615 research outputs found
Saturation of fishbone instability by self-generated zonal flows in tokamak plasmas
Gyrokinetic simulations of the fishbone instability in DIII-D tokamak plasmas
find that self-generated zonal flows can dominate the nonlinear saturation by
preventing coherent structures from persisting or drifting in the energetic
particle phase space with mode down-chirping. Results from the simulation with
zonal flows agree quantitatively, for the first time, with experimental
measurements of the fishbone saturation amplitude and energetic particle
transport. Moreover, the suppression of the microturbulence by fishbone-induced
zonal flows is likely responsible for the formation of an internal transport
barrier that was observed after fishbone bursts in this DIII-D experiment.
Finally, gyrokinetic simulations of a related ITER baseline scenario show that
the fishbone induces insignificant energetic particle redistribution and may
enable high performance scenarios in ITER burning plasma experiments
Observation of confined current ribbon in JET plasmas
we report the identification of a localised current structure inside the JET
plasma. It is a field aligned closed helical ribbon, carrying current in the
same direction as the background current profile (co-current), rotating
toroidally with the ion velocity (co-rotating). It appears to be located at a
flat spot in the plasma pressure profile, at the top of the pedestal. The
structure appears spontaneously in low density, high rotation plasmas, and can
last up to 1.4 s, a time comparable to a local resistive time. It considerably
delays the appearance of the first ELM.Comment: 10 pages, 6 figure
Aerodynamic investigations of ventilated brake discs.
The heat dissipation and performance of a ventilated brake disc strongly depends
on the aerodynamic characteristics of the flow through the rotor passages. The
aim of this investigation was to provide an improved understanding of ventilated
brake rotor flow phenomena, with a view to improving heat dissipation, as well
as providing a measurement data set for validation of computational fluid
dynamics methods. The flow fields at the exit of four different brake rotor
geometries, rotated in free air, were measured using a five-hole pressure probe
and a hot-wire anemometry system. The principal measurements were taken using
two-component hot-wire techniques and were used to determine mean and unsteady
flow characteristics at the exit of the brake rotors. Using phase-locked data
processing, it was possible to reveal the spatial and temporal flow variation
within individual rotor passages. The effects of disc geometry and rotational
speed on the mean flow, passage turbulence intensity, and mass flow were
determined. The rotor exit jet and wake flow were clearly observed as
characterized by the passage geometry as well as definite regions of high and
low turbulence. The aerodynamic flow characteristics were found to be reasonably
independent of rotational speed but highly dependent upon rotor geometry
LOCUST-GPU predictions of fast-ion transport and power loads due to ELM-control coils in ITER
The LOCUST-GPU code has been applied to study the fast-ion transport and loss caused by resonant magnetic perturbations in the high-performance Q= 10 ITER baseline scenario. The unique computational efficiency of the code is exploited to calculate the impact of the application of the ITER ELM-control-coil system on neutral beam heating efficiency, as well as producing detailed predictions of the resulting plasma-facing component power loads, for a variety of operational parameters—the toroidal mode number n0, mode spectrum and absolute toroidal phase of the imposed perturbation. The feasibility of continually rotating the perturbations is assessed and shown to be effective at reducing the time-averaged power loads.Through careful adjustment of the relative phase of the applied perturbation in the three rows of coils, peak power loads are found to correlate with reductions in NBI heating efficiency for n= 3 fields. Adjusting the phase this way can increase total NBI system efficiency by approximately 2-3% and reduce peak power loads by up to 0.43 MWm-2. From the point of view of fast-ion confinement, n= 3 ELM control fields are preferred overall to n= 4 fields.In addition, the implementation of 3D magnetic fields in LOCUST is also verified by comparison with the SPIRAL code for a DIII-D discharge with ITER-similar shaping and n= 3 perturbation
- …