Plasma detachment study of high density helium plasmas in the Pilot-PSI device

In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3 mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50 % and 20 %, respectively, at ne = 8•10 20 m −3 . These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In lowdensity conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ionneutral friction and electron-ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p-n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1 mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.

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“…Langmuir probes are widely used to measure various plasma properties [25], and have also previously been employed in Magnum-PSI [26,27].…”

Section: Flush-mounted Langmuir Probementioning

confidence: 99%

Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

et al. 2021

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“…Current experimental campaigns in Magnum-PSI are aimed at understanding the physics occurring during the change from sheath-limited to detached plasma regimes, similar to previous studies in Pilot-PSI [14]. Also, extensive campaigns investigating the power handling capabilities of liquid Sn and Li plasma facing components (PFCs) and the physics of liquid metal vapour shielding have been performed in both machines [8,9].…”

Section: Introductionmentioning

confidence: 99%

Plasma radiation studies in Magnum-PSI using resistive bolometry

et al. 2018

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Both the physics of divertor detachment and vapour shielding are characterized by a relatively large amount of radiation produced in the divertor. The linear plasma generator Magnum-PSI is well-suited to study such processes due to its ITER-divertor relevant plasma conditions, simplified geometry and diagnostic accessibility. The need the quantify the plasma radiated power close to the target surface motivated the development of a 4-channel resistive bolometer for Magnum-PSI, and marks the first deployment of such a diagnostic on a linear device. An axially resolved measurement of plasma emission at arbitrary distances from the target surface is now possible. The radial position of the detector can be varied, hereby viewing the full diameter of the plasma column or down to a central region. The overall system design is discussed alongside a comparison of the spectral absorbance of carbon-coated versus non-coated Au/Al bolometer sensors. Despite low electron temperatures of the plasma (1-5 eV), the observed power densities were found to be 10-37 times the sensor noise floor of ∼0.1 W m −2. A synthetic diagnostic based on collisional radiative model calculations from ADAS could well match observed values from H and Ne plasmas while the measured values for Ar and He were more difficult to reproduce. The obtained findings allow for approximate power balance calculations in Magnum-PSI indicating that maximally ∼47 % and ∼14 % of the total power is lost by radiation in the cases of Ar and Ne/He respectively. The results demonstrate the feasibility of resistive bolometry in low temperature high density plasma regions and on long timescales (>450 s) which is of relevance to ITER. Due to long-term temperature drifts which were observed, a planned upgrade involves the installation of a shutter and FPGAbased electronics for increased accuracy.

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“…The anomaly is considered due to fluctuation of space potential and/or plasma resistivity between a probe tip and a reference electrode . On the other hand, when the potential fluctuation was lower than T e , the single‐probe measurement could be applicable even in the recombining plasmas without any anomaly of the probe in the I ‐ V characteristics . Single‐probe measurements are sometimes inapplicable, and sometimes applicable, and the possibility could be determined by the amplitude of potential fluctuation, which depends on the discharge system and condition in each device.…”

Section: Introductionmentioning

confidence: 99%

Double‐probe measurement in recombining plasma using NAGDIS‐II

Hayashi

Nishikata

Ohno

et al. 2019

Contributions to Plasma Physics

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We have studied the validity of the double-probe method in recombining plasmas. Electron temperature (T e ) measured with a double probe was quantitatively evaluated by taking into account the influences of plasma potential fluctuation, plasma resistivity, and electron density fluctuation on the current-voltage characteristics. Differential potential fluctuation and plasma resistivity between two electrodes have a minor effect on T e especially when the inter-distance is small (typically 1 mm). Scattering of measured T e due to the density fluctuation was sufficiently suppressed by making the data acquisition time long (typically 4 s) and taking the average. There is a good agreement between T e measured with the optimized double-probe method and that with laser Thomson scattering diagnostics. KEYWORDSdouble probe, electron temperature, recombining plasma 1

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Plasma detachment study of high density helium plasmas in the Pilot-PSI device

Cited by 28 publications

References 59 publications

Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

Plasma radiation studies in Magnum-PSI using resistive bolometry

Double‐probe measurement in recombining plasma using NAGDIS‐II

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