1,214 research outputs found
Fast growing double tearing modes in a tokamak plasma
Configurations with nearby multiple resonant surfaces have broad spectra of
linearly unstable coupled tearing modes with dominant high poloidal mode
numbers m. This was recently shown for the case of multiple q = 1 resonances
[Bierwage et al., Phys. Rev. Lett. 94 (6), 65001 (2005)]. In the present work,
similar behavior is found for double tearing modes (DTM) on resonant surfaces
with q >= 1. A detailed analysis of linear instability characteristics of DTMs
with various mode numbers m is performed using numerical simulations. The mode
structures and dispersion relations for linearly unstable modes are calculated.
Comparisons between low- and higher-m modes are carried out, and the roles of
the inter-resonance distance and of the magnetic Reynolds number S_Hp are
investigated. High-m modes are found to be destabilized when the distance
between the resonant surfaces is small. They dominate over low-m modes in a
wide range of S_Hp, including regimes relevant for tokamak operation. These
results may be readily applied to configurations with more than two resonant
surfaces.Comment: 11 pages, 15 figure
Classification of radiating compact stars
A classification of compact stars, depending on the electron distribution in velocity space and the density profiles characterizing their magnetospheric plasma, is proposed. Fast pulsars, such as NP 0532, X-ray sources such as Sco-X1, and slow pulsars are suggested as possible evolutionary stages of similar objects. The heating mechanism of Sco-X1 is discussed in some detail
Dynamics of resistive double tearing modes with broad linear spectra
The nonlinear evolution of resistive double tearing modes (DTMs) with safety
factor values q=1 and q=3 is studied in a reduced cylindrical model of a
tokamak plasma. We focus on cases where the resonant surfaces are a small
distance apart. Recent numerical studies have shown that in such configurations
high-m modes are strongly unstable. In this paper, it is first demonstrated
that linear DTM theory predicts the dominance of high-m DTMs. A semi-empirical
formula for estimating the poloidal mode number of the fastest growing mode,
m_peak, is obtained from the existing linear theory. Second, using nonlinear
simulations, it is shown that the presence of fast growing high-m modes leads
to a rapid turbulent collapse in an annular region, whereby small magnetic
island structures form. Furthermore, consideration is given to the evolution of
low-m modes, in particular the global m=1 internal kink, which can undergo
nonlinear driving through coupling to fast growing linear high-m DTMs. Factors
influencing the details of the dynamics are discussed. These results may be
relevant for the understanding of the magnetohydrodynamic (MHD) activity near
the minimum of q and may thus be of interest to studies concerned with
stability and confinement in advanced tokamaks.Comment: 11 pages, 10 figure
Thermo-Rotational Instability in Plasma Disks Around Compact Objects
Differentially rotating plasma disks, around compact objects, that are
imbedded in a ``seed'' magnetic field are shown to develop vertically localized
ballooning modes that are driven by the combined radial gradient of the
rotation frequency and vertical gradients of the plasma density and
temperature. When the electron mean free path is shorter than the disk height
and the relevant thermal conductivity can be neglected, the vertical particle
flows produced by of these modes have the effect to drive the density and
temperature profiles toward the ``adiabatic condition'' where
. Here is the plasma temperature and
the particle density. The faster growth rates correspond to steeper
temperature profiles such as those produced by an internal
(e.g., viscous) heating process. In the end, ballooning modes excited for
various values of can lead to the evolution of the disk into a
different current carrying configuration such as a sequence of plasma rings
Theoretical Resolution of Magnetic Reconnection in High Energy Plasmas
The formation of macroscopic reconnected magnetic structures (islands) have
been observed in advanced experiments on weakly collisional, well confined
plasmas while established theories of the drift-tearing modes, which depend
strongly on the electron temperature gradient and can describe the formation of
these structures, had predicted practically inaccessible excitation thresholds
for them in these regimes. The relevant theoretical dilemma is resolved as
mesoscopic modes that depend critically on the ratio of the transverse (to the
magnetic field) to the longitudinal thermal
conductivity{D^e_{\perp}/D^e_{\|}, can produce large scale magnetic
reconnection. These modes are envisioned to emerge from a background, which can
be coherent, of collisionless microscopic reconnecting modes driven by the
electron temperature gradient, that create a sequence of adjacent strings of
magnetic islands and increase considerably the ratio {D^e_{\perp}/D^e_{\|}
over its classical value. The mesoscopic reconnecting mode is treated by a
singular perturbation analysis involving three asymptotic regions and the small
parameters and , where
, is the magnetic diffusion coefficient,
,
, is the transverse mode
number, \texttt{v}^{2}_{A}=B^{2}/(4\pi{nm}_{i})} and .Comment: To be published in "Collective Phenomena in Macroscopic Systems",
Eds. G. Bertin, et al., Publ. World Scientific, 2007. Preprinted here with
the permission of the editor
Interpretation of the I-Regime and transport associated with relevant heavy particle modes
The excitation of a novel kind of heavy particle [1, 2] mode at the edge of the plasma column is
considered as the signature of the I-con nement Regime [3{7]. The outward transport of impurities
produced by this mode is in fact consistent with the observed expulsion of them from the main
body of the plasma column (a high degree of plasma purity is a necessary feature for fusion burning
plasmas capable of approaching ignition). Moreover, the theoretically predicted mode phase velocity,
in the direction of the electron diamagnetic velocity, has been con rmed by relevant experimental
analyses [8] of the excited
uctuations (around 200 kHz). The plasma \spontaneous rotation" in the
direction of the ion diamagnetic velocity is also consistent, according to the Accretion Theory [9] of
this phenomenon, with the direction of the mode phase velocity. Another feature of the mode that
predicted by the theory is that the I-Regime exhibits a knee of the ion temperature at the edge of
the plasma column but not one of the particle density as the mode excitation factor is the relative
main ion temperature gradient exceeding the local relative density gradient. The net plasma current
density appearing in the saturation stage of the relevant instability, where the induced particle and
energy
uxes are drastically reduced, is associated with the signi cant amplitudes of the poloidal
magnetic eld
uctuations [6, 7] observed to accompany the density
uctuations. The theoretical
implications of the signi cant electron temperature
uctuations [10] observed are discussed.United States. Dept. of Energ
Reduced magnetohydrodynamic theory of oblique plasmoid instabilities
The three-dimensional nature of plasmoid instabilities is studied using the
reduced magnetohydrodynamic equations. For a Harris equilibrium with guide
field, represented by \vc{B}_o = B_{po} \tanh (x/\lambda) \hat{y} + B_{zo}
\hat{z}, a spectrum of modes are unstable at multiple resonant surfaces in the
current sheet, rather than just the null surface of the polodial field , which is the only resonant surface in 2D or in
the absence of a guide field. Here is the asymptotic value of the
equilibrium poloidal field, is the constant equilibrium guide field,
and is the current sheet width. Plasmoids on each resonant surface
have a unique angle of obliquity . The resonant
surface location for angle is x_s = - \lambda \arctanh (\tan \theta
B_{zo}/B_{po}), and the existence of a resonant surface requires . The most unstable angle is oblique, i.e. and , in the constant- regime, but parallel, i.e.
and , in the nonconstant- regime. For a fixed angle
of obliquity, the most unstable wavenumber lies at the intersection of the
constant- and nonconstant- regimes. The growth rate of this mode is
, in which
, is the Alfv\'{e}n speed, is the current sheet
length, and is the Lundquist number. The number of plasmoids scales as .Comment: 9 pages, 8 figures, to be published in Physics of Plasma
Active black holes: Relevant plasma structures, regimes and processes involving all phase space
The presented theory is motivated by the growing body of experimental information on the characteristics, connected with relevant spectral, time, and space resolutions, of the radiation emission from objects considered as rotating black holes. In the immediate surroundings of these objects, three plasma regions are identified: an innermost Buffer Region, an intermediate Three-regime Region, and a Structured Peripheral Region. In the last region, a Composite Disk Structure made of a sequence of plasma rings corresponding to the formation of closed magnetic surfaces is considered to be present and to allow intermittent accretion flows along the relevant separatrices. The nonlinear âMaster Equationâ describing composite disk structures is derived and solved in appropriate asymptotic limits. A ring configuration, depending on the state of the plasma at the microscopic level: (i) can be excluded from forming given the strongly nonthermal nature of the electron distribution (in momentum space) within the Three-regime Region allowing the onset of a spiral structure; the observed High Frequency Quasi Periodic Oscillations are associated with these tridimensional structures; (ii) may be allowed to propagate to the outer edge of the Buffer Region where successive rings carrying currents in opposite directions are ejected vertically (in opposite directions) and originate the observed jets; or (iii) penetrates in the Three-regime Region and is dissipated before reaching the outer edge of the Buffer Region. The absence of a coherent composite disk structure guiding accretion in the presence of a significant magnetic field background is suggested to characterize quiescent black holes.United States. Dept. of Energ
Comparison between resistive and collisionless double tearing modes for nearby resonant surfaces
The linear instability and nonlinear dynamics of collisional (resistive) and
collisionless (due to electron inertia) double tearing modes (DTMs) are
compared with the use of a reduced cylindrical model of a tokamak plasma. We
focus on cases where two q = 2 resonant surfaces are located a small distance
apart. It is found that regardless of the magnetic reconnection mechanism,
resistivity or electron inertia, the fastest growing linear eigenmodes may have
high poloidal mode numbers m ~ 10. The spectrum of unstable modes tends to be
broader in the collisionless case. In the nonlinear regime, it is shown that in
both cases fast growing high-m DTMs lead to an annular collapse involving small
magnetic island structures. In addition, collisionless DTMs exhibit multiple
reconnection cycles due to reversibility of collisionless reconnection and
strong ExB flows. Collisionless reconnection leads to a saturated stable state,
while in the collisional case resistive decay keeps the system weakly dynamic
by driving it back towards the unstable equilibrium maintained by a source
term.Comment: 15 pages, 9 figure
Induced scattering of short radio pulses
Effect of the induced Compton and Raman scattering on short, bright radio
pulses is investigated. It is shown that when a single pulse propagates through
the scattering medium, the effective optical depth is determined by the
duration of the pulse but not by the scale of the medium. The induced
scattering could hinder propagation of the radio pulse only if close enough to
the source a dense enough plasma is presented. The induced scattering within
the relativistically moving source places lower limits on the Lorentz factor of
the source. The results are applied to the recently discovered short
extragalactic radio pulse.Comment: submitted to Ap
- âŠ