4 research outputs found
Nonlinear Energetic Particle Transport in the Presence of Multiple Alfvenic Waves in ITER
This work presents the results of a multi mode ITER study on Toroidal Alfven
Eigenmodes, using the nonlinear hybrid HAGIS-LIGKA model. It is found that main
conclusions from earlier studies of ASDEX Upgrade discharges can be transferred
to the ITER scenario: global, nonlinear effects are crucial for the evolution
of the multi mode scenario. This work focuses on the ITER 15 MA baseline
scenario with with a safety factor at the magnetic axis of 0.986. The
least damped eigenmodes of the system are identified with the gyrokinetic,
non-perturbative LIGKA solver, concerning mode structure, frequency and
damping. Taking into account all weakly damped modes that can be identified
linearly, nonlinear simulations with HAGIS reveal strong multi mode behavior:
while in some parameter range, quasi-linear estimates turn out to be reasonable
approximations for the nonlinearly relaxed energetic particle profile, under
certain conditions low-n TAE branches can be excited. As a consequence, not
only grow amplitudes of all modes to (up to orders of magnitude) higher values
compared to the single mode cases but also, strong redistribution is triggered
in the outer radial area between 0.6 and 0.85, far above
quasi-linear estimates.Comment: 14 pages, 20 figures; To be published as special issue in PPCF
12/2015 for EPS Lisbon invited tal
Nonlinear alfv\'enic fast particle transport and losses
Magnetohydrodynamic instabilities like Toroidal Alfv\'en Eigenmodes or
core-localized modes such as Beta Induced Alfv\'en Eigenmodes and Reversed
Shear Alfv\'en Eigenmodes driven by fast particles can lead to significant
redistribution and losses in fusion devices. This is observed in many ASDEX
Upgrade discharges. The present work aims to understand the underlying
resonance mechanisms, especially in the presence of multiple modes with
different frequencies. Resonant mode coupling mechanisms are investigated using
the drift kinetic HAGIS code [Pinches 1998]. Simulations were performed for
different plasma equilibria, in particular for different q profiles, employing
the availability of improved experimental data. A study was carried out,
investigating double-resonant mode coupling with respect to various overlapping
scenarios. It was found that, depending on the radial mode distance,
double-resonance is able to enhance growth rates as well as mode amplitudes
significantly. Small radial mode distances, however can also lead to strong
nonlinear mode stabilization of a linear dominant mode. With the extended
version of HAGIS, losses were simulated and directly compared with experimental
loss measurements. The losses' phase space distribution as well as their
ejection signal is consistent with experimental data. Furthermore, it allowed
to characterize them as prompt, resonant or stochastic. It was found that
especially in multiple mode scenarios (with different mode frequencies),
abundant incoherent losses occur in the lower energy range, due to a broad
phase-space stochastization. The incoherent higher energetic losses are
"prompt", i.e. their initial energy is too large for confined orbits.Comment: 7 pages, 6 figures, Reviewed Conference Proceedings (Joint Varenna -
Lausanne International Workshop on the Theory of Fusion Plasmas) to be
published in IOP's "Journal of Physics: Conference Series
Multi-mode Alfv\'enic Fast Particle Transport and Losses: Numerical vs. Experimental Observation
In many discharges at ASDEX Upgrade fast particle losses can be observed due
to Alfv\'enic gap modes, Reversed Shear Alfv\'en Eigenmodes or core-localized
Beta Alfv\'en Eigenmodes. For the first time, simulations of experimental
conditions in the ASDEX Upgrade fusion device are performed for different
plasma equilibria (particularly for different, also non-monotonic q profiles).
The numerical tool is the extended version of the HAGIS code [Pinches'98,
Br\"udgam PhD Thesis, 2010], which also computes the particle motion in the
vacuum region between vessel wall in addition to the internal plasma volume.
For this work, a consistent fast particle distribution function was implemented
to represent the strongly anisotropic fast particle population as generated by
ICRH minority heating. Furthermore, HAGIS was extended to use more realistic
eigenfunctions, calculated by the gyrokinetic eigenvalue solver LIGKA
[Lauber'07]. The main aim of these simulations is to allow fast ion loss
measurements to be interpreted with a theoretical basis. Fast particle losses
are modeled and directly compared with experimental measurements
[Garc\'ia-Mu\~noz'10]. The phase space distribution and the mode-correlation
signature of the fast particle losses allows them to be characterized as
prompt, resonant or diffusive (non-resonant). The experimental findings are
reproduced numerically. It is found that a large number of diffuse losses occur
in the lower energy range (at around 1/3 of the birth energy) particularly in
multiple mode scenarios (with different mode frequencies), due to a phase space
overlap of resonances leading to a so-called domino [Berk'95] transport
process. In inverted q profile equilibria, the combination of radially extended
global modes and large particle orbits leads to losses with energies down to
1/10th of the birth energy.Comment: 16 Pages, 17 Figure