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Particle
transport in the edge plasma and scrape-off layer will play a key role
in the performance and operation of a tokamak fusion reactor: setting
the width of the scrape-off layer density profile and its impurity screening
characteristics, regulating the energetic particle fluxes onto first-wall
components and associated impurity generation rates, and determining
the effectiveness of the divertor in receiving particle exhaust and
controlling neutral pressures in the main-chamber. The processes which
govern particle transport involve plasma turbulence, phenomena which
can not yet be reliably computed from a first-principles numerical simulation.
Thus, in order to project to a reactor-scale experiment, such as ITER,
one must first develop an understanding of particle transport phenomena
based on experimental measurements in existing plasma fusion devices.
Over the past few years of research, a number of fundamental advances
in the understanding of the cross-field particle transport physics have
occurred, replacing crude, incorrect, and often misleading transport
models such as the “constant diffusion coefficient” model
with a more appropriate description of the phenomenon. It should be
noted that this description applies to transport processes in the absence
of ELM phenomenon, i.e., physics underlying the “background”
plasma state. In this letter, we first review the experimental support
for this understanding which is based extensively on data from L-mode
discharges and from H-mode discharges at time intervals without ELMs.
We then comment on its implications for ITER. |
