Fengju Sun

Experimental Study on Multigap Multichannel Gas Spark Closing Switch for LTD

In this paper, a multigap multichannel gas spark closing switch (MMCS) has been modified to improve the synchronous closing characteristics of ten switches designed for linear transformer driver (LTD). Convexo-convex discal electrodes have been used instead of the plane toroidal electrodes to improve the electric field across the gaps. A new endo-grooved plane bracing structure has been designed for fixing the electrodes instead of the small ball anchoring method to ensure the length of the gap uniformity. In order to observe the discharge process of the MMCS, the transparent lucite insulation chambers instead of the nylon ones have been used. A test circuit comprising a single discharge circuit called LTD brick has been established to investigate the self-breakdown and triggered breakdown characteristics of ten switches. The results show that when switches are filled with pure nitrogen of 0.14 MPa and charged to plusmn60 kV, the synchronous discharge of ten MMCSs has output a current pulse with an amplitude of about 100 kA and a rise time of 100 ns.

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.

Effect of Cavity-Triggering Sequences on Output Parameters of LTD-Based Drivers

This paper explores the feasibility of improving the load current parameters by optimizing the cavity-triggering sequences for a linear transformer driver (LTD) module with 60 cavities connected in series. The mechanism behind this was analyzed. The results demonstrate that it is necessary for the improvement in the output parameters of the LTD to postpone the closure of the switches in the cavities later than the arrival of the electromagnetic pulse propagating downstream. However, this requires the switches to withstand pulsed voltages higher than their charge voltage before closing. When the LTD switches withstood a pulsed voltage peak of about 400 kV for several tens of nanoseconds before closing, the LTD module could generate a current pulse with a rise time of 12.5 ns and a peak of 1340 kA on the matched load.

Optimized Design of Azimuthal Transmission Lines for the Cell Driven by Two PFLs in Induction Voltage Adders

In this paper, a full-size 3-D electromagnetic model (without electrons) and an equivalent circuit model of a single cell in magnetically insulated induction voltage adders has been established. The simulation results of these two models agree well with each other. On the basis of these two models, azimuthal transmission lines for a cell driven by two pulse forming lines (PFLs) have been optimized. The optimization process aims at matching the impedance to the feed PFLs, ensuring uniform potential distribution along the insulator-stack, maximizing the symmetric current flow around the bore, and minimizing the waveform distortion of the injected pulse. A formula is presented to estimate the azimuthal impedance and an asymmetric coefficient is also defined to quantify the asymmetric extent of the feed pulses. The profiles of the total four candidate azimuthal lines have been simulated and analyzed. The simulation results indicate a relatively low feed asymmetry for all the four profiles. However, considering the waveform quality of the cell output voltage, the azimuthal line profiles with four points connected or wholly connected to the cathode plate are the most suitable for induction cells with two PFL feeding. And the corresponding coefficients

300-kA Fast Linear Transformer Driver Stage

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Effect Analysis of Switch Prefire in Linear Transformer Drivers

are 3.1% and 7.6%, respectively. It is concluded that, when optimizing azimuthal lines for cells with two PFL feeding, the principal consideration should be matching the impedance instead of symmetrizing the current flow.

Study on Firing Conditions of Multigap Gas Switch for Fast Linear Transformer Driver

A modular linear transformer driver (LTD) is a novel fast-discharge high-pulsed power source for primarily stored energy. In LTDs, a large number of stages (or cavities) can be connected in series or in parallel conveniently to directly generate high power pulses of 50-200 ns with the required voltage or current. This paper introduces an LTD stage developed by us recently. This LTD stage consists of 14 bricks in parallel. Each brick contains two 40-nF capacitors and a compact multigap gas spark switch in series. The switch is a modification of our original version, and the diameter and the height of which have been reduced to 98 and 127 mm, respectively. The circuit model of this stage is established in PSPICE, and its output parameters are obtained through simulation. Driving a rough-matched resistive load of 0.27 Ω, its output current peak is 300 kA with an overall rise time of about 110 ns at a charged voltage of the capacitors of about ±90 kV, agreeing well with its simulation result.

Trigger Method Based on Secondary Induced Overvoltage for Linear Transformer Drivers

In this paper, the circuit model capable of dealing with magnetizing characteristics of the magnetic cores and switch prefire is established for linear transformer driver (LTD) cavities in PSPICE. Through simulation, the effects of switch prefire on other components in LTD and its output current pulse have been analyzed associated with the variation of energy loss in magnetic cores. Simulation results indicate that a switch prefire in one cavity would produce oscillations added to the charge voltage of the other switches in this cavity and cavities adjacent immediately. The magnitudes of such oscillations have an intimate relation with the magnetic core loss equivalent resistance. The loss equivalent resistances in the range from 4 to 6 have produced moderate voltage oscillations. Experiments on the 300-kA LTD cavity developed by the Northwest Institute of Nuclear Technology proved that such oscillations added to the charge voltage of other switches are likely to increase the breakdown probability of them.

2.8-MV Low-Inductance Low-Jitter Electrical-Triggered Gas Switch

As one of the critical elements of fast linear transformer driver (FLTD), gas switch directly influences the output parameters, stability, and reliability of generators based on FLTD technology. In this paper, a multigap gas switch with six gaps in series has been investigated to study the proper firing conditions to get better performance for FLTD. The self-breakdown voltage with different gas pressure has been studied to derive the working ratio of the switch. The triggered breakdown delay and jitter with different charge voltage, gas pressure, and working ratio have been studied. The results indicate that when the switch is triggered by a 100-kV trigger voltage pulse under 180-kV charge voltage and 0.325-MPa pure nitrogen (70% working ratio), the delay time and the jitter are 34 and 1 ns, respectively.

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

In this paper, a triggering method based on secondary induced overvoltage for a linear transformer driver (LTD) is proposed and investigated by simulation. A detailed circuit model established for the LTD is chosen to study the effect of some factors on the feasibility of this triggering method. The considered factors include the permeability, equivalent resistance of energy loss, saturation of magnetic cores, and the delay of gas switches in LTD cavities. Simulation results indicate that high equivalent resistance and high permeability of magnetic cores are helpful in realizing this trigger method. However, the resistance plays a leading role. An equivalent resistance of more than 1.5 Ω favors the realization of this trigger. However, a relative permeability of even 25 has not produced a fatal menace to this triggering method. The feasibility of this triggering necessitates the available volt-second product of the present magnetic cores to be expanded twice. As for gas switches, the interval given by secondary induced overvoltage would likely be long enough to complete their breakdown. From simulation results, it is shown that LTD switches could be rapidly closed under the action of such an overvoltage pulse with a rise time of less than 10 ns.