Linear Transformer Driver Cavity Research Articles

Numerical Study on Key Factors Affecting Synchronization of Multiple Spark-Gap Switches in Single-Stage LTD

Gas switch jitter in fast linear transformer driver (LTD) has a great influence on output current waveform. Aiming at explaining the phenomenon that the breakdown jitter increases when gas switches are synchronously triggered in a single-stage LTD cavity, key factors that affect gas switch breakdown jitter were analyzed in this article. Based on Monte Carlo analysis, a novel LTD circuit model with switch breakdown delay and jitter function was developed, and its accuracy was verified by experimental results. Key factors that result in breakdown jitter increasing were analyzed by changing the gas switch self-jitter, breakdown sequence, and loop inductance. It is found that the gas switch breakdown process can influence each other when triggered in a single-stage LTD cavity. The simulation results showed that the gas switch self-jitter and load inductance have a great influence on the branches-jitter, where the branches-jitter decreased from 12.01 to 5.81 ns when the load inductance decreased from 5 to 4 nH. The simulation results gave an in-depth understanding of the branches-jitter increasing in a single-stage LTD cavity.

5.8-GW Discharge Brick for Linear Transformer Driver

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%.

Modeling and tests of nested transmission lines for current adding on a four-stage linear transformer driver.

Linear transformer driver (LTD) technology allows a pulsed-power generator to be transportable due to its salient features in compactness and modular design. To further reduce the footprint of an MA-class pulsed-power generator, nested transmission lines were designed and tested for current adding in a four-stage gas-insulated LTD module. The current adder assembly contained two modules that were charged in opposite polarities. Each module held two LTD cavities that shared a common electrode of the nested transmission line with deionized water insulation. Post-hole convolutes were installed for the aggregation of the output current of different modules. More specifically, numerical simulations were conducted to calculate the nested line inductance, which revealed that the total system inductance was ∼10 nH in the nested geometry. Experimentally, testing on the four-stage LTD prototype showed that the LTD module can deliver a 1.2 MA current peak with a rise time of 140ns to a short circuit load under the charging voltage of ±50kV, which validated the applicability of using nested lines for current adding in an MA-class LTD module.

Characterization of a 200-nF unipolar, high current, low inductance capacitor switch assembly.

Capacitor switch assemblies (CSAs) are a viable alternative to the standard Linear Transformer Driver (LTD) brick-the prime power "building block" in LTD-based high current drivers. In a CSA, the geometrical advantages combined with the switch's placement within a CSA produce a significantly lower inductance structure compared to the LTD brick. Lower inductances enable higher peak currents and faster rise times when all other parameters are held constant, thus making low inductance a crucial feature in high current systems. In this article, the experimental characterization of a 200-nF, 38-nH CSA is presented. When charged to 56 (57 kV), this CSA generates a 76 (64-kA) current peak into 0.28 (0.46-Ω) resistive loads. Comparing the experimental data's waveforms with those of an analytic circuit model, close agreement with underdamped RLC circuit theory is obtained. Based on this CSA's performance, the experimental data are extrapolated to model the performance of a bipolar 100-nF (±90 kV) CSA and a unipolar 200-nF (90 kV) CSA and compared with a standard 40-nF, bipolar (±100 kV) LTD brick. Furthermore, circuit simulations are performed for a 2.2-m diameter LTD cavity with 20 dual polarity (±100 kV) standard bricks and 23 dual polarity (±90 kV) CSAs. An increase by a factor of 2 in the power output is obtained for the CSA-LTD cavity over the brick LTD cavity while maintaining the same cross-sectional area. This has a large potential impact on the pulsed power generator design, both for smaller university-scale machines and proposed next-generation [60 Mega Amp (MA)] accelerators.

First results from a 760-GW linear transformer driver module for Z-pinch research

In this paper, the results of tests on a 0.76-TW linear transformer driver (LTD) module for Z-pinch research are presented for the first time. Ten LTD cavities, each generating a 1-MA/90-kV pulse on a matched load, were connected in series with a magnetically insulated voltage adder to drive the e-beam diode. Three inner stalks with different radii were tested, and the results indicate that the output parameters of the ten cavities are sensitive to the cathode radii. As an intermediate step, a high-current pulse with 832 kV/912 kA/130 ns was obtained on the e-beam diode. To date, this is the maximum power generated directly by a fast LTD with mega-ampere current output.

A gas-insulated mega-ampere-class linear transformer driver with pluggable bricks.

This paper presents the design and test of a gas-insulated linear transformer driver (LTD) cavity aimed at the Z-pinch experimental device CZ-34. The LTD cavity has a diameter of 2290 mm and a height of 346 mm. It consists of 23 main bricks and 1 trigger brick. Each main brick is comprised of two 100 nF capacitors connected electrically in series with a field-distortion gas switch. The trigger brick is comprised of two 50 nF capacitors connected in series with a compact multi-gap gas switch. All bricks are placed in the cavity filled with compressed SF6 and are pluggable like drawers. The trigger pulse generated by the trigger brick passes through an azimuthal transmission line to the trigger ring and makes the main bricks discharge synchronously. The LTD cavity can deliver ∼1 MA current pulse with a rise time of 115 ns to 0.08 Ω liquid resistance load when the charging voltage is ±100 kV, which is in good agreement with the circuit simulation results. Experimental results demonstrate the successful application of using gas insulation and pluggable bricks. The technical feasibility of the charging configuration, triggering method, and isolation resistors is verified. There is little difference in output performance as return-current rods replaced the outside metal cylinder, which provides a new path for the design of LTD cavities in series.

Analyzing and Improving the Output Waveform of Linear Transformer Driver Cavity

The linear transformer driver (LTD) is widely used in the field of pulsed power, especially for generating intense electron beam. The design of an LTD with quasi-rectangular voltage output is presented. The Blumlein pulse-forming network (BPFN) with L-type configuration replaced traditional capacitors to produce a quasi-rectangular voltage waveform; the adding voltage is achieved through a linear transformer (LT) that couples the voltage from multiple primaries to a single secondary. Since the output waveforms of BPFN and LT determine the output waveform of LTD, the effects of the configuration of the BPFN and the parameters of the LT on the output voltage waveform are studied. At the same time, according to the results, the output voltage waveform is improved by optimizing the inductance of the BPFN and selecting the appropriate parameters of the LT. After that, an electron beam generator based on six-stage LTD cavities is developed, and a well-shaped quasi-rectangular waveform is obtained at a planar diode load.

A repetitive electron beam driver based on 6-stage linear transformer driver cavities.

A repetitive electron beam driver based on 6-stage Linear Transformer Driver (LTD) cavities is presented. Each cavity consists of two Blumlein pulse forming networks (BPFNs) sharing a laser trigger switch. Voltage adding is obtained by means of Linear Transformer (LT) technology. The LTD cavity consists of two BPFNs connected in parallel, an LT with a ratio of 1:1, and a laser-triggered spark switch. The energy efficiency of a single LTD was investigated. The results show that the energy efficiency of an L-type BPFN and an LT was 94.3% and 92.8%, respectively. An electron beam driver consisting of 6 such cavities was developed with an output voltage of 740 kV, a current of 12.3 kA, and a power of 9.1 GW. The repetition frequency operation results show that the electron beam driver can operate stably for 20 s at 25 Hz with a jitter of approximately 1.7 ns.

The Story of the LTD Development

In this article, the story of linear transformer driver (LTD) development is presented, which began in the 1980s when the inductive energy storage (IES) systems with plasma opening switches (POSs) were the focus of pulsed power research all over the world. The accompanying problems of this technology resulted in the development of microsecond LTDs by Boris Kovalchuk, which was the first crucial step toward the development of this new technology. Next came the fast (~100 ns) LTDs, eliminating completely the need for any pulse compression and power amplification. Most recently, the LTD technology was further developed to generate fast rising output pulses with a flat or trapezoidal (rising or falling) top. Such LTD cavities, called Square Pulse LTDs, are better suited for applications such as flash radiography, Z-pinch, high-power microwaves, etc. A number of recent new suggestions important for the viability of LTD technology are also presented.

100 GW linear transformer driver cavity: Design, simulations, and performance

100 GW linear transformer driver cavity: Design, simulations, and performance

A Circuit Particle-in-Cell Coupled Simulation of a Magnetically Insulated Transmission Line System

We have developed a circuit particle-in-cell (PIC) coupled model for simulating a pulsed power system, based on a circuit algorithm of a pulsed power supply and a PIC simulation for a magnetically insulated transmission line (MITL) system. The coupled algorithm comprises an external circuit algorithm based on BERTHA and a coupled algorithm for the PIC and circuit interface based on the Mur absorbing boundary condition. In this paper, a simple coaxial MITL with a single linear transformer driver (LTD) was simulated to validate the coupled model compared with PSPICE and BERTHA simulations. The numerical results show that the circuit–PIC coupled model achieved almost the same outputs on the load. The coupled model was then applied to an MITL system driven by ten LTD cavities. The simulation results and experimental results were in agreement, which confirms that the circuit–PIC coupled model could provide a reference for the structural design of experimental apparatuses.

Conceptual design of a 15-TW pulsed-power accelerator for high-energy-density–physics experiments

We have developed a conceptual design of a 15-TW pulsed-power accelerator based on the linear-transformer-driver (LTD) architecture described by Stygar [W. A. Stygar et al., Phys. Rev. ST Accel. Beams 18, 110401 (2015)]. The driver will allow multiple, high-energy-density experiments per day in a university environment and, at the same time, will enable both fundamental and integrated experiments that are scalable to larger facilities. In this design, many individual energy storage units (bricks), each composed of two capacitors and one switch, directly drive the target load without additional pulse compression. Ten LTD modules in parallel drive the load. Each module consists of 16 LTD cavities connected in series, where each cavity is powered by 22 bricks connected in parallel. This design stores up to 2.75 MJ and delivers up to 15 TW in 100 ns to the constant-impedance, water-insulated radial transmission lines. The transmission lines in turn deliver a peak current as high as 12.5 MA to the physics load. To maximize its experimental value and flexibility, the accelerator is coupled to a modern, multibeam laser facility (four beams with up to 5 kJ in 10 ns and one beam with up to 2.6 kJ in 100 ps or less) that can provide auxiliary heating of the physics load. The lasers also enable advanced diagnostic techniques such as X-ray Thomson scattering and multiframe and three-dimensional radiography. The coupled accelerator-laser facility will be the first of its kind and be capable of conducting unprecedented high-energy-density–physics experiments.

Explanation of multi-pulse behavior in an LTD based on compact PFNs

A six cavity inductive voltage adder with pulsed power incorporated into a single assembly, linear transformer driver (LTD), has been developed. Each LTD cavity is driven by a single pulse forming network and spark gap switch. The output pulse is approximately square in shape due to the pulse forming networks (PFNs) used for the drivers. In some cases, when unreliable switch firing occurred, multiple output pulses were observed. The pulses stem from each cavity firing at a different time. A circuit model which explains the multi-pulse behavior and details the current path is developed. This observation shows the potential for an LTD to discharge each cavity individually to the load thus providing extremely high repetition rates on a common cathode stalk.

Factors affecting the output pulse flatness of the linear transformer driver cavity systems with 5th harmonics

We describe the study we have undertaken to evaluate the effect of component tolerances in obtaining a voltage output flat top for a linear transformer driver (LTD) cavity containing 3rd and 5th harmonic bricks [A. A. Kim et al., in Proc. IEEE Pulsed Power and Plasma Science PPPS2013 (San Francisco, California, USA, 2013), pp. 1354--1356.] and for 30 cavity voltage adder. Our goal was to define the necessary component value precision in order to obtain a voltage output flat top with no more than $\ifmmode\pm\else\textpm\fi{}0.5%$ amplitude variation.

Design of a 5-MA 100-ns linear-transformer-driver accelerator for wire array Z-pinch experiments

The linear-transformer-driver (LTD) is a recently developed pulsed-power technology that shows great promise for a number of applications. These include a $Z$-pinch-driven fission-fusion-hybrid reactor that is being developed by the Chinese Academy of Engineering Physics. In support of the reactor development effort, we are planning to build an LTD-based accelerator that is optimized for driving wire-array $Z$-pinch loads. The accelerator comprises six modules in parallel, each of which has eight series 0.8-MA LTD cavities in a voltage-adder configuration. Vacuum transmission lines are used from the interior of the adder to the central vacuum chamber where the load is placed. Thus the traditional stack-flashover problem is eliminated. The machine is 3.2 m tall and 12 m in outer diameter including supports. A prototype cavity was built and tested for more than 6000 shots intermittently at a repetition rate of 0.1 Hz. A novel trigger, in which only one input trigger pulse is needed by utilizing an internal trigger brick, was developed and successfully verified in these shots. A full circuit modeling was conducted for the accelerator. The simulation result shows that a current pulse rising to 5.2 MA in 91 ns (10%--90%) can be delivered to the wire-array load, which is 1.5 cm in height, 1.2 cm in initial radius, and 1 mg in mass. The maximum implosion velocity of the load is $32\text{ }\text{ }\mathrm{cm}/\ensuremath{\mu}\mathrm{s}$ when compressed to 0.1 of the initial radius. The maximum kinetic energy is 78 kJ, which is 11.7% of the electric energy stored in the capacitors. This accelerator is supposed to enable a radiation energy efficiency of 20%--30%, providing a high efficient facility for research on the fast $Z$ pinch and technologies for repetition-rate-operated accelerators.

Conceptual designs of two petawatt-class pulsed-power accelerators for high-energy-density-physics experiments

Here, we have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator’s water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator’s physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The acceleratormore » generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield thermonuclear fusion; i.e., a yield that exceeds the energy initially stored by the accelerator’s capacitors. 2D MHD simulations suggest Z 800 will deliver 8.0 MJ to the liner, and achieve a yield on the order of 440 MJ. Z 300 and Z 800, or variations of these accelerators, will allow the international high-energy-density-physics community to conduct advanced inertial-confinement-fusion, radiation-physics, material-physics, and laboratory-astrophysics experiments over heretofore-inaccessible parameter regimes.« less

Trigger Method Based on Secondary Induced Overvoltage for Linear Transformer Drivers

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.

Optimization of Cavity Combination for 20 MA LTD-Based Accelerators

Based on a transmission line code, a circuit model is proposed that could serve as the basic method for the analysis of linear transformer driver (LTD)-based accelerators. By using 1 MA, 100 kV LTD cavities, the peak load current is optimized for a total of N cavities between 500 and 1200. The simulation results suggest that, with the same number of cavities, the peak current changes obviously with the types of combinations, and the maximum change can be as large as 1.2 MA. The results also show that, for the cases considered, the optimized peak current as a function of the total number of cavities agrees with the exponential associate, and the peak current for one level LTD cannot be enhanced infinitely. Furthermore, it is found that, to obtain a 20 MA peak load current, at least 1029 LTD cavities (49 in series and 21 in parallel connection) are needed. Finally, the typical parameters of the optimized design are compared to those of the existing Z accelerator.

Square pulse linear transformer driver

The linear transformer driver (LTD) technological approach can result in relatively compact devices that can deliver fast, high current, and high-voltage pulses straight out of the LTD cavity without any complicated pulse forming and pulse compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. The usual LTD architecture [A. A. Kim, M. G. Mazarakis, V. A. Sinebryukhov, B. M. Kovalchuk, V. A. Vizir, S. N Volkov, F. Bayol, A. N. Bastrikov, V. G. Durakov, S. V. Frolov, V. M. Alexeenko, D. H. McDaniel, W. E. Fowler, K. LeCheen, C. Olson, W. A. Stygar, K. W. Struve, J. Porter, and R. M. Gilgenbach, Phys. Rev. ST Accel. Beams 12, 050402 (2009); M. G. Mazarakis, W. E. Fowler, A. A. Kim, V. A. Sinebryukhov, S. T. Rogowski, R. A. Sharpe, D. H. McDaniel, C. L. Olson, J. L. Porter, K. W. Struve, W. A. Stygar, and J. R. Woodworth, Phys. Rev. ST Accel. Beams 12, 050401 (2009)] provides sine shaped output pulses that may not be well suited for some applications like $z$-pinch drivers, flash radiography, high power microwaves, etc. A more suitable power pulse would have a flat or trapezoidal (rising or falling) top. In this paper, we present the design and first test results of an LTD cavity that generates such a type of output pulse by including within its circular array a number of third harmonic bricks in addition to the main bricks. A voltage adder made out of a square pulse cavity linear array will produce the same shape output pulses provided that the timing of each cavity is synchronized with the propagation of the electromagnetic pulse.

Effect Analysis of Switch Prefire in 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.