Catalyzing nuclear fusion via nanoplasmonics?

The goal of achieving inertial confinement fusion is driving the development of increasingly powerful high-energy lasers. In a new approach, François Bayard, Jean-Enric Ducret, and Rodolphe Antoine in France explored crossing activation barriers at the nuclear fusion scale—comparable to enzymes in biology or by catalysts in chemistry.

The combustion of hydrogen requires a temperature of 850°C, which happens at ambient temperature if catalyzed onto a single site with platinum. Drawing a parallel to the Lawson criterion where the temperature of a deuterium tritium plasma must rise above 50,000,000°C to trigger net nuclear fusion, Bayard reasoned that if a similar effect was found and demonstrated for fusion, laser fusion could become achievable via less energy. In 2007 he coined for this effect the word “ERTIGO,” which stands for energy, vertiginous activation barrier, and science levels comparable to the Virgo gravitational wave detector.

To catalyze fusion, the team experimented with the formation of clusters containing nanoparticle molecules to be energized by laser irradiation (see below). They demonstrated that the mediating function of ERTIGO, if it existed, would act directly on the nuclei and the electrons involved in the fusion process.

Nanoplasmonics and fusion

The foundations of laser fusion catalysis research are:

• Electrons mediate the transmission of laser energy to protons: any mechanism facilitating an intense energizing of the electrons could trigger the catalyzing effect.
• The Coulomb repulsion between light nuclei should be reduced by electron shielding, which influences the cross section favorably.
• The electromagnetic field of optical radiation generates a force that optically induced phenomena can amplify: Electrons and protons can be manipulated by modulating the ponderomotive acceleration.

Localized surface plasmon resonances (LSPRs) are nonpropagating oscillations of conduction electrons that remain confined around metallic nanostructures. By modifying the shape, dimensions, and chemical composition of the nanomaterials involved, LSPRs can influence the frequency of the laser light focused on a target and enhance its absorption. Resonances of metals such as silver, gold, and platinum exhibit a strong absorption band (or multiple bands) in the visible and near-infrared, which is why they are used extensively in nanophotonics for chemical and biosensing but also as catalysts in chemistry.

Because of the oscillation of the electrons, and depending on the nanoparticle configuration, an electric field wave incident on a suitable gold particle can drive the increase of the electric near field. In simulations, when femtosecond pulsed lasers are used for particle field enhancement with oscillations lasting few femtoseconds, surface electric fields can rise to currents above 10¹² Vm^-1 and speed up electrons and ions. Electrons accumulate at the dielectric interface where the gold nanoparticle discontinues.

At the nanoscale, plasmonic gold nanoparticles amplify the electromagnetic field of lasers to fields orders of magnitude considerably higher than those of the laser pulse itself.

Effects of this mechanism of interest in fusion science are:

• The electrons resonate along the nanoparticle, which entices the protons to move at the same frequency and delivers laser energy (laser wakefield acceleration) to them. Electrons and ions move with the same laser wake speed, which results in a kinetic energy increase of the protons vs. the electrons in the near field. By selecting a laser with parameters that can be played with, amplitudes and energy can be tuned via laser pulse duration.
• The localized increase of the electron density causes a shielding effect, which reduces the Coulomb barrier strength between ions.
• A dense accumulation of the electrons at the nanoparticle extremes and a correspondingly intense ion acceleration can trigger Coulomb explosions that further accelerate the ions involved in the dynamic.

Norbert Kroó at the Wigner Research Center for Physics (RCP) in Hungary proposed exploring these plasmonic effects to improve the conditions for plasmonic fusion. And the Nano Plasmonic Inertial Laser Fusion Experiment (NAPLIFE) is pursuing laser-driven fusion with a nonthermal laser wakefield collider that uses targets of gold nanoantennas. NAPLIFE, as part of the Hungarian National Laboratory Program, is a collaboration between the Wigner RCP, the University of Szeged in Hungary, BME in Hungary, the University of Debrecen in Hungary, the University of Bergen in Norway, and the Frankfurt Institute of Advanced Study in Germany.

For current fusion experiments, target normal sheath acceleration (TNSA) still is the leading acceleration mechanism. It involves a laser beam directed onto a single target surface that vaporizes only some front plasma. A laser wakefield collider’s target is illuminated from two opposite sides (which are reciprocally accelerated to compression over a nanosecond timespan while protons attain energies of GeV).

To trigger a detonation that expands uniformly with the speed of light within the target itself inside a laser wakefield collider, the NAPLIFE experiment aims to enhance the absorption of laser energy by using nanoplasmonic effects. Tuned by a weighted doping of suitable nanoparticles, the enhanced laser light field drives a simultaneous ignition in the target.

Based on a theory developed by László Csernai and Daniel Strottman in 2015, this configuration removes the obstacle of Rayleigh-Taylor instabilities and the need to compress the target beforehand to suit the cross-section conditions for fusing a specific fuel type. Compression happens during the explosion while the cross section gets more favorable because of the effects of the electron polarization cloud of the nanoparticle. Particle-in-cell kinetic model simulations via the EPOCH code confirmed the efficiency of the particles in enhancing the laser light absorption and the expected proton acceleration.

Attila Bonyár and colleagues at the Budapest University of Technology and Economics fabricated targets for laser experiments by doping the gold nanoparticles in a photopolymer mix of urethane dimethacrylate (UDGMA) diluted with a triethylene glycol dimethacrylate monomer (TEGDMA) and photoinitiators. To optimize light absorption for the 795-nm laser wavelength and for the copolymer refraction index, consistency and dimensions of the nanoparticle rods were selected with two aspect ratios: 25 × 75 nm and 25 × 85 nm. The polymer was pressed for photopolymerization on a silicon glass (see Fig. 1).

At Wigner RCP, the first irradiation experiments were carried out in 2024 using a Coherent Ti:sapphire chirp-pulsed amplification laser with center wavelength of 795 nm and pulse energy of 30 mJ. By an equal illumination, the volume increase of the craters at focus within the targets of the doped specimens—with respect to the undoped ones—justifies the conclusion of a higher laser intensity that triggered nuclear fusion within the doped ones.

Within the craters of the doped targets, Raman spectroscopy found new bands in the 2000- to 2500-cm^-1 spectral region. The bands were interpreted as revelatory of carbon deuterium and nitrogen deuterium vibrations or of the unfolding of nuclear reactions within the polymer because of the laser radiation enhancement generated by the nanorods. Laser-induced breakdown spectroscopy also detected deuterium ions in the backward scattered plasma.

The results of experiments at the Extreme Light Infrastructure–Attosecond Light Pulse Source (ELI-ALPS), a Hungarian research facility in Szeged co-financed by the European Commission, were even more significant. In nanocomposite targets with a thickness of 160 µm and a content of 0.185-m/m% gold nanorods and 2.5-m/m% boron nitride nanoparticles, the rods were treated with 3-mecaptopropyl trimethyloxysilane to enable mutual particle bonding.

With a constant focus diameter and pulse energy of 3 µm and 25 mJ, illumination times varied between 12 to 360 fs with a peak intensity at the focus of 8.3 × 10¹⁸ W/cm². In targets with added boron nitride, at a laser pulse length of 100 to 125 fs and peak irradiation intensity of I = 4 × 10¹⁵ W/cm², a sudden drop of the proton count was noticed because of the protons expended in the p¹¹B fusion reaction. Bolstering this conclusion, the counts of the CR 39 detector showed impact traces with approximate diameter of α particles (12 µm). (See Figs. 2, 3, and 4.)

ERTIGO or fusion catalysis?

When I asked Professors Tamás Biró and Norbert Kroó, two NAPLIFE researchers, whether their experiments demonstrated the existence of ERTIGO, they’d noticed the root of the idea is similar, but catalysis assumes a change in the effective reaction rate. By using nanoparticles, they successfully manipulate the electromagnetic fields and the concentration of the laser pulse energy. A quantum tunneling effect on the fusion channels' cross sections can be imagined but exerts a minor impact (and was already known in astrophysics).

Biró and Kroó found that, when considering the novelty of the whole process, an enhancement on the energy distribution of the proton corresponds to a “catalysis” from the viewpoint of the possible yield. The catalytic effect consists of screening the plasmonic hot spots, the ponderomotive acceleration of the protons, and the correlated motion of the electrons in the localized plasmon oscillations on the source.

Interestingly, according to simulations and to the analyses of the post-reaction crater size, there is a high probability that Coulomb explosions were triggered during the experiments. But the achieved and activated fusion reaction channel thresholds have not yet been studied experimentally.

Further work is needed before the effect of nanoplasmonics in laser nuclear fusion can be accepted as parallel to catalysis in chemistry, but it has proved to be remarkably similar so far.

FURTHER READING

F. Bayard et al., “A possible way to a new nuclear reactor: the 'ertigo' concept,” poster presented at the WATOC conference in Santiago (2014); https://doi.org/10.13140/rg.2.2.27901.77283.

M. J. Guffey and A. Y. Wong, arXiv:2106.08127 (2020); https://doi.org/10.48550/arXiv.2106.08127.

N. Kroó et al., Sci. Rep., 14, 30087 (2024); https://doi.org/10.1038/s41598-024-80070-5.