92 research outputs found
Towards a Simple Model of Compressible Alfvenic Turbulence
A simple model collisionless, dissipative, compressible MHD (Alfvenic)
turbulence in a magnetized system is investigated. In contrast to more familiar
paradigms of turbulence, dissipation arises from Landau damping, enters via
nonlinearity, and is distributed over all scales. The theory predicts that two
different regimes or phases of turbulence are possible, depending on the ratio
of steepening to damping coefficient (m_1/m_2). For strong damping
(|m_1/m_2|<1), a regime of smooth, hydrodynamic turbulence is predicted. For
|m_1/m_2|>1, steady state turbulence does not exist in the hydrodynamic limit.
Rather, spikey, small scale structure is predicted.Comment: 6 pages, one figure, REVTeX; this version to be published in PRE. For
related papers, see http://sdphpd.ucsd.edu/~medvedev/papers.htm
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
LED Monitoring System for the BTeV Lead Tungstate Crystal Calorimeter Prototype
We report on the performance of a monitoring system for a prototype
calorimeter for the BTeV experiment that uses Lead Tungstate crystals coupled
with photomultiplier tubes. The tests were carried out at the 70 GeV
accelerator complex at Protvino, Russia.Comment: 12 pages, 8 figures, LaTeX2e, revised versio
The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations
Long-lived >100 MeV emission has been a common feature of most Fermi-LAT
detected gamma-ray bursts (GRBs), e.g., detected up to ~10^3s in long GRBs
080916C and 090902B and ~10^2s in short GRB 090510. This emission is consistent
with being produced by synchrotron emission of electrons accelerated to high
energy by the relativistic collisionless shock propagating into the weakly
magnetized medium. Here we show that this high-energy afterglow emission
constrains the preshock magnetic field to satisfy 1(n/1cc)^{9/8}
mG<B<10^2(n/1cc)^{3/8}mG, where n is the preshock density, more stringent than
the previous constraint by X-ray afterglow observations on day scale. This
suggests that the preshock magnetic field is strongly amplified, most likely by
the streaming of high energy shock accelerated particles.Comment: 9 pages, JCAP accepte
Modeling the Radio and X-ray Emission of SN 1993J and SN 2002ap
Modeling of radio and X-ray observations of supernovae interacting with their
circumstellar media are discussed, with special application to SN 1993J and SN
2002ap. We emphasize the importance of including all relevant physical
mechanisms, especially for the modeling of the radio light curves. The
different conclusions for the absorption mechanism (free-free or synchrotron
self-absorption), as well as departures from an CSM, as
inferred by some authors, are discussed in detail. We conclude that the
evidence for a variation in the mass loss rate with time is very weak. The
results regarding the efficiencies of magnetic field generation and
relativistic particle acceleration are summarized.Comment: 10 pages, 2 figures. Uses svmult.cls. To appear in proceedings of IAU
Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia,
Spain, eds. J. M. Marcaide and K. W. Weile
Synchronization of coupled limit cycles
A unified approach for analyzing synchronization in coupled systems of
autonomous differential equations is presented in this work. Through a careful
analysis of the variational equation of the coupled system we establish a
sufficient condition for synchronization in terms of the geometric properties
of the local limit cycles and the coupling operator. This result applies to a
large class of differential equation models in physics and biology. The
stability analysis is complemented with a discussion of numerical simulations
of a compartmental model of a neuron.Comment: Journal of Nonlinear Science, accepte
Particle Acceleration in Pulsar Wind Nebulae: PIC modelling
We discuss the role of particle-in-cell (PIC) simulations in unveiling the
origin of the emitting particles in PWNe. After describing the basics of the
PIC technique, we summarize its implications for the quiescent and the flaring
emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be
emerging that, in addition to the standard scenario of particle acceleration
via the Fermi process at the termination shock of the pulsar wind, magnetic
reconnection in the wind, at the termination shock and in the Nebula plays a
major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae"
edited by D. Torres for Springer, based on the invited contributions to the
workshop held in Sant Cugat (Barcelona), June 14-17, 201
Status of the Novosibirsk terahertz FEL
The first stage of Novosibirsk high-power free-electron laser (FEL) was commissioned in 2003. It is based on a
normal conducting CW energy recovery linac. Now the FEL provides electromagnetic radiation in the wavelength
range of 120…180 micrometers. An average power is 400 W. The minimum measured line width is 0.3%, which is
close to the Fourier-transform limit. A user-beamline assembly is in progress, parts of the full-scale machine are
manufactured. The latter will operate in the near IR region and provide higher average powerВ 2003 году в Новосибирске заработала первая очередь мощного лазера на свободных электронах (ЛСЭ).
Машина построена на базе линака-рекуператора непрерывного действия. В настоящее время ЛСЭ работает в
диапазоне длин волн 120…180 мкм, его средняя мощность достигает 400 Вт. Минимальная измеренная ши-
рина полосы излучения составляет 0.3%, что близко к теоретическому минимуму. В настоящее время
монтируются каналы разводки излучения для пользователей, части полномасштабной машины запущены в
производство. Полномасштабная машина будет работать в ближнем ИК-диапазоне и обладать большей
мощностью.В 2003 році в Новосибірську заробила перша черга потужного лазера на вільних електронах (ЛВЕ).
Машина побудована на базі лінака-рекуператора безперервної дії. Зараз ЛВЕ працює в діапазоні довжин
хвиль 120...180 мкм, його середня потужність досягає 400 Вт. Мінімальна виміряна ширина смуги
випромінювання становить 0.3%, що близько до теоретичного мінімуму. Зараз монтуються канали
розведення випромінювання для користувачів, частини повномасштабної машини запущені у виробництво.
Повномасштабна машина буде працювати в ближньому ІЧ-діапазоні і мати більшу потужність
Experimental investigation and validation of neutral beam current drive for ITER through ITPA Joint Experiments
Joint experiments investigating the off-axis neutral beam current drive (NBCD) capability to be utilized for advanced operation scenario development in ITER were conducted in four tokamaks (ASDEX Upgrade (AUG), DIII-D, JT-60U and MAST) through the international tokamak physics activity (ITPA). The following results were obtained in the joint experiments, where the toroidal field, B t, covered 0.4-3.7 T, the plasma current, Ip, 0.5-1.2 MA, and the beam energy, Eb, 65-350 keV. A current profile broadened by off-axis NBCD was observed in MAST. In DIII-D and JT-60U, the NB driven current profile has been evaluated using motional Stark effect diagnostics and good agreement between the measured and calculated NB driven current profile was observed. In AUG (at low δ ∼ 0.2) and DIII-D, introduction of a fast-ion diffusion coefficient of Db ∼ 0.3-0.5 m2 s-1 in the calculation gave better agreement at high heating power (5 MW and 7.2 MW, respectively), suggesting anomalous transport of fast ions by turbulence. It was found through these ITPA joint experiments that NBCD related physics quantities reasonably agree with calculations (with Db = 0-0.5 m2 s-1) in all devices when there is no magnetohydrodynamic (MHD) activity except ELMs. Proximity of measured off-axis beam driven current to the corresponding calculation with Db = 0 has been discussed for ITER in terms of a theoretically predicted scaling of fast-ion diffusion that depends on Eb/Te for electrostatic turbulence or βt for electromagnetic turbulence. © 2011 IAEA, Vienna
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