Researchers report on D-D fusion neutron emission in a plasma device with an energy input of only 0.1 Joules, within a range where fusion events have been considered very improbable. The results presented here are the consequence of scaling rules we have derived, thus being the key point to assure the same energy density plasma in smaller devices than in large machines. The Nanofocus (NF)—our device—was designed and constructed at the P4 Lab of the Chilean Nuclear Energy Commission. Two sets of independent measurements, with different instrumentation, were made at two laboratories, in Chile and Argentina. The neutron events observed are 20σ greater than the background. The NF plasma is produced from a pulsed electrical discharge using a submillimetric anode, in a deuterium atmosphere, showing empirically that it is, in fact, possible to heat and compress the plasma. The strong evidence presented here stretches the limits beyond what was expected. A thorough understanding of this could possibly tell us where the theoretical limits actually lie, beyond conjectures. Notwithstanding, a window is thus open for low cost endeavours for basic fusion research. In addition, the development of small, portable, safe nonradioactive neutron sources becomes a feasible issue.
The energy input, to drive a PF, typically ranges from kilojoules to megajoules. Most of the experimental studies have been focused on facilities that use tens to hundreds of kilojoules. By observing that some scaling laws hold for the PF plasma, some years ago, we considered the possibility of developing lower energy input devices. The key point was to assure the same energy density for the pinch. The pinch radius and length are proportional to the anode radius a, and its volume, proportional to a3. We observed that the ion density in the pinch n and the ratio E/a3 (E is the stored energy in the capacitor bank) are approximately invariant for devices from 1 kJ to 1 MJ. Because the pinch temperature is essentially given by the energy per ion and is therefore proportional to E/(a3n), this invariance suggests that most nuclear and atomic reactions occurring in large plasma foci should also be expected in a miniaturized pinch, given the proper scaled design. We concluded that it was possible to scale plasma foci in a wide range of energies and sizes, and keeping the same value for ion density, magnetic field, plasma sheath velocity, Alfvén speed, and temperature. Notwithstanding, plasma stability will depend on the size and energy of the device. Following the line of reasoning outlined, we were able to build fully operational PF devices with energy inputs of tens of joules. In 400 J and 50 J PF devices with deuterium, neutrons were produced and accurately measured.15,16 By determining the neutron energy by time-of-flight techniques, thus resulting in 2.51 ± 1.0 MeV for the PF-400J and 2.71 ± 1.8 MeV for the PF-50J, we confirmed that the neutrons had a D-D fusion reaction origin. At present, other laboratories are doing research using PF devices in the range of tens to hundreds of joules.
(a) A sketch of the NF discharge device. The driven capacitor (5 nF) is composed of two parallel plates (lower plate: anode; upper plate: cathode). A 0.42 mm diameter copper cylinder is covered with quartz, attached to the centre of the anode plate, and passes through a small hole in the cathode centre. Plasma is formed between the top of the anode and the cathode base. (b) The NF chamber (pointed in the photograph). (c) A time-integrated photograph of the discharge. Note the bright spot on the anode top.
The Nanofocus (NF) was constructed following the scaling rules that we have explained earlier. It consists of a pair of brass electrodes of 200 mm diameter, separated by four 80-m dielectric polyvinylidenefluoride films, the whole acting as a 4.9 nF capacitor for driving the discharge . A copper cylinder of 0.42 mm diameter, covered with a quartz tube, is attached to the centre of the anode plate and passes through a small hole in the cathode centre. The anode is enclosed within a small vacuum chamber filled with gas at low pressure—deuterium for neutron emission.
The overall device dimensions are ∼20 cm × 20 cm × 5 cm.
They have enough evidence that the Nanofocus device produces and compresses the transient plasma in a way similar to Z-pinches and other plasma focus devices. Moreover, in contrast to higher energy PF devices, the NF could present enhanced stability due to resistive effects. In the present article, they report evidence of neutron emission from this extremely small PF device.
The empirical scaling laws established from larger plasma focus devices permit them to predict the neutron yield, ranging from 10 million to a trillion neutrons for devices with energy ranges from kJ to MJ. By using neutron emission data from various devices in a range of energies from 1 kJ to 1 MJ, and currents from 100 kA to 1 MA, the total neutron yield Y becomes proportional to some power of the peak current. They estimated a yield of about 200 neutrons per pulse for discharges in deuterium when using a device with a current of 5 kA. This amount of neutrons is below the detectable level for usual activation-based detectors.
They can detect neutron yields lower than one thousand neutrons per shot.
According to the detectors calibration, the total neutron yield is estimated to be 100 ± 40 neutrons.
These results, and those obtained for a 50 J PF, permit us to anticipate that a device working at a few joules, with one thousand to ten thousand shots at a frequency of 10 to 100 Hz, will produce a neutron yield of ten thousand to one million events per second, although a more accurate determination of the absolute neutron yield could be desirable for this extrapolation. A deuterium-tritium mixture would boost this by up to 100 times. For applications, some technological issues remain to be tackled, particularly to improve reproducibility of neutron emission for periods longer than minutes when the NF operates at hundreds of Hz. Scientific questions raised by the results presented here are pertinent. The scale rules they applied here have proven to be a powerful tool for expanding the boundaries beyond what was expected. But, is there a lower bound on the size of a PF to generate the conditions for nuclear fusion? In much smaller devices, the surface/volume ratio seems to be more favorable for plasma heating and compression. Could this actually increase plasma energy density and improve the output in fusion reactions and radiation? How stable will the plasma be? These are some of the questions that will guide the future work in this line of research.