Experimental set-up

Experiments have been carried out on the Pico2000 laser facility at the LULI laboratory. This installation synchronizes two laser beams as described above, for use in the same vacuum chamber. The long high-energy pulse delivers 400 J in 1.5–4 ns, square pulse at 0.53 μm of laser light. It was focused with an f/8 lens through a random-phase plate producing a focal spot diameter around 100 μm (full width at half maximum) and an average intensity of 5 × 1014 W cm−2. It was used to produce a plasma from a natural boron target (20% of 10B and 80% of 11B) placed at an incidence of 45° from the laser pulse propagation axis. The boron plasma expanded in vacuum producing an electron density profile from zero to solid (~6 × 1023 cm−3). The short-laser pulse delivered 20 J in 1 ps with high contrast at 0.53 μm wavelength. It was tightly focused on target reaching intensities ~6 × 1018 W cm−2 to produce a proton beam by the target normal sheath acceleration (TNSA) mechanism29. Thin foils of aluminium, plastic and plastic covered by a thin layer of gold were irradiated at normal incidence. Details of the set-up are shown in Fig. 1. The two beams were set at a relative angle of 112.5° from each other. The distance between the thin foil and the boron target was 1.5 mm. The time delay between the two beams was adjusted between 0.25 and 1.2 ns so that the proton beam interacted with a plasma state in various conditions of ionization and temperature. Shots were done with either the short pulse only so that the proton beam interacted with solid boron, or with the two laser beams so that the proton beam interacted with boron plasma.

Figure 1: Experimental set-up. Scheme of the experimental set-up showing the laser beam configuration, the target arrangement and the diagnostics (CR39 track detectors and a magnetic spectrometer). The picosecond pulse arrives from the left and generates a proton beam in the first 20-μm Al foil, which has an impact on the boron plasma produced by the nanosecond pulse arriving from the bottom. The second 10-μm Al foil protects the first one from irradiation by the nanosecond beam. Full size image

Diagnostics

Track detectors CR39 (ref. 30) covered by aluminium foils of various thickness between 6 and 80 μm were used to collect impacts by both protons and α-particles. Six detectors were used for each shot with angles 0°, 15°, 35°, 70°, 100° and 170° from the picosecond beam axis (0° is the forward direction). A magnetic spectrometer, with a magnetic field of 0.5 T, was placed along the normal of the boron target to analyse the α-particle spectrum. An aluminium filter with 12 μm thickness was placed in front of the slit of the spectrometer to block low-energy ions: boron below 11 MeV, carbon below 14 MeV, oxygen below 19 MeV and aluminium below 23 MeV. The tracks observed inside the spectrometer are therefore mainly ascribable to protons and α-particles, which have an impact at the same position when having the same entrance point and same energy inside the spectrometer. Taking into account the loss of energy in the aluminium filter at the entrance of the spectrometer, α-particles with energy between 3.3 and 7.5 MeV and protons between 0.9 and 5 MeV could be measured. This is also the method by which we characterized the proton beam spectra in preparation for fusion shots.

Plasma characterization

In the main part of the experiment, the objective was to study the number of p11B reactions between the proton beam accelerated by the picosecond laser and the boron for different prepared target conditions. The expansion of the boron plasma produced by the nanosecond pulse was characterized by time-integrated X ray pinhole images in the range ~3–5 keV. Typically, the overall extension of the boron plasma was around 200 μm after 1 ns. This diagnostic was also very useful for controlling the alignment and superposition of the two beams as shown in Fig. 2 where three plasmas are observed along the direction of propagation of the picosecond beam: the first one comes from the 20-μm Al foil, which is used to produce the proton beam, the second one comes from a second 10-μm Al foil that was inserted to protect the rear part of the first Al foil from the nanosecond-scattered photons by the boron target and the third one is the boron plasma. Without the 10-μm Al shield, the proton beam could not be produced in the two-beam irradiation shots because light scattered from the nanosecond pulse modified the rear surface of the Al foil, cleaning up all the hydrogen-rich impurities31. Finally, an estimate of the electronic temperature of the boron plasma was obtained from the shift of the time-resolved-stimulated Brillouin backscattering (SBS) spectra of the nanosecond pulse32. They were recorded with a high-dispersion spectrometer and a streak camera. A typical example of such time-resolved SBS spectrum is shown in Fig. 3 in the case of a 4-ns pulse irradiating the boron target. The spectral shift of the SBS light was analysed using the ion-acoustic velocity formula, providing an electron temperature of T e ~0.7±0.15 keV.

Figure 2: Observation of the multiple plasmas. Time-integrated X ray pinhole image of the three plasmas along the direction of propagation of the picosecond beam. From left to right, we observe the heated parts of the first Al foil that produces the proton beam, the second Al foil that protects the first one and the boron plasma. Full size image

Figure 3: Stimulated Brillouin scattering spectrum. Time-resolved spectrum of stimulated SBS of the nanosecond pulse from the boron plasma. The light is collected in the focusing optics of the nanosecond beam in the backward direction. The laser pulse is at 0.53 μm with an intensity of 5 × 1014 W cm−2. Full size image

Experimental results

The total number of tracks observed in the magnetic spectrometer, per unit of surface on the CR39, as a function of the α-particle energy is shown in Fig. 4 for various shot conditions. No particle can be observed in the hatched part because of the aluminium filter in front of the spectrometer. Shots with no boron target behind the Al foil (yellow triangles) display almost no track, which means that very few protons are accelerated at an angle of 100° from the pico beam axis as expected from the TNSA process. In the case of shots with the picosecond beam alone, in which the proton beam interacts with a solid boron (blue diamonds), the number of tracks is close to the noise level, indicating very weak activity. Shots with the two beams, in which the proton beam interacted with boron plasma, demonstrate a large increase of the number of tracks by a factor of more than a hundred, in the highest case compared with the previous ones. Three time delays between the two beams have been tried: 0.25 ns (open circles), 1 ns (green triangles) and 1.2 ns (blue squares) showing that the highest number of tracks was obtained for the longest time delay, which corresponds to the highest temperature and ionization state of the boron plasma.

Figure 4: α-particle spectra. The total number of tracks observed in the magnetic spectrometer (with an entrance slit of 1 mm2) per unit of surface of CR39 as a function of the α-particle energy for six shot configurations: yellow triangles, shot with no boron; blue diamonds, interaction of the proton beam with solid boron; blue square and green triangles, interaction of the proton beam with plasma boron and time delay between the two beams of 1 and 1.2 ns, respectively; red circles, ibid+the proton beam is produced in a foam rather than aluminium foil; open circles, short delay (0.25 ns) between the nano and the pico pulses. The error bars in energy are given by the width of the CR39 on which the number of impacts has been counted; the error bars in the number of impacts are given by the shot to shot fluctuations (~±10). The low-energy domain has no counts as the entrance slit is protected by a 12-μm Al foil. Full size image

These results were complemented by the analysis of the CR39 detectors, which were positioned outside the spectrometer, close to the entrance slit. Tracks were observed only behind aluminium filters of thickness smaller than 24 μm. If scattering of the proton beam by the boron plasma had sent protons in the spectrometer, tracks would have been recorded for all the aluminium thicknesses as the proton beam includes a continuous spectrum of energy up to ~10 MeV (see Methods section), which can cross a thickness of aluminium larger than 80 μm. This is not the case for the produced α-particles. Given our proton spectrum, the kinetic energy range of α-particles produced in p11B reactions12 is 0.5–8 MeV. Considering the exponential decrease of proton yield with energy, there is little, if any, production of α-particles with energy larger than 7.1 MeV required to cross 36 μm or more of aluminium. α-particles with typical fusion energy between 3.3 and 5.4 MeV can cross 12 and 24 μm of aluminium as observed in the p11B shots. The absence of high-energy proton signature in control detectors placed near to the spectrometer is our evidence that scattered protons are not producing the track signature inside the spectrometer.

A rough estimate of the fusion rate can be obtained from the number of tracks in the spectrometer and the solid angle of observation (δΩ=1.1 × 10−5 sr). The highest fusion rate measured in this scheme was 9 × 106 sr−1, which is orders of magnitude higher than previous observations20. However, as pointed out in ref. 21, the choice of the detection energy region of the reaction products can underestimate the total yield as α-particles with energy lower than 3.3 MeV are not taken into account. In our experimental conditions, the low-energy α-particles are not expected to escape the plasma, and further those having relatively small energy when escaping may not be observed leading to an underestimate of the absolute fusion yield21. External α-particle detectors can only observe fusion products emitted within the plasma in a backward hemisphere at an energy allowing escape from the plasma. This means that α-particles propagating in the forward direction, into the thick solid target, cannot be observed directly. These are accounted for by consideration of the solid angle of observation of the spectrometer. Test shots were performed with either the nano or the pico pulse alone on the boron target to measure the possible reactions in the hot plasma, and the number of tracks in both cases was below 10. This demonstrates that the observed high number of particles in the two-beam experiments is definitely the consequence of the interaction of the proton beam with the boron plasma.