A new look for magnetic-mirror plasma confinement

Controlling thermonuclear fusion requires confining hot plasmas at high densities and high temperatures for sufficiently long periods of time. Tokamaks, such as the one under construction at ITER, provide that confinement through a strong magnetic field that loops around in a closed, toroidal geometry; the plasma’s charged ions and electrons follow the field lines in tight spirals. (See the articles by Don Batchelor, Physics Today, February 2005, page 35 , and by Richard Hazeltine and Stewart Prager, Physics Today, July 2002, page 30 .) A different strategy for magnetic confinement uses a cylindrical solenoid capped at each end by a magnetic mirror, a region of higher field that forces the charged particles to slow and reverse direction. The electron temperature is the main factor limiting the plasma confinement time and thus the power efficiency of a fusion reactor. Concerns over the attainable electron temperature were a factor in magnetic mirrors largely falling out of favor in the 1980s. Peter Bagryansky and colleagues now report more than tripling the electron temperature—up to 900 eV from their previous 250 eV and well above early estimated limits—of the deuterium plasma in their 7-m-long magnetic-mirror reactor at the Budker Institute of Nuclear Physics in Novosibirsk, Russia. Key to the group’s achievement were a novel system that resonantly heated the electrons by high-power microwaves and a new technique to mitigate the plasma’s so-called flute instability. The results, say the researchers, show promise for such uses as developing and testing fusion materials and reprocessing nuclear waste. (P. A. Bagryansky et al., Phys. Rev. Lett. 114, 205001, 2015, doi:10.1103/PhysRevLett.114.205001 .)