Plasma focus based repetitive source of fusion neutrons and hard x-rays
© Raspa et al. 2008
Received: 30 June 2007
Accepted: 29 October 2008
Published: 29 October 2008
A plasma focus device capable of operating at 0.2 pulses per second during several minutes is used as a source of hard x-rays and fast neutrons. An experimental demonstration of the use of the neutrons emissions for radiation probing of hydrogenated substances is presented, showing a particular application in detecting water concentrations differences in the proximity of the device by elastic scattering. Moreover, the device produces ultrashort hard x-rays pulses useful for introspective images of small objects, static or in fast motion, suitable for the identification of internal submillimetric defects. Clear images of metallic objects shielded by several millimeters iron walls are shown.
PACS Codes: 29.25.Dz,52.59.Px
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 [1, 2] they were intensively studied as nuclear fusion devices and were described in detail, among other references, in [3–5]. 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 [3–5].
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  and detection of hidden dangerous or illegal substances including drugs, weapons and plastic explosives . The plasma focus neutron emission is also suitable for dark matter research , radioisotopes production  and neutron therapy .
Other reported applications are radiographies of biological samples [11–13], small pieces made of different materials , and a rotating car wheel . Introspective images of small metallic pieces [16, 17] even through several millimeter thick metallic walls  were also produced. Additionally, a radiography of an aluminum turbine rotating at 6120 rpm using a plasma focus pulse was reported . 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 [16, 20]. 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 [21, 22], medical radiation therapy applications , industrial non-destructive testing [24, 25], as well as, thin film deposition .
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  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 . More recently, Rapezzi et al developed a mobile repetitive device for industrial applications .
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.
Device design and output characteristics
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 . 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.
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.
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.
Applications: Methods and Results
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.
Prospection by fast neutrons scattering
Hard x-rays introspective imaging of metallic pieces
Plasma focus operated in single shot mode has demonstrated to be a hard x-ray source suitable to obtain introspective images of small metallic objects.
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.
This research was supported by PLADEMA – CNEA, and Universidad de Buenos Aires. VR, FDL, PK and AT are doctoral fellows of CONICET. CM and AC are members of CONICET.
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