Jose Armando Boedo

Transport by intermittent convection in the boundary of the DIII-D tokamak

Intermittent plasma objects (IPOs) featuring higher pressure than the surrounding plasma, and responsible for ∼50% of the E×BT radial transport, are observed in the scrape off layer (SOL) and edge of the DIII-D tokamak [J. Watkins et al., Rev. Sci. Instrum. 63, 4728 (1992)]. Conditional averaging reveals that the IPOs, produced at a rate of ∼3×103u200as−1, are positively charged and also polarized, featuring poloidal electric fields of up to 4000 V/m. The IPOs move poloidally at speeds of up to 5000 m/s and radially with E×BT/B2 velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The IPOs appear in the SOL of both L and H mode discharges and are responsible for nearly 50% of the SOL radial E×B transport at all radii; however, they are highly reduced in absolute amplitude in H-mode conditions.

Fluctuation-driven transport in the DIII-D boundary

Cross-field fluctuation-driven transport is studied in edge and scrape-off layer (SOL) plasmas in the DIII-D tokamak using a fast reciprocating Langmuir probe array allowing local measurements of the fluctuation-driven particle and heat fluxes. Two different non-diffusive mechanisms that can contribute strongly to the cross-field transport in the SOL of high-density discharges are identified and compared. The first of these involves intermittent transport events that are observed at the plasma separatrix and in the SOL. Intermittence has qualitatively similar character in L-mode and ELM-free H-mode. Low-amplitude ELMs observed in high-density H-mode produce in the SOL periods with cross-field transport enhanced to L-mode levels and featuring intermittent events similar to those in L-mode. The intermittent transport events are compatible with the concept of plasma filaments propagating across the SOL due to E×B drifts. The intermittent character of the transport in the SOL is also in agreement with predictions of the non-linear numerical simulations performed with an imposed driving flux. Another type of non-diffusive transport is often seen in high-density H-modes with prolonged ELM-free periods, where the transport near the separatrix is dominated by quasi-coherent modes driving particle and/or heat fluxes exceeding L-mode levels. These modes may play an important role by providing particle and/or heat exhaust between ELMs.

Scaling of plasma turbulence suppression with velocity shear

The scaling of plasma turbulence, turbulent particle flux and cross-phase with shear in the edge of the TEXTOR-94 tokamak is obtained from measurements from fast scanning probes and compared with various existing analytical theories. It is found that the scaling can be expressed as a second order polynomial and that the cross-phase plays a key role in the suppression of particle flux. The variable rate of shear, kept below the value required to produce a low to high particle confinement transition, was obtained by changing, on a shot to shot basis, the voltage applied to an electrode introduced 4 cm into the plasma.

Physics of confinement improvement of plasmas with impurity injection in DIII-D

External impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence. Suppression of the long wavelength turbulence and transport reduction are attributed to synergistic effects of impurity induced enhancement of E × B shearing rate and reduction of toroidal drift wave turbulence growth rate. A prompt reduction of density fluctuations and local transport at the beginning of impurity injection appears to result from an increased gradient of toroidal rotation enhancing the E × B shearing. Transport simulations carried out using the National Transport Code Collaboration demonstration code with a gyro-Landau fluid model, GLF23, indicate that E × B shearing suppression is the dominant transport suppression mechanism.

Fluctuations and anomalous transport (in tokamaks, particularly TEXT)

Links between turbulent fluctuations and transport processes in tokamaks, particularly in TEXT, are discussed. In the plasma edge probes allow detailed measurements of the quantities required to directly determine the fluctuation driven fluxes. The total flux of particles is well explained by the measured electrostatic fluctuation driven flux. However, a satisfactory model to explain the origin of the fluctuations has not been identified. The processes responsible for determining the edge electron heat flux are less clear. In the confinement region experimental observations are restricted to measurements of density and potential fluctuations and their correlations. Three distinct features have been identified: (1) the ubiquitous broadband turbulence and (2) a quasi-coherent feature, both of which propagate in the electron diamagnetic drift direction, and (3) an ion feature which by definition propagates in the ion drift direction. The characteristics of the measured fluctuations are discussed and compared with the predictions of various models. Comparisons between measured particle, electron heat and ion heat fluxes, and those fluxes predicted to result from the measured fluctuations, are made.

Physics of the detached radiative divertor regime in DIII-D

This paper summarizes results from a two-dimensional (2D) physics analysis of the transition to and stable operation of the partially detached divertor (PDD) regime induced by deuterium injection in DIII-D. The analysis [1] shows that PDD operation is characterized by a radiation zone near the X-point at -15 eV which reduces the energy flux into the divertor and thereby also reduces the target plate heat flux, an ionization zone below the X-point which provides a deuterium ion source to fuel parallel flow down the outer divertor leg, an ion-neutral interaction zone in the outer leg which removes momentum and energy from the flow and finally a volume recombination zone above the target plate which reduces the particle flux to the low levels measured on the plates and thereby also contributes to reduction in target plate heat flux.

Experimental investigations on the role of flow shear in improved confinement

Edge transport and confinement changes induced by radial electric fields, externally imposed in TEXTOR-94 by means of electrode biasing, were investigated. The edge profiles of the electric field were measured continuously by a specially developed nine-tip Langmuir probe, thus allowing the study of the relative spatial and temporal dynamics of and its radial gradient and possible ensuing transport changes. A particle transport barrier is found to be built up as the electric field gradient increases, thus strengthening the conjecture that stabilization is a viable mechanism for improved confinement in tokamaks.

Divertor geometry effects on detachment in TCV

Experimental observations and some preliminary results from B2-Eirene code modelling of divertor detachment in the TCV tokamak are reported, with emphasis on the aspects of this detachment related to divertor geometry. The contribution is restricted to deuterium fuelled ohmic plasmas for which the CB drift is directed away from the X-point of lower single-null, open diverted equilibria with fixed elongation and triangularity. Unlike more conventional diverted equilibria, however, the configurations described are characterised by both a very short divertor poloidal depth on a vertical target and a very long poloidal depth on a horizontal target. Results are presented from density ramp discharges in which the outer (high poloidal depth) divertor is already in the high recycling regime at the start of the density ramp and in which varying degrees of detachment are obtained depending on the magnitude of the imposed outer divertor flux expansion. In contrast, the inner divertor remains in the large Dart attached for all densities

H-mode threshold and dynamics in the National Spherical Torus Experiment

Edge parameters play a critical role in high confinement mode (H-mode) access, which is a key component of discharge optimization in present day toroidal confinement experiments and the design of next generation devices. Because the edge magnetic topology of a spherical torus (ST) differs from a conventional aspect ratio tokamak, H-modes in STs exhibit important differences compared with tokamaks. The dependence of the National Spherical Torus Experiment (NSTX) [C. Neumeyer et al., Fusion Eng. Des. 54, 275 (2001)] edge plasma on heating power, including the low confinement mode (L-mode) to H-mode (L-H) transition requirements and the occurrence of edge-localized modes (ELMs), and on divertor configuration is quantified. Comparisons between good L-modes and H-modes show greater differences in the ion channel than the electron channel. The threshold power for the H-mode transition in NSTX is generally above the predictions of a recent International Tokamak Experimental Reactor (ITER) [ITER Physics Basis Edi...

The influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit nGW) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at r=a6 0:6) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.