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James D. Callen
Kerst Professor of Engineering Physics and Physics

James Callen is the recipient of the 2003 Byron Bird Award for his contributions to the theoretical prediction and experimental identification of neoclassical tearing modes (NTMs) in controlled fusion plasmas. NTMs are now recognized as a major potential obstacle to operation of a tokamak fusion reactor.

A tokamak is essentially a toroidal magnetic bottle used for holding high-temperature plasma. The magnetic fields are produced by a combination of currents flowing in external coils and currents flowing within the plasma itself. Researchers have been studying tokamaks for more than 50 years with the goal of creating a suitable magnetic bottle for a stable, sustained fusion reaction — a problem that has been compared to trying to hold Jell-O with rubber bands.

NTMs are one of the most serious instabilities in a steady-state high-performance tokamak plasma. The instability produces macroscopically large deformations which lead to confinement degradation and, in some cases, termination of the plasma discharge. Callen developed the theory of these instabilities in a series of papers primarily with his postdoctoral students in 1985. Callen, Chris Hegna (now Associate Professor of Engineering Physics) and other co-authors then detailed the phenomenology in a series of papers throughout the early 1990s. Callen, and Z. Chang also developed models for quantifying the energy confinement degradation due to magnetic islands and applied the theory to Tokamak Fusion Test Reactor (TFTR) plasmas. Subsequently, Chang, Callen, Hegna and colleagues at Princeton reported definitive experimental evidence of the instability in a 1995 paper, now considered a classic. To a large degree, most of the present research on NTMs builds upon the original interpretation and theoretical insights introduced by Callen and his postdocs.

Callen earned his PhD in nuclear engineering in 1968 from Massachusetts Institute of Technology and did postdoctoral research at the Institute for Advanced Study, Princeton, New Jersey. After three years on the MIT faculty and seven years with Oak Ridge National Laboratory, he joined the University of Wisconsin-Madison's Departments of Engineering Physics (then Nuclear Engineering) and Physics in 1979. Callen is also a member of the National Academy of Engineering.