Turbulent fluctuations during pellet injection into a dipole confined plasma torus

Abstract

We report measurements of the turbulent evolution of the plasma density profile following the fast injection of lithium pellets into the Levitated Dipole Experiment (LDX) [Boxer et al., Nat. Phys. 6, 207 (2010)]. As the pellet passes through the plasma, it provides a significant internal particle source and allows investigation of density profile evolution, turbulent relaxation, and turbulent fluctuations. The total electron number within the dipole plasma torus increases by more than a factor of three, and the central density increases by more than a factor of five. During these large changes in density, the shape of the density profile is nearly “stationary” such that the gradient of the particle number within tubes of equal magnetic flux vanishes. In comparison to the usual case, when the particle source is neutral gas at the plasma edge, the internal source from the pellet causes the toroidal phase velocity of the fluctuations to reverse and changes the average particle flux at the plasma edge. An edge particle source creates an inward turbulent pinch, but an internal particle source increases the outward turbulent particle flux. Statistical properties of the turbulence are measured by multiple microwave interferometers and by an array of probes at the edge. The spatial structures of the largest amplitude modes have long radial and toroidal wavelengths. Estimates of the local and toroidally averaged turbulent particle flux show intermittency and a non-Gaussian probability distribution function. The measured fluctuations, both before and during pellet injection, have frequency and wavenumber dispersion consistent with theoretical expectations for interchange and entropy modes excited within a dipole plasma torus having warm electrons and cool ions.

Inward diffusion driven by low frequency fluctuations in self-organizing magnetospheric plasma

“…Similar phenomena have been observed in the CTX [12] and the LDX [8] as interchange and entropy modes. In the CTX, it has been reported that the instantaneous particle transport is intermittent, and the detailed time evolutions of the perturbed flux-tube number and potential indicate that convective transport dominates in the dipole configuration.…”

“…The fluctuation level increases obviously near the cut-off density, suggesting the density dependence of the fluctuations. The particle transport characterizes a turbulent diffusion driven by the electric-field fluctuations which are related to the presence of entropy modes observed in a laboratory magnetospheric plasma of the LDX [7,8].…”

Local gyrokinetic study of electrostatic microinstabilities in dipole plasmas

“…Previous theoretical and experimental results in Refs. [3][4][5] show that the dipole confinement is good even under the stochastic motion of particles due to the collision and turbulence, at least under the present laboratory low temperature and density parameters. However, the confinement properties of dipole field under fusion parameters, e.g., high temperature and density and the gradient of them, are still open questions.…”

“…Dipole magnetic fields widely exist in the Universe, such as in the planetary magnetospheres. The idea of using strong dipole field configuration for magnetic confinement of laboratory plasmas for fusion is proposed theoretically by Hasegawa [1,2] and several experimental devices have also been built since then, such as the Levitated Dipole Experiment (LDX) [3][4][5] at MIT, the Collisionless Terrella Experiment (CTX) [6] at Columbia University and Ring Trap-1 (RT-1) [7,8] at the University of Tokyo. The dipole configuration is also used to confine electron-positron pair plasmas in the laboratory [9].…”

A generalized Hasegawa–Mima equation in curved magnetic fields

“…This hypothesis effectively restricts the applicability of the HM equation to plasmas with a small density gradient and to magnetic fields with small curvature or field inhomogeneities. However, experimental observations pertaining to plasmas confined by dipole magnetic fields [16,17] suggest the existence of drift wave turbulence and zonal flows in systems where both the electron spatial density and the magnetic field are characterized by strong gradients over spatial scales comparable to that of electric field and density fluctuations (these low frequency fluctuations are often referred to as entropy modes [18]). In principle, an accurate description of electromagnetic turbulence in such setting could be obtained with the aid of nonlinear gyrokinetic theory [19,20,21].…”