A Collaborative National Center for Fusion & Plasma Research
March 29, 2017,
4:15pm to 5:30pm

MBG Auditorium

COLLOQUIUM: The long and the short of collisions in strong magnetic fields

Professor Daniel Dubin
University of California - San Diego

For plasmas in a strong magnetic field, like-particle collisions approximately-conserve a cyclotron adiabatic invariant that limits the sharing of energy between cyclotron motion and motion parallel to the field. In this talk I will consider two experiments that measure different aspects of these strongly-magnetized collisions. Short range collisions, with impact parameters less than the cyclotron radius, can break the cyclotron invariant. I will discuss how these collisions can be used to measure the Salpeter enhancement factor[1] for nuclear reaction rates, even though there are no nuclear reactions happening in the experiment. This enhancement factor is important in a range of astrophysical settings, but has not been previously measured. A second experiment probes long-range collisions, with impact parameters large compared to the cyclotron radius. I will show that these collisions enhance several transport coefficients, including thermal conduction and viscosity, as well as the collisional slowing of charges moving parallel to the magnetic field. This enhanced slowing was observed for the first time this year [2] in an experiment that measures the collisional damping of magnetized plasma oscillations.
[1]E.E. Salpeter, Aust. J. Phys. 7, 373 (1954)
[2] M. Affolter et al., Phys. Rev. Lett 117, 155001 (2016)

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