Low Inductance Capacitor Bank Research Articles

Small-Size High-Current Generators for X-Ray Backlighting

The paper deals with the soft X-ray backlighting based on the X-pinch as a powerful tool for physical studies of fast processes. Proposed are the unique small-size pulsed power generators operating as a low-inductance capacitor bank. These pulse generators provide the X-pinch-based soft X-ray source (hν = 1–10 keV) of micron size at 2–3 ns pulse duration. The small size and weight of pulse generators allow them to be transported to any laboratory for conducting X-ray backlighting of test objects with micron space resolution and nanosecond exposure time. These generators also allow creating synchronized multi-frame radiographic complexes with frame delay variation in a broad range.

Development and characterization of a high yield transportable pulsed neutron source with efficient and compact pulsed power system.

The results of characterization experiments carried out on a newly developed dense plasma focus device based intense pulsed neutron source with efficient and compact pulsed power system are reported. Its high current sealed pseudospark switch based low inductance capacitor bank with maximum stored energy of ∼10 kJ is segregated into four modules of ∼2.5 kJ each and it cumulatively delivers peak current in the range of 400 kA-600 kA (corresponding to charging voltage range of 14 kV-18 kV) in a quarter time period of ∼2 μs. The neutron yield performance of this device has been optimized by discretely varying deuterium filling gas pressure in the range of 6 mbar-11 mbar at ∼17 kV/550 kA discharge. At ∼7 kJ/8.5 mbar operation, the average neutron yield has been measured to be in the order of ∼4 × 109 neutrons/pulse which is the highest ever reported neutron yield from a plasma focus device with the same stored energy. The average forward to radial anisotropy in neutron yield is found to be ∼2. The entire system is contained on a moveable trolley having dimensions 1.5 m × 1 m × 0.7 m and its operation and control (up to the distance of 25 m) are facilitated through optically isolated handheld remote console. The overall compactness of this system provides minimum proximity to small as well as large samples for irradiation. The major intended application objective of this high neutron yield dense plasma focus device development is to explore the feasibility of active neutron interrogation experiments by utilization of intense pulsed neutron sources.

Nanopowder production by gas-embedded electrical explosion of wire

A small electrical explosion of wire (EEW) setup for nanopowder production is constructed. It consists of a low inductance capacitor bank of 2 μF–4 μF typically charged to 8 kV−30 kV, a triggered gas switch, and a production chamber housing the exploding wire load and ambient gas. With the EEW device, nanosize powders of titanium oxides, titanium nitrides, copper oxides, and zinc oxides are successfully synthesized. The average particle size of synthesized powders under different experimental conditions is in a range of 20 nm−80 nm. The pressure of ambient gas or wire vapor can strongly affect the average particle size. The lower the pressure, the smaller the particle size is. For wire material with relatively high resistivity, such as titanium, whose deposited energy Wd is often less than sublimation energy Ws due to the flashover breakdown along the wire prematurely ending the Joule heating process, the synthesized particle size of titanium oxides or titanium nitrides increases with overheat coefficient k (k = Wd/Ws) increasing.

Measurement of effective blast energy for direct initiation of spherical gaseous detonations from high-voltage spark discharge

In this study, effective energy from spark discharge for direct blast initiation of spherical gaseous detonations is investigated. In the experiment, direct initiation of detonation is achieved via a spark discharge from a high-voltage and low-inductance capacitor bank and the spark energy is estimated from the analysis of the current output. To determine the blast wave energy from the powerful spark, the time-of-arrival of the blast wave in air is measured at different radii using a piezoelectric pressure transducer. Good agreement is found in the scaled blast trajectories, i.e., scaled time c o·t/R o where c o is the ambient sound speed, as a function of blast radius R s/R o between the numerical simulation of a spherical blast wave from a point energy source and the experimental results where the explosion length scale R o is computed using the equivalent spark energy from the first 1/4 current discharge cycle. Alternatively, by fitting the experimental trajectories data, the blast energy estimated from the numerical simulation appears also in good agreement with that obtained experimentally using the 1/4 cycle criterion. Using the 1/4 cycle of spark discharge for the effective energy, direct initiation experiments of spherical gaseous detonations are carried out to determine the critical initiation energy in C2H2–2.5O2 mixtures with 70 and 0% argon dilution. The experimental results obtained from the 1/4 cycle of spark discharge agree well with the prediction from two initiation models, namely, the Lee’s surface energy model and a simplified work done model. The main source of discrepancy in the comparison can be explained by the uncertainty of cell size measurement which is needed for both the semi-empirical models.

Development of the Repetitive Pulsed Power System for Spallation Neutron Source Beam Extraction Fast Kicker

The spallation neutron source (SNS) is a next-generation high-intensity beam facility. The extraction kicker system is a high peak power, high average power, high-precision pulse-waveform, low beam impedance, and high repetition rate pulsed power system. It has been successfully designed and developed at Brookhaven National Laboratory, Upton, NY. The system consists of fourteen extraction magnet sections inside the ring vacuum chamber and fourteen identical high-voltage modulators located in the service building. The design features a Blumlein pulse-forming-network (PFN) based topology, a low beam impedance termination, a fast current switching thyratron, and low inductance capacitor banks. It has a maximum charging voltage of 50 kV, an open circuit output of 100 kV, and a designed maximum pulsed current output of 4 kA per modulator. The overall system output will reach multiple GW peak power with a 60 pulse/s repetition rate. A prototype modulator has been successfully built and tested well above the SNS requirement. The first batch of production modulators have passed all required tests at manufacture facility and delivered to Oak Ridge National Laboratory, Oak Ridge, TN, and the rest of the production units are under construction. The technical challenges and development results are presented in this paper.

TEMPORAL CORRELATION OF NEUTRONS, ION BEAM, AND HIGH VOLTAGE PROBE SIGNALS IN A LOW ENERGY PLASMA FOCUS

A simple, low inductance (~50 nH) capacitor bank is developed for plasma focus operation. The bank comprises three modules, each consisting of two 2µF, 40 kV capacitors along with a field distortion type pressurized sparkgap. The system is employed to investigate the temporal correlation of neutrons, ion beam, high voltage probe, and Rogowski coil signals. From a careful analysis of the data, the high voltage probe signal is found to coincide well with the Rogowski coil signal, whereas the ion beam and the neutron pulses are delayed by 20–30 nsec. These observations suggest that in low energy plasma foci, ion beam generation and neutron emission occur after the maximum compression has taken place and the pinch filament has started to break, most probably due to the onset of sausage type instability.

Comparison of characteristics of an x-ray preionized XeCl laser, pumped by water transmission line and low inductance capacitor bank

With a high uniformity of x-ray preionization, a long pulse duration and a high specific energy (9 J l -1), an XeCl excimer laser output has been obtained by using a compact low inductance capacitor bank as the laser excitation circuit. The superior features of the low inductance capacitor bank (LICB) compared to the water transmission line (WTL) are attractive for repetitively pulsed XeCl excimer lasers.

The TUM/LRT electromagnetic launchers

In the laboratory of the Lehrstuhl fur Raumfahrttechnik LRT (Astronautical Institute) of the Technische Universitat Munchen TUM (Technical University Munich) different types of electromagnetic launchers are being developed and operated : (1) An electromagnetic accelerator which accelerates particles in the mass range of 10-8g < m < 10 g to velocities up to 500 m/s ; (2) an electrothermal accelerator, where particles of masses of 10-4g < m < 10-1g reach velocities up to 3 km/s ; (3)a rail gun which accelerates particles in the mass range of 10-1g < m < 10 g up to 3,5 km/s ; (4) a plasma-dynamic launcher where particles in the mass range of 10-8g < m < 10-3g can be accelerated to velocities up to 20 km/s. All these systems use a fast, low inductance capacitor bank for energy storage. The first section of this paper gives a description of the four accelerator systems together with the present performance data and limitations. The current development work is discussed. In the second section the plasma diagnostics, that are used in the accelerator types (2), (3) and (4) are described together with the plasma shielding, that is necessary in the impact section. The next section describes the specific impact diagnostics, that had to be developed for the plasma accelerator. In the final section the research programs are described, in which these electro-magnetic accelerators are used.

Measurements of a 70 T pulsed magnetic system with long operational life

A pulsed, high current machine has recently been developed for producing pulsed magnetic fields in the 70 Tesla range, with long operational life of the load inductor. The current source, which drives a single-turn solenoid, is a high voltage, low inductance capacitor bank. Connections from the capacitor bank to the inductor are by way of low inductance multichannel rail spark gap switches and low inductance buswork. Peak magnetic fields of 72 Tesla have been measured in the bore of the single-turn inductor. These measurements have been made using the magneto-optical, Faraday, effect. The pulsed magnetic machine has recently been used to develop enclosure welds on tubing which is being evaluated as possible cladding material in nuclear reactors.

Transient Surface Temperature Response of Liquid Crystal Films

Abstract Various commercially available microencapsulated cholesteric liquid crystal films and coatings were tested to determine their response to rapid heating. The rapid response data are required for the diagnostic devices to be used to measure the spatial energy distribution in lasers and electron beams. These films, which reversibly change color at discrete temperatures, were placed on tapered stainless steel strips that were pulse heated to a predetermined level in 30 μsec by discharging through the steel strip the stored charge from a high-voltage low-inductance capacitor bank. The taper introduced a temperature gradient along the length of the strip with the higher temperature at the narrow end. A high-speed movie camera and time fiducial system recorded the color change movement along the length of the bar as a function of time. The response time is the time required for the material to achieve its event temperature when the ratio of event temperature less initial temperature to substrate temperature rise is 0.9. These data are reported in tabular form for each material tested. In general, the response is inversely proportional to the film thickness squared since this is one of the limiting factors for heat diffusion. Various techniques to reduce film and coating thickness and enhance response were demonstrated and are discussed in the paper.

A Reliable Flashlamp Pumped Tunable Organic Dye Laser

An improved flashlamp pumped dye laser is described. Reliable long term operation has been attained by using a low inductance capacitor bank. A linewidth of 0.3 Å and a beam divergence of 2.5×10−3 rad result from more uniform optical pumping in the flashlamp. Tuning ranges of several organic dyes are presented.

Determining the Dynamic Elastic Modulus at Elevated Temperatures by Using Rapid Heating Techniques

A unique method of determining the dynamic elastic modulus at elevated temperatures is reported. It consists of discharging a low-inductance capacitor bank through a preheated thin rod of the test material, which is clamped at one end and free at the other. The transient i2R heating during bank discharge takes place in a time interval short compared to the mechanical response time of the rod, inducing longitudinal oscillations. The periodic motion of the free end of the rod interrupts a collimated light beam directed over the rod and onto an SD-100 photodiode whose output is recorded on a 561A Tektronix oscilloscope. This gives a direct measure of the modulus since it is inversely proportional to the square of the period of oscillation. The theory, experimental apparatus, and method are discussed and sample data are given for tests made on type 304 stainless steel to illustrate the accuracy and usefulness of this technique. The data are compared with those of other investigators using other methods, and excellent agreement was obtained. This method is particularly useful for testing hazardous materials where remote operation is required for safety.

Measurements of thermally induced stress waves in a thin rod using birefringent coatings

When an elastic body is heated rapidly, significant inertial stresses are developed if the imposed heating rates cause a substantial temperature change in times which are short relative to the mechanical response time of the body. This work describes a method for inducing and measuring the thermally induced elastic stress waves in an unrestrained thin rod. Rapid heating was accomplished electrically by discharging a low-inductance capacitor bank (0.1 μH, 2800 J) through the rod. Utilizing the frozen-stress technique, an initial fringe pattern was introduced into thin strips of a birefringent material securely bonded to each side of the rod. The longitudinal strain oscillations were measured by direct observation of the movement of the fringe patterns with a high-speed framing camera. Interpretation of these measurements required a dynamic calibration and application of a dynamic correction factor for the reinforcing effect of the coating. Oscillations with periods from 35 to 250 μsec and stress amplitudes up to 900 psi were measured and compared with the uncoupled thermoelastic theory. Good agreement was obtained, and it was concluded that this technique is suitable for transient measurements in the presence of large magnetic fields which normally restrict the use of electronic methods.

Low-inductance capacitor banks and linear pinched discharges

Details are given of low inductance capacitors, and capacitor banks with energy storages in the range 1 to 10 kJ capable of giving maximum currents of 1.2 MA and rates of rise of 6 × 1012 A/S. The working voltage in the work described is 10 kV, but the techniques have been used at higher voltages, up to 40 kV. An outline of the measurement techniques, used in the study of linear pinched discharges in low-pressure deuterium with these banks, is given.