169 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
Gyrokinetic simulations of neoclassical electron transport and bootstrap current generation in tokamak plasmas in the TRIMEG code
For magnetic confinement fusion in tokamak plasmas, some of the limitations
to the particle and energy confinement times are caused by turbulence and
collisions between particles in toroidal geometry, which determine the
"anomalous" and the neoclassical transport, respectively. In this work, we
focus on the implementation of neoclassical physics in the gyrokinetic code
TRIMEG, which is a TRIangular MEsh-based Gyrokinetic code that can handle both
the closed and open field line geometries of a divertor tokamak. We report on
the implementation of a simplified Lorentz collision operator in TRIMEG. Since
the code uses an unstructured mesh, a procedure for calculating the flux
surface averages of particle and energy fluxes and the bootstrap current is
derived without relying on the poloidal coordinate, which is useful also for
other simulations in unstructured meshes. With the newly implemented collision
operator, we study electron transport and bootstrap current generation for
various simplified and realistic geometries. In comparison to neoclassical
theory, good agreement is obtained for the large aspect ratio case regarding
the particle and energy fluxes as well as the bootstrap current. However, some
discrepancies are observed at moderate aspect ratio and for a case with the
realistic geometry of the ASDEX Upgrade tokamak. These deviations can be
explained by different treatments and approximations in theory and simulation.
In this paper, we demonstrate the capability to calculate the electron
transport and bootstrap current generation in TRIMEG, which will allow for the
self-consistent inclusion of neoclassical effects in gyrokinetic simulations in
the future
Full and gyrokinetic particle simulations of Alfv\'en waves and energetic particle physics
In this work, we focus on the development of the particle-in-cell scheme and
the application to the studies of Alfv\'en waves and energetic particle physics
in tokamak plasmas. The and full schemes are formulated on the
same footing adopting mixed variables and the pullback scheme for
electromagnetic problems. The TRIMEG-GKX code [Lu et al. J. Comput. Phys. 440
(2021) 110384] has been upgraded using cubic spline finite elements and full
and schemes. The EP-driven TAE has been simulated for the
ITPA-TAE case featured by a small electron skin depth , which is a challenging parameter regime of
electromagnetic simulations, especially for the full model. The simulation
results using the scheme are in good agreement with previous work.
Excellent performance of the mixed variable/pullback scheme has been observed
for both full and schemes. Simulations with mixed full EPs
and electrons and thermal ions demonstrate the good features of this
novel scheme in mitigating the noise level. The full scheme is a natural
choice for EP physics studies which allows a large variation of EP profiles and
distributions in velocity space, providing a powerful tool for kinetic studies
using realistic experimental distributions related to intermittent and
transient plasma activities.Comment: 27 pages, 8 figure
Observation of many-body long-range tunneling after a quantum quench
Quantum tunneling constitutes one of the most fundamental processes in
nature. We observe resonantly-enhanced long-range quantum tunneling in
one-dimensional Mott-insulating Hubbard chains that are suddenly quenched into
a tilted configuration. Higher-order many-body tunneling processes occur over
up to five lattice sites when the tilt per site is tuned to integer fractions
of the Mott gap. Starting from a one-atom-per-site Mott state the response of
the many-body quantum system is observed as resonances in the number of doubly
occupied sites and in the emerging coherence in momentum space. Second- and
third-order tunneling shows up in the transient response after the tilt, from
which we extract the characteristic scaling in accordance with perturbation
theory and numerical simulations.Comment: 22 pages, 7 figure
Alpha particle driven Alfv\'enic instabilities in ITER post-disruption plasmas
Fusion-born alpha particles in ITER disruption simulations are investigated
as a possible drive of Alfv\'enic instabilities. The ability of these waves to
expel runaway electron (RE) seed particles is explored in the pursuit of a
passive, inherent RE mitigation scenario. The spatiotemporal evolution of the
alpha particle distribution during the disruption is calculated using the
linearized Fokker-Planck solver CODION coupled to a fluid disruption
simulation. These simulations are done in the limit of no alpha particle
transport during the thermal quench, which can be seen as a most pessimistic
situation where there is also no RE seed transport. Under these assumptions,
the radial anisotropy of the resulting alpha population provides free energy to
drive Alfv\'enic modes during the quench phase of the disruption. We use the
linear gyrokinetic magnetohydrodynamic code LIGKA to calculate the Alfv\'en
spectrum and find that the equilibrium is capable of sustaining a wide range of
modes. The self-consistent evolution of the mode amplitudes and the alpha
distribution is calculated utilizing the wave-particle interaction tool HAGIS.
Intermediate mode number () Toroidal Alfv\'en Eigenmodes (TAEs)
are shown to saturate at an amplitude of up to \% in
the spatial regimes crucial for RE seed formation. We find that the mode
amplitudes are predicted to be sufficiently large to permit the possibility of
significant radial transport of runaway electrons
The impact of fusion-born alpha particles on runaway electron dynamics in ITER disruptions
In the event of a tokamak disruption in a D-T plasma, fusion-born alpha
particles take several milliseconds longer to thermalise than the background.
As the damping rates drop drastically following the several orders of
magnitudes drop of temperature, Toroidal Alfven Eigenmodes (TAEs) can be driven
by alpha particles in the collapsing plasma before the onset of the current
quench. We employ kinetic simulations of the alpha particle distribution and
show that the TAEs can reach sufficiently strong saturation amplitudes to cause
significant core runaway electron transport in unmitigated ITER disruptions. As
the eigenmodes do not extend to the plasma edge, this effect leads to an
increase of the runaway electron plateau current. Mitigation via massive
material injection however changes the Alfven frequency and can lead to mode
suppression. A combination of the TAE-caused core runaway electron transport
with other perturbation sources could lead to a drop of runaway current in
unmitigated disruptions
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