Graphene key to laser ion acceleration

Thanks to the superlative properties of graphene, researchers from the University of Osaka, in collaboration with the National Institutes for Quantum Science and Technology, Kobe University, and the National Central University in Taiwan were able to achieve energetic ion acceleration.

Typical particle acceleration works by using electric fields to speed up and increase the energy of a beam of particles, which are then steered and focused by magnetic fields. The beam of particles travels inside of a vacuum in a metal beam pipe; this is crucial as it maintains an air and dust-free environment for the beams of particles to travel unobstructed.

Electric fields spaced around the accelerator switch from positive to negative at a given frequency, creating radio waves that accelerate particles in bunches like waves of an ocean. The particles can then be directed at fixed targets, such as a thin piece of metal foil, or two beams of particles can be collided, such as the famous Large Hadron Collider.

It has long been known that a thinner target is required for higher energy in laser ion acceleration theory. However, it has been difficult to directly accelerate ions with an extremely thin target regime since the noise components of an intense laser destroy the targets before the main peak of the laser pulse.

Often, it is necessary to use plasma mirrors, which remove these noise components, to realize efficient ion acceleration with an intense laser.

With the hopes of removing the necessity of plasma mirrors, the researchers developed a large-area suspended graphene (LSG) as a target of laser ion acceleration.

"Atomically thin graphene is transparent, highly electrically and thermally conductive, and lightweight, while also being the strongest material," said study author Wei-Yen Woon. "To date, graphene has seen a variety of applications, including those in transportation, medicine, electronics, and energy. We demonstrate another disruptive application of graphene in the field of laser-ion acceleration, in which the unique features of graphene play an indispensable role."

To date, hundreds of industrial processes use particle accelerators – from manufacturing computer chips to cross-linking of plastic for shrink wrap.

Beyond its commercial uses, tens of millions of patients receive accelerator-based diagnoses and therapy each year in hospitals and clinics around the world. The two primary roles for particle accelerators in medial applications are the production of radioisotopes for medical diagnoses and therapy and as sources of beams of electrons, protons, and heavier charged particles for medical treatment.

Generally, when an intense laser pulse interacts with a thin film target, it instantaneously turns a thin front layer of the film into a dense plasma.

In this layer, where the laser's and plasma's frequencies are equal, the laser pulse is strongly absorbed, terminating its propagation into denser plasma regions. Thus, the thinner the target, the less loss of energy.

"The outcomes of this research are applicable to the development of compact and efficient laser-driven ion accelerators for cancer therapy, laser nuclear fusion, high energy physics, and laboratory astrophysics," said study lead author Yasuhiro Kuramitsu. "Direct acceleration of energetic ions without a plasma mirror evidently shows the robustness of LSG. We will use the atomic-thin LSG as a target mount to accelerate other materials which cannot stand by themselves."