Magnetic fields have been extensively studied for their ability to confine highly energetic plasmas, most notably for the purpose of sustaining nuclear fusion reactions. The efficiency of magnetic confinement is given by β—the plasma pressure divided by the magnetic field pressure—since the fusion power output increases as β. One of the first classes of magnetic configurations to be studied were “cusp” configurations for their inherent stability at high β. In the 1950s, Harold Grad at New York University conjectured that a cusp configuration might exhibit dramatically better plasma confinement at high β (β∼1) than low β (β β cusp can demonstrate better confinement of high-energy plasma electrons.

With the use of a high-energy (7 keV) electron beam as a diagnostic, we measure the time-resolved confinement property of the cusp via hard-x-ray intensity as β increases from zero to the order of unity and then deceases back toward zero. Abrupt changes in the time derivative of the x-ray intensity clearly show the transition from poor confinement to enhanced confinement and back to poor confinement in a single shot. The estimated improvement in the confinement time is about 50 times or more, consistent with Grad’s theoretical predictions. Our experimental results also provide tantalizing evidence that once the transition to improved confinement occurs, the improved confinement persists even after a significant reduction in β. These results represent critical progress toward an understanding of the plasma dynamics in a high-β cusp; they resolve a key impediment to using high-energy electron beams for plasma heating.

We expect that our work will pave the way for small and efficient fusion reactors that employ high-β plasmas.