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Friday, 28 October 2011

The Energy that Drives the Stars – Different Technologies for Unique Demands

Lawrence Berkeley National Laboratory
Oct 27, 2011

The NDCX-II accelerator is specifically designed to study warm dense matter. By using an induction accelerator and a neutralized drift compression system, the ion pulse can be shaped to deliver most of its energy to the target surface.

A video simulation of how the NDCX-II accelerator and neutralized drift compression system shape ion pulses to deliver most of their energy on target.


Berkeley Lab, a partner in the Heavy Ion Fusion Sciences Virtual National Laboratory (HIFS VNL) with Lawrence Livermore and the Princeton Plasma Physics Laboratory, has been a leader in developing a special kind of accelerator for experiments aimed at fusion power, called an induction accelerator. The induction principle is like a string of transformers with two windings, where the accelerator beam itself is the second winding. Induction accelerators can handle ions with suitable kinetic energy at higher currents (many more charged particles in the beam), much more efficiently than RF accelerators.

“Choosing the best kind of accelerator and the best kind of target are just the start of the fusion-power challenge,” says Seidl. “To put the right amount of energy on the target in the right pattern, scores of beams are needed – and it must be possible to focus them tightly onto a target, only a few millimeters wide, at a distance of several meters. New targets have to be injected into the chamber five to ten times each second, and the chamber has to be designed so the energy from ignition is recovered. Meanwhile the final beam-focusing elements have to be protected from the explosion debris, the energetic particles, and the x-rays.”

Some of these challenges would be easier to meet if the target didn’t have to be hit from both sides at once. Researchers are encouraged by indications that target burning, hot enough to spark and sustain ignition, can be initiated with fewer beams illuminating the target from only one side.

This side of fusion: warm dense matter

While investigating approaches to heavy-ion fusion, Berkeley Lab and its partners in the HIFS VNL are also tackling other scientific questions related to heating matter to high temperatures with ion beams. The current research program is designed to produce a state of matter that’s on the way to fusion but not as hot – a state perhaps facetiously called warm dense matter, which is “warm” (10,000 degrees Kelvin or so) only by comparison to the millions of degrees typical of fusion reactions.

Not a heavy-ion experiment, the Neutralized Drift Compression Experiment II (NDCX-II) instead uses an induction linear accelerator to accelerate and compress bunches of very light lithium ions to moderate energies. NDCX-II confronts a problem common to all accelerators, the space-charge problem, in which particles of the same charge – positive, in the case of atomic ions – repel each other; the bunches try to blow themselves up. For a given number of ions per bunch, this sets a lower limit on the pulse length.
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