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R. E. Ergun - One of the best experts on this subject based on the ideXlab platform.
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ANTENNA RADIATION NEAR THE LOCAL PLASMA FREQUENCY BY LANGMUIR WAVE Eigenmodes
The Astrophysical Journal, 2012Co-Authors: David M. Malaspina, Iver H. Cairns, R. E. ErgunAbstract:Langmuir waves (LWs) in the solar wind are generated by electron beams associated with solar flares, interplanetary shock fronts, planetary bow shocks, and magnetic holes. In principle, LWs localized as Eigenmodes of density fluctuations can emit electromagnetic (EM) radiation by an antenna mechanism near the local plasma frequency fp and twice the local plasma frequency. In this work, analytic expressions are derived for the radiated electric and magnetic fields and power generated near fp by LW Eigenmodes. The EM wave power emitted near fp is predicted as a function of the eigenmode length scale L, maximum electric field, driving electron beam speed, and the ambient plasma density and temperature. The escape to a distant observer of fp radiation from a localized Langmuir eigenmode is also briefly explored as a function of the plasma conditions.
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Localized Langmuir Eigenmodes and Solar Radio Bursts
PLANETARY RADIO EMISSIONS VII, 2011Co-Authors: David M. Malaspina, S. H. Hess, R. E. ErgunAbstract:Observed spatial- and frequency-domain signatures of the most intense solar wind Langmuir waves can be described as localized, discrete-frequency Eigenmodes trapped in a parabolic density fluctuation. Electric field waveforms from spacecraft in the solar wind are compared with one- and three-dimensional solutions and, in many cases, can be represented by 1-3 of the lowest order Eigenmodes. The spatial scale of eigenmode wave packets is on the order of tens of Langmuir wavelengths, allowing them to draw energy directly from the unstable electron distributions associated with a solar type III radio bursts and implying that Langmuir waves can grow in a strongly inhomogeneous medium. The currents generated by localized Langmuir Eigenmodes emit coherent electromagnetic radiation as antennas at the fundamental and at twice the local plasma frequency. STEREO observations demonstrate that the currents required for eigenmode antenna radiation are present and have strengths within an order of magnitude of theoretical predictions. The eigenmode antenna radiation mechanism implies that, of all the Langmuir waves excited by an electron beam, relatively few localized antenna radiators may account for a majority of observed emission from an extended radio source. Finally, the possibility that turbulence may ultimately play a strong role in the generation of Langmuir waves and the radio emissions associated with solar type II and type III radio bursts is investigated.
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Growth of the Langmuir cavity Eigenmodes in the solar wind
Journal of Geophysical Research, 2010Co-Authors: Sebastien Hess, David M. Malaspina, R. E. ErgunAbstract:[1] The Langmuir waves in the solar wind are known to be at the origin of many types of solar radio emissions, in particular solar type II and type III radio bursts. In situ observations have shown that the largest amplitude Langmuir waves often appear as localized wave packets. Several models have been proposed to explain this localization of the waves, such as kinetic localization, nonlinear wave-wave processes, or eigenmode formation. The latter theory states that the wave packets are Eigenmodes of density cavities generated by background turbulence. In the present paper we investigate the wave growth of the localized Langmuir Eigenmodes.
E.d. Fredrickson - One of the best experts on this subject based on the ideXlab platform.
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Compressional Alfvén Eigenmodes in rotating spherical tokamak plasmas
Plasma Physics and Controlled Fusion, 2017Co-Authors: Håkan Smith, E.d. FredricksonAbstract:Spherical tokamaks often have a considerable toroidal plasma rotation of several tens of kHz. Compressional Alfven Eigenmodes in such devices therefore experience a frequency shift, which if the plasma were rotating as a rigid body, would be a simple Doppler shift. However, since the rotation frequency depends on minor radius, the Eigenmodes are affected in a more complicated way. The eigenmode solver CAE3B (Smith et al 2009 Plasma Phys. Control. Fusion 51 075001) has been extended to account for toroidal plasma rotation. The results show that the eigenfrequency shift due to rotation can be approximated by a rigid body rotation with a frequency computed from a spatial average of the real rotation profile weighted with the eigenmode amplitude. To investigate the effect of extending the computational domain to the vessel wall, a simplified eigenmode equation, yet retaining plasma rotation, is solved by a modified version of the CAE code used in Fredrickson et al (2013 Phys. Plasmas 20 042112). In summary, both solving the full eigenmode equation, as in the CAE3B code, and placing the boundary at the vessel wall, as in the CAE code, significantly influences the calculated eigenfrequencies.
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Observation of new branch of toroidal Alfven Eigenmodes in TFTR
Nuclear Fusion, 1995Co-Authors: E.d. Fredrickson, R. Nazikian, Chio-zong Cheng, M. G. Bell, R.v. Budny, Z. Chang, E. Mazzucato, A.c. Janos, K. M. McguireAbstract:Experimental observations are presented of a new branch of the toroidal Alfven eigenmode spectrum during ICRF heating of plasmas on TFTR. The identification of the second branch is based largely on direct measurements of the toroidal mode numbers of the toroidal Alfven Eigenmodes and by the differences in the time evolution of the frequency spectrum between the new branch and the original toroidal Alfven Eigenmodes. The new branch has so far only been observed in relatively low edge q (=4-4.5) plasmas
Håkan Smith - One of the best experts on this subject based on the ideXlab platform.
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Localization of fast magnetosonic Eigenmodes in spherical tori
2017Co-Authors: Tünde Fülöp, Håkan Smith, Mietek Lisak, Dan AndersonAbstract:Edge-localized fast magnetosonic Eigenmodes (FME) may be responsible for the observed sub-ion cyclotron emission (SICE) in recent NSTX experiments, [1,2]. These modes can be driven unstable by resonant interaction with a small population of energetic ions, having an anisotropic distribution in velocity space. Radially localized modes are important not only to explain the observed SICE, but also because these modes might open a possibility for transferring energy from the fusion products to the background ions. The observation of sub-ion cyclotron emission in NSTX at frequencies about half of the ion cyclotron frequency, calls for an extension of the previous eigenmode analysis to be valid also this frequency range. In the present paper, we extend the eigenmode analysis to be valid for spherical tokamak geometry and sub-ion cyclotron frequencies. The radial and poloidal structure of these Eigenmodes is analyzed, by solving the eigenmode equation using a variational approach. The eigenmode equation The eigenmode equation for the perturbed magnetic eld in a cold, inhomogeneous and magnetized plasma with one ion species can be obtained from the Maxwell equations by assuming the perturbed quantities X to depend on time as exp ( i!t), ! is the wave frequency, and by introducing the elliptic-toroidal co-ordinates , # and ’, where is a radial co-ordinate such that = q
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Compressional Alfvén Eigenmodes in rotating spherical tokamak plasmas
Plasma Physics and Controlled Fusion, 2017Co-Authors: Håkan Smith, E.d. FredricksonAbstract:Spherical tokamaks often have a considerable toroidal plasma rotation of several tens of kHz. Compressional Alfven Eigenmodes in such devices therefore experience a frequency shift, which if the plasma were rotating as a rigid body, would be a simple Doppler shift. However, since the rotation frequency depends on minor radius, the Eigenmodes are affected in a more complicated way. The eigenmode solver CAE3B (Smith et al 2009 Plasma Phys. Control. Fusion 51 075001) has been extended to account for toroidal plasma rotation. The results show that the eigenfrequency shift due to rotation can be approximated by a rigid body rotation with a frequency computed from a spatial average of the real rotation profile weighted with the eigenmode amplitude. To investigate the effect of extending the computational domain to the vessel wall, a simplified eigenmode equation, yet retaining plasma rotation, is solved by a modified version of the CAE code used in Fredrickson et al (2013 Phys. Plasmas 20 042112). In summary, both solving the full eigenmode equation, as in the CAE3B code, and placing the boundary at the vessel wall, as in the CAE code, significantly influences the calculated eigenfrequencies.
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Compressional Alfvén eigenmode structure in spherical tokamaks
Plasma Physics and Controlled Fusion, 2009Co-Authors: Håkan Smith, Erwin VerwichteAbstract:The two-dimensional structure of compressional Alfven Eigenmodes (CAEs) below the ion cyclotron frequency is studied numerically by solving the cold plasma Hall-MHD equations for a realistic spherical tokamak equilibrium. The simplest of the computed Eigenmodes have a standing wave-like structure and the higher frequency solutions show a more travelling wave-like behaviour. The effects of the equilibrium current and the Hall terms in the eigenmode equation are investigated, and these terms are found to shift the frequencies of Eigenmodes with different sign of the toroidal mode number away from each other. A classification scheme is proposed which relates each spherical tokamak eigenmode to the three mode numbers of the corresponding large aspect ratio circular cross section eigenmode obtained by gradually decreasing the elongation and increasing the aspect ratio.
W W Heidbrink - One of the best experts on this subject based on the ideXlab platform.
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convective beam ion losses due to alfven Eigenmodes in diii d reversed shear plasmas
Plasma Physics and Controlled Fusion, 2011Co-Authors: D C Pace, W W Heidbrink, R K Fisher, Marianela Garciamunoz, M A Van ZeelandAbstract:Coherent losses of neutral beam ions are observed at frequencies corresponding to toroidal and reversed-shear Alfven Eigenmodes (RSAEs) in DIII-D. Reversed-shear profiles are created by injecting beam power during the plasma current ramp. Beam ion losses stemming from Alfven eigenmode activity contribute to flattening of the energetic ion density profile in such discharges. This is the first observation of convective beam ion losses due to RSAEs. The energies and pitch angles of lost ions are measured and found to exist within a well-defined region of phase space. Loss flux signals decrease in time as current penetrates and Alfven eigenmode activity becomes more core localized. Preliminary Monte Carlo simulations of energetic ion interactions with measured mode structures show the dominant loss mechanism is a transition from a counter-passing orbit to a trapped orbit that is lost to the wall.
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measurements modelling and electron cyclotron heating modification of alfven eigenmode activity in diii d
Nuclear Fusion, 2009Co-Authors: M A Van Zeeland, N. N. Gorelenkov, W W Heidbrink, R. Nazikian, C Z Cheng, C T Holcomb, A.w. Hyatt, G J Kramer, M E Austin, J LohrAbstract:Neutral beam injection into reversed magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including toroidicity and ellipticity induced Alfven Eigenmodes (TAE/EAE, respectively) and reversed shear Alfven Eigenmodes (RSAE) as well as their spatial coupling. These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and strong drive due to multiple higher order resonances. It is found that ideal MHD modelling of eigenmode spectral evolution, coupling and structure are in excellent agreement with experimental measurements. It is also found that higher radial envelope harmonic RSAEs are clearly observed and agree with modelling. Some discrepancies with modelling such as that due to up/down eigenmode asymmetries are also pointed out. Concomitant with the Alfvenic activity, fast ion (FIDA) spectroscopy shows large reductions in the central fast ion profile, the degree of which depends on the Alfven eigenmode amplitude. Interestingly, localized electron cyclotron heating (ECH) near the mode location stabilizes RSAE activity and results in significantly improved fast ion confinement relative to discharges with ECH deposition on axis. In these discharges, RSAE activity is suppressed when ECH is deposited near the radius of the shear reversal point and enhanced with deposition near the axis. The sensitivity of this effect to deposition power and current drive phasing as well as ECH modulation are presented.
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Coupling of global toroidal Alfvén Eigenmodes and reversed shear Alfvén Eigenmodes in DIII-D
Physics of Plasmas, 2007Co-Authors: M. A. Van Zeeland, M. A. Makowski, N. N. Gorelenkov, W W Heidbrink, E Ruskov, R. Nazikian, G.r. Mckee, G J Kramer, M E Austin, A D TurnbullAbstract:Reversed shear Alfvén Eigenmodes (RSAEs) are typically thought of as being localized near the minima in the magnetic safety factor profile, however, their spatial coupling to global toroidal Alfvén Eigenmodes(TAEs) has been observed in DIII-D discharges. For a decreasing minimum magnetic safety factor, the RSAE frequency chirps up through that of stable and unstable TAEs. Coupling creates a small gap at the frequency degeneracy point forming two distinct global modes. The core-localized RSAE mode structure changes and becomes temporarily global. Similarly, near the mode frequency crossing point, the global TAE extends deeper into the plasma core. The frequency splitting and spatial structure of the two modes throughout the various coupling stages, as measured by an array of internal fluctuation diagnostics, are in close agreement with linear ideal MHD calculations using the NOVA code. The implications of this coupling for eigenmode stability is also investigated and marked changes are noted throughout the coupling process.
M A Van Zeeland - One of the best experts on this subject based on the ideXlab platform.
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convective beam ion losses due to alfven Eigenmodes in diii d reversed shear plasmas
Plasma Physics and Controlled Fusion, 2011Co-Authors: D C Pace, W W Heidbrink, R K Fisher, Marianela Garciamunoz, M A Van ZeelandAbstract:Coherent losses of neutral beam ions are observed at frequencies corresponding to toroidal and reversed-shear Alfven Eigenmodes (RSAEs) in DIII-D. Reversed-shear profiles are created by injecting beam power during the plasma current ramp. Beam ion losses stemming from Alfven eigenmode activity contribute to flattening of the energetic ion density profile in such discharges. This is the first observation of convective beam ion losses due to RSAEs. The energies and pitch angles of lost ions are measured and found to exist within a well-defined region of phase space. Loss flux signals decrease in time as current penetrates and Alfven eigenmode activity becomes more core localized. Preliminary Monte Carlo simulations of energetic ion interactions with measured mode structures show the dominant loss mechanism is a transition from a counter-passing orbit to a trapped orbit that is lost to the wall.
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measurements modelling and electron cyclotron heating modification of alfven eigenmode activity in diii d
Nuclear Fusion, 2009Co-Authors: M A Van Zeeland, N. N. Gorelenkov, W W Heidbrink, R. Nazikian, C Z Cheng, C T Holcomb, A.w. Hyatt, G J Kramer, M E Austin, J LohrAbstract:Neutral beam injection into reversed magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including toroidicity and ellipticity induced Alfven Eigenmodes (TAE/EAE, respectively) and reversed shear Alfven Eigenmodes (RSAE) as well as their spatial coupling. These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and strong drive due to multiple higher order resonances. It is found that ideal MHD modelling of eigenmode spectral evolution, coupling and structure are in excellent agreement with experimental measurements. It is also found that higher radial envelope harmonic RSAEs are clearly observed and agree with modelling. Some discrepancies with modelling such as that due to up/down eigenmode asymmetries are also pointed out. Concomitant with the Alfvenic activity, fast ion (FIDA) spectroscopy shows large reductions in the central fast ion profile, the degree of which depends on the Alfven eigenmode amplitude. Interestingly, localized electron cyclotron heating (ECH) near the mode location stabilizes RSAE activity and results in significantly improved fast ion confinement relative to discharges with ECH deposition on axis. In these discharges, RSAE activity is suppressed when ECH is deposited near the radius of the shear reversal point and enhanced with deposition near the axis. The sensitivity of this effect to deposition power and current drive phasing as well as ECH modulation are presented.
