Effects of Ohmic current in the TJ-II stellarator

Abstract

TJ-II plasmas respond in a very controllable way to the external induction of a loop voltage in electron cyclotron resonance heating conditions. First experiments showed that the confinement improves (degrades) when a negative (positive) electric field is induced. Combining loop voltage induction with electron cyclotron current drive, the cause is now proven to be the plasma current but not the induced electric field. It is also found that high positive currents (⪆7 kA) invert trends in plasma density, no longer deteriorating the confinement. If, as suggested, the current acts via magnetic shear modification, then both its magnitude and sign affect the confining properties of the plasma. Several ideas are put forward based upon magneto-hydrodynamic, particle orbits and drift wave turbulence arguments.

Electron internal transport barrier formation and dynamics in the plasma core of the TJ-II stellarator

“…By contrast, in stellarators the effect of the sign of the magnetic shear on these modes is unknown. In TJ-II, ongoing experimental and theoretical work is in progress to explore the effects of the plasma current and the magnetic shear on plasma confinement [17]. Future experiments are planned to induce a positive OH current in order to investigate e-ITB formation in configurations with positive magnetic shear.…”

Integrated data analysis at TJ-II: The density profile

“…Net currents do sometimes flow inside the plasma, typically smaller than a few kA. These currents are associated with (a) the bootstrap current 13 , (b) an external Ohmic drive 14,15 , (c) the drive by NBI (usually small), (d) the drive by ECRH (usually small except in current drive experiments) 16 .…”

“…(15). The critical radius r c (f ) is the solution of η(f, r) = 0, and the corresponding critical density is (ω = 2πf ):…”

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

“…Electron heat transport in TJ-II ECH plasmas is comparable to the neoclassical values (1-5 m 2 /s) around midradius [8]. An estimate of turbulent particle transport near the plasma edge region [9] based on its density fluctuation levels ( 20%) might explain the experimental values there, but at the edge region (ρ > 0.9) a large convective, ripple induced, particle transport seems to be dominant [10]. Figure 2 shows the time evolution of mode-number resolved frequencies detected with a set of Mirnov coils during several dynamic configuration scans like the one just described.…”