PerspectiveApplied Physics

Laser-Driven Particle Accelerators

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Science  21 Apr 2006:
Vol. 312, Issue 5772, pp. 374-376
DOI: 10.1126/science.1126051

For many years, the high-power-laser community has been pursuing the goal of producing miniaturized particle accelerators. Limitations of conventional technology mean that kilometer-sized accelerators are required for high-energy physics research (see the bottom figure). Similarly, major installations are required for medical applications such as particle beam treatment of tumors, which excludes all but the largest research hospitals. By contrast, laser-plasma-based techniques can support accelerating electric fields at least four orders of magnitude larger than those of conventional techniques, leading to the hope that particle accelerators could one day become a commonplace tool.

Although the goal is attractive, these plasma schemes have, until recently, suffered from many shortcomings, such as poorly controlled particle energy, high divergence, and low luminosity. A host of results in the past year has provided renewed impetus, with demonstrations of quasi monochromatic, high-brightness beams in the multi-MeV energy range. Now, Toncian et al. (1) at the University of Düsseldorf have demonstrated an imaginative technique for tunable energy selection and focusing of proton beams in the multi-MeV energy range. As reported on page 410 of this issue, they show that the use of a simple capillary lens can lead to dramatic results.

Lens arrangements for proton beams are not new; but until now, there has not been a way to focus picosecond-time-scale beams with currents that are orders of magnitude higher than those of conventional beams and that require focusing over micrometer spatial scales. In the system developed by Toncian et al., a proton beam is created by irradiating a metal foil with laser pulses, and the protons are focused by the electric fields created inside a small cylinder that is irradiated on the side by a second laser (see the top figure). This new idea provides a simple tool to exploit the dramatic advances in laser-driven hadron acceleration seen over recent years.

It is easy to imagine the breadth of science disciplines that would benefit from a new generation of miniature accelerators. Potential applications include the imaging and treatment of cancerous cells, offering scale reductions and far simpler beam orientation compared with conventional cyclotrons. Nearer term applications involve the use of these ultrashort proton beams to heat matter isochorically (that is, at constant density) to produce extreme thermodynamic states of interest for fundamental atomic physics studies, laboratory astrophysics, and the fast-ignition approach to fusion energy production (2). Similarly, such beams have recently been used as a unique probe of electromagnetic field structures in relativistic plasma physics studies (3). The technique demonstrated by Toncian et al. provides researchers with the means to control these emerging sources to an unprecedented degree.

Laser acceleration of protons occurs as a result of the extremely high electric field gradients created on either the front or rear surface of thin foils illuminated by subpicosecond laser pulses, whose focused intensities exceed 1018 W cm−2. The laser ionizes and subsequently accelerates electrons to relativistic velocities. These are confined to form a beam by the azimuthal magnetic field created in the interaction. The ensuing charge separation sets up the electric field to accelerate the proton or ion species in the foil, with an overall energy efficiency between 1 and 10%. Although such laser intensities are incredibly high, they are now routinely produced at high repetition rate on table-top-sized systems. This reflects the dramatic progress seen in laser technology, where an order-of-magnitude increase in peak laser power has been achieved roughly every 3 years.

Over the past few months, we have seen very encouraging experimental results on the production of quasi mono-energetic proton beams in the MeV range (4), narrow-band ion acceleration (5), and a study of the scaling of proton energy with laser parameters (6). This builds on similarly dramatic results with laser-driven electron beams that were reported 18 months ago (79). The results reported by Toncian et al. now provide a method for simple control of laser-produced particle beams without having to rely on access to specific plasma physics regimes that are still only partially understood. This should allow greater deployment of controlled particle-beam sources over the near term, at least within the laser research community. This will likely lead to a wealth of new studies using the beams as unique probes and localized heating sources.

Large and small.

(Left) Conventional accelerator at Fermilab. (Center) Part of the linear accelerator beamline. (Right) Benchtop laser particle accelerator for multi-MeV experiments.

CREDIT: FERMILAB
Compact accelerator.

(Top) Schematic of the laser-plasma interaction process that produces a beam of energetic electrons, protons, and ions. (Bottom) Diagram of the cylindrical lens focusing system used by Toncian et al. to focus a proton beam.

CREDIT: C. BICKEL/SCIENCE.

Such rapid progress demonstrates that laser-driven systems may indeed be able to move from a scientific curiosity to real-world applications in due course. Caution is needed, however, in extrapolating current results to potential applications. Plasma physics has a long history of throwing unwanted surprises at its practitioners, and most of the applications require a level of stability and reproducibility that is a long way from having been demonstrated. Much research across a broad front is still required, but it is now time to start to link this work to the analyses of how such techniques may be developed into reliable systems.

In particular, it will be important to strike an appropriate balance between the basic research needed to progress these ideas and the required industrialization of the techniques. As an example of this balance, European researchers have just been granted funds from the European Union to pursue the development of laser-driven electron accelerators into the GeV energy regime, with a view to creating reproducible monochromatic beams. Alongside this, a demonstration proton oncology laser system known as “Pro-Pulse” is being pursued by a team led by the Laboratoire d'Optique Appliquée (LOA) in France, with the goal of raising the energy of the proton beam to the required 70- to 250-MeV level. These two ambitious projects will help demonstrate whether laser acceleration of particles is a viable route for fundamental physics studies and clinical applications.

Looking further into the future, an exciting new proposal is being developed by a consortium of researchers led by Mourou (at the LOA) for ultrarelativistic particle beam lines based on exawatt-class laser technology. This project, known as Extreme Light Infrastructure, is currently under consideration as part of the European Research Infrastructure roadmap process (10). Its goal is to provide multiple accelerator beam lines delivering high-brightness electron, gamma, and proton sources for a wide range of user applications.

It is clear that we are still a number of years away from exploitation of these laser-driven accelerators, but this should not detract from the major advances demonstrated over the past few months. Unprecedented research attention is being paid to this area, which is already paying dividends as demonstrated by the innovative techniques reported here. This is definitely a field to watch.

References and Notes

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