Edge characteristics and global confinement of electron cyclotron resonance heated plasmas in the TJ-II Stellarator

Abstract

The edge parameters of electron cyclotron resonance heated plasmas in the TJ-II stellarator are reported. Data from atomic beam diagnostics and electrical probes have been used for edge and scrape-off layer characterization. Scans in heating power and plasma density for H and He plasmas have been performed, for a given magnetic configuration. A linear increase of the diffusion coefficient at the last-closed magnetic surface with the ratio of injected power to plasma density and a similar value of that parameter for the two atomic species investigated were obtained. Global particle confinement times between 3 and 15 ms have been deduced, and transition to an enhanced confinement mode in H plasmas has been observed under some conditions. The role of high-energy particle losses, due to trapping into the relatively high magnetic ripple, in the global energy balance of TJ-II plasmas is addressed.

Extended estimations of neoclassical transport for the TJ-II stellarator: The bootstrap current

“…The values obtained are very close to those obtained experimentally in Ref. [38] in which the particle confinement time was found to be approximately τ P ≈ 10 5 × (P in /n e ) −1.7 ms, where P in is the ECRH injected power in kW and n e the line integrated density in 10 19 m −3 . They are also in agreement with a predictive transport code [39] using the plasma profiles, absolutely calibrated H α signals and EIRENE code simulations to estimate flux surface averaged particle sources [40].…”

Long‐Range Correlations During Plasma Transitions in the TJ‐II Stellarator

“…Transition to an improved confinement regime has been observed in TJ-II for some ECRH plasma conditions [19], [20]. In agreement with this result, it has been shown that the development of sheared flows at the plasma edge of the TJ-II requires a critical value of plasma density or density gradient that depends on global plasma parameters [9], [10], [21], [22].…”

Impact of lithium-coated walls on plasma performance in the TJ-II stellarator

“…The application of a few dry discharges was required for recovery, but this was only transiently obtained. A second factor in play was the presence of the Enhanced Particle Confinement (EPC) mode, characterized by a sudden increase of particle confinement at a critical density in the order of 0.6 Â 10 19 m À3 [10]. Transition into this mode was found to strongly depend on fuelling pulse shape and amplitude [11] and, its development can be eventually suppressed by proper tailoring of the gas puffing.…”