Pulsed sources of fusion neutrons and x-rays, as well as energetic electron and ion beams can be produced by means of plasma focus devices. After its invention in the 60's 12 they were intensively studied as nuclear fusion devices and were described in detail, among other references, in 345. During the last decade, they began to be investigated as convenient sources for several technological applications, to be cited below, of the mentioned radiations.

Essentially these devices are plasma accelerator guns, where the plasma is generated in a low pressure (1–10 mbar) atmosphere by a pulsed powerful capacitive discharge between a pair of coaxial cylindrical electrodes. The Lorentz force drives a plasma sheath along the electrodes, which radially collapses at the symmetry axis forming a hot dense plasma pinch of about 100 ns. Intense pulses of electromagnetic radiation as well as ions and electrons beams are emitted as a result of the focusing process. The electromagnetic emission from the collapsing plasma has a very high brightness and exhibits a broadband spectra ranging from visible light to soft x-rays. Fusion reactions can be obtained if deuterium or deuterium-tritium mixtures are used, with the consequent emission of fast neutrons. The kinetic energy of the emitted neutrons being 2.45 MeV in the first case and 2.45 MeV or 14.1 MeV in the second one. Both emissions are almost monochromatic 345.

Since the neutron and also the hard x-ray burst duration of this source is in the 10–100 ns range, and that its emission can be turned off, the plasma focus becomes an interesting alternative to commercially available radioisotopic sources of both neutrons and hard x-rays.

Due to the high effective cross section of hydrogen for neutron dispersion, plasma focus neutron pulses can be used as radiation probes to detect hydrogenated substances by means of neutron scattering. Examples of potential applications of this radiation are soil humidity studies 6 and detection of hidden dangerous or illegal substances including drugs, weapons and plastic explosives 7. The plasma focus neutron emission is also suitable for dark matter research 8, radioisotopes production 9 and neutron therapy 10.

Other reported applications are radiographies of biological samples 111213, small pieces made of different materials 14, and a rotating car wheel 15. Introspective images of small metallic pieces 1617 even through several millimeter thick metallic walls 18 were also produced. Additionally, a radiography of an aluminum turbine rotating at 6120 rpm using a plasma focus pulse was reported 19. The tomographic reconstruction of both surface and volume of small metallic objects was investigated as another non-conventional imaging application of the hard x-ray emission of plasma focus devices 1620. The spatial resolution of the digitalized images has demonstrated to be suitable for 3-dimensional tomographic reconstruction of an object with just 8 projections.

Other capabilities of the plasma focus emissions include lithography with ten nanometers resolution 2122, medical radiation therapy applications 23, industrial non-destructive testing 2425, as well as, thin film deposition 26.

Compact and portable plasma focus are being developed to work efficiently on the field at relative low costs when compared to nuclear reactors or linear accelerator based sources. Moreover, the feasibility of combining neutron and x-ray scannings simultaneously in a single device is a unique advantage of plasma foci.

However, plasma foci still presents several challenges that must be overcome in order to extend the uses of these devices as x-ray and neutron source for commercial and industrial applications. In order to be useful over the widest range of applications, a plasma focus device should be able to operate at a discrete repetition rate 27 in order to produce average neutron emission rates about (10⁷ – 10¹⁰) neutrons/sec, and intense penetrating x-rays beams. Lee et al presented a 16 Hz plasma focus operating in neon 21. More recently, Rapezzi et al developed a mobile repetitive device for industrial applications 28.

In the current paper a repetitive plasma focus device operating with deuterium aimed to a pulsed source of neutrons and hard x-rays is presented. The repetition rate is controlled by means of a variable clock synchronized with a triggering system and a power supply, which ensures a regular operation up to 0.2 Hz. Moreover, the feasibility of technological application are analyzed.

A 4.7 kJ small chamber Mather-type plasma focus was used as repetitive radiation source. The gas chamber was filled with 4.0 mbar of an admixture of 2.5% (in volume) of argon in deuterium. The cylindrical chamber, 1 dm³, is made of a 3-mm thick stainless steel tube. The 2-mm thick front of the chamber is a stainless steel disk, 100 mm OD, used as the hard x-ray emission window. The electrodes are concentric cylinders, 85 mm length, 38 and 73 mm OD respectively, made of hollow electrolytic copper and twelve 3-mm diameter brass rods, respectively. The anode base is made of brass. A 50-mm OD cylindrical Pyrex insulator, 4 mm thick, 30 mm length, is located covering the anode at the base. The chamber design is optimized for hard x-ray and fast neutron production 29. The footprint of the whole device is 0.60 m², its height being 1.3 m.

The capacitor bank is composed by 3 modules of six 0.7 μF Maxwell capacitors. The bank was charged using a 10 kW Maxwell CCDS power supply. The modules were connected in parallel to the discharge chamber through 3 Maxwell spark gaps (model 40264), which were triggered simultaneously by means of a car ignition coil.

The device was able to operate regularly at 30 kV at a repetition rate up to 0.2 Hz during runs of 2 minutes maintaining the temperature within reasonable ranges. The external temperature of the anode reached 30°C above room temperature after each run. The continuous compressed air flow needed to set the spark-gap operating voltage and cleaning, was sufficient to cool this element. The external surface of the spark-gap plastic encasing body heated up only few degrees over the room temperature, whereas the chamber refrigerated by natural air convection with the environment.

A microprocessor-based control system synchronized both the power supply and the triggering system (Fig. 1). The peak current delivered was in every shot ~360 kA. Higher frequencies are in principle possible to achieve introducing a cooling system of the chamber and the electrodes. The higher frequency achieved during the tests was 1 Hz during 20 seconds.

The discharge current was monitored by a non-integrating Rogowski coil. The x-ray emission was detected with a NE102A scintillator optically coupled to a photomultiplier (PMT) polarized at 800 V and placed 3.9 m away from the chamber. The Rogowski coil and the PMT signals were acquired with a Tektronix TDS540A oscilloscope. Both the PMT and the oscilloscope are placed inside a Faraday cage.

Figure 2 shows the Rogowski coil and PMT signals acquired during a 24 shot sequence at 0.2 Hz. It can be observed that both the amplitude and the first quarter of period of the Rogowski coil signal, are repetitive from shot to shot within ± 50 kA/μs and ± 50 ns, respectively. The pinch time occurs at (1.48 ± 0.12) μs, close to the peak current time, which is (1.27 ± 0.05) μs. After integrating the Rogowski coil signal, it results that the current at pinch is always above 90% of the maximum current value.

As it usually happen in plasma focus discharges focusing not always occur in every shot, specially when working in rep-rate mode without replacing the working gas. In the case shown in figure 2, 4 shots out of 24 failed to focus, whereas very intense current dips were attained in 16 of the others. The remaining 4 focalizations were not particularly intense. The PMT signals show that x-ray pulses are produced around 35 ns after the pinch, lasting each about 50 ns FWHM.

The neutron yield was determined by silver activation 30. The detector consists of a 30 × 30 × 15 cm³ rectangular box filled with paraffin (see figure 3), in which four Victoreen 1B85 Geigers, each surrounded by a 300 μm thick silver foil and polarized to 800 V, are allocated. The Geigers are placed, respectively, inside four holes (2 cm in diameter by 9 cm in depth) drilled on the diagonals of one of the 30 × 30 cm² faces, at 9 cm from the face center. The amount of paraffin allows for the necessary neutron thermalization to match the silver resonance peak for neutron capture, with a beta decay, located at 5 eV. The background counts of this detector are 66 ± 8 in a time interval of 20 sec.

Figure 4 shows the measured neutron yield per shot recorded during four independent runs lasting 3 to 5 minutes of 0.2 Hz operation. The average neutron yield is about 10⁸ neutrons per shot during the first 20 shots. After that the yield gradually decreases until it reaches half that number around the shot 60. This may be in part due to the build up of impurities in the filling gas along the run, which slows down the current sheath, resulting in a pinch time delay and a corresponding decrease of the pinch current, or, due to a degradation of the focusing efficiency caused by such impurities. Each run was done with the same gas filling, but it was fully refreshed before starting the run.

Both neutron and x-ray emissions of the plasma focus device were tested for different technological applications with a potential industrial or therapeutical use. The following sections describe the obtained results.

A repetitive plasma focus device capable of emitting fusion neutrons and hard x-rays was presented, showing that it can be operated at a moderate repetition rate for several minutes. The performance and thermal conditions of the device is stable during 2 to 3 minute runs. Higher shot frequencies can in principle be achieved if some thermal management is provided to cool the heat dissipated in every shot.

It was demonstrated that the emitted neutrons can be used to detect the presence of water near the discharge chamber by neutron scattering. In principle, this procedure can be extended to detect other hydrogenated substances such as explosives or drugs.

Radiographic images could be obtained from the hard x-ray emissions showing submillimetric spatial resolutions with expositions times around 50 ns. The energy and intensity of the x-rays are sufficiently high for the inspection of metallic objects located behind or inside several millimeters of iron or steel. These characteristics are suitable to develop non-intrusive detection systems of internal defects and the imaging of fast rotating components.