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Protons bring fusion into view

28 Feb 2008
Proton radiography experiment at OMEGA

Fusion is a word that rarely crops up in headlines devoted to tackling climate change, and there are good reasons for this. In theory, a fusion power plant could harness the energy released when certain light nuclei bind together to produce the same annual output as a typical coal-fired plant — while burning a million times less fuel by mass and releasing no greenhouse gases. But despite 50 years of research, finding a way to sustain and contain fusion reactions remains a major technological challenge, in part because they require temperatures of hundreds of millions of degrees.

Researchers in the US have now developed an imaging technique that could help bring fusion power to fruition. Richard Petrasso and colleagues at the Massachusettes Institute of Technology and Wolfgang Theobold and colleagues at the University of Rochester have used “proton radiography” to map the electromagnetic structure of the extremely hot, dense plasmas in which fusion reactions take place. The technique has revealed hitherto unseen magnetic and electric fields, and could help researchers to get fusion plasmas to ignite — the key to electricity generation.

Inertial confinement

The MIT-Rochester technique applies to inertial-confinement fusion (ICF), which is one of two possible routes to a fusion reactor. The idea behind ICF is to bombard fuel capsules (typically containing deutrium and tritium) with high-powered laser pulses so that they implode, generating a small volume of hot, dense plasma in which the deutrium and tritium nuclei can overcome their electrical replusion and produce a helium nucleus plus a free neutron. Since these reaction products are lighter than the original nuclei, copious energy is released via Einstein’s mass–energy equivalence.

The other, more advanced, approach to fusion power involves magnetic confinement. Here, the fuel is heated slowly and the plasma confined in large, doughnut shaped devices called tokamaks using strong magentic fields. The culmination of this approach is the International Thermonuclear Experimental Reactor (ITER) currently being built at Cadarache in France and due to switch on within a decade.

‘Backlighter’ capsule

In the new work, the MIT and Rochester researchers used 36 beams at the high-powered OMEGA laser facility at Rochester to symmetrically implode ICF fuel capsules (Science 319 1223). The same beams also struck a different capsule 1 cm away which was filled with deuterium and helium-3 gas. Protons released from this “backlighter” capsule all have the same (known) energy, so by measuring the deflection of the positively charged protons that had transited some plasma the team was able to map the electromagentic fields present in ICF implosions for the first time.

The resulting images reveal complex magnetic fields comprising many radial filaments with an enormous (60 T) field strength. In addition, the team saw evidence for centrally directed electric fields with a strength of about 1 GV/m close to the capsule surface. Although the team does not understand the origin or evolution of the structures, Petrasso thinks that such large fields could potentially impact the movement of charged particles, plasma and energy and therefore affect the symmetry of the capsule implosion.

“In order to to ignite the plasma we need to compress the capsule radius by a factor of about 30 in a near spherically symmetric fashion,” he says. “The coherent electric field could give us a sensitive probe on the pressure gradient within the capsule which might help us tailor the implosion to achieve ignition,” he says. Once ignition is achieved, which will likely be at the National Ignition Facility at the Lawrence Livermore National Laboratory in 2010-2012, Petrassso says the next challenge will to be to address how such energy could be used for generating economic electrical power.

Unexpected fields

“This paper is an outstanding example of the need for fundamental measurements in complex scientific subjects,” says plasma expert Paul Drake at the University of Michigan, who was not involved in the work. “Applying proton radiography to ICF implosions has revealed large magnetic fields under conditions where no one expected them, and large electric fields where such fields were previously ignored,” he says. “Understanding the two effects will lead to potentially important improvements in target designs for inertial fusion.”

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