Plasma Temperature Profiles Research Articles

Multiradar observations of the polar tongue of ionization

We present a global view of large‐scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low‐latitude, auroral, and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high‐latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high‐latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m−3) and low electron and ion temperatures (∼2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high‐latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was ∼50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m−2 s−1) were observed in the topside ionosphere from the SED/TOI plume within the cusp.

Equations for nonlinear MHD convection in shearless magnetic systems

A closed set of reduced dynamic equations is derived that describe nonlinear low-frequency flute MHD convection and resulting nondiffusive transport processes in weakly dissipative plasmas with closed or open magnetic field lines. The equations obtained make it possible to self-consistently simulate transport processes and the establishment of the self-consistent plasma temperature and density profiles for a large class of axisymmetric nonparaxial shearless magnetic devices: levitated dipole configurations, mirror systems, compact tori, etc. Reduced equations that are suitable for modeling the long-term evolution of the plasma on time scales comparable to the plasma lifetime are derived by the method of the adiabatic separation of fast and slow motions.

Electron cyclotron emission simulation from TCABR plasmas

Electron cyclotron emission due to electrons described by a particular distribution function has been studied. The latter presents an extended tail generated by the interaction of the Lower Hybrid wave with the plasma as compared to the Maxwellian distribution function. For this purpose a new code has been developed which calculates for an arbitrary distribution function the intensity of radiation arriving at the plasma edge, the emission profile (as a function of position) and the optical depth (as a function of frequency) using the full dielectric tensor for a magnetized plasma. The electron distribution function is obtained by solving the Fokker-Planck equation in the frame of the quasilinear theory using a slab model. Results obtained for TCABR-like parameters show changes in the emission localized at positions where electron distribution function has been modified by the waves. Main parameter governing the changes in the electron cyclotron emission is the wave power. Changes in the plasma temperature and density profiles do not alter the emission profiles substantially. Reconstructed electron temperature profile has been obtained from the code radiation emission simulation, showing good agreement with the imposed temperature profile. The present results also showed that the changes in the emission profile in the region where the Lower Hybrid wave deposes its energy as compared with the emission profile of the plasma with Maxwellian distribution function are not so strong.

Turbulence in the Divertor Region of Tokamak Edge Plasma

AbstractResults of recent modeling of tokamak edge plasma with the turbulence code BOUT are presented. In previous studies with BOUT, the background profiles of plasma density and temperature were set as flux surface functions. However, in the divertor region of a tokamak, the temperature is typically lower and density is higher than those at the mid‐plane. To account for this in the present study, a poloidal variation of background plasma density and temperature is included to provide a more realistic model. For poloidally uniform profiles of the background plasma, the calculated turbulence amplitude peaks near outer mid‐plane, while in the divertor region the amplitude is small. However, present simulations show that as the background plasma profiles become more poloidally non‐uniform, the amplitude of density fluctuations, ñi, starts peaking in the divertor. It is found that in the divertor region the amplitude of ni fluctuations grows approximately linearly with the local density of the background plasma, ni0, while the amplitudes of Te and ϕ fluctuations are positively correlated with the local electron temperature, Te0. Correlation analysis shows that plasma turbulence above and below the x‐point is largerly uncorrelated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent advances in the LHD experiment

In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (>10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

Maximum Entropy Estimation of Electron Cyclotron Emission Spectra from Incomplete Interferograms in ELMy H-mode Tokamak Experiment

To measure the electron temperature profile of tokamak plasma in a high confinement mode with edge localized mode (ELMy H-mode), a method of data analysis is presented on the fast-scanning Fourier transform spectrometer (FTS), and results in the JT-60U tokamak are described. In the ELMy H-mode, where the FTS interferogram of electron cyclotron emission (ECE) becomes incomplete due to nonthermal emission pulses of ELM, the spectrum of ECE is estimated by using the maximum entropy method (MEM) of nonparametric type. The optimization of the regularization parameter is studied in terms of adopting the generalized cross validation (GCV) from the linear regularization of Tikhonov–Phillips. A study is also performed on a method to remove the nonthermal emission pulses from interferogram data before the MEM analysis, and on an extension of minimum cross entropy in order to meet with the missing of interferogram values in the region of zero path difference. From the application to experimental data of many shots, it is found that the electron temperature profiles can be automatically measured using the FTS during the ELMy H-mode operation of the tokamak.

Enthalpy Probe Technique for Thermal Plasma Diagnostics

The measuring principle and experimental results of the enthalpy probe technique for thermal plasma diagnostics are presented. Its calibration and errors are discussed. Typical results are presented for the system operation in an Ar/H2(5%H2) plasma arc jet under a reactor chamber pressure of 101.3 kPa. The plasma temperature and velocity profiles are measured. The center temperature and velocity are 6600 K and 850 m/s for plasma power 9 kW at axial location of 17 mm.

Modeling of a helicon plasma source

A two-dimensional axisymmetric helicon plasma source with three-dimensional wave components is modeled. Plasma fluid equations are solved self-consistently with helicon wave equations. Electron inertia is taken into account to simulate the propagation of electrostatic wave components. The purpose of this paper is to describe the model in detail and present the results to verify wave propagation characteristics, power deposition mechanisms, especially bulk versus peripheral power absorption. We also study the effect of wave absorption characteristics on discharge plasma density and temperature profiles.

A proposal for both plasma ion- and electron-temperature diagnostics under simultaneous incidence of particles and x-rays into a semiconductor on the basis of a proposed model for a semiconductor detector response

A method is proposed for obtaining radial profiles of plasma temperatures of both plasma ion (Ti) and electron (Te) simultaneously by the use of a semiconductor detector array. The method is based on our developed particle-response model for a semiconductor detector; in particular, the response theory is constructed for giving the applicability in particle energies ranging down to a kiloelectronvolt. Calculated results from our model are in fairly good agreement with experimental data on the detector response of incident particle beams with energies in the range 100 eV to a few kiloelectronvolts. On the basis of the verification of the proposed model, an idea of the use of a developed semiconductor detector array covered with `reliably unbreakable' ultrathin SiO2 `dead-layer filters' having various nanometre-order thicknesses is applied for simultaneous Ti and Te analyses by using charge-exchange neutral particles and x-rays from plasmas. Radial profiles of Ti and Te are obtained in a single plasma discharge alone, and the data reliability is independently cross-checked by a radial scan of a conventional charge-exchange neutral-particle analyser system as well as a 50-channel microchannel plate x-ray diagnostics system in the GAMMA 10 tandem mirror.

Neutral transport simulations of gas puff imaging experiments

Visible imaging of gas puffs has been used on the Alcator C-Mod tokamak to characterize edge plasma turbulence, yielding data that can be compared with plasma turbulence codes. Simulations of these experiments with the DEGAS 2 Monte Carlo neutral transport code have been carried out to explore the relationship between the plasma fluctuations and the observed light emission. By imposing two-dimensional modulations on the measured time-average plasma density and temperature profiles, we demonstrate that the spatial structure of the emission cloud reflects that of the underlying turbulence. However, the photon emission rate depends on the plasma density and temperature in a complicated way, and no simple scheme for inferring the plasma parameters directly from the light emission patterns is apparent. The simulations indicate that excited atoms generated by molecular dissociation are a significant source of photons, further complicating interpretation of the gas puff imaging results.

ELM energy and particle losses and their extrapolation to burning plasma experiments

Analysis of Type I ELMs from present experiments shows that ELM energy losses decrease with increasing pedestal plasma collisionality ( ν ∗ ped ) and/or increasing τ Front ∥, where ( τ ∥ Front=2π Rq 95/ c s,ped ) is the typical ion transport time from the pedestal to the divertor target. ν ∗ ped and τ Front ∥ are not the only parameters that affect the ELMs, also the edge magnetic shear influences the plasma volume affected by the ELMs. ELM particle losses are influenced by this ELM affected volume and are weakly dependent on other pedestal plasma parameters. ‘Minimum’ Type I ELMs, with energy losses acceptable for ITER, where there is no change in the plasma temperature profile during the ELM, are observed for some conditions in JET and DIII-D. The duration of the divertor ELM power pulse is well correlated with τ Front ∥ and not with the duration of the ELM-associated MHD activity. Similarly, the time scale of ELM particle fluxes is also determined by τ Front ∥. The extrapolation of present experimental results to ITER is summarised.

Time- and Space-Resolved Spectroscopic Diagnostics of Magnetically Confined Plasmas Measurements of Temporally and Spatially Resolved Electron Temperatures Using a Matrix-Type Semiconductor Detector

A new matrix-type semiconductor x-ray detector was developed for the purpose of the measurements of temporally and spatially resolved electron temperatures (Te) during a single plasma shot. This detector is fabricated using precise formation of thin dead layers (SiO2) of six different thicknesses aligned compactly on its surface. Each ”row” has seven channels for the measurements of plasma x-ray radial profiles so as to make x-ray tomographic reconstructions. These various SiO2 layers are proposed to be utilized as ultrathin ”x-ray absorption filters”. This novel idea enables us to analyze x-ray tomography data including the Te region down to a few tens eV. The comparison of tomographically reconstructed data from the respective detector rows provides the temporal evolution of energy-resolved x-ray or Te profiles using ” the absorption method ”. Observations of internal core plasma structural behavior during the magnetohydrodynamic (MHD) destabilization are carried out by the use of a matrix-type semiconductor detector. Furthermore, a method is proposed for obtaining radial profiles of plasma temperatures of both plasma ions and electrons simultaneously.

Correction to “2D plasma sheet ion density and temperature profiles for northward and southward IMF” by Simon Wing and Patrick T. Newell

Correction to “2D plasma sheet ion density and temperature profiles for northward and southward IMF” by Simon Wing and Patrick T. Newell

Investigation of the connection between plasma temperature and electrode temperature in metal-halide lamps

Spatial profiles of electrode temperatures and plasma temperatures have been measured on `real' HID lamps filled with a commercial metal-halide compound. The absolute accuracy of pyrometric determination of electrode tip temperatures was ±30 K, while the determination of plasma core temperatures, using a modified Bartels method, has an accuracy of ±100 K.We could deduce a close correlation between the plasma temperature in front of an electrode Tp and its tip temperature Tt due to the influence of the cataphoresis. If Tp is reduced at the cathode the Tt value has also lowered, whereas Tp at the anode is raised together with its Tt data. This correlation disappears at ballast frequencies above 100 Hz, whereas the cataphoresis influence on Tp continues up to 500 Hz. Based on the latter limit, a rough estimation of the cataphoresis velocity delivers 700 cm s-1. As a tentative interpretation, we suggest that the connection between Tp and Tt is caused by an increase of the ion part of the total current at the cathode due to Na accumulation before it. Thus, the cathode has to emit fewer electrons and works at a lower temperature.Further results are the temporal behaviour of Tt depends on the ballast type. For vertical operation the strong influence of convection on Tt has also to be taken into account. Above 100 Hz, where only convection plays a role, the upper electrode Tt exceeds the Tt value of the lower electrode by nearly 400 K. This discrepancy one may explain, tentatively, by convection heating of the upper electrode and convection cooling of the lower one.

Electron cyclotron radiative transfer in fusion plasmas

The radiative transfer in a system at local thermodynamic equilibrium is investigated on the basis of the solution of the (geometrical optics) transfer equation, accounting for the non-local nature of the radiative process due to both re-absorption of the emitted radiation and reflectivity of the walls of the system. The specific case of the electron cyclotron (EC) radiation in a cylindrical fusion plasma with specularly reflecting walls for which an analytical solution can be derived, is addressed and, in particular, the radial profile of the net power density radiated is evaluated by making use of an improved expression for the EC absorption coefficient. A detailed numerical analysis, carried out by varying both the wall reflection coefficient and the radial profile of the plasma temperature, reveals that a reversal of the net power density profile can occur on the plasma outboard for sufficiently high wall reflectivity. From a comparison with bremsstrahlung radiation profiles it is apparent that a local treatment of EC power emission is needed for sufficiently hot plasmas as expected, e.g. in the so-called advanced regimes of DT tokamak reactors. Furthermore, an exact approach is used to check the accuracy of approximate EC net power density profiles as calculated with the CYTRAN code showing that the latter provides a globally reasonable approximation. Evaluating the total EC radiated power from the exact local approach shows that its scaling with the reflection coefficient is very well described by a scaling following from a recently established global model for the EC radiation, which improves the well-known Trubnikov scaling. The results obtained are discussed in view of their possible relevance to affecting the plasma temperature profile.

2D plasma sheet ion density and temperature profiles for northward and southward IMF

We previously developed a method for inferring plasma sheet ion temperature and density from ionospheric observations. Using this method and DMSP observations for 1992, we present 2D profiles of the equatorial plasma sheet ion density and temperature for southward and northward IMF for b2i > 64° or Kp < ∼3. During periods of northward IMF, cold dense ions can be found plentifully along the plasma sheet flanks. However, during periods of southward IMF, the presence of these cold dense ions is noticeably diminished, especially at the dusk flank where the density peak is less discernable. These cold dense ions have been previously interpreted in terms of magnetosheath ion entry into the plasma sheet. Our result suggests that any mechanism proposed to transport magnetosheath ions from dusk LLBL to the plasma sheet along the dusk flank during periods of northward IMF would have to be able to do so efficiently over a large spatial scale.

One-dimensional electromagnetic solitons in a hot electron-positron plasma

The set of relativistic hydrodynamic equations for a two-species plasma is derived with the aim to investigate the interaction between arbitrary amplitude electromagnetic (EM) fields and hot plasmas. The equations are then specialized in order to study the existence of solitonlike EM distributions in a one-dimensional electron-positron plasma. It is found that: (i) a nonzero temperature makes possible the existence of nondrifting soliton-like solutions, otherwise impossible in a strictly cold plasma; (ii) in an ultrarelativistic plasma, extremely large concentrations of EM energy densities can be achieved; (iii) correspondingly, the temperature profile of the background plasma develops strong nonuniformities.

Application of magnetically-broadened hydrogenic line profiles to computational modeling of a plasma experiment

Magnetically-broadened hydrogenic line profiles are used, in conjunction with a non-local thermodynamic equilibrium (NLTE) code, to model an optically thick phenomenon in the Alcator C-Mod tokamak fusion experiment. The line profiles, including magnetic field effects as well as Doppler and Stark broadening, have been calculated using Beline and are used in Cretin, a multi-dimensional NLTE radiation transfer code, to study the influence of a magnetic field on emergent spectrum. A midplane Alcator C-Mod multifaceted axisymmetric radiation from the edge (MARFE), in which the magnetic field strength was changed dynamically from 5.4 T down to 2.7 T , is simulated and compared with experimental data. Assuming hydrodynamic and radiative equilibrium, MARFE simulations are performed in one-dimension with fixed plasma density and temperature profiles. Magnetic broadening effects on the brightness of optically thick Lyman series lines are quantified.

An efficient tabulated collisional radiative equilibrium radiation transport model suitable for multidimensional hydrodynamics calculations

A computationally efficient method for transporting radiation in multidimensional plasmas has been developed and evaluated. The basis of this method is a uniform plasma approximation that allows one to utilize existing escape probability techniques that are successfully used in one-dimensional (1D) calculations to approximately solve the multidimensional radiation transport problem. This method is superior to diffusion methods because (1) the probability of escape technique insures that the plasma goes to the correct optically thin and thick limits, (2) the effects of line absorption due to photoexcitations are modeled, and (3) this method uses source functions that are based on a self-consistent nonlocal thermodynamic equilibrium calculation, not an ad hoc assumption that the source functions are Planckian. This method is highly efficient because equation of state information from 1D calculations is tabulated as a function of plasma internal energy, ion density, and the line probability of escape from a uniform plasma, and then used in multidimensional calculations. Given the internal energy and ion density, and by calculating the line probability of escape from a zone of the multidimensional plasma, the equation of state, including emissivities and absorption coefficients, of the zone is determined from the table. Total radiative power, K-shell radiative power, total radiative yield, K-shell radiative yield, and plasma density and temperature profiles obtained from 1D Z-pinch calculations employing this method are in good agreement with the same powers, yields, and profiles calculated using a full radiation transport model. This method has been implemented in the 2D plasma radiating imploding source model code [F. L. Cochran et al., Phys. Plasmas 2, 2765 (1995)] to determine the influence of radiation transport in argon Z-pinch experiments performed on the Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories.

Effects of plasma non-homogeneity on the physical properties of sputtered thin films

The plasma generated in a two-target closed field unbalanced magnetron sputtering system for thin film deposition is characterized by means of Hall probes and cylindrical Langmuir probes as a function of the position inside the vacuum system. The plasma potential, electron density and temperature profiles in different locations are measured by two diagnostic systems equipped with cylindrical Langmuir probes. The plasma non-homogeneity due to the presence of magnetic field gradients is evaluated. In order to test the effects of measured plasma non-homogeneity on the physical properties of sputter-deposited coatings, several substrates are put inside the chamber in regions characterized by different plasma density and plasma potential. The composition, microstructure and morphology of TiN x films grown onto these substrates are then studied by means of nuclear techniques (RBS, n-RBS, NRA, ERDA), X-ray diffraction (XRD) and secondary electron microscopy (SEM). The mechanical properties are determined by micro-scratch test and nanoindentation and correlated to the local plasma parameters.