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...