Electric and Magnetic Separatrices in NNPs : 20 years of Wave Couplings, Dissipation, and Transport

Over the past 50 years, NNP experiments have provided the research flexibility to follow scientific opportunities based on the internal logic of plasma theory, leading to

quantitative tests, and significant connections with other branches of physics. This talk will give an brief survey of the past 20 years of research at UCSD on "neoclassical" transport effects, where particle diffusion arises from drift-orbit steps which are much larger than the particle cyclotron radii. Experimentally, an applied theta-asymmetry (such as a magnetic "tilt") creates tailored drift orbits; separately applied electric or magnetic "ripples" create energy separatrices between distinct populations of z-trapped particles; and separatrix crossings generate filamentation of the distinct velocity functions, giving entropic dissipation and transport. The separatrix crossings can be thought of as due to the inevitable "random" particle-particle energy-scattering collisions; or due to "chaotic" changes in the separatrix energy encountered by a particle. The chaotic scatterings can arise from theta-dependent "ruffles" in the separatrix, which may be experimentally controlled; or from temporal changes in the separatrix energy, which may also be controlled, or may be just plain "noisy". A wide variety of experiments have identified the magnetic field scalings for these processes, ranging from B^-1/2 to B^-3. The chaotic processes may be dominant in low collisionality (ν_c) plasmas, and "slosh-driven" (f_sl) heating across a well-diagnosed Θ-symmetric separatrix has been observed⁽¹⁾ at rate ( ν_c f_sl )^1/2 .



1) F. Anderegg et al, PRL 123, 105002 (2019)