Tuesday December 1, 2020 at 4:00pm, Lothar Schmitz, Department of Physics & Astronomy, UCLA, "Reducing the L-H Transition Power Threshold in ITER—What Can We Learn from Microscopic Transition Physics?" Contact Meg Murphy for Zoom meeting link and information.
Abstract: High confinement mode (H-mode) operation is essential for the International Thermonuclear Experimental Reactor (ITER) and future burning plasmas, yet no predictive physics-based model exists so far for the required L-H transition threshold power. We demonstrate here on DIII-D that fast (0.1-1 ms) electric field transients, quantitatively consistent with the radial polarization current, initiate the transition once the transient E×B shearing rate exceeds the plasma frame turbulence decorrelation rate [1]. Edge turbulence is subsequently suppressed as the Reynolds stress increases, allowing the edge transport barrier to form.
Significant differences in transition dynamics are observed between Helium and Hydrogen plasmas (important for pre-nuclear physics operation in ITER). In recent experiments, we have demonstrated that the observed high power threshold in hydrogen (a substantial challenge for ITER) could be reduced by Helium dilution. Initial experiments also indicate that the L-H power threshold can be reduced at low ion collisionality via Neoclassical Toroidal Viscosity (NTV) from applied n=3 non-resonant magnetic fields (NRMF).
[1] L. Schmitz et al., Phys. Rev. Lett. 108, 155002 (2012).
Tuesday November 17, 2020 at 3:00pm, Nuno F. Loureiro, MIT, Boston MA, "New Insights into Plasma Turbulence." Contact Meg Murphy for Zoom meeting link and information.
Abstract: The current understanding of plasma turbulence in the MHD regime suggests that turbulent eddies become progressively more elongated structures in the plane perpendicular to the local mean magnetic field. Using recent results from reconnection theory, we argue that such eddies must inevitably fall prey to the tearing instability. As a result, there is a transition, in the inertial range, to a tearing (reconnection)-mediated turbulent cascade, where the spectral scaling k^{-11/5} is predicted. These results can be extended to collisionless plasmas; in the different regimes of interest that have been explored, the magnetic energy spectral scaling is invariably found to be in the range k^{-8/3} to k^{-3}, consistent with many observations.
SoCal Plasma Zoom Seminar: Tuesday November 3, 2020 at 9:00am, David Hughes (Leeds University UK), "Inertialess Dynamos in Rotating Convection." Contact Meg Murphy for Zoom meeting link and information. Co-sponsored by UC San Diego, UCI, UCLA.