Plasma Temperature Profiles Research Articles

Mach probe measurements of peripheral plasma rotation evolution during L–H transition and ITB decay in the TUMAN-3M tokamak

Radial electric field and associated sheared plasma rotation is known to play crucial role in the turbulent transport suppression and improved confinement development. Toroidal and poloidal rotation velocity evolutions at the plasma edge were studied on the TUMAN-3M tokamak using four-tip movable Mach probe. Measurements were performed in two modes of plasma confinement: H-mode transition (featured by peripheral transport barrier formation), and transient internal transport barrier (ITB) formation; both in ohmic heating scenario. The measurements were performed in the vicinity of a last closed flux surface, both in scrape-off layer and core plasmas. In ohmic H-mode, virtually no perturbation of toroidal rotation velocity was observed, whereas poloidal rotation velocity profile underwent gradual evolution after the transition. Surprisingly, poloidal rotation changed not in tact with radial electric field, but followed density gradient built up at the edge transport barrier region. In transient ITB scenario, a noticeable perturbation of toroidal rotation velocity was observed just after the ITB decay, indicating, probably, the outflux of the toroidal momentum due to confinement degradation. The Mach probe data obtained in both the scenarios are compared to available data on plasma density and temperature profiles, radial electric field and rotation velocity of impurity ions.

Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process

In this study, a plasma electrolytic oxidation (PEO) process was used to produce oxide coatings on commercially pure aluminium (1100 alloy) at a pulsed dc power mode. The effects of process parameters (i.e. current density and treatment time) on the plasma discharge behaviour during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285–800 nm). The elements present in the plasma were identified. Stark shifts of spectral lines and line intensity ratios were utilized to determine the plasma electron concentrations and temperatures, respectively. The plasma electron temperature profile, coating surface morphology and coating composition were used to interpret the plasma discharging behaviour. The different coating morphologies and compositions at different coating surface regions are explained in terms of three types of discharge, which originate either at the substrate/coating interface, within the upper layer, or at the coating top layer. The high spike peaks on the plasma intensity and temperature profiles corresponded to discharges originated from the substrate/coating interface, while the base line and small fluctuations were due to discharges at the coating/electrolyte interface.

Schlieren technique applied to the arc temperature measurement in a high energy density cutting torch

Plasma temperature and radial density profiles of the plasma species in a high energy density cutting arc have been obtained by using a quantitative schlieren technique. A Z-type two-mirror schlieren system was used in this research. Due to its great sensibility such technique allows measuring plasma composition and temperature from the arc axis to the surrounding medium by processing the gray-level contrast values of digital schlieren images recorded at the observation plane for a given position of a transverse knife located at the exit focal plane of the system. The technique has provided a good visualization of the plasma flow emerging from the nozzle and its interactions with the surrounding medium and the anode. The obtained temperature values are in good agreement with those values previously obtained by the authors on the same torch using Langmuir probes.

Simulation study of HL-2A-like plasma using integrated predictive modeling code

Self-consistent simulations of HL-2A-like plasma are carried out using 1.5D BALDUR integrated predictive modeling code. In these simulations, the core transport is predicted using the combination of Multi-mode (MMM95) anomalous core transport model and NCLASS neoclassical transport model. The evolution of plasma current, temperature and density is carried out. Consequently, the plasma current, temperature and density profiles, as well as other plasma parameters, are obtained as the predictions in each simulation. It is found that temperature and density profiles in these simulations are peak near the plasma center. In addition, the sawtooth period is studied using the Porcilli model and is found that before, during, and after the electron cyclotron resonance heating (ECRH) operation the sawtooth period are approximately the same. It is also observed that the mixing radius of sawtooth crashes is reduced during the ECRH operation.

Northward interplanetary magnetic field plasma sheet entropies

The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate northward interplanetary magnetic field plasma sheet specific entropy s = p/ργ or p/nγ (for a single ion species). Profiles of s for hot and cold populations are considered. The hot ion population s profile suggests a duskward heat flux that is consistent with the curvature and gradient drift. In contrast, s of the cold population is approximately conserved in the X direction but has a strong gradient toward the midnight meridian. The total entropy (S = ∫p1/γ dl/B) and s under various entry mechanisms are estimated for comparisons with observed values. The cold population s is higher than that of the magnetosheath, suggesting the entry process may heat the ions. Cusp reconnection by itself may not increase the magnetosheath ion s, but cusp reconnection with kinetic Alfvén waves (KAW) may provide the necessary heating. Localized reconnection in Kelvin‐Helmholtz vortices may increase s if the plasma expands nonadiabatically into the large magnetospheric flux tube volume. KAWs may lead to diffusion and heating consistent with the observations. The cold population s increases by a factor of 5 from the flanks to the midnight meridian, which provides a constraint for transport mechanism(s) within the plasma sheet. A simple calculation is performed to demonstrate that the spatial gradients of s in the plasma sheet could result from the temporal dependence of transport processes in the magnetopause boundary layer.

Application of calibration-free laser-induced breakdown spectroscopy to radially resolved spectra from a copper-based alloy laser-induced plasma

In this work, the Calibration-Free approach for Laser-Induced Breakdown Spectroscopy (CF-LIBS) was applied for the first time to radially resolved spectra emitted by a laser-induced plasma. The radial profiles of plasma temperature and electron number density were used to calculate the local relative concentration of the elements of interest. We analyzed a set of profiles of the local spectral emission coefficient obtained previously by means of spatial deconvolution of the spectra from a copper-based alloy (Cu 93, Fe 5, Mn 1, Ni 1 wt.%) laser-induced plasma. A spatially integrated spectrum of the same plasma was also analyzed for comparison purpose. The relative abundance of the minor components Fe, Mn and Ni was calculated. The results obtained from the central region of the plasma were closer to the nominal concentrations than those obtained from the spatially integrated spectrum. However, an increasing deviation was observed towards the plasma edge. It is proposed that this deviation could be the result of a gradual departure from Local Thermodynamic Equilibrium (LTE) conditions due the significant decrease of the electron density at the external shells of the plasma.

Simulations of ITER with combined effects of internal and edge transport barriers

Predictive simulations of ITER with the presence of both an edge transport barrier (ETB) and an internal transport barrier (ITB) are carried out using the BALDUR integrated predictive modelling code. In these simulations, the boundary is taken at the top of the pedestal, where the pedestal values are described using theory-based pedestal models. These pedestal temperature models are based on three different pedestal width scalings: magnetic and flow shear stabilization (Δ ∝ ρis2), flow shear stabilization () and normalized poloidal pressure (). The pedestal width scalings are combined with a pedestal pressure gradient scaling based on the ballooning mode limit to predict the pedestal temperature. A version of the semi-empirical Mixed Bohm/gyroBohm (Mixed B/gB) core transport model that includes ITB effects is used to compute the evolution of plasma profiles. In this model, the anomalous transport in the core is stabilized by the influence of Er × B flow shear and magnetic shear, which results in the formation of ITB. The combination of the Mixed B/gB core transport model with ITB effects, together with the pedestal model, is used to simulate the time evolution of plasma current, temperature, and density profiles for ITER standard type I ELMy H-mode discharges. It is found that ITER fusion performance using the BALDUR code with Mixed B/gB transport model without the presence of ITB is quite pessimistic (Fusion Q ∼ 3). The presence of ITB is crucial and can result in a significant improvement, which is needed for achieving a target Fusion Q of 10. The improvement due to the presence of ITB is almost the same for all simulations with those three pedestal temperature models. This is caused by the predicted pedestal temperature from each pedestal temperature model varying just slightly. The presence of ITB has a strong impact on both temperature profiles, especially near the centre of the plasma, but has a small impact on electron, deuterium, tritium and carbon density profiles, except the helium density profile. The formation of ITB does not impact on the pedestal. It is also found that during a sawtooth crash, the temperature profiles drop significantly, but there is a small change in the density profiles. However, the sawtooth oscillation has no impact on the pedestal. When the auxiliary heating power is turned off, it is found that significant fusion power is sustained.

Pellet – Plasma Interaction: an Analysis of Pellet Injection Experiments by Means of a Multi‐Dimensional MHD Pellet Code

AbstractThis work is aimed at the analysis of pellet injection experiments by means of a magnetohydrodynamic code that computes the processes of pellet ablation, expansion, ionization, and magnetic confinement of the ablated substance. In particular, experimental pellet injection scenarios stemming from the tokamak ASDEX‐Upgrade (D2 pellets) and the stellarator W7‐AS (carbon pellets) are analyzed by means of this time‐dependent quasithree‐dimensional MHD pellet code. Pellet penetration depths measured in ASDEX‐Upgrade at LFS (low‐field‐side) and HFS (high‐field‐side) pellet injection, are compared with computational results.It was found that the penetration depths of deuterium pellets observed in about twenty randomly selected LFS and HFS injection scenarios could accurately be reproduced by a single code in which drift effects were intentionally ignored. This may be understood as an indication of the negligible effect drift exerts on the ablation process. At the same time, the distribution of the ablated and partially ionized substance may very well be affected by the drift of the ionized particles in the direction of the decreasing magnetic field.With the help of the Dα radiation patterns observed, the magnitudes of the heat fluxes affecting the pellet and the ablation cloud were estimated. The calculations show that the thermal heat fluxes (electron and ion) acting along the magnetic field lines are not sufficient to explain the size of the radiating filaments observed.The effect of extra‐thermal electrons on the ablation history of pellets injected in W7‐AS is demonstrated: the particle end energy fluxes causing measured pellet ablation profiles are determined.Based on a statistically well‐designed computational data set, an empirical scaling of the penetration depths in terms of the background plasma parameters was attempted. It is shown that the form of the temperature profile of the background plasma cannot be ignored while deriving scaling laws; the specification of the central or the bulk temperature alone is insufficient.Quantitative data are provided for the magnitudes of various additional processes such as the anomalous thermal conduction across the field lines or the grad(B)‐induced drift of the shielding cloud surrounding the pellet.The modelling of supersonic gas jets, a fuelling method that appears to be an alternative to pellet injection, is also considered. Since the massive stream of a cold gas represents a disturbance for the recipient plasma similar to that of a pellet, its dynamics can be described by the same means as the evolution of pellet clouds. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimizing a Thomson scattering diagnostic for fast dynamics and high background

The Madison Symmetric Torus (MST) presents challenging conditions for Thomson scattering (TS) measurements. The MST plasmas are reversed-field pinches (RFPs) with electron density n(e)<3x10(13) cm(-3), typically 1x10(13) cm(-3). The TS system was designed to measure from 10 eV to 2 keV; however, six polychromators were upgraded from four to eight spectral channels to resolve to 10 keV. There is no diverter or vertical field, so wall interaction results in high background light both from ion and neutral bremsstrahlungs and from line radiation. Also during standard plasmas, the RFP exhibits regular reconnection sawteeth events during which the plasma current, density, and temperature profiles are flattened. These events are of interest both due to the reconnection physics and to their contribution to the MST equilibrium and confinement. These events occur over 100 microS and exhibit large changes in background light and fast changes in temperature. During improved confinement plasmas, there are no sawteeth; the background is low but the temperature can be over an order of magnitude higher. Data analysis of the system has been developed to accommodate both the large dynamic range of the temperature, the fast dynamics, and the fast changing, high amplitude background. Special attention has been paid to the sources of error, in particular, the contribution of the background. A response-function method reduces the measured uncertainty by a factor of 2. Numerical techniques have been developed which are extremely robust. Two methods are used, a conventional chi(2) minimization using a Levenberg-Marquardt algorithm coupled with Monte Carlo modeling for the error bar and a Bayesian statistics method. The Bayesian method computes the probability distribution for the measured photons and electron temperature and this information can be used to ensemble data and will allow future integrated data analysis efforts.

Stratification-driven instabilities with bi-kappa distribution functions in the Io plasma torus

We extend our previous studies on plasma transport in the fast rotating magnetosphere of Jupiter. The present study uses a new kinetic Io plasma torus model in order to test the local stability criteria against stratification-driven low-frequency modes in this environment. Compared to the model used in our previous application, the new model introduces bi-kappa velocity distributions and plasma pressure anisotropies in order to reproduce most of the available observational data obtained in the torus. This generates radial profiles of plasma densities and temperatures that slightly differ from those used previously. Here we examine the implications of non bi-Maxwellian velocity distributions and of pressure anisotropies for the centrifugal interchange instability and for the resulting transport of plasma in the Io torus and further demonstrate how our theoretical findings can be applied to realistic environments.

Predictions of H-mode performance in ITER

Time-dependent integrated predictive modelling is carried out using the PTRANSP code to predict fusion power and parameters such as alpha particle density and pressure in ITER H-mode plasmas. Auxiliary heating by negative ion neutral beam injection and ion-cyclotron heating of He3 minority ions are modelled, and the GLF23 transport model is used in the prediction of the evolution of plasma temperature profiles. Effects of beam steering, beam torque, plasma rotation, beam current drive, pedestal temperatures, sawtooth oscillations, magnetic diffusion and accumulation of He ash are treated self-consistently. Variations in assumptions associated with physics uncertainties for standard base-line DT H-mode plasmas (with Ip = 15 MA, BTF = 5.3 T and Greenwald fraction = 0.86) lead to a range of predictions for DT fusion power PDT and quasi-steady state fusion QDT (≡PDT/Paux). Typical predictions assuming Paux = 50–53 MW yield PDT = 250–720 MW and QDT = 5–14. In some cases where Paux is ramped down or shut off after initial flat-top conditions, quasi-steady QDT can be considerably higher, even infinite. Adverse physics assumptions such as the existence of an inward pinch of the helium ash and an ash recycling coefficient approaching unity lead to very low values for PDT. Alternative scenarios with different heating and reduced performance regimes are also considered including plasmas with only H or D isotopes, DT plasmas with toroidal field reduced 10% or 20% and discharges with reduced beam voltage. In full-performance D-only discharges, tritium burn up is predicted to generate central tritium densities up to 1016 m−3 and DT neutron rates up to 5 × 1016 s−1, compared with the DD neutron rates of 6 × 1017 s−1. Predictions with the toroidal field reduced 10% or 20% below the planned 5.3 T and keeping the same q98, Greenwald fraction and βn indicate that the fusion yield PDT and QDT will be lower by about a factor of two (scaling as B3.5).

ASCOT simulations of electron energy distribution at the divertor targets in an ASDEX Upgrade H-mode discharge

Electron energy distribution at the divertor targets was calculated with the Monte Carlo code ASCOT for an ASDEX Upgrade H-mode discharge. The scrape-off layer (SOL) plasma background was obtained from the edge fluid code SOLPS. The orbit-following of test particles was performed assuming a fixed Maxwellian plasma background, accounting for the effects of the magnetic geometry, Coulomb interaction with the background and a prescribed electric potential. The energies recorded from the electrons impinging on the divertor targets indicated that, close to the plates, there should be strong deviations from the thermal Maxwell–Boltzmann distribution. In addition, details of the magnetic geometry and parallel plasma temperature and density profiles were observed to significantly impact the target energy distributions. The obtained discrepancy between the Monte Carlo and fluid results indicates a lack of self-consistency in fluid modelling due to kinetic effects in a medium-collisionality SOL.

On the Use of Sweeping Langmuir Probes in Cutting-Arc Plasmas—Part II: Interpretation of the Results

A semiempirical Langmuir probe model is introduced that is particularly adapted to high-energy-density cutting arcs, for which, as we have shown in part I, the ion current collected by negatively biased probes shows no plateau in the ion branch of the current-voltage (I-V) probe characteristic, and the signal amplitude is independent of the probe radius. According to the model, the ion drag due to the high-velocity plasma flow around the probe limits the effectively collecting area to a small fraction of the probe surface. If, according to the experimental evidence, this fraction is made independent of the probe radius, then its value results proportional to the probe bias, and so no plateau is found, at least as long as the collecting area is less than (half) the probe surface, which happens only at rather high probe bias. The model requires the determination of the function relating the electric field (in the region between the unperturbed plasma and the space-charge sheath close to the probe) to the parameters of the problem. Dimensional analysis together with empirical information allow to restrict the form of this function to leave only an auxiliary dimensionless function, which can be argued to be practically constant and whose value can be determined between rather tight bounds. As an example, radial profiles of plasma temperature and density are obtained by applying the proposed model to the experimental values of a I-V probe characteristic obtained in part I. The derived temperature profile is in good agreement with a previous published numerical simulation for a similar cutting torch.

Substorm plasma sheet ion pressure profiles

[1] The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate substorm growth, expansion, early-recovery, and late-recovery phases. During the growth phase, the pressure peaks at the inner edge of the plasma sheet. The premidnight pressure peak is associated with the temperature peak, while the postmidnight peak is associated with the density enhancement. After the substorm onset, the pressure at the inner edge diminishes. Instead, the pressure peaks at premidnight from X = −10 to −40 RE, which can be associated with temperature enhancement. During the early and late recovery phases, the pressure peaks at postmidnight, which is associated with a cold, dense ion population, possibly resulting from ion outflow and the substorm current systems. In the near-Earth region, the entropy decreases after substorm onset, but the specific entropy appears to be roughly conserved.

Study of the dependence of the specific output power of a copper chloride laser on the radial temperature profile of a gas plasma

The design of a copper chloride laser is described, and the laser is optimised by studying the dependence of its output power on the buffer gas type. The voltage and current of the laser discharge at the optimum buffer gas pressure are measured. The influence of the diaphragm diameter on the specific output power is studied after optimisation of switch parameters. When an diaphragm producing the optimal temperature gradient in the laser gas-discharge tube, the record specific output power of 123 W L-1 is obtained without any admixtures.

Magnetohydrodynamic Simulation of a Coaxial High-Density Plasma Opening Switch

The plasma dynamics and the magnetic-field penetration in a coaxial plasma opening switch (POS) are studied self-consistently based on the two-dimensional magnetohydrodynamic (MHD) equation conjunction with the generalized Ohm's law. The energy transfers between the plasma and the magnetic field are considered during the penetration of the magnetic field, in which both the Ohmic heating and the heat transfer are included in the energy-balance equation. The profiles of magnetic field, plasma density, and plasma temperature are simulated with the initial plasma density n0=1016 cm-3. For the high-density plasma, it is shown that the convection effect dominates the magnetic-field penetration; the plasma near the cathode is rapidly compressed by the magnetic pressure, and the temperature of the plasma in the shock region increases noticeably because of the compression effect. Furthermore, the increase of temperature in the shock region enhances the magnetic-field penetration near the cathode. The energy analysis shows that more than half of the input energy is stored as magnetic-field energy, and a significant amount of the input energy goes into the plasma thermal energy. The scaling relation between the maximal conduction current and the initial plasma density is also studied, and the simulation results agree with the theoretical law and experimental data well

Radial compression and torque-balanced steady states of single-component plasmas in Penning-Malmberg traps

Penning-Malmberg traps provide an excellent method to confine single-component plasmas. Specially tailored, high-density plasmas can be created in these devices by the application of azimuthally phased rf fields (i.e., the so-called “rotating wall” technique). Recently, we reported a regime of compression of electron (or positron) plasmas in which the plasma density increases until the E×B rotation frequency, ωE (with ωE∝ plasma density), approaches the applied frequency, ωRW. Good compression is achieved over a broad range of rotating wall frequencies, without the need to tune to a mode in the plasma. The resulting steady-state density is only weakly dependent on the amplitude of the rotating-wall drive. Detailed studies of these states are described, including the evolution of the plasma temperature, peak density, and density profiles during compression; and the response of the plasma, once compressed, to changes in frequency and rotating-wall amplitude. Experiments are conducted in a 4.8T magnetic field with ∼109 electrons. The plasmas have initial and final temperatures of ∼0.1eV. They can be compressed to steady-state densities &amp;gt;1010cm−3 and plasma radii &amp;lt;200μm. The outward, asymmetry-driven plasma transport rate, Γo, of the compressed plasmas is independent of density, n, in contrast to the behavior at lower densities where Γo∝n2. The implications of these results for the creation and confinement of high-density electron and positron plasmas and the creation of finely focused beams are discussed.

Convective Stability of Galaxy‐Cluster Plasmas

Recent studies suggest that active galactic nuclei and cosmic rays drive convection in galaxy-cluster plasmas and that convection heats cluster cores and regulates the plasma temperature profile. In this paper, we investigate the convective stability of the intracluster medium. Previous work has described how magnetic fields modify the convective stability criterion by causing heat to diffuse primarily along magnetic field lines. We extend earlier analyses by including cosmic rays and cosmic-ray diffusion. We consider an equilibrium in which the gravitational acceleration is in the --direction, the magnetic field is perpendicular to , and the magnetic pressure is much less than the thermal pressure. We use a local linear stability analysis to show that the convective stability criterion is nkBdT/dz + dpcr/dz > 0, where n, T, and pcr are the plasma particle density, temperature, and cosmic-ray pressure, respectively. We also present approximate solutions to the dispersion relation and describe the different waves and instabilities present in the plasma. Because the stability criterion differs markedly from the Schwarzschild criterion, galaxy-cluster plasmas can be convective even though the specific entropy increases outward.

Finding the radial parallel temperature profile in a non-neutral plasma using equilibrium calculations on experimental data

In 1992, Eggleston et al. [D. L. Eggleston et al., Phys. Fluids B 4, 3432 (1992)] reported on a technique for measuring the radial temperature profile in a pure-electron plasma confined in a Malmberg-Penning trap by partially dumping the plasma onto a charge collector at the end of the trap. For short plasmas and short confining rings, the assumptions in their paper are violated and a more general calculation is needed. This paper presents a variation of the standard equilibrium calculation to find the temperature profile of a pure-electron plasma. Eggleston’s shortcut “evaporation” temperature method is found to require a correction factor that can be calculated using methods described in this paper. For typical conditions, the evaporation method overstates the actual temperature by a factor ranging from 1.1 to 1.5 or more, depending on the plasma’s total charge and temperature and the geometry of the trap.

Numerical Modeling of the High-Intensity Transferred Arc with a Water-Cooled Constrictor Tube

Stable and axi-symmetrical DC high-intensity transferred arcs with a coaxial water-cooled constrictor tube have been used to study the arc characteristics for many years. All the previous modeling studies concerning the high-intensity transferred arcs were restricted to the near-anode region. Modeling results are presented in this paper concerning the characteristics of the whole high-intensity transferred arc, referring to a recent experiment. It is shown that the computed flow and temperature fields for different flow rates of the working gas are overall similar, but a fully developed flow regime can only be achieved in the water-cooled constrictor tube at low working-gas flow rates. The predicted radial profiles of plasma temperature at the cross section near the constrictor-tube exit compare favorably with available experimental data, but corresponding comparison about the plasma axial-velocity profiles shows appreciable difference, revealing that there may exist considerable errors in the plasma velocity measurements using a sweeping Pitot tube.