A new type of instability mode, related to suprathermal electrons, was found in extremely low-density plasmas, (n e = (0.2-0.5) × 10 19 m −3 ) with electron cyclotron resonance heating and current drive (ECRH/ECCD) in the TJ-II heliac. The quasi-monochromatic density and plasma potential oscillations in the frequency range 20-120 kHz tend to have several "branches" with constant frequency shift between them. The typical amplitude of the mode induced potential oscillations was estimated to be Δϕ EM ∼ 20 V. The mode branches have an individual and finite radial extent, odd low poloidal structure (m ≤ 5) and angular phase velocity of poloidal rotation of about 8 × 10 4 rad/s in the ion diamagnetic drift direction. The contribution of the mode to the turbulent particle flux Γ E×B for the observed wave vectors k θ < 3 cm −1 was found to be small in comparison with the contribution from broadband turbulence.
Energetic ion driven Alfvén Eigenmodes (AEs) in the NBI-heated plasma at the TJ-II heliac were studied by Heavy Ion Beam Probing (HIBP) in the core, and by Langmuir and Mirnov probes (LP and MP) at the edge. HIBP observed the locally (∼ 1 cm) resolved AE at radii-0.5 < ρ < 0.9. The set of AE branches with low poloidal numbers (m < 8) was detected by MP. The most plausible candidates are global, helical and toroidal AEs. AEs on the density, electric potential and poloidal magnetic field oscillations were detected by HIBP at frequencies 50 kHz < f AE < 300 kHz with a high resolution (< 5 kHz). The amplitude of the AE potential oscillations δϕ AE ∼ 10 V was estimated. The MP and HIBP data have a high coherency at f AE. When the density rises, AE frequency is decreasing, f AE ∼ n −1/2 e , but the cross-phase between the density and potential remains permanent. Poloidally resolved potential measurements by HIBP and LP shows high coherency and finite crossphase at f AE , resulting in finite electric field δE pol. Depending on the cross-phase between δn e and δE pol , AEs may bring small or significant contribution to the turbulent particle flux Γ E×B for the observed k θ < 3 cm −1 .
In the last years, lithium wall conditioning has been carried out in several fusion devices by different techniques, providing in many instances record values of plasma parameters and enhanced plasma reproducibility and opening the possibility of developing high radiative, low recycling liquid divertor concepts of high potential for future reactors. This concept has been termed the Li Tokamak Reactor. Compared to tokamaks, stellarator plasmas show distinct features in their interaction with the surrounding materials. The lack of disruptions and type I ELMs make them more reliable for reactor operation. So it is the lack of MHD-driven density limit. On the other side, the intrinsic radiative character of the density limit of stellarators and the tendency to central impurity accumulation makes wall-material selection paramount. In the present work, the plasma performance of the TJ-II Heliac under Li-coated wall conditions is described. Compared to previous coatings, lithium has produced the best plasma performance to date, leading to the achievement of record values in plasma density and energy confinement. Plasma profiles free from impurity accumulation have been obtained under specific fuelling schemes. Future research lines in this direction, with impact on the design of a Li stellarator reactor concept, are also addressed.
Recent experiments in the Advanced Toroidal Facility (ATF) torsatron [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] have emphasized the role of magnetic configuration control in transport studies. Long-pulse plasma operation up to 20 sec has been achieved with electron cyclotron heating (ECH). With neutral beam injection (NBI) power of ≥1 MW, global energy confinement times of 30 msec have been obtained with line-average densities up to 1.3×1020 m−3. The energy confinement and the operational space in ATF are roughly the same as those in tokamaks of similar size and field. The empirical scaling observed is similar to gyro-reduced Bohm scaling with favorable dependences on density and field offsetting an unfavorable power dependence. The toroidal current measured during ECH is identified as the bootstrap current. The observed currents agree well with predictions of neoclassical theory in magnitude and in parametric dependence. Variations of the magnetic configuration in discharges heated by ECH alone and by NBI change plasma transport and plasma profiles. Magnetic fluctuations respond to the concomitant pressure profile variations. Comparative studies of edge fluctuations in the Texas experimental tokamak (TEXT) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] and the ATF stellarator showed remarkable similarity in the levels of fluctuations and the existence of a velocity shear layer.
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