Proton beams from a nanotube accelerator

Carbon nanotubes (CNTs) are hardy and versatile, with remarkable material and electronic properties. And they could be useful in some extreme conditions as well. Two physicists in Japan, Masakatsu Murakami of Osaka University and Motohiko Tanaka of Chubu University, propose using a CNT as a shotgun barrel to shoot a beam of protons. Their scheme nests two small hydrogen-rich fragments—which could be water ice, paraffin, or some other low-Z material but were modeled as hydrogen nanotubes—within a larger CNT that has gold atoms chemically adsorbed in its wall. The assembled structure is then zapped from the side with an ultraintense femtosecond laser pulse. As shown in quantitative three-dimensional simulations, the laser partially ionizes the gold and fully ionizes the hydrogen and carbon in the assemblage; after a few swings of the laser's electric field, significant numbers of electrons (white in the schematic) are blown off and form a cloud around the CNT. The now highly ionized coaxial structure generates a Coulomb potential in which the protons (blue) from the low-Z shotgun shells are squeezed toward the axis and accelerated out both ends of the CNT. The simulations indicate that even a non-optimized setup can produce highly collimated beams of nearly monoenergetic protons—1.5 MeV for the parameters used. Such beams are of great interest in fields as diverse as medicine, fusion energy, and materials engineering. (M. Murakami, M. Tanaka, Appl. Phys. Lett. 102, 163101, 2013.)—Stephen G. Benka