single pulse and two pulses, respectively. The results indicate that, in the case of the single-point feed, the best current uniformity with an azimuthal variation of 19.3% is acquired when azimuthal lines connect to cathode plates with lower half circumferences. In the case of two-pulse synchronous feed, the current uniformity becomes better, and a current with an azimuthal variation of 11.8% is achieved. Moreover, the effects of asynchronous feed on current uniformity are also experimentally investigated. The results imply that, for given injecting pulses with the duration of 70-80 ns, a time deviation less than 30 ns could be acceptable, without obvious degradation on the feed uniformity and current addition. In addition, the influences of the current uniformity and amounts of feed pulses on cell equivalent inductances are evaluated. The experiment results show that, the equivalent inductance would nearly keep a value of 130 nH as the current variation is less than 50%. However, the extreme asymmetry of feed currents or the increase of amounts of feed pulses would produce additional inductance.

Circuit Model of Magnetically-Insulated Induction Voltage Adders Based on the Transmission Line Code

Based on the transmission line code (TLCODE), a 1-D circuit model used for the analysis of magnetically-insulated induction voltage adders (MIVAs) was developed. Using cells improved from those of the JianGuang-I facility, a 10-stage MIVA was conceptually designed, and its equivalent TLCODE circuit model was described in detail with consideration of the magnetic insulation. To verify the effectiveness of this model, simulation results were compared with those of a PSPICE model (with no electron emission) under both synchronously triggering mode and ideal time-sequence triggering mode. The comparisons show that calculation results of these two models are accord with each other perfectly (within 0.1%). In addition, features of the calculation results were particularly analyzed and verified by the existent literatures. Considering the magnetic-insulation process, the inner stalk of the 10-stage MIVA was designed, and its output parameters were simulated with various electron emission thresholds (150, 200, and 250 kV/cm). Furthermore, qualitative analysis was done to present the models abilities of the magnetic-insulation treatment.

Characteristics Study of Multigaps Gas Switch With Corona Discharge for Voltage Balance

A multigaps gas switch (MGS) with corona discharge for voltage balance is developed. The self-breakdown characteristics of the switch under different gas pressures and the volt-ampere characteristics of corona discharge are studied. The experiment results show that to increase the shot times can improve the self-breakdown voltage stability of the switch. After conditioning the switch self-breakdown voltage relative standard deviation is less than 2%, the voltage balance has a good effect; the volt-ampere characteristics of multigaps corona discharge is similar to single gap. The delay time and jitter of the MGS with corona discharge for voltage balance in different conditions have been tested. The trigger stability is fine when switch charging at ±100 kV.

Note: Multi-gap gas switch with low trigger-threshold voltage by mounting resistors and capacitors in parallel with switch gaps

To reduce the trigger threshold voltage of the multi-gap gas switch used for linear transformer drivers, a method is proposed by mounting resistors and capacitors in parallel with the switch gaps. Based on the circuit model of the six-gap gas switch, the gap voltage distribution during the triggering process is analyzed. When the multi-gap gas switch is triggered, the voltage distribution between gaps is mainly determined by the stray capacitance between electrodes. In such condition, the trigger voltage is not fully applied on the trigger gap, and as a consequence, a higher trigger voltage is required for obtaining a low jitter. The effects of capacitor parameters on the triggering characteristics of the switch are experimentally investigated. Compared with the original switch design, the results indicate that at a charging voltage of ±80 kV and operating at 60% of the self-breakdown voltage, the trigger voltage is reduced from 110 kV to 75 kV while the 3.2 ns jitter of the switch is preserved.

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.

Experimental Study and Electromagnetic Model of a 1-MV Induction Voltage Cavity

Magnetically insulated induction voltage adders are pulsed-power accelerators widely used in various fields. Preliminary cavity experiments concerning the current uniformity have been done on a prototype induction cavity. The cavity is single-point fed by a 1.0-MV/200-kA/60-ns pulse of Chenguang accelerator. Three different configurations of azimuthal lines are tested. The results indicate that the azimuthal variations of feed currents for all these configurations are less than 40%. And under this extent of nonuniformity, the current distributions slightly affect the cavity output voltage. An electromagnetic (EM) model is proposed for this prototype cavity. The characteristics of cavity output responses and current uniformity are simulated and analyzed. The EM model could reproduce the major experimental results well.

Exponential Impedance-Transformer Stalks Used for Magnetically Insulated Induction Voltage Adders

An exponential impedance-transformer stalk was proposed for magnetically insulated induction voltage adders (MIVAs). Based on a 10-cell TLCODE model, performances of stalks with linear, stepped, and exponential profiles were quantified and analyzed as a function of m (the ratio of the adder output impedance to the one that is driver matched, m ≥ 1). Simulation results show that when m ≥ 1.5, stalks with the exponential profile result to the highest adder output voltages with a reverse voltage ~7.5% of the dominative voltage, and the η (the ratio of the subtraction between adder peak voltages with exponential and stepped profiles to the one with stepped profile) increases as the m goes up (when m = 5, η is as much as 8%). Simulation results also indicate that with exponential impedance profile, the highest feeding voltage of the 10 cells is ~2 MV when 1 ≤ m ≤ 10, and it always occurs at one of the last five cells. Moreover, it is found that with the exponential profile, the reverse coefficient β (the maximum ratio of the reverse peak voltages to the nominal) is ~9.5%.

Engineering design and primary experiment study of a 2.6-MV field-distortion oil switch

A 2.6-MV field-distortion oil switch was developed and tested. Engineering design theories of this switch, including electrode areas, oil anode-cathode (A-K) gaps, insulator-surface electric field, and A-K gaps of the pre-gas switch, were described in detail. Structure designs of the switch were presented. Electric-field distributions of the oil A-K gap and insulator surface were analyzed and discussed. Also, switch breakdown processes were estimated. Finally, performances of this switch were tested on the “JianGuang-I” facility (2.3MV/350ns). Preliminary experiment results show that with a 250kV trigger voltage, the switch-breakdown time delay is about 132ns and its jitter (the standard deviation) is less than 4.4ns.

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.