Suprathermal ion transport in simple magnetized torus configurations

Abstract

Inspired by suprathermal ion experiments in the basic plasma experiment TORPEX, the transport of suprathermal ions in ideal interchange mode turbulence is theoretically examined in the simple magnetized torus configuration. We follow ion tracer trajectories as specified by ideal interchange mode turbulence imported from a numerical simulation of drift-reduced Braginskii equations. Using the variance of displacements, σ2(t)∼tγ, we find that γ depends strongly on suprathermal ion injection energy and the relative magnitude of turbulent fluctuations. The value of γ also changes significantly as a function of time after injection, through three distinguishable phases: ballistic, interaction, and asymmetric. During the interaction phase, we find the remarkable presence of three regimes of dispersion: superdiffusive, diffusive, and subdiffusive, depending on the energy of the suprathermal ions and the amplitude of the turbulent fluctuations. We contrast these results with those from a “slab” magnetic geometry in which subdiffusion does not occur during the interaction phase. Initial results from TORPEX are consistent with data from a new synthetic diagnostic used to interpret our simulation results. The simplicity of the simple magnetized torus makes the present work of interest to analyses of more complicated contexts ranging from fusion devices to astrophysics and space plasma physics.

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

“…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].…”

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

“…At TORPEX, the experimental setup is ready to explore the different fast ion transport regimes that are predicted by theory [18,19] under different experimental conditions. For example, the ratio of fast ion energy to electron temperature can easily be changed.…”

“…A comprehensive theoretical investigation of the behavior of the fast ions as tracer particles in TORPEX plasmas was recently completed [18,19]. The study was performed in plasmas dominated by ideal interchange turbulence [21].…”

“…For example, in TORPEX, the tracer particle trajectories are integrated in a turbulent electrostatic field resulting from the ideal interchange driven turbulence that is calculated by 2D simulations of the drift reduced Braginskii fluid equations [20,21]. Once the particles reach the collector, a synthetic diagnostic that mimics the detector computes 2D or 3D [18,19] profiles of the fast-ion current density for comparison with the data.…”

“…Thanks to the toroidal movable system of the source, a 3D beam profile can be reconstructed showing the contraction and expansion of the beam width, due to the gyromotion of the fast particles, and the broadening effect of the plasma on top of it [11,18,19].…”