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 $\delta$ 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.

We have designed, assembled, and tested a 4-MA, 60-ns fast linear transformer driver (LTD), which is the first operating generator featuring multiple LTD modules connected in parallel. The LTD-based accelerator comprises six modules in parallel, each of which has ten-stage cavities stacked in series. The six LTD modules are connected to a water tank of diameter 6 m via a 3-m-long impedance-matched deionized water-insulated coaxial transmission line. In the water tank, the electrical pulses are transmitted down by six horizontal tri-plate transmission lines. A 2.1-m-diameter two-level vacuum insulator stack is utilized to separate the deionized water region from the vacuum region. In the vacuum, the currents are further transported downstream by a two-level magnetically insulated transmission-line and then converged through four post-hole convolutes. Plasma radiation loads or bremsstrahlung electron beam diodes serve as loads that are expected to generate intense soft X rays or warm X rays. The machine is 3.2 m in height and 22 m in outer diameter, including support systems such as a high-voltage charge supply, magnetic core reset system, trigger system, and support platform for inner stalk installation and maintenance. A total of 1440 individual ±100-kV multi-gap spark switches and 2880 individual 100-kV capacitors are employed in the accelerator. A total of 12 fiber-optic laser-controlled trigger generators combining photoconductive and traditional gas spark switch technologies are used to realize the synchronous discharge of the more than 1000 gas switches. At an LTD charge voltage of ±85 kV, the accelerator stores an initial energy of about 300 kJ and is expected to deliver a current of 3–5 MA into various loads. To date, the LTD facility has shot into a thick-walled aluminum liner load and a reflex triode load. With a thick-walled aluminum liner of inductance 1.81 nH, a current with peak up to 4.1 MA and rise time (10%–90%) of about 60 ns has been achieved. The current transport efficiency from the insulator stack to the liner load approaches 100% during peak times. The LTD accelerator has been used to drive reflex triode loads generating warm X rays with high energy fluence and large radiation area. It has been demonstrated that this LTD is a promising and high-efficiency prime pulsed power source suitable for use in constructing the next generation of large-scale accelerators with currents of tens of megaamperes.

A 5-MA-LTD-based, high current accelerator is modeled using transmission line code. And the machine performances are presented for driving short-circuit loads, $Z$ -pinch loads, and bremsstrahlung electron beam diodes. With simulation analysis, it is expected that the short-circuit load current can reach 4.7 MA in 100 ns with $\pm$ 80 kV LTD charge voltage, and the peak of $Z$ -pinch load current when the machine is conducted with a 0.45 mg wire-array can be high as 4.2 MA, the peak kinetic energy is 49 kJ, and the energy coupling efficiency is estimated as 38%. The physical model of the reflex triode load which is typical electron beam diode is presented in detail. By incorporating the physics-based reflex triode model into a 3-D transient electromagnetic model, the field-circuit coupling simulation can be performed and the results are compared with the circuit model. The results from these two methods are in good agreement. The peak load current when the machine is configured with a $\sim$ 0.15 $\Omega $ reflex triode load is $\sim$ 2.8 MA. The energy coupling efficiency to the reflex triode load is 35.8%.

A 4 MV flash x-ray radiographic machine based on induction voltage adders has been developed. The configuration and design of this machine are reviewed. A three-dimensional, fully electromagnetic model and a circuit simulation model are established to compare with the experiments. The simulation results are in overall agreement with the electrical measurements. The pulsed power performances and output fluctuations of this machine over successive shot sequences are demonstrated. Among the 54 shots, the average peak output voltage is $4.4\pm 0.3\mathrm{MV}$ ($1\text{−}\sigma$) and the average diode current is $81.6\pm 4.5\mathrm{kA}$ ($1\text{−}\sigma$). Four typical malfunction modes are identified shot by shot including the diode-impedance collapse, insulator flashover, core saturation, and drive mistiming. Some remarkable features from each fault mode are recognized. The first-to-last time spreads of the four drive pluses, $t_{\mathrm{spread}}$, are chosen to quantify the drive synchronization and the influences of the $t_{\mathrm{spread}}$ on the peak voltages and diode currents are summarized from the almost 100 shots since the machine was commissioned. It is found that, in order to achieve a voltage of up to 4 MV, $t_{\mathrm{spread}}$ should not exceed 25 ns, which is approximately twice the time for electromagnetic wave propagation from the first cavity to the last cavity in vacuum. In addition, the rise time and FWHM duration of output voltages varying with $t_{\mathrm{spread}}$ are given. The results indicate that the rise time changes little at the beginning but increases exponentially once the $t_{\mathrm{spread}}$ exceeds 30 ns. The FWHM duration nearly increases linearly with $t_{\mathrm{spread}}$.

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.