88 research outputs found
Electron Modulation Instability in the Strong Turbulent Regime for Electron Beam Propagation in Background Plasma
We study collective processes for an electron beam propagating through a
background plasma using simulations and analytical theory. A new regime where
the instability of a Langmuir wave packet can grow locally much faster than ion
frequency ({\omega}_pi) is clearly identified. The key feature of this new
regime is an Electron Modulational Instability that rapidly creates a local
Langmuir wave packet, which in its turn produces local charge separation and
strong ion density perturbations because of the action of the ponderomotive
force, such that the beam-plasma wave interaction stops being resonant. Three
evolution stages of the process and observed periodic burst features are
discussed. Different physical regimes in the plasma and beam parameter space
are clearly demonstrated for the first time.Comment: 19 pages, 3 figure
Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagation in Background Plasma
Electron-beam plasma interaction has long been a topic of great interest. The
validities of Quasi-Linear (QL) theory and Weak Turbulence (WT) theory are
limited by the requirement of sufficiently dense mode spectrum and small wave
amplitude. In this paper, by performing a large number of high resolution
two-dimensional (2D) particle-in-cell (PIC) simulations and using analytical
theories, we extensively studied the collective processes of a mono-energetic
electron beam emitted from a thermionic cathode propagating through a cold
plasma. We show that initial two-stream instability between the beam and
background cold electrons is saturated by wave trapping rather than QL theory.
Further evolution occurs due to strong wave-wave nonlinear processes. We show
that the beam-plasma interaction can be classified into four different physical
regimes in the parameter space for the plasma and beam parameters. The
differences between the different regimes are analyzed in detail. For the first
time, we identified a new regime in strong Langmuir turbulence featured by what
we call Electron Modulational Instability (EMI) that creates a local Langmuir
wave packet faster than ion frequency ({\omega}_pi) and ions initially do not
respond to EMI in the initial growing stage. On a longer timescale, the action
of the ponderomotive force produces very strong ion density perturbations so
that the beam-plasma wave interaction stops being resonant. Consequently, in
this EMI regime beam-plasma interaction is a periodic burst (intermittent)
process. The beams are strongly scattered, and the Langmuir wave spectrum is
significantly broadened, which gives rise to the strong heating of bulk
electrons. Some interesting phenomena in the strong turbulent regime are also
discussedComment: 65 pages, 19 figure
The mechanism of tribological interaction of a ferromagnet in a directed magnetic field
The mechanism of tribological model of ferromagnetic material ШX-15 on a neutral countertile is substantiated, the effect of wear on the zone of actual contact under the influence of magnetic field (MF) in the M10Г2к oil is shown. With the direction of the MF to the surface of the sample, all the wear products involved both the scratchs and thin films. If the direction is from the sample the scratchs is not observed but there is a fine-grained structure. Friction surface without MF has a rough topography with the formation of SF with thickness up to 10 μm. In the use of oil after the friction in the MF, it is not find thick SF on the friction surface of the sample, and instead of them there are thin SF spots, which suggests the use of fine fraction SF
Ca2+- and Volume-sensitive Chloride Currents Are Differentially Regulated by Agonists and Store-operated Ca2+ Entry
Using patch-clamp and calcium imaging techniques, we characterized the effects of ATP and histamine on human keratinocytes. In the HaCaT cell line, both receptor agonists induced a transient elevation of [Ca2+]i in a Ca2+-free medium followed by a secondary [Ca2+]i rise upon Ca2+ readmission due to store-operated calcium entry (SOCE). In voltage-clamped cells, agonists activated two kinetically distinct currents, which showed differing voltage dependences and were identified as Ca2+-activated (ICl(Ca)) and volume-regulated (ICl, swell) chloride currents. NPPB and DIDS more efficiently inhibited ICl(Ca) and ICl, swell, respectively. Cell swelling caused by hypotonic solution invariably activated ICl, swell while regulatory volume decrease occurred in intact cells, as was found in flow cytometry experiments. The PLC inhibitor U-73122 blocked both agonist- and cell swelling–induced ICl, swell, while its inactive analogue U-73343 had no effect. ICl(Ca) could be activated by cytoplasmic calcium increase due to thapsigargin (TG)-induced SOCE as well as by buffering [Ca2+]i in the pipette solution at 500 nM. In contrast, ICl, swell could be directly activated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeable DAG analogue, but neither by InsP3 infusion nor by the cytoplasmic calcium increase. PKC also had no role in its regulation. Agonists, OAG, and cell swelling induced ICl, swell in a nonadditive manner, suggesting their convergence on a common pathway. ICl, swell and ICl(Ca) showed only a limited overlap (i.e., simultaneous activation), although various maneuvers were able to induce these currents sequentially in the same cell. TG-induced SOCE strongly potentiated ICl(Ca), but abolished ICl, swell, thereby providing a clue for this paradox. Thus, we have established for the first time using a keratinocyte model that ICl, swell can be physiologically activated under isotonic conditions by receptors coupled to the phosphoinositide pathway. These results also suggest a novel function for SOCE, which can operate as a “selection” switch between closely localized channels
Direct Implicit and Explicit Energy-Conserving Particle-in-Cell Methods for Modeling of Capacitively-Coupled Plasma Devices
Achieving entire large scale kinetic modelling is a crucial task for the
development and optimization of modern plasma devices. With the trend of
decreasing pressure in applications such as plasma etching, kinetic simulations
are necessary to self-consistently capture the particle dynamics. The standard,
explicit, electrostatic, momentum-conserving Particle-In-Cell method suffers
from tight stability constraints to resolve the electron plasma length (i.e.
Debye length) and time scales (i.e. plasma period). This results in very high
computational cost, making this technique generally prohibitive for the large
volume entire device modeling (EDM). We explore the Direct Implicit algorithm
and the explicit Energy Conserving algorithm as alternatives to the standard
approach, which can reduce computational cost with minimal (or controllable)
impact on results. These algorithms are implemented into the well-tested
EDIPIC-2D and LTP-PIC codes, and their performance is evaluated by testing on a
2D capacitively coupled plasma discharge scenario. The investigation revels
that both approaches enable the utilization of cell sizes larger than the Debye
length, resulting in reduced runtime, while incurring only a minor compromise
in accuracy. The methods also allow for time steps larger than the electron
plasma period, however this can lead to numerical heating or cooling. The study
further demonstrates that by appropriately adjusting the ratio of cell size to
time step, it is possible to mitigate this effect to acceptable level
Numerical thermalization in 2D PIC simulations: Practical estimates for low temperature plasma simulations
The process of numerical thermalization in particle-in-cell (PIC) simulations
has been studied extensively. It is analogous to Coulomb collisions in real
plasmas, causing particle velocity distributions (VDFs) to evolve towards a
Maxwellian as macroparticles experience polarization drag and resonantly
interact with the fluctuation spectrum. This paper presents a practical
tutorial on the effects of numerical thermalization in 2D PIC applications.
Scenarios of interest include simulations which must be run for many thousands
of plasma periods and contain a population of cold electrons that leave the
simulation space very slowly. This is particularly relevant to many low
temperature plasma discharges and materials processing applications. We present
numerical drag and diffusion coefficients and their associated timescales for a
variety of grid resolutions, discussing the circumstances under which the
electron VDF is modified by numerical thermalization. Though the effects
described here have been known for many decades, direct comparison of
analytically derived, velocity-dependent numerical relaxation timescales to
those of other relevant processes has not often been applied in practice due to
complications that arise in calculating thermalization rates in 1D simulations.
Using these comparisons, we estimate the impact of numerical thermalization in
several example low temperature plasma applications including capacitively
coupled plasma (CCP) discharges, inductively coupled plasma (ICP) discharges,
beam plasmas, and hollow cathode discharges. Finally, we discuss possible
strategies for mitigating numerical relaxation effects in 2D PIC simulations
Search for High-Mass Resonances Decaying to τν in pp Collisions at √s=13 TeV with the ATLAS Detector
A search for high-mass resonances decaying to τν using proton-proton collisions at √s=13 TeV produced by the Large Hadron Collider is presented. Only τ-lepton decays with hadrons in the final state are considered. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb−1. No statistically significant excess above the standard model expectation is observed; model-independent upper limits are set on the visible τν production cross section. Heavy W′ bosons with masses less than 3.7 TeV in the sequential standard model and masses less than 2.2–3.8 TeV depending on the coupling in the nonuniversal G(221) model are excluded at the 95% credibility level
Operation and performance of the ATLAS Tile Calorimeter in Run 1
The Tile Calorimeter is the hadron calorimeter covering the central region of the ATLAS experiment at the Large Hadron Collider. Approximately 10,000 photomultipliers collect light from scintillating tiles acting as the active material sandwiched between slabs of steel absorber. This paper gives an overview of the calorimeter’s performance during the years 2008–2012 using cosmic-ray muon events and proton–proton collision data at centre-of-mass energies of 7 and 8TeV with a total integrated luminosity of nearly 30 fb−1. The signal reconstruction methods, calibration systems as well as the detector operation status are presented. The energy and time calibration methods performed excellently, resulting in good stability of the calorimeter response under varying conditions during the LHC Run 1. Finally, the Tile Calorimeter response to isolated muons and hadrons as well as to jets from proton–proton collisions is presented. The results demonstrate excellent performance in accord with specifications mentioned in the Technical Design Report
Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC
Light-by-light scattering (γγ right arrow γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) right arrow Pb( ) + Pb( )γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions
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