Peripheral Region Of Plasma Research Articles

Numerical investigation into the peripheral energetic-particle-driven MHD modes in Heliotron J with free boundary hybrid simulation

The interaction between energetic particles (EPs) and EP-driven magnetohydrodynamic (MHD) instabilities in Heliotron J, a low-shear helical axis stellarator/heliotron, is investigated with MEGA, a hybrid MHD-EP simulation code with the free boundary condition, on the last closed flux surface. The n/m = 1/2 energetic particle mode (EPM) and the n/m = 2/4 global Alfvén eigenmode (GAE) in the peripheral plasma region of Heliotron J are successfully modeled with the free boundary condition. The free boundary condition affects the EP driving rate of the n/m = 1/2 EPM and the n/m = 2/4 GAE through the changes in the mode spatial profile. Under the fixed boundary condition, the linear growth rate of the EP-driven MHD mode with low mode numbers is underestimated. The interaction between EP and these experimentally-observed modes is kinetically analyzed. It is found that the strongest EP–shear Alfvén wave interactions arise from the toroidicity-induced resonances in the high-velocity region. These high-velocity EPs can efficiently interact with the peripheral EP-driven mode. The additional toroidally-asymmetric resonances are localized in the low-velocity region; therefore, their effects are weak for the bump-on-tail EP velocity distribution function.

Solute dispersion in two-fluid flowing through tubes with a porous layer near the absorbing wall: Model for dispersion phenomenon in microvessels

Present study concerns the solute dispersion analysis in a two-fluid model of blood flow through tubes with a thin porous layer near the absorbing wall. A thin porous layer near the wall represents an endothelial glycocalyx layer the composition of which may be attributed to the accumulation of carbohydrates, absorbed plasma proteins and macromolecules in a layer at the wall that affects the plasma flow (Secomb et al., 1998. A model for red blood cell motion in glycocalyx-lined capillaries. Am. J. Physiol. 274 (Heart Circ. Physiol. 43), H1016-H1022; Pries et al., 2000. The endothelial surface layer. Eur. J. Physiol. 440, 653–666). Two-fluid model for blood flow is considered in which the central region of the blood vessel is occupied by Herschel-Bulkley fluid with variable viscosity and a peripheral region of plasma surrounded over the central region is occupied by a Newtonian fluid with constant viscosity. Governing equations are solved analytically for all the regions (central, intermediate and porous regions) and the approach of Sankarasubramanian and Gill (Sankarasubramanian, R, Gill, W N, 1973. Unsteady convective diffusion with interphase mass transfer. Proc. R. Soc. London Ser. A 333, 115–132) is followed to solve the diffusion equation by series expansion method. The impact of porous layer (and hence the porous layer parameters), varying viscosity, plasma layer and wall absorption parameters on the diffusion process like convective, dispersion and mean concentration have been analyzed. A comparative investigation of solute dispersion between Newtonian and non-Newtonian fluids has also been discussed. It is perceived that the presence of such a layer reduces the convective and axial dispersion for a highly reactive wall. A remarkable observation is that a reduced porosity near the wall contributes towards reduced average concentration of the solvent. The analysis of the porous layer inside the arterial wall may be used to understand the diffusion process for flow through arteries with a deposition of a porous layer near the absorbing wall.

Formation of nonmonotonic profiles of densities and fluxes of charged particles and ambipolar field reversal in argon dusty plasmas

In the present work, various scenarios of the spatial distributions of argon dusty plasma parameters are considered, focusing on the case of a large density of dust particles. It is shown that in this case a scenario with the inversion of the ambipolar field sign, considered in earlier studies by authors for plasma with heavy negative ions, can be realized. In this case, the ionization inside the dust cloud becomes small in comparison with the volume loss of charges inside the dust cloud, and the lack of charges is compensated by their arrival from the outer peripheral plasma region. This transition is accompanied by the phenomenon of the reversal of the sign of the radial electric field at the boundary of the dust cloud, and the formation of nonmonotonic profiles of the electron density with a shift of the maximum electron density from the center to the periphery. The radial stability of the dust particles cloud is provided by the balance of the increasing dust charge density gradient in the presence of the pushing out ambipolar field.

Direct measurements of internal structures of born-locked modes and the key role in triggering tokamak disruptions

The evolution of the magnetic topology between the outer separatrix of a large m = 2/n = 1 island (m and n: poloidal and toroidal numbers) and the last closed flux surface after mode locking is, for the first time, directly measured. Edge locked island chains with multiple helicity and very narrow widths are discovered to be destabilized and govern the cooling process in the plasma peripheral region. These edge small nonrotating islands are initially well separated from the 2/1 island, leading to a long quasistationary phase, but later trigger thermal quench immediately after they start to overlap with the 2/1 island, producing a broad stochastic layer deep into the plasma midradius.

Temperature gradient driven Alfvén instability producing inward energy flux in stellarators

Destabilizing influence of plasma inhomogeneity on Alfvén eigenmodes in stellarators is considered. It is found that the diamagnetic frequency can strongly increase due to the resonance interaction of particles and Alfvén eigenmodes. This occurs when the particle resonance velocity exceeds the thermal velocity, in which case the role of superthermal particles enlarges. Then Alfvén eigenmodes can be destabilized even in the absence of the energetic ion population. It is shown that in the case of the temperature distribution with a large gradient at the periphery, the destabilized mode can channel the energy from the peripheral plasma region to the inner region. A stability analysis employing a model temperature profile of the ions was carried out for the Wendelstein 7-X stellarator. It indicates that the considered mechanism could lead to an Alfvén instability accompanied with the inward energy flux in the first W7-X experiments where long-lasting high-frequency oscillations were observed.

Asymptotic-Preserving scheme for a strongly anisotropic vorticity equation arising in fusion plasma modeling

The electric potential is an essential quantity for the confinement process of tokamak plasmas, with important impact on the performances of fusion reactors. Understanding its evolution in the peripheral region – the part of the plasma interacting with the wall of the device – is of crucial importance, since it governs the boundary conditions for the burning core plasma. The aim of the present paper is to study numerically the evolution of the electric potential in this peripheral plasma region. In particular, we are interested in introducing an efficient Asymptotic-Preserving (AP) numerical scheme capable to cope with the strong anisotropy of the problem as well as the non-linear boundary conditions, and this with acceptable computational costs. This work constitutes the numerical follow-up of the more mathematical paper by C. Negulescu, A. Nouri, Ph. Ghendrih, Y. Sarazin, Existence and uniqueness of the electric potential profile in the edge of tokamak plasmas when constrained by the plasma-wall boundary physics.

Characteristics of MHD instabilities for high beta plasmas in inward shifted LHD configurations

Characteristics of the MHD instabilities for high beta LHD (large helical device) plasmas in the inward shifted configurations have been investigated by numerical simulations. On the condition that the magnetic Reynolds number is lower than the experimental value, the time evolution of the MHD instabilities is as follows: (1) resistive ballooning modes, the toroidal mode number of which is higher than the LHD’s toroidal pitch (), are destabilized in the plasma peripheral region; (2) low toroidal modes typified by are destabilized by the nonlinear mode coupling of the resistive ballooning modes the toroidal mode number of which is adjacent to each other where n is the toroidal mode number. In particular, the velocity of the mode is finite at the magnetic axis so that there is a large velocity directed to the magnetic axis; and (3) when the high n modes are saturated, the destabilized low n modes, which have a global mode structure and shift the magnetic axis, also begin to be saturated. The self mode coupling of the low n modes induces the core crush and transports the plasma from the core region to the peripheral region. Since the nonlinear MHD phenomena are dominated by the resistive modes, the MHD phenomena for the experimental high magnetic Reynolds number are considered to be milder than our numerical results. Thus, when the experimental high magnetic Reynolds number is taken in the simulation, the core crush may be suppressed so that the numerical results are expected to become close to the experimental results where the stable high beta plasmas are obtained.

Observation of the inward propagation of spontaneous toroidal flow from the plasma boundary in LHD

Spontaneous generation of toroidal flow from the separatrix and its inward radial propagation in association with the change in the electron temperature gradient have been observed near the plasma boundary in the modulated electron cyclotron heating (MECH) experiment in the Large Helical Device. The observations presented in this paper provide strongly the supports of the hypothesis for the conversion of the poloidal flow to the toroidal one at the plasma peripheral region, exhibiting a clear delay response on MECH in the time derivation of the toroidal flow (V̇ϕ≡∂Vϕ/∂t) to the poloidal one (V̇θ≡∂Vθ/∂t) at a normalized radius of ρ≈ 0.97. The ratio of order unity for |V̇ϕ/V̇θ| during a conversion phase from poloidal flow into toroidal one is consistent with that predicted by a quasi-stationary theoretical model based on the turbulent instability, regardless of its sign and spatial structure. The present work demonstrates a new dynamic response in the plasma momentum transport and represents a significant confirmation of its non-local nature.

Resistive interchange mode destabilized by helically trapped energetic ions and its effects on energetic ions and bulk plasma in a helical plasma

A resistive interchange mode of the structure (, : poloidal and toroidal mode numbers, respectively) with a bursting character and rapid frequency chirping in the range less than 10 kHz is observed for the first time in the edge region of the net current-free, low beta LHD (Large Helical Device) plasmas during high power injection of perpendicular neutral beams. The mode resonates with the precession motion of helically trapped energetic ions (EPs), following the resonant condition. The radial mode structure is recognized to be similar to that of the pressure-driven resistive interchange mode, of which radial displacement eigenfunction quite localizes around the mode rational surface, and evolves into an odd-type (or island-type) during the late of frequency chirping phase. This beam driven mode is excited when the beta value of helically trapped EPs exceeds a certain threshold. This instability is thought to be a new branch of resistive interchange mode destabilized by the trapped energetic ions. The radial transport, i.e. redistribution and losses, of helically trapped energetic ions induced by the mode transiently generates significant radial electric field near the plasma peripheral region. The large shear of thus generated radial electric field is thought to contribute to the observed suppression of micro-turbulence and transient increases of the temperature of fully ionized carbon impurity ions and electron density, suggesting improvement of bulk plasma confinement.

Resistive Interchange Modes Destabilized by Helically Trapped Energetic Ions in a Helical Plasma

A new bursting m=1/n=1 instability (m,n: poloidal and toroidal mode numbers) with rapid frequency chirping down has been observed for the first time in a helical plasma with intense perpendicular neutral beam injection. This is destabilized in the plasma peripheral region by resonant interaction between helically trapped energetic ions and the resistive interchange mode. A large radial electric field is induced near the edge due to enhanced radial transport of the trapped energetic ions by the mode, and leads to clear change in toroidal plasma flow, suppression of microturbulence, and triggering an improvement of bulk plasma confinement.

Blood flow in stenosed arteries with radially variable viscosity, peripheral plasma layer thickness and magnetic field

A novel approach of combined mathematical and computational models has been developed to investigate the oscillatory two-layered flow of blood through arterial stenosis in the presence of a transverse uniform magnetic field applied. Blood in the core region and plasma fluid in the peripheral layer region are assumed to obey the law of Newtonian fluid. An analytical solution is obtained for velocity profile and volumetric flow rate in the peripheral plasma region and also wall shear stress. Finite difference method is employed to solve the momentum equation for the core region. The numerical solutions for velocity, flow rate and flow resistance are computed. The effects of various parameters associated with the present flow problem such as radially variable viscosity, hematocrit, plasma layer thickness, magnetic field and pulsatile Reynolds number on the physiologically important flow characteristics namely velocity distribution, flow rate, wall shear stress and resistance to flow have been investigated. It is observed that the velocity increases with the increase of plasma layer thickness. An increase or a decrease in the velocity and wall shear stress against the increase in the value of magnetic parameter (Hartmann number) and hematocrit is dependent on the value of t. An increase in magnetic field leads to an increase in the flow resistance and it decreases with the increase in the plasma layer thickness and pulsatile Reynolds number. The information concerning the phase lag between the flow characteristics and how it is affected by the hematocrit, plasma layer thickness and Hartmann number has, for the first time, been added to the literature.

Zeeman polarimetry measurement for edge current density determination using Li-beam probe on JT-60U

Zeeman polarimetry system using Li-beam probe has been developed for the edge current density measurement in the JT-60U tokamak, which measures the polarization angle alpha (related to the pitch angle of the magnetic field) by means of photoelastic modulators, etalons, and phase sensitive detection using digital lock-in amplifiers with the accuracy in the alpha of Delta alpha approximately 0.1 degrees. The diagnostic has 20-channel viewing chords covering the plasma peripheral region of normalized minor radius r/a approximately 0.8-1 with a spatial resolution of up to approximately 1 cm. Li-beam injection with beam current of up to approximately 5 mA has been achieved. A new tuning method of the wavelength for the etalon has been demonstrated, scanning the beam acceleration voltage and keeping a beam current constant during a single shot. The peak wavelength of the etalon is adjusted in the direction to both blue- and redshifts by changing the angle of incidence and increasing the temperature, respectively. Time evolution of the edge current density profile has been determined for the current ramp experiment in the Ohmically heated discharges. In addition, the edge current density profile with the local peak of j(ped) approximately 0.15-0.25 MA/m(2) at r/a approximately 0.9 has been identified in the H-mode plasma, which is correlated with large pressure gradient in the pedestal region.

Excitation of stable Alfvén eigenmodes by application of alternating magnetic field perturbations in the Compact Helical System

Stable toroidicity-induced Alfvén eigenmodes (TAEs) with low toroidal mode number (n=1 and n=2) were excited by application of alternating magnetic field perturbations generated with a set of electrodes inserted into the edge region of neutral beam injection heated plasmas on the Compact Helical System [K. Nishimura, K. Matsuoka, M. Fujiwara et al., Fusion Technol. 17, 86 (1990)]. The gap locations of TAEs excited by the electrodes are in the plasma peripheral region of ρ>0.7 (ρ is the normalized minor radius) where energetic ion drive is negligibly small, while some AEs are excited by energetic ions in the plasma core region of ρ<0.4. The damping rate of these stable TAEs derived from plasma responses to applied perturbations is fairly large, that is, ∼9% to ∼12% of the angular eigenfrequency. This large damping rate is thought to be dominantly caused by continuum damping and radiative damping.

Two -fluid nonlinear mathematical model for pulsatile blood flow through catheterized arteries

The pulsatile flow of blood through a catheterized artery is analyzed, assuming the blood as a two-fluid model with the suspension of all the erythrocytes in the core region as a Herschel-Bulkley fluid and the peripheral region of plasma as a Newtonian fluid. The resulting system of the nonlinear implicit system of partial differential equations is solved by perturbation method. The expressions for shear stress, velocity, flow rate, wall shear stress and longitudinal impedance are obtained. The variations of these flow quantities with yield stress, catheter radius ratio, amplitude, pulsatile Reynolds number ratio and peripheral layer thickness are discussed. The velocity and flow rate are observed to decrease, and the wall shear stress and resistance to flow increase when the yield stress increases. The plug flow velocity and flow rate decrease, and the longitudinal impedance increases when the catheter radius ratio increases. The velocity and flow rate increase while the wall shear stress and longitudinal impedance decrease with the increase of the peripheral layer thickness. The estimates of the increase in the longitudinal impedance are significantly lower for the present two-fluid model than those of the single-fluid model.

Parameters of peripheral plasma fluctuations in the FT-2 tokamak

Probe measurements in a peripheral plasma region of the FT-2 tokamak have been performed in experiments with lower hybrid heating and during the subsequent transition to an improved confinement regime. The average characteristics of high-density plasma structures and the velocity of their propagation in the radial and poloidal directions are determined by the conditional averaging technique and compared to the results obtained using some other methods of data processing. It is established that a quasi-coherent mode can exist on the inner side of the torus.

Measurement of ion cyclotron emissions by use of ICRF heating antennas in LHD

Ion cyclotron emissions (ICEs) were clearly detected in the Large Helical Device (LHD) during perpendicular neutral beam (NB) injection. Antennas for the ion cyclotron range of frequencies (ICRF) heating were used as probes. The frequencies of ICEs were proportional to magnetic field strength. The location of ICE excitation was a peripheral region of plasma and the source particles were lost-ions injected by the perpendicular NB.

A two-fluid model for pulsatile flow in catheterized blood vessels

The pulsatile flow of blood through a catheterized artery is analyzed, assuming the blood as a two-fluid model with the suspension of all the erythrocytes in the core region as a Casson fluid and the peripheral region of plasma as a Newtonian fluid. The resulting non-linear implicit system of partial differential equations is solved using perturbation method. The expressions for shear stress, velocity, flow rate, wall shear stress and longitudinal impedance are obtained. The variations of these flow quantities with yield stress, catheter radius ratio, amplitude, pulsatile Reynolds number ratio and peripheral layer thickness are discussed. It is observed that the velocity distribution and flow rate decrease, while, the wall shear, width of the plug flow region and longitudinal impedance increase when the yield stress increases. It is also found that the velocity increases, but, the longitudinal impedance decreases when the thickness of the peripheral layer increases. The wall shear stress decreases non-linearly, while, the longitudinal impedance increases non-linearly when the catheter radius ratio increases. The estimates of the increase in the longitudinal impedance are considerably lower for the present two-fluid model than those of the single-fluid model.

Two-fluid Herschel-Bulkley model for blood flow in catheterized arteries

The steady flow of blood through a catheterized artery is analyzed, assuming the blood as a two-fluid model with the core region of suspension of all the erythrocytes as a Herschel-Bulkley fluid and the peripheral region of plasma as a Newtonian fluid. The expressions for velocity, flow rate, wall shear stress and frictional resistance are obtained. The variations of these flow quantities with yield stress, catheter radius ratio and peripheral layer thickness are discussed. It is observed that the velocity and flow rate decrease while the wall shear stress and resistance to flow increase when the yield stress or the catheter radius ratio increases when all the other parameters held constant. It is noticed that the velocity and flow rate increase while the wall shear stress and frictional resistance decrease with the increase of the peripheral layer thickness. The estimates of the increase in the frictional resistance are significantly much smaller for the present two-fluid model than those of the single-fluid model.

Electron Bernstein Wave Heating on the Internal Coil Device Mini-RT

The possibility of mode conversion from electromagnetic wave to electron Bernstein wave is investigated for plasmas confined with in an internal coil device Mini-RT. A steep density gradient, which plays an important role for the EBW mode conversion, is produced at the plasma peripheral region by changing confined magnetic configuration. When the electron density gradient is steep at the upper hybrid resonance layer, electron temperature has a peak near the top of the electron density slope. This consistent to be due to the mode conversion to EBW and associate heating.

Optimization of incident wave polarization for ECRH in LHD

This paper reports on the results of an experimental investigation into electron cyclotron resonance heating (ECRH) using the fundamental O-mode in the large helical device (LHD). The aim of the experiment is to understand the mechanism of the selective excitation of the O-mode EC wave and, thereby, optimize the performance of the ECRH. The polarization angle and the ellipticity of the electric field of the incident high power millimetre waves were varied in the experiment using a pair of grating polarizers. The responses of the plasma stored energy and electron temperature were measured to evaluate the effects. The optimum values of the polarization angle and the ellipticity for the ECRH were identified experimentally. A simple model of power branching between the O- and X-modes is introduced in order to interpret the observed experimental results. Specific to the ECRH in the LHD configuration, the effect of magnetic shear in the plasma peripheral region is considered.