Filamentary current detection in stellarator plasmas

Abstract

In this work we present the experimental setup, combining edge, and x-ray diagnostics, used to localize the currents generated by fast electrons confined near the plasma periphery in TJ-II plasmas. The method is similar to that used for the experimental determination of the magnetic field mapping in stellarators, just intercepting the trajectories of test electrons. The fact that stellarators can keep magnetic structures “frozen” in vacuum during the field plateau enables the detection of local currents near the plasma periphery in the TJ-II stellarator.

Filaments in the edge confinement region of TJ-II

“…Again, propagation is visible in a specific direction, matching the externally imposed biasing (at t 1200, 1213, 1225 ms). This result indicates clearly that the filaments have a toroidal extension at least as long as the separation between the two probes (of the order of 5 m), which is consistent with earlier results based on the correlation [7].…”

“…This is interesting: whereas during biasing, the plasma rotates such that information flows from probe B to probe D, when biasing is off, information flows from probe D to probe B (although weaker and somewhat slower). In section 4, we will propose an explanation for this phenomenon: in fact, the filamentary structure may be so long that it performs more than a single toroidal transit; again, this would be consistent with earlier work [7], in which parallel correlation lengths of up to about 50 m were deduced. This means that a single filamentary structure may cross a probe pin several times as it rotates poloidally.…”

“…This was done by performing scans of the rotational transform within a discharge, while the Langmuir probes were inserted at a fixed radial position inside the plasma, and determining the (long range) correlation between the two probes at remote locations. The filamentary structure could then be deduced using a simple model for the expected correlation produced by filaments [7]. Thus, we found that the size of the filamentary structures along the field lines is L || 40 − 50 m (implying 4 to 5 toroidal turns), whereas the (poloidal) extension of the structures perpendicular to the field lines is L ⊥ 7 cm.…”

“…In TJ-II, filaments have been observed indirectly using soft X-ray measurements [3] and visualized using fast cameras in the edge region [4,5]. Similarly, filaments in the edge region of the W7-X stellarator were also visualized recently [6].…”

Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

“…Finally, an effort has been made to find possible diagnostic artifacts. It is known that high energy electrons get efficiently confined inside the magnetic islands [18], so their effect would follow the resonances as they move through the plasma in the configuration scans. While the radiative temperature obtained by ECE measurements might be affected, it is very unlikely that such high energy particles would affect the Thomson Scattering diagnostic.…”