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The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod is designed to drive off-axis current in the core plasma, e.g. r/a<=0.8, however several interesting observations indicate that the LH waves are depositing significant power in the Scrape Off Layer (SOL) in high density L-Mode discharges. These phenomena are observed in accessible plasmas below the so-called “density limit” attributed to Parametric Decay Instabilities as ω→2ωlh. Parallel current densities of ∼500 kA/m2 are measured on divertor Langmuir probes located just outside the separatrix during high power LH operation above ne∼1×1020 m-3. These SOL currents rise at the same density at which the line integrated hard X-ray Bremsstrahlung emission drops sharply. Ray tracing/Fokker-Planck simulations over-estimate the line integrated X-ray emissivity by a factor of ∼500 at ne = 1.4 1020 m-3 as compared to a factor of 3–4 at ne = 0.6×1020 m. Ray tracing simulations including a realistic SOL show similar trends in X-ray emission compared with the experimental observations. [ABSTRACT FROM AUTHOR]

As the first divertor tokamak in China, HL-2A device has been successfully operated and stably sustained in the single-null divertor configuration under good feedback control of plasma current and position. Up to now, the following operation parameters have been achieved: Ip = 400 kA, Bt = 2.6 T and discharge duration Td = 2980ms. An excellent repeatability of discharge has been achieved and about 90% Greenwald density limit has been reached. The HL-2A SN divertor configuration can be simulated by the SWEQU code and identified by CCD imaging system and CF code. The experimental results proved that the divertor configuration and siliconization could strongly decrease the plasma radiation and impurity emission and reduce the recycling effectively. Density limit disruption has been analyzed and some interesting phenomena, such as asymmetric cold pulse perturbation and snake-like perturbation, have been observed during Molecular Beam Injection. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Alfvén Waves (AW) were launched in tokamak (TCABR) density limit plasmas for the first time. Experimental evidence of plasma heating is backed up by calculations from an 1-D numerical cylindrical code, based on the toroidal electric field diffusion. Simultaneously, increase in the density limit and plasma pressure with negligible impurities level launched by the AW antennas were also observed, without major appearance of a resistive disruption. The increase in the density limit and the heating might be related to the expected edge and off-axis AW power deposition, respectively, in agreement with the calculation performed by an 1-D numerical code linked to ASTRA. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]