Measurements of interactions between waves and energetic ions in basic plasma experiments

Heidbrink, W. W.; Boehmer, H.; McWilliams, R.; Preiwisch, A.; Zhang, Y; Zhao, L.; Zhou, Shuangliu; Bovet, Alexandre; Fasoli, A.; Furno, I.; Gustafson, Kyle; Ricci, Paolo; Carter, T; Leneman, D.; Tripathi, Shreekrishna; Vincena, S.

doi:10.1088/0741-3335/54/12/124007

Cited by 9 publications

(13 citation statements)

References 33 publications

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“…A small spreading in the initial parameters creates a small spreading of the ion Larmor radii and a small initial phase difference in the gyromotion among the ions. This induces an oscillation of the beam width at the cyclotron frequency [18,21]. Figure 2 shows time-averaged suprathermal ion current profiles in the presence of plasma, measured at different toroidal locations for two different energies, E = 30 eV and E = 70 eV.…”

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confidence: 99%

Nondiffusive transport regimes for suprathermal ions in turbulent plasmas

et al. 2015

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The understanding of the transport of suprathermal ions in the presence of turbulence is important for fusion plasmas in the burning regime that will characterize reactors, and for space plasmas to understand the physics of particle acceleration. Here, three-dimensional measurements of a suprathermal ion beam in the toroidal plasma device TORPEX are presented. These measurements demonstrate, in a turbulent plasma, the existence of subdiffusive and superdiffusive transport of suprathermal ions, depending on their energy. This result stems from the unprecedented combination of uniquely resolved measurements and first-principles numerical simulations that reveal the mechanisms responsible for the nondiffusive transport. The transport regime is determined by the interaction of the suprathermal ion orbits with the turbulent plasma dynamics, and is strongly affected by the ratio of the suprathermal ion energy to the background plasma temperature. Diffusion of tracers in a neutral fluid was observed by Brown and explained through collisional theory by Einstein and Smoluchowski [1,2]. Classical diffusive transport, originating from scale-fixed random walks with a typical step size, , and a typical waiting time between steps, τ , leads to a linear scaling of the mean-squared displacement with time and a diffusion coefficient given by 2 /τ . In many complex systems such as fusion and space plasmas [3], scale lengths or time scales are not well defined, thus transport cannot be modeled as a classical diffusive process. Generalizations of the classical diffusion model, such as Lévy walks [4][5][6] or fractional Lévy motion [7], allow introducing power-law distributions of the step sizes or waiting times and long-range temporal correlations, introducing a non-Gaussian and non-Markovian character. These generalizations result in nondiffusive transport characterized by a mean-squared displacement (variance of displacement) of an ensemble of individuals that does not necessarily scale linearly with time: (r(t) − r(0)) 2 ∝ t γ , with γ = 1 generally, where r(t) represents the positions of individuals and · indicates the ensemble average. When γ > 1 or γ < 1, the transport is called superdiffusive or subdiffusive, respectively. For the special case of classical diffusion γ = 1.Using time-resolved measurements we have recently shown that suprathermal ions are more sensitive to the intermittent turbulent structures in the basic plasma device TORPEX when their energy is smaller [8]. In this Rapid Communication, we present measurements of suprathermal ion transport in TORPEX carried out in three spatial dimensions and with varying input energies. We show that, as the ion energy is increased, the transport varies from subdiffusive to superdiffusive as predicted by numerical simulations.Although earlier experimental and numerical studies suggest that the transport of suprathermal ions in fusion devices and astrophysical plasmas is generally nondiffusive [9-12], * alexandre.bovet@epfl.ch direct measurements of suprathermal ion transport...

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Nondiffusive transport regimes for suprathermal ions in turbulent plasmas

et al. 2015

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“…In astrophysical plasmas, the understanding of solar energetic particles and of cosmic ray transport still has some gray areas [3,4]. For example, during impulsive solar energetic particle events, large fluctuations in intensity, called dropouts, were observed and are not fully understood [5].Experimental and numerical evidence suggests that suprathermal ion cross-field transport can be nondiffusive [6][7][8][9][10], characterized by a variance of particle displacements that does not scale linearly with time h½rðtÞ − rð0Þ 2 i ∝ t γ , with the transport exponent γ ≠ 1 generally. Here, rðtÞ indicates the particle positions.…”

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confidence: 99%

“…Experimental and numerical evidence suggests that suprathermal ion cross-field transport can be nondiffusive [6][7][8][9][10], characterized by a variance of particle displacements that does not scale linearly with time h½rðtÞ − rð0Þ 2 i ∝ t γ , with the transport exponent γ ≠ 1 generally. Here, rðtÞ indicates the particle positions.…”

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Time-Resolved Measurements of Suprathermal Ion Transport Induced by Intermittent Plasma Blob Filaments

2014

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Suprathermal ion turbulent transport in magnetized plasmas is generally nondiffusive, ranging from subdiffusive to superdiffusive depending on the interplay of the turbulent structures and the suprathermal ion orbits. Here, we present time-resolved measurements of the cross-field suprathermal ion transport in a toroidal magnetized turbulent plasma. Measurements in the superdiffusive regime are characterized by a higher intermittency than in the subdiffusive regime. Using conditional averaging, we show that, when the transport is superdiffusive, suprathermal ions are transported by intermittent field-elongated turbulent structures that are radially propagating. Understanding turbulent transport of suprathermal ions, i.e., ions with energies greater than the quasi-Maxwellian background plasma, is of paramount importance for a variety of laboratory and natural systems. In future fusion reactors such as ITER and DEMO, a good confinement of suprathermal ions, created by fusion reactions or additional heating, is necessary to reach and control burning plasma conditions [1,2]. In astrophysical plasmas, the understanding of solar energetic particles and of cosmic ray transport still has some gray areas [3,4]. For example, during impulsive solar energetic particle events, large fluctuations in intensity, called dropouts, were observed and are not fully understood [5].Experimental and numerical evidence suggests that suprathermal ion cross-field transport can be nondiffusive [6][7][8][9][10], characterized by a variance of particle displacements that does not scale linearly with time h½rðtÞ − rð0Þ 2 i ∝ t γ , with the transport exponent γ ≠ 1 generally. Here, rðtÞ indicates the particle positions. For 0 < γ < 1 the transport is subdiffusive, for 1 < γ < 2 superdiffusive. Classical diffusion corresponds to γ ¼ 1. In fusion experiments and astrophysical plasmas, measurements are limited to a few positions by high-temperature and diagnostics accessibility. In those cases, it is not possible to characterize the transport by the temporal evolution of the variance of displacements and information about the transport has to be inferred from the time trace statistics [11][12][13].In this Letter, we present first time-resolved measurements of the cross-field transport of suprathermal ions in a turbulent magnetized plasma. Previously, in the TORPEX device [14], by using three-dimensional time-averaged measurements of the width of a suprathermal ion beam in combination with numerical simulations (an example is shown in Fig. 1), we have shown that the transport of suprathermal ions varies from superdiffusive to subdiffusive as their energy is increased [8,10,[15][16][17][18][19]. We consider here two suprathermal ion energies, 30 and 70 eV, for which the radial transport was identified to be superdiffusive (γ ¼ 1.20) and subdiffusive (γ ¼ 0.51), respectively [10,18]. We show that the time traces of the suprathermal ion current show a clear difference in intermittency. Using the technique of conditional average sampling (CAS) [20,21],...

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“…The source is based on a two grid accelerating system with a thermionic emitter similar to the one used at the LAPD at UCLA [13,14]. The emitter is made from lithium aluminosilicate, purchased from HeatWave Labs Inc [15].…”

Section: Fast Ion Sourcementioning

confidence: 99%

“…A schematic of the experimental setup is detailed in figure 1 of [14]. The source is moved between discharges and a high-resolution poloidal cross-section of the fast ion current profile is measured by the detector for each toroidal position of the source.…”

Section: Fast Ion Sourcementioning

confidence: 99%

Three-dimensional measurements of non-diffusive fast ion transport in TORPEX

Bovet

Furno

Fasoli

et al. 2013

Plasma Phys. Control. Fusion

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In the basic plasma physics device TORPEX, progress in the fundamental understanding of supra-thermal ion transport is achieved by extensive sets of three-dimensional (3D) data, together with numerical simulations of supra-thermal ion tracers in fluid turbulent fields. A miniaturized lithium 6+ ion source injects fast ions with energies up to 1 KeV and a double-gridded energy analyzer is used as a detector. The source is mounted on a toroidally movable system and the detector can be moved in the poloidal cross-section, allowing one to reconstruct 3D fast ion current profiles. Synchronous detection is used to enhance the signal-to-noise ratio. A modulated biasing voltage is applied to the fast ion source and an analog lock-in amplifier is used to demodulate the detector signal. The analog lock-in amplifier is specially designed to remove the capacitive noise associated with the voltage modulation. Radial transport of the fast ions, associated with plasma turbulence, is characterized. A synthetic diagnostic allows comparing the experimental results with numerical simulations of the fast ion transport in a global fluid simulation of the TORPEX plasma. A good agreement is shown.

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Measurements of interactions between waves and energetic ions in basic plasma experiments

Cited by 9 publications

References 33 publications

Nondiffusive transport regimes for suprathermal ions in turbulent plasmas

Nondiffusive transport regimes for suprathermal ions in turbulent plasmas

Time-Resolved Measurements of Suprathermal Ion Transport Induced by Intermittent Plasma Blob Filaments

Three-dimensional measurements of non-diffusive fast ion transport in TORPEX

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