215 research outputs found
Test-particle acceleration in a hierarchical three-dimensional turbulence model
The acceleration of charged particles is relevant to the solar corona over a
broad range of scales and energies. High-energy particles are usually detected
in concomitance with large energy release events like solar eruptions and
flares, nevertheless acceleration can occur at smaller scales, characterized by
dynamical activity near current sheets. To gain insight into the complex
scenario of coronal charged particle acceleration, we investigate the
properties of acceleration with a test-particle approach using
three-dimensional magnetohydrodynamic (MHD) models. These are obtained from
direct solutions of the reduced MHD equations, well suited for a plasma
embedded in a strong axial magnetic field, relevant to the inner heliosphere. A
multi-box, multi-scale technique is used to solve the equations of motion for
protons. This method allows us to resolve an extended range of scales present
in the system, namely from the ion inertial scale of the order of a meter up to
macroscopic scales of the order of km (th of the outer scale of
the system). This new technique is useful to identify the mechanisms that,
acting at different scales, are responsible for acceleration to high energies
of a small fraction of the particles in the coronal plasma. We report results
that describe acceleration at different stages over a broad range of time,
length and energy scales.Comment: 12 pages, 8 figures, ApJ (in press
Status report of the baseline collimation system of CLIC. Part II
Important efforts have recently been dedicated to the characterisation and
improvement of the design of the post-linac collimation system of the Compact
Linear Collider (CLIC). This system consists of two sections: one dedicated to
the collimation of off-energy particles and another one for betatron
collimation. The energy collimation system is further conceived as protection
system against damage by errant beams. In this respect, special attention is
paid to the optimisation of the energy collimator design. The material and the
physical parameters of the energy collimators are selected to withstand the
impact of an entire bunch train. Concerning the betatron collimation section,
different aspects of the design have been optimised: the transverse collimation
depths have been recalculated in order to reduce the collimator wakefield
effects while maintaining a good efficiency in cleaning the undesired beam
halo; the geometric design of the spoilers has been reviewed to minimise
wakefields; in addition, the optics design has been optimised to improve the
collimation efficiency. This report presents the current status of the the
post-linac collimation system of CLIC. Part II is mainly dedicated to the study
of the betatron collimation system and collimator wakefield effects.Comment: 25 pages, 13 figure
Observation of mesoscopic conductance fluctuations in YBaCuO grain boundary Josephson Junctions
Magneto-fluctuations of the normal resistance R_N have been reproducibly
observed in high critical temp erature superconductor (HTS) grain boundary
junctions, at low temperatures. We attribute them to mesoscopic transport in
narrow channels across the grain boundary line. The Thouless energy appears to
be the relevant energy scale. Our findings have significant implications on
quasiparticle relaxation and coherent transport in HTS grain boundaries.Comment: Revised version, minor changes. 4 pages, 4 figure
Magnetic moment non-conservation in magnetohydrodynamic turbulence models
The fundamental assumptions of the adiabatic theory do not apply in presence
of sharp field gradients as well as in presence of well developed
magnetohydrodynamic turbulence. For this reason in such conditions the magnetic
moment is no longer expected to be constant. This can influence particle
acceleration and have considerable implications in many astrophysical problems.
Starting with the resonant interaction between ions and a single parallel
propagating electromagnetic wave, we derive expressions for the magnetic moment
trapping width (defined as the half peak-to-peak difference in the
particle magnetic moment) and the bounce frequency . We perform
test-particle simulations to investigate magnetic moment behavior when
resonances overlapping occurs and during the interaction of a ring-beam
particle distribution with a broad-band slab spectrum.
We find that magnetic moment dynamics is strictly related to pitch angle
for a low level of magnetic fluctuation, , where is the constant and uniform background magnetic field.
Stochasticity arises for intermediate fluctuation values and its effect on
pitch angle is the isotropization of the distribution function .
This is a transient regime during which magnetic moment distribution
exhibits a characteristic one-sided long tail and starts to be influenced by
the onset of spatial parallel diffusion, i.e., the variance
grows linearly in time as in normal diffusion. With strong fluctuations
isotropizes completely, spatial diffusion sets in and
behavior is closely related to the sampling of the varying magnetic field
associated with that spatial diffusion.Comment: 13 pages, 10 figures, submitted to PR
Coherent quasiparticle transport in grain boundary junctions employing high-Tc superconductors
Magneto-fluctuations of the normal resistance RN have been reproducibly
observed in YBa2Cu3O7-d biepitaxial grain boundary junctions at low
temperatures. We attribute them to mesoscopic transport in narrow channels
across the grain boundary line, occurring in an unusual energy regime. The
Thouless energy appears to be the relevant energy scale. Possible implications
on the understanding of coherent transport of quasiparticles in HTS and of the
dissipation mechanisms are discussed.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Ion diffusion and acceleration in plasma turbulence
Particle transport, acceleration and energisation are phenomena of major
importance for both space and laboratory plasmas. Despite years of study, an
accurate theoretical description of these effects is still lacking. Validating
models with self-consistent, kinetic simulations represents today a new
challenge for the description of weakly-collisional, turbulent plasmas. We
perform two-dimensional (2D) hybrid-PIC simulations of steady-state turbulence
to study the processes of diffusion and acceleration. The chosen plasma
parameters allow to span different systems, going from the solar corona to the
solar wind, from the Earth's magnetosheath to confinement devices. To describe
the ion diffusion, we adapted the Nonlinear Guiding Center (NLGC) theory to the
2D case. Finally, we investigated the local influence of coherent structures on
particle energisation and acceleration: current sheets play an important role
if the ions Larmor radii are on the order of the current sheets size. This
resonance-like process leads to the violation of the magnetic moment
conservation, eventually enhancing the velocity-space diffusion.Comment: 24 pages, 16 figure
First determination of the one-proton induced Non-Mesonic Weak Decay width of p-shell {\Lambda}-Hypernuclei
Previous studies of proton and neutron spectra from Non-Mesonic Weak Decay of
eight Lambda-Hypernuclei (A = 5-16) have been revisited. New values of the
ratio of the two-nucleon and the one-proton induced decay widths,
Gamma_2N/Gamma_p, are obtained from single proton spectra, Gamma_2N/Gamma_p =
0.50 +/- 0.24, and from neutron and proton coincidence spectra,
Gamma_2N/Gamma_p = 0.36 +/- 0.14stat +0.05sys -0.04sys , in full agreement with
previously published ones. With these values, a method is developed to extract
the one-proton induced decay width in units of the free Lambda decay width,
Gamma_p/Gamma_Lambda, without resorting to Intra Nuclear Cascade models but by
exploiting only experimental data, under the assumption of a linear dependence
on A of the Final State Interaction contribution. This is the first systematic
determination ever done and it agrees within the errors with recent theoretical
calculations.Comment: 16 pages, 3 figures, 2 table
Vlasov simulations of multi-ion plasma turbulence in the solar wind
Hybrid Vlasov-Maxwell simulations are employed to investigate the role of
kinetic effects in a two-dimensional turbulent multi-ion plasma, composed of
protons, alpha particles and fluid electrons. In the typical conditions of the
solar-wind environment, and in situations of decaying turbulence, the numerical
results show that the velocity distribution functions of both ion species
depart from the typical configuration of thermal equilibrium. These
non-Maxwellian features are quantified through the statistical analysis of the
temperature anisotropy, for both protons and alpha particles, in the reference
frame given by the local magnetic field. Anisotropy is found to be higher in
regions of high magnetic stress. Both ion species manifest a preferentially
perpendicular heating, although the anisotropy is more pronounced for the alpha
particles, according with solar wind observations. Anisotropy of the alpha
particle, moreover, is correlated to the proton anisotropy, and also depends on
the local differential flow between the two species. Evident distortions of the
particle distribution functions are present, with the production of bumps along
the direction of the local magnetic field. The physical phenomenology recovered
in these numerical simulations reproduces very common measurements in the
turbulent solar wind, suggesting that the multi-ion Vlasov model constitutes a
valid approach to the understanding of the nature of complex kinetic effects in
astrophysical plasmas
emission rates in absorptions at rest on Li, Li, Be, C and O
An experimental study of the reaction
on Li, Li, Be, C and O -shell nuclei is
presented. The data were collected by the FINUDA spectrometer operating at the
DANE -factory (LNF-INFN, Italy). Emission rates for the reaction in
the mentioned nuclei are measured and compared with the few existing data. The
spectra of several observables are discussed; indications of Quasi-Free
absorptions by a pair embedded in the nucleus can be obtained from
the study of the missing mass distributions.Comment: Version accepted by PR
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