5,597 research outputs found
Decay of geodesic acoustic modes due to the combined action of phase mixing and Landau damping
Geodesic acoustic modes (GAMs) are oscillations of the electric field whose
importance in tokamak plasmas is due to their role in the regulation of
turbulence. The linear collisionless damping of GAMs is investigated here by
means of analytical theory and numerical simulations with the global
gyrokinetic particle-in-cell code ORB5. The combined effect of the phase mixing
and Landau damping is found to quickly redistribute the GAM energy in
phase-space, due to the synergy of the finite orbit width of the passing ions
and the cascade in wave number given by the phase mixing. When plasma
parameters characteristic of realistic tokamak profiles are considered, the GAM
decay time is found to be an order of magnitude lower than the decay due to the
Landau damping alone, and in some cases of the same order of magnitude of the
characteristic GAM drive time due to the nonlinear interaction with an ITG
mode. In particular, the radial mode structure evolution in time is
investigated here and reproduced quantitatively by means of a dedicated initial
value code and diagnostics.Comment: Submitted to Phys. Plasma
On the mutual effect of ion temperature gradient instabilities and impurity peaking in the reversed field pinch
The presence of impurities is considered in gyrokinetic calculations of ion
temperature gradient (ITG) instabilities and turbulence in the reversed field
pinch device RFX-mod. This device usually exhibits hollow Carbon/Oxygen
profiles, peaked in the outer core region. We describe the role of the
impurities in ITG mode destabilization, and analyze whether ITG turbulence is
compatible with their experimental gradients.Comment: 19 pages, 9 figures, accepted for publication in Plasma Phys.
Control. Fusio
Effect of turbulence on electron cyclotron current drive and heating in ITER
Non-linear local electromagnetic gyrokinetic turbulence simulations of the
ITER standard scenario H-mode are presented for the q=3/2 and q=2 surfaces. The
turbulent transport is examined in regions of velocity space characteristic of
electrons heated by electron cyclotron waves. Electromagnetic fluctuations and
sub-dominant micro-tearing modes are found to contribute significantly to the
transport of the accelerated electrons, even though they have only a small
impact on the transport of the bulk species. The particle diffusivity for
resonant passing electrons is found to be less than 0.15 m^2/s, and their heat
conductivity is found to be less than 2 m^2/s. Implications for the broadening
of the current drive and energy deposition in ITER are discussed.Comment: Letter, 5 pages, 5 figures, for submission to Nuclear Fusio
Understanding the core density profile in TCV H-mode plasmas
Results from a database analysis of H-mode electron density profiles on the
Tokamak \`a Configuration Variable (TCV) in stationary conditions show that the
logarithmic electron density gradient increases with collisionality. By
contrast, usual observations of H-modes showed that the electron density
profiles tend to flatten with increasing collisionality. In this work it is
reinforced that the role of collisionality alone, depending on the parameter
regime, can be rather weak and in these, dominantly electron heated TCV cases,
the electron density gradient is tailored by the underlying turbulence regime,
which is mostly determined by the ratio of the electron to ion temperature and
that of their gradients. Additionally, mostly in ohmic plasmas, the Ware-pinch
can significantly contribute to the density peaking. Qualitative agreement
between the predicted density peaking by quasi-linear gyrokinetic simulations
and the experimental results is found. Quantitative comparison would
necessitate ion temperature measurements, which are lacking in the considered
experimental dataset. However, the simulation results show that it is the
combination of several effects that influences the density peaking in TCV
H-mode plasmas.Comment: 23 pages, 12 figure
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