1,106 research outputs found
Impurity flows and plateau-regime poloidal density variation in a tokamak pedestal
In the pedestal of a tokamak, the sharp radial gradients of density and
temperature can give rise to poloidal variation in the density of impurities.
At the same time, the flow of the impurity species is modified relative to the
conventional neoclassical result. In this paper, these changes to the density
and flow of a collisional impurity species are calculated for the case when the
main ions are in the plateau regime. In this regime it is found that the
impurity density can be higher at either the inboard or outboard side. This
finding differs from earlier results for banana- or Pfirsch-Schl\"uter-regime
main ions, in which case the impurity density is always higher at the inboard
side in the absence of rotation. Finally, the modifications to the impurity
flow are also given for the other regimes of main-ion collisionality.Comment: 15 pages, 5 figures, submitted to Physics of Plasma
Kinetic and finite ion mass effects on the transition to relativistic self-induced transparency in laser-driven ion acceleration
We study kinetic effects responsible for the transition to relativistic
self-induced transparency in the interaction of a circularly-polarized
laser-pulse with an overdense plasma and their relation to hole-boring and ion
acceleration. It is demonstrated using particle-in-cell simulations and an
analysis of separatrices in single-electron phase-space, that ion motion can
suppress fast electron escape to the vacuum, which would otherwise lead to
transition to the relativistic transparency regime. A simple analytical
estimate shows that for large laser pulse amplitude the time scale over
which ion motion becomes important is much shorter than usually anticipated. As
a result, the threshold density above which hole-boring occurs decreases with
the charge-to-mass ratio. Moreover, the transition threshold is seen to depend
on the laser temporal profile, due to the effect that the latter has on
electron heating. Finally, we report a new regime in which a transition from
relativistic transparency to hole-boring occurs dynamically during the course
of the interaction. It is shown that, for a fixed laser intensity, this dynamic
transition regime allows optimal ion acceleration in terms of both energy and
energy spread.Comment: Added new material. 15 pages, 12 figure
Measurements of a Quantum Dot with an Impedance-Matching On-Chip LC Resonator at GHz Frequencies
We report the realization of a bonded-bridge on-chip superconducting coil and
its use in impedance-matching a highly ohmic quantum dot (QD) to a
measurement setup. The coil, modeled as a lumped-element resonator, is
more compact and has a wider bandwidth than resonators based on coplanar
transmission lines (e.g. impedance transformers and stub tuners) at
potentially better signal-to-noise ratios. In particular for measurements of
radiation emitted by the device, such as shot noise, the 50 larger
bandwidth reduces the time to acquire the spectral density. The resonance
frequency, close to 3.25 GHz, is three times higher than that of the one
previously reported wire-bonded coil. As a proof of principle, we fabricated an
circuit that achieves impedance-matching to a load
and validate it with a load defined by a carbon nanotube QD of which we measure
the shot noise in the Coulomb blockade regime.Comment: 7 pages, 6 figure
Interpretation of runaway electron synchrotron and bremsstrahlung images
The crescent spot shape observed in DIII-D runaway electron synchrotron
radiation images is shown to result from the high degree of anisotropy in the
emitted radiation, the finite spectral range of the camera and the distribution
of runaways. The finite spectral camera range is found to be particularly
important, as the radiation from the high-field side can be stronger by a
factor than the radiation from the low-field side in DIII-D. By
combining a kinetic model of the runaway dynamics with a synthetic synchrotron
diagnostic we see that physical processes not described by the kinetic model
(such as radial transport) are likely to be limiting the energy of the
runaways. We show that a population of runaways with lower dominant energies
and larger pitch-angles than those predicted by the kinetic model provide a
better match to the synchrotron measurements. Using a new synthetic
bremsstrahlung diagnostic we also simulate the view of the Gamma Ray Imager
(GRI) diagnostic used at DIII-D to resolve the spatial distribution of
runaway-generated bremsstrahlung.Comment: 21 pages, 11 figure
Synchrotron radiation from a runaway electron distribution in tokamaks
The synchrotron radiation emitted by runaway electrons in a fusion plasma
provides information regarding the particle momenta and pitch-angles of the
runaway electron population through the strong dependence of the synchrotron
spectrum on these parameters. Information about the runaway density and its
spatial distribution, as well as the time evolution of the above quantities,
can also be deduced. In this paper we present the synchrotron radiation spectra
for typical avalanching runaway electron distributions. Spectra obtained for a
distribution of electrons are compared to the emission of mono-energetic
electrons with a prescribed pitch-angle. We also examine the effects of
magnetic field curvature and analyse the sensitivity of the resulting spectrum
to perturbations to the runaway distribution. The implications for the deduced
runaway electron parameters are discussed. We compare our calculations to
experimental data from DIII-D and estimate the maximum observed runaway energy.Comment: 22 pages, 12 figures; updated author affiliations, fixed typos, added
a sentence at the end of section I
Dome-building volcanic activity in the Oas-Gutai Neogene Volcanic Area, Eastern Carpathians, Romania
A complex dome-building volcanic activity
developed during a 5 Myr time interval (13.2-
8.0 Ma) in OaĆ-GutĂąi Mts., associated to the
intermediate volcanism of the OaĆ-GutĂąi Neogene
volcanic area (OG). Numerous domes were built
up in the entire volcanic region also triggering
both non-explosive and explosive fragmentation
volcanic processes. The volcanic forms consist
of extrusive domes, lava domes and dome-
flows/coulées and cryptodomes predominantly
as solitary domes, or compound domes and dome
complexes. The domes are comprised of andesites,
dacites and rhyolites (acid andesites and dacites
are prevalent). The volcanic rocks show a calc-
alkaline and medium to high-K character and
typical subduction-zone geochemical signatures.
Overall, either subaerial or subaqueous, the dome
growth and collapse associated with fragmental
explosive or non-explosive processes, was
dominantly responsible for most of the volcanic
products. Dome emplacement in submarine setting
is commonly associated with marginal auto-
brecciation, much subordinated explosive events
and subsequent resedimentation. Overall, the
dome-building volcanic activity in OG is recorded
to a monogenetic-type of volcanism. The series
of dome-building events which were triggered
and controlled by magma-mixing and -mingling
processes developed from time to time in different
locations of O
DREAM: a fluid-kinetic framework for tokamak disruption runaway electron simulations
Avoidance of the harmful effects of runaway electrons (REs) in
plasma-terminating disruptions is pivotal in the design of safety systems for
magnetic fusion devices. Here, we describe a computationally efficient
numerical tool, that allows for self-consistent simulations of plasma cooling
and associated RE dynamics during disruptions. It solves flux-surface averaged
transport equations for the plasma density, temperature and poloidal flux,
using a bounce-averaged kinetic equation to self-consistently provide the
electron current, heat, density and RE evolution, as well as the electron
distribution function. As an example, we consider disruption scenarios with
material injection and compare the electron dynamics resolved with different
levels of complexity, from fully kinetic to fluid modes.Comment: 32 pages, 11 figure
Low Mach-number collisionless electrostatic shocks and associated ion acceleration
The existence and properties of low Mach-number () electrostatic
collisionless shocks are investigated with a semi-analytical solution for the
shock structure. We show that the properties of the shock obtained in the
semi-analytical model can be well reproduced in fully kinetic Eulerian
Vlasov-Poisson simulations, where the shock is generated by the decay of an
initial density discontinuity. Using this semi-analytical model, we study the
effect of electron-to-ion temperature ratio and presence of impurities on both
the maximum shock potential and Mach number. We find that even a small amount
of impurities can influence the shock properties significantly, including the
reflected light ion fraction, which can change several orders of magnitude.
Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity
ions.Comment: In Plasma Physics and Controlled Fusio
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