413 research outputs found
Consistency between deep crustal heating of strange stars in superbursters and soft X-ray transients
No full textContext.Both superbursters and soft X-ray transients probe the
process of deep crustal heating in compact stars. It was recently
shown that the transfer of matter from crust to core in a strange
star can heat the crust and ignite superbursts provided certain
constraints on the strange quark matter equation of state are
fulfilled.
Aims.We derive corresponding constraints on the equation of state for
soft X-ray transients assuming their quiescent emission is powered
in the same way, and further discuss the time dependence of this
heating mechanism in transient systems.
Methods.We approach this using a simple parametrized model for deep crustal
heating in strange stars assuming slow neutrino cooling in the core and
blackbody photon emission from the surface.
Results.The constraints derived for hot frequently accreting soft X-ray
transients are always consistent with those for superbursters. The
colder sources are consistent for low values of the quark
matter binding energy, heat conductivity and neutrino
emissivity. The heating mechanism is very time dependent which may
help explain cold sources with long recurrence times. Thus deep
crustal heating in strange stars can provide a consistent
explanation for superbursters and soft X-ray transients
Axisymmetric oscillations at L-H transitions in JET: M-mode
No full textL to H transition studies at JET have revealed an n = 0, m = 1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the magnetic oscillation is present a weak ELM-less H-mode regime is obtained, with a clear increase of density and a weak electron temperature pedestal. It is an intermediate state between L and H-mode. In ICRH heated plasmas or low density NBI plasmas the magnetic mode and the pedestal can remain steady (with small oscillations) for the duration of the heating phase, of order 10 s or more. The axisymmetric magnetic oscillation has period ∼0.5-2 ms, and poloidal mode number m = 1: it looks like a pedestal localised up/down oscillation, although it is clearly a natural oscillation of the plasma, not driven by the position control system. Electron cyclotron emission, interferometry, reflectometry and fast Li beam measurements locate the mode in the pedestal region. Dα, fast infrared camera and Langmuir probe measurements show that the mode modulates heat and particle fluxes to the target. The mode frequency appears to scale with the poloidal Alfvén velocity, and not with sound speed (i.e. it is not a geodesic acoustic mode). A heuristic model is proposed for the frequency scaling of the mode. We discuss the relationship between the M-mode and other related observations near the L-H transition
Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation
No full textIn this paper, we present a method for numerical computation of collective Thomson scattering (CTS). We developed a forward model, eCTS, in the electrostatic approximation and benchmarked it against a full electromagnetic model. Differences between the electrostatic and the electromagnetic models are discussed. The sensitivity of the results to the ion temperature and the plasma composition is demonstrated. We integrated the model into the Bayesian data analysis framework Minerva and used it for the analysis of noisy synthetic data sets produced by a full electromagnetic model. It is shown that eCTS can be used for the inference of the bulk ion temperature. The model has been used to infer the bulk ion temperature from the first CTS measurements on Wendelstein 7-X.</p
Parametric decay instability near the upper hybrid resonance and anomalous mm-wave scattering in tokamak and stellarator plasmas
No full textParametric decay instability near the upper hybrid resonance and anomalous millimeter-wave scattering in tokamak and stellarator plasmas
No full textObservation and Modelling of the Onset of Parametric Decay Instabilities during Gyrotron Operation at ASDEX Upgrade
No full textWe investigate parametric decay instabilities (PDIs) occurring for gyrotron radiation near the upper hybrid resonance at the ASDEX Upgrade tokamak. The PDIs are observed through anomalous millimeter-wave scattering which is recorded using the high-resolution, fast acquisition collective Thomson scattering system installed at ASDEX Upgrade, and an experiment in which such observations are made during a scan of the toroidal magnetic field is performed. A previously published theoretical model is used to calculate the gyrotron power necessary to excite PDIs in the experiment; the theoretical model is capable of predicting whether or not PDIs will be observed at a given toroidal magnetic field with a high degree of accuracy
Parametric decay instability near the upper hybrid resonance in magnetically confined fusion plasmas
No full textParametric Decay Instabilities during Electron Cyclotron Resonance Heating at ASDEX Upgrade
No full textObservation and Modelling of the Onset of Parametric Decay Instabilities during Gyrotron Operation at ASDEX Upgrade
No full textWe investigate parametric decay instabilities (PDIs) occurring for gyrotron radiation near the upper hybrid resonance at the ASDEX Upgrade tokamak. The PDIs are observed through anomalous millimeter-wave scattering which is recorded using the high-resolution, fast acquisition collective Thomson scattering system installed at ASDEX Upgrade, and an experiment in which such observations are made during a scan of the toroidal magnetic field is performed. A previously published theoretical model is used to calculate the gyrotron power necessary to excite PDIs in the experiment; the theoretical model is capable of predicting whether or not PDIs will be observed at a given toroidal magnetic field with a high degree of accuracy
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