The comparison between otherwise-identical D and DT plasmas is a major strength of this work. This rise in electron temperature without external heating represents the first direct observation of alpha heating in a magnetically confined plasma. They found that the temperature of electrons decreased in the D plasma but kept climbing in the DT plasma. They then compared the two plasmas in the period immediately after this heating had been switched off. The researchers operated the JET tokamak using a DT plasma, or a comparable D-only plasma, and applied external heating in the form of an injected beam of neutral particles. The new results from Kiptily and colleagues show that this goal was achieved (Fig. However, the successes of those early experiments were questioned, making a less ambiguous demonstration of alpha heating (plasma heating induced by alpha particles) a primary goal of a 2021 DT campaign at JET. The first attempts at realizing tokamak self-heating came at the Tokamak Fusion Test Reactor and at the JET tokamak when these two devices were first operated with DT fuel in the 1990s. Demonstrating self-heating and burning-plasma physics in a tokamak is a key goal for fusion researchers. The tokamak is widely considered as a leading candidate for a fusion reactor, with large ongoing tokamak constructions including the multidecade international-collaboration project ITER and the more recently conceived and heavily privately funded SPARC. Magnetic confinement fusion, by contrast, relies on keeping the hot fusing plasma sustainably contained by having the plasma ions and electrons spiral along magnetic-field lines-most popularly in a donut-shaped vessel called a tokamak. A reactor based on this approach would have to be pulsed, with fusion implosions repeating several times per second. In inertial confinement fusion, lasers compress a tiny pellet of DT fuel to high density and temperature, relying on the inertia of the assembled material to keep it together long enough for fusion reactions to propagate through the fuel. The goal of achieving energy production from controlled fusion on Earth relies on the created alpha particles remaining in the plasma and heating the fusion fuel to keep the reactions going, while the kinetic energy of neutrons escaping the plasma is converted to electrical energy. When D and T fuse, an alpha particle (a helium-4 nucleus) and a neutron are generated, carrying the released energy in the form of kinetic energy. The fusion of two heavy hydrogen isotopes-deuterium (D) and tritium (T)-presents the most promising path to a fusion reactor, both because of the relative ease in getting these isotopes to fuse and because of the large amount of energy released in each reaction. This goal could now be within reach thanks to direct evidence for fusion-induced heating of electrons in magnetically confined plasmas obtained by Vasily Kiptily and colleagues at the UK-based Joint European Torus (JET) facility. This step has recently been demonstrated for inertially confined plasmas (see Research News: Ignition First in a Fusion Reaction) but has so far remained elusive for magnetically confined ones. A key step in the race toward realizing the dream of such a reactor is the creation of a burning plasma-one in which the fusion reactions themselves supply most of the heating needed to keep the plasma at fusion-relevant temperatures. ×Ī fusion reactor would generate electricity using the energy released by nuclear-fusion reactions occurring in a plasma. The free neutron escaped the tokamak, whereas the alpha particle remained inside the vessel and heated electrons that spiraled along magnetic-field lines (green). In each fusion reaction, a deuterium nucleus (one proton plus one neutron) merged with a tritium nucleus (one proton plus two neutrons) to generate a free neutron and an alpha particle (two protons plus two neutrons). The red, blue, and pink balls represent protons, neutrons, and electrons, respectively. APS/ Alan Stonebraker Figure 1: Kiptily and colleagues observed fusion reactions in a donut-shaped vessel called a tokamak (gray).
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