Inductive Voltage Adder Research Articles

The angular distribution of characteristic parameters in eccentric magnetically insulated transmission lines

In pulse power modules designed with linear transformer driver (LTD) and magnetically insulated inductive voltage adder (MIVA) technologies, the phenomenon of electrode eccentricity in long-distance magnetically insulated transmission lines (MITLs) cannot be overlooked due to its impact on pulse power transmission. The paper conducts a theoretical analysis of the distribution of electromagnetic fields, vacuum impedance, and characteristic parameters such as self-limited flow in eccentric MITLs with respect to the angular direction. Additionally, it constructs an eccentric MITL model to validate the theoretical findings.

Characterization of the failure shots of an induction voltage adder

Characterization of the failure shots of an induction voltage adder

Branch module for an inductive voltage adder for driving kicker magnets with a short circuit termination

For driving kicker magnets terminated in a short circuit, a branch module for an inductive voltage adder has been designed and assembled. The module has been designed for a maximum charging voltage of 1.2 kV and an output current of 200 A considering the current doubling due to the short circuit termination. It features three consecutive modes of operation: energy injection, freewheeling, and energy extraction. Therefore, the topology of the branch module consists of two independently controlled SiC MOSFET switches and one diode switch. In order not to extend the field rise time of the kicker magnet significantly beyond the magnet fill time, the pulse must have a fast rise time. Hence, the switch for energy injection is driven by a gate boosting driver featuring a half bridge of GaN HEMTs and a driving voltage of 80 V. Measurements of the drain source voltage of this switch showed a fall time of 2.7 ns at a voltage of 600 V resulting in a voltage rise time of 5.4 ns at the output terminated with a resistive load. To meet both the rise time and current requirements, a parallel configuration of four SiC MOSFETs was implemented.

Dual-pulse generation from a velvet cathode with a new inductive voltage adder for x-ray flash radiography applications

The evolution of two intense pulsed electron beams for an x-ray flash radiography application is characterized in a coupling of experimental and simulation studies. For this purpose, experiments were performed at Commissariat \`a l'Energie Atomique et aux Energies Alternatives with a new dual-pulse inductive voltage adder named ``Mi2.'' This experimental setup has been designed and built to produce two intense pulsed electron beams of 2 kA, 700 keV, and 80 ns full width at half maximum from a velvet cold cathode. The delay between the pulses ranges from tens of nanoseconds to a few microseconds. This study quantifies the effect of the plasma dynamics on the beam current of the second pulse, which represents an important parameter from the beam transport point of view. A multidimensional particle-in-cell simulation model is used to provide a numerical analysis in order to determine the influence of velvet plasma dynamics on the second pulse. Analysis of the experimental results indicates that the motion of the electron extraction zone within the expanding plasma remains lower than 1 mm for $3\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ delay, inducing a minor impact on the beam current extracted from the velvet cathode. Based on the developed model, this plasma expansion induces an increase of 5% for the current extracted from the second pulse with a delay of $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ for two pulses having the same voltage, which is consistent with linear induction accelerator injector parameters for multiple pulse use.

Characterization of self-magnetic pinch radiographic diode performance on RITS-6 at Sandia National Laboratories. II. Coupling between the inductive voltage adder and the SMP load

The self-magnetic pinch (SMP) diode is a type of radiographic diode used to generate an intense electron beam for radiographic applications. At Sandia National Laboratories, SMP was the diode load for the six-cavity radiographic integrated test stand inductive voltage adder (IVA) driver operated in a magnetically insulated transmission line (MITL). The MITL contributes a flow current in addition to the current generated within the diode itself. Extensive experiments with a MITL of 40 Ω load impedance [T. J. Renk et al., Phys. Plasmas 29, 023105 (2022)] indicate that the additional flow current leads to results similar to what might be expected from a conventional high-voltage interface driver, where flow current is not present. However, when the MITL flow impedance was increased to 80 Ω, qualitatively different diode behavior was observed. This includes large retrapping waves suggestive of an initial coupling to low impedance as well as diode current decreasing with time even as the total current does not. A key observation is that the driver generates total current (flow + diode) consistent with the flow impedance of the MITL used. The case is made in this paper that the 80 Ω MITL experiments detailed here can only be understood when the IVA-MITL-SMP diode is considered as a total system. The constraint of fixed total current plus the relatively high flow impedance limits the ability of the diode (whether SMP or other type) to act as an independent load. An unexpected new result is that in tracking the behavior of the electron strike angle on the converter as a function of time, we observed that the conventional cIVx “Radiographic” radiation scaling (where x ∼ 2.2) begins to break down for voltages above 8 MV, and cubic scaling is required to recover accurate angle tracking.

Characterization of self-magnetic pinch (SMP) radiographic diode performance on RITS-6 at Sandia National Laboratories. I. Diode dynamics, DC heating to extend radiation pulse

Radiographic diodes focus on an intense electron beam to a small spot size to minimize the source area of energetic photons for radiographic interrogation. The self-magnetic pinch (SMP) diode has been developed as such a source and operated as a load for the six-cavity radiographic integrated test stand (RITS-6) inductive voltage adder driver. While experiments support the generally accepted conclusion that a 1:1 aspect diode (cathode diameter equals anode–cathode gap) delivers optimum SMP performance, such experiments also show that reducing the cathode diameter, while reducing spot size, also results in reduced radiation dose, by as much as 50%, and degraded shot reproducibility. Analysis of the effective electron impingement angle on the anode converter with time made possible by a newly developed dose-rate array diagnostic indicates that fast-developing oscillations of the angle are correlated with early termination of the radiation pulse on many of the smaller-diameter SMP shots. This behavior as a function of relative cathode size persists through experiments with output voltages and currents up to 11.5 MV and 225 kA, respectively, and with spot sizes below approximately few millimeters. Since simulations to date have not predicted such oscillatory behavior, considerable discussion of the angle behavior of SMP shots is made to lend credence to the inference. There is clear anecdotal evidence that DC heating of the SMP diode region leads to stabilization of this oscillatory behavior. This is the first of two papers on the performance of the SMP diode on the RITS-6 accelerator.

Design and performance of a pulsed power-driven x-ray source for flash radiography

The design and test results of a pulsed power-driven X-ray source which is named Hawkeye-I are described. The Hawkeye-I is developed to execute flash X-ray radiography. It is a six-stage induction voltage adder and terminated by a positive polarity rod-pinch diode. The six stages are identical and the prime energy of each stage is provided by a tesla transformer elevating the output voltage from a discharge brick which consists of two $4\text{ }\text{ }\ensuremath{\mu}\mathrm{F}$ capacitors and a three-electrode gas switch. Comparing to the induction voltage adders driven by Marx generators, the numbers of gas switches and capacitors in the energy storage sections of Hawkeye-I decrease significantly. The Hawkeye-I can produce a maximum output voltage of about 4.2 MV. The corresponding X-ray source spot size and dose at 1 m are 1.4 mm and 17.9 rad, respectively. Fifty consecutive shots have been conducted to evaluate the reliability of the machine and 98% of the shots are successful. Furthermore, as the energy propagating in the accelerator is mainly determined by seven laser-triggered gas switches, the Hawkeye-I is capable of controlling the timing of the X-ray pulse accurately.

Requirements for an Inductive Voltage Adder as Driver for a Kicker Magnet with Short Circuit Termination

Requirements for an Inductive Voltage Adder as Driver for a Kicker Magnet with Short Circuit Termination

Simulation, experiment, and performance of a 4 MV induction voltage adder machine for flash x-ray radiography

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 $4.4\ifmmode\pm\else\textpm\fi{}0.3\text{ }\text{ }\mathrm{MV}$ ($1\text{\ensuremath{-}}\ensuremath{\sigma}$) and the average diode current is $81.6\ifmmode\pm\else\textpm\fi{}4.5\text{ }\text{ }\mathrm{kA}$ ($1\text{\ensuremath{-}}\ensuremath{\sigma}$). 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 ${t}_{\mathrm{spread}}$.

Production of pulsed high-energy neutron bursts from beam–target interaction using a 15 MeV HERMES III ion beam

Intense pulsed neutron fluences are generated by a high-energy (>10 MeV) proton beam using the beam-target method on the HERMES III facility at Sandia National Laboratories [J. J. Ramirez et al., in Proceedings of the 7th International Conference on High Power Particle Beams (Kernforschungszentrum, Karlsruhe GmbH, Karlsruhe, Germany, 1988), p. 148]. In order to generate the high-energy proton beam, a radial ion diode previously developed and fielded at the 6-MeV level in negative polarity was extended in performance to the 15-MeV level. This performance increase is described along with the development of a more durable hardware set to withstand the much more potent 15-MeV proton beam. An extensive series of simulations is developed to characterize the neutrons produced by the proton–target interaction. Particle-in-cell simulations describe the electron and ion dynamics, while Monte Carlo simulations characterize the neutron output. Due to differing estimates of proton beam voltage and current between the respective simulations, we make an approximate estimate of 13.5-MeV to 15-MeV and 120-kA ion beams at peak power in a 40 ns FWHM pulse. Simulations indicate that a total of 1.7 × 1013 neutrons are generated into 4π. Comparison of the neutron output predictions with a limited set of neutron flux measurements suggests a flux level of ∼1 × 1010 neutrons/cm2 to 10 × 1010 neutrons/cm2 over an approximately few tens of cm2 area at the relevant application location. This effort also contributes to physics understanding of the use of inductive voltage adder platforms to drive ion-beam diode loads.

An induction voltage adder with MHz repetition rates and nanosecond duration based on printed circuit board stacked Blumlein lines.

By utilizing the radio-frequency metal-oxide-semiconductor field-effect transistor switch, the multi-stage stacked printed circuit board Blumlein lines, and the induction voltage adder, a voltage generator with MHz repetition rates and nanosecond duration was designed and fabricated. The shunting current of the multi-stage stacked Blumlein lines, which consists of the coupling current at the load ends and the leakage current at the switch ends, is clarified. The circuit simulation and experiment of the three-stage stacked Blumlein lines are carried out; the result shows that the experiment and simulation agree well. The five-stage IVA prototype is designed for generating voltage pulses of ∼10 kV in amplitude and 8-11 ns in duration with MHz burst repetition rates. The voltage generator with MHz repetition rates and nanosecond duration demonstrated an important application prospect to reduce the electron beam initial emittance of the high energy accelerator.

Development of a 4-MV, 80-kA-Induction Voltage Adder for Flash X-ray Radiography

An induction voltage adder (IVA) has been developed to drive rod-pinch diodes (RPDs), to produce X-rays for flash radiography. The IVA facility consists of four-stage induction cavities stacked in series, each of which is single-point driven by a 6- $\Omega $ pulseline. The prime pulsed-power source comprises two Marx generators and four de-ionized water pulselines. Each Marx generator fast charges two pulselines to about 2.2 MV in less than 370 ns. Four electrically triggered, SF6-insulated gas switches serve as energy-transfer switches from pulse-forming lines to output transmission lines, and thus without an additional intermediate energy storage. A stepped inner stalk is utilized to make the vacuum impedance of the secondary transmission line increase from $30~\Omega $ at the first cavity to $120~\Omega $ at the last cavity, to prevent the electron emission from cathode surfaces. The RPD is chosen to create bremsstrahlung X-rays, which are believed to be optimal at voltage levels of about 4 MV. In this article, the design details and experimental results concerning the main components, such as the Marx/PFLs, IVA, and RPD, are demonstrated. The electrical performances of the facility are characterized. Presently, the IVA provides voltages of up to 4.3 MV and currents of up to 85 kA. The X-ray dose is up to 15.5 Rad [lithium fluoride (LiF)] at 1 m with a spot size of 1.4 mm and a FWHM time of 55 ns.

Development of a GW-Level Solid-State Long Pulse Generator

Solid-state pulsed power systems are characterized by high repeatability, high reliability, long lifetime, free maintenance, and easy usage. Such systems with moderate peak power (100-MW level) are widely applied in the high average power region; however, many issues are still to be researched to obtain GW-level peak power. In this paper, a high power solid-state pulse generator delivering a GW-level quasi-rectangle pulse is developed to investigate the electrical characteristics of the generator: 1) the system design is accomplished based on a reasonable setting of a power compression ratio combining semiconductor switches and magnetic switches and verified via circuit simulation; 2) key technologies such as serially connected thyristor switches, magnetic pulse compressor (MPC), Blumlein type low impedance pulse forming network (LIPFN), and induction voltage adder (IVA) are investigated for subsystem construction; and 3) at present, the experimental results achieved on a dummy load are a peak pulsed power of 5.2 GW, a pulsewidth of 170 ns, a repetitive rate of 20 Hz, and an operation time of 5 s with good waveform repeatability. The generator can be potentially applied for high power microwave (HPM) generation, X-ray sources, and industrial applications such as material surface treatment and pulse electric field (PEF) sterilization.

Determination of Magnetically Insulated Transmission Line Cathode Current of a 1.0-MV Pulsed Power Generator

We have employed B-dot monitors to measure the magnetically insulated transmission line (MITL) cathode current on a 1.0-MV pulsed power machine, whose MITL anode and cathode electrodes are separated completely by the insulation stack. The MITL anode and cathode conductors are connected together by the B-dot sensors’ signal transmission cables, when transmitting the outputs of the B-dot probes out of the MITL anode–cathode (A-K) gap. It provides a leakage current path and disturbs the MITL power flow unavoidably. During our experiment, the impedance of the leakage current path is increased by winding the signal transmission cables along one helical isolated inductor frame. The leakage current flowing along the outer conductors of the cables is significantly reduced, and the outputs of B-dot sensors could be delivered out of the MITL A-K gap effectively. The experiment results suggest that the B-dot sensors are able to diagnose the MITL cathode current successfully, even though the performance of the B-dot probes on this generator is not as perfect as that of the monitors employed on an induction voltage adder. Finally, particle-in-cell simulation is performed to check the accurateness of the measured MITL cathode current. It demonstrates that the peak value of the cathode current given by the B-dot monitors agrees with the simulation result very well.

Design of a 1-MV induction cavity and validation of the two-dimensional circuit model

We present the design and test results of an induction cavity, which is a full-scale prototype cell of a prospective six-stage induction voltage adder. This induction cell runs in positive polarity and the output transmission line operates in the vacuum insulation state. The design methods and configurations of the azimuthal transmission line, insulation stack, magnetic core, cathode plate, and output transmission line in the induction cell are given in detail. The induction cavity is driven by one pulsed power system consisting of a tesla transformer, intermediate storage capacitor, laser triggered gas switch, pulse forming line, self-breaking oil switch, and a water transmission line. A maximum voltage about 1.05 MV could be achieved across one $7.0\text{ }\text{ }\mathrm{\ensuremath{\Omega}}$ water resistor load. A full circuit model including the primary energy storage section and pulse forming section is proposed. The circuit model also includes the two-dimensional circuit model of the induction cavity [Phys. Rev. Accel. Beams 20, 020401 (2017)]. The voltage and current waveforms across the feed port, azimuthal transmission line, insulation stack, and resistor load obtained during the experiment are compared with the circuit simulation results. It demonstrates that they can agree with each other very well.

Contribution of the backstreaming ions to the self-magnetic pinch (SMP) diode current

The results presented here were obtained with a self-magnetic pinch (SMP) diode mounted at the front high voltage end of the RITS accelerator. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulse of six 1.3 MV inductively insulated cavities. The RITS driver together with the SMP diode has produced x-ray spots of the order of 1 mm in diameter and doses adequate for the radiographic imaging of high area density objects. Although, through the years, a number of different types of radiographic electron diodes have been utilized with SABER, HERMES III and RITS accelerators, the SMP diode appears to be the most successful and simplest diode for the radiographic investigation of various objects. Our experiments had two objectives: first to measure the contribution of the back-streaming ion currents emitted from the anode target and second to try to evaluate the energy of those ions and hence the Anode-Cathode (A-K) gap actual voltage. In any very high voltage inductive voltage adder utilizing MITLs to transmit the power to the diode load, the precise knowledge of the accelerating voltage applied on the A-K gap is problematic. This is even more difficult in an SMP diode where the A-K gap is very small (∼1 cm) and the diode region very hostile. The accelerating voltage quoted in the literature is from estimates based on the measurements of the anode and cathode currents of the MITL far upstream from the diode and utilizing the para-potential flow theories and inductive corrections. Thus, it would be interesting to have another independent measurement to evaluate the A-K voltage. The diode's anode is made of a number of high-Z metals in order to produce copious and energetic flash x-rays. It was established experimentally that the back-streaming ion currents are a strong function of the anode materials and their stage of cleanness. We have measured the back-streaming ion currents emitted from the anode and propagating through a hollow cathode tip for various diode configurations and different techniques of target cleaning treatment: namely, heating at very high temperatures with DC and pulsed current, with RF plasma cleaning, and with both plasma cleaning and heating. We have also evaluated the A-K gap voltage by energy filtering technique. Experimental results in comparison with LSP simulations are presented.

PIC simulation of the vacuum power flow for a 5 terawatt, 5 MV, 1 MA pulsed power system

In this paper, a 5 Terawatt, 5 MV, 1 MA pulsed power system based on vacuum magnetic insulation is simulated by the particle-in-cell (PIC) simulation method. The system consists of 50 100-kV linear transformer drive (LTD) cavities in series, using magnetically insulated induction voltage adder (MIVA) technology for pulsed power addition and transmission. The pulsed power formation and the vacuum power flow are simulated when the system works in self-limited flow and load-limited flow. When the pulsed power system isn’t connected to the load, the downstream magnetically insulated transmission line (MITL) works in the self-limited flow, the maximum of output current is 1.14 MA and the amplitude of voltage is 4.63 MV. The ratio of the electron current to the total current is 67.5%, when the output current reached the peak value. When the impedance of the load is 3.0 Ω, the downstream MITL works in the self-limited flow, the maximums of output current and the amplitude of voltage are 1.28 MA and 3.96 MV, and the ratio of the electron current to the total current is 11.7% when the output current reached the peak value. In addition, when the switches are triggered in synchronism with the passage of the pulse power flow, it effectively reduces the rise time of the pulse current.

Effects of current asymmetry on the electron sheath of magnetically-insulated induction voltage adders

The current asymmetry resulting from the special injection manner of cavities in magnetically-insulated induction voltage adders (MIVA) would affect the electron sheath and power flow of the magnetically-insulated transmission lines (MITL). In this article, based on the two-dimensional Creedon equilibrium flow of the magnetic insulation, the effects of current asymmetry on the electron sheath are evaluated. It is found that, the current asymmetry affects the electron sheath more greatly than the charge centroid and flow impedance. For the given parameters of MITL (e.g., the line voltage V 0 is 4.1 MV, and the minimal anode current I amin is 132 kA), the asymmetric coefficient of the electron sheath is about 79.4% when the asymmetric coefficient of cathode currents, δ c, is 35%. And the flow impedance decreases by 5.6% compared with the case of no current asymmetry. As the δ c increases to 69.5%, the AK gap is short circuit at some positions due to the off-center electron sheath. It is also found that, the lower of the line voltage V 0 and the minimal anode current I amin are, the larger of the influence extent of current asymmetry is. As the line voltage decreases from the 4.2 to 1.4 MV, the critical value of asymmetric coefficients of cathode currents leading to the gap closure, decreases from 69.5% to 35%.

Effects of cell-driving jitters on the output voltage of magnetically insulated induction voltage adders

Based on a 12-cell transmission line code model, the effects of cell-driving jitters on the output voltage (both the rise time and the flat-top waveform) of magnetically insulated induction voltage adders (MIVAs) were analyzed in detail. Simulation results show that the rise time (0.1–0.9) of the MIVA output voltage is a function of $\sigma $ (the standard deviation of the cell-driving jitters) when N (the number of cells connected in series for an MIVA) is small. When cell-driving jitters follow a normal probability distribution, the rise time of the output voltages also accord well with normal distributions. Parameters of these distributions are analytically deduced and presented. For any experimental shot, there is a fiducial probability that the rise time falls into the analytic solution range (within a certain error range). With the same error range, the fiducial probability decreases as $\sigma $ increases, while probabilities that the rise time are higher or lower than this range increase synchronously. Simulation results also indicate that cell-driving jitters have fewer effects on the flat-top peak of the MIVA output voltage. However, a series of ripple voltages is added on the flat-top waveform, and the average peak of these ripple voltages increases as $\sigma $ increases.

A method to calculate the electron sheath profile of the nonaxisymmetrical magnetic insulation

The nonaxisymmetrical magnetic insulation would occur due to the disalignment of inner electrodes in long magnetically insulated transmission lines, or the nonuniform distributions of injected currents in induction cavities of magnetically insulated induction voltage adders (MIVA). The electron sheath profile is a very important parameter to characterize the nonaxisymmetrical magnetic insulation. In the past, the three-dimensional particle in cell simulation was usually used to determine the electron sheath profile, which is extremely time-consuming and inefficient. In this paper, a fast and efficient calculation method is proposed. The classical one-dimensional Creedon theory of the magnetic insulation equilibrium is generalized to a two-dimensional plane of (r, ) via introducing a parameter defined as the azimuthal mode number. Two-dimensional Creedon is developed to model the asymmetric magnetic insulation of the MIVA. Provided the azimuthal distributions of magnetic flux density on the cathode, which is in proportion to the cathode current, the two-dimensional Creedon model is numerically solved. A numerical solution method to calculate the electron sheath profile is proposed, and then the calculation error is also given. As the azimuthal distribution of magnetic flux density on the cathode meets a cosine function, the profile of the electron sheath is approximate to the Gauss function. As the nonuniform portion of cathode current increases, the electron sheath becomes more eccentric, and the calculation error is also much larger.