Nonlinear Kinetic Ion Response to Small Scale Magnetic Islands in Tokamak Plasmas: Neoclassical Tearing Mode Threshold Physics.

A new drift-kinetic theory of the ion response to magnetic islands in tokamak plasmas is presented. Small islands are considered, with widths w much smaller than the plasma radius r, but comparable to the trapped ion orbit width ρ_{bi}. An expansion in w/r reduces the system dimensions from five down to four. In the absence of an electrostatic potential, the ions follow stream lines that map out a drift-island structure that is identical to the magnetic island, but shifted by an amount ∼ few ρ_{bi}. The ion distribution function is flattened across these drift islands, not the magnetic island. For small islands, w∼ρ_{bi}, the shifted drift islands result in a pressure gradient being maintained across the magnetic island, explaining previous simulation results [E. Poli et al., Phys. Rev. Lett. 88, 075001 (2002)PRLTAO0031-900710.1103/PhysRevLett.88.075001]. To maintain quasineutrality an electrostatic potential forms, which then supports a pressure gradient in the electrons also. This influence on the electron physics is shown to stabilize small magnetic islands of width a few ion banana widths, providing a new threshold mechanism for neoclassical tearing modes-a key result for the performance of future tokamaks, including ITER.

Nonlinear Stability and Saturation of Ballooning Modes in Tokamaks.

The theory of tokamak stability to nonlinear "ballooning" displacements of elliptical magnetic flux tubes is presented. Above a critical pressure profile the energy stored in the plasma may be lowered by finite (but not infinitesimal) displacements of such tubes (metastability). Above a higher pressure profile, the linear stability boundary, such tubes are linearly and nonlinearly unstable. The predicted saturated flux tube displacement can be of the order of the pressure gradient scale length. Plasma transport from these displaced flux tubes may explain the rapid loss of confinement in some experiments.

Explosive instability and erupting flux tubes in a magnetized plasma.

The eruption of multiple flux tubes in a magnetized plasma is proposed as a mechanism for explosive release of energy in plasmas. A significant fraction of the linearly stable isolated flux tubes are shown to be metastable in a box model magnetized atmosphere in which ends of the field lines are embedded in conducting walls. The energy released by destabilizing such field lines can be a large proportion of the gravitational energy stored in the system. This energy can be released in a fast dynamical time.

Rapid, minimally invasive adult voluntary male circumcision: A randomised trial.

BACKGROUND: Voluntary medical male circumcision (VMMC) is a priority HIV preventive intervention. To facilitate male circumcision scale- up, the World Health Organization is actively seeking circumcision techniques that are quicker, easier, and safer than open surgical methods. OBJECTIVE: To compare conventional open surgical circumcision with suturing with a minimally invasive technique using the Gomco circumcision clamp plus tissue adhesive. METHODS: We conducted a non-blinded randomised controlled trial comprising 200 male volunteers >18 years of age, seen at the outpatient university teaching clinic of the Catholic University of Mozambique. We compared two interventions - open surgical circumcision with suturing v. Gomco instrument plus tissue adhesive. Our primary outcome was intraoperative time and our secondary outcomes included: ease of performance, post-operative pain, adverse events, time to healing, patient satisfaction and cosmetic result. RESULTS: The intraoperative time was less with the Gomco/tissue adhesive technique (mean 12.8 min v. 22.5 min; p<0.001). Adverse events were similar except that wound disruption was greater in the Gomco/tissue adhesive group, with no difference in wound healing at 4 weeks. Levels of satisfaction were high in both groups. The cosmetic result was superior in the Gomco/tissue adhesive group. CONCLUSIONS: This study has important implications for the scale-up of VMMC services. Removing the foreskin with the Gomco instrument and sealing the wound with cyanoacrylate tissue adhesive in adults is quicker, is an easier technique to learn, and is potentially safer than open surgical VMMC. A disposable plastic, Gomco-like device should be produced and evaluated for use in resource-limited settings.

Kinetic instabilities that limit ß in the edge of a tokamak plasma: a picture of an H-mode pedestal.

Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak have sufficient resolution to capture plasma evolution during the short period between edge-localized modes (ELMs). Immediately after the ELM, steep gradients in pressure, P, and density, n(e), form pedestals close to the separatrix, and they then expand into the core. Local gyrokinetic analysis over the ELM cycle reveals the dominant microinstabilities at perpendicular wavelengths of the order of the ion Larmor radius. These are kinetic ballooning modes in the pedestal and microtearing modes in the core close to the pedestal top. The evolving growth rate spectra, supported by gyrokinetic analysis using artificial local equilibrium scans, suggest a new physical picture for the formation and arrest of this pedestal.

Design of a new Nd:YAG Thomson scattering system for MAST.

A new infrared Thomson scattering system has been designed for the MAST tokamak. The system will measure at 120 spatial points with approximately 10 mm resolution across the plasma. Eight 30 Hz 1.6 J Nd:YAG lasers will be combined to produce a sampling rate of 240 Hz. The lasers will follow separate parallel beam paths to the MAST vessel. Scattered light will be collected at approximately f/6 over scattering angles ranging from 80 degrees to 120 degrees. The laser energy and lens size, relative to an existing 1.2 J f/12 system, greatly increases the number of scattered photons collected per unit length of laser beam. This is the third generation of this polychromator to be built and a number of modifications have been made to facilitate mass production and to improve performance. Detected scattered signals will be digitized at a rate of 1 GS/s by 8 bit analog to digital converters (ADCs.) Data may be read out from the ADCs between laser pulses to allow for real-time analysis.

Binocular rivalry in migraine.

Cortical hyperexcitability in migraine could arise from abnormally weak inhibition or from strengthened intracortical excitatory mechanisms. The present study employed binocular rivalry to differentiate between these possibilities. Rivalry between static oriented grating patterns was examined in migraine with aura (MA), migraine without aura (MoA) and headache-free control participants. A non-significant trend toward elevated mean dominance intervals (monocular percepts, in seconds) was seen in both migraine groups at all contrasts. Second, significant interocular differences in rivalry dominance durations were seen in the MoA group compared with controls; this difference also approached significance in the MA group. Finally, both MA and MoA exhibited significantly greater visual discomfort than the control group in the presence of both static stripes and flickering visual stimuli. The rivalry results provide no support for weakened intracortical inhibition in migraine. Optical or neural precortical differences in the eyes' input strengths paired with enhanced recurrent cortical excitation can explain these findings.

Evolution of filament structures during edge-localized modes in the MAST Tokamak.

Edge-localized modes (ELMs) are repetitive instabilities that occur in the outer region of tokamak plasmas. This Letter provides new information on and the implications of the evolution of the filament structures observed during ELMs in the MAST tokamak. The filaments exist for the time over which particles are being released into the scrape off layer. They start off at the plasma edge rotating at the velocity of the pedestal, and then decelerate toroidally and accelerate radially outwards. As the filaments propagate radially they remain aligned with the local magnetic field line.

Spatial and temporal structure of edge-localized modes.

This Letter provides information on the spatial and temporal structure of periodic eruptions observed in magnetically confined laboratory fusion plasmas, called edge-localized modes (ELMs), and highlights similarities with solar eruptions. Taken together, the observations presented in this Letter provide strong evidence for ELMs being associated with a filamentlike structure. These filaments are extended along a field line, are generated on a 100 micros time scale, erupt from the outboard side, and connect back into the plasma. Such structures are predicted by a theoretical model based on the "ballooning" instability, developed for both solar and tokamak applications.

Theory for explosive ideal magnetohydrodynamic instabilities in plasmas.

Flux tubes confined in tokamaks are observed to erupt explosively in some plasma disruptions and edge localized modes. Similar eruptions occur in astrophysical plasmas, for example, in solar flares and magnetospheric substorms. A single unifying nonlinear evolution equation describing such behavior in both astrophysical and tokamak plasmas is derived. This theory predicts that flux tubes rise explosively, narrow, and twist to pass through overlying magnetic field lines without reconnection.

Role of flow shear in the ballooning stability of tokamak transport barriers.

A tokamak's confinement time is greatly increased by a transport barrier (TB), a region having a high pressure gradient and usually also a strongly sheared plasma flow. The pressure gradient in a TB can be limited by ideal magnetohydrodynamic instabilities with a high toroidal mode number n ("ballooning modes"). Previous studies in the limit n--> infinity showed that arbitrarily small (but nonzero) flow shears have a large stabilizing influence. In contrast, the more realistic finite n ballooning modes studied here are found to be insensitive to sub-Alfvénic flow shears, provided the magnetic shear s approximately 1 (typical for TBs near the plasma's edge). However, for the lower magnetic shears that are associated with internal transport barriers, significantly lower flow shears will influence ballooning mode stability, and flow shear should be retained in the analysis of their stability.

Neoclassical tearing physics in the spherical tokamak MAST.

Results from MAST provide a first test of neoclassical tearing mode physics in the spherical tokamak (ST). The mode accounts for the main performance limit in conventional tokamaks. Its behavior in the ST is remarkably well described by existing theoretical models, although it is more readily seeded by sawtooth events in these scenarios. Modeling confirms the significance of stabilizing field-curvature effects. This provides good grounds for optimism that with suitable control of profiles, it may be possible to avoid these modes in the ST.

Finite Larmor-radius theory of magnetic island evolution.

Gyrokinetic theory is used to investigate the effect of the polarization drift on magnetic island evolution. Three regimes are found. For island phase velocities between the ion- and electric-drift velocities, the polarization current is shown to be stabilizing. For phase velocities between the electric- and electron-drift velocities, the island emits drift waves. This results in a radiative drag force. For all other phase velocities the polarization current is destabilizing, in agreement with the fluid limit.

Dynamics of travelling waves in visual perception.

Nonlinear wave propagation is ubiquitous in nature, appearing in chemical reaction kinetics, cardiac tissue dynamics, cortical spreading depression and slow wave sleep. The application of dynamical modelling has provided valuable insights into the mechanisms underlying such nonlinear wave phenomena in several domains. Wave propagation can also be perceived as sweeping waves of visibility that occur when the two eyes view radically different stimuli. Termed binocular rivalry, these fluctuating states of perceptual dominance and suppression are thought to provide a window into the neural dynamics that underlie conscious visual awareness. Here we introduce a technique to measure the speed of rivalry dominance waves propagating around a large, essentially one-dimensional annulus. When mapped onto visual cortex, propagation speed is independent of eccentricity. Propagation speed doubles when waves travel along continuous contours, thus demonstrating effects of collinear facilitation. A neural model with reciprocal inhibition between two layers of units provides a quantitative explanation of dominance wave propagation in terms of disinhibition. Dominance waves provide a new tool for investigating fundamental cortical dynamics.

An inverse oblique effect in human vision.

In the classic oblique effect contrast detection thresholds, orientation discrimination thresholds, and other psychophysical measures are found to be smallest for vertical or horizontal stimuli and significantly higher for stimuli near the +/-45 degrees obliques. Here we report a novel inverse oblique effect in which thresholds for detecting translational structure in random dot patterns [Glass, L. (1969). Moiré effect from random dots. Nature, 223, 578-580] are lowest for obliquely oriented structure and higher for either horizontal or vertical structure. Area summation experiments provide evidence that this results from larger pooling areas for oblique orientations in these patterns. The results can be explained quantitatively by a model for complex cells in which the final filtering stage in a filter-rectify-filter sequence is of significantly larger area for oblique orientations.

Detecting shape deformation of moving patterns.

This study measured thresholds for the discrimination of rigidly and nonridgidly rotating patterns in two dimensions. The stimuli employed were closed contours created by the sum of two 'radial frequency' components and sensitivity to their deformation was measured as a function of the difference in the angular velocities of the components. Results show that thresholds do not depend on the specific shape of the pattern. To quantify the influence of local computations versus global pooling, thresholds were measured with parts of the pattern covered by (invisible) pie-shaped apertures. One finds thresholds are not simply a function of the total amount of pattern visible but exhibit a dependence on the number of apertures. Moreover, sensitivity to deformation could neither be fully explained on the basis of local computations nor by linear global summation. A simultaneous masking paradigm was employed to elucidate potential mechanisms involved in the computation of deformation. While 1D masks (horizontal gratings and translating random dots) only marginally elevate thresholds, rotating and expanding motion significantly impairs sensitivity. This indicates that detectors tuned to radial and circular motion are involved in the computation of shape deformation.

An fMRI study of the selective activation of human extrastriate form vision areas by radial and concentric gratings.

The ventral form vision pathway of the primate brain comprises a sequence of areas that include V1, V2, V4 and the inferior temporal cortex (IT) [1]. Although contour extraction in the V1 area and responses to complex images, such as faces, in the IT have been studied extensively, much less is known about shape extraction at intermediate cortical levels such as V4. Here, we used functional magnetic resonance imaging (fMRI) to demonstrate that the human V4 is more strongly activated by concentric and radial patterns than by conventional sinusoidal gratings. This is consistent with global pooling of local V1 orientations to extract concentric and radial shape information in V4. Furthermore, concentric patterns were found to be effective in activating the fusiform face area. These findings support recent psychophysical [2,3] and physiological [4,5] data indicating that analysis of concentric and radial structure represents an important aspect of processing at intermediate levels of form vision.