W7-X DCD Colloquium

Tuesday 27 Feb 2024, 16:30 → 17:30 Europe/Berlin

7.2-001 (IPP-HGW)

Thierry Kremeyer (Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald)

Meeting Location: 7.2-001

https://eu01web.zoom.us/j/91303515504?pwd=eStUUG83c2pXOHMrOEpocXBkVU9nQT09

Zoom-ID: 913 0351 5504

PW: divertor

2024-02-20_Divertor_a_priori_first_principles_part_I+II_kremeyer_WiP.pptx

1. Divert Plasma Particles

Particles can be diverted from the plasma toroid, by creating a divertor toroid that is connected to the plasma toroid by a loop of x-points. This divertor toroid can be created through a coil in the O-point, interference of two other toroids, resonance, or chaos.

We discussed the difference between chaos, stochasticity and ergodicity. While chaos and stochasticity both involve randomness and unpredictability, they arise from different underlying mechanisms. Chaos arises from deterministic nonlinear dynamics, whereas stochasticity arises from inherent randomness in the system or its environment. Ergodicity, on the other hand, concerns the long-term behavior and averaging properties of a system over time.

Chaotic field lines do not have to close in on themselves (https://pubs.aip.org/aip/pop/article/7/6/2279/104156/Magnetic-field-lines-Hamiltonian-dynamics-and) Therefore in a chaotic field, the connected X-points do not have to close on a toroidal loop.

A Cantori is a separatrix surface that seperates different regions in the phase space. 

Cantori are the invariant sets remaining after destruction of the KAM surfaces and create partial barriers to transport in chaotic fields.                https://w3.pppl.gov/~shudson/Papers/Conference/2007PPPL/CantoriPresentation.pdf

If an X point is on this surface, it can be heteroclinic or homoclinic. The in and out going field lines that do not belong to our plasma toroid, can be used as divertor legs. These divertor legs are also called turnstiles. 

The exiting and entering flux tubes can be adjacent as is generally expected but can also have the unexpected feature of entering or exiting at separate locations of the cantori. Not only can there be two types of turnstiles, but pseudo turnstiles can also exist. A pseudo turnstile is formed when a cantorus has a sufficiently large, although limited, radial excursion to strike a surrounding chamber wall. [A. Punjabi and A. H. Boozer, Phys. Plasmas 27, 012503 (2020)]

A good recent reference is Kelly Garcias recent publication: „Exploration of non-resonant divertor features on the compact toroidal hybrid https://iopscience.iop.org/article/10.1088/1741-4326/ad0160/meta

 

 

1. Neutralize Plasma Particles

Only neutral particles can be exhausted. To form neutral particles, plasma can recombine on the target surface, or in the volume.

Surface recombination can happen through surface/gas recombination or reflection. Both processes deposit the plasma energy on the wetted area of the target. Neutral particles leave the surface in a directed manner. Particle reflection coefficients are part of the Trim database with the reflection angle being equal to the incidence angle (https://www.eirene.de/cgi-bin/trim/trim.cgi?dat=h_on_w). Gas recombination is released in a cosine distribution.

 

Volume recombination has an isotropic neutral particle and radiated energy release from the point of neutralization.

In an infinitely long divertor chamber with a finite neutral pressure, incoming plasma will first loose energy through ionization of neutral gas and dissociation, cooling the plasma. At colder temperatures MAD and MAR become relevant, until EIR becomes dominant below ~1eV. https://iopscience.iop.org/article/10.1088/1741-4326/acd394/pdf

                Volume recombination has the advantage of an isotropic energy release and can be used to cancel physical sputtering. At the same time the isotropic particle release prohibits dedicated direct particle collection and must rely on indirect ways.