151 research outputs found
The effect of ECRH on the electron velocity distribution function
Electron cyclotron resonance heating (ECRH) is a mature technology that has progressed constantly over a period of forty years, particularly as a tool in magnetic confinement fusion. As with other heating methods, this technique has seen a steady increase in the sophistication of its applications, from bulk heating through profile tailoring and finally to distribution function engineering. By comparison with other techniques, ECRH presents the significant advantages of good coupling, localized power deposition, easy launching and precise directionality. This paper reviews some recent applications related to third harmonic ECRH and highlights the role of the relaxation dynamics of suprathermal electrons, both in real space and in velocity space, in regulating the overall effect of ECRH on fusion plasmas. A technique for direct visualization of these relaxation phenomena, using modulated ECRH, is described and demonstrated
Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales
Here we report on the results of the WEBT photo-polarimetric campaign
targeting the blazar S5~0716+71, organized in March 2014 to monitor the source
simultaneously in BVRI and near IR filters. The campaign resulted in an
unprecedented dataset spanning \,h of nearly continuous, multi-band
observations, including two sets of densely sampled polarimetric data mainly in
R filter. During the campaign, the source displayed pronounced variability with
peak-to-peak variations of about and "bluer-when-brighter" spectral
evolution, consisting of a day-timescale modulation with superimposed hourlong
microflares characterized by \,mag flux changes. We performed an
in-depth search for quasi-periodicities in the source light curve; hints for
the presence of oscillations on timescales of \,h and \,h do
not represent highly significant departures from a pure red-noise power
spectrum. We observed that, at a certain configuration of the optical
polarization angle relative to the positional angle of the innermost radio jet
in the source, changes in the polarization degree led the total flux
variability by about 2\,h; meanwhile, when the relative configuration of the
polarization and jet angles altered, no such lag could be noted. The
microflaring events, when analyzed as separate pulse emission components, were
found to be characterized by a very high polarization degree () and
polarization angles which differed substantially from the polarization angle of
the underlying background component, or from the radio jet positional angle. We
discuss the results in the general context of blazar emission and energy
dissipation models.Comment: 16 pages, 17 Figures; ApJ accepte
The 2015-2016 outburst of the classical EXor V1118 Ori
After a quiescence period of about 10 years, the classical EXor source V1118
Ori has undergone an accretion outburst in 2015 September. The maximum
brightness (DV > 4 mag) was reached in 2015 December and was maintained for
several months. Since 2016 September, the source is in a declining phase.
Photometry and low/ high-resolution spectroscopy were obtained with MODS and
LUCI2 at the {\it Large Binocular Telescope}, with the facilities at the Asiago
1.22 and 1.82 m telescopes, and with GIANO at the {\it Telescopio Nazionale
Galileo}. The spectra are dominated by emission lines of \hi\ and neutral
metallic species. From line and continuum analysis we derive the mass accretion
rate and its evolution during the outburst. Considering that extinction may
vary between 1.5 and 2.9 mag, we obtain m_acc= 0.32.0 10 m_sun/yr, in
quiescence and m_acc= 0.21.9 10 m_sun/yr, at the outburst peak. The
Balmer decrement shape has been interpreted by means of line excitation models,
finding that from quiescence to outburst peak, the electron density has
increased from 2 10 cm to 4 10 cm. The
profiles of the metallic lines are symmetric and narrower than 100 km s,
while \hi\, and \hei\,\,lines show prominent wings extending up to 500 km
s. The metallic lines likely originate at the base of the accretion
columns, where neutrals are efficiently shielded against the ionizing photons,
while faster ionized gas is closer to the star. Outflowing activity is
testified by the detection of a variable P Cyg-like profile of the H
and \hei\, 1.08\,m lines.Comment: Accepted by Ap
First measurements of oblique ECE with a real-time moveable line-of-sight on TCV
Electron cyclotron (EC) emission (ECE) radiometers viewing perpendicular to the magnetic field are common on nearly all tokamaks for measuring the electron temperature with good spatio-temporal resolution. Two such radiometers are installed on TCV, one looking from the low field side (LFS) and the other from the high field side (HFS). The HFS radiometer is especially sensitive to non-Maxwellian emission in the presence of the strong EC current drive (ECCD) provided by the 3-MW second-harmonic (X2) EC system as the nonthermal radiation is not reabsorbed by the bulk when passing to the receiver. Simultaneous HFS and LFS measurements allow higher-order modeling of the electron distribution function as more constraints are provided by the dual measurements; however, the asymmetric nature of the electron distribution function required for ECCD to occur is not directly put in evidence by these lines of sight. Oblique ECE measurements of an asymmetric nonthermal electron distribution, on the other hand, are expected to also be asymmetric and can provide important information on the current-carrying features of the nonthermal population. A dedicated receiving antenna has been installed allowing real-time swept oblique ECE on TCV in both the co- and counter-looking directions. Proof-of-principle experiments are described in which Doppler-shifted emission is measured
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