A Polytropic Model for Space and Laboratory Plasmas Described by Bi-Maxwellian Electron Distributions

Zhang, Yunchao; Charles, Christine; Boswell, Roderick

doi:10.3847/0004-637x/829/1/10

Cited by 12 publications

(14 citation statements)

References 26 publications

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“…Little and Choueiri [33] suggested the possibility of performance improvements, arguing that the particle motion does not correspond to adiabatic cooling, but rather isothermal behavior reflected in the excessively small Nusselt number in which electron heat conduction along the magnetic field overwhelms convection, which was originally derived by Litvinov [35]. On the contrary, Zhang et al [34,36] concluded that the nozzle device with electric double layer is already in adiabatic expansion, which ensures no heat transfer into the system, and non-local electron kinetics very far from a local thermodynamic equilibrium for a nearly collisionless plasma is responsible for the low γ e value. Recently, there has been an attempt to investigate the relationship between the thermodynamic change of the electrons, i.e.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons

Kim

Ryu

et al. 2018

New J. Phys.

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Thermodynamics of a magnetically expanding plasma (magnetic nozzle (MN)) has been investigated considering the existence of confined electrons bouncing back and forth inside a potential well formed by a combination of external magnetic field and self-generating ambipolar electrostatic potential. The properties of confined electrons are distinguished from that of the adiabatically expanding electrons with γ e ≈5/3 by the separate measurement of each species using a double-sided planar Langmuir probe. Relationship between the electron pressure versus electron density averaged over electron energy probability functions (eepfs) clearly reveals that the confined electrons in MN have a nearly isothermal characteristic. Existence of isothermally behaving confined electrons together with adiabatically expanding electrons separates the MN system into two regions with different thermodynamic properties; one is a nearly adiabatic region located near the nozzle throat and the other is nearly isothermal region located far from the nozzle. A transition of electron thermodynamic property along a distance from the nozzle throat can be explained with conservation of magnetic moment of electrons bounced back by ambipolar electrostatic potential. Coexistence of the nearly adiabatic electrons with Maxwellian eepf and the nearly isothermal electrons with high energydepleted eepf makes the overall eepf shape low energy-populated eepf, indicating a need for careful analysis on the measured eepfs near the nozzle throat. In spite of significant contribution of confined electrons to eepf and overall electron thermodynamics, it is found that the confined electrons behaving isothermally do not contribute to the generation of ambipolar electrostatic potential which is important for ion acceleration in MN. The present study suggests that ion acceleration should not be directly inferred from the value of polytropic exponent γ e because thermodynamic property of a MN is influenced by isothermally behaving confined electrons as well as adiabatically expanding electrons.

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Section: Introductionmentioning

confidence: 99%

Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons

Kim

Ryu

et al. 2018

New J. Phys.

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“…Rather, it expands isothermally, implying that heating of the plasma occurs as it propagates through interplanetary space [4][5][6]. Many laboratory experiments under adiabatic conditions [7][8][9] have also shown a nearly isothermal expansion with γ ∼ 1.0 − 1.2 in magnetic nozzles, and the relation with astrophysical plasmas has been discussed [10]. However, in these expanding adiabatic systems, it appears that electric fields may have a significant nonlocal effect on the dynamics of the electrons; many of them are trapped in the system by the ambipolar and wall sheath electric fields, allowing an isothermal equilibrium to be established after sufficient time has elapsed, while some of the electrons having the energy overcoming the potential drop can escape from the system.…”

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confidence: 99%

Adiabatic Expansion of Electron Gas in a Magnetic Nozzle

et al. 2018

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A specially constructed experiment shows the near perfect adiabatic expansion of an ideal electron gas resulting in a polytropic index greater than 1.4, approaching the adiabatic value of 5/3, when removing electric fields from the system, while the polytropic index close to unity is observed when the electrons are trapped by the electric fields. The measurements were made on collisionless electrons in an argon plasma expanding in a magnetic nozzle. The collision lengths of all electron collision processes are greater than the scale length of the expansion, meaning the system cannot be in thermodynamic equilibrium, yet thermodynamic concepts can be used, with caution, in explaining the results. In particular, a Lorentz force, created by inhomogeneities in the radial plasma density, does work on the expanding magnetic field, reducing the internal energy of the electron gas that behaves as an adiabatically expanding ideal gas.

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“…Finally, the thermal speed was set to increase with the plasma density according to the adiabatic relationship (i.e., v th ∝ n 1/3 ), which is widely used to describe space plasmas within the heliosphere (e.g., Parker 1961;Zhang et al 2016). We then constructed the VDFs in the Cartesian instrument frame with resolution ∆u x ×∆u y ×∆u z = 500 × 500 × 500 km 3 s −3 and analyzed them as explained in Sections 3.4 and 3.5 to derive the electron bulk parameters.…”

Section: Model Resultsmentioning

confidence: 99%