This paper presents a pedagogical review of the physics of mesoscopic transport events and their role in the breakdown of Fick’s Law for turbulent transport in magnetically confined plasma. It is now ...clear that the conventional picture of localized turbulence and quasi-linear calculation of fluxes fails to address and account for the phenomenology of tokamak transport. One key issue is the observed departure from the expected gyro-Bohm transport scaling. The causes of this breakdown of Fickian thinking include turbulent avalanching and pulse propagation (turbulence spreading). Both are mesoscopic transport events, and both tend to de-localize the flux–gradient relation. Turbulence spreading is the process of self-scattering and expansion of a slug or other local exciton of turbulence. Spreading is described by theoretically-motivated, phenomenological reaction–diffusion models for the turbulence activity (intensity) field, much in the spirit of Ginzburg–Landau theory. Such models imply that spreading will occur by propagation of intensity fronts. After discussing the basic theory, this paper presents several critical tests of turbulence spreading models using gyrokinetic simulation. Applications include rho-star scaling, penetration of transport barriers and core-edge coupling. Relevant experiment–theory comparisons are addressed, as well. Avalanching refers to a process whereby correlated topplings of nearby localized cells overturn sequentially and drive a burst of transport. Avalanching is a process intrinsic to systems that support a broad range of scales
l
between a cell size Δ and system size
L
,
i.e
. Δ <
l
<
L
. Avalanching is also a natural way to produce transport events on scales that exceed the cell size or correlation length. Therefore, the PDF (probability distribution function) of avalanches as a function of
l
is a crucial quantity, necessary for predicting confinement in a system like ITER, with a very large-scale separation between
L
and Δ. Avalanching emerged from the theory of selforganized criticality but is a more general phenomenon. The paper traces the intellectual prehistory of avalanching through the advent of self-organized criticality. Special focus is devoted to reduced continuum models of avalanching. The physics of avalanching in confined plasma is discussed in detail, via several multi-faceted comparisons to flux-driven fluid and gyrokinetic simulations. The dominance of bursty, large transport events in the flux is identified. Evidence for avalanching in basic and confinement experiments is summarized. The paper concludes with sections on selected special topics, a discussion of the relation between turbulence spreading and avalanching, and a list of possible future directions. Throughout the paper, an effort is made to set fusion theory and phenomenology in the context of ideas discussed in the broader scientific community.
With the advent of direct laser writing using two-photon polymerization, the generation of high-resolution three-dimensional microstructures has increased dramatically. However, the development of ...stimuli-responsive photoresists to create four-dimensional (4D) microstructures remains a challenge. Herein, we present a supramolecular cholesteric liquid crystalline photonic photoresist for the fabrication of 4D photonic microactuators, such as pillars, flowers, and butterflies, with submicron resolution. These micron-sized features display structural color and shape changes triggered by a variation of humidity or temperature. These findings serve as a roadmap for the design and creation of high-resolution 4D photonic microactuators.
We derive a new nonlinear criterion for the occurrence of fast relaxation (crash) events at the edge of high-confinement-mode plasmas. These fast relaxation events called ELMs (edge-localized modes) ...evolve from ideal magnetohydrodynamics (MHD) instabilities, but the crash is not due only to linear physics. We show that for an ELM crash to occur, the coherence time of the relative phase between potential and pressure perturbations must be long enough to allow growth to large amplitude. This phase coherence time is determined by both linear and nonlinear dynamics. An ELM crash requires that the instability growth rate exceed a critical value, i.e., gamma > gamma sub(c) where gamma sub(c)is set by 1/tau sub(c)and tau sub(c)is the phase coherence time. For 0 < gamma < gamma sub(c) MHD turbulence develops and drives enhanced turbulent transport. The results indicate that the shape of the growth rate spectrum gamma (n) is important to whether the result is a crash or turbulence. We demonstrate that ELMs can be mitigated by reducing the phase coherence time without changing linear instability. These findings also offer an explanation of the occurrence of ELM-free H-mode regimes.
Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea that the bulk of galactic cosmic rays is accelerated in such remnants by a Fermi mechanism, also ...known as diffusive shock acceleration. However, the W44 expands into weakly ionized dense gas, and so a significant revision of the mechanism is required. Here, we provide the necessary modifications and demonstrate that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is calculated and successfully fitted to the Fermi Observatory data. The parent proton spectrum is best represented by a classical test particle power law ∝E(-2), steepening to E(-3) at E(br)≈7 GeV due to deteriorated particle confinement.
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