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High-confinement (H-mode) operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit and reduced core transport regions needed for advanced tokamak operation. Experimental results from DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (Intemational Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation that is ELM-free and yet has good density and impurity control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τ[sub E]), yielding a quiescent double barrier regime. By slowly ramping the input power, we have achieved β[sub N]H[sub 89]:7 for up to 5 times the τ[sub E] of 150 ms. β[sub N]H[sub 89] values of 7 substantially exceed the value of 4 routinely achieved in the standard ELMing H mode. The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density and impurity control in the quiescent H mode is possible because of the presence of an edge magnetohydrodynamic (MHD) oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected. [ABSTRACT FROM AUTHOR]

In DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improvedunderstanding for electron thermal transport within the plasma. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in the electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the T3 profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm-1), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wave number, but it is computed to be robustly stable near the axis. Consequently, in the plasmas we have examined, calculations of drift wave linear stability do not explain the observed transport near the axis in plasmas with or without additional electron heating, and there are probably other processes controling transport in this region.. [ABSTRACT FROM AUTHOR]

Linear cavity optically pumped far-infrared (FIR) lasers have been used extensively for plasma diagnostic measurements. However, recent experiments have suffered from frequency pulling induced by feedback of FIR radiation. The application of a ring cavity to FIR lasers is investigated as a means of improving frequency stability, particularly in the presence of strong feedback. Unidirectional traveling wave laser operation is observed, and so feedback radiation does not interact directly with the dominant cavity mode. The resultant frequency pulling is observed to be minimal (∼5 kHz) in comparison with a linear cavity (Δf ∼ 100 kHz) when a feedback power of Δf ∼ 2% is studied. In addition, the ring laser eliminates the two-photon light shift, and, therefore, significantly (a factor of 10) improves the absolute frequency stability near the absorption line center. [ABSTRACT FROM AUTHOR]