Divertor Langmuir Probe Measurements on COMPASS-D

Silva, C. G.; Fielding, S.J.; Axon, K.B.; Booth, M.

doi:10.1002/ctpp.19980380112

Cited by 6 publications

(8 citation statements)

References 8 publications

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“…The agreement between the two methods is satisfactory in most cases. Peak temperatures at the outer target in the range of 50 eV are consistently observed by both methods (see an example of such measurements in figure 19(c)), which also agrees with older measurements performed at COMPASS-D [20]. This indicates that the LFS SOL is in the sheath-limited regime with almost constant T e along the magnetic field lines.…”

Section: Comparison With Ball-pen Probe Measurementssupporting

confidence: 87%

“…So far all the simulations targeted the geometry of the LPs in the combined probe array. For completeness, we have extended the study to include geometry of the swept probe array, which was installed in COMPASS already during its operation in Culham [20] and was retained after re-installation in Prague. Originally, the probes in the swept array had an approximately circular cross-sections protruding 2.0 mm above the divertor tile with a probe length equal to 7.0 mm and probe width 3.0 mm.…”

Section: Probe Geometrymentioning

confidence: 99%

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On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

et al. 2022

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The problem of power exhaust is one of the grand challenges of nuclear fusion research today. In order to understand the physics phenomena occurring in the scrape-off layer and the divertor regions of tokamaks, it is essential to correctly determine the divertor plasma parameters, which are often measured by swept Langmuir probes. While the construction and operation of this diagnostic can be straightforward, the data analysis using 3- or 4-parameter fits presents a challenge and can potentially lead to erroneous values of electron temperature and ion saturation current. In this work, we present modelling and experiments aimed at determination of conditions for proper analysis of swept Langmuir probes using these two fitting models. Particle-in- cell modelling was employed to evaluate the sheath-expansion effects for particular probe geometry and plasma conditions, yielding a semi-empirical rule capable of predicting its magnitude. Experiments with unusually wide range of swept voltage in the divertor of the COMPASS tokamak explored the magnitude of voltage range required for successful analysis with either three or four-parameter fitting. With the use of our new semi-empirical rule, it is possible to improve the four-parameter fit reliability in situations where the available voltage range is limited. In addition, we introduce the tangent method - an independent and fast method of electron temperature estimation, which allows to reliably determine the available voltage range and as such assist the more complex method of probe analysis.

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Section: Comparison With Ball-pen Probe Measurementssupporting

confidence: 87%

Section: Probe Geometrymentioning

confidence: 99%

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

et al. 2022

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“…Compared to these larger tokamaks, COMPASS has a relatively short connection length from the OMP to the outer target (typically < 5 m), which may facilitate transport in the sheath-limited regime. Indeed, a previous analysis performed during its operation in Culham, UK [29] has concluded that its SOL can operate in either the sheath-or the conduction-limited regime. To distinguish between the two regimes, we use the SOL collisionality parameter ν * = 10 −16 n u L/T 2 u defined by equation 4.105 in [30], where n u is the upstream electron density, L is the connection length and T u is the upstream electron temperature.…”

Section: Uncertainties In the Databasementioning

confidence: 99%

Scaling of L-mode heat flux for ITER and COMPASS-U divertors, based on five tokamaks

et al. 2020

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This contribution aims to improve existing scalings of the L-mode power decay length λ q o m p , especially for plasma configurations with strike points at the ITER-relevant location—closed vertical divertor targets. We propose 13 new λ q o m p scalings based on data from the tokamaks JET, EAST, MAST, Alcator C-mod and COMPASS, and validate them against the output of the 2D turbulence code HESEL. The analysis covers 500 divertor heat flux profiles (obtained by probes or IR cameras), measured in L-mode discharges with varying 12 global plasma parameters (all well predictable). We find that the two previously published scalings (Eich 2013 J. Nucl. Mat. 438 S72) and (Scarabosio 2013 J. Nucl. Mat. 438 S426), which were based on outer target data from AUG and JET, describe the JET, C-mod and COMPASS profiles well. This holds not only at the outer horizontal and vertical targets, but surprisingly also at the inner vertical targets. In contrast, EAST, HESEL and especially MAST data are poorly described by these two scalings. We therefore derive 13 new scalings, which account for 85–92 % of the measured λ q o m p variability across all five tokamaks. Although each of the scalings is based on a different parameter combination, their predictions for the ITER and COMPASS-Upgrade tokamaks are very similar. Just before the L-H transition in the ITER baseline scenario, the presented scalings predict values λ q o m p = 3.0 ± 0.5 mm. For the COMPASS-Upgrade tokamak, all the scalings predict λ q o m p = 2.1 ± 0.5 mm with a single exception of the scaling based on the stored plasma energy which predicts only 1.2 mm for both tokamaks. We encourage the reader to use as many of these scalings as possible, depending on available data. In attached plasma and using significant assumptions, our results imply steady-state surface-perpendicular heat flux around 10 MW/m2 for ITER, and 20 MW/m2 for COMPASS-Upgrade.

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“…The rooftop-shaped LPs have a 20-degree chamfer with a total exposed area above the divertor target of S LP = 22 mm 2 and projected area (on one side) of S LP = 2.8 mm 2 to reduce the impact of parallel heat flux and avoid the risk of a self-emitting regime. The design of LPs partially follows the previous design of standard divertor Langmuir probes operating on COMPASS [11]. Similar rooftop-shaped LPs are also used in the divertor of the DIII-D tokamak [8] and on the ASDEX Upgrade high heat flux probe head [6].…”

Section: Design Of New Divertor Ball-pen and Langmuir Probesmentioning

confidence: 99%

“…The Langmuir probe (LP) has been implemented in most fusion experiments, in particular at the boundary [5,6,7] and in the divertor region [8,9,10] of tokamaks, as it can provide high-resolution direct measurements of several plasma parameters. In the COMPASS tokamak, 39 standard LPs are routinely operated [11,12] to measure either the floating potential or the ion saturation current with high temporal resolution. Each probe can also provide the electron temperature by the swept probe technique, but at low temporal resolution ~1 kHz [13].…”

Section: Introductionmentioning

confidence: 99%

Electron temperature and heat load measurements in the COMPASS divertor using the new system of probes

et al. 2017

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A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy Hmode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ = 7. The ELM energy density  || was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of  || were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

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Divertor Langmuir Probe Measurements on COMPASS-D

Cited by 6 publications

References 8 publications

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

Scaling of L-mode heat flux for ITER and COMPASS-U divertors, based on five tokamaks

Electron temperature and heat load measurements in the COMPASS divertor using the new system of probes

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