Parametric instabilities in a turbulent plasma

1980 ◽  
Vol 23 (3) ◽  
pp. 463-473
Author(s):  
E. Galin
Keyword(s):  
Energy Absorption ◽  
High Frequency ◽  
Turbulent Plasma ◽  
Turbulence Spectrum ◽  
Pump Frequency ◽  
Threshold Conditions ◽  
Parametric Instabilities ◽  
Optimum Threshold

We make some investigations regarding the anomalous energy absorption in a turbulent plasma through high-frequency parametric instabilities. We establish the optimum threshold conditions with respect to the pump frequency. The influence of the turbulence spectrum on the parametric phenomena is investigated.

High-frequency operation of pulsatile ventricular assist devices: A computational study on circular and elliptically shaped pumps

2019 ◽  
Vol 42 (12) ◽  
pp. 725-734 ◽  
Author(s):  
Christian Loosli ◽  
Stephan Rupp ◽  
Bente Thamsen ◽  
Mathias Rebholz ◽  
Gerald Kress ◽  
...  
Keyword(s):  
High Frequency ◽  
Flow Patterns ◽  
Ventricular Assist Devices ◽  
Shear Stresses ◽  
Residence Times ◽  
Pump Frequency ◽  
Ventricular Assist ◽  
Assist Devices ◽  
High Frequency Operation ◽  
Wall Shear

Pulsatile positive displacement pumps as ventricular assist devices were gradually replaced by rotary devices due to their large volume and high adverse event rates. Nevertheless, pulsatile ventricular assist devices might be beneficial with regard to gastrointestinal bleeding and cardiac recovery. Therefore, aim of this study was to investigate the flow field in new pulsatile ventricular assist devices concepts with an increased pump frequency, which would allow lower stroke volumes to reduce the pump size. We developed a novel elliptically shaped pulsatile ventricular assist devices, which we compared to a design based on a circular shape. The pump size was adjusted to deliver similar flow rates at pump frequencies of 80, 160, and 240 bpm. Through a computational fluid dynamics study, we investigated flow patterns, residence times, and wall shear stresses for different frequencies and pump sizes. A pump size reduction by almost 50% is possible when using a threefold pump frequency. We show that flow patterns inside the circular pump are frequency dependent, while they remain similar for the elliptic pump. With slightly increased wall shear stresses for higher frequencies, maximum wall shear stresses on the pump housing are higher for the circular design (42.2 Pa vs 18.4 Pa). The calculated blood residence times within the pump decrease significantly with increasing pump rates. A smaller pump size leads to a slight increase of wall shear stresses and a significant improvement of residence times. Hence, high-frequency operation of pulsatile ventricular assist devices, especially in combination with an elliptical shape, might be a feasible mean to reduce the size, without any expectable disadvantages in terms of hemocompatibility.

The Use of High Frequency Energy Absorption to Measure Limb Musculature

1993 ◽  
pp. 359-362 ◽  
Author(s):  
Kim F. Michaelsen ◽  
Rita Wellens ◽  
Alex F. Roche ◽  
Allan Northeved ◽  
Jacob Culmsee ◽  
...  
Keyword(s):  
Energy Absorption ◽  
High Frequency ◽  
Limb Musculature

scholarly journals Variation in altitude of high-frequency enhanced plasma line by the pump near the 5th electron gyro-harmonic

2019 ◽  
Author(s):  
Jun Wu ◽  
Jian Wu ◽  
Michael T. Rietveld ◽  
Ingemar Haggstrom ◽  
Haisheng Zhao ◽  
...  
Keyword(s):  
Electron Temperature ◽  
High Frequency ◽  
Langmuir Wave ◽  
Decisive Role ◽  
Bragg Condition ◽  
Pump Frequency ◽  
Ionospheric Heating ◽  
Lower Altitude ◽  
Incoherent Scatter ◽  
Plasma Line

Abstract. During an ionospheric heating campaign carried out at the European Incoherent Scatter Scientific Association (EISCAT), the ultra high frequency incoherent scatter (IS) radar observed a systematic variation in the altitude of the high-frequency enhanced plasma line (HFPL), which behaves depending on the pump frequency. Specifically, the HFPL altitude becomes lower when the pump lies above the 5th gyro-harmonic. The analysis shows that the enhanced electron temperature plays a decisive role in the descent in the HFPL altitude. That is, on the traveling path of the enhanced Langmuir wave, the enhanced electron temperature can only be matched by the low electron density at a lower altitude so that the Bragg condition can be satisfied, as expected from the dispersion relation of Langmuir wave.

scholarly journals THE CHARACTERISTIC OF PARAMETRIC INSTABILITIES EXCITED BY HIGH FREQUENCY PUMPS

1981 ◽  
Vol 30 (11) ◽  
pp. 1448
Author(s):  
CHEN LIU ◽  
ZHOU YU-MEI ◽  
CAI SHI-DONG
Keyword(s):  
High Frequency ◽  
Parametric Instabilities

Small amplitude transverse wave propagation in a weakly Langmuir turbulent plasma

1969 ◽  
Vol 3 (4) ◽  
pp. 593-601
Author(s):  
N. Bel ◽  
J. Heynaerts
Keyword(s):  
High Frequency ◽  
Small Amplitude ◽  
Cold Plasma ◽  
Plasma Frequency ◽  
Turbulent Plasma ◽  
Distribution Functions ◽  
Isotropic Plasma ◽  
Particle Correlation ◽  
Special Cases ◽  
Corrective Term

The high frequency conductivity tensor of an isotropic plasma is derived taking into account particle correlations at the lowest consistent order in the parameter ωp/ω these correlations describe a weakly Langmuir turbulent plasma. Two special cases are investigated in which the two-particle correlation function is related to the turbulent electrostatic field spectrum. Particular distribution functions and spectra are considered and approximate dispersion relations are derived in both cases in ‘the cold plasma limit’. The importance of the corrective term is discussed in terms of three dimensionless parameters measuring the strength of the turbulence, the shape of the spectrum, and the frequency. The effect could be important for frequencies not too far from the plasma frequency.

scholarly journals Artificial optical emissions at HAARP for pump frequencies near the third and second electron gyro-harmonic

2005 ◽  
Vol 23 (5) ◽  
pp. 1585-1592 ◽  
Author(s):  
M. J. Kosch ◽  
T. Pedersen ◽  
J. Hughes ◽  
R. Marshall ◽  
E. Gerken ◽  
...  
Keyword(s):  
High Frequency ◽  
Optical Emission ◽  
Particle Interactions ◽  
Auroral Ionosphere ◽  
Radio Waves ◽  
Pump Frequency ◽  
Optical Emissions ◽  
The Third ◽  
Active Experiments ◽  
First Time

Abstract. High-power high-frequency radio waves beamed into the ionosphere cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutral oxygen causing artificial optical emissions identical to natural aurora. Pumping at electron gyro-harmonic frequencies has special significance as many phenomena change their character. In particular, artificial optical emissions become strongly reduced for the third and higher gyro-harmonics. The High frequency Active Auroral Research Program (HAARP) facility is unique in that it can select a frequency near the second gyro-harmonic. On 25 February 2004, HAARP was operated near the third and passed through the second gyro-harmonic for the first time in a weakening ionosphere. Two novel observations are: firstly, a strong enhancement of the artificial optical emission intensity near the second gyro-harmonic, which is opposite to higher gyro-harmonics; secondly, the optical enhancement maximum occurs for frequencies just above the second gyro-harmonic. We provide the first experimental evidence for these effects, which have been predicted theoretically. In addition, irregular optical structures were created when the pump frequency was above the ionospheric critical frequency.Keywords. Active experiments – Auroral ionosphere – Wave-particle interactions

Observation of Turbulent Waves in a Helium Plasma by Optical Spectroscopy

1975 ◽  
Vol 30 (10) ◽  
pp. 1271-1278
Author(s):  
W. R. Rutgers
Keyword(s):  
Energy Density ◽  
Field Strength ◽  
High Frequency ◽  
Low Frequency ◽  
Turbulent Plasma ◽  
Helium Plasma ◽  
High Frequency Oscillations ◽  
Low Frequency Oscillations ◽  
Frequency Domains ◽  
Turbulent Heating

Abstract From the combined Stark-Zeeman pattern of helium allowed and forbidden optical lines the frequency spectrum, the field strength and the dominant polarization of microfields were determined in a turbulent plasma. Two frequency domains of oscillations were found in a turbulent heating experiment: low-frequency oscillations with dominant polarization perpendicular to the current direction and high-frequency oscillations (f~fpe) with random polarization. The r.m.s. field strength of the oscillations is between 2 kV/cm and 10 kV/cm. The energy density of turbulent microfields amounts to 1‰ of the thermal energy density.

Inverse Energy Cascade of Fast Magnetosonic Turbulence in the Heliosheath

2020 ◽  
Author(s):  
Bertalan Zieger
Keyword(s):  
Solar Wind ◽  
High Resolution ◽  
Shock Waves ◽  
High Frequency ◽  
Collisionless Plasma ◽  
Energy Cascade ◽  
Fast Mode ◽  
Turbulence Spectrum ◽  
Inverse Energy Cascade ◽  
Voyager 2

<p>The solar wind in the heliosheath beyond the termination shock (TS) is a non-equilibrium collisionless plasma consisting of thermal solar wind ions, suprathermal pickup ions (PUI) and electrons. In such multi-ion plasma, two fast magnetosonic wave modes exist: the low-frequency fast mode that propagates in the thermal ion component and the high-frequency fast mode that propagates in the suprathermal PUI component [<em>Zieger et al.</em>, 2015]. Both fast modes are dispersive on fluid and ion scales, which results in nonlinear dispersive shock waves. In this talk, we briefly review the theory of dispersive shock waves in multi-ion collisionless plasma. We present high-resolution three-fluid simulations of the TS and the heliosheath up to 2.2 AU downstream of the TS. We show that downstream propagating nonlinear magnetosonic waves grow until they steepen into shocklets (thin current sheets), overturn, and start to propagate backward in the frame of the downstream propagating wave, as predicted by theory <em>[McKenzie et al</em>., 1993; <em>Dubinin et al.</em>, 2006]. The counter-propagating nonlinear waves result in fast magnetosonic turbulence far downstream of the shock. Since the high-frequency fast mode is positive dispersive on fluid scale, energy is transferred from small scales to large scales (inverse energy cascade). Thermal solar wind ions are preferentially heated by the turbulence. Forward and reverse shocklets in the heliosheath can efficiently accelerate both ions and electrons to high energies through the shock drift acceleration mechanism. We validate our three-fluid simulations with in-situ high-resolution Voyager 2 magnetic field and plasma observations at the TS and in the heliosheath. Our simulations reproduce the magnetic turbulence spectrum with a spectral slope of -5/3 observed by Voyager 2 in frequency domain [<em>Fraternale et al</em>., 2019]. However, since Taylor’s hypothesis is not true for fast magnetosonic perturbations in the heliosheath, the inertial range of the turbulence spectrum is not a Kolmogorov spectrum in wave number domain. </p>

Wavelet Based De-Noising Using Self-Optimizing Method for ECG Signal

2013 ◽  
Vol 416-417 ◽  
pp. 1214-1219 ◽  
Author(s):  
Fang Yu Xiao ◽  
Wei Tang ◽  
Na Fu
Keyword(s):  
High Frequency ◽  
The Self ◽  
Threshold Function ◽  
Ecg Signal ◽  
Signal To Noise ◽  
Optimal Method ◽  
Visual Appearance ◽  
Threshold Concept ◽  
The Dead ◽  
Optimum Threshold

This paper presents a new ECG demising algorithm based on the self-optimizing method. This paper discusses the optimum threshold concept and the optimum threshold is decided by signal and threshold function. Based on the concept, this paper provides the ECG optimal method concrete steps. Through the dead value at high frequency phenomenon that is observed by experiment can identify the terminal value. Experiments show that the proposed method improves the signal-to-noise ratios. Moreover, the de-noising signals have a smooth and visual appearance.

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