1,105 research outputs found
Electromagnetic waves destabilized by runaway electrons in near-critical electric fields
Runaway electron distributions are strongly anisotropic in velocity space.
This anisotropy is a source of free energy that may destabilize electromagnetic
waves through a resonant interaction between the waves and the energetic
electrons. In this work we investigate the high-frequency electromagnetic waves
that are destabilized by runaway electron beams when the electric field is
close to the critical field for runaway acceleration. Using a runaway electron
distribution appropriate for the near-critical case we calculate the linear
instability growth rate of these waves and conclude that the obliquely
propagating whistler waves are most unstable. We show that the frequencies,
wave numbers and propagation angles of the most unstable waves depend strongly
on the magnetic field. Taking into account collisional and convective damping
of the waves, we determine the number density of runaways that is required to
destabilize the waves and show its parametric dependences.Comment: 22 pages, 11 figures, to be published in Physics of Plasma
Impurity transport in trapped electron mode driven turbulence
Trapped electron mode turbulence is studied by gyrokinetic simulations with
the GYRO code and an analytical model including the effect of a poloidally
varying electrostatic potential. Its impact on radial transport of high-Z trace
impurities close to the core is thoroughly investigated and the dependence of
the zero-flux impurity density gradient (peaking factor) on local plasma
parameters is presented. Parameters such as ion-to-electron temperature ratio,
electron temperature gradient and main species density gradient mainly affect
the impurity peaking through their impact on mode characteristics. The poloidal
asymmetry, the safety factor and magnetic shear have the strongest effect on
impurity peaking, and it is shown that under certain scenarios where trapped
electron modes are dominant, core accumulation of high-Z impurities can be
avoided. We demonstrate that accounting for the momentum conservation property
of the impurity-impurity collision operator can be important for an accurate
evaluation of the impurity peaking factor.Comment: 30 pages, 10 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
Effective Governance of Global Financial Markets:An Evolutionary Plan for Reform
Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature - a âbump' - can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here, we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivation
Finite bias Cooper pair splitting
In a device with a superconductor coupled to two parallel quantum dots (QDs)
the electrical tunability of the QD levels can be used to exploit non-classical
current correlations due to the splitting of Cooper pairs. We experimentally
investigate the effect of a finite potential difference across one quantum dot
on the conductance through the other completely grounded QD in a Cooper pair
splitter fabricated on an InAs nanowire. We demonstrate that the electrical
transport through the device can be tuned by electrical means to be dominated
either by Cooper pair splitting (CPS), or by elastic co-tunneling (EC). The
basic experimental findings can be understood by considering the energy
dependent density of states in a QD. The reported experiments add
bias-dependent spectroscopy to the investigative tools necessary to develop
CPS-based sources of entangled electrons in solid-state devices.Comment: 4 pages, 4 figure
Optical bandgap engineering in nonlinear silicon nitride waveguides
Silicon nitride is awell-established material for photonic devices and
integrated circuits. It displays a broad transparency window spanning from the
visible to the mid-IR and waveguides can be manufactured with low losses. An
absence of nonlinear multi-photon absorption in the erbium lightwave
communications band has enabled various nonlinear optic applications in the
past decade. Silicon nitride is a dielectric material whose optical and
mechanical properties strongly depend on the deposition conditions. In
particular, the optical bandgap can be modified with the gas flow ratio during
low-pressure chemical vapor deposition (LPCVD). Here we show that this
parameter can be controlled in a highly reproducible manner, providing an
approach to synthesize the nonlinear Kerr coefficient of the material. This
holistic empirical study provides relevant guidelines to optimize the
properties of LPCVD silicon nitride waveguides for nonlinear optics
applications that rely on the Kerr effect
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