Blob current structures in TORPEX plasmas: experimental measurements and numerical simulations

Abstract

In the simple magnetized torus TORPEX, field-aligned blobs originate from ideal interchange waves and propagate radially outward due to ∇B and curvature induced drifts. Time-resolved two-dimensional measurements of the field-aligned current density J associated with blobs are obtained from conditionally sampled data from a single-sided Langmuir probe and a specially designed current probe. The profile of J exhibits an asymmetric dipolar structure, which originates from the polarization of the blob and is consistent with sheath boundary conditions. The asymmetry results from the non-linear dependence of J at the sheath edge upon the floating potential. Using internal measurements, we directly confirm the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity. To investigate the effect of the observed asymmetry of J on the blob motion, we carried out numerical simulations of seeded blobs, using a two-field fluid model, which evolves electron density and vorticity. Simulations are performed spanning a wide range of blob sizes covering both regimes. We use either the complete or a linearized form for the sheath dissipation term in the vorticity equation. The structure of the parallel current density and plasma potential is found to be different in the two cases. Asymmetric profiles are observed in simulations with the complete form, while symmetric profiles are obtained when a linearized form is used. Negligible effects are, however, observed in terms of blob radial velocity. The relevance of the present results for fusion devices is also discussed.

Plasma turbulence in the scrape-off layer of the ISTTOK tokamak

“…We remark that the V f PDFs are rather symmetric, possibly due to the bipolar nature of V f associated with the intermittent events, and display Gaussian properties. 46 We then consider the I sat and V f power spectral density, evaluated as the square of the absolute value of the temporal Fourier transform. These are shown in Figs.…”

Blob dynamics in the TORPEX experiment: a multi-code validation

“…This experimental setup has been used to investigate the parallel current structure associated with blobs, as presented in Ref. [10].…”

Boundary conditions for plasma fluid models at the magnetic presheath entrance

“…Plasmas 19, 122307 (2012) profiles form at the limiters, suggesting that the set of boundary conditions for the plasma-wall transition is compatible with the GBS model equations. Also, the plasma potential can fluctuate at the limiter and thus allows for finite parallel currents, as typically observed at the edge of basic plasma physics experiments 35 and at the limiters or divertors of tokamaks.…”