Role of reynolds stress-induced poloidal flow in triggering the transition to improved ohmic confinement on the HT-6M tokamak

Time and space resolved measurements of electrostatic Reynolds stress, radial electric field E(r), and plasma rotations have been performed across the transition to improved Ohmic confinement in the Hefei Tokamak-6M (HT-6M). The first experimental evidence of the correlation between the enhanced Reynolds stress gradient and the poloidal flow acceleration in the edge plasma is presented. The results indicate that the turbulence-induced Reynolds stress might be the dominant mechanism to create the sheared poloidal flow and E(r), which may further trigger the transition.     

ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A new model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. The model for the pedestal and ELMs is used in the ASTRA integrated transport code to follow the time evolution of tokamak discharges from L-mode through the transition from L-mode to H-mode, with the formation of the H-mode pedestal, and, subsequently, to the triggering of ELMs. Turbulence driven by the ion temperature gradient mode, resistive ballooning mode, trapped electron mode, and electron temperature gradient mode contributes to the anomalous thermal transport at the plasma edge in this model. Formation of the pedestal and the L-H transition is the direct result of flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by MHD instabilities. Two mechanisms for triggering ELMs are considered: ELMs are triggered by ballooning modes if the pressure gradient exceeds the ballooning threshold or by peeling modes if the edge current density exceeds the peeling mode threshold. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Mott transition in anharmonic confinement

Two effects are identified that affect the visibility of the Mott transition in an atomic gas in an optical lattice confined in a power-law potential. The transition can be made more pronounced by increasing the power law, but at the same time, experimental uncertainty in the number of particles will induce corresponding fluctuations in the measured condensate fraction. Calculations in two dimensions indicate that a potential slightly more flat-bottomed than a quadratic one is to be preferred for a wide range of particle number fluctuation size.     

Eigenvalue confinement in the problem of linear baroclinic instability for oceanic zonal flows

Summary The classical problem of baroclinic instability is examined from the point of view of eigenvalue confinement in the complex plane. The analysis is performed for a baroclinic zonal flow perturbed by a monochromatic plane wave, harmonic in the latitude. The main results, obtained from thea priori properties of the eigenvalue equation, yield estimates of the phase velocity and the growth rate of the perturbation. As a consequence, the spectrum results to be confined within a bounded region of the complex plane, which depends upon the zonal flux only through its extreme values and the maximum of the shear. The larger is the wave number of the meridional structure of the perturbation, the narrower is the obtained confinement: in the case of higher harmonics the present estimates sharpen the results known from the literature.

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Riassunto Si riesamina il problema classico dell'instabilità baroclinica lineare dal punto di vista della localizzazione dei relativi autovalori nel piano complesso. L'analisi è eseguita per un generico flusso zonale baroclinico perturbato da un'onda piana monocromatica, armonica nella latitudine. I risultati principali, ottenuti esaminando le proprietàa priori dell'equazione agli autovalori, forniscono stime sulla velocità di fase e il tasso di accrescimento della perturbazione. Ne consegue la localizzazione dello spettro in una regione limitata del piano complesso che risulta dipendere dal flusso zonale solo tramite i suoi valori estremi ed il massimo dello shear. Il confinamento ottenuto risulta tanto piú stretto quanto maggiore è il numero d'onda della struttura meridionale della perturbazione: nel caso delle armoniche superiori si migliorano le stime esistenti in letteratura.

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Резюме Исследуется классическая проблема бароклинной неустойчивости с точки зрения ограничения собственных значений в комплексной плоскости. Проводится анализ бароклинного зонного течения, возмущенного монохроматической плоской волной, гармонической по широте. Основные результаты, полученные из свойств уравнения для собственных значений, дают оценки для фазовой скорости и интенсивности роста возмущения. Воледствии этого, полученный спектр заключен внутри ограниченной области комплексной области, которая зависит от зонного потока только через его экстремальные значения и максимум сдвига. В случае высших гармоник полученные оценки позволяют улучшить результаты, имеющиеся в литературе.

     

Strong confinement and oscillations in two-component bose-einstein condensates

We present a new model of Bose-Einstein condensate dynamics based on strong confinement near the ground state. The model is based on a combined particle-wave view of the condensate and predicts oscillations in a two-component condensate, based on interference of nonspreading wave packets moving within a pair of tilted nearly square potentials. The oscillations are similar to those recently reported for a magnetically trapped 87Rb condensate, and the model's predictions give good quantitative agreement with the experiments.     

Colloidal glass transition observed in confinement

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.     

Tokamak residual zonal flow level in near-separatrix region

Residual zonal flow level is calculated for tokamak plasmas in the near-separatrix region of a diverted tokamak. A recently developed method is used to construct an analytic divertor tokamak configuration. It is shown that the residual zonal flow level becomes smaller but still keeps finite near the separatrix because the neoclassical polarisation mostly due to the trapped particles goes larger in this region.     

On the confinement phase transition for a colourless bag

Critical temperatures forSU(N)-singlet bags are determined and a deconfinement phase transition is exhibited.     

Axion oscillations and the quark-hadron phase transition

We consider the possibility that the quark-hadron phase transition occurs when the axion field passes through the minimum of its potential during its oscillation cycle. If this were to occur, the axion field would gain no energy from the associated increase in mass thus permitting the cosmological bound on the axion decay constant to be raised. However, we find that the probability of this happening is small.     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

Intramicellar glass transition and liquid dynamics in soft confinement

We explore the dynamics of viscous propylene glycol (PG) near its glass transition for the case of soft spatial confinement. The supercooled liquid is geometrically restricted by the reverse micelles of a glass-forming PG/AOT/decalin microemulsion, with the intramicellar dynamics being probed by triplet state solvation dynamics. While hard confinement by porous solids is known to result in slower dynamics and an increased glass transition temperature T(g) of PG, the nanodroplets suspended in a more fluid environment display faster structural relaxation, equivalent to a reduction of T(g) as observed in freestanding polymer films.