Upstream Plasma Parameters Research Articles

Analysis and design of fast flow liquid Li divertor for fusion nuclear science facility (FNSF) using coupled plasma boundary and LM MHD/heat transfer codes *

The SOLPS-ITER code is utilized to analyze the boundary plasma associated with a fast-flow lithium (Li) divertor configuration in the fusion nuclear science facility (FNSF) tokamak and identify operational regimes with acceptable divertor and core conditions. Plasma transport from the SOLPS-ITER code has been coupled with a liquid metal (LM) MHD/heat transfer code to model a Li open-surface divertor design and assess its impact on the scrape-off-layer (SOL) and core plasma performance. Simulations with only Neon (Ne) impurity seeding have been conducted to evaluate its impact on meeting FNSF design demands for the divertor and upstream plasma parameters. Simulation results indicate that Ne seeding significantly mitigates divertor heat flux but potentially reduces both upstream electron and main ion density due to fuel dilution. The combined application of Ne seeding and deuterium (D2) puffing is required to satisfy the FNSF design requirements on upstream density ( ne,sepOMP ∼1× 1020 m−3) and divertor energy flux ( q⊥,maxOdiv < 10 MW m−2). D2 puffing plays a role in counteracting upstream density drops and augmenting energy and momentum losses through atomic and molecular processes.The inlet Li flow velocity is systematically varied across a wide range to identify acceptable flows and corresponding LM surface temperatures. This comprehensive analysis identifies the acceptable Li flow parameters, LM surface temperature, and emitted Li fluxes necessary to meet the major design constraints. The emitted Li fluxes exhibit minimal impact on the main plasma at surface temperatures up to approximately ∼525 ∘C, corresponding emitted Li fluxes of up to φ Li ∼2 ×1023 atoms s−1. Uncertainties in the Li emission processes from the surface are also investigated, primarily influencing Li loss in the lower surface temperature range ( <525∘ C), with simulation results indicating a minor impact on the divertor and upstream plasma. Conversely, evaporation predominantly drives the Li loss processes at higher surface temperature ranges ( >525∘ C), contaminating both the divertor and upstream plasma.

The Plasma Environment Surrounding the Reiner Gamma Magnetic Anomaly

AbstractReiner Gamma is a prime target for low‐orbiting spacecraft or even surface‐landed missions in the near future. The region hosts a prominent lunar swirl that is co‐located with a strong and well‐structured magnetic anomaly. We simulate and discuss Reiner Gamma's near‐surface plasma environment at different altitudes above the lunar surface using the fully kinetic particle‐in‐cell code iPIC3D. The input magnetic field model is based on orbital‐altitude observations from the Kaguya and Lunar Prospector missions. We develop eight simulation cases, representing the distinct plasma regimes. Reiner Gamma is exposed to along a typical orbit, including different solar wind incidence angles and the magnetosheath crossing. We show that the plasma environment is vastly different at different altitudes and depends critically on the upstream plasma parameters, consistent with the predictions of the solar wind standoff model. Our work helps to define measurement requirements for a possible future low‐orbiting or lander mission to the Reiner Gamma area or similarly magnetized regions of the lunar surface.

Heat Fluxes in Collisionless Magnetohydrodynamic Shock Waves

Properties of solutions for parallel magnetohydrodynamic (MHD) shock waves in collisionless plasma with heat fluxes obtained in the 8-moment MHD approximation are discussed. The domain of upstream shock-wave plasma parameters in which there are solutions that preserve physical meaning of the downstream plasma parameters is determined. New results have also been obtained for the heat fluxes behind the front of parallel shock waves in collisionless plasma.

Study of end-cell plasma parameters of GAMMA 10/PDX by the LINDA code

This research numerically investigates the plasma behavior during neutral gases puffing into the west end-cell of the linear plasma confinement device GAMMA 10/PDX. A multi-fluid code named ‘LINDA’ is developed and applied at the west end-cell of the device to explore the processes of plasma detachment and energy loss mechanism during plasma-neutral interactions. The ion heat flux limiter is newly introduced in the LINDA code. In the paper, we investigate the influence of the ion heat flux limiter in our divertor plasma models to understand how the ion heat flux limiter affects the computed plasma parameters. The upstream plasma parameters are The effect of ion heat flux limiting reduces the ion temperature and heat flux in the direction of the target plate. The ion temperature profile is shown to be similar when the heat flux limiting factor (αi) is varied from 0.2 to 2.0. The effect of Hydrogen (H) and Argon (Ar) gas puffing has been investigated under the condition of ion heat flux limiting factors αi = 1.0. The ion temperature is shown to be similar between Ar and without Ar puffing. On the other hand, Te reduces remarkably for Ar puffing by enhancing the radiation cooling of Ar. The ion and electron loss are greatly enhanced for combined puffing of Ar and H. The ionization loss is found to be slightly higher and the charge-exchange loss is found to be slightly lower during the ion heat flux limiting case. These outcomes may contribute to understanding the plasma detachment processes in fusion devices.

First electron temperature and density measurements of D-module plasma in GAMMA 10/PDX using Thomson scattering and microwave interferometer systems

We developed an yttrium aluminium garnet (YAG)-Thomson scattering (TS) system for measuring an electron temperature and density in the divertor simulation experimental module (D-module) plasma in GAMMA 10/PDX. We applied a frequency multiplied microwave interferometer (MIF) system for measuring a line integrated electron density in D-module. In D-module, there are many electrostatic probes on the V-shaped tungsten target plate. However, they cannot measure the upstream plasma parameters. By using the YAG-TS system and MIF system, we can obtain the upstream plasma parameters. In the argon gas puffing D-module plasma, we successfully obtained the electron temperature and density of upstream plasma for the first time.

Effects of the gas puffing neutral on the plasma parameters in the end-cell of GAMMA 10/PDX by using the multi-fluid code “LINDA”

This research numerically investigates the behavior of plasma flow in GAMMA 10/PDX. The impact of impurity gases seeding into the divertor region on the plasma parameters and the energy loss processes has been investigated numerically by using the multi-fluid code “LINDA”. The neutral particles are injected into the bulk hydrogen plasma and the upstream plasma parameters on the z-axis are ni=1.0×1019m−3, Ti=100eV,Te=30eV. A reduction in the plasma temperature has been observed with the increasing H neutral injection. The ionization and Charge-exchange (CX) power loss increase almost linearly with the increasing H neutral density, which induce a reduction in the plasma temperature. During H seeding, Ar and Ne are seeded simultaneously to observe the influence of Ar and Ne on the electron and ion energy loss mechanism. In this study, the fraction of impurity density to upstream plasma density (nz/ni) is used to be 10%. A comparison between Ar and Ne shows that Ar is effective on the electron cooling. The radiation power loss of Ar is found to be higher than that of Ne. The radiation cooling effect and CX loss significantly reduces the electron and ion temperature, respectively. A slight influence of Ar and Ne seeding on the ion energy loss terms has been observed. The electron energy loss terms increase significantly during impurity gas Ar and Ne seeding. These outcomes represent the impact of gas seeding for reducing the plasma energy on the divertor target plates.

Rankine‐Hugoniot Relations in Multispecies Plasma With Gyrotropic Anisotropic Downstream Ion Distributions

AbstractIn the observed heliospheric shocks downstream distributions are usually measured in the regions where the kinematic relaxation has already happened but isotropy is not achieved. Rankine‐Hugoniot relations, connecting the upstream and downstream plasma parameters, should be able to take into account the downstream ion anisotropy. In a multispecies plasma different species behave differently at the ramp crossing and when collisionlessly gyrating in the downstream region. The downstream density and pressure of each species can be expressed in terms of the magnetic compression and the cross‐shock potential, which allows us to derive new Rankine‐Hugoniot relations.

Kinetic modeling of Langmuir probe characteristics in a laboratory plasma near a conducting body

Results are presented from kinetic modeling of Langmuir probe characteristics measured in the vicinity of a conducting sphere, in a well controlled laboratory plasma. In the experiment, the interaction of a drifting argon plasma with a conducting spherical sphere is studied using two cylindrical Langmuir probes located upstream and downstream of the sphere. Plasma parameters upstream of the sphere are determined by fitting an experimentally measured characteristic with the one obtained from simulations. Using the same upstream plasma parameters as input in simulations, the characteristic computed in the wake of the sphere is compared with the measured one. In addition to validating our simulation model for this type of experiment, the comparison serves to illustrate the importance of effects often neglected in the interpretation of Langmuir probe measurements, such as plasma with gradients and Earth magnetic field.

Dust environment of an airless object: A phase space study with kinetic models

The study of dust above the lunar surface is important for both science and technology. Dust particles are electrically charged due to impact of the solar radiation and the solar wind plasma and, therefore, they affect the plasma above the lunar surface. Dust is also a health hazard for crewed missions because micron and sub-micron sized dust particles can be toxic and harmful to the human body. Dust also causes malfunctions in mechanical devices and is therefore a risk for spacecraft and instruments on the lunar surface. Properties of dust particles above the lunar surface are not fully known. However, it can be stated that their large surface area to volume ratio due to their irregular shape, broken chemical bonds on the surface of each dust particle, together with the reduced lunar environment cause the dust particles to be chemically very reactive. One critical unknown factor is the electric field and the electric potential near the lunar surface. We have developed a modelling suite, Dusty Plasma Environments: near-surface characterisation and Modelling (DPEM), to study globally and locally dust environments of the Moon and other airless bodies. The DPEM model combines three independent kinetic models: (1) a 3D hybrid model, where ions are modelled as particles and electrons are modelled as a charged neutralising fluid, (2) a 2D electrostatic Particle-in-Cell (PIC) model where both ions and electrons are treated as particles, and (3) a 3D Monte Carlo (MC) model where dust particles are modelled as test particles. The three models are linked to each other unidirectionally; the hybrid model provides upstream plasma parameters to be used as boundary conditions for the PIC model which generates the surface potential for the MC model. We have used the DPEM model to study properties of dust particles injected from the surface of airless objects such as the Moon, the Martian moon Phobos and the asteroid RQ36. We have performed a (v0, m/q)-phase space study where the property of dust particles at different initial velocity (v0) and initial mass per charge (m/q) ratio were analysed. The study especially identifies regions in the phase space where the electric field within a non-quasineutral plasma region above the surface of the object, the Debye layer, becomes important compared with the gravitational force. Properties of the dust particles in the phase space region where the electric field plays an important role are studied by a 3D Monte Carlo model. The current DPEM modelling suite does not include models of how dust particles are initially injected from the surface. Therefore, the presented phase space study cannot give absolute 3D dust density distributions around the analysed airless objects. For that, an additional emission model is necessary, which determines how many dust particles are emitted at various places on the analysed (v0, m/q)-phase space. However, this study identifies phase space regions where the electric field within the Debye layer plays an important role for dust particles. Overall, the initial results indicate that when a realistic dust emission model is available, the unified lunar based DPEM modelling suite is a powerful tool to study globally and locally the dust environments of airless bodies such as planetary moons, Mercury, asteroids and non-active comets far from the Sun.

Callisto plasma interactions: Hybrid modeling including induction by a subsurface ocean

AbstractBy using a hybrid plasma solver (ions as particles and electrons as a fluid), we have modeled the interaction between Callisto and Jupiter's magnetosphere for variable ambient plasma parameters. We compared the results with the magnetometer data from flybys (C3, C9, and C10) by the Galileo spacecraft. Modeling the interaction between Callisto and Jupiter's magnetosphere is important to establish the origin of the magnetic field perturbations observed by Galileo and thought to be related to a subsurface ocean. Using typical upstream magnetospheric plasma parameters and a magnetic dipole corresponding to the inductive response inside the moon, we show that the model results agree well with observations for the C3 and C9 flybys, but agrees poorly with the C10 flyby close to Callisto. The study does support the existence of a subsurface ocean at Callisto.

Thomson scattering measurement of a shock in laser-produced counter-streaming plasmas

We report the first direct measurement of temporally and spatially resolved plasma temperatures at a shock as well as its spatial structure and propagation in laser-produced counter-streaming plasmas. Two shocks are formed in counter-streaming collisionless plasmas early in time, and they propagate opposite directions. This indicates the existence of counter-streaming collisionless flows to keep exciting the shocks, even though the collisional effects increase later in time. The shock images are observed with optical diagnostics, and the upstream and downstream plasma parameters of one of the shocks are measured using Thomson scattering technique.

Comparisons between MHD model calculations and observations of Cassini flybys of Titan

The Cassini spacecraft passed by Titan on 26 October 2004 (Ta flyby) and 13 December 2004 (Tb flyby). In both cases the Cassini Orbiter entered Titan's ionosphere and flew through Titan's dynamic wake region. In this paper, we present our simulation results of these two flybys using our three‐dimensional multispecies MHD model. This model is a slightly updated version of the model outlined by Ma et al. (2004a); the main difference is the inclusion of magnetic diffusion into the equations. The calculations used the best available upstream plasma and magnetic field parameters obtained by the Cassini instrument complement. The calculated parameters, corresponding to the Cassini flybys near the closest approach, are compared with the relevant observed values. There is a reasonably good but clearly not perfect agreement between the measured and calculated values. Some of these differences are believed to be due to the uncertainties and time variability associated with the upstream parameters and some differences must definitely be the result of the uncertainties in the parameters selected for the model, as well as the limitations associated with the MHD approximations.

Walén and slow‐mode shock analyses in the near‐Earth magnetotail in connection with a substorm onset on 27 August 2001

We have examined Cluster spacecraft 1 (C1) magnetic field and particle observations from 27 August 2001 when the Cluster spacecraft separation was less than 0.27 RE and the spacecraft were located near the equatorial plane of the postmidnight magnetotail at xGSM ∼ −19 RE and yGSM ∼ −2.1 RE. A detailed examination of observations recorded between 0350 UT and 0435 UT is performed with special emphasis on the plasma flow reversal from strong tailward flows to earthward flows in the approximate time interval 0400 UT to 0410 UT. Evidence for the expansion phase onset of an isolated substorm between ∼0406 UT and ∼0408 UT was reported by Baker et al. [2002]. During the 0400–0410 UT interval, C1 was located in the plasma sheet where the observed plasma density N ∼ 0.3 cm−3 and the plasma ion β ∼ 1.5. The Walén analysis applied to the tailward flow interval from 0400:36 UT to 0403:14 UT is consistent with the ions being accelerated to ∼73% of the Alfvén speed across a slow‐mode shock connected to a near‐Earth neutral line located on the earthward side of the spacecraft. The occurrence of a small plasmoid‐type magnetic flux rope during the leading edge of the tailward flows provides further support in favor of an active region of magnetic reconnection earthward of Cluster. The more field‐aligned earthward flows between 0406 UT and 0408 UT, however, failed to satisfy the Walén test. Rankine‐Hugoniot analyses of upstream and downstream plasma and magnetic field parameters confirm the presence of a slow‐mode shock in connection with the passage of the tailward flow region but not with the 0406 UT to 0408 UT earthward flow interval. The confirmed shock satisfies the critical slow‐mode requirements: MI* ≤ 1.0 and MSM* &gt; 1.0 on the upstream side and MSM* &lt; 1.0 on the downstream side. Here, MI* and MSM* are the intermediate Alfvén speed and slow‐mode phase speed Mach numbers in the frame of the shock. The failure of both the Walén test and the Rankine‐Hugoniot analysis on the earthward flow portion of the plasma reversal event may be associated with the strong earthward ∇B of the inner magnetosphere. The successful joint Walén and slow shock analyses on the tailward flows within the plasma sheet present further evidence in favor of Petschek‐type reconnection at distances xGSM &gt; −19 RE of the near‐Earth magnetotail.

Studies of detached plasmas on the ULS divertor simulator

We report on studies of detached plasma operation in the UMIST linear system (ULS). The ULS, designed to study a range of edge physics issues relevant to tokamak divertors, is capable of producing plasmas with electron densities and temperatures in the range 10 17–10 19 m −3 and 2–15 eV, respectively. Previous studies of the interaction between the hydrogen plasma and low-pressure hydrogen gas have identified a regime where molecular activated recombination (MAR) processes dominate plasma losses. Here we report on studies in which the upstream plasma parameters are varied such that three-body and radiative electron–ion recombination (EIR) dominates. Initial modelling of the recombination region is undertaken using a simplified version of the one-dimensional electron energy and continuity equations. We determine the factors that govern the threshold between MAR and EIR dominated detached regimes in terms of upstream plasma parameters and compare this with predictions based on the competing effects of electron cooling and recombination.

Planetary bow shocks: Asymptotic MHD Mach cones

A direct approach for determining the asymptotic MHD Mach cone is formulated and solved. An implicit analytical solution enables the calculation of the asymptotic downstream slope of MHD Mach cones at any clock angle for arbitrary Ms, Ma, and ϑbv. The solution obtained includes all previously known symmetric cases. The elongation and shift of the asymptotic fast mode shock cross-section are studied for a wide range of upstream plasma parameters as well as its unusual ‘chopped’ shape under certain conditions. Our results may be useful for planetary shock modeling and MHD numerical codes verification.

Injection and acceleration of thermal protons at quasi‐parallel shocks: A hybrid simulation parameter survey

Collisionless shocks that propagate along the mean magnetic field are known to accelerate some fraction of the incident charged particles directly from the thermal pool to energies that are considerably higher than the energy at which the plasma rams into the shock. Using hybrid simulations, we address two issues: (1) the dependence of the injection/acceleration of thermal protons to energies much higher than the plasma ram energy on various shock parameters such as Mach number, plasma beta, etc., and (2) the effect of the high‐energy particles, accelerated directly from the thermal population by the shock, on the macroscopic properties of the shock, most notably, on the density compression. We find that for supercritical Mach numbers the acceleration of the thermal plasma is efficient enough that the back pressure due to the energetic particles can significantly increase the density compression across the shock, above the value expected from the simple Rankine‐Hugoniot prediction. Additionally, at low Alfvén Mach number, where the acceleration of the thermal plasma is inefficient, the density compression is smaller than the simple Rankine‐Hugoniot prediction owing to the nonresonant fire hose instability. The acceleration efficiency increases with Mach number except at very high Alfvén Mach numbers, where it begins to decrease for Mach numbers greater than ∼ 10. This is due to the presence of a fixed, free‐escape boundary that limits the size of the foreshock region measured in units of the mean‐free paths of the accelerated particles. Additionally, we find that regardless of the upstream plasma parameters, the acceleration efficiency increases with both the density compression ratio across the shock and the distance to the free‐escape boundary measured in units of the mean‐free path of the energetic particles. Both of these are consistent with analytic theory and numerical models that use a phenomenological scattering law.

Assessment of the DIVIMP ‘onion-skin’ model in the JET Mark I divertor

The DIVIMP-NIMBUS [1] ‘Onion-Skin’ model has been used to predict upstream plasma parameters using as input measurements by Langmuir probes in the divertor target tiles. Measurements of density and temperature (both T i and T e ) across the SOL close to the stagnation point at the top of the torus have been made using a reciprocating probe carrying both Langmuir and retarding field analyzer (RFA) heads. In addition, a lithium beam diagnostic has been used to measure radial distributions of density upstream. Ohmic, L-mode and H-mode discharges have been studied, and good agreement with code predictions in the three cases has been found. The heat transport coefficient, χ ⊥( r), has been evaluated across the SOL, and is found to increase with increasing distance from the separatix.

Proxy studies of energy transfer to the magnetosphere

The transfer of energy into the magnetosphere is studied using as proxy the Am geomagnetic index and multilinear regressions and correlations with solar wind data. In particular, the response of Am to the reconnection mechanism is examined in relation to the orientation of the interplanetary magnetic field as well as the upstream plasma parameters. A functional dependence of Am on clock angle, the orientation of the IMF in the plane perpendicular to the flow, is derived after first correcting the index for nonreconnection effects due to dynamic pressure and velocity. An examination of the effect of upstream magnetosonic Mach number shows the reconnection mechanism to become less efficient at high Mach numbers. The reconnection mechanism is shown to be slightly enhanced by higher dynamic pressures.

Survey of coherent ion reflection at the quasi‐parallel bow shock

Ions coherently reflected off the Earth's bow shock have previously been observed both when the upstream geometry is quasi‐perpendicular and when it is quasi‐parallel. In the case of quasi‐perpendicular geometry, the ions are reflected in a nearly specular manner and are quickly carried back into the shock by the convecting magnetic fleld. In the quasi‐parallel geometry, however, near‐specularly reflected ions' guiding center velocities would on the average be directed away from the shock, allowing the ions to escape into the upstream region. The conditions under which coherent reflection occurs and the subsequent coupling of the reflected ions to the incoming solar wind plasma are important factors when assessing the contribution of the reflected ions to the downstream temperature increase and the shock structure. The survey presented in this paper, along with previously reported observations, suggests that near‐specularly reflected ions are indeed an important aspect of energy dissipation at the Earth's quasi‐parallel bow shock. We find that (1) cool, coherent, near‐specularly reflected ion beams are detected over nearly the full range of upstream plasma parameters commonly found at the Earth's bow shock; (2) the beams are typically observed only near the shock ramp or some shock‐like feature; and (3) the observed beam velocities are almost always consistent with what one would expect for near‐specularly reflected ions after only a small fraction of a gyroperiod following reflection. The second and third points indicate that the beams spread very quickly in velocity space. This spread in velocities could be due either to interactions between the beam and incoming solar wind ions or to some initially small velocity spread in the beam. In addition we present an observation of two simultaneous beams, one outgoing from the shock and one incoming towards the shock. They were detected when the local geometry was quasi‐perpendicular, even though the averaged field geometry was quasi‐parallel. This observation indicates that local variations in the magnetic field are in some cases responsible for returning the specularly reflected ions to the downstream region, allowing them to contribute directly to the downstream thermalization.

Plasma Heating by a Magnetosonic Shock Wave through Resonant Ion Acceleration

Resonant wave-particle interactions in magnetosonic shock waves are studied by theory and simulation. The number of ions trapped by a perpendicular laminar shock in a finite beta plasma is evaluated, and the amount of shock heating due to resonant ion acceleration is analytically obtained in terms of the Alfven Mach number and upstream plasma parameters. Some effects of trapped ions on shock waves are also discussed. In addition, it is shown that a laminar oblique shock can reflect some electrons by a magnetic mirror effect, in spite of a large positive potential in the shock region. These theoretical predictions are confirmed by a fully electromagnetic particle simulation with full ion and electron dynamics in one spatial dimension.