Horacio Bruzzone

Conceptual engineering of plasma-focus thermonuclear pulsors

The basic engineering criteria for conceptual design of plasma focus devices is derived from a thermonuclear model, and applied successfully to the operation of small neutron pulsors. The theory is able to explain the variation of the neutron yield with the gas pressure in deuterium-filled chambers, the current evolution, and the electrode geometrical parameters. The performance of a prototype designed to optimize the flux/fluence ratio is presented, contrasting the experimental outcomes with the model. A set of effective design parameters is deduced, which ensure a band confidence of 20%.

A plasma focus driven by a capacitor bank of tens of joules

As a first step in the design of a repetitive pulsed neutron generator, a very small plasma-focus device has been designed and constructed. The system operates at low energy (160 nF capacitor bank, 65 nH, 20–40 kV, and ∼32–128 J). The design of the electrode was assisted by a computer model of Mather plasma focus. A single-frame image converter camera (5 ns exposure) was used to obtain plasma images in the visible range. The umbrellalike current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher. The calculations indicate that yields of 104–105 neutrons per shot are expected with discharges in deuterium.

A lumped parameter model of plasma focus

In this paper, a fast running computer model of a plasma focus device is presented. The model is based on the snowplow model constructed with effective parameters validated against experimental results. A pinch model is included to calculate the temporal evolution of the focal variables. The resulting neutron yield predictions are compared against available data at different pressures, electrode length, and capacitor voltages from experimental measurements, finding good agreements. The model ultimately calculates the neutron production given the geometric parameters and the filling deuterium pressure.

Small-chamber 4.7 kJ plasma focus for applications

The performance of a small-chamber Plasma Focus designed to maximize the output fluence and to be used as portable radiation generator is presented. The neutron yield; the x-ray intensity; the applied voltage and the flowing current between the electrodes, were measured at different deuterium filling pressures. Voltage spikes of about 10 times the amplitude of the initial interelectrode voltage were obtained at focalization time, thus indicating very good focusing. The mean neutron yield was 3×108 neutrons of 2.45 MeV per shot, corresponding to a 106 neutrons/cm2 fluence on the external surface of the chamber. Both, the measured current sheath kinematics and neutron production are compared with numerical models, showing very good agreement. The x-ray emission was applied to obtain radiographies of metallic objects, whereas the neutron output was used for low-Z elements detection.

On Probable Fusion Mechanisms in a Mather-Type Plasma Focus

An experimental study on plasma focus fusion mechanisms is presented in this paper. Simple diagnostic techniques such as current derivative, voltage, and time-integrated neutron detectors are used. This paper allows us to obtain information and fusion mechanisms present in a medium energy plasma focus [Auto Confined Plasma (PACO), 2 kJ, 31 kV, and T=2.9 μs]. The current sheet (CS) inductance is calculated through the anode voltage and current derivative signals, for some 80 shots performed within the pressure range 0.8-2.1 mbar of deuterium. It is concluded that the CS inductance is consistent with the behavior of a unique plasma sheet moving between two coaxial electrodes. From the information collected, it is concluded that the main fusion mechanism in this device is the beam-target one.

The effect of transmission lines and switching action on the electrical signals in a powerful capacitive discharge

A numerical model for the equivalent electrical circuit of a capacitive discharge that includes the effects of finite closure time of switches and finite transit time in the transmission line is presented. An approximate analytical method to predict the oscillations that can be excited in the circuit is developed. The model yields complex behavior over time of the magnitudes of currents and voltages in pulsed circuits in the simple circuit configurations studies. Some of the oscillations predicted by the calculations admit simple physical interpretations in limiting cases (and are then predictable without the need of complex calculations); however, the rest of the observed features are not amenable to easy interpretation. The numerical solutions of the model are compared with experimental signals measured in a plasma focus device. Good agreement with many of the observed features is noted. The model can be extended easily to other types of circuits (Marx generators, Blumlein lines, etc.). >

Design and construction of a very small plasma focus in the limit of low energy

In order to achieve the design of a repetitive neutron pulses source for a substance detector, a single shot very small plasma focus device has been designed and constructed. The plasma focus operates in the limit of low energy (160nF capacitor bank, 25–50nH, 30–40kV, ∼100J). The design of the electrode was assisted by a simple model of a Mather plasma focus. A neutron yield of 104–105 is expected when the discharge is operated with deuterium.

Time-Varying Inductance of the Plasma Sheet in the PF1000 Plasma-Focus Device

The time-varying inductance of the plasma-electrode system in the large plasma focus (PF) device PF1000 is experimentally determined from voltage and current-time derivative signals. The signals were acquired in several shots performed at the same conditions (2.4 mbar of D2, 24 kV). Using these results, the temporal evolution of the voltage drop on the pinch column is also assessed. The maximum values of these voltage drops exceed 100 kV in all the shots.

Electrical Behavior of an Ultralow-Energy Plasma-Focus Device

Measurements of electrical signals in discharges performed in a miniature plasma-focus device (5 nF capacitor bank, central electrode diameter and effective free length 2.0 and 0.5 mm, respectively, no outer electrode) are reported. A lumped model is built that gives good agreement between the model and the recorded signals. The main conclusions are as follows: the voltage at which the chamber disk capacitor is charged is limited by the gas filling pressure inside the chamber and not just by the voltage in the capacitor energy source, and the behavior of the discharge current within the vacuum chamber depends on the whole system and not only on the last portion of the circuit, even for longer transmission line lengths.

Temporal Correlations Between Hard X-Ray and Neutron Pulses in the PACO Plasma Focus Device

An experimental study on the fluctuations in the instants of time and amplitudes of the peaks in the voltage, current time derivative, X-rays, and neutron signals during a 150-shot series in a 2 kJ Plasma Focus device is presented. The results show a reasonable correlation between the time of the voltage peak and that of X-ray emission, in agreement with the picture of the formation of an electron beam accelerated by this voltage. However, the time differences between X-rays and neutron maximum emission are difficult to interpret in terms of a common origin.