Focused laser power boosts ion acceleration

August 7, 2015
A laser beam (red, coming from the left) shines on an ultrathin diamond-like carbon foil coated on one side with a layer of nanotubes. The impact of the laser beam ejects high-energy ions from the uncoated side of the carbon foil. The additional focus provided by the nanotube coating enhances the efficiency of this laser-driven particle acceleration. Credit: Isabella Cortrie

An international team of physicists has used carbon nanotubes to enhance the efficiency of laser-driven particle acceleration. This significant advance brings compact sources of ionizing radiation for medical purposes closer to reality.

The interaction of high-intensity light with solid targets could someday serve as the basis of table-top sources of high-energy ions for medical applications. An international team led by physicists of the LMU affiliated with the Munich-Centre for Advanced Photonics (MAP), a Cluster of Excellence based in Munich, and in cooperation with scientists from the Max Planck Institute of Quantum Optics, has taken another step towards this goal. They have done so by boosting the efficiency of a technique that uses extremely intense pulses of laser light to eject packets of high-energy ions from diamond-like foils. In their experiment, the researchers coated one side of the foil with carbon nanotubes. Upon laser irradiation, the layer acts like a lens to focus and concentrate the light energy on the foil, which results in the production of much more energetic ion beams. This makes experiments with high-energy carbon ions on cells feasible for the first time, and brings light-driven generation of closer to practical application.

Light is an enormously powerful and versatile source of energy. When high-intensity pulses of are fired at ultrathin diamond-like carbon (DLC) foils, they punch through the foil, stripping electrons from the atoms. The negatively charged electron cloud then drags a stream of positively charged carbon ions along, accelerating them to speeds of up to 10% of the speed of light. The bursts of carbon ions produced by the radiation pressure exerted on the foil by ultrashort laser pulses could be used to treat tumors, provided the ions pack sufficient energy. At present, the only machines capable of producing such high-energy ion beams are large and highly expensive particle accelerators. Laser-based technologies are as yet unable to generate beams of comparable quality. However, light-driven approaches offer a possible route to much more compact and far less costly ion sources for medical applications in the future.

To reach this goal, laser physicists need to increase pulse intensities, and find ways to ensure that much more of the incident light energy is delivered in concentrated form to the carbon foil target. MAP physicists have now taken a significant step toward the latter objective. Each laser pulse fired at the target lasts for 50 femtoseconds (a femtosecond equals a millionth of a billionth of a second), and consists of about 20 oscillations of the optical field. This means that not all of the electromagnetic energy associated with the optical pulse reaches the target at the same time. It arrives in dispersed form, so that the radiation pressure acting on the target atoms rises gradually to a maximum, then drops off again. Since only the peak energy is sufficiently high to rip ions from the foil, the process is not terribly efficient.

Ultrathin foils of diamond-like carbon were first used in studies of radiation pressure acceleration five years ago. For the latest experiments, technicians at the MAP Service Centre used vapor deposition to coat the front of each foil with carbon nanotubes. The nanotubes come to lie on the surface in a higgledy-piggledy fashion, like blades of grass in a haystack. But the plasma formed when the laser pulse impinges on the nanotube coating effectively acts like a lens. As a result, the power of the incident pulse is concentrated sufficiently to permit immediate ionization of the underlying carbon foil. In addition, the nanotube coating focuses the light pulse onto a very small area of the target. These two effects together enhance the energy of the carbon ions ejected from the foil to around 200 MeV (mega electron volts) – significantly higher than was previously attainable. In a collaborative effort involving researchers from Germany, the UK, Spain and China, the experiments were carried out with the ASTRA-Gemini laser at the Rutherford Appleton Laboratory in Didcot (UK), as part of the Laserlab Europe Program.

The higher energies now available make it possible, for the first time, to carry out experiments on cells with beams of carbon ions. However, because the radiation must pass through healthy tissue before it reaches a tumor, energies of at least one GeV (gigaelectronvolt) will be required for clinically relevant applications, about five times higher than that attained in the latest experiments. But boosting power output to this level is not an impossible dream. On the basis of the expertise available at the Munich-Centre for Advanced Photonics, a new Centre for Advanced Laser Applications (CALA) is now being built on the high-tech campus in Garching. CALA will house a novel ultrashort pulse system, called ATLAS 3000, which is designed to provide laser pulses with powers of up to three petawatt. In combination with the energy enhancement made possible by the nanotube-coated carbon foils, this system promises to bring the era of light-based sources of ionizing radiation a lot closer.

Explore further: World-largest petawatt laser completed, delivering 2,000 trillion watts output

More information: "Ion Acceleration Using Relativistic Pulse Shaping in Near-Critical-Density Plasmas." Phys. Rev. Lett. 115, 064801 (2015), 3 August 2015 DOI: 10.1103/PhysRevLett.115.064801

Related Stories

A novel source of X-rays for imaging purposes

June 16, 2015

Physicists at LMU Munich and the Max Planck Institute of Quantum Optics have validated a novel laser-driven means of generating bright and highly energetic X-ray beams. The method opens up new ways of imaging the fine structure ...

Laser research shows promise for cancer treatment

August 21, 2012

(Phys.org) -- Scientists at Los Alamos National Laboratory have observed for the first time how a laser penetrates dense, electron-rich plasma to generate ions. The process has applications for developing next generation ...

Japanese team fires world's most powerful laser

July 29, 2015

(Phys.org)—A team of researchers and engineers at Japan's Osaka University is reporting that they have successfully fired what they are claiming is the world's most powerful laser. In their paper published in the journal ...

Recommended for you

Carefully crafted light pulses control neuron activity

November 17, 2017

Specially tailored, ultrafast pulses of light can trigger neurons to fire and could one day help patients with light-sensitive circadian or mood problems, according to a new study in mice at the University of Illinois.

Strain-free epitaxy of germanium film on mica

November 17, 2017

Germanium, an elemental semiconductor, was the material of choice in the early history of electronic devices, before it was largely replaced by silicon. But due to its high charge carrier mobility—higher than silicon by ...

New imaging technique peers inside living cells

November 16, 2017

To undergo high-resolution imaging, cells often must be sliced and diced, dehydrated, painted with toxic stains, or embedded in resin. For cells, the result is certain death.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.