Langmuir probe measurements of the electron energy distribution function in magnetized gas discharge plasmas

Popov, Tsv K; Ivanova, P. K.; Dimitrova, M.; Kovačič, J.; Gyergyek, T.; Čerček, M.

doi:10.1088/0963-0252/21/2/025004

Cited by 53 publications

(59 citation statements)

References 13 publications

(32 reference statements)

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“…There are more complicated Langmuir probe theories for magnetized electron collection that may yield a more accurate plasma potential. [28][29][30] However, they are prohibitively difficult to implement in the mapping of a ring-cusp discharge since the magnetic field strength and orientation vary significantly throughout the chamber. In addition, the errors associated with the probe analysis are systematic and the results are sufficient for a qualitative comparison between the various discharge conditions.…”

Section: Discharge Mappingmentioning

confidence: 99%

Miniature ion thruster ring-cusp discharge performance and behavior

Dankongkakul

Wirz

2017

Journal of Applied Physics

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Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1-3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales. Published by AIP Publishing. https://doi

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Section: Discharge Mappingmentioning

confidence: 99%

Miniature ion thruster ring-cusp discharge performance and behavior

Dankongkakul

Wirz

2017

Journal of Applied Physics

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“…Moreover, Eq. (36) (directly linked to the Druyvesteyn formula [36,40,45,46]) is the diffusionless limit of Eq. (35).…”

Section: B Corrections For the Langmuir Probes Interpretationsmentioning

confidence: 99%

“…[17,18,44] where bi-modal distribution functions (distribution function of 2 populations of particles at different temperatures) have been used. Other results are published in the literature describing effects of non-Maxwellians on the Langmuir probe measurements [36][37][38][39][40][41][42][43][44][45][46][47]. The bi-modal approximation is the first efficient way to describe NMDFs, but it assumes the superposition of two populations of particles at the Maxwellian equilibrium and the self-consistency is very restricted since in this bi-modal description the plasma is enough collisional to assure a MDF for each species but do not allow collisions between these two populations.…”

mentioning

confidence: 99%

“…(ii) Another example is the discrepancy of the Langmuir probe interpretation [36][37][38][39][40][41][42][43][44][45][46][47] with respect to the Thomson scattering measurements [48,49] of the electron temperature which has been associated with the presence of super-thermal particles [17,18] (i.e., bulk and super-thermal populations, both at different thermodynamic equilibrium). The Langmuir probe is one of the most used diagnostic for low temperature plasmas.…”

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confidence: 99%

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Kinetic corrections from analytic non-Maxwellian distribution functions in magnetized plasmas

Izacard

2016

Physics of Plasmas

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In magnetized plasma physics, almost all developed analytic theories assume a Maxwellian distribution function (MDF) and in some cases small deviations are described using the perturbation theory. The deviations with respect to the Maxwellian equilibrium, called kinetic effects, are required to be taken into account specially for fusion reactor plasmas. Generally, because the perturbation theory is not consistent with observed steady-state non-Maxwellians, these kinetic effects are numerically evaluated by very CPU-expensive codes, avoiding the analytic complexity of velocity phase space integrals. We develop here a new method based on analytic non-Maxwellian distribution functions constructed from non-orthogonal basis sets in order to (i) use as few parameters as possible, (ii) increase the efficiency to model numerical and experimental non-Maxwellians, (iii) help to understand unsolved problems such as diagnostics discrepancies from the physical interpretation of the parameters, and (iv) obtain analytic corrections due to kinetic effects given by a small number of terms and removing the numerical error of the evaluation of velocity phase space integrals. This work does not attempt to derive new physical effects even if it could be possible to discover one from the better understandings of some unsolved problems, but here we focus on the analytic prediction of kinetic corrections from analytic non-Maxwellians. As applications, examples of analytic kinetic corrections are shown for the secondary electron emission, the Langmuir probe characteristic curve, and the entropy. This is done by using three analytic representations of the distribution function: the Kappa (KDF), the bi-modal or a new interpreted non-Maxwellian (INMDF) distribution function. The existence of INMDFs is proved by new understandings of the experimental discrepancy of the measured electron temperature between two diagnostics in JET. As main results, it is shown that (i) the empirical formula for the secondary electron emission is not consistent with a MDF due to the presence of super-thermal particles, (ii) the super-thermal particles can replace a diffusion parameter in the Langmuir probe current formula and (iii) the entropy can explicitly decrease in presence of sources only for the introduced INMDF without violating the second law of thermodynamics. Moreover, the first order entropy of an infinite number of super-thermal tails stays the same as the entropy of a MDF. The latter demystifies the Maxwell's demon by statistically describing non-isolated systems.The observation of non-equilibrium distribution functions is possible in a large number of areas other than laboratory plasma physics such as in astrophysics plasmas, hydrodynamic fluids and molecular dynamics, atomic physics, condensed matter, chemical reactions and in statistics (see Refs. [1][2][3][4][5]). We focus here on laboratory plasma physics with charged particles in a magnetic field relevant to tokamaks and spherical tokamaks for the purpose of energy production by fusion ener...

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“…The I-V curves of collecting probes provide information about parameters such as the local plasma potential, the electron or ion temperatures and the charged species density [131][132][133]. These probes are widely used for the diagnostics of fully [134][135][136] and weakly ionized plasmas [137][138][139]. Theoretically, the operation of Langmuir probes does not depend on the shape of the signal used to bias the probe.…”

Section: Analysis Of Certain Discrepancies On the Characterisation Ofmentioning

confidence: 99%

Propagator integral method for plasma physics kinetic equations with practical applications

Muñoz¹

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Langmuir probe measurements of the electron energy distribution function in magnetized gas discharge plasmas

Cited by 53 publications

References 13 publications

Miniature ion thruster ring-cusp discharge performance and behavior

Miniature ion thruster ring-cusp discharge performance and behavior

Kinetic corrections from analytic non-Maxwellian distribution functions in magnetized plasmas

Propagator integral method for plasma physics kinetic equations with practical applications

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