NONLINEAR PROCESSES: Raman amplification sets sights on laser fusion

A pulse-amplification process designed for the highest-intensity short-pulse regime looks like it could lend its power to the effort toward laser fusion.

Probe and pump pulses couple via a plasma wave
Probe and pump pulses couple via a plasma wave

A pulse-amplification process designed for the highest-intensity short-pulse regime looks like it could lend its power to the effort toward laser fusion.

There is an increasing array of techniques in the high-intensity community that attempts to sidestep the single most limiting factor to research in the field: damage thresholds. Even chirped-pulse amplification, the gold standard for creating short, high-intensity pulses, starts damaging its own optics when intensities get near 1012 W/cm2.

One technique to handle these tremendous intensities that is only just seeing its powers put to the test is Raman amplification in plasma. It is a relatively straightforward nonlinear interaction that takes advantage of the fact that plasmas can handle 100,000 times greater intensity than can solid-state optical components.

First proposed by a group at Princeton University (Princeton, NJ) in 1999, the technique relies on counterpropagating beams in a plasma in a process called stimulated Raman backscattering.1 A relatively low-intensity, short-duration “seed pulse” and a much higher-energy “pump” pulse couple through plasma waves (see figure). As they collide, energy is funneled from the pump pulse into the seed pulse, which suffers little temporal broadening and thus gets a significant boost in intensity. The approach has widely been considered for the case of amplifying femtosecond pulses using picosecond pump pulses.

Enlarging the interaction area
A group at the Rutherford Appleton Laboratory’s Central Laser Facility (Didcot, England) had been considering the various effects in these plasma interactions, wondering about the effects of increasing spot sizes—a common approach to reduce intensities in large-scale amplifier systems. “We wanted to properly look at all kinds of instabilities that might come into play if the spot diameter of the laser pulse becomes larger,” says Raoul Trines of the Central Laser Facility. “Most of the earlier research was aimed at getting very high intensities, but not too much was dedicated to larger spot sizes.”

Their simulations showed that, even with the instabilities in the plasma brought about by higher interaction areas, the Raman amplification process could bring large pulses into the multi-petawatt regime.2 That work hinged on amplification of compressed pulses of just 25–50 fs in duration, and Trines said that many applications benefit from similarly high intensities but with deliberately longer durations—namely, the kind of inertialconfinement fusion currently being championed at the National Ignition Facility (Livermore, CA). “If you wish to compress a small target for fusion to a sufficiently high temperature and density so the fusion process might start, pulses that have a very high intensity but only last for 25 fs is not enough; it simply takes longer for the target to get compressed,” says Trines.

Other pursuits also benefit from the same kind of high-temperature compression that ICF is attempting, such as studies to simulate planetary cores and extreme astrophysical situations. So Trines and his coworkers from the Institute of Plasmas and Nuclear Fusion (Lisbon, Portugal) and the University of St. Andrews (Fife, Scotland) considered the approach of amplifying longer pulses.

“The question was what can we do, starting from that work where the compressed pulse was 25 fs in length, to maintain a similar intensity but over a full picosecond—40 times as long,” says Trines.

The answer, the team’s new simulations have shown, is to make sure the intensity of the pump is kept low and to keep the ratio between pump- and seed-beam duration at a factor of 1000. For example, to obtain a 2 ps seed beam using a 2 ns pump, the intensity has to be kept below a relatively tame 1011 W/cm2. The result, Trines says, “strongly indicates that it is possible to use the Raman amplification process to start from a nanosecond laser pulse and compress it to a picosecond in duration with an efficiency of up to 50%.”3

The simulations actually showed efficiencies as high as 60%, but Trines concedes that an actual experiment is unlikely to hit the idealized targets the simulations suggest—although he and his colleagues are eager to try. “We’re hoping to secure time on a laser system and do the experiment,” he says, “but the main problem is that suitable laser systems are hugely oversubscribed.”

REFERENCES

1. V.M. Malkin et al., Phys. Rev. Lett. 82, 4448 (1999).
2. R.M.G.M. Trines et al., Nature Physics 7, 87 (2011).
3. R.M.G.M. Trines et al., Phys. Rev. Lett. 107, 105002 (2011).

FIGURE. Probe (blue) and pump (red) pulses couple via a plasma wave, transferring energy from the pump to the probe pulse.

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