Main Current Pulse Research Articles

Understanding the impact of an applied axial magnetic field on efficient current coupling on the Z machine

Magnetized liner inertial fusion (MagLIF) is an attractive concept for producing thermonuclear fusion reactions. The MagLIF platform involves the operation of Helmholtz coils to apply a 15 Tesla axial magnetic field to the load region, where a cylindrical, fuel-filled metal liner is imploded by a ∼20 MA current pulse. The fringe field from these coils extends into the transmission line that delivers the current to the target. We investigated the extent to which this applied field disturbs the nominal power flow within that transmission line. A simplified model of the geometry shows that adding the applied magnetic field results in magnetic field lines that connect the cathode to the anode, suggesting electrons may not be magnetically insulated in this region. Particle-in-cell simulations indicated the addition of the applied magnetic field would not significantly impact the current delivery to the load. Velocimetry was used to experimentally assess the current delivery with and without the applied magnetic field. We find no measurable effects of the applied field on current delivery in the configuration investigated in this study. Published by the American Physical Society 2024

A wide dynamic range diamond quantum sensor as an electric vehicle battery monitor.

To demonstrate the application capability of the diamond quantum sensor as an electric vehicle (EV) battery monitor, we (i) investigated the measurable current in a real car noise level and (ii) compared the linearity with conventional sensors. Consequently, (i) we could measure a 20 mA current pulse even under an external magnetic field of 80 µT, which is larger than that of 50 µT around the EV battery module in a real car during driving. The 20 mA pulse measurement corresponds to the EV battery state of charge estimation accuracy of 0.2% in the standard driving pattern, which is smaller than the present level of 10%. (ii) The linearity degradation seen in the Hall sensor near the upper limit of the measurement range was not seen in the diamond sensor. Although the Hall sensor and the shunt resistor showed linearity degradation in the current range of several tens of amperes or less, the degradation was smaller for the diamond sensor. The transverse magnetic field effect in the diamond sensor on the linearity was estimated to be less than 0.01% for a several-degree misalignment of the sensor surface to the magnetic field direction and under a 340 A current. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

On the mechanisms of the influence of preliminary ionization on the plasma dynamics of nanosecond capillary discharges and the properties of discharge-based EUV lasers

In this paper, we present the results of numerical investigations into the influence of preionization on the properties of extreme ultraviolet (EUV) lasers based on nanosecond capillary discharges. Prior to application of the main current pulse, gas inside the capillary is usually preionized by a separate current pulse with longer duration and lower amplitude, which creates plasma with minimal density on the capillary axis and maximal at the capillary wall. Magnetohydrodynamic simulations were performed for a range of prepulse parameters that defined different degrees of inhomogeneity of this initial profile. It was found that the plasma density distribution at the start of the main current pulse affects the cylindrical shock wave that takes place during the compression stage of a capillary discharge: A lower degree of radial inhomogeneity results in a steeper front of the shock wave. It is further shown that a steeper wave front results in a more concave electron density profile moments before the shock wave collapses on the capillary axis, when the EUV laser pulse presumably takes place, which may lead to a decrease in the rate of refraction losses. The proposed interpretation of the obtained numerical results correlates well with the available experimental data on the dependence of EUV laser pulse intensity and duration on the preliminary pulse amplitude.

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.

MA-class linear transformer driver for Z -pinch research

A linear transformer driver (LTD) generator capable of delivering up to 0.9 MA current pulses with 160 ns rise time has been assembled and commissioned at University of California San Diego. The machine is an upgrade of the LTD-III pulser from Sandia National Laboratories, consisting of 40 capacitors and 20 spark gap switches, arranged in a 20-brick configuration. The driver was modified with the addition of a new trigger system, active premagnetization of the inductive cores, a vacuum chamber with multiple diagnostic ports, and a vacuum power feed to couple the driver to plasma loads. The new machine is called compact experimental system for $Z$-pinch and ablation research (CESZAR). The driver has a maximum bipolar charge voltage of $\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }\mathrm{kV}$, but for reliability and testing, and to reduce the risk of damage to components, the machine was operated at $\ifmmode\pm\else\textpm\fi{}60\text{ }\text{ }\mathrm{kV}$, producing 550 kA peak currents with a rise time of 170 ns on a 3.5 nH short circuit. While the peak current is scaled down due to the reduced charge voltage, the pulse shape and circuit parameters are close to the results of the cavity and power feed models but suggest a slightly higher inductance than predicted. The machine was then used to drive wire array $Z$-pinch and gas puff $Z$-pinch experiments as initial dynamic plasma loads. The evolution of the wire array $Z$ pinch is consistent with the general knowledge of this kind of experiment, featuring wire ablation and stagnation of the precursor plasma on axis. The gas puff $Z$ pinches were configured as a single, hollow argon gas shell, in preparation for more structured gas puff targets such as multispecies, multishell implosions. The implosion dynamics agree generally with 1D magnetohydrodynamics simulation results, but large zippering and magneto-Rayleigh-Taylor instabilities are observed. The CESZAR load region was designed to accommodate many load types to be driven by the machine, which makes it a versatile platform for studying $Z$-pinch plasmas. The completion of the CESZAR driver allows plasma experiments on energy coupling from LTD machines to plasma loads, instability mitigation techniques and magnetic field distributions in $Z$ pinches, and interface dynamics in multispecies implosions.

Implosion of auto-magnetizing helical liners on the Z facility

In the first auto-magnetizing liner implosion experiments on the Z Facility, precompressed internal axial fields near 150 T were measured and 7.2-keV radiography indicated a high level of cylindrical uniformity of the imploding liner's inner surface. An auto-magnetizing (AutoMag) liner is made of discrete metallic helical conductors encapsulated in insulating material. The liner generates internal axial magnetic field as a 1–2 MA, 100–200 ns current prepulse flows through the helical conductors. After the prepulse, the fast-rising main current pulse causes the insulating material between the metallic helices to break down ceasing axial field production. After breakdown, the helical liner, nonuniform in both density and electrical conductivity, implodes in 100 ns. In-flight radiography data demonstrate that while the inner wall maintains cylindrical uniformity, multiple new helically oriented structures are self-generated within the outer liner material layers during the implosion; this was not predicted by simulations. Furthermore, liner stagnation was delayed compared to simulation predictions. An analytical implosion model is compared with experimental data and preshot simulations to explore how changes in the premagnetization field strength and drive current affect the liner implosion trajectory. Both the measurement of >100 T internal axial field production and the demonstration of cylindrical uniformity of the imploding liner's inner wall are encouraging for promoting the use of AutoMag liners in future MagLIF experiments.

Blowfly yaw control via electrical stimulation of the H1 lobula plate tangential cell.

Eliciting predictable flight responses in insects via exogenous stimulation of the nervous system is an area of both scientific and engineering interest. Blowflies in particular possess an excellent biological flight control system, making them an ideal system for characterising responses to stimulation. Here we demonstrate a means of electrically controlling Calliphoridae-Protophormia terranovae wing behaviour, generating a repeatable yaw response via biphasic electrical stimulation of the H1 lobula plate tangential cell (LPTC). We found that a 10 mA current pulse at a frequency of 30-270 Hz produces a yaw response in the preferred direction of H1 in a tethered blowfly preparation, and the magnitude of the yaw response is proportional to the frequency of the stimulus. This result suggests that these LPTCs, which encode optic flow, may be a viable interface for controlling fly flight behaviour. This platform could find application not only for micro air vehicles (MAVs), but also in developing flight models or for studying neurological control of fly flight behaviour.

Preconditioned wire array Z-pinches driven by a double pulse current generator

Suppression of the core-corona structure and wire ablation in wire array Z-pinches is investigated using a novel double pulse current generator ‘Qin-1’ facility. The ‘Qin-1’ facility allows coupling a ∼10 kA 20 ns prepulse generator with a ∼0.8 MA 160 ns main current generator. The tailored prepulse current preheats wires to a gaseous state and the time interval between the prepulse and the main current pulse allows formation of a more uniform mass distribution for the implosion. The implosion of a gasified two aluminum-wire array showed no ablation phase and allowed all array mass to participate in the implosion. The initial perturbations formed from the inhomogeneous ablation were suppressed, however, the magneto Rayleigh–Taylor (MRT) instability during the implosion was still significant and further researches on the generation and development of the MRT instabilities of this gasified wire array are needed.

The Electric Explosion of Conductors in Megagauss Magnetic Fields

The results are presented from an experimental study of the explosion of copper conductors performed at ~2 MA current pulse with a rise time of 100 ns using the MIG generator. Using a HSFC Pro optical camera, large-scale instabilities with wavelengths of 0.2–0.5 mm are recorded on the surface of the conductor. The instabilities are tongue-shaped ejections of plasma expanding at the speed of sound in the direction opposite the gradient of the magnetic field. The obtained data are analyzed using two-dimensional magnetohydrodynamic simulation. It is shown the emergence of the observed structure was likely due to the growth of flute instabilities.

Long-lived laser-induced arc discharges for energy channeling applications

Laser filamentation offers a promising way for the remote handling of large electrical power in the form of guided arc discharges. We here report that it is possible to increase by several orders of magnitude the lifetime of straight plasma channels from filamentation-guided sparks in atmospheric air. A 30 ms lifetime can be reached using a low-intensity, 100 mA current pulse. Stability of the plasma shape is maintained over such a timescale through a continuous Joule heating from the current. This paves the way for applications based on the generation of straight, long duration plasma channels, like virtual plasma antennas or contactless transfer of electric energy.

Characteristics of gain in Ne-like Ar 69.8 nm laser pumped by capillary discharge

In this paper, the theoretical calculation model of the gain coefficient of Ne-like Ar 69.8 nm laser is established. With the collisional-radiative model, the rate equations for the 46.9 nm and 69.8 nm lasers are built by considering the 4 levels of the 2s2p6 1S0, 2p53p 1S0, 2p53p 3P2, and 2p53s 1P1. The gain coefficients per ion density of 46.9 nm and 69.8 nm lasers are calculated on the basis of the rate equations. The results show that the 46.9 nm laser has potential of higher gain than the 69.8 nm laser at an electron temperature of 200 eV. The gain coefficients per ion density at different electron temperatures are also calculated. Under the same electron density, the higher electron temperature is favorable for increasing the gain coefficients per ion density of the 69.8 nm laser. Meanwhile there is also an optimal electron density corresponding to the maximum gain coefficient per ion density of the 69.8 nm laser at a given electron temperature. Then a one-dimensional cylindrical symmetry Lagrangian magneto-hydrodynamics (MHD) code is utilized to simulate the Z-pinch process. The radial distributions of the electron temperatures, the electron densities and the Ne-like Ar ion densities are calculated with the MHD code at the different initial pressures. According to the rate equations for the 69.8 nm laser and the simulation results of the MHD code, the gain coefficient distribution of 69.8 nm laser in the radial direction of the plasma can be determined when the plasma is compressed to a minimum radius. According to the experimental parameters, the maximum gain coefficient of 69.8 nm laser is calculated to be 0.32 cm-1 when the main pulse current is 12 kA. The relationship between the radial distribution of gain coefficient of 69.8 nm laser and the initial pressure is also simulated. The theoretical results show that the optimal initial pressure is in a range of 12-14 Pa, in which the amplitude of gain coefficient is maximum. The experiments about 69.8 nm laser are conducted with Al2O3 capillary which has an inner diameter of 3.2 mm and a length of 35 cm. A main current of 12 kA with a rise time of 32 ns is produced by the main pulse generator, which consists of a Marx generator and a Blumlein line filled with de-ionized water. The Blumlein line is pulse-charged by a ten-stage Marx generator and discharges through the capillary by a self-breakdown main switch pressurized with N2 gas. To reduce the amplitude of main current, we reduce the charging voltage of the Marx generator and increase the conducting inductance of the main switch. Prior to the operation of the main current pulse, the capillary filled with Ar is predischarged by a current of~20 A. The 69.8 nm laser intensity as a function of initial pressure is measured by a 1-m grazing incidence Rowland spectrograph. The experimental results show that the optimum pressure is 16 Pa which is similar to the theoretical result. In addition, the gain coefficient (0.4 cm-1) measured in experiment is slightly higher than that (0.32 cm-1) of the theoretical calculation.

Dynamics of pre-ionized fast capillary discharge

The goal of this work is to determine the best conditions for pre-ionization of the nitrogen filled capillary plasma column applying an external exponentially damped or high-frequency alternating current. As we supposed, optimal pre-ionization conditions are achieved when the plasma is quiescent, motionless and isothermal, near the local thermodynamical equilibrium. At the time of optimal conditions for the pre-ionization plasma column, the main pulse is applied. This approach enables us to estimate the influence of such prepared plasma on the value of emitted energy during the main current pulse. For modeling of plasma during the pre-pulse and main pulse, the magneto-hydro-dynamics (MHD) NPINCH code [1] and the radiative-MHD Z* code [2] were used. The computer results are used for further improvement of x-ray–ultraviolet-capillary sources designed in IPP ASCR and CTU FNSPE laboratories in Prague.

Neuroendocrine homeostasis after vagus nerve stimulation in rats

The vagus nerve is important in maintaining HPA axis and sympatho-adrenal system (SAS) homeostasis, however little is known about the effect of vagus nerve stimulation (VNS), as used therapeutically, on these functions. Accordingly, the effect of VNS on plasma indices of HPA axis (ACTH, corticosterone), and SAS (norepinephrine, epinephrine) function were evaluated in rats. Male rats, on day-0 (D0), underwent surgeries for implantation of catheters into the right jugular vein and programmable (VNP) or non-programmable (VND) neurocybernetic devices encircling the left cervical vagus. On D7, after a blood sample, the device in VNP rats was programmed to deliver 500 μs width, 0.25 mA current pulses at 20 Hz ('on' 30s, 'off' 5 min) followed by timed blood samples during the next 90 min. In acute studies, VNS was stopped at 60 min and the rats were perfused at 90 min to evaluate neuronal Fos immunoreactivity (Fos-IR). In chronic studies, the probe remained active. In these rats, the HPA axis response to airpuff-startle stressor (D17) and anterior pituitary CRF-receptor binding (D26) were evaluated. During acute VNS, plasma indices of HPA axis and SAS activity, as well as Fos-IR activation pattern in brain regions known to increase after stress, were not different between VND and VNP rats. During chronic VNS, stress-induced HPA axis responses exhibited a tendency toward faster recovery to baseline in VNP rats. Therapeutic VNS is not a stressor and does not compromise HPA axis or SAS homeostasis. Chronic VNS may facilitate development of efficient feedback mechanisms.

Controlled transport of superparamagnetic beads with spin-valves

Trapping, release, and transport of individual, or ensembles of, 2.8 μm superparamagnetic beads (SPB) functionalized with streptavidin were demonstrated with an addressable array of spin-valve (SV) traps integrated into a microfluidic channel. The linear array consists of two staggered lines of 1 μm × 8 μm SVs toggled “on” or “off” with 10 ms and 150 mA or −100 mA current pulses, respectively. The SPB is trapped when the SV is “on” and released or ignored when the SV is “off”. This “switchable permanent magnet” offers a low power alternative to other precision microfluidic transport devices.

Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

A new wire array configuration has been used to create thin shell-like implosions in a cylindrical array. The setup introduces a ~5 kA, ~25 ns current prepulse followed by a ~140 ns current-free interval before the application of the main (~1 MA) current pulse. The prepulse volumetrically heats the wires which expand to ~1 mm diameter leaving no dense wire core and without development of instabilities. The main current pulse then ionizes all the array mass resulting in suppression of the ablation phase, an accelerating implosion, and no trailing mass. Rayleigh-Taylor instability growth in the imploding plasma is inferred to be seeded by μm-scale perturbations on the surface of the wires. The absence of wire cores is found to be the critical factor in altering the implosion dynamics.

Measurement of high data rate performance in perpendicular magnetic recording

Two-port network analysis is used to probe the dynamics of the perpendicular recording head. Two identical heads are aligned to each other with poletip to poletip on their air-bearing-surface sides, while the spacing is about the typical flying height. With two-port network analysis, forward transmission coefficient measurement shows the nonuniformity of the frequency response across the footprint of the poletip. It is suggested that the spatial distribution of the fast-field pulse can be optimized by constructing the fast write pulse with proper frequency components. In addition, the head-to-media recording performance is characterized by contact scanning recording tester. A single 0.15 ns/50 mA current pulse is good enough to complete the recording.

Evaluation of Si and SiC SGTOs for High-Action Army Applications

The U. S. Army Research Laboratory (ARL) has been exploring silicon and silicon carbide Super gate turn-off thyristors (SGTOs) for high power pulse switching required by Army survivability and lethality applications. Silicon SGTOs (3.5 cm2) were pulsed at 5 kA with a half-sine current waveform measuring 1 ms at the base. The recovery time, or Tq, of the devices was evaluated from the point at which the main current pulse fell to zero. Using a driver designed to provide both turn-on and turn-off signals, the Tq was reduced to 10 mus. Smaller silicon carbide SGTOs (0.16 cm2) were similarly evaluated for wide-pulse performance. They were switched several times at a peak current above 300 A, with an unassisted Tq time of 30 mus. This paper provides details of the aforementioned pulse switching, as well as a description of continuing evaluations involving parallel devices and larger test beds.

Capillary discharge soft X-ray lasing in Ne-like Ar pumped by low main current pulses

Under the condition of the low main current pulse, the capillary discharge soft X-ray laser is obtained. The laser energy of the same discharge current amplitude at different Ar pressures and of different current amplitudes at the same Ar pressure are compared, respectively. At different Ar pressures, the amplitude of the main current pulse evidently impacts the soft X ray, especially at a pressure higher than 30 Pa. In order to obtain the laser output at a high Ar pressure, the amplitude should be increased. However, at the same Ar pressure, it might have an optimum current, in which case the laser spike is the highest and the most stationary. The laser achieving time at different amplitudes is also discussed. As far as we know, this is the first time the influence of the amplitude of the main current pulse has been analyzed.

On surface elementary processes and polymer surface modifications induced by double pulsed dielectric barrier discharge

In this paper, polyethylene terephthalate and polyethylene films were plasma treated using a dielectric barrier discharge (DBD), which presents a second current pulse with inverse polarity induced at the decreasing applied voltage flank in addition to the main current pulse. The surface modifications of polymer samples were pointed out by two complementary methods: the contact angle method and atomic force microscopy (AFM) technique. The influence of the voltage pulse parameters as width, falling rate and frequency on the secondary discharge formation was studied and correlated with polymer surface modifications. The plasma treatment is improved when the discharge is driven by a voltage pulses with very fast falling flank. After plasma treatment, roughness of the PE films is not considerably changed; only topographical modifications of PE treated samples being observed, while treated PET surfaces present an increased roughness and a new granular structure, with much smaller features compared to the pristine ones.

The effects of the prepulse on capillary discharge extreme ultraviolet laser

In the past few years collisionally pumped extreme ultraviolet (XUV) lasers utilizing a capillary discharge were demonstrated. An intense current pulse is applied to a gas-filled capillary, inducing magnetic collapse (Z pinch) and formation of a highly ionized plasma column. Usually, a small current pulse (prepulse) is applied to the gas in order to preionize it prior to the onset of the main current pulse. In this paper we investigate the effects of the prepulse on a capillary discharge Ne-like Ar XUV laser (46.9nm). The importance of the prepulse in achieving suitable initial conditions of the gas column and preventing instabilities during the collapse is demonstrated. Furthermore, measurements of the amplified spontaneous emission (ASE) properties (intensity and duration) in different prepulse currents revealed unexpected sensitivity. Increasing the prepulse current by a factor of 2 caused the ASE intensity to decrease by an order of magnitude and to nearly disappear. This effect is accompanied by a slight increase in the lasing duration. We attribute this effect to axial flow in the gas during the prepulse.