Laser and graphene accelerated protons to 132 million electronvolts

A laser pulse struck a target of suspended graphene layers. Graphene is a sheet of carbon one atom thick. The pulse accelerated protons, positively charged particles, to a measured maximum energy of 132 megaelectronvolts (MeV), or 132 million electronvolts. That energy corresponds to a speed close to half the speed of light. The result, published in Progress of Theoretical and Experimental Physics by Takumi Minami, Che-Men Chu, Kentaro Sakai and colleagues, advances experiments with this class of relatively long laser pulses. It is not a beam ready to treat patients.

An electronvolt is a unit of energy used on the scale of particles. The target had several graphene layers suspended across a small gap. The aim was to use the laser to generate intense electric fields and give the protons more time to gain energy.

At Osaka University, the researchers irradiated targets made of four, eight or sixteen graphene layers with a pulse lasting about 1.5 picoseconds; one picosecond is one trillionth of a second. Track detectors recorded marks left by particles, a spectrometer separated signals by energy, and machine learning applied to the images helped identify the rare tracks of high-energy protons. The highest reading was 132 MeV.

The proposed explanation comes from two-dimensional simulations. After the laser turned part of the target into plasma—ionized matter in which electrons freed from atoms coexist with positive ions—a region of electric field would move through the material and keep pushing protons for more than three picoseconds. “Surfing” on that field is a metaphor for a particle remaining in the region that accelerates it. The field’s motion was calculated, not filmed directly.

A release by Japan’s National Institutes for Quantum Science and Technology, which does not name an individual reporter, calls 132 MeV a record for the category of relatively long pulses. It also cites 150 MeV for short-pulse lasers under different conditions. The 132 MeV result is therefore not an absolute world record, and the figures are not a straightforward comparison of efficiency between devices.

The experiment demonstrates that this arrangement can accelerate some protons to high energy. It has not demonstrated a plentiful, uniform or controllable beam for clinical use: the small group of protons with the highest energies contained far fewer particles than in a comparison study. The decisive next step is to repeat the acceleration with a sufficient number of protons and useful beam quality, and to test the moving field predicted by the simulation with additional measurements.