Self-shielding of a plasma-exposed surface during extreme transient heat loads

Zielinski, JJ Jakub; Meiden, H.J. van der; Morgan, T.W.; Hoen, M. H. J. 't; Schram, D.C.; Temmerman, G.C. De

doi:10.1063/1.4869486

Cited by 11 publications

(8 citation statements)

References 18 publications

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“…ELM sputtered impurities were shown to cool the plasma in the JET tokamak, leading to temporary plasma detachment [13]. In the linear plasma generator Pilot-PSI, which produces ELM-like hydrogen plasmas, target outgassing was suggested to shield the tungsten surface from the intense plasma heat load [14]. These works underline the importance of near-target plasma-neutral interactions in understanding the ELM induced heat load on PFCs, motivating the current study.…”

Section: Introductionmentioning

confidence: 74%

Power deposition behavior of high-density transient hydrogen plasma on tungsten in Magnum-PSI

Li¹,

Morgan²,

Berg-Stolp³

et al. 2021

Plasma Phys. Control. Fusion

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The lifetime of plasma-facing components (PFCs) will have a strong influence on the efficiency and viability of future fusion power plants. However, the PFCs suffer from thermal stresses and physical sputtering induced by edge-localized modes (ELMs). ELMs in future fusion devices are expected to occur with a high plasma density compared to current day devices such that coupling of recycling neutrals and plasma ions will be strong. Because of the scale hierarchy of future fusion devices compared to the present ones, the influence of this coupling is difficult to predict. Here, we investigate the ELM-like hydrogen plasma induced heat loads on tungsten in the linear device Magnum-PSI, producing ∼1 ms plasma pulses with electron densities up to 3.5 × 10 21 m −3 . A combination of time-resolved Thomson scattering and coherent Thomson scattering was used to acquire plasma parameters in front of the target. Moreover, a fast infrared camera coupled to finite element thermal analyses allowed to determine the deposited heat loads on the target. We found a significant inconsistency between the plasma power calculated with a conventional collisionless sheath model and the absorbed power by the target. Moreover, plasma stagnation upstream and plasma cooling downstream were observed during the pulses. The observations are explained based on ionization and elastic collisions between the recycling neutrals and plasma ions. The results highlight the impact of plasma-neutral interaction on the power deposition behavior of ELM-like hydrogen plasma on tungsten.

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Section: Introductionmentioning

confidence: 74%

Power deposition behavior of high-density transient hydrogen plasma on tungsten in Magnum-PSI

Li¹,

Morgan²,

Berg-Stolp³

et al. 2021

Plasma Phys. Control. Fusion

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“…On the other hand in the case of JET (T i ∼ 800 eV, B φ ∼ 3 T, ρ L ∼ 1.4 mm) and the ITER outer vertical target (T i ∼ 5 keV, B φ ∼ 6 T, ρ L ∼ 1.7 mm) the Larmor radius is similar to or greater than the misalignment, and therefore Larmor smoothing may be important. While Larmor smoothing is therefore not expected to play a large role in the Pilot-PSI results, the pulsed load may provide additional local recycling effects [19], as well as possibly complex effects on the local electric fields [12,20]. Therefore it was important to determine whether the power loading onto a misaligned edge was well described by the OA under the well diagnosed and controlled conditions which are available in Pilot-PSI.…”

Section: Pilot-psi Experimentsmentioning

confidence: 99%

Power deposition on misaligned castellated tungsten blocks in the Magnum-PSI and Pilot-PSI linear devices

Morgan¹,

Berg²,

Temmerman³

et al. 2017

Nucl. Fusion

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For the final design of the ITER divertor it is important to determine whether shaping of each tungsten monoblock to eliminate leading edges is required or not. In order to aid this decision, two experiments were performed in DIFFER's linear plasma devices to study heat loads on misaligned water cooled blocks at glancing incidence. First, a series of tungsten blocks were exposed to a high parallel heat flux (26 MW m −2 ) hydrogen plasma beam in the Magnum-PSI linear device. The blocks were exposed at an oblique angle between 4 • and 7 • with respect to the plasma beam, approaching the low angle of incidence expected in the ITER divertor strike-point regions. One block was vertically misaligned with respect to the others by a value between 0 and 1.2 mm, and the surface temperature evolution monitored. This was compared to a finite element thermo-mechanical model of the system, to investigate any discrepancies between the two. The importance of Larmor orbit smoothing effects could also be considered, as the Larmor radius was larger than half the misalignment height. Next, a row of identical tungsten blocks were exposed at an angle of 3 • to ∼500-700 μs pulsed hydrogen plasmas of peak power between 275-625 MW m −2 in the Pilot-PSI linear device, to simulate the effect of edge localized mode (ELM) transients. One block was misaligned with respect to the others by 1 mm. In all cases, the model and experiment were found to be in excellent agreement (to ∼1%), demonstrating clearly that an optical approximation for plasma power loading on misaligned edges is appropriate. A simple model including Larmor smoothing was applied, the results of which give a similar prediction to the optical approximation for the temperature of the misaligned edges, within the error of the temperature measurements, though a small decrease in edge temperature is calculated. Therefore it can be concluded that the application of the optical approximation is generally appropriate as an input physics assumption for the study of shaping alternatives.

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“…Entrained neutrals will be expected to mostly return to the target. As ionized particles are trapped in the magnetic field most ionized tin species should also be drawn-back to the negatively biased target by the sheath field in the case of prompt ionization or entrained in the plasma flow due to the large flow velocity towards the target [54,55] and re-deposited. It should be noted that this depends mostly on electron temperature and density rather than ion species, and thus would also be applicable to dominantly light-ion plasmas, such as in fusion reactors, rather than pure argon plasmas as here.…”

Section: Discussionmentioning

confidence: 99%

Tin re-deposition and erosion measured by cavity-ring-down-spectroscopy under a high flux plasma beam

Kvon¹,

Al²,

Bystrov³

et al. 2017

Nucl. Fusion

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Cavity-Ring-Down Spectroscopy (CRDS) was implemented to measure the re-deposition of liquid tin under a high flux plasma beam in the linear plasma device Pilot-PSI. A capillary porous system (CPS) consisting of a molybdenum cup and tungsten meshes (pores diameters of 0.2 mm and 0.44 mm) was filled with tin and exposed to argon plasma. The absorption of a UV laser-beam at 286.331 nm was used to determine a number of sputtered neutral tin atoms. The incoming flux of argon ions of ~50 eV was 1.6−2.7x10 23 m-2 s-1 , and the sample temperature measured by pyrometry varied from 850 °C to 1200 °C during exposures. The use of CRDS for measuring absolute number of particles under such plasma exposure was demonstrated for the first time. The number of sputtered tin particles in the cavity region assuming no losses would be expected to be 5.5x10 11 −1.2x10 12 while CRDS measurements showed only 5.7−9.9x10 8. About 98−99.8% of sputtered particles were therefore found to not reach the CRDS observation volume. Spectroscopic ratios of Sn I to Sn II ions, as well as equilibrium considerations, indicate that fast ionization as well as plasma entrainment of neutrals is responsible for the discrepancy. This would lead to high re-deposition rates, implying a lowered contamination rate of core plasma and lower required replenishment rates at high-flux conditions than would otherwise be expected.

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Self-shielding of a plasma-exposed surface during extreme transient heat loads

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Cited by 11 publications

References 18 publications

Power deposition behavior of high-density transient hydrogen plasma on tungsten in Magnum-PSI

Power deposition behavior of high-density transient hydrogen plasma on tungsten in Magnum-PSI

Power deposition on misaligned castellated tungsten blocks in the Magnum-PSI and Pilot-PSI linear devices

Tin re-deposition and erosion measured by cavity-ring-down-spectroscopy under a high flux plasma beam

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