For several decades, researchers have been using the world’s most powerful lasers to try to recreate the fusion reaction that occurs in stars. The process, called inertial confinement fusion (ICF), uses multiple laser beams to compress and heat a small, spherical target containing nuclear fuel, in order to ignite thermonuclear fusion. In principle, the heat released from the reaction could provide an alternative energy source, but the challenges to achieving ignition are many. One, in particular, is to understand and control a process first identified in the mid-1990s called crossed-beam energy transfer (CBET), in which the laser beams exchange energy with each other as they overlap in the plasma. In laser-driven fusion experiments, CBET occurs just before the laser beams deposit their energy into the target. The effect can therefore modify the finely tuned symmetry of the beams or cause energy to leak out of the target.
Now, Igor Igumenshchev from the Laboratory for Laser Energetics at the University of Rochester in New York and colleagues are proposing a new technique to mitigate the negative effects of CBET. In Physical Review Letters, Igumenshchev et al. use simulations to show that dynamically reducing—or “zooming”—the spot size of the lasers as they interact with the target would reduce energy transfer between the beams, without disrupting the symmetry of the laser illumination—a negative side effect of some existing proposals for correcting CBET.
The CBET process is relatively simple to explain. When two laser beams overlap in a plasma [ed. see above], they create a beat wave. Free charges in the plasma accumulate where the beat wave’s electric field is weakest (a result of what is known as the ponderomotive force) and this accumulation of charges modulates the refractive index, in effect creating a Bragg diffraction grating for the lasers. Because this grating is created by the beat wave, the Bragg condition is, by construction, always satisfied. Moreover, if there is a wavelength separation between the laser beams, or the plasma flows, the grating can move at (or close to) the speed of sound. The moving grating will then scatter light from one beam in the exact direction of the other beam, effectively transferring energy from one laser beam to the other.
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What the Rochester group is proposing now [6] may prove to be an even more effective way of controlling CBET than simply reducing the laser spot size. Igumenshchev et al.’s key insight is that the negative effects of CBET and radiation asymmetry occur at different times during the target’s implosion. The symmetry of the laser illumination is crucial in the first few nanoseconds after the lasers interact with the target, when nonuniformities can drive low-frequency perturbations on the surface of the spherical target, which grow as it implodes and reduce the implosion performance. At later times, though, the plasma corona has sufficiently expanded to smooth out radiation nonuniformities. But it also offers a favorable terrain for CBET to occur. Igumenshchev et al.’s proposal is to drive the first few nanoseconds of the laser pulse with large spots that are roughly the same diameter as the size of the target; then, at later times, switch to spot sizes that are ∼30–40% smaller, in order to reduce CBET and maintain good energy coupling to the target. The authors show, using 2D hydrodynamics simulations, that tailoring the spot size in this way yields an implosion performance that is almost as good as the ideal case: a purely spherically symmetric implosion.
How would this tuning of the spot size be achieved in practice? The authors imagine a new type of optical element called a “zooming phase plate,” which would produce a different spot size depending on whether a laser lands on the central part of the plate or the outer edges. With such a plate, it would be possible to send two consecutive laser pulses toward the plasma: a first pulse that hits only the outer area of the plate and produces a large spot on the target, and a second pulse that covers the center of the plate and produces a 30–40% smaller spot on the target.
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