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Deals with a study aimed to show experimental and modeling results which indicate that significant neutral uniformity variations can occur in high density plasma (HDP) processing tools. Origin of the neutral pumping mechanism; Importance of highly uniform plasma ion and reactive neutral densities in HDP reactors; Measurement of radial profiles of plasma density, electron temperature and neutral pressure; Data interpretation; Conclusion.

The intermediate oscillatory phase during the L–H transition, termed the I-phase, is studied in the EAST superconducting tokamak using a newly developed dual gas puff imaging (GPI) system near the L–H transition power threshold. The experimental observations suggest that the oscillatory behaviour appearing at the L–H transition could be induced by the synergistic effect of the two components of the sheared m, n = 0 E × B flow, i.e. the turbulence-driven zonal flow (ZF) and the equilibrium flow. The latter arises from the equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outwards into the far scrape-off layer (SOL) and inwards into the core plasma. The turbulence Reynolds stress is directly measured using the GPI system during the I-phase, providing direct evidence of kinetic energy transfer from turbulence to ZFs at the plasma edge. The GPI observations strongly suggest that the SOL transport physics and the evolution of pressure gradient near the separatrix play an important role in the L–I–H transition dynamics. To highlight these new physics, the previous predator–prey model is extended to include a new equation for the SOL physics. The model successfully reproduces the L–I–H transition process with several features comparing favourably with GPI observations. [ABSTRACT FROM AUTHOR]

Examines the paradigm of shear suppression of turbulence as the mechanism for the low to high (L to H) confinement mode transition, by quantitative comparison of the predictions of the paradigm with experimental results from the DIII-D tokamak. Triggering of a VxB rotation due to the L to H transition; Spatial dependence of the turbulence reduction.

Abstract: The one-dimensional time-delay-estimation (TDE) velocity technique infers the velocity of plasma fluctuations using cross-correlations between two spatially separated signals, and here this technique is evaluated using experimental results and numerical simulations. Probe arrays and a fast-framing imaging camera are used to measure the azimuthal fluctuation propagation speed in a cylindrical magnetized plasma device. The time-averaged TDE velocity field obtained in this way is found to be approximately 30% larger than Mach probe measurements of the plasma fluid velocity, suggesting that the TDE method infers the plasma E × B velocity and presumably a diamagnetic flow. The TDE technique is also applied to turbulent simulation data with known velocity fields. The results show that for the TDE parameters chosen here, the TDE technique can be used to infer the large-scale, slowly varying velocity, but that small scale or turbulent velocity fields cannot be reliably inferred. [Copyright &y& Elsevier]

We create a model for H retention depth profiles in W and subsequently model how this profile changes after isotope exchange. This is accomplished by calculating how trapping defects in W accumulate D (or H) inventory as W is being exposed to plasma. Each trapping site is characterized by a trapping rate and a release rate, where the only free parameters are the distribution of these trapping sites in the material. The filled trap concentrations for each trap type are modeled as a diffusion process because post-mortem deuterium depth profiles indicate that traps are filled well beyond the ion implantation zone (2–5 nm). Using this retention model, an isotope exchange rate is formulated. The retention model and isotope exchange rate are compared to low temperature isotope exchange experiments in tungsten with good agreement. The limitations of the current model highlight important physics and motivate future work. [ABSTRACT FROM AUTHOR]

Abstract: The effect of H isotope exchange and radiation damage on the retention of D in W was examined in the PISCES linear plasma device. W samples were treated with D plasma at low sample temperatures (473K), with a fluence of 1026 ions/m2 and ion energies of 150eV. Each sample was then exposed to varying doses of H plasma with similar sample temperature and plasma conditions to fluences ranging from 0 to 1026 ions/m2, to examine the effectiveness of isotope exchange as a means of tritium removal. The D(3He, p)4He nuclear reaction was used to measure D concentration profiles up to a depth of 7.7μm. Thermal desorption spectroscopy (TDS) was used to determine the D retained throughout the bulk of the sample. Isotope exchange allows for a unique study of atomic migration by separately examining the diffusion of implanted atoms from those bombarding the surface. D atoms are exchanged out of traps as a result of H plasma bombardment and diffuse until either falling into another trap or reaching the surface to recombine and escape. Radiation damage at levels of 0.01, 0.1, and 1 displacements per atom (dpa) was carried out before plasma exposure on some samples with 2MeV Cu ions as a surrogate for damage caused by fusion neutrons. The Cu ion damage was compared to damage induced by 6MeVW ions to see if there is an effect of Cu contamination on retention. We saw little difference in Cu versus W ion damage at low dpa, but at 1dpa, where Cu content reached 65appm, contamination seems to be significant. Retention measurements showed that ion damage has little effectiveness on isotope removal at these sample temperatures; however, there is evidence to suggest that the trapping mechanisms in W change as damage is increased. [Copyright &y& Elsevier]

Abstract: Hydrogen isotope exchange experiments in tungsten samples were conducted in the PISCES linear plasma device to examine this technique as a means of tritium removal. Tungsten samples were first exposed to deuterium plasma in typical divertor conditions to a fluence of 1026 ions/m2 while maintaining a sample temperature below 373K and subsequently exposed to hydrogen plasma at varying fluences (1023–1026 ions/m2). Bulk retention was measured by thermal desorption spectroscopy (TDS), and the D(3He, p)4He reaction was used to obtain concentration profiles of deuterium. The effects of neutron damage were simulated by exposing some samples to 2.5MeV Cu ions. Ion induced damage increases deuterium inventory in the near surface region and decreases diffusion into the bulk. Although displacement damage allows more deuterium to be retained near the surface where much of the isotope exchange takes place, the efficiency of isotope exchange in the bulk was reduced. [Copyright &y& Elsevier]