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

Abstract

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],...

Nondiffusive transport regimes for suprathermal ions in turbulent plasmas

“…Superdiffusive ions show a high degree of intermittency, while subdiffusive ones do not, consistently with the picture of a transport governed by heavy-tailed statistics in the former and not in the latter [7]. Conditionally averaged measurements confirm that the intermittency is due to the interaction with the blobs and that the effect of blobs is larger on 30 eV ions than on 70 eV ions [8].…”

“…This anticorrelated radial motion is at the origin of subdiffusive transport [7,26,27]. Time-resolved measurements in the interaction phase reveal a clear difference between the intermittency level of the suprathermal ion current time traces of the 30 and the 70 eV ions [8]. Superdiffusive ions show a high degree of intermittency, while subdiffusive ones do not, consistently with the picture of a transport governed by heavy-tailed statistics in the former and not in the latter [7].…”

“…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.…”

Superdiffusive transport in laboratory and astrophysical plasmas

“…For ions of 30 eV, after the superdiffusive phase, a phase where the transport is close to diffusive (↵ R = 0.92 ± 0.04) is visible in figure 1(e) after approximately 1 m. This phase appears when the size of the beam becomes sufficiently large that ions sample regions of the plasma with a different fluctuation amplitude, originating an average transport close to diffusive (Bovet et al 2014a(Bovet et al ,b, 2015.…”

“…If one takes a one-dimensional cut of the trajectory in the plane perpendicular to the magnetic field, its fractal dimension is D cut F = D F 1 = µ 1. We can try to relate the fractal dimension of the distribution of superdiffusive ions to the observed superdiffusion exponent ↵ R ' 1.24 (Bovet et al 2014a), so that D F = µ = 2/↵ ' 1.61. Crossing such a structure, as happens in TORPEX because of the transverse displacements of the plasma column due to turbulence, can give an intermittent signal as the one found for lithium ions in the case of energy equal to 30 eV (Bovet et al 2014b), with a predicted fractal dimension in time of order D cut F = µ 1 ' 0.6.…”