Radial Profiles Of Electron Density Research Articles

Modelling of the internal dynamics and density in a tens of joules plasma focus device

Using MHD theory, coupled differential equations were generated using a lumped parameter model to describe the internal behaviour of the pinch compression phase in plasma focus discharges. In order to provide these equations with appropriate initial conditions, the modelling of previous phases was included by describing the plasma sheath as planar shockwaves. The equations were solved numerically, and the results were contrasted against experimental measurements performed on the device PF-50J. The model is able to predict satisfactorily the timing and the radial electron density profile at the maximum compression.

Edge and core Thomson scattering systems and their calibration on the ASDEX Upgrade tokamak

A new 10 channel Thomson scattering (TS) system was installed on the ASDEX Upgrade tokamak to measure radial profiles of electron density and temperature at the plasma edge with high radial resolution. Together with the already existing TS system, which is now used for the core plasma, electron density and temperature profiles extending from the edge to the core are now obtained in a single discharge. The TS systems are relatively calibrated by an optical parametric oscillator.

Tailoring the air plasma with a double laser pulse

We present a comprehensive model of plasma dynamics that enables a detailed understanding of the ways the air plasma induced in the atmosphere in the wake of a laser-induced filament can be controlled by an additional laser pulse. Our model self-consistently integrates plasma-kinetic, Navier−Stokes, electron heat conduction, and electron−vibration energy transfer equations, serving to reveal laser−plasma interaction regimes where the plasma lifetime can be substantially increased through an efficient control over plasma temperature, as well as suppression of attachment and recombination processes. The model is used to quantify the limitations on the length of uniform laser-filament heating due to the self-defocusing of laser radiation by the radial profile of electron density. The envisaged applications include sustaining plasma guides for long-distance transmission of microwaves, standoff detection of impurities and potentially hazardous agents, as well as lightning control and protection.

Observation and numerical analysis of plasma parameters in a capillary discharge-produced plasma channel waveguide

We observed the parameters of the discharge-produced plasma in cylindrical capillary. Plasma parameters of the waveguide were investigated by use of both a Normarski laser interferometer and a hydrogen plasma line spectrum. A space-averaged maximum temperature of 3.3 eV with electron densities of the order of 1017 cm−3 was observed at a discharge time of 150 ns and a maximum discharge current of 200 A. One-dimensional dissipative magnetohydrodynamic (MHD) code was used to analyze the discharge dynamics in the gas-filled capillary discharge waveguide for high-intensity laser pulses. Simulations were performed for the conditions of the experiment. We compared the temporal behavior of the electron temperature and the radial electron density profiles, measured in the experiment with the results of the numerical simulations. They occurred to be in a good agreement. An ultrashort, intense laser pulse was guided by use of this plasma channel.

Bayesian modelling of a thermal helium beam for measurement of electron density and temperature in the W7-X divertor plasma

In Greifswald/Germany W7-X, a new stellarator-type fusion plasma experiment, is currently being built. For the investigation of the divertor plasma two thermal helium beams are foreseen. This diagnostic is routinely used on several fusion plasma experiments and is capable of measuring radial profiles of electron density and temperature with good spatial and temporal resolution in the range of typical edge plasma parameters ne = 1018–1019 m−3 and Te = 20–200 eV. The penetration depth of the beam is limited by electron collisional ionization of the helium atoms and amounts to 3–8 cm in this parameter range. In this paper we investigate the beam propagation for detached plasma conditions in the W7-X divertor region (based on a background plasma simulated with a 3D plasma and neutral transport code EMC3/EIRENE), in which the electron density in the divertor may well exceed 1020 m−3, as observed in the predecessor experiment W7-AS. In this regime the beam penetration drops to 1–2 cm. Through a Bayesian approach, we include uncertainties of all rate coefficients for electronic excitation and ionization used in the collisional–radiative model of atomic helium based on a steady-state approximation valid for a relaxed thermal or supersonic beam. Bayesian inversion of simulated signals for W7-X conditions provides a reliable quantitative estimation of the propagation of uncertainties of the atomic data to the ne and Te errors as well as input for potential improvements of the diagnostic setup. For example, the temperature error at Te = 5 eV and ne = 1020 m−3 can be reduced from approximately 50% to 9% by absolute calibration of the observation system and fitting of three absolute line intensities instead of two line intensity ratios to the model.

Confinement of electron plasma by levitating dipole magnet

A recent experiment on the Ring Trap 1 device has demonstrated long-term (exceeding 300 s) confinement of non-neutral (pure electron) plasma in a dipole magnetic field; particles diffuse inward, steepening the density gradient and self-organizing into a stable vortex structure [Z. Yoshida et al., Phys. Rev. Lett. 104, 235004 (2010)]. In this study, the internal structures of the plasma are experimentally investigated, and it is shown that the observations are consistent with rigidly rotating charged particle clump. The radial profiles of electrostatic potential and electron density consistently show that the drift velocity has homogeneous angular frequency in the confinement region. The electrostatic fluctuations also rotate rigidly with a phase velocity that agrees with the drift velocity. The magnetospheric system should have a wide application in confining single-species and even multiple-species charged particles.

Optical measurements for turbulence characterization in RFX-mod edge

The edge of the reversed field pinch experiment RFX-mod (R=2 m, a=0.46 m) is characterized by a complex magnetic topology and strong pressure gradients. A set of fast spectroscopic diagnostics has been designed to characterize both main plasma edge parameters and turbulence, including coherent structures (blobs) and radial profiles of the thermodynamic variables. Measurements of the edge radial profiles of electron density (n(e)), temperature (T(e)), and pressure (P(e)) are obtained by a thermal helium beam diagnostic, which locally measures the ratios of the spectral lines emitted by a neutral He cloud puffed into hydrogen plasma discharges. A gas puff imaging system allows the high frequency (1 MHz bandwidth) measurement of the fluctuations of He I emissivity in the same positions. The system measures both the intermittent behavior of the edge turbulence and n(e), T(e), and P(e) profiles of the corresponding coherent structures, with a resolution down to 6 μs. The optical diagnostics are complemented by a set of triaxial magnetic coils to simultaneously characterize the edge magnetic field.

A Closer Look at the Structure of Sterically Stabilized Liposomes: A Small-Angle X-ray Scattering Study

The evaluation of the radial electron density profile of a drug containing a sterically stabilized liposomal system is described. Using synchrotron small-angle X-ray scattering, we were able to characterize the hydrophilic shell of the polyethylene glycol chains. Using a Gaussian model for describing the electron density profile along the normal of the bilayer, we got an asymmetric distribution of PEGylated lipids in accordance with theoretical considerations. Moreover, we used anomalous X-ray scattering to study the localization of a hydrophobic drug (a kinase inhibitor), which revealed that these molecules are mainly located in the hydrocarbon chain region of the phospholipid bilayer.

Design of new Thomson scattering diagnostic system on COMPASS tokamak

Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get radial profiles of both electron temperature and electron density. The core TS system will measure at 24 spatial points with ≈10 mm resolution across the core plasma while the edge TS system will measure at 32 spatial points with ≈ (3–5) mm resolution across the edge plasma. Two independent Nd:YAG lasers (1.5 J, 30 Hz each) could be combined to produce a repetition rate of 60 Hz. The lasers will follow separate beam paths to the COMPASS vessel. Scattered light will be collected by two objectives: one for the core region, the second for the plasma edge. Both TS systems are based on Nd:YAG/APD (Avalanche photodiodes) technology. Scattered light is spectrally resolved in polychromators equipped with a set of spectral filters and APDs. The output signal is digitized by fast sampling (GS/s) 8 bit analog digital converters (ADCs).

ECRH Superposition on Linear Cylindrical Helicon Plasma in the LMD-U

Electron cyclotron resonance heating (ECRH) is superimposed on a linear cylindrical helicon plasma in the Large Mirror Device-Upgrade (LMD-U) to study the fluctuation characteristics of high density helicon plasma with/without ECRH injection. The radial profiles of electron density and electron temperature rose by < 10 %, and the drift wave frequency decreased with the ECRH injection. Bicoherence analysis reveals that the nonlinear interaction between drift waves and broad frequency band components exists with or without ECRH injection.

Solubilization of Poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} in Water by Nonionic Amphiphiles

In the presence of the nonionic alkyloxyethylene surfactant n-dodecylpentaoxyethylene glycol ether (C12E5), the anionic conjugated polyelectrolyte (CPE) poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) dissolves in water, leading to a blue shift in fluorescence and dramatic increases in fluorescence quantum yields above the surfactant critical micelle concentration (cmc). No significant changes were seen with a poly(ethylene oxide) of similar size to the surfactant headgroup, confirming that specific surfactant-polyelectrolyte interactions are important. From UV-visible and fluorescence spectroscopy, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and electrical conductivity, together with our published NMR and small-angle neutron scattering (SANS) results, we provide a coherent model for this behavior in terms of breakup of PBS-PFP clusters through polymer-surfactant association leading to cylindrical aggregates containing isolated polymer chains. This is supported by molecular dynamics simulations, which indicate stable polymer-surfactant structures and also provide indications of the tendency of C12E5 to break up polymer clusters to form these mixed polymer-surfactant aggregates. Radial electron density profiles of the cylindrical cross section obtained from SAXS results reveal the internal structure of such inhomogeneous species. DLS and cryo-TEM results show that at higher surfactant concentrations the micelles start to grow, possibly partially due to formation of long, threadlike species. Other alkyloxyethylene surfactants, together with poly(propylene glycol) and hydrophobically modified poly(ethylene glycol), also solubilize this polymer in water, and it is suggested that this results from a balance between electrostatic (or ion-dipole), hydrophilic, and hydrophobic interactions. There is a small, but significant, dependence of the emission maximum on the local environment.

A simple scale height model of the electron density in Saturn's plasma disk

A study of electron densities in Saturn's inner magnetosphere is presented using measurements of the upper hybrid resonance frequency obtained from the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft. The study uses data from the first 16 months of operation in orbit around Saturn. The distribution of density data spans latitudes up to 20° and ranges from 3.6 ≤ L ≤ 8.6. The results are compared to a simple centrifugal potential model for the plasma density. The measurements for 5 ≤ L ≤ 8.6 yield a good fit to an equatorial electron density profile that varies as n0 = (51,880) L−4.1 cm−3 and a plasma scale height that varies as H = (0.047) L1.8 RS, where RS is the radius of Saturn. The measurements for L &lt; 5 are more variable, most likely due to plasma injection effects by Saturn's moon Enceladus which is a known source of neutral gas. A contour map of the electron densities derived from the centrifugal potential model is presented over the entire range of L values and latitudes analyzed, using cubic polynomials to represent the radial profiles of the equatorial electron density and the plasma scale height.

Abel inversion of a holographic interferogram for determination of the density profile of a sheared-flow Z pinch

A holographic interferometer is used to determine the radial electron number density profile of a sheared-flow Z pinch. Chord-integrated density information is recorded during a plasma pulse using the expanded beam of a pulsed ruby laser and holographic techniques. An Interactive Data Language (IDL) computer routine that requires only minimal user interaction is used to measure the resulting fringe shift in the reconstructed interferogram. This chord-integrated density information is inverted using an Abel inversion to determine the radial electron density profile. The density profiles obtained show a radially symmetric plasma column with an electron density of 1016–1017cm−3 above the background plasma density. Holographic measurements are made at different times on separate plasma pulses to track the evolution of the density profile over time. These measurements are corroborated by time-dependent measurements made using a He–Ne interferometer.

Study of an edge transport barrier by Langmuir probes in the compact helical system

The edge transport barrier (ETB) produced by the L–H transition was measured by a triple Langmuir probe (LP) at two toroidal sections of the compact helical system (CHS), of which diagnostic method has good time and spatial resolutions. The radial profiles of electron density (ne), electron temperature (Te) and space potential (Vs) in the ETB region have different shapes at two different toroidal sections. These profiles are deformed inside the ETB region at one location and are formed with rather smooth variations at the other. These deformations gradually disappear in the deep H-phase (after ∼15 ms from the transition) and the profiles inside the ETB become similar at both sections. The deformation seems linked to the presence of a non-rotating magnetic island at the rational surface of the rotational transform ι/2π = 1.

Characteristic Formation of Edge Transport Barrier in the Compact Helical System

The structure of the Edge Transport Barrier (ETB) was measured by means of a triple Langmuir probe in the Compact Helical System, this diagnostic method achieving high time and spatial resolutions. The radial profiles of electron temperature and electron density show a steep gradient inside the normalized minor radius ρ ∼ 0.96, having a plateau of Te at 0.96 <ρ< 1 and a dip of ne at 0.95 <ρ< 0.98. The radial electric field clearly changed in the H-phase in the region at ρ< 0.96 and 0.99 <ρ< 1, and its shear increased around ρ ∼ 0.97. The characteristic profile evolutions suggest an interaction between ETB formation and a magnetic island related to ι/(2π) = 1. Based on these data, the precise position of the ETB is not determined definitely; that is, it is not established whether it exists inside the magnetic island or closely inside the last closed flux surface (ρ = 1).

Field profiles of symmetric surface waves in diffusion-controlled regime of a non-isothermal plasma columns

The field distribution of a symmetric surface wave (SW) propagating on a weakly ionized unmagnetized plasma placed in a cylindrical ideal waveguide, in which a quartz tube is surrounded by vacuum, is presented in the diffusion-controlled regime. This is done to better understand the effect of some physical parameters such as electron temperature, SW frequency, etc on the plasma column. The graphs of electric and magnetic fields versus the plasma column radius under the radial electron density profile dependency are obtained in the commercial radio frequency region where spatial dispersion might be important.

Laser Induced Tunnel Ionization and Electron Density Evolution in Air

An indigenously built Ti:sapphire laser system delivering 50 mJ, 100 fs pulses at a repetition rate of 10 Hz, is employed to tunnel ionize air at 1 atm pressure and form a plasma channel. The laser is line focused using a cylindrical lens to a spot size of 20 µm×1 mm. A folded wave interferometry is used to deduce the radial electron density profile by measuring the phase shift and employing Abel inversion. The maximum value of chord-integrated electron density and radial electron density are estimated to be approximately equal to 3.0×10 16 cm -2 and 4.0×10 18 cm -3 respectively with density scale length of 20 µm. These results have been theoretically interpreted by developing a unified formalism of tunnel ionization. The experimental results are in reasonable agreement with the theory.

Experimental evidence of parametric decay processes in the variable specific impulse magnetoplasma rocket (VASIMR) helicon plasma source

Decay waves have been observed in the megahertz range in the helium plasma generated by the variable specific impulse magnetoplasma rocket magnetoplasma thruster. They are measured using one of the tips of a triple probe connected to a 50 Ω input of a spectrum analyzer via a dc block (a small capacitor). The maximum amplitude of all waves is in the center of the plasma and does not appear correlated to the radial electron density or temperature profiles. The waves seem to be generated close to the helicon antenna that was 91 cm “upstream” from the measuring Langmuir probe. A possible explanation is parametric decay of the large amplitude helicon wave that also generates the plasma.

Electrostatic fluctuation measurements in the periphery of reversed field pinch plasma

AbstractPeripheral plasma electrostatic fluctuations are studied with an inserted electrostatic probe array in the STP‐3(M) Reversed Field Pinch, and radial profiles of electron density, electron temperature, and plasma space potential are measured. The particle flux driven by electrostatic fluctuation is calculated with the correlation between the fluctuation of electron density and fluctuation of plasma space potential, yielding the result that the increase of electrostatic fluctuation at the edge causes particle diffusions. The agreement between the Er × B drift velocity of the radial electric field by the gradient of plasma space potential and the ion thermal velocity at the edge indicates ion motion dominates the particle diffusion. The decrease of the toroidal phase velocity at the edge and the confinement degradation shows the radial outside flow of the Er × Bp drift, where Et is generated by the toroidal flow fluctuation of the Er × B drift. Therefore, the electrostatic fluctuation caused by the radial propagation of plasma space potential is thought of as the dominance of the radial particle flux. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(1): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.20019

Numerical simulation of cableguns using mach2

A radiation-driven ablation model was developed for the MHD code MACH2 to provide a numerical simulation for cableguns. Ablation from the insulator surface is driven by radiation from an optically thin gray gas in the computational domain adjacent to the insulator surface. Two parameters required for the model-specific opacity and vapor layer transmissivity-were determined from baseline experiments. Using these parameter values, numerical simulations for five additional cablegun configurations were compared with experimental measurements obtained using a two-beam laser interferometer. Equations of state models for copper-Teflon and polyethylene plasmas were calculated for use in these simulations. Comparisons were made for radial profiles of electron density, plume velocity, and plume width. Based on the results obtained in this study, it appears that MACH2 simulations can be used to provide reasonable estimates of cablegun plume properties that are difficult or impossible to obtain experimentally, such as the spatial flow details inside the cavity or the temporal distribution of mass loss.