Density Scale Length Research Articles

Three-halves harmonic emission from femtosecond laser produced plasmas

Measurements of three-halves harmonic radiation (3ω0/2) produced by femtosecond, Ti:sapphire laser pulses (⩽2×1017 W/cm2) in long density scale length plasmas generated from solid aluminum targets are presented. The 3ω0/2 emission yield shows excellent agreement with theories of the two-plasmon decay instability in the predominantly linear regime.

Optimization of Kalpha bursts for photon energies between 1.7 and 7 keV produced by femtosecond-laser-produced plasmas of different scale length.

The conversion efficiency of a 90 fs high-power laser pulse focused onto a solid target into x-ray Kalpha line emission was measured. By using three different elements as target material (Si, Ti, and Co), interesting candidates for fast x-ray diffraction applications were selected. The Kalpha output was measured with toroidally bent crystal monochromators combined with a GaAsP Schottky diode. Optimization was performed for different laser intensities as well as for different density scale lengths of a preformed plasma. These different scale lengths were realized by prepulses of different intensities and delay times with respect to the main pulse. Whereas the Kalpha yield varied by a factor of 1.8 for different laser intensities, the variation of the density scale length could provide a gain factor up to 4.6 for the Kalpha output.

An Edge Pedestal Model

An edge pedestal model is proposed in which the density and temperature gradient scale lengths and the density and temperature widths in the edge pedestal are determined by the physics of: 1) the local transport and atomic physics processes and the particle and heat fluxes flowing through the pedestal; 2) MHD pressure gradient constraints; 3) neutral particle penetration into the plasma edge pedestal; and 4) MHD pedestal β-limit constraints.

A criterion for the breakdown of vibrational equilibrium in expansions

The ratio of the mean time between collisions of a typical molecule to the scale time of the density change of an element moving with the flow has long been used as a criterion for the breakdown of translational equilibrium in gaseous expansions. The pressure tensor becomes anisotropic when this parameter ceases to be very small in comparison with unity. It is proposed in this paper that a breakdown parameter for an internal mode is obtained simply by multiplying the translational parameter by the relaxation collision number for that mode. Emphasis is placed on the breakdown of vibrational equilibrium because this is of practical significance in many applications. For steady flows, the vibrational breakdown parameter is the product of a constant of order unity, the relaxation collision number, the speed ratio or Mach number, and the Knudsen number based on the scale length of the density variation. The Mach number term becomes nonlinear if the scale length of the area variation is employed in place of the density scale length. The direct simulation Monte Carlo (DSMC) method is used to make a series of calculations that support the use of this parameter to predict the breakdown of vibrational equilibrium in expansions.

Gyrokinetic particle-in-cell simulation of Alfvénic ion-temperature-gradient modes in tokamak plasma

A linearized gyrokinetic particle-in-cell code is developed to study the excitations of electromagnetic instabilities due to finite ion temperature gradients in tokamak plasmas. The code employs the δf scheme to solve the gyrokinetic equations for ion dynamics; while electrons are modeled as massless fluid. The instability threshold condition of the Alfvénic ion-temperature-gradient (ITG) mode as well as the finite β stablization of electrostatic ITG mode are studied. Here β=8πp/B2 is the ratio between the plasma and magnetic pressure. In the β→0 limit, our code recovers the electrostatic results. As β increases, the destablization of kinetic ballooning mode due to finite ion temperature gradient is observed. The simulation results also reveal two possible critical values of ηi, where ηi=d ln Ti/d ln n is the ratio between the density scale length to the temperature scale length; one of the critical values is for the predominantly electrostatic ITG mode and one for the Alfvénic ITG mode, where ηi,cAITG<ηi,cESITG. The transition domain between the Alfvénic ITG mode and the ITG mode, meanwhile, is found to depend on β and ηi.

High Frequency Behavior of La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> with Giant Magnetoimpedance Effect

The generation of coherent soft X rays is studied using a terawatt picosecond Nd:glass laser system. Two different methods are investigated as candidates for efficient generation of such radiation, namely, longitudinally pumped transient collisional excitation nickel-like molybdenum X-ray laser, and high-harmonic generation from solid-vacuum interfaces. In the course of experiments on longitudinally pumped X-ray lasers, unexpected jetlike structures are observed in the visible emission of the molybdenum plasma, extending over a length of several millimeters. An interesting characteristic of this phenomena is that clear jets are observed only for longitudinal pump intensities between 5 x 10(14) and 7 x 10(14) W/cm(2). The effects of a finite scale length density gradient on surface harmonics is also investigated. The efficiency of harmonic generation from near-solid density plasma is found to increase by a factor of 2 to 3 when using prepulses. The scale length of the preplasma is simulated using a one-dimensional hydrodynamic code, and the increase in efficiency is verified to be in accordance with particle-in-cell simulation results.

Coherent short wavelength radiation via picosecond Nd:glass lasers

The generation of coherent soft X rays is studied using a terawatt picosecond Nd:glass laser system. Two different methods are investigated as candidates for efficient generation of such radiation, namely, longitudinally pumped transient collisional excitation nickel-like molybdenum X-ray laser, and high-harmonic generation from solid–vacuum interfaces. In the course of experiments on longitudinally pumped X-ray lasers, unexpected jetlike structures are observed in the visible emission of the molybdenum plasma, extending over a length of several millimeters. An interesting characteristic of this phenomena is that clear jets are observed only for longitudinal pump intensities between 5 × 1014 and 7 × 1014 W/cm2. The effects of a finite scale length density gradient on surface harmonics is also investigated. The efficiency of harmonic generation from near-solid density plasma is found to increase by a factor of 2 to 3 when using prepulses. The scale length of the preplasma is simulated using a one-dimensional hydrodynamic code, and the increase in efficiency is verified to be in accordance with particle-in-cell simulation results.

Effect of neutral atoms on tokamak edge plasmas

Neutral atoms can significantly influence the physics of tokamak edge plasmas, e.g., by affecting the radial electric field and plasma flow there, which may, in turn, be important for plasma confinement. Earlier work [Fülöp et al., Phys. Plasmas 5, 3969 (1998)], assuming short mean-free path neutrals and Pfirsch–Schlüter ions, has shown that the ion-neutral coupling through charge-exchange affects the neoclassical flow velocity significantly. However, the mean-free path of the neutrals is not always small in comparison with the radial scale length of densities and temperatures in the edge pedestal. It is therefore desirable to determine what happens in the limit when the neutral mean-free path is comparable with the scale length. In the present work a self-similar solution for the neutral distribution function allowing for strong temperature and density variation is used, following the analysis of Helander and Krasheninnikov [Phys. Plasmas 3, 226 (1995)]. The self-similar solution is possible if the ratio of the mean-free path to the temperature and density scale length is constant throughout the edge plasma. The resulting neutral distribution function is used to investigate the neutral effects on the ion flow and electrostatic potential as this ratio varies from much less than one to order unity.

The nonlinear dynamics of the modulational instability of drift waves and the associated zonal flows

The linear and nonlinear dynamics of zonal flows and their interactions with drift wave turbulence is considered in the simple but illuminating generalized Charney–Hasegawa–Mima model due to Smolyakov et al. [Phys. Plasmas 7, 1349 (2000)]. Two positive definite, exact, integral invariants associated with the full generalized Charney–Hasegawa–Mima system are derived. For an initial monochromatic drift wave pump with small but finite amplitude, a modulational instability can occur, characterized by growing zonal flow and sideband perturbations (i.e., a four-wave interaction). The pump threshold for instability is readily satisfied, depending on the zonal flow wave number. The fully nonlinear Charney–Hasegawa–Mima equations are solved with a numerical scheme which is validated by demonstrating the conservation of the two exact invariants. The simulations show that the validity of the four-wave model is limited to approximately three instability growth times. The radial structure of the zonal flow can be “jet-like” or highly oscillatory depending upon the ratio of the system size to the density scale length and initial conditions. It is found that zonal flows can be dramatically reduced if the most unstable zonal flow wave number does not fit into the system.

Evolution of ηi(r,t) in JT-60U reverse shear discharges

Evolution of the ion temperature gradient parameter (ηi) has been studied in JT-60U reverse shear (RS) discharges experimentally. The ηi(r,t) behaviours for RS with and without internal transport barrier (ITB) discharges were observed, where the ITB appeared as steep gradients in the ion temperature profile, the electron density and temperature profiles. The relationship between the electron density scale length (Lne) and the ion temperature scale length (LTi) for different heating methods (NBI only and NB{} + {}ICRF) in RS discharges was compared with each other. It was observed that ηi is a key parameter to characterize ITB formation for the two cases. Kinetic stability for the toroidal drift-type modes (s<0 and ηi<ηicrit) was identified experimentally in the shear reversal plasma with ITB discharges.

An edge pedestal model based on transport and atomic physics

A model is presented for the calculation of the characteristic scale lengths from transport considerations in the edge pedestal region of high confinement (H-mode) plasmas. The model is based on the requirements of heat and particle removal through the edge. Atomic physics effects on edge density and temperature gradient scale lengths are taken into account. An empirical fit for the width of the edge pedestal transport barrier is employed. Model problem calculations indicate that the model predicts the magnitudes and some trends of characteristic gradient scale lengths observed in current experiments.

Reduction in transport by the parallel velocity shear instability due to reversed magnetic shear

A nonlocal theory of the electrostatic parallel velocity shear instability in a three-dimensional slab with a uniformly sheared magnetic field has been developed. It is shown that in the limit of a weak parallel velocity gradient, the linear growth rate can be increased depending upon the direction of the magnetic shear (ŝ) with respect to the radial curvature of the parallel velocity profile (d2v∥/dx2). When these parameters have the same sign, the growth rate can actually be stronger than in the limit of no magnetic shear. In this limit of increased instability, the eigenmode is broadened, thus producing enhanced transport. This effect should be observable when the scale length of the curvature is of order ∼Lsρs. For strong parallel velocity gradients that are more typical of flows in tokamaks, the effect of the varying Doppler shift becomes more prominent on the stability of the mode, the net result being that the sensitivity of the growth rates on the sign of the magnetic shear becomes insignificant. This effect, however, is effectively offset when a finite density gradient is included. When the density scale length is of order the scale length of v∥, the growth rate is moderately reduced, but becomes dependent again upon the sign of the magnetic shear.

Absorption of femtosecond laser pulses in interaction with solid targets.

We have studied the effects of the plasma density scale length on the absorption mechanism of the femtosecond (fs) laser pulses interacting with solid targets. Experiments and particle-in-cell (PIC) simulations demonstrate that the vacuum heating is the main absorption in the plasma in the interaction of fs laser pulses with solid targets when no prepulses are applied. The energy spectrum of hot electrons ejected out of or injected into the plasma show a bitemperature distribution. While the first temperature of the two groups of hot electrons can be attributed to the "pull-and-push" exertion of the laser field, the second temperature refers to the electrons accelerated by the static part (in front of the target) and the oscillating part (in the plasma layer) of the laser-induced electric field, respectively. PIC simulations also show that with an appropriate density scale length, the femtosecond laser energy can be absorbed locally through different mechanisms.

Effect of the plasma density scale-length on harmonic generation in laser-plasma interaction

The conversion efficiency of harmonics emitted by laser driven plasma was investigated via PIC (Particle In Cell) simulation. We calculated the efficiencies of harmonics generated in plasma with different scale- lengths irradiated by p-polarized, intense (but still non-relativistic) laser pulses of different durations. We found that the plasma scale-length plays an important role in harmonic generation, but this effect is quite different for different harmonic order, different pulse duration or different plasma density.

Internal transport barriers on Alcator C-Mod

The formation of internal transport barriers (ITBs) has been observed in the core region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement to low confinement mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes). Experiments with ion cyclotron range of frequencies power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for ten energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been observed in ohmically heated H-mode plasmas. All of these ITB events have strongly peaked density profiles with a minimum in the density scale length occurring near r/a=0.5 and have improved confinement parameters in the core region of the plasma.

Radial scalelengths of the galactic thin and thick disc with 2MASS data

This paper presents a global analysis of the 2MASS (Two Micron All Sky Survey) data as observed in seven fields at different galactic latitudes in our Galaxy. The data allow the preliminary determination of the scale parameters, which lead to strong constraints on the radial and vertical structure of the galactic thin and thick disc. The interpretation of star counts and colour distributions of stars in the near-infrared with the synthetic stellar population model gives strong evidence that the galactic thin disc density scalelength (h R ) is rather short (2.8 ± 0.3 kpc). The galactic thick disc population is revisited in the light of new data. We find the thick disc to have a local density of 3.5 ± 2.0 per cent of the thin disc, exponential scaleheight (h z ) of 860±200 pc and exponential scalelength (h R ) of 3.7 ± 0.8 0.5 kpc.

High-power high-frequency-induced Langmuir turbulence in the smooth ionosphere at Arecibo. II. Low duty cycle, altitude-resolved, observations

This is the second of two papers comprising a theoretical and observational study of new, altitude-resolved, observations at Arecibo of Langmuir turbulence induced in the ionosphere by a new, more powerful, high frequency heater operated at very low duty cycles. Altitude resolution of 150 m in incoherent scatter radar spectra is made possible by the coded-long-pulse method. Here we present the first observation at Arecibo of the well-developed parametric decay instability and the Langmuir decay instability cascade features in the Thomson scatter radar power spectrum, of the plasma line, at the unmodified matching altitudes under near-cold start conditions. The dependence of the plasma line spectra on altitude, pump power, and density scale length have been studied. The temporal growth and saturation of the spectra during heating and the decay of the spectra in the afterglow of heating has also been studied in detail. Comparisons are made here with the theoretical predictions of the companion paper I [DuBois et al., Phys. Plasmas 8, 791 (2001)]. From these comparisons and a comparison with recent observations at both Arecibo and Tromso/, we conclude that all the predictions of modern Langmuir turbulence theory for the radar spectral signatures of the turbulence in a smooth ionosphere have now been verified.

Experimental investigations of the SOL plasma in the MAST tokamak

Initial target plasma parameters have been obtained using Langmuir probe arrays in MAST DND plasmas. A clear in–out asymmetry was observed in the target parameters. All plasma parameter scale lengths were significantly elongated on the outboard side due to strong poloidal flux expansion. The electron temperature scale length λ Te is typically 5–8 times broader than the density scale length λ ne, suggesting a significant difference between heat and particle diffusivities in the SOL. A rough estimation of the SOL collisionality indicates that the inner SOL may be collisional ( ν *∼50) whereas the outer SOL is marginally collisional ( ν *∼7). The peak power density is about 4–5 times higher (∼1 MW m −2) on the inboard side, which has a SOL power density scale length ∼10 times narrower (∼0.5 cm) than the outboard. The total power flow was approximately up–down symmetric, whilst the in–out power asymmetry was observed to be P in: P out∼1:6.5, exceeding the ratio of ∼1:3 for the inboard and outboard separatrix surface areas. A total of about 2/3 of the power entering the SOL reached the strike points (300 kW out of P SOL ∼450 kW ).

Effect of plasma scale length on multi-MeV proton production by intense laser pulses.

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at I lambda(2) = 5 x 10(19) W cm(-2) has been studied. With an unperturbed foil, protons with energy >20 MeV were formed in an exponential energy spectrum with a temperature of 2.5+/-0.3 MeV. When a plasma with a scale length of 100 microm was preformed on the back of the foil, the maximum proton energy was reduced to <5 MeV and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.

Dynamical Behavior of Higher Harmonic Generation from a Picosecond-Laser-Produced Solid Surface Plasma

We have studied experimentally the efficiency of the harmonics from solid surface plasma by varying the electron density scale length produced during the interaction of a 2.2 ps, 100 mJ, 1054 nm, p-polarized laser pulse with deposited Al targets. Although third harmonic radiation can also be observed when a single beam is used, an approximately 5 ps time difference between the prepulse and the main pulse results in an increase in the harmonic signal by a factor of 2–3, which then decreases after 10 ps. The results for the 4th- and 5th-order harmonic generation are also similar. We have also calculated the electron density scale length by using the one-dimensional (1D) hydrodynamic code HYADES. The calculation results are in good agreement with our experimental results. It is found that it is possible to generate high efficiency harmonics by controlling the density gradient using a double laser pulse.