The paradigm of shear suppression of turbulence as the mechanism for the low to high confinement mode (L to H) transition is examined by quantitative comparison of the predictions of the paradigm with experimental results from the DIII-D tokamak [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. The L to H transition trigger is V×B rotation, not the main ion pressure gradient. The radial electric field Er shear increases before the fluctuation suppression, consistent with increasing Er shear as the cause of the turbulence suppression. The spatial dependence of the turbulence reduction is consistent with shear suppression for negative Er shear. For positive Er shear, the turbulence suppression is consistent with the effect of Er curvature for modes for which an Er well is destabilizing. Finally, the transport barrier depends on the phase angle between the density and potential fluctuations inside the Er well, an effect not included in existing L to H transition models.

Combined theoretical and experimentalwork has resulted in thecreation of a paradigm which has allowedsemi-qusntitative understandingof the edge confinementimprovement that occursin the H-mode. Shear in the E × B flowof the fluctuations in theplasma edge can lead to decorrelation ofthefluctuations, decreaae!d radialcorr._ lation lengthsand reduced turbulenttransport.Changes in the radialelectric field, the densityfluctuations and the edge transportconsistent with shear stabilization of turbulencehave been seen ha severaltokamaks, The purpose of thispaper isto discuss the most recentdata in the light of the basicparadigm of electric field shear stabilization and to critically cx)mparethe experimentalresults with various theories.

Large 70 Hz Type-I edge localized modes (ELMs) are converted into small 130 Hz oscillations using edge resonant magnetic perturbations (RMPs) from a coil with currents 60.4% I p in double null DIII-D plasmas. When the RMP is properly phased with respect to the background field errors, all but a few isolated ELM-like events are suppressed. The impulsive pedestal energy loss DE ELM /Dt 1/2 to the scrape-of layer is reduced a factor of P20 relative to the Type-I ELMs and the core confinement is unaffected by the perturbation field. Significant changes in the properties of the ELMs are also observed when edge RMPs are applied to lower single null plasmas but the nature of these changes are much more complex. Both lower single null and double null plasmas are being studied to determine how ELM control techniques based on the application of edge RMPs can be expected to scale to future devices such as ITER.

The reduction in size of Type I edge localized modes (ELMs) with increasing density is explored in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] for the purpose of studying the underlying transport of ELM energy. The separate convective and conductive transport of energy due to an ELM is determined by Thomson scattering measurements of electron density and temperature in the pedestal. The conductive transport from the pedestal during an ELM decreases with increasing density, while the convective transport remains nearly constant. The scaling of the ELM energy loss is compared with an edge stability model. The role of the divertor sheath in limiting energy loss from the pedestal during an ELM is explored. Evidence of outward radial transport to the midplane wall during an ELM is also presented.

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A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ⩽3 MW. Formation of MARFEs on closed field lines was avoided with low safety factor (q) operation but was observed at high q, qualitatively consistent with theory. By using pellet fueling and optimizing discharge parameters to avoid each of these limits, an operational space was accessed in which density ∼1.5×Greenwald limit was achieved for 600 ms, and good H-mode confinement was maintained for 300 ms of the density flat-top. More significantly, the density was successfully increased to the limit where a central radiative collapse was observed, the most fundamental density limit in tokamaks.

‘Puff-and-pump’ radiating divertor scenarios, applied to both upper single-null (SN) and double-null (DN) H-mode plasmas, result in a 30–60% increase in radiated power with little or no decrease in τ E . Argon was injected into the private flux region of the upper divertor, and plasma flow into the upper divertor was enhanced by a combination of deuterium gas puffing upstream of the divertor targets and particle pumping at the targets. For the same constant deuterium injection rate, argon penetrated the main plasma of SNs more rapidly and reached a higher steady-state concentration when the B × ∇ B -ion drift direction was towards the divertor (V ∇B↑) rather than away from the divertor (V ∇B↓). We also found that the initial rate at which argon accumulated inside DN plasmas was more than twice that of comparable SN plasmas having the same B × ∇ B -ion drift direction. In DNs, the radiated power was not shared equally between divertors during argon injection. Only when the B × ∇ B ion drift direction was away from the divertor were both significant increases in divertor radiated power and an accumulation of argon in the divertor observed, based on spectroscopic measurements of Ar II. Our data suggest that an unbalanced DN shape where the B × ∇ B -ion drift is directed away from the dominant divertor may provide the best chance of successfully coupling a radiating divertor approach with a higher performance H-mode plasma.

We present theglob.al analysis ofs recent survey of theH-mode power threshold inDIII-Dusing D°.-., D _ NBI after boronization ofthevacuumvessel. Single parameter scans ofBT, Ip,density, andplasma shapehavebeencarried outontheDIII-D tokamakforneutral beam heated single-null anddouble-null diverted plasmas. Insinglenull discharges, thepowerthreshold isfoundtoincrease approxinmtely linearly withBT and n= butremains independent ofZp. Indouble-null discharges, thepowerthreshold is foundtobeappraximately independent ofbothBT andhe.Various shapeparameters such asplasma-wall gapshadonlys weakeffect onthepowerthreshold. Imbalancing thedouble null configuration resulted ina large increase inthethreshold power. * Sand|a National Laboratories. t Massachusetts Institute of Technolo_j,. UnJver_ty ofCalifornia_ Los Angeles.