One mystery of magnetic plasma confinement solved

The fusion of nuclei generates prodigious amounts of energy, as in the Sun’s core. Harnessing that energy is the primary goal of researchers who work on tokamaks—large toroidal machines in which a plasma can be confined by magnetic fields and held at high enough temperatures and densities to engender fusion. (See the article by Donald Batchelor in Physics Today, February 2005, page 35 .) Yet the goal remains elusive, in no small part due to myriad instabilities that arise on a multitude of length and time scales, degrading the magnetic confinement of the hot plasmas. In the 1990s researchers discovered that sheared rotation of the plasma and a specific plasma current profile could generate so-called internal transport barriers (ITBs) that inhibit plasma transport across a magnetic surface within a tokamak. The nonlinear, multiscale physics at the heart of ITBs, however, proved to be difficult to unravel. In particular, still unsolved was the mystery of precisely why the ion energy transport was suppressed with ITBs but the particle and momentum transport of the ions was not. Now Gary Staebler and the tokamak team at the DIII-D National Fusion Facility (shown here) operated by General Atomics in San Diego, California, have demonstrated that instabilities at intermediate length scales near the ion gyro-radius (about a millimeter) survive the strong rotation shear and provide the electron energy and momentum transport across the ITB. Using a multiscale quasi-linear transport model, the researchers were able to accurately predict the ion and electron densities and temperatures as well as the rotation of a real ITB experiment at the DIII-D. The upshot is that gyrokinetic turbulence theory works, even in ITBs. (G. M. Staebler et al., Phys. Plasmas 21, 055902, 2014, doi:10.1063/1.4875334 .)