Leopoldo Soto

Neutron emission from a fast plasma focus of 400 Joules

In dynamic pinches, short-duration high-temperature and high-density plasmas are produced, which can emit x rays and intense neutron pulses ~when deuterium is used in the discharge!. A plasma focus ~PF! is a particular pinch discharge in which a high pulsed voltage is applied to a low pressure gas between coaxial cylindrical electrodes. The central electrode is the anode partially covered with a coaxial insulator. The discharge starts over the insulator surface, and afterwards the current sheath is magnetically accelerated along the coaxial electrodes. After the current sheath runs over the ends of the electrodes the plasma is compressed in a small cylindrical column ~focus!. In most of the devices these three stages last a few microseconds. The pinch compression should be coincident with peak current ~really with the magnetic flow ! in order to achieve the best efficiency. The pinch generates beams of ions and electrons, and ultrashort x-ray pulses. Using deuterium gas, plasma focus devices produce fusion D‐D reactions, generating fastneutrons pulses (;2.5 MeV) and protons ~leaving behind 3 He and 3 H). The neutrons burst usually lasts about tens to hundreds of nanoseconds. The emitted neutrons can be applied to perform radiographs and substance analysis, taking advantage of the penetration and activation properties of this neutral radiation. The plasma focus is a pulsed neutron source especially suited for applications because it reduces the danger of contamination of conventional isotopic radioactive sources. A passive radioactive source of fast neutrons with similar energy ~for instance 252 Cf with similar mean energy or Am/Be with a harder spectrum! emits continuously, causing inconveniences in handling and storing. In turn, plasma-focus generators do not have activation problems for storage and handling. During the last 30 years, substantial effort and resources

Demonstration of neutron production in a table-top pinch plasma focus device operating at only tens of joules

Neutron emission from a deuterium plasma pinch generated in a very small plasma focus (6 mm anode diameter) operating at only tens of joules is presented. A maximum current of 50 kA is achieved 140 ns after the beginning of the discharge, when the device is charged at 50 J (160 nF capacitor bank, 38 nH, 20–30 kV, 32–72 J). Although the stored energy is very low, the estimated energy density in the plasma and the energy per particle in the plasma are of the same order as in higher energy devices. The dependence of the neutron yield on the filling pressure of deuterium was obtained for discharges with 50 and 67 J stored in the capacitor bank. Neutrons were measured by means of a system based on a 3He proportional counter in current mode. The average neutron yield for 50 J discharges at 6 mbar was (1.2 ± 0.5) × 104 neutrons per shot, and (3.6 ± 1.6) × 104 for 67 J discharges at 9 mbar. The maximum energy of the neutrons was (2.7 ± 1.8) MeV. Possible applications related to substance detection and others are discussed.

Studies on scalability and scaling laws for the plasma focus: similarities and differences in devices from 1 MJ to 0.1 J

A comprehensive analysis of scaling laws for plasma focus devices producing neutrons is presented. Similarities and differences in plasma focus devices working with stored energies ranging from 1 MJ to 0.1 J are found. First, a brief review listing the most important results achieved by the Thermonuclear Plasma Department of the Chilean Nuclear Energy Commission, CCHEN, is presented. The aim of the work at CCHEN has been to characterize the physics of dense plasma foci and also to carry out the design and construction of smaller devices—in terms of both input energy and size—capable of providing dense hot plasmas. Certain scaling rules have been found from this research. These rules combined with other scaling laws have been applied to design and construct plasma focus devices with storage energy in a region never explored before (tens of joules and less than 1 J). Thus, a comprehensive analysis also including results from other groups is presented. In particular, all the devices, from the largest to the smallest, maintain the same value of ion density, magnetic field, plasma sheath velocity, Alfven speed and the quantity of energy per particle. Therefore, fusion reactions are even possible to obtain in ultraminiature devices (driven by generators of 0.1 J for example), as they are in the larger devices (driven by generators of 1 MJ). However, the stability of the plasma pinch highly depends on the size and energy of the device.

Research on pinch plasma focus devices of hundred of kilojoules to tens of joules

At present the Plasma Physics and Plasma Technology Group of the Comision Chilena de Energia Nuclear (CCHEN) has the experimental facilities in order to study fast dense transient discharges in a wide range of energy and current, namely: I) energy from hundred of kilojoules to tens of joules, II) current from megaamperes to tens of kiloamperes. Also several diagnostics have been implemented. An overview of the work being carried out on dense pinch plasma focus discharges at the Comision Chilena de Energia Nuclear is presented. The plasma energy density and scaling laws for the neutron yield are discussed. Possible applications of the radiation emitted are also discussed.

Experimental study of the hard x-ray emissions in a plasma focus of hundreds of Joules

An experimental study on hard x-ray production in a small plasma focus device operating in a few hundreds of Joule range is presented. A threshold in the voltage drop on the pinch was observed for x-ray emission. A comparison with Dreicer theory for electrons runaway in plasmas yields significant agreement. The study was performed at a constant pressure (1.8 mbar) of deuterium with three different anode lengths.

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.

Pinch evidence in a fast and small plasma focus of only tens of joules

The pinch evidence in a deuterium-filled plasma focus (PF) of only tens of joules is presented. The system operates at a very low energy in the tens of joules range (160 nF capacitor bank, 38 nH, 20–35 kV, 32–98 J, ~150 ns current rise time), maintaining the same energy density as in large devices. The typical dip in the current derivative signal and the typical peak in the voltage signal observed in PF devices with energies of 1–1000 kJ, which are associated with pinch compression, were observed in a deuterium-filled PF operating at 50 and 67 J. The time to pinch and time to the peak current versus deuterium filled pressure were also obtained.

Optical observations of the plasma motion in a fast plasma focus operating at 50 J

Plasma images with 5 ns exposure time were obtained in a very small plasma focus. The plasma focus operates at low energy (160 nF capacitor bank, 65 nH, 25 kV, 50 J). A single frame ICCD camera was used to obtain plasma images in the visible range. The umbrella-like 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 velocity of the radial collapse is of the order of 105 m s−1. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher.

System for measurement of low yield neutron pulses from D-D fusion reactions based upon a 3He proportional counter

A conventional neutron detection technique was adapted to measure low neutron yields from D–D fusion pulses. This method uses a 3He proportional counter surrounded by a paraffin moderator. Electric signals generated in the 3He tube are fed into a preamplifier. The output of the preamplifier is directly connected to a digital oscilloscope. The time-integrated signals represent the charge generated in the 3He tube which is proportional to the total neutron yield. The integration time is determined by the preamplifier and moderator characteristics within some hundreds of microseconds. No meaningful neutron background was detected during this time window. The system, previously calibrated, was used to measure the neutron yield (<106 neutron/pulse) generated in a fast and very small plasma focus device designed to operate with energies of tens of Joules. Neutron yields as low as 103 neutrons per pulse were measured.

Characterization of the axial plasma shock in a table top plasma focus after the pinch and its possible application to testing materials for fusion reactors

The characterization of plasma bursts produced after the pinch phase in a plasma focus of hundreds of joules, using pulsed optical refractive techniques, is presented. A pulsed Nd-YAG laser at 532 nm and 8 ns FWHM pulse duration was used to obtain Schlieren images at different times of the plasma dynamics. The energy, interaction time with a target, and power flux of the plasma burst were assessed, providing useful information for the application of plasma focus devices for studying the effects of fusion-relevant pulses on material targets. In particular, it was found that damage factors on targets of the order of 104 (W/cm2)s1/2 can be obtained with a small plasma focus operating at hundred joules.