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.
Highlights
In the laboratory, the inward turbulent particle pinch occurs when the toroidally confined magnetized plasma is produced and sustained by external heating and particle sources
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
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
Summary
The inward turbulent particle pinch occurs when the toroidally confined magnetized plasma is produced and sustained by external heating and particle sources. During pellet injection and using the same method to interpret the probe array, the direction of the particle flux reverses These observations are consistent with theoretical and computational expectations of turbulent transport due to drift-interchange and entropy modes in a dipole-confined plasma with relatively cool ions, Te ) Ti, as is believed to be the case in the LDX device. The electron’s collisionless curvature heat flux (first described by Eq (5.13) in Braginskii’s chapter titled “Transport Processes in a Plasma,” Ref. 38) provides the underlying physical explanation for the reversal of the toroidal phase velocity of the fluctuations during lithium pellet injection We summarize these results, point-out opportunities for further study, and discuss the importance of these measurements to the understanding of magnetically confined high-temperature plasmas. Because the energetic electrons drift quickly around the torus, about half of the energetic electrons within the dipole plasma torus will come into contact with pellet as it passes through the plasma
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