Theory of a cylindrical Langmuir probe parallel to the magnetic field and its calibration with interferometry

Usoltceva, M.; Faudot, E.; Ledig, J; Devaux, S.; Heuraux, S.; Zadvitskiy, G.; Ochoukov, R.; Moritz, J.; Crombé, Kristel; Noterdaeme, Jean-Marie

doi:10.1063/1.5038666

Cited by 6 publications

(8 citation statements)

References 5 publications

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“…Microwave interferometry is a very common tool for measuring plasma density due to its robustness, wide coverage of the density range, non-intrusive measurement approach, and low susceptibility to external impacts. In fusion-related plasma physics, many small-and medium-size tokamaks (JET, 1 DIII-D 2 COMPASS, 3 COMPASS-U, 4 KSTAR, 5 and SUNIST 6 ), stellarators (TJ-II, 7 Uragan-3M, 8 and Uragan-2M 9 ), and other devices (Aline, 10 LDX, 11 and a capacitive RF plasma device 12 ) use (or used) interferometric diagnostics in the microwave range for measuring density in the central region of the plasma.…”

Section: Introductionmentioning

confidence: 99%

A new technique for tokamak edge density measurement based on microwave interferometer

Usoltceva

Heuraux

Khabibullin

et al. 2022

Review of Scientific Instruments

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A novel approach for density measurements at the edge of a hot plasma device is presented-Microwave Interferometer in the Limiter Shadow (MILS). The diagnostic technique is based on measuring the change in phase and power of a microwave beam passing tangentially through the edge plasma, perpendicular to the background magnetic field. The wave propagation involves varying combinations of refraction, phase change, and further interference of the beam fractions. A 3D model is constructed as a synthetic diagnostic for MILS and allows exploring this broad range of wave propagation regimes. The diagnostic parameters, such as its dimensions, frequency, and configuration of the emitter and receiver antennas, should be balanced to meet the target range and location of measurements. It can be therefore adjusted for various conditions, and here, the diagnostic concept is evaluated on a chosen example, which was taken as suitable to cover densities of ∼10 15 to 10 19 m −3 on the edge of the ASDEX Upgrade tokamak. Based on a density profile with a fixed ra dial sh ape, ap propriate fo r experimental density approximation, a database of synthetic diagnostic measurements is built. The developed genetic algorithm genMILS of density profile reconstruction using the constructed database has quite low errors. It is estimated as ∼5% to 15% for density ≥10 17 m −3 . Therefore, the new diagnostic technique (with a dedicated data processing algorithm) has a large potential in practical applications in a wide range of densities, with low errors in the numerical model and in the method of density reconstruction, so the total error and the density estimation accuracy are expected to be defined mostly by experimental uncertainties. FIG. 15. Dependence of the refractive index on density. Gray dashed line shows the cut-off density, nc = 2.74 × 10 19 m −3 .

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Section: Introductionmentioning

confidence: 99%

A new technique for tokamak edge density measurement based on microwave interferometer

Usoltceva

Heuraux

Khabibullin

et al. 2022

Review of Scientific Instruments

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“…This sensitivity to the magnetic field also changes the collection surface for each species. [ 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 .…”

Section: Experimental Designmentioning

confidence: 99%

“…This subsection refers to the plots Figure 10a,b. Unfortunately, the floating potential is the consequence of current balance on the probe ( I e = ∣ I i ∣) and depends implicitly on the collection surfaces, but as electrons and ions do not reach the probe in the same way (effective collection surfaces are different [ 22,23,32 ] ), it is more complicated to evaluate on which part of the probe the collection is carried out. Moreover, the floating potential depends more on transverse electron fluxes than others: Unmagnetized ion flux collected across the probe area is low due to low ion velocity, and the longitudinal electron flux magnetically connected to the probe is only collected on the front area of the probe depending on the probe radius and electron Larmor radius, which are very small as well (

π r_{p}^{2} ≪ 2 π L_{p} r_{p}

).…”

Section: Two‐dimensional Contour Plots Of Plasma Parametersmentioning

confidence: 99%

“…Finally, in order to compute the probe floating potential using only measured quantities, we assume unmagnetized cold ions, which are collected by the whole probe and magnetized, and Maxwellian electrons, which are collected by only a small fraction of the probe, which we assume to represent approximately 10% of the probe [ 22,23 ] but is actually the effective probe collecting area for the parallel flux at such magnetic field magnitudes (50–100 mT). In addition to those magnetized electrons, we must include the transverse electron current density, which we assume to be collected by only a portion κ of the probe.…”

Section: Two‐dimensional Contour Plots Of Plasma Parametersmentioning

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%

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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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Comparison of Langmuir probe and laser Thomson scattering for electron property measurements in magnetron discharges

2019

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Electron property measurements made by Langmuir probes and laser Thomson scattering have been compared in weakly magnetized plasma conditions using a planar unbalanced magnetron with the aim of assessing the accuracy of the probe diagnostic. The measurements were performed at several locations within the magnetic field configuration, the magnetic null region (≲1 mT) on the discharge axis and inside the last closed flux surface boundary with fields up to 33 mT. There was good diagnostic agreement during High Power Impulse Magnetron Sputtering, but significant discrepancies were observed for DC magnetron operation, even at the magnetic null. For some discharge conditions, the electron density determined by Thomson scattering was over an order of magnitude greater than the plasma density obtained from the Langmuir probe, using both ion and electron collection theories. In addition, the low energy part of the electron energy distribution function determined by the probe was depleted. The possible reasons for the discrepancies are discussed, with the conclusion being that the plasma was significantly perturbed by the probe stem. The range of plasma densities and electron temperatures measured in this study were 0.4–54 × 1017 m−3 and 0.2–5.9 eV, respectively.

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Theory of a cylindrical Langmuir probe parallel to the magnetic field and its calibration with interferometry

Cited by 6 publications

References 5 publications

A new technique for tokamak edge density measurement based on microwave interferometer

A new technique for tokamak edge density measurement based on microwave interferometer

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

Comparison of Langmuir probe and laser Thomson scattering for electron property measurements in magnetron discharges

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