3.1. Study of the $^{11}B(p,\alpha)2\alpha$ fusion reaction channel in plasma

The study and optimization of the $^{11}B(p,\alpha)2\alpha$ reaction in plasma will be conducted through experimental campaigns that will be performed using the high-energy, long-pulse PALS (Prague Asterix Laser System) laser, whose characteristics are summarized in Table 1.

Table 1. PALS laser specifications (Ref [23])

These experiments will focus on studying the $^{11}B(p,\alpha)2\alpha$ reaction in two configurations: ‘in-plasma’ and ‘pitcher-catcher’, within the same laser shot, utilizing the developed targets. The ‘in-plasma’ configuration refers to scenarios where fusion reactions occur directly within the target irradiated by the main laser pulse. In this setup, protons and ¹¹B isotopes are accelerated inside the energetic and expanding plasma generated by the laser–target interaction. In contrast, the ‘pitcher-catcher’ configuration involves fusion reactions induced by protons exiting the primary target and striking a secondary boron target positioned along the normal axis of the primary target.

This approach allows for a direct comparison of fusion reaction rates under different conditions within the same laser shot. Specifically, it enables quantification of the difference between fusion reactions occurring in the plasma and those in a standard ‘pitcher-catcher’ scenario, where protons interact with a cold, solid boron target. The experimental set-up is depicted in Figure 3, which shows a CAD rendering of the interior section of the PALS interaction chamber. The laser pulse (illustrated as a green cone) will be focused onto a thick solid target (yellow cylinder) using a focusing lens. This target, inclined at a 25^∘ angle relative to the laser main axis, is composed of hydrogen and ¹¹B. Upon interaction with the laser, the solid target will transit into a plasma state, potentially initiating the $^{11}B(p,\alpha)2\alpha$ fusion reaction in the ‘in-plasma’ configuration. A portion of the protons accelerated during the laser–target interaction will propagate along the target normal axis and strike a secondary target, which is mounted on a dedicated holder. The secondary target consists of a natural boron slab, measuring $2 \times 2$ cm and 2 mm in thickness. If protons have sufficient kinetic energy, they will interact with the boron, triggering the $^{11}B(p,\alpha)2\alpha$ reaction and producing alpha particles, as part of the ‘pitcher-catcher’ configuration. The alpha particles generated will be detected using a CR39 track detector housed in a dedicated holder (highlighted in blue in Figure 3). To analyse the energy spectra of the emitted alpha particles, aluminium filters of varying thicknesses (ranging from 0 to 35 µm) will be placed in front of each detection aperture in the CR39 detector. In the experiments, time-of-flight diagnostic systems will also be employed, utilizing diamond and silicon carbide detectors positioned at various angles with respect to the normal of the primary target. The set-up will include three individual diamond detectors, one silicon carbide detector and a matrix of six diamond detectors. The effective detection areas of the detectors range from 1 to 6.5 mm². In particular, the diamond matrix is composed of six detectors, each with an active area of 6.3 mm², resulting in a total effective detection area of 37.8 mm². Filters with a thickness of 6 $\mu m$ will be placed in front of the ToF detectors, while a set of filters ranging from 3 to 25 $\mu m$ will be placed in front of the matrix detectors. The use of aluminium absorber serves to block low-energy ions and to separate the contribution of alpha particles. Additionally, for the detection of alpha particles in a mixed-signal regime, an ‘a posteriori’ analysis based on signal deconvolution will be employed to accurately isolate the different contributions. This set-up was previously used during an earlier experimental campaign, whose analysis is still ongoing, and it will be adopted in upcoming experiments.

Figure 3. Layout of the experimental set-up that will be used for the study of the $^{11}B(p,\alpha)2\alpha$ reaction in a laser-generated plasma.

A detailed view of the target system, including the primary target, the secondary target and the CR39 detector holder, is shown in Figure 4.

Figure 4. Rendering of the target system designed for the purposes of the FUSION experiments at the PALS facility.

3.2. Study of the protons and alphas stopping power in a borated plasma

In FUSION, a set of measures will be dedicated to estimating the proton and alpha-stopping power in a borated plasma. In this case, a plasma will be generated in a vacuum using a 10 mJ, 6 ns laser pulse interacting with a solid target. After the initial expansion, the plasma plume will be characterized while, simultaneously, a synchronized proton or alpha microbeam from a conventional linear accelerator will cross it at the intersection point with the interferometer axis (Figure 5).

Figure 5. Experimental set-up for the proton and alpha stopping power measurements in a borated plasma. A 6 ns laser pulse generates a plasma plume that is crossed by a sub-nanosecond bunched ions beam. Energy loss in the plasma will be measured in single particle mode by silicon-carbide detectors.

The energy loss of the traversing ions will be measured with a single event modality as a function of the plasma characteristics (temperature, density, etc.) by Time-of-Flight measurements and using Silicon Carbide detector as particles energy detectors. Alpha and proton beams will be provided by the ‘Singletron’ accelerators available at the Physics Department in Catania, Italy. A pulsed beam (bunch FWHM < 1 ns) will be obtained thanks to a chopper-bunching system funded, designed and realized within the FUSION project.

The determination of stopping power for ions within plasma entails a meticulous examination of single-particle interactions within a well-defined plasma state. To achieve this, the development of a chopper-bunching system is imperative, enabling the controlled delivery of one or a few particles at a time with a temporal resolution of approximately 1 ns. This system will rely on a rapid electrostatic deflection mechanisms to manipulate particle beams crossing a pinhole. Initially, the continuous beam emitted by the Singletron undergoes bending via a switching magnet, followed by collimation to better define the beam emittance. Subsequently, the beam is deflected through a pair of electrodes and ultimately selected by a small collimator. The mechanical structure of the chopper has been designed (left side of Figure 6) and the whole system has then been simulated to evaluate both beam dynamics and time resolution. The mechanical sections of the bunching system have been manufactured, delivered and finally assembled as shown in Figure 6 (right side). The system is now ready to be installed along the Singletron experimental beamline of the Physics Department of Catania University (Italy) and the preliminary measurements are planned in 2025.

Figure 6. Left picture: mechanical design of the chopper-bunching system as defined during the first year of the FUSION project. Right: bunching system vacuum chamber delivered, assembled and ready to be installed in the experimental area.

4.1. The diamond matrix

A TOF detector matrix, composed of six diamond-based detectors in a sandwich configuration (metal/intrinsic diamond/metal), was developed and fabricated at the University of Rome ‘Tor Vergata’. The detectors feature a high-purity intrinsic diamond layer grown homoepitaxially on a single-crystal diamond substrate using microwave-enhanced chemical vapour deposition. The intrinsic CVD diamond layer was precisely laser-cut from the substrate, achieving a thickness of 35 $\mu m$. Such thickness was selected to optimize the temporal resolution and ensure the complete absorption of protons up to 2.5 MeV and alpha particles up to 10 MeV. The detectors were assembled on a PCB board with an integrated bias-tee (R = 1 MΩ, C = 1 µF) and housed within an aluminium casing mounted on a KF-100 vacuum flange to shield against electromagnetic pulse (EMP) noise (see Figure 7-left). Six aluminium filters of varying thicknesses, i.e. 3, 6, 15, 18, 20 and 25 µm, were positioned in front of the detectors. The differing stopping powers of Al filters allow for enhanced analysis of particle energy spectra by distinguishing particles based on their mass and energy (Ref [Reference Verona24]). Additionally, the set-up includes two slots for CR-39 solid-state detectors, mounted on the inner side of the flange with identical Al filters, enabling comparative measurements. The TOF diamond matrix was successfully used to study and characterize the $^{11}B(p,\alpha)2\alpha$ reaction in plasma during the first experimental campaign at the PALS facility (see 3.1). The device was positioned at 53^∘ relative to the target normal direction and placed 107 cm away from the target. The outputs were connected to a fast oscilloscope via six double-shielded RG223 cables. Figure 7-right shows a typical example of ToF signals captured by the TOF diamond matrix during the laser-target interaction. The figure clearly displays a distinct, narrow photopeak, which marks the starting point for the ToF measurement. Following this, a broader signal is visible, representing protons and ions with varying energies as they reach the detector. Signals from the detectors with different filters exhibit a noticeable reduction in both amplitude and time, demonstrating the system’s effectiveness in differentiating and analysing the reaction products.

Figure 7. Left: aluminium shielding flange with CR39 and Diamond detectors inserted. Right: typical ToF signals from Diamond detector array with different aluminium filters applied.

4.2. Thompson spectrometer

Within the FUSION project, we developed a diagnostic device, dedicated to the detection of alpha particles, that is based on Thomson spectrometry (i.e. a well-established method for revealing and differentiating accelerated protons and heavy ions in laser-plasma experiments). This device is capable of retrieving information on the proton spectrum and the spectrum of heavy ions, discriminating particles according to their mass-to-charge ratio A/Z. The alpha particles, generated by the $^{11}B(p,\alpha)2\alpha$ fusion reaction, will be detected on the A/Z=2 parabolic curve. However, the alpha particles must be differentiated from the other heavy ions with A/Z=2 generated in the laser-plasma interaction, especially C⁶⁺, which are a common product when intense laser pulses interact with solid targets. This requires a high sensitivity of the device and/or additional discrimination techniques. Since one of the major challenges of alpha particle detection in $^{11}B(p,\alpha)2\alpha$ experiments is represented by the low particle flux (Ref [Reference Consoli17]), this device has been designed, aiming at a high sensitivity and at the possibility to be implemented in proximity to the laser-plasma interaction point (Ref [Reference Scisciò18]). Taking inspiration from the Thomson Spectrometer (TS) prototypes developed by ENEA in the last years (Ref [Reference Consoli25]), it features ad-hoc shielding case for the strong EMP generated inside the interaction chamber and the ionizing radiation, which interfere with the magnetic and electric field of the device and are typically responsible for the background noise on the device detector, respectively. The schematic of the device is reported in Figure 8A. The parameters are conceived for allowing proton detection at an energy from hundreds of keV, up to several MeVs for protons and similarly for alpha particles produced by $^{11}B(p,\alpha)2\alpha$, and have been optimized by numerical simulations (electromagnetic simulations and particle tracking). The deflecting dipole features a magnetic field of ≈ 400 mT and an electric field of >1 MV/m. The dipole geometry has been dimensioned for avoiding electrical breakdown even at multiple kV voltage. The simulated deflection of ions with A/Z=1 and A/Z=2 (see Figure 8A) indicates enough energy resolution for allowing the implementation of differential filters on a spatial scale of a few millimeters (Ref [Reference Salvadori26]), in the energy range that is typical for the alpha particles produced by $^{11}B(p,\alpha)2\alpha$ fusion reactions. The position of the detector inside the device will be adaptable by using an extension of the drift space after the deflecting dipole, allowing a further enhancement of energy resolution and magnification of the obtained parabolic traces (see Figure 8C). Moreover, the detector plane will be equipped with special frames for implementing single or multiple CR39-arrays (shown in Figure 8B), allowing a more detailed discrimination of the fusion reaction products. The device is currently at the assembly phase and will be tested during an experimental session planned for 2025.

Figure 8. A. Schematic design of the alpha particle detection device. B. Improved design of the detector plane, allowing the use of a CR39 array. C. Parabolic traces of the deflected ions, computed by a particle tracking simulation, at the end of the drift space after the deflecting dipole (i.e. at the position of the particle detector of the device).

5.1. Borated pills from University of Catania

5.1.1. Targets chemical formulation

The targets consist of disks 13 mm in diameter, with thickness of 3 mm. Targets of reference (only resin) and resin+boron, were prepared. The resin+boron targets were obtained by using a mixture powder (2:1 wt. ratio) of: i) solid acrylic resin (Henry Schein Krugg, Italy, code 413004937) and ii) elemental amorphous boron (Sigma Aldrich Merck, Italy code 15580, with a purity ≥95 $\%$). The grain size of the elemental boron used was less than 1 µm, while that of the acrylic resin was a few tens of microns. Each target pill was prepared using about 0.420 g of the resin powder (for the reference sample) or of the mixture powder (B50 target sample). The powders were placed into a pellet die of 13 mm in diameter and added with 250 µL of liquid resin (Henry Schein Krugg, Italy, code 413004938), obtaining pastes. Furthermore, the pastes were pressed at 0.5 GPa for 10 minutes at room temperature. During this step the polymerization of the resin occurs. Figure 9 shows the pictures of the target pills of reference and B50 samples, respectively.

Figure 9. Pictures of the target pills (13 mm in diameter and 3 mm thin). A. Reference sample (only resin) and B. B50 sample (resin + elemental boron, at 2:1 wt. ratio).

5.1.2. Characterization techniques

The scanning electron microscopy (SEM) was used for the characterization of the pill surface morphology. The SEM pictures were obtained by a SEM Gemini Field Emission SEM Carl Zeiss SUPRATM 25 (FEG-SEM, Carl Zeiss Microscopy GmbH, Jena, Germany) set to the in-lens mode. An acceleration voltage of 5 keV and a working distance of 7 mm were used. Raman characterizations were obtained by a Horiba Scientific instrument model 1024 × 256-OE, equipped with a He–Ne laser model HNL225R, by THORLABS Inc. (43 Sparta Ave, Newton, New Jersey, U.S.A.), operating at a wavelength of 632.8 nm.

The surface of the reference target, visible from optical micrograph of Figure 10A and SEM of Figure 10C, shows the presence of insulating grains, bound together, which have dimensions of several micrometers. This morphology reflects the grain size of the solid acrylic resin which is, in fact, tens of micrometers large. The optical and SEM micrograph of the surface of the B50 target (Figure 10B and D) show the boron particles with an average size of less than 1 µm, mixed with the resin particles, which have much larger dimensions. This latter data are visible in the optical microscopy of Figure 10B.

Figure 10. Low-resolution optical micrographs of: A. reference and B. resin/boron 2:1 wt. target; C. and D. corresponding high-resolution SEM micrographs.

Figure 11A shows the Raman spectra of boron alone (red line) and resin alone (black line). The spectrum of boron shows the peaks centred at 1067, 915, 801, 656, 431, 387 and 317 (cm⁻¹) that can be attributed to a mixture of amorphous boron and α-B and γ-B phases (Refs [Reference Iannin27, Reference Bykova, Parakhonskiy and Dubrovinskaia28]). The spectrum of the resin shows the peaks assigned to -CH₃ stretching (2954 cm⁻¹), C=O stretching (1728 cm⁻¹), –CH₃ symmetric bending (1462 cm⁻¹), C–C stretching of the main chain (970 cm⁻¹), C–C asymmetric stretching (811 cm⁻¹), O–C=O deformation of the ester group (598 cm⁻¹), rotation of the ester group (490 cm⁻¹), skeletal deformation (373 cm⁻¹), all characteristic of the polyacrylate molecular structure (Refs [Reference Socrates29–Reference Nagasawa, Koda and Nomura31]). Furthermore, given the morphology of the B50 target, to study the composition in different regions, we performed Raman spectra in this target over a resin grain (point 1 in Figure 11B) and in the junction region of three grains (point 2 in Figure 11B). The spectrum of the B50 target are acquired at point 1 of Figure 11B (blue) shows boron signals, while the resin signals are strongly attenuated. The spectrum acquired at points 2 of Figure 11B (magenta) shows boron signals, while the resin signals are almost absent. According to these data, the B50 targets appear to be composed of resin grains covered with boron, with a higher content of boron in between the various resin grains.

Figure 11. A. Raman spectra of boron (red line), resin (black line) and B50 target acquired at points 1 and 2 in the picture B (blue and magenta). The Si peak is due to the substrate used for the analysis of boron powder; B. corresponding optical micrograph of the points 1 and 2.

5.2. Foam coupled boarated targets (ENEA and FBK Institute)

The ENEA and the Bruno Kessler Foundation (FBK) Institutes designed and realized a target by depositing a foam layer on top of an Si–H–B substrate. The foam layer acts as a suitable hydrogen source to increase the contribution of available protons for the $^{11}B(p,\alpha)2\alpha$ reaction.

These targets have a surface of about $1\,\mathrm{cm}\,\times\,1\,\mathrm{cm}$. The deposited foam layer thickness is around 100 µm and its density ranges from 15 mg/cm³ to 1 g/cm³. Figure 12 shows a sketch of these targets with the foam deposited on top and confined by washer support.

Figure 12. Sketch of the foam target.

5.3. Borated nanoparticles targets (ELI-beamlines)

Hexane nanoparticles (NPs) were synthesized using a Gas Aggregation Cluster Source (GAS) with an RF generator and plasma power settings of 30 and 100 W. At 100 W, NPs with a diameter of 130 nm were obtained, while at 30 W, the diameter increased to 650 nm. Inside the GAS, there was a mixture of argon gas and Hexane in the pressure ratio Ar/Hx=15.7. These NPs were then deposited onto a boron nitride (BN) substrate, resulting in a cumulative NP thickness of approximately 10 µm after 30 minutes of deposition. The unique deposition and production method of these NPs resulted in a mesoporous structure with a density of approximately 0.1 g/cm³. The BN substrate itself has a thickness of 1.5 mm. The hydrogen-rich Hexane NPs are used to accelerate protons from the surface while the BN substrate is used as a source of ¹¹B to enhance the $^{11}B(p,\alpha)2\alpha$ fusion events. These targets are ready and were tested during the first FUSION experimental campaign.

5.4. Targets from University of Salento and IMM-CNR of Lecce

During 2023 the University of Salento and IMM-CNR of Lecce were engaged on the design and the realization of PMMA fibres bundles, characterized by good uniformity and low degree of defects: especially pores (which can lead to missing active material) and beads (which can induce non-uniform density distribution). The activity consisted of the optimization of the fabrication procedure of PMMA fibres, eventually doped by boron, ready to be implemented as potential targets. We have studied two kinds of fibre samples: (a) aligned and (b) randomly oriented fibres (see Figure 13). The starting point is the preparation of the solution in which PMMA is initially dissolved. The optimization process led to fibres with good quality and a high degree of reproducibility. PMMA concentration plays a crucial role, influencing directly the final aspect of the fibres, especially their uniformity and the absence of defects such as pores. By varying the concentration from 100 to 800 mg/ml, we observed different quality of the fibres: too low concentration leads to irregular and broken fibres, whereas the highest concentration leads to inhibition of fibre extrusion from the needle. The ideal concentration is 375 mg/ml, leading to high-quality fibres, uniform and defect-free. In Figure 13, SEM images of the best random (a) and aligned (b) fibres are shown.

Figure 13. Electrospun fibres obtained by a solution of PMMA in chloroform (375 mg/ml) spun from an 18 G needle. A. SEM image of a random field of electrospun fibres, evidencing the good quality of the fibres characterized by absence of beads and ruptures. B. Representative SEM image of aligned fibres showing the absence of defects, a good uniformity and general quality of the size and the shape.

5.5. Ammonia borane targets (central laser facility)

Borane (BnHm) molecules, which contain high amounts of hydrogen and boron, are promising candidates for proton–boron fusion fuels. One example is diborane ( $B_2 H_6$), which has been used as a fusion fuel in magnetically confined plasma experiments (e.g., by TAE Inc.). However, diborane is a flammable and toxic gas, requiring careful handling.

As an alternative, Ammonia Borane (BNH ₆), also known as ‘AB’, has been proposed at the Central Laser Facility (CLF), as a solid, easy-to-handle fusion fuel for laser-driven sources of alpha particles (Ref [Reference Turcu, Margarone, Giuffrida, Picciotto, Spindloe, Robinson and Batani32]). AB is a stable powder under ambient conditions, with a density of 0.78 g/cm³. This makes it suitable for compression into solid disc-shaped pellets to serve as laser targets. Disc-shaped AB pellets, with a diameter of 12 mm and thickness of 1.2 mm, have been manufactured at the CLF .

Further work was carried out at the FBK, where AB was synthesized and compressed into disc targets, then irradiated with laser pulses to generate alpha particles for the first time (Ref [Reference Picciotto, Valt, Molloy, Gaiardo, Milani, Kantarelou, Giuffrida, Nersisyan, McNamee and Kennedy33]). In this experiment, conducted using the QUB 8 J/pulse, 800 fs laser, a normalized alpha flux of $6\times10^7$ alphas/J/sr was achieved. This result is comparable to the highest alpha particle yields reported in the literature for laser-driven proton–boron fusion using boron-containing targets. In Figure 14, a microscope image of an Ammonia Borane target.

Figure 14. An Ammonia Borane solid target, compressed from powder into 12 mm diameter discs with a thickness of 0.16 mm, was segmented into smaller laser targets. The image shows one of these target segments, highlighting both the edge profile and the typical surface morphology discussed in (Ref [Reference Turcu, Margarone, Giuffrida, Picciotto, Spindloe, Robinson and Batani32]).