Modification Of Density Profile Research Articles

Ponderomotively induced step-like modification of electron density profile in the plasma resonance region

Ponderomotively induced step-like modification of electron density profile in the plasma resonance region

Density profiles of a colloidal liquid at a wall under shear flow

Using a dynamical density functional theory we analyse the density profile of a colloidal liquid near a wall under shear flow. Due to the symmetries of the system considered, the naive application of dynamical density functional theory does not lead to a shear induced modification of the equilibrium density profile, which would be expected on physical grounds. By introducing a physically motivated dynamic mean field correction we incorporate the missing shear induced interparticle forces into the theory. We find that the shear flow tends to enhance the oscillations in the density profile of hard spheres at a hard wall and, at sufficiently high shear rates, induces a non-equilibrium transition to a steady state characterized by planes of particles parallel to the wall. Under gravity, we find that the centre of mass of the density distribution increases with shear rate, i.e. shear increases the potential energy of the particles.

Dependence on pulse duration and foil thickness in high-contrast-laser proton acceleration

Experimental measurements of proton acceleration with high intensity and high-contrast short laser pulses have been carried out over an order of magnitude range in target thickness and laser pulse duration. The dependence of the maximum proton energy with these parameters is qualitatively supported by two-dimensional particle-in-cell simulations. They evidence that two regimes of proton acceleration can take place, depending on the ratio between the density gradient and the hot electron Debye length at the rear target surface. As this ratio can be affected by the target thickness, a complex interplay between pulse duration and target thickness is observed. Measurements and simulations support unexpected variations in the laser absorption and hot electron temperature with the pulse duration and laser intensity, for which density profile modification at the target front surface is the controlling parameter.

Filamentation of laser beams and excitation of ion acoustic wave in non-paraxial region

This paper contains the work on the filamentation of a laser beam and its effect on excitation of ion acoustic waves in a hot collision less plasma in non-paraxial region. To consider the non-paraxial region, the amplitude and the eikonal part of the beams are expended in terms of r2 and r4. On account of the non-uniform intensity distribution along the wave front of the laser beam, the ponderomotive force becomes finite. This leads to modification of the plasma density profile in the plane transverse to the beam axis and hence also the modification in the propagation characteristics of the waves propagating in the plasma. It is shown that the Filamentation of the waves can occur and this considerably affects the ion acoustic wave.

Using the Steepened Plasma Profile and Wave Breaking Threshold in Laser‐Plasma Interaction

AbstractIn this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self–focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of competitive oscillations between drift mode and flute mode in linear cylindrical ECR plasma

A competitive coexistence between a drift mode and a flute mode has been investigated experimentally in a bounded cylindrical electron cyclotron resonance (ECR) plasma. This competitive cycle was repeated during a whole continuous discharge. Competitive phenomena appeared in plasmas when the density scale length was close to the critical value: the drift mode is dominant and the flute mode is suppressed just below a critical value, but above this value, the flute mode grows abruptly and the drift mode is suppressed. In this phase, the density gradient increases temporally. A small crash takes place when the density scale length reaches a limit value, and then the density profile goes back to the previous states and thus the drift mode is excited and the flute mode is suppressed. The direct nonlinear coupling between the drift mode and the flute mode is weaker than the process of the second harmonic generation of each mode, suggesting that the growth and decay of them are closely related to the modification of the radial density profile.

Dynamic transport simulation code including plasma rotation and radial electric field

A new one-dimensional transport code named TASK/TX, which is able to describe dynamic behavior of tokamak plasmas, has been developed. It solves simultaneously a set of flux-surface averaged equations composed of Maxwell’s equations, continuity equations, equations of motion, heat transport equations, fast-particle slowing-down equations and two-group neutral diffusion equations. The set of equations describes plasma rotations in both toroidal and poloidal directions through momentum transfer and evaluates the radial electric field self-consistently. The finite element method with a piecewise linear interpolation function is employed with a fine radial mesh near the plasma surface. The Streamline Upwind Petrov–Galerkin method is also used for robust calculation. We have confirmed that the neoclassical properties are well described by the poloidal neoclassical viscous force. The modification of density profile during neutral beam injection is presented. In the presence of ion orbit loss, the generation of the inward radial electric field and torque due to radial current is self-consistently calculated.

Attainment of High Normalized Current by Current Profile Manipulation in the Pegasus Toroidal Experiment

Large stable values of normalized current IN are achievable in spherical tori due to the large gradient of the toroidal field across the plasma. This allows access to high Troyon beta limits. Values of IN > 12 MA/m-T are expected to be stable to ideal MHD modes in the ultra-low-A Pegasus Toroidal Experiment. In previous experiments, the toroidal field utilization (Ip/Itf) was found to be limited roughly to unity by the onset of large-scale low-order tearing modes during the current ramp, which limited IN to roughly 6 MA/m-T. Three techniques have now been developed that allow access to higher values of IN, primarily through modification of the current density profile. The first of these techniques employs a fast rampdown of the low-inductance toroidal field coil to rapidly decrease Itf. The other techniques employ gas-fueled washer stack guns as plasma sources. The second technique uses these guns to provide preionization at low toroidal field, while the third relies on the guns as helicity sources to form ST plasmas non-inductively. Using these techniques, values of Ip/Itf > 2 have been obtained.

Turbulent transport of alpha particles in reactor plasmas

A systematic study of the behavior of energetic ions in reactor plasmas is presented. Using self-consistent gyrokinetic simulations, in concert with an analytic asymptotic theory, it is found that alpha particles can interact significantly with core ion-temperature-gradient turbulence. Specifically, the per-particle flux of energetic alphas is comparable to the per-particle flux of thermal species (deuterium or helium ash). This finding opposes the conventional wisdom that energetic ions, because of their large gyroradii, do not interact with the turbulence. For the parameters studied, a turbulent modification of the alpha-particle density profile appears to be stronger than turbulent modification of the alpha-particle pressure profile. Crude estimates indicate that the alpha density modification, which is directly proportional to the core turbulence intensity, could be in the range of 15% at midradius in a reactor. The corresponding modification of the alpha-particle pressure profile is predicted to be smaller (in the 1% range).

Density and temperature profile modifications with electron cyclotron power injection in quiescent double barrier discharges on DIII-D

Quiescent double barrier (QDB) conditions often form when an internal transport barrier is created with high-power neutral-beam injection into a quiescent H-mode (QH) plasma. These QH-modes offer an attractive, high-performance operating scenario for burning plasma experiments due to their quasi-stationarity and lack of edge localized modes. Our initial experiments and modelling using ECH/ECCD in QDB shots were designed to control the current profile and, indeed, we have observed a strong dependence on the q-profile when EC-power is used inside the core transport barrier region. While strong electron heating is observed with EC power injection, we also observe a drop in the other core parameters, namely ion temperature and rotation, electron density and impurity concentration. At onset and termination of the EC pulse, dynamically changing conditions are induced that provide a rapid evolution of Te/Ti profiles accessible with 0.3 < (Te/Ti)axis < 0.8 observed in QDB discharges. We are exploring the correlation and effects of observed density profile changes with respect to these time-dependent variations in the temperature ratio. Increases in the measured ion thermal and particle diffusivities inside the barrier region during an ECH pulse correlate with electron heating and a rise in the core Te/Ti ratio as the ion temperature and density profiles flatten with this change in transport. The change in transport is consistent with a destabilization of ITG turbulence as inferred from the reduction of the stability threshold due to the change in Te/Ti.

J×B heating by non-relativistic linearly polarized laser light

The J×B heating mechanism of plasma under the radiation of non-relativistic linearly polarized laser light is studied analytically and a self-consistent theory including the effects of two kinds of ponderomotive forces is developed. Meanwhile, the J×B heating mechanism with the density profile modification is discussed, and the relation curve of the corresponding absorption coefficient vis the laser intensity is obtained. The result shows that the absorption coefficient fabs owing to the J×B heating is about 2.8% when the laser intensity factor A0＝20.

The effect of the use of different electrode materials for edge-plasma biasing on plasma density and floating potential modifications

The effect of the use of different electrode materials for edge-plasma biasing on plasma density and floating-potential profile modifications has been recently investigated on the CASTOR tokamak. Dependences of biasing current, edge plasma density and floating potential on biasing voltage have been measured. Induced relaxation events and fast (10–50) kHz oscillations are shown and surface changes of the biasing electrodes are discussed in the paper.

The Electron Cyclotron Resonant Heating System on the DIII-D Tokamak

In the DIII-D electron heating and current drive installation, up to six gyrotron microwave generators in the 1-MW class at pulse lengths up to 5 s have been operated simultaneously. The frequency for all the gyrotrons is 110 GHz, corresponding to the second harmonic of the electron gyrofrequency at 2 T. The peak generated power has been >4 MW with peak injected power slightly greater than 3 MW. The radio frequency (rf) generators are located remotely and are connected to the tokamak by up to 100 m of evacuated circular corrugated waveguide carrying the HE1,1 mode with overall transmission efficiency, including coupling to the waveguide, of up to 75%. Ancillary equipment for polarization control, beam switching, power monitoring, control of launch direction, and system protection has been developed.The system has been used to support a wide variety of physics experiments, including control of magnetohydrodynamic modes, current density profile modifications, basic plasma heating and current drive, transport studies, and rf-assisted start-up. The gyrotron complex is being upgraded by the acquisition of additional tubes with 5- to 10-s pulse length capability.

Dynamics of pedestal perturbations by ELMs and edge harmonic oscillations in DIII-D

Edge density profile modifications associated with edge localized modes (ELMs) and edge harmonic oscillations (EHOs) have been measured using fast profile reflectometry in DIII-D H-mode plasmas. Specifically, high time (down to 10 µs) and spatial (∼4 mm) resolution density profile measurements from 0 × 1019 to 3.1 × 1019 m−3 provide us with new tools to study the dynamics of pedestal perturbations. During ELMs, it has been observed that the SOL density profile expands outwards to the vessel wall while the pedestal density reduces. These measurements provide direct evidence for enhanced particle radial transport to the vessel wall at the onset of ELMs. A large SOL density profile radial expansion velocity is observed during ELMs. A comparison of the velocities for different pedestal densities is made. The density scale length, density at the wall, and density fluctuations during ELMs are determined as a function of pedestal density, as well as particle transport during ELMs. Measurements in quiescent H-mode plasmas indicate that the pedestal density profile is modulated at the fundamental frequency of the EHO. A comparison between reflectometer and beam emission spectroscopy measurements regarding the EHO density fluctuation profile is presented.

Nonlinear dynamics of intense laser pulses in a pair plasma

We derive equations for nonlinearly interacting intense laser pulses and an electron–positron plasma, taking into account the combined action of the relativistic particle mass increase and the plasma density profile modification by the relativistic light ponderomotive force. The physical system is described by a modified nonlinear Schrödinger equation (NLSE). The latter predicts a modulational instability and one-dimensional light envelope solitons. In multi-dimensions, we observe the interesting phenomena of light beam focusing, light trapping in self-created density holes, and light beam filamentation. Our results should be useful in understanding the nonlinear propagation of intense laser beams in an electron–positron plasma which could be created by next generation intense lasers.

ECCD power necessary for the neoclassical tearing mode stabilization in ITER

The stabilization of the neoclassical tearing mode (NTM) by an electron cyclotron current drive (ECCD) has been numerically studied for the parameters of ITER in order to evaluate the necessary EC power for the stabilization of NTMs in ITER. The time evolution of an island width of NTM is calculated by the modified Rutherford equation. The modification of the background current density profile by the EC current located on the rational surface is calculated by the current diffusion equation and the variation of the tearing stability index Δ′ is taken into account. When the EC power is higher than a threshold value, NTMs with any island width can be fully stabilized. The dependence of the threshold power on the parameters in the modified Rutherford equation is examined. The threshold power depends significantly on the parameters of the bootstrap current term in the modified Rutherford equation. The injected EC current decreases the value of Δ′ through the background current modification, which results in the reduction of the threshold power. The effects of the peaked nature of the EC current density profile and the EC power modulation on the threshold power are investigated. When the width of the EC current density profile becomes half, the threshold power is reduced to half or less. The EC power modulation is found to be inessential for the threshold power reduction if the EC current density profile can be peaked. As regards the maximum value of the threshold power in the range of parameters, an EC power of about 25 MW is found to be sufficient for the simultaneous stabilization of both the m/n = 3/2 and 2/1 NTMs in ITER.

Nonlinearly coupled upper-hybrid and magnetoacoustic waves in collisional magnetoplasmas.

The nonlinear interaction between large amplitude electrostatic upper-hybrid (UH) waves and magnetoacoustic perturbations in a collisional magnetoplasma is considered. For this purpose, we present a nonlinear Schrödinger equation for the UH wave envelope in the presence of the electron density, electron temperature, and compressional magnetic field perturbations of the dispersive magnetoacoustic waves, and derive an equation for the latter taking into account the combined influence of the UH ponderomotive force and the differential electron Joule heating in the UH wave electric field. The coupled equations are employed to investigate modulational instabilities of a constant amplitude UH wave, and the density profile modifications due to the UH ponderomotive and thermal forces. It is found that the thermal instability has its maximum at a wavelength half the one of the UH wave, and a numerical solution of the coupled time-dependent system of equations reveals that localized density cavities are also created with a spacing approximately half the wavelength of the pump wave.

Ultrashort pulse reflectometry for density profile and fluctuation measurements on SSPX

A broadband, multichannel ultrashort pulse reflectometry (USPR) diagnostic on the Sustained Spheromak Physics eXperiment device has recently undergone a number of system upgrades, which has resulted in significant improvements in the signal-to-noise ratio of the USPR signals and a dramatic reduction in the number of “lost signals” in which the amplitude of the reflected wave form drops below detection threshold. This has greatly enhanced the ability of USPR to study relatively fast density profile modifications, and allows the simultaneous monitoring of multiple density layers with as short as a 3 μs pulse repetition rate. This article provides details of the upgraded USPR system together with density profiles and fluctuation data.

High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other parameters are derived from the corresponding density profiles. Measurements performed in the presence of type I ELMs reveal that the edge density profile modifications occur in the same time scale of the Dα radiation at the divertor. The collapse and recovery of the pedestal structure during each ELM is resolved. The values of the pedestal width (around 4–5 cm) measured in-between ELMs are similar to those obtained previously in similar ASDEX Upgrade discharges. Also, the evolution of the pedestal density given by reflectometry is found to be in good agreement with the changes of the line integrated density measured by the interferometer.

Modification of the electron density profile near the upper hybrid layer during radio wave heating of the ionosphere

High frequency (HF) radio waves can modify the electron density profile of the ionosphere in the upper hybrid resonance region (UHR). The spatial and temporal development of this modification was investigated using a multi‐frequency Doppler diagnostic system at the TROMSØ facility [Lobachevsky et al., 1992]. We present the results of the simulations of the density profile modification near the upper hybrid resonance height for the parameters relevant to the experiment and show that the spatio‐temporal development of the density modification is quantitatively modelled by the mode conversion and the radio wave heating.