Lower Single Null Plasmas Research Articles

Effects of plasma configuration, ELM and gas puffing on LHW coupling during H-mode in EAST

Couplings of lower hybrid wave (LHW) with different divertor configurations are studied in EAST. With an anti-clockwise toroidal magnetic field and similar plasma parameters, experimental results show that the best coupling occurs in the lower single null (LSN) configuration, whereas the worst occurs in the double-null plasma. Furthermore, for the case of clockwise toroidal magnetic field, the coupling of LHW becomes better in the upper single null configuration and worse in the LSN plasma. Such phenomena show that the LHW coupling with different divertor configurations is possibly related to the flux induced by Er × Bt and edge recycling intensity represented by Da, where Er is the radial electric field in the scrape-off layer. In addition, various edge-localized modes (ELMs), including its intensity and frequency, have impacts on LHW coupling. With increasing ELM frequency in low edge recycling, the intensity of Da would decrease and the associated coupling of LHW should deteriorate. For the case of comparable edge density, the coupling of LHW is almost not influenced by the ELM crash. Results indicate that the changes among Da intensity, ELM frequency and the reflection coefficients of LHW power are self-consistent. Studies show that by gas puffing the nearby LH grill can improve the coupling of LHW during H-mode in EAST. Meanwhile, it is observed that the frequency of ELM should decrease and the plasma confinement should be improved with proper gas puffing, whereas excessive gas puffing should increase the frequency of ELM during H-mode in the case of good LHW coupling. Results also indicate a degradation in confinement performance at increasing puffing rate.

Measurements of ion cyclotron parametric decay of lower hybrid waves at the high-field side of Alcator C-Mod

Ion cyclotron parametric decay instability (PDI) of lower hybrid (LH) waves is surveyed using edge Langmuir probes on the Alcator C-Mod tokamak. The measurement is carried out simultaneously at the high-field side (HFS) and low-field side (LFS) mid-plane of the tokamak, as well as in the outer divertor region. Different LH spectra are observed depending on the location of the probes and magnetic configuration in L-mode plasmas, with drift direction downward. In lower single null (LSN) plasmas, strong ion cyclotron PDI occurring at the HFS is observed for the first time. This instability is characterized by a frequency separation in sidebands corresponding to the ion cyclotron frequency (ωci) near the HFS scrape-off layer and develops with threshold-like behavior as density increases. In inner wall limited (IWL) plasmas, this HFS instability shows a higher density threshold compared with that in LSN plasmas. The pump width becomes broadened even in the absence of the sidebands. In upper single null plasmas with similar plasma parameters, ion cyclotron PDI sidebands have a frequency separation corresponding to ωci near the LFS and are weaker than those observed in the LSN and IWL plasmas. Correlation between the onset of ion cyclotron PDI and the observed loss of lower hybrid current drive efficiency (Wallace et al 2012 Phys. Plasmas 19 062505) is discussed.

Suppression of large edge localized modes with edge resonant magnetic fields in high confinement DIII-D plasmas

Large sub-millisecond heat pulses due to Type-I edge localized modes (ELMs) have been eliminated reproducibly in DIII-D for periods approaching nine energy confinement times (τE) with small dc currents driven in a simple magnetic perturbation coil. The current required to eliminate all but a few isolated Type-I ELM impulses during a coil pulse is less than 0.4% of plasma current. Based on magnetic field line modelling, the perturbation fields resonate with plasma flux surfaces across most of the pedestal region (0.9 ⩽ ψN ⩽ 1.0) when q95 = 3.7 ± 0.2, creating small remnant magnetic islands surrounded by weakly stochastic field lines. The stored energy, βN, H-mode quality factor and global energy confinement time are unaltered by the magnetic perturbation. Although some isolated ELMs occur during the coil pulse, long periods free of large Type-I ELMs (Δt > 4–6 τE) have been reproduced numerous times, on multiple experimental run days in high and intermediate triangularity plasmas, including cases matching the baseline ITER scenario 2 flux surface shape. In low triangularity, lower single null plasmas, with collisionalities near that expected in ITER, Type-I ELMs are replaced by small amplitude, high frequency Type-II-like ELMs and are often accompanied by one or more ELM-free periods approaching 1–2 τE. Large Type-I ELM impulses represent a severe constraint on the survivability of the divertor target plates in future burning plasma devices. Results presented in this paper demonstrate that non-axisymmetric edge magnetic perturbations provide a very attractive development path for active ELM control in future tokamaks such as ITER.