Ion orbit modelling of ELM heat loads on ITER divertor vertical targets

Gunn, J.; Carpentier-Chouchana, S.; Dejarnac, R.; Escourbiac, F.; Hirai, T.; Komm, M.; Kukushkin, A.S.; Panayotis, S.; Pitts, R.A.

doi:10.1016/j.nme.2016.10.005

Cited by 32 publications

(41 citation statements)

References 9 publications

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“…Within a Larmor radius of any plasma-facing surface in a tokamak, due to the removal of ions from the plasma, there is a poloidal ion flux directed in the ion diamagnetic direction (clockwise when looking in the toroidal direction, and when B×∇B is directed downward). At the ITER inner vertical target (IVT) (Figure 2), as shown previously [7], this ion flux strikes the lower TG edges due to Larmor gyration. The electron component of the heat flux, assuming that it can be described by the guiding center, or "optical" approximation (the Larmor radius being negligibly small compared to the TG width), strikes the upper edge due to the inclination of the magnetic flux surfaces.…”

Section: Introductionsupporting

confidence: 68%

A study of planar toroidal–poloidal beveling of monoblocks on the ITER divertor outer vertical target

et al. 2019

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The design of the monoblocks constituting the ITER divertor vertical targets comprises a simple toroidal (i.e. toroidally-facing) bevel of 0.5 mm in order to magnetically shadow poloidal (i.e. poloidally-running) leading edges, arising from radial misalignments between toroidally neighbouring blocks, from parallel heat loads between and during ELMs. Previous studies suggest that excessive heating of long toroidal edges could also occur, possibly leading to melting during ELMs. Furthermore, despite the toroidal bevel, tiny regions of the poloidal leading edges known as "optical hot spots", accessible along magnetic field lines through toroidal gaps, remain exposed to parallel heat flux from ELMs. The intense heat flux onto those optical hot spots could be large enough to trigger tungsten boiling. A possible solution at the outer vertical target is to implement a planar toroidal-poloidal bevel that would hide all poloidal and toroidal edges and eliminate the optical hot spot. It will be demonstrated that a reasonable "shallow" toroidal-poloidal bevel solution solves all these problems with minimal trade-offs, under the condition that monoblocks on neighbouring plasma-facing units be well aligned poloidally in order to prevent the appearance of exposed leading edges, meaning, in the worst case, a stepwise downward shift of each toroidally upstream plasmafacing unit by -2±2 mm with respect to their downstream neighbours. A more deeply beveled solution has also been studied that is immune to poloidal misalignments, but which comprises important trade-offs in terms of higher heat load to the main wetted surface, and excessive ELM heat loads onto the magnetically shadowed side of the toroidal gaps. Unfortunately, due the inclination of magnetic flux surfaces, the planar toroidal-poloidal beveling solution does not work at the inner vertical target, meaning that its application at the outer target alone leaves the inner toroidal gaps unprotected. This, together with the technologically challenging requirement for a high degree of poloidal alignment of toroidally neighbouring plasma-facing units, has led to a decision not to apply the poloidal-toroidal bevel solution on the ITER vertical targets.

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Section: Introductionsupporting

confidence: 68%

A study of planar toroidal–poloidal beveling of monoblocks on the ITER divertor outer vertical target

et al. 2019

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“…Simplifying models can offer more immediate calculations. For example, taking into account gyro-orbit losses at the wall, but ignoring the electric field, one can solve for distribution functions at the wall analytically, assuming an incoming Maxwellian (Parks & Lippmann 1994) or more refined boundary conditions (Gunn et al 2017). However, in neglecting the electric field this model assumes that some ions can reach the target travelling tangentially, 1 as the left ion in figure 1(a) does.…”

Section: Introductionmentioning

confidence: 99%

Large gyro-orbit model of ion velocity distribution in plasma near a wall in a grazing-angle magnetic field

Geraldini

2021

J. Plasma Phys.

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A model is presented for the ion distribution function in a plasma at a solid target with a magnetic field $\boldsymbol {B}$ inclined at a small angle, $\alpha \ll 1$ (in radians), to the target. Adiabatic electrons are assumed, requiring $\alpha \gg \sqrt {Zm_{e}/m_{i}}$ , where $m_{e}$ and $m_{i}$ are the electron and ion mass, respectively, and $Z$ is the charge state of the ion. An electric field $\boldsymbol {E}$ is present to repel electrons, and so the characteristic size of the electrostatic potential $\phi$ is set by the electron temperature $T_{e}$ , $e\phi \sim T_{e}$ , where $e$ is the proton charge. An asymptotic scale separation between the Debye length $\lambda _{D} = \sqrt {\epsilon _0 T_{{e}} / e^{2} n_{{e}} }$ , the ion sound gyro-radius $\rho _{s} = \sqrt { m_{i} ( ZT_{e} + T_{i} ) } / (ZeB)$ and the size of the collisional region $d_{c} = \alpha \lambda _{\textrm {mfp}}$ is assumed, $\lambda _{D} \ll \rho _{s} \ll d_{c}$ . Here $\epsilon _0$ is the permittivity of free space, $n_{e}$ is the electron density, $T_{i}$ is the ion temperature, $B= |\boldsymbol {B}|$ and $\lambda _{\textrm {mfp}}$ is the collisional mean free path of an ion. The form of the ion distribution function is assumed at distances $x$ from the wall such that $\rho _{s} \ll x \ll d_{c}$ , that is, collisions are not treated. A self-consistent solution of the electrostatic potential for $x \sim \rho _{s}$ is required to solve for the quasi-periodic ion trajectories and for the ion distribution function at the target. The large gyro-orbit model presented here allows to bypass the numerical solution of $\phi (x)$ and results in an analytical expression for the ion distribution function at the target. It assumes that $\tau =T_{i}/(ZT_{e})\gg 1$ , and ignores the electric force on the quasi-periodic ion trajectory until close to the target. For $\tau \gtrsim 1$ , the model provides an extremely fast approximation to energy–angle distributions of ions at the target. These can be used to make sputtering predictions.

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“…Predictions of target conditions such as power density, particle flux density and impact energy during Edge-Localized Modes (ELMs) in future fusion devices relying on H-mode plasmas is essential for the design of plasma facing components (PFC) and operational scenarios. Plasma-wall interaction issues like phase transition of the PFC material, erosion or impurity sputtering, are expected to be dominantly due to ELMs [1,2]. In this context, a reliable model allowing precise predictions of target conditions during ELMs to assess these phenomena would be very useful.…”

Section: Introductionmentioning

confidence: 99%

Experimental validation of an analytical kinetic model for edge-localized modes in JET-ITER-like wall

et al. 2018

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The design and operation of future fusion devices relying on H-mode plasmas requires reliable modelling of Edge-Localized Modes (ELMs) for precise prediction of divertor target conditions. An extensive experimental validation of simple analytical predictions of the time evolution of target plasma loads during ELMs has been carried out here in more than 70 JET-ITER-Like Wall H-mode experiments with a wide range of conditions. Comparisons of these analytical predictions with diagnostic measurements of target ion flux density, power density, impact energy and electron temperature during ELMs are presented in this paper and show excellent agreement. The analytical predictions tested here are made with the "Free-Streaming" kinetic model (FSM) which describes ELMs as a quasi-neutral plasma bunch expanding along the magnetic field lines into the Scrape-Off Layer without collisions. Consequences of the FSM on energy reflection and deposition on divertor targets during ELMs are also discussed.

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Ion orbit modelling of ELM heat loads on ITER divertor vertical targets

Cited by 32 publications

References 9 publications

A study of planar toroidal–poloidal beveling of monoblocks on the ITER divertor outer vertical target

A study of planar toroidal–poloidal beveling of monoblocks on the ITER divertor outer vertical target

Large gyro-orbit model of ion velocity distribution in plasma near a wall in a grazing-angle magnetic field

Experimental validation of an analytical kinetic model for edge-localized modes in JET-ITER-like wall

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