Effects of the Chodura sheath on tungsten ionization and emission in tokamak divertors

Guterl, J.; Johnson, Curtis A.; Ennis, D.A.; Loche, S.; Snyder, P. B.

doi:10.1002/ctpp.201900140

Cited by 3 publications

(3 citation statements)

References 18 publications

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“…These spectroscopically inferred values of W net erosion can be compared to re-deposition models. Also overlaid in figure 7 are the analytic approximations assuming that W prompt re-deposition is dominated the electrostatic forces of the magnetic pre-sheath [49], described by the scaling law in equation (1), or by gyromotion, as postulated in [8]. While both models tend to approximately capture the measured W erosion behavior, the W + gyro-orbit model under-predicts prompt c at low values of / , W iz l r while as discussed above, the measurements are expected to provide an upper limit.…”

Section: And /mentioning

confidence: 99%

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

Abrams,

Guterl,

Abe

et al. 2023

Mater. Res. Express

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Fundamental mechanisms governing the erosion and prompt re-deposition of tungsten impurities in tokamak divertors are identified and analyzed to inform the lifetime of tungsten plasma-facing components in ITER and other future devices. Various experiments conducted at DIII-D to benchmark predictive models are presented, leveraging the DiMES removable sample exposure probe capability and the Metal Rings Campaign, in which toroidally symmetric rows of tungsten-coated tiles were installed in the DIII-D divertor. In tokamak divertors, the width of the electric sheath is of the order of the main ion Larmor radius, and a vast majority of sputtered tungsten impurities are typically ionized within the sheath. Therefore, W prompt redeposition is mainly governed by the ratio of the characteristic ionization mean-free path of neutral tungsten to the width of the sheath. In-situ monitoring of the prompt redeposition of tungsten impurities in divertors is demonstrated via the use of WII/WI line ratios and the ionizations/photon (S/XB) method in L-mode discharges. Even with this relatively limited set of emission measurements, net erosion measurements were found to be a consistent upper bound to an analytic scaling based on the ratio of the W ionization length, λ iz , and the width of the magnetic sheath rather than the ratio of λ iz and the W+ gyro-radius. In the far-scrape-off layer (SOL) of the ITER divertor, however, it is calculated that the measurement of photon emissions associated with the ionization of tungsten impurities up to W 5 + may be required. Finally, W deposition patterns on DiMES collector probes, interpreted via DIVIMP-WallDYN modelling, reveal the key roles of progressive W erosion/re-deposition staps and E × B drifts in regulating long-range high-Z material migration.

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Section: And /mentioning

confidence: 99%

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

Abrams,

Guterl,

Abe

et al. 2023

Mater. Res. Express

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“…The sheath model can modify calculated GCR coefficients at high electron densities. Previous efforts to model the 400.9 nm neutral tungsten S/XB within the CS assumed that tungsten populations were in equilibrium concentrations throughout the sheath [27]. The model presented here differs by including time-dependent effects that can arise from the low density environment of the sheath.…”

Section: Time Dependent Tungsten Emission and Ionization Within The S...mentioning

confidence: 99%

“…Note that steady-state S/XB values have been utilized in most tungsten spectroscopic erosion measurements where no sheath was considered; this could induce errors as neutral tungsten emits mostly within the sheath. The steady-state sheath model including only the effects of the density gradient within the sheath is equivalent to the work by Guterl et al [27]. The fully time-dependent solution includes both the effects of density as well as time-dependent effects assuming the model discussed in section 3.…”

Section: Time Dependent Tungsten Emission and Ionization Within The S...mentioning

confidence: 99%

Time-dependent collisional radiative modeling of tungsten in the magnetic sheath for erosion diagnosis

et al. 2023

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Tungsten is the material of choice for the divertors in ITER, SPARC and future fusion reactors. Accurate diagnosis of tungsten erosion and migration is important for first wall life time, slag production and core performance. The addition of a magnetic presheath requires time-dependent collisional radiative effects to be included for accurate neutral tungsten collisional radiative modeling. Gross erosion measurements could be modified by a factor of 10 due to the inclusion of time-dependent effects for ITER relevant divertor conditions. A simple sputtering model and sheath density model are developed to investigate time-dependent collisional radiative effects. Neutral tungsten spectral lines populated from different metastable levels depend on model parameters leading to potential spectroscopic diagnostics of plasma parameters. Electron temperatures inferred from spectroscopic line ratios are in agreement with Langmuir probe measurements in the Compact Toroidal Hybrid.

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Analysis of ion orbits in front of a negative planar electrode immersed in an oblique magnetic field

et al. 2022

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The orbital motion approach is used to analyze the ion impact on a negatively biased planar wall immersed in a strongly magnetized plasma. It is assumed that the given homogeneous magnetic field forms a small angle with a planar negatively biased electrode, while the inhomogeneous electric field is perpendicular to the electrode. Spatial dependence of the electric field is modeled in such a way that the electric field exhibits two-scale behavior, which is characteristic of plasma sheath problems. The equation of motion of a singly charged deuterium ion is solved for a large variety of parameters and initial conditions. The effects of electrode bias, magnetic flux density, magnetic field angle, initial velocity, electric field scaling, and electrode bias are investigated. It is found that the impact angles of ions are distributed over a surprisingly wide range, and in a vast majority of cases, the angle of impact is several times larger than the magnetic field angle. The study is relevant for the analysis of ion flow to the electrodes in fusion plasmas, i.e., divertors of tokamaks.

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Effects of the Chodura sheath on tungsten ionization and emission in tokamak divertors

Cited by 3 publications

References 18 publications

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

Time-dependent collisional radiative modeling of tungsten in the magnetic sheath for erosion diagnosis

Analysis of ion orbits in front of a negative planar electrode immersed in an oblique magnetic field

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