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Solar flare induced interplanetary shock and helium enriched driver gas observed on 13 February 1967, discussing wind velocity and plasma acceleration
A generalization of the low-density corona-model theory of the intensities of satellite lines of the resonance lines emitted by multiply charged helium-like ions in the X-ray spectra of high-temperature plasmas is developed in order to take account of electron collisional transitions between close autoionizing levels. A formalism for determining the population densities of the autoionizing levels and the satellite line intensities is presented, and the quantum mechanical calculation of autoionization, radiative decay and electron-induced collisional transitions is discussed. Generalized emission rate coefficients corresponding to dielectronic recombination and inner-shell-electron collisional excitation are tabulated as functions of electron density and temperature. The procedure is then used to illustrate the effects of angular-momentum-changing electron collisions on the intensities of the satellites of the 1s2p 1P to 1s2 1S resonance line of the ions Al XII and Si XIII. It is concluded that the collisional transitions have their most significant effect on the intensities of satellite transitions from metastable autoionizing states, which may be useful in determining plasma densities.
Radially propagating shocks of solar flare origin are analyzed using plasma and charged particle measurements on Pioneer 10 to show that protons and helium nuclei are continuously accelerated to energies of up to roughly 70 MeV per nucleon. The discovery that electrons are simultaneously accelerated to relativistic energies places new constraints on models for acceleration by these quasi-perpendicular shocks in the outer heliosphere.
This paper discusses the basic approach to developing a comprehensive electromagnetic model for a microwave electrothermal discharge. The discharge and surrounding region are modeled as a section of waveguide loaded with a uniform, cold, lossy plasma. A characteristic equation has been derived describing all possible system modes. In particular the TM(01) mode is numerically solved. Propagation and attenuation constants for the TM(01) mode are found numerically for a given range of electron-neutral collision frequencies and plasma densities. Results are used to interpret a number of experimental observations in helium. Specifically, they are used to explain high coupling efficiencies and ease of cavity tuning at high pressures.
Experimental and theoretical studies on the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) have continued through a NASA led collaborative program involving several research groups. In the experimental area, performance characterization of the VASIMR helicon plasma source has been obtained over a portion of the parameter space, with helium and hydrogen propellant. Density (10(exp 18) - 10(exp 19)/ cubic meter) and temperature (5 eV) were measured at moderate degree of ionization in two separate experimental devices. Helicon design improvement and optimization will be discussed. Experiments with the ion cyclotron resonance heating (ICRH) subsection have begun and preliminary results will be discussed. Theoretical picture and integrated numerical simulation continue to be refined to account for the main physics elements of the VASIMR, including RF absorption and particle acceleration with subsequent detachment in the magnetic nozzle.
The accretion rate of the black hole candidate X-ray binary GX 339-4 in the 'off' state is low, and an advection dominated accretion flow (ADAF) is present. Hydrogen-like and helium-like iron Kbalpha; emission lines at 6.7 and 6.95 keV from hot plasma of ADAF can be produced by recombination-cascade processes with moderately high intensities, which are markedly distinguished from the fluorescent iron Kα line at approximately 6.4 keV. We show that the observational features of GX 339-4 can be explained by the ADAF model, if the iron abundance is more than 10 times the solar value, though the reason for such a high abundance is still unclear. We suggest that the increase of the accretion rate makes GX 339-4 change from off, low, intermediate, to high and very high states, and the line center of iron Kbalpha; will therefore shift from approximately 6.83 to approximately 6.4 keV, i.e. to the fluorescent disc-line, since the disappearance of the ADAF due to its high accretion rate.
The study of multiscale pickup ion phase-mixing in the lunar plasma wake with a hybrid model is the main subject of our investigation in this paper. Photoionization and charge exchange of protons with the lunar exosphere are the ionization processes included in our model. The computational model includes the self-consistent dynamics of the light (hydrogen ions or hydrogen molecule ion or helium ion), and heavy (sodium ion ) pickup ions. The electrons are considered as a fluid. The lunar interior is considered as a weakly conducting body. In this paper we considered for the first time the cumulative effect of heavy neutrals in the lunar exosphere (e.g., Aluminum, Argon), an effect which was simulated with one species of sodium ion but with a tenfold increase in total production rates. We find that various species produce various types of plasma tail in the lunar plasma wake. Specifically, sodium ion and helium ion pickup ions form a cycloid-like tail, whereas the hydrogen ion or hydrogen molecule ion pickup ions form a tail with a high density core and saw-like periodic structures in the flank region. The length of these structures varies from 1:5 R(sub M) to 3:3 R(sub M) depending on the value of gyro radius for hydrogen ion or hydrogen molecule ion pickup ions. The light pickup ions produce more symmetrical jump in the density and magnetic field at the Mach cone which is mainly controlled by the conductivity of the interior, an effect previously unappreciated. Although other pickup ion species had little effect on the nature of the interaction of the Moon with the solar wind, the global structure of the lunar tail in these simulations appeared quite different when the hydrogen molecule ion production rate was high.
Experimental observations have been made during steady-state operation of the NASA Lewis Bumpy Torus experiment at input powers up to 150 kW in deuterium and helium gas, and with positive potentials applied to the midplane electrodes. This steady-state ion heating method utilizes a modified Penning discharge operated in a bumpy torus confinement geometry such that the plasma is acted upon by a combination of strong electric and magnetic fields. Experimental investigation of a deuterium plasma revealed electron temperatures from 14 to 140 eV and ion kinetic temperatures from 160 to 1785 eV. At least two distinct modes of operation exist, each of which is associated with a characteristic range of background pressure and electron temperature. Experimental data show that the average ion residence time in the plasma is virtually independent of the magnetic field strength.
Turbulent impinging jets have been proposed to cool high heat flux plasma-facing components such as the solid tungsten target plates of the divertor in long-pulse magnetic fusion energy reactors. In particular, the T-tube modular divertor, originally developed by the ARIES Team, consists of two concentric cylindrical tubes where helium flows through a slot in the inner tube, forming an approximately planar jet that impinges upon and cools the inner surface of the pressure boundary (namely, the outer tube) and the ~15-cm 2 plasma-facing W target. The objective of this work is to demonstrate that large eddy simulations (LESs) accurately simulate the thermal transport in canonical flows that comprise the cooling flow in the T-tube, as well as validate temperatures from LES with experimental measurements in a simplified T-tube geometry. Wall‑resolved LESs, validated by experimental data and verified by direct numerical simulations (DNSs), provide benchmark data for two canonical flows in the T‑tube, namely, planar impinging and wall jets, for Reynolds numbers Re B = 4 × 10 3 to 2 × 10 4 . Our LES results are within 4% to 12% root-mean-square error (RMSE) of surface Nusselt number distributions (Nu) from experiments and DNSs. The validated LES results are then used as the ground truth to evaluate four Reynolds‑averaged Navier-Stokes (RANS) turbulence closures, namely, the k‑ω SST, realizable k‑ε, GEKO, and γ‑SST models. The k‑ω SST model has the best overall performance in terms of heat transfer, giving surface Nu within 12% RMSE of the LES results for high‑ReB impinging jets and reduced overprediction in the wall‑jet region. The GEKO model with default constants has the next best performance, providing slightly better Nu predictions for low ReB impinging jets (versus k-ω SST) but worse overall performance over the full range of ReB studied here. The realizable k‑ε turbulence model significantly overestimates turbulence near the stagnation point, while the γ‑SST model suppresses near‑wall production, biasing the simulations toward simulating laminar surface heat transfer. Simulations of the simplified T‑tube show that LES and RANS simulations with the k‑ω SST model give nearly identical average heat transfer coefficients (HTCs) over the impingement surface. The realizable k‑ε model predicts significantly lower wall temperatures due to overestimation of HTC in the outlet flow.
Microwave-heated plasmas in convergent nozzles are analyzed using a coupled Maxwell and Navier-Stokes solver to examine relevant issues associated with microwave thermal propulsion. Parametric studies are conducted to understand the effect of power, pressure, and plasma location with respect to the nozzle throat. For nozzles in the 0.5 to 3 N range with helium flow, results show that specific impulses up to 550-650 sec are possible, with further increases being limited by severe wall-heating. Coupling efficiencies of over 90 percent are consistently obtained, with overall efficiencies ranging from 40 percent to 80 percent. Size scale-up studies (done by scaling the frequency from 2.45 GHz to 0.91 GHz) indicate that plasma migration toward the walls occurs more frequently for the lower frequency. Increasing the cavity aspect ratio and detuning the cavity are found to be effective ways of keeping the plasma on-axis.
We report results from a systematic analysis of thermal expansion of plasma-exposed tungsten based on molecular-dynamics simulations using models of tungsten with distributions of helium (He) bubbles in the tungsten matrix. We distinguish between two approaches of filling the bubbles with He, where the amount of He in the bubble can or cannot vary with temperature. In the former case, the thermal expansion coefficient decreases monotonically with the porosity and He content of the tungsten matrix, while in the latter case, the thermal expansivity increases monotonically with increasing porosity and He content. The latter condition, where the He content in the bubble is determined at the implantation temperature and remains constant with varying temperature in the tungsten matrix, is consistent with He species transport in tungsten used as a plasma-facing component (PFC) in nuclear fusion reactors and implies the development of biaxial compressive thermal strains in the PFC material that contribute to accelerating the growth of a nanostructure on PFC tungsten surfaces. Furthermore our analysis advances the fundamental understanding of thermal expansion in PFC tungsten and contributes to the development of a thermophysical property database for properly incorporating effects of realistic heat loads into modeling the dynamical response of PFC tungsten under fusion reactor operating conditions.
Doubly ionized helium is the 2nd most abundant element in the solar wind. The solar wind’s properties are a function of the source region on the Sun from which it emanates. This includesthe abundances of solar wind Helium and rarer, heavier elements. The speed of the solar wind, when observed near Earth, is also related to the type of source region from which the solar wind originated. Early solar wind models are unable to explain how the solar wind achieved the asymptotically fastest, non-transient speeds observed at 1 AU and the solar wind must be accelerated during transit through interplanetary space. Only recently have observations shown that the solar wind from different source regions is accelerated by different mechanisms. However, the solar wind speed is also known to be an insufficient for identifying the type of solar source from which it originated, likely because these different acceleration mechanisms accelerate the solar wind into two different speed regimes that overlap. Furthermore, the mechanism that provides the coronal plasma sufficient energy to accelerate into the solar wind is still undetermined. These mysteries are further complicated by the Sun’s evolution through its 11-year activity cycle and the impact of the solar cycle on in situ observations. Using solar wind observations of the helium abundance from the Wind spacecraft and heavy ion abundances from the Advanced Composition Explorer (ACE) along with observations of the Sunspot Number – a long-studied proxy for the Sun’s activity – we will discuss the imprint of different solar wind source regions on observations collected at 1 AU. We will draw inferences about the role of helium in the acceleration of coronal plasma into the solar wind. Using the Alfvénicity – a metric related to the solar wind acceleration that occurs during transit through interplanetary space – we will derive a solar wind classification scheme for near-Earth observations that is related to the differences in the source regions from which it emanates.
A fusion prototypic neutron source (FPNS) will play a fundamental role in testing and developing fusion materials that can withstand the harsh fusion environment. We conducted a thorough assessment to determine whether blanket materials would benefit from FPNS testing. Because of the cross sections for the various blanket materials, lithium-containing materials can generally be tested adequately using existing fission irradiation capabilities instead of waiting for or relying solely on future FPNS experiments. However, non-lithium blanket materials, such as beryllium multipliers and flow channel inserts, should be tested under a fusion neutron spectrum.
Rare-earth barium copper oxide (REBCO) high-temperature superconducting (HTS) magnets are considered a game changer for the capability of generating magnetic fields exceeding 20 T at or above liquid helium temperature, combined with the potential for substantially reduced manufacturing and operational costs. Princeton Plasma Physics Laboratory is dedicated to developing large-bore high-field superconducting magnet systems to support forefront physics research. Here, our current project focuses on a φ72-mm cold-bore REBCO insert comprising 21 dry-wound double-pancake coils, designed to generate at least 8 T when nested within a 12-T outsert magnet. A significant challenge for high-field REBCO magnets is the time-varying-magnetic-field-induced screening currents (SC) in the REBCO conductors, which can cause localized strain and stress concentrations. This paper presents a numerical study of the SC-induced strain in the REBCO insert magnet, confirming that the SC-induced strain can be substantially suppressed by energizing the REBCO insert before the outsert magnets. We discuss and reveal the mechanism behind this reduction. By applying this strategy, we expect to unleash the potential of the REBCO insert magnet to generate up to 12 T in a 12-T background field.
Microwave-heated plasmas in convergent nozzles are analyzed using a coupled Maxwell and Navier-Stokes solver to examine relevant issues associated with microwave thermal propulsion. Parametric studies are conducted to understand the effect of power, pressure, and plasma location with respect to the nozzle throat. For nozzles in the 0.5 to 3 N range with helium flow, results show that specific impulses up to 550-650 seconds are possible, with further increases being limited by severe wall-heating. Coupling efficiencies of over 90 percent are consistently obtained, with overall efficiencies ranging from 40 percent to 80 percent. Size scale-up studies-done by scaling the frequency from 2.45 GHz to 0.91 GHz-indicate that plasma migration toward the walls occurs more frequently for the lower frequency. Increasing the cavity aspect ratio and detuning the cavity are found to be effective ways of keeping the plasma on axis.
Data obtained by OGO 5 are used to confirm IMP 6 observations of an inverse dependence of the helium-to-hydrogen temperature ratio in the solar wind on the ratio of solar-wind expansion time to the Coulomb-collision equipartition time. The analysis is then extended to determine the relation of the difference between the hydrogen and helium bulk velocities (the differential flow vector) with the ratio between the solar-wind expansion time and the time required for Coulomb collisions to slow down a beam of ions passing through a plasma. It is found that the magnitude of the differential flow vector varies inversely with the time ratio when the latter is small and approaches zero when it is large. These results are shown to suggest a model of continuous preferential heating and acceleration of helium (or cooling and deceleration of hydrogen), which is cancelled or limited by Coulomb collisions by the time the plasma has reached 1 AU. Since the average dependence of the differential flow vector on the time ratio cannot explain all the systematic variations of the vector observed in corotating high-velocity streams, it is concluded that additional helium acceleration probably occurs on the leading edge of such streams.
The laser ablation/ion storage facility at the UNLV Physics Department is dedicated to the study of atomic processes in low temperature plasmas. Our current program is directed to the study of charge transfer of multiply charged ions and neutrals that are of importance to astrophysics at energies less than 1 eV (about 10(exp 4) K). Specifically, we measure the charge transfer rate coefficient of ions such as N(2+), Si(3+), Si(3+), with helium and Fe(2+) with molecular and atomic hydrogen. All these ions are found in a variety of astrophysical plasmas. Their electron transfer reactions with neutral atoms can affect the ionization equilibrium of the plasma.
The objective of this project is to develop, and integrate, high-performance simulation tools capable of predicting plasma-facing component (PFC) operating lifetime and the impact of the evolving surface morphology of tungsten-based PFCs on plasma contamination, including the dynamic recycling of fuel species and tritium retention, in future magnetic fusion devices. Establishing a fundamental physical understanding and developing predictive capabilities of plasma-surface interactions (PSI) requires simultaneously addressing complex and diverse physics occurring over a wide range of length (Angstroms to meters) and time (femtoseconds to years) scales, as well as integrating extensive physical processes across the plasma–surface interface. This requires development of not only detailed physics models and computational strategies at each scale, but also algorithms and methods to couple them effectively in a way that can be robustly validated. Deploying these tools requires the continued development and coupling of leadership-scale computational codes to describe the boundary plasma and the evolving PFC surface, as well as a host of simulations that bridge disparate scales to address complex physical and computational issues at the plasma–surface interface in multi-component materials systems for magnetic fusion energy development beyond ITER.