HIGH-POWER LASERS: Laser creates quasi-monoenergetic proton beam

With lasers offering ever-shorter pulses and higher peak intensities, high-energy radiation/matter interaction has become widely studied, for example to induce nuclear fusion. An outgrowth of this research deals with laser particle acceleration. Targets irradiated by sufficiently intense laser pulses can emit electrons and ions whose energy can readily surpass 100 MeV. A laser-field-driven separation of positive and negative charge in the target plasma gives rise to a plasma wake field that exhibits electrical-field strengths exceeding those realizable in electromagnetic-particle accelerators by several orders of magnitude. With the advent of tabletop terawatt lasers, particle accelerators that fill halls could, in principle, be shrunk to comfortable laboratory size.

One problem is the large spread in energies of the emitted particles; research has pushed this energy spread down to a few percent.¹ Now, researchers at Friedrich-Schiller University (Jena, Germany) have succeeded in a considerable further improvement, at least for protons. Applying a single laser pulse, they generated 108 protons with a 1.2 MeV peak energy and a mere 300 keV, full width at half maximum, superimposed on a broad background.² In this case, the proton acceleration was achieved by so-called target-normal sheath acceleration (TNSA), which occurs when a sheath-like cloud of electrons is expelled from the target. When a laser pulse is focused onto the target, electrons are freed and accelerated so that they leave the thin target in the direction of the laser pulse, thus giving rise to a strong electric field on the rear side of the target (see figure). Protons, frequently present due to hydrocarbon contamination, are rapidly released and accelerated.

PMMA dots localize protons

In their experiment, the researchers carried out a proposal to use a microstructured double-layer target, with the first layer made of titanium to provide the electron cloud, and the second, which supplies the protons, made of polymethyl methacrylate (PMMA, which contains sufficient amount of hydrogen).³ Most of the PMMA layer was removed by laser ablation, leaving a grid of PMMA dots, each narrower than the region of the laser-expelled electron cloud so that protons released from the dots mostly experienced the maximal field strength.

The laser pulses struck the target exactly opposite a fresh PMMA dot. To adjust those positions shot by shot, the dots were doped with rhodamine dye so that their position could be traced, while the target was moved, by fluorescence excited by a second laser.

For the accelerating laser, the researchers used the JETI (Jena Ti:sapphire) laser, which delivers pulses of 600 mJ and 80 fs duration at a maximum repetition rate of 10 Hz. The irradiance at the focus reached 3 × 10¹⁹W/cm². Energy and mass analysis were performed using a parabola spectrometer.

The authors consider their results as proof of principle, intending to pursue further improvement and scalability. Using a petawatt tabletop laser expected to be available in a few years, highly monoenergetic proton beams with energies greater than 100 MeV should be achievable that could be used practically, for example in tumor therapy. In addition, they could serve as powerful injection sources for electromagnetic accelerators.

REFERENCES

1. See papers from three groups in Nature 431, 535 (2004).

2. H. Schwoerer et al., Nature 439, 445 (2006).

3. T. Zh. Esirkepov et al., Phys. Rev. Lett. 89, 175003 (2002).