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In high-current pulsed-power accelerators, magnetically insulated transmission lines (MITLs) are used to transmit hundreds of terawatts of pulsed power from the multimodule prime sources and multilevel insulator stacks to various physical loads at the central region. At the inner-MITL (iMITL) region, whose lineal current density can reach several MA/cm, energy loss on electrodes will be severe. A variational calculus technique is used to optimize the iMITL profile to minimize the energy loss on the electrodes. The optimized iMITL is compared with the common L-shaped iMITL. The results show that the energy loss of the optimized iMITL is 40%–50% less than that of L-shaped iMITL at the same operating conditions. The effects of peak current and its rise time on the energy loss are investigated. The advantage of the optimized iMITL is greater when the peak current and rise time are higher. As the peak current is fixed at 35 MA and the rise time increases from 100 to 200 ns, the ratio of the energy loss to the input energy from the L-shaped iMITL increases from 7.85% to 19.07%, whereas the energy-loss ratio of the optimized iMITL always does not exceed 13%. As the current rise time is fixed at 150 ns and the peak varies between 25 and 45 MA, the energy-loss ratio of the L-shaped iMITL ranges from 9.80% to 19.75%, but the energy-loss ratio of the optimized iMITL is always within 14%.

Multigap gas spark switches are commonly utilized in linear transformer drivers (LTDs) because of their outstanding static and triggering performance. The number of discharge channels in the switch gap has an effect on the inductance and dissipated energy of the switches. To explore the multichannel discharge characteristics of the switch gaps, a two-gap gas switch (TGGS) with one trigger gap and one self-breakdown gap is designed. The trigger experiments are conducted on the TGGS under various switch working coefficients and nanosecond trigger pulses with different rise rates. The number of discharge channels in the switch gap is recorded by a intensified charge-coupled device (ICCD) camera. The frequency distribution of the number of discharge channels in the trigger gap and the self-breakdown gap is shown under different operating conditions. The experimental results indicate that the number of discharge channels in the self-breakdown gap is significantly more than that in the trigger gap. The preionization obviously increases the number of discharge channels in the trigger gap. The influence of the number of discharge channels in the switch ON the discharge loop current is discussed, and the inductance and dissipated energy of spark channels in the switch gap under different trigger settings are calculated. Increasing the trigger voltage rise rate, the working coefficient, or introducing preionization can correspondingly promote the number of discharge channels in the switch gap, which reduces the inductance and dissipated energy of the switch and thus improves the rise rate and amplitude of the brick current.

A four-level axial insulator stack is designed for a 15 MA Z-pinch accelerator. The height and diameter are 1.5 and 3.4 m. The two upper levels operate at 2.5 MV, while the two bottom levels operate at 3.0 MV. The insulator ring stages in series, the vacuum flares, and water flares are optimized in detail. By reiterative optimizations, the two upper levels consist of five 5-cm-thick insulator rings and four 0.8-cm-thick grading rings. While the two bottom levels consist of six insulator rings and five grading rings with the same individual insulator and grading ring. The inductance of each level is 14.02, 15.48, 18.71, and 20.88 nH, respectively. The equivalent inductance is 4.21 nH with the four levels in parallel. The voltage variations from average for the four levels are within ±4.9%, ±4.9%, ±2.1%, and ±2.2%, respectively. At the average operating electric field of 100 kV/cm, the predicted flashover probability for the two upper level and the two bottom level insulator stacks are about 0.29% and 0.53%, respectively.

The development of discharge that starts near the cathode in a homogeneous electric field air gap under nanosecond pulse voltage is studied using a 2.5-D particle-in-cell/Monte Carlo collisional model. The simulation is first performed in the absence of photoionization reactions. It is found that runaway electrons are produced in the head of the streamer when the local electric field strength is sufficiently high. Runaway electrons ionize the area in front of the streamer tip while producing abundant preionization electrons, which significantly accelerate the rate of discharge propagation. Photoelectrons and runaway electrons perform an analogous role in preionizing gas, but the latter is more efficient because of their directionality. A more complete discharge process is simulated in the presence of photoionization reactions. Electrons with relatively high energy appear in the bipolar end of the discharge channel while the discharge continuously progresses toward the anode. During the later moment of discharge, the streamer expands to the anode at an extremely fast velocity as a result of the combined action of photoionization and runaway electrons located at the negative streamer tip. The simulation results reveal the discharge mechanisms of the air gap applied with a homogeneous electric field under a nanosecond voltage pulse and provide a comprehensive understanding of the fast breakdown of air gaps.

Rod pinch diode (RPD) is an intense text X-ray point source with a millimeter spot diameter, tens of nanosecond pulsewidth, and tens of keV to MeV photon energy, resulting in extraordinary flash radiography performance. Classical RPD is featured with a high-impedance vacuum gap between the annular cathode and the central anode rod, which limits the electron current and electrical power density, therefore is not conducive to the improvement of brightness. Plasma prefilling can effectively reduce the RPD impedance, thereby increasing the electron current and realizing an impedance match of RPD with low-impedance, high-power-density drivers. In this article, an experimental study on text plasma-prefilled RPD is carried out based on a fast linear transformer driver (FLTD) with relatively low charging voltage; the RPD anode and cathode are initially connected by metal wires, which are transformed into plasma shortly after the onset of discharge current. A quasi-spherical X-ray spot is obtained with $\sim $ 0.45 mm diameter, $\sim $ 30 ns pulsewidth, 0.6 rad@1 m (LiF) dose, and 55–60 keV equivalent mono-energy photon energy. The radiography performance is further characterized by a tungsten text rolled-edge and slit plate. The preliminary results proved the feasibility of text low-impedance RPD as a flash radiography source within the relatively low photon energy range.

Magnetic field measurement in Z-pinch plasma has drawn considerable interest because it is a key problem in the research of dynamic behaviors and the coupling of magnetic field and plasma. In this study, the measurement sensitivity of the rotation angle in Z-pinch plasma was improved by considering the second polarization effect caused by optical elements. The magnetic field distribution of preconditioned single-wire Z-pinch plasma was determined by this method. The Faraday rotation of the 1064-nm probe laser was measured by the beam-splitting polarization method when the plasma acted as a Faraday medium. A second polarization effect on the rotation angle was realized by taking the different transmittances and reflectivities of the $p$ and $s$ components of the beam splitters into account. This configuration doubled the sensitivity of the Faraday rotation measurement. The diagnostic errors produced by the techniques and data processing were quantified. The early stage of the preconditioned imploding plasma was monitored, and the distributions of areal electron density, Faraday rotation, and magnetic field were measured. The experimental results demonstrated that the current distribution can be divided into three parts by two dense shells. The majority of the current was distributed outside the dense shell which existed inside the coronal plasma. In addition, only a small amount of current was distributed in the wire core.

Accurate adjustment of long magnetic insulation transmission lines (MITLs) is crucial in high-power pulsed systems, which must guarantee insulation and high transmission efficiency of current under complicated electromagnetic environment. In this study, a spiral inductance-supported insulation structure in a coaxial MITL was designed, which achieved high efficiency of current transmission up to 90%. Moreover, the current transmission efficiency of the MITL under various working states and specific mechanism of the current loss was investigated by combining circuit with particle-in-cell (PIC) simulations. Results show that the current loss of the inductance-supported MITL in the under-matched state is caused by the inductance current on the support structure, as the electron charge layer is strongly constrained by magnetic field. However, the charge constraint among MITL electrodes is relatively weak in self-limited flow. The electrons bombard the supporting rod, which results in charge accumulation and unneglectable current loss in self-limited flow. This work uncovers the mechanism of current loss in the spiral inductance-supported MITL, which provides guidance for engineering design.

In this article, the dimensionless ratio $E/{\textit{cB}}$ at the starting point of constant-gap section of the magnetically insulated transmission line (MITL) at the time of peak MITL voltage was used as a figure of merit of a disk MITL. Taking a 15-MA driver as an example, the equivalent circuit model and the impedance iterative method are established by means of the TL-code circuit simulations. The vacuum impedance of the four-level outer-MITLs is determined. In particular, when the target ratio ( $E/{\textit{cB}})_{i}$ is set to 0.25 (the ratio of flow current to anode current $I_{f}/I_{a}$ is 3.12%), the impedance of the four-level MITLs is 1.70, 2.12, 2.88, and $3.16~\Omega $ . When ( $E/{\textit{cB}})_{i}$ of the four-level MITLs is set to 0.15, 0.18, 0.20, and 0.22 (the ratio $I_{f}/I_{a}$ is 1.23%), the impedances are 2.68, 2.89, 3.06, and $3.02~\Omega $ , respectively.

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 ±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 ~2.8 MA. The energy coupling efficiency to the reflex triode load is 35.8%.

Discharge brick is the basic discharge unit of fast linear transformer driver (LTD). It is important to reduce the scale and cost of LTD device by increasing discharge current and brick power of the brick. In this article, a discharge brick is designed for LTD, including a 200-kV three-electrode field-distortion gas switch (TFGS) and two 100-kV/100-nF capacitors. At the same time, a low inductance liquid resistor load (LRL) is designed for simulating the actual working condition of discharge brick in LTD cavity. The brick inductance is tested with 200 nH, and when operating voltage at ±100 kV, the peak discharge current and brick power can reach 62 kA and 5.8-GW, respectively. The trigger test of 10 000 shots with different switch structure has been proceeded, and the pre-fire rate is lower than 0.3%.

The combination of energetic material (EM) and electrical wire explosion (EWE) is an efficient and economical method to generate strong shock waves (SWs). The idea resembles the exploding bridge-wire (EBW) detonator, but in this case, the EWE operates with much higher electrical energy storage, and the EMs of much lower sensitivity can be directly ignited, which provides a practical compromise among limited energy storage, strong SWs, and safe usage of explosives. The hybrid load based on EWE ignited EMs has been successfully applied for reservoir stimulation in exploitation of both conventional and unconventional oil/gas resources. In this brief review, we summarize the EWE characteristics, SW characteristics, ignition mechanism, and engineering applications of this hybrid load, and give suggestions for future research directions.