Proper performance prediction for ITER

In his letter in the February 2006 issue of PHYSICS TODAY (page 10), David Montgomery expresses concern about the theoretical basis for predicting the performance of ITER, the planned international magnetic fusion energy experiment. He focuses specifically on the use of ideal (nondissipative) magnetohydrodynamics (MHD) as the zeroth-order description of equilibrium in magnetically confined plasmas. He correctly points out that such an ordering is not generally possible in the mechanics of neutral fluids.

Although in principle that concern could be legitimate, in practice a wide range of experiments has confirmed that the ideal MHD model excellently describes the equilibrium of high-temperature magnetically confined plasmas. Global and detailed local measurements of plasma equilibrium show excellent agreement with ideal theory. Fast instabilities are also well described by ideal theory; slower, dissipative instabilities require more elaborate description, but under the usual operating conditions, the basic ordering that starts from ideal equilibrium is not violated. Ideal MHD calculations are routinely used in major magnetic confinement experiments to assemble and interpret measurements of magnetic fields, pressure gradients, and flow speeds, and to predict stability boundaries. Those MHD codes track experimental behavior consistently and in detail.

Why does ideal MHD provide a legitimate zeroth-order description of magnetically confined plasmas? The answer is tied to researchers' broad success in using magnetic fields to confine plasmas in relatively quiescent states free of large-scale MHD instability. By contrast, the evolution of globally turbulent neutral fluids depends on a dynamic balance between ideal forces, acceleration, and dissipation. In magnetically confined plasmas, the ideal equilibrium forces—j × B, ∇p, and to a lesser degree ρ(v · ∇)v—are measured to balance each other and dominate strongly over acceleration and nonideal dissipation terms, confirming experimentally the ordering procedure that Montgomery questions. Violation of that ordering corresponds to global acceleration on an ideal MHD time scale, and so to rapid plasma loss, which is observed only under circumstances in which ideal MHD stability is compromised. Even Montgomery's model calculations support the use of an ordering that starts from ideal equilibrium MHD, since the results he displays show a very low flow speed, in the range of 1 m/s.

One of the most exciting developments in fusion-energy research over the past decade has been the increasing agreement between magnetized plasma theory and confinement experiments. Recent progress in achieving predictive understanding of confined plasma behavior seems to us, an experimentalist and a theorist, truly remarkable. Theory—including MHD theory, more elaborate fluid models, and fully kinetic plasma descriptions—does not play a "decorative" role today in high-temperature plasma physics; such tools provide an essential guide in the design and execution of experiments. Theory and advanced computing, particularly through the US Department of Energy's Scientific Discovery Through Advanced Computing and Leadership-Class Computing initiatives, are key elements of US preparations for ITER operation.