Nuclear reactions involving the close approach of charged particles have much smaller cross sections than do chemical reactions at ambient thermal energy. Obtaining reasonable reaction rates for nuclear fusion by thermal means thus requires achieving simultaneously high densities and temperatures. Such extreme conditions are difficult to control in thermal plasmas, laser-compressed fuel pellets, or within small clusters of ordinary molecules subject to intense laser excitation. Even in a molecular cluster, much of the absorbed laser energy is thermalized through intra-cluster collisions. As a result, the cluster is blown apart, decreasing fusion yields. A novel approach to circumvent the confinement problem in thermal plasmas has been suggested: a solid state plasma confined by the quantum pinch effect in a semiconducting wire.

Fusion reactions also can be induced by non-thermal means. For example, charged particle beams can be collided at appropriately high energy to carry out fusion reactions in the laboratory. Alternatively, fusion can be catalyzed by achieving a high spatial density, as happens for the nuclei within a muonic molecule. When a muon replaces the electron, it brings the nuclei ~200 times closer together than in an ordinary molecule, greatly enhancing the spontaneous nuclear reaction rate even at low temperature. In many ways, the ground state of such a molecule is the ideal situation for fusion because the phase space density of the reacting species takes on the largest possible value consistent with quantum mechanics. While greeted by much excitement when it was discovered in the 1950s, muon-catalyzed fusion still just falls a bit short of practicality because of the insufficient lifetime of the muon.

Fusion does not occur to a measurable extent in the ground state of normal molecules bound by electrons because of the lower density of nuclei (~ 1/ų, not 1/pm³) and the low vibrational energy (meV, not keV) compared to muonic molecules. In this paper we will explore whether laser pulse shaping could allow quantum control to enhance intramolecular nuclear collision rates, starting from normal internuclear distances.

—Berrios et al.

Peter Wolynes of Rice, Martin Gruebele of Illinois and Illinois alumnus Eduardo Berrios of Chile simulated reactions in two dimensions that, if extrapolated to three, might just produce energy efficiently from deuterium and tritium or other elements.

Their paper appears in the festschrift edition of Chemical Physical Letters dedicated to Ahmed Zewail, Gruebele’s postdoctoral adviser and a Nobel laureate for his work on femtochemistry, in which femtosecond-long laser flashes trigger chemical reactions.

The femtochemical technique is central to the new idea that nuclei can be pushed close enough to overcome the Coulomb barrier that forces atoms of like charge to repel each other. When that is accomplished, atoms can fuse and release heat through neutron scattering. When more energy is created than it takes to sustain the reaction, sustained fusion becomes viable.

Using shaped laser pulses might make it possible to nudge atoms in a deuterium/tritium molecule close enough to fuse, according to a new study. Researchers at Rice University, the University of Illinois at Urbana-Champaign and the University of Chile suggested that quantum-controlled fusion may provide a possible new path toward the production of energy through nuclear fusion. (Credit: Gruebele Group/University of Illinois at Urbana-Champaign) Click to enlarge.

The trick is to do all this in a controlled way, and scientists have been pursuing such a trick for decades, primarily by containing hydrogen plasmas at sun-like temperatures (at the US Department of Energy’s National Ignition Facility and the International Thermonuclear Experimental Reactor effort in France) and in large facilities.

The new paper describes a basic proof-of-principle simulation that shows how, in two dimensions, a shaped-laser pulse would push a molecule of deuterium and tritium, its nuclei already poised at a much smaller internuclear distance than in a plasma, nearly close enough to fuse.

… we performed quantum wavepacket propagation in a 2-D toy model of two field-bound nuclei in the presence of a time-dependent 800 nm laser pulse that was shaped to exert coherent control over the nuclear wavepacket. The collision probability is enhanced by about 3 orders of magnitude by the best coherent control pulse, and by up to 20 orders of magnitude relative to an electron-bound molecule. Since muonic fusion is already not far from break-even for net energy production, shaped VUV laser pulses, when they become available, could also be an efficient means of enhancing muonic fusion by coherent control.

—Berrios et al.

Wolynes said 2-D simulations were necessary to keep the iterative computations practical, even though doing so required stripping electrons from the model molecules.

The best way to do it would be to leave the electrons on to help the process and control their motions, but that is a higher-dimensional problem that we—or someone—will tackle in the future

—Peter Wolynes

Without the electrons, it was still possible to bring nuclei within a small fraction of an angstrom by simulating the effects of shaped 5-femtosecond, near-infrared laser pulses, which held the nuclei together in a “field-bound” molecule.

For decades, researchers have also investigated muon-catalyzed fusion, where the electron in the deuterium/tritium molecule is replaced by a muon. Think of it as a 208-times heavier electron. As a result, the molecular bond distance shrinks by a factor of 200, poising the nuclei even better for fusion.

Sadly, muons don’t live forever, and the increased fusion efficiency just falls short of breaking even in energy output. But when shaped vacuum ultraviolet laser pulses become as available as the near-infrared ones we simulated here, quantum control of muonic fusion may get it over the threshold.

—Martin Gruebele

Because the model works at the quantum level—where subatomic particles are subject to different rules and have the characteristics of both particles and waves—the Heisenberg uncertainty principle comes into play. That makes it impossible to know the precise location of particles and makes tuning the lasers a challenge, Wolynes said.

It’s clear the kind of pulses you need have to be highly sculpted and have many frequencies in them. It will probably take experimentation to figure out what the best pulse shape should be, but tritium is radioactive, so no one ever wants to put tritium in their apparatus until they’re sure it’s going to work.

—Peter Wolynes

Wolynes said he and Gruebele, whose lab studies protein folding, cell dynamics, nanostructure microscopy, fish swimming behavior and other topics, have been thinking about the possibilities for about a decade, even though nuclear fusion is more of a hobby than a profession for both.

We’re not starting a company … yet. But there may be angles here other people can think through that would lead to something practical even in the short term, such as production of short alpha particle pulses that could be useful in research applications.

—Peter Wolynes

Berrios, lead author of the paper, is a research scientist at the University of Chile, Santiago. Wolynes is the D.R. Bullard-Welch Foundation Professor of Science, a professor of chemistry, of biochemistry and cell biology, of physics and astronomy and of materials science and nanoengineering at Rice and a senior investigator at Rice's National Science Foundation-funded Center for Theoretical Biological Physics. Gruebele is the head of chemistry, the James R. Eiszner Endowed Chair in Chemistry and a professor of physics, biophysics and computational biology at Illinois.

The National Science Foundation supported the research.

Resources

• Eduardo Berrios, Martin Gruebele, Peter G. Wolynes (2017) “Quantum controlled fusion,” Chemical Physics Letters doi: 10.1016/j.cplett.2017.02.045