Void-Filled Material Stops Intense Electron Beam
New experiments show that porous materials consisting mostly of empty space can absorb the energy carried by an ultraintense electron beam more effectively than porous media with higher mass densities. The finding contradicts the prevailing notion that denser and thicker obstacles always provide more stopping power and suggests that the microstructure of a material fundamentally changes its electron-stopping ability. Simulations by the experimental team revealed the physical mechanisms behind this “anomalous-stopping” effect, which the researchers believe provides a new way to control the propagation of electron beams in extreme environments [1].
The study focuses on relativistic electron beams (REBs), which travel at close to the speed of light. REBs that carry currents in the mega-ampere regime can deliver petawatts (1015 watts) of power to a small target in a pulse lasting for a few picoseconds. This high intensity makes them ideal for creating and probing extreme states of matter that exist in stars, planetary cores, or nuclear events. The short bursts of intense energy provided by REBs are also used in inertial-confinement fusion—a scheme in which high-power lasers heat a fuel pellet and trigger nuclear fusion.
Such ultraintense REBs are generated at the XingGuang-III facility in China by firing ultrashort pulses of a high-power laser beam at a solid target. The energy delivered by the laser instantly ionizes the surface of the target, creating a large population of hot electrons that are propelled at relativistic speeds into the material. Under these extreme conditions, electrons don’t behave like isolated particles that interact through two-body collisions. Instead, the propagation of the beam through the material is dominated by collective plasma effects. As a result, strong electromagnetic fields generated by the fast-moving electrons control both the shape of the beam and the loss of energy.
Recent experiments at XingGuang-III and elsewhere have started to explore the interactions of these REBs with porous foams. Previous studies have shown that the pore structure produces random variations in the electromagnetic fields, generating complex beam effects that are not seen with nonporous targets. So far, however, little is known about the electron stopping power of these microstructured materials.
To investigate the energy-loss mechanisms at play, Ke Jiang of Shenzhen Technology University in China and his colleagues recorded the electrons that emerged after an ultraintense REB had traveled through each of two types of porous foam: one consisting mostly of pores and having a mass density of just 5 mg/cm3, and one having a smaller volume of pores and a density of 200 mg/cm3. Electron spectra recorded behind the target showed that the low-density foam transmitted fewer electrons with lower energies than the denser version. Measurements of electrons emitted at an angle with respect to the central beam axis also revealed that more electrons were scattered by the low-density foam, which reduced the number moving in the forward direction.
The loss of energy within the low-density foam was orders of magnitude higher than predicted by conventional theories, which assume that the stopping power of a material depends mainly on its density. To better understand the anomalous-stopping effect, Jiang and colleagues simulated the interactions between the REB and the porous material. These simulations showed that the enhanced stopping power results from electrical currents that flow through the solid portions of the foam. These currents generate strong magnetic fields within the pores that deflect, scatter, and trap the beam electrons.
While this study focused on randomly structured foams, Jiang and colleagues believe that the porous microstructure could be designed to control how and where the electron beam deposits its energy. “Our longer-term goal is to move toward precise control, or even ‘programming,’ of intense electron-beam transport and energy deposition through material design,” Jiang says.
According to Jiang, such control of REBs could lead to new strategies for regulating the fast-ignition process for nuclear fusion. He also imagines exploiting the radiation emitted when electrons are deflected by strong magnetic fields to produce tabletop sources of ultrabright x rays or gamma rays.
Peter Norreys from the University of Oxford in the UK believes that the work will be significant for fast-ignition experiments. It provides “a strong motivation for future studies into the use of porous and liquid-filled foams for experiments in inertial-fusion energy,” he says.
–Susan Curtis
Susan Curtis is a freelance science writer based in Bristol, UK.
References
- K. Jiang et al., “Experimental observation of anomalous stopping of mega-ampere electron current in porous materials,” Phys. Rev. Lett. 136, 185102 (2026).