Historical research guide · 0440 · 2019

Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment

R. M. Magee · A. Necas · R. Clary · S. Korepanov · S. Nicks · T. Roche · M. C. Thompson · M. W. Binderbauer · T. Tajima

In one minute

Richard Magee and his colleagues at TAE Technologies were running C-2U, a field-reversed configuration — a self-organised ring of plasma held together largely by its own currents — and heating it by firing beams of fast hydrogen atoms through it. The textbook expectation is that those fast ions give their energy to electrons first, which then warm the fuel ions slowly by collision. What the team measured instead was fusion neutrons arriving far too early, and far too many of them, for that route. The fast ions were driving a wave in the plasma outside the core, ringing at harmonics of the ion cyclotron frequency, and the wave handed its energy straight to the deuterium fuel. A neutral particle analyser with 39 energy channels found the fingerprint: a tail of deuterium ions near 12 keV, about one percent of the population but carrying roughly a tenth of the plasma’s heat. The team then confirmed the mechanism by deliberately flattening the beam’s velocity spread and watching the effect fall away.

Connection

Chapter 9 treats the self-organised plasmoid as a real, controllable object, and here is one being run as a fusion device whose internal wave structure does useful work. For chapter 12 it is the cleanest published demonstration that a fusion rate can be raised by engineering the coupling — where the energy goes — rather than by raising the temperature of the whole plasma.

From the source

C-2U is a fast-ion-dominated device by design: the ratio of fast-ion orbit radius to plasma radius is about one, where a tokamak sits at a few percent, and the injected beam power per unit plasma volume is roughly an order of magnitude larger than in the Joint European Torus. By about one millisecond into the discharge the fast-ion pressure is approximately equal to the thermal pressure of the plasma.Introduction, first two paragraphs; Fig. 1
The measured neutron rate exceeds the calculated thermonuclear rate from very early in the discharge, on a timescale far shorter than collisional heating of the fuel ions by the beam can explain. The plasma is pure deuterium and the beams are pure hydrogen, so there is no beam-target contribution, a point the team confirmed experimentally by firing the beams into the deuterated vessel wall and measuring zero neutron signal.Observations, first paragraphs; Fig. 2c
A high-resolution neutral particle energy analyser with 39 energy channels per species measured the cause directly: at about one millisecond a broad deuterium tail appears, centred near 12 keV, holding roughly one percent of the background ion population. At about 12 keV per particle the tail carries around 500 joules, some ten percent of the plasma’s total thermal energy, which means about five percent of the injected beam energy ends up in the tail of the fuel-ion distribution. The bulk temperature, taken from a Gaussian fit, does not change.Observations; Fig. 4; Simulation and theory
A particle-in-cell simulation identifies the wave as the ion Bernstein mode, appearing at many harmonics of the ion cyclotron frequency, and reproduces the effect: in eight microseconds, less than one hundredth of a beam slowing-down time, the fusion reaction rate rises by a factor of 30 above the thermonuclear value. The team read the simulation as showing the mode is more active in the mirror plasma outside the core than within it, which is also why it does not degrade confinement of the core.Simulation and theory; Fig. 5a and Fig. 5c
The model’s key prediction — that free energy in the beam drives the wave — was tested directly. Staggering the primary energies of the five neutral beams flattened the fast-ion velocity distribution, reducing the positive-gradient parameter from 2.5 to 1.8 at the same total injected current and power of 6.8 megawatts. The flattened case showed markedly less activity in the ion cyclotron band and roughly half the fusion enhancement of the peaked case.Simulation and theory, dedicated experiment; Fig. 6
The authors name alpha channelling as the more important application: in a burning plasma the helium ash must be removed to avoid diluting the fuel, but its energy should stay behind. A fast-ion-driven wave that channels energy from alphas directly into fuel ions would do both at once, and this result is the first observation of plasma ions gaining energy from a beam-driven wave in a magnetic fusion device.Discussion, closing paragraphs

The way in

https://doi.org/10.1038/s41567-018-0389-0

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

Published as Nature Physics 15, pages 281 to 286 (March 2019); received 21 February 2017, accepted 30 November 2018, published online 14 January 2019. The article is closed at the publisher and the only licence deposited for the DOI is Springer’s text-and-data-mining user licence, which is not a licence to readers, so no text of the paper is reproduced here. SOURCE REACHED. A copy of the published article is posted openly on a University of California, Irvine faculty site and was read in full for this sheet; the summary, the claims and the locators come from that reading and use the paper’s own section headings and figure numbers. All nine authors write from TAE Technologies, Inc., Foothill Ranch, California. The Crossref creator field for this DOI names only the first author, and the skeleton for this sheet omitted Toshiki Tajima; the full author list is restored above from the article itself.

How to cite it

R. M. Magee, A. Necas, R. Clary, S. Korepanov, S. Nicks, T. Roche, M. C. Thompson, M. W. Binderbauer, T. Tajima (2019) Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment doi:10.1038/s41567-018-0389-0

Where it sits in the curriculum

Fusion machines: pinches, focus devices and inertial drivers