On Electrical Probes Used in Magnetized Plasma Diagnostics

Mihăilă, Ilarion; Solomon, M. L.; Costin, C.; Popa, G.

doi:10.1002/ctpp.201310017

Cited by 6 publications

(8 citation statements)

References 19 publications

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“…[ 22,23 ] In some cases, the electron and ion current growth can even be similar in amplitude. [ 39 ] As was shown in a previous paper, using a thin cylindrical probe in a magnetized plasma can lead to wrong estimation of the plasma parameters due to electron density depletion [ 18–21,39,40 ] once the applied voltage overcomes the plasma potential, V > ϕ p . Therefore, it is more convenient and reliable to work with the ion part of the I ( V ) than with the electron part in magnetized conditions.…”

Section: Experimental Designmentioning

confidence: 99%

“…However, the problem is more challenging when performing these measurements in a magnetized plasma [ 17–23 ] and even more when connected to a RF antenna/electrode. [ 24–30 ] Indeed, turning on the magnetic field breaks down isotropy, and the probe measurement is then only defined locally (measured parameters can drastically change when moving across magnetic field lines).…”

Section: Introductionmentioning

confidence: 99%

“…In some cases, a bump rises on the I ( V ) in this potential range due to density depletion of the biased plasma connected to the probe during a voltage ramp. [ 21,32 ] This knowledge of numerous cylindrical probe theories has been reviewed to interpret probe data and to draw potential, density, and temperature contour plots around an RF electrode.…”

Section: Introductionmentioning

confidence: 99%

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Experimental and theoretical study of density, potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Ledig

Faudot

Moritz

et al. 2020

Contributions to Plasma Physics

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Potential and density structures in the vicinity of a RF electrode/antenna in a magnetized plasma are investigated using a RF compensated cylindrical Langmuir probe. These measurements were performed in the ALINE plasma device in which only electrons can be considered as well magnetized. Very precise 2-D maps of the plasma parameters are drawn thanks to a 3-D automatic manipulator on which the probe is mounted. The effect of the tilted magnetic angle between the RF biased surface and the magnetic lines is also studied thanks to a tilting electrode. Comparison of several simplistic models with the experiments proved the reliability of simple Langmuir probe measurements in such a RF and magnetized environment (space potential v.s. tilting angle of the antenna with respect to magnetic field lines, and recovering of the floating potential structure using measured currents). A fluid model based on total current density, and ion diffusion equations over the biased flux tube, provides the same density structures in front of the electrode than the measurements. Those density structures display a "bunny ears" shape, and can be explained using transverse RF and collisional current behaviour: in front of the antenna the transverse ion currents deplete the magnetized flux tube, while at the edge of the biased flux tube, the same currents rise the density. 36 probes can be used to get the plasma's main pa-37 rameters, and probe measurements are technically 38 speaking relatively easy to perform. There is al-39 ready a large documentation on this subject 10-16 40 especially in a steady and non magnetized plasma 41 where a Langmuir probe measurement can provide 42 a robust estimation of the whole bulk plasma den-43 sity, temperature and potentials (floating V fl and 44 plasma φ p ones). 45 But the problem is more challenging when 46 performing these measurements in a magnetized 47 plasma 17-23 and even more when connected to a 48 radio-frequency (RF) antenna/electrode 24-30. In-49 deed, turning on the magnetic field breaks down 50 isotropy and the probe measurement is then only 51 defined locally (measured parameters can drasti-52 cally change when moving across magnetic field 53 lines). It is then needed to perform measurements 54 into several areas of the plasma in order to access 55 to the full structure of plasma parameters. But the 56 most difficult issue is to understand how the current 57 is collected by a cylindrical probe when it is aligned 58 with magnetic field. This topic has been addressed 59 by several authors in the case of strongly magne-60 tized electrons 22,31. But in the case of weakly mag-61 netized ions, such as in small plasma discharges, 62 the ion part in the I(V) characteristics remains the 63 best way to deduce the plasma density, particularly 64 in RF environment because ions are less sensitive 65 to RF oscillations 24. On the electrons part of the 66 characteristics, cylindrical probes exhibits a strong 67 157 trode inclination, and that the ion larmor radius is of 158 the order of 400 µm at 94 mT). ...

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Section: Experimental Designmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Experimental and theoretical study of density, potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Ledig

Faudot

Moritz

et al. 2020

Contributions to Plasma Physics

View full text Add to dashboard Cite

show abstract

“…When using a cylindrical probe aligned with the magnetic field lines, the knowledge of the probe's area is not enough. The current collected by the probe depends on the probe's length [6,7], but it also depends on the ratio between the Larmor radius of the charged particles (a c ) and the probe's radius (r p ). If a c << r p , probe's collecting area can be identified with probe's top area ( 2 p r π ).…”

Section: Estimation Of the Charged Particles Flux To The Target Surfacementioning

confidence: 99%

Tailoring the charged particle fluxes across the target surface of Magnum-PSI

Costin

Anita

Popa

et al. 2016

Plasma Sources Sci. Technol.

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Linear plasma generators are plasma devices designed to study fusion-relevant plasma-surface interactions. The first requirement for such devices is to operate with adjustable and well characterized plasma parameters. In the linear plasma device Magnum-PSI, the distribution of the charged particle flux across the target surface can be tailored by the target bias. The process is based on the radial inhomogeneity of the plasma column and it is evidenced by electrical measurements via a 2D multi-probe system installed as target. Typical results are reported for a hydrogen discharge operated at 125 A and confined by a magnetic field strength of 0.95 T in the middle of the coils. The probes were biased in the range of -80 to -30 V, while the floating potential of the target was about -35 V. The results were obtained in steady-state regime of Magnum-PSI, being time-averaged over any type of fluctuations. Depending on the relative value of the target bias voltage with respect to the local floating potential in the plasma column, the entire target surface can be exposed to ion or electron dominated flux, respectively, or it can be divided into two adjacent zones: one exposed to electron flux and the other to ion flux. As a consequence of this effect, a floating conductive surface that interacts with an inhomogeneous plasma is exposed to non-zero local currents despite its overall null current and it is subjected to internal current flows.

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“…1. Of those works, very few tried to examine cylindrical probes. [2][3][4][5][6][7][8] Two articles published by Laframboise and Rubinstein 2,3 can be considered as foundational theoretical papers on the subject of a cylindrical Langmuir probe in magnetized plasmas. We will make an attempt to compare our model to theirs.…”

Section: Introductionmentioning

confidence: 99%

Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

et al. 2018

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Langmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 μm and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B→ on a linear plasma device Aline, with magnetic fields of 0.0024–0.1 T and plasma densities of 1015–1017 m−3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the “intersection” method of plasma potential evaluation, and the robustness of the conventional “1st derivative” method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density.

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On Electrical Probes Used in Magnetized Plasma Diagnostics

Cited by 6 publications

References 19 publications

Experimental and theoretical study of density, potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Experimental and theoretical study of density, potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Tailoring the charged particle fluxes across the target surface of Magnum-PSI

Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

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