Probing internal transport barriers with heat pulses in JET

The first electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion internal transport barriers (ITB) have been performed in JET using ion cyclotron resonance heating in mode conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by subcritical transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation.     

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.     

Critical threshold behavior for steady-state internal transport barriers in burning plasmas

Burning tokamak plasmas with internal transport barriers are investigated by means of integrated modeling simulations. The barrier sustainment in steady state, differently from the barrier formation process, is found to be characterized by a critical behavior, and the critical number of the phase transition is determined. Beyond a power threshold, alignment of self-generated and noninductively driven currents occurs and steady state becomes possible. This concept is applied to simulate a steady-state scenario within the specifications of the International Thermonuclear Experimental Reactor.     

The problem of transport phenomena in polycrystalline semiconductor thin films with potential barriers in the case where the carriers gas degenerated

The character of the potential relief near crystallite boundaries, on which the surface states are localized for the polycrystalline films with degenerated current carriers gas was found. It was shown than in the case (?′-E_F)/E_F ? 1 the barriers on the crystallite boundaries can be approximated by rectangular barriers. In the scattering mechanism the carriers with Fermi energy on such rectangular barriers was grounded and film model allowing to calculate the energy dependence of relaxation time was suggested. Conductivity and thermoelectric coefficients were calculated. The developed theory explains the experimental results, observed in degenerated films of A^IVB^VI and A₂^VB₃^VI semiconductors. In particular, weakening of the dependence σ(T) in films and the effect of thermoelectric coefficients increasing in comparison with monocrystals at the same carrier concentration were explained.     

A probe-based method for measuring the transport coefficient in the tokamak edge region

A new method for measuring the diffusion coefficient in the edge plasma of fusion devices is presented. The method is based on studying the decay of the plasma fluctuation spectrum inside a small ceramic tube having its mouth flush with a magnetic surface and its axis aligned along the radial direction. The plasma fluctuations are detected by an electrode, radially movable inside the tube. In the experiment described herein, which was performed in the edge region of the CASTOR tokamak, the electrode measured the floating potential. The experimental arrangement is the same used for the direct plasma potential measurements according to the “Ball-pen probe” [1], the design of which is based on the Katsumata probe principle. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma. Retracting the electrode into the tube, the signal power spectrum displays a decay. This decay is different for different frequencies, and is exponential. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive a value of the diffusion coefficient. The measurement were performed at different radial positions in the CASTOR edge region, so that a radial profile of the diffusion coefficient was obtained. Typical values ofD are of (2–3) m²/s, consistent with expectations from the global particle balance. The radial profile shows a tendency of the diffusion coefficient to increase going deeper into the plasma.     

Evolution of statistical properties of ion temperature gradient driven turbulence during the formation of internal transport barrier in a tokamak

Statistical properties of ion temperature gradient driven turbulence and anomalous flux are analysed in plasmas with low and improved energy confinement. The turbulence is simulated using the three dimensional fluid code, where the source is prescribed and the turbulence and temperature profile evolve self-consistently. The transition to the improved core confinement with an internal transport barrier is obtained by increasing the heating power. It is found that the momentums of the probability distribution function of flux fluctuations reduce to their Gaussian values when the transport barrier on the temperature profile forms. The formation of the oscillating structure with the properties of linear wave has been found at very high heating power. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002.     

The discrete ordinates method for anisotropic scattering in the neutron transport theory: The critical sphere problem

The spherical harmonics method for anisotropic scattering in the neutron transport theory related to the critical sphere problem was investigated by Yildiz [The spherical harmonics method for anisotropic scattering in neutron transport theory: the critical sphere problem. JQSRT 2001;71:25-37]. Some numerical results and figures that they provided are incorrect. The correct numerical results for the critical radius are obtained and tabulated for different scattering parameters by using the discrete ordinates method.     

Thermal wave method as a tool for determining the polarization profile in ferroelectric materials

The thermal square wave method based on irradiation of the crystal under investigation by a squarely modulated thermal wave at a particular frequency is described in detail. The choice of parameters for the quantitative determination of the pyroelectric characteristics of crystals is justified. The method is used to determined the conditions for the formation of a system of antiparallel domains in strontium barium niobate crystals.     

Electrical turbulence as a result of the critical curvature for propagation in cardiac tissue

In cardiac tissue, the propagation of electrical excitation waves is dependent on the active properties of the cell membrane (ionic channels) and the passive electrical properties of cardiac tissue (passive membrane properties, distribution of gap junctions, and cell shapes). Initiation of cardiac arrhythmias is usually associated with heterogeneities in the active and/or passive properties of cardiac tissue. However, as a result of the effect of wave front geometry (curvature) on propagation of cardiac waves, inexcitable anatomical obstacles, like veins and arteries, may cause the formation of self-sustained vortices and uncontrolled high-frequency excitation in normal homogeneous myocardium. (c) 1998 American Institute of Physics.     

Finite-size scaling as a tool for the search of the critical endpoint of QCD in heavy-ion data

We briefly discuss the role played by the finiteness of the system created in high-energy heavyion collisions (HIC’s) in the experimental search of the QCD critical endpoint and, in particular, the applicability of the predicting power of finite-size scaling plots in data analysis of current HIC’s.     

Evidence for internal resistive barriers in a crystal of the giant dielectric constant material: CaCu3Ti4O12

Complex impedance spectra were obtained on a crystal of CaCu₃Ti₄O₁₂ (CCTO) from 289 to 456 K. As in the case of ceramic CCTO, these spectra can be interpreted as arising from a conducting material containing insulating barriers. This is then further evidence for the existence of planar defects within crystals of CCTO that act as insulating barriers and produce the large dielectric constant through a space charge mechanism.     

The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.     

平行因子分析法在太湖水体三维荧光峰比值分析中的应用

以太湖水样三维荧光光谱数据为例,提出在采用平行因子分析法(PARAFAC)处理后的荧光数据中提取荧光峰强度计算荧光峰比值进行水环境分析评价的方法,较直接在水样原始荧光谱图中获取的荧光峰强度更加准确客观。天然水体中各水样间由于受荧光团复杂多样性等因素的影响,某类荧光物质荧光峰的激发发射波长位置并不是固定不变的,就同一水样而言各类荧光峰之间的相互重叠干扰也将影响到荧光峰强度和位置的准确判断。而在PARAFAC模型各因子中提取相应荧光峰值可以保证各水样间同类荧光物质荧光峰在同一位置又有效减弱同一水样中各类荧光物质荧光峰之间的相互干扰,更加高效准确的利用荧光峰比值进行水环境分析。区域差异性分析时水样因子得分比值的区域变化与原始荧光峰比值变化趋势是一致的。     

Domain wall propagation in antiferroelectric donor chains as a possible explanation for the 1-D transport properties of complex TCNQ salts

The structures are explored of viewing the structure of the permanent electric dipoles on the donor molecules in complex TCNQ salts as a set of independent, Ising-type, antiferro-electric chains. The chain character will entail the thermal excitation of 1-d domains. The walls in between these will produce extra local polarized charges on the TCNQ chainsm, so that wall propagation should contribute to the conductivity. The observed temperature and frequency dependences of conductivity and permeability can likewise be explained.     

Recent trends in the applications of sonochemical reactors as an advanced oxidation process for the remediation of microbial hazards associated with water and wastewater: A critical review

Water is one of the major sources that spread human diseases through contamination with bacteria and other pathogenic microorganisms. This review focuses on microbial hazards as they are often present in water and wastewater and cause various human diseases. Among the currently used disinfection methods, sonochemical reactors (SCRs) that produce free radicals combined with advanced oxidation processes (AOPs) have received significant attention from the scientific community. Also, this review discussed various types of cavitation reactors, such as acoustic cavitation reactors (ACRs) utilizing ultrasonic energy (UE), which had been widely employed, involving AOPs for treating contaminated waters. Besides ACRs, hydrodynamic cavitation reactors (HCRs) also effectively destroy and deactivate microorganisms to varying degrees. Cavitation is the fundamental phenomenon responsible for initiating many sonochemical reactions in liquids. Bacterial degradation occurs mainly due to the thinning of microbial membranes, local warming, and the generation of free radicals due to cavitation. Over the years, although extensive investigations have focused on the antimicrobial effects of UE (ultrasonic energy), the primary mechanism underlying the cavitation effects in the disinfection process, inactivation of microbes, and chemical reactions involved are still poorly understood. Therefore, studies under different conditions often lead to inconsistent results. This review investigates and compares other mechanisms and performances from greener and environmentally friendly sonochemical techniques to the remediation of microbial hazards associated with water and wastewater. Finally, the energy aspects, challenges, and recommendations for future perspectives have been provided.     

Perfluorinated surfactants as model charged systems for understanding the effect of confinement on proton transport and water mobility in fuel cell membranes. A study by QENS

We have investigated the dynamical properties of water confined in mesomorphous phases of perfluorinated sulfonic surfactants. These systems mimic the physico-chemical properties of the perfluorinated Nafion membranes which are used as electrolyte in fuel cells. As the surfactants offer the advantage to self-assemble in well defined organized phases (such as hexagonal and lamellar phases), they could be used as model charged systems to understand the structure-transport relationship in complex real materials. Indeed, the geometry as well as the typical confinement size can be easily controlled and tuned through water concentration and temperature. A QENS study of hexagonal and lamellar phases has been performed on both time-of-flight and backscattering spectrometers to cover a dynamic range from picoseconds to nanoseconds. Analysis of the data with localized translational diffusion models shows the existence of a strong confinement effect that depends on the geometry. Typical confinement sizes and diffusion coefficients can be extracted from the QENS analysis and compared to the Nafion membrane.