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{{Quantum book backlink|Plasma and fusion physics}}
'''Tokamak edge physics and recycling asymmetries''' concerns the coupled behavior of plasma transport, neutral recycling, and divertor asymmetry in the boundary region of magnetically confined fusion plasmas. In tokamaks, the outermost plasma layer — the scrape-off layer (SOL) — connects the confined plasma to plasma-facing components and governs how particles and heat are exhausted to the divertor.<ref name="ricci2025">{{cite book
|last=Ricci
|first=Paolo
|title=Introduction to Plasma Physics for Controlled Fusion
|chapter=Scrape-Off Layer Dynamics
|publisher=Springer
|year=2025
|doi=10.1007/978-981-97-9609-0_18
|url=https://doi.org/10.1007/978-981-97-9609-0_18
}}</ref><ref name="krieger2025">{{cite journal
|last1=Krieger
|first1=K.
|last2=Brezinsek
|first2=S.
|last3=Coenen
|first3=J. W.
|last4=et al.
|title=Scrape-off layer and divertor physics: Chapter 5 of the special issue: on the path to tokamak burning plasma operation
|journal=Nuclear Fusion
|volume=65
|issue=4
|pages=043001
|year=2025
|doi=10.1088/1741-4326/adaf42
|url=https://doi.org/10.1088/1741-4326/adaf42
}}</ref>

The SOL contains open magnetic field lines that guide plasma toward divertor targets, where ions are neutralized and recycled back into the plasma as neutrals. Because cross-field drifts, parallel flows, geometry, and plasma rotation are all important in this region, the resulting neutral and particle distributions are often strongly asymmetric between the inboard high-field side (HFS) and outboard low-field side (LFS).<ref name="ippSOL">{{cite web
|title=Physics of the Scrape-Off Layer
|website=Max Planck Institute for Plasma Physics
|url=https://www.ipp.mpg.de/5458169/pds
|access-date=2026-04-16
}}</ref><ref name="rognlien2010">{{cite conference
|last1=Rognlien
|first1=T. D.
|last2=Bodi
|first2=K.
|last3=Cohen
|first3=R. H.
|last4=et al.
|title=Advances in Understanding Tokamak Edge/Scrape-Off Layer Transport
|book-title=Proceedings of the 23rd IAEA Fusion Energy Conference
|year=2010
|url=https://www-pub.iaea.org/MTCD/Meetings/PDFplus/2010/cn180/cn180_papers/thd_p3-05.pdf
}}</ref>
[[File:Tokamak randphysics and recycling-asymmetrics-1.jpg|thumb|400px|Conceptual illustration of tokamak edge physics, showing scrape-off layer flows, drift-driven transport, plasma rotation, and resulting recycling asymmetries between the high-field and low-field sides of the divertor.]]

==Abstract==
Coupled fluid-kinetic simulations for high-confinement DIII-D plasmas show that plasma rotation and edge drifts jointly play a major role in shaping the poloidal distribution of recycling neutrals.<ref name="richards2025">{{cite journal
|last1=Emdee
|first1=E. D.
|last2=Horvath
|first2=L.
|last3=Bortolon
|first3=A.
|last4=Gerrú
|first4=R.
|last5=Wilkie
|first5=G. J.
|last6=Haskey
|first6=S. R.
|last7=Laggner
|first7=F. M.
|title=Combined Influence of Rotation and Scrape-Off Layer Drifts on Recycling Asymmetries in Tokamak Plasmas
|journal=Physical Review Letters
|volume=135
|issue=22
|year=2025
|date=2025-11-24
|doi=10.1103/zjpv-vxwd
|url=https://pubmed.ncbi.nlm.nih.gov/41385680/
|pmid=41385680
}}</ref><ref name="stangeby2000">{{cite book
|last=Stangeby
|first=P. C.
|title=The Plasma Boundary of Magnetic Fusion Devices
|publisher=Institute of Physics Publishing
|year=2000
|isbn=978-0750305594
|url=https://worldcat.org/isbn/9780750305594
}}</ref>

When either effect is included by itself, the calculated asymmetry is enhanced only moderately. When both are included simultaneously, however, the inboard–outboard difference becomes much stronger, in substantially better agreement with experimentally observed Lyman-<math>\alpha</math> emission patterns and divertor fueling asymmetries in DIII-D.<ref name="richards2025" /><ref name="llama2021">{{cite journal
|last1=Rosenthal
|first1=A. M.
|last2=Brons
|first2=S.
|last3=Effenberg
|first3=F.
|last4=et al.
|title=A 1D Lyman-alpha profile camera for plasma edge neutral studies on the DIII-D tokamak
|journal=Review of Scientific Instruments
|volume=92
|issue=3
|pages=033523
|year=2021
|doi=10.1063/5.0024115
|url=https://doi.org/10.1063/5.0024115
}}</ref><ref name="llamaCal2021">{{cite journal
|last1=Laggner
|first1=F. M.
|last2=Bortolon
|first2=A.
|last3=Rosenthal
|first3=A. M.
|title=Absolute calibration of the Lyman-α measurement apparatus at DIII-D
|journal=Review of Scientific Instruments
|volume=92
|issue=3
|pages=033522
|year=2021
|doi=10.1063/5.0038134
}}</ref>

The main physical mechanism is not drift transport alone, but the way in which drifts and core-driven rotation alter '''parallel SOL flow''' and '''radial momentum transport''', increasing deuterium flux toward the inner divertor and reducing it at the outer divertor under favorable drift orientation.<ref name="richards2025" />

==Edge plasma structure==
The tokamak boundary consists of several coupled regions:

* core plasma
* edge pedestal
* scrape-off layer (SOL)
* divertor legs and divertor targets

The '''separatrix''' marks the boundary between closed magnetic field lines and open field lines. Once particles cross the separatrix, they enter the SOL and stream rapidly along the magnetic field toward plasma-facing surfaces.<ref name="ricci2025" /><ref name="ippSOL" />

Because the magnetic field, connection length, cross-field transport, and neutral source distribution are not poloidally uniform, the inner and outer divertor legs often behave differently. Such asymmetries are a standard feature of tokamak edge physics and are sensitive to magnetic configuration and drift direction.<ref name="krieger2025" /><ref name="luo2012">{{cite journal
|last1=Luo
|first1=G.-N.
|last2=Wang
|first2=L.
|last3=Ding
|first3=R.
|last4=et al.
|title=Divertor asymmetry and scrape-off layer flow in various divertor configurations in EAST
|journal=Physics of Plasmas
|volume=19
|issue=5
|pages=052507
|year=2012
|doi=10.1063/1.4706935
|url=https://doi.org/10.1063/1.4706935
}}</ref>

==Drift physics==
Charged particles in tokamaks experience several systematic drifts, including <math>\mathbf{E}\times\mathbf{B}</math> drift and magnetic-gradient/curvature drifts. In diverted plasmas, the direction of ion <math>\mathbf{B}\times\nabla B</math> drift relative to the X-point strongly affects where particles, momentum, and radiation accumulate in the divertor region.<ref name="asakura2006">{{cite conference
|last=Asakura
|first=N.
|title=Scrape-off layer plasma flow and drifts in the tokamak divertor configuration
|book-title=17th International Toki Conference Proceedings
|year=2007
|url=https://www.nifs.ac.jp/itc/itc17/file/PDF_proceeding/I-05_asakura.pdf
}}</ref><ref name="luo2012" />

Modeling and experiment both show that favorable drift orientation can substantially increase plasma density and particle flux in one divertor leg relative to the other, reversing when the field configuration is changed.<ref name="luo2012" /><ref name="rognlien2017">{{cite journal
|last1=Rognlien
|first1=T. D.
|last2=McLean
|first2=A. G.
|last3=Fenstermacher
|first3=M. E.
|last4=et al.
|title=Comparison of 2D simulations of detached divertor plasmas with divertor Thomson measurements in the DIII-D tokamak
|journal=Nuclear Materials and Energy
|volume=12
|pages=44–50
|year=2017
|doi=10.1016/j.nme.2016.12.002
|url=https://doi.org/10.1016/j.nme.2016.12.002
}}</ref>

==Plasma rotation==
Tokamak plasmas commonly exhibit '''toroidal rotation''', driven by mechanisms such as neutral beam injection and intrinsic momentum transport. This rotation is not confined to the core: via viscous coupling and radial momentum transport, part of the momentum reaches the edge and modifies SOL parallel flow patterns.<ref name="richards2025" /><ref name="churchill2017">{{cite journal
|last1=Churchill
|first1=R. M.
|last2=Canik
|first2=J. M.
|last3=Chang
|first3=C. S.
|last4=et al.
|title=Kinetic simulations of scrape-off layer physics in the DIII-D tokamak
|journal=Nuclear Materials and Energy
|volume=12
|pages=978–983
|year=2017
|doi=10.1016/j.nme.2016.12.013
|url=https://doi.org/10.1016/j.nme.2016.12.013
}}</ref>

This is important because parallel flow, rather than local drift alone, can dominate the final redistribution of particles reaching the divertor entrance and targets.<ref name="richards2025" />

==Momentum transport==
The radial transport of parallel ion momentum may be written schematically as

<math>
\Gamma^m_{i,r} = m_i V_{\parallel i}\Gamma_{i,r} - m_i n_i \nu^{\text{visc}}_i \frac{\partial V_{\parallel i}}{\partial r}
</math>

where <math>V_{\parallel i}</math> is the parallel ion velocity, <math>n_i</math> is the ion density, and <math>\nu^{\text{visc}}_i</math> is an effective viscosity.<ref name="richards2025" />

This form makes clear that edge flows depend not only on particle transport but also on viscous momentum coupling between neighboring flux surfaces. In the 2025 DIII-D study, enhanced radial momentum transport was the key link through which rotation strengthened the recycling asymmetry.<ref name="richards2025" />

==Neutral recycling and Lyman-<math>\alpha</math> diagnostics==
At the divertor targets and nearby walls, ions are neutralized and re-emitted as neutrals. These recycled neutrals can penetrate back into the plasma edge, become ionized, and thereby influence fueling, edge density buildup, and emission structure.<ref name="ricci2025" /><ref name="krieger2025" />

In DIII-D, absolutely calibrated Lyman-<math>\alpha</math> diagnostics such as '''LLAMA''' have made it possible to infer neutral density and ionization-rate profiles with good spatial resolution, providing a direct constraint on edge transport models.<ref name="llama2021" /><ref name="llamaCal2021" /><ref name="llamaUpgrade2024">{{cite journal
|last1=Rosenthal
|first1=A. M.
|last2=Brons
|first2=S.
|last3=Effenberg
|first3=F.
|last4=et al.
|title=Upgrade of the Lyman-alpha diagnostic system on DIII-D for main chamber neutral studies
|journal=Review of Scientific Instruments
|volume=95
|issue=8
|pages=083507
|year=2024
|doi=10.1063/5.0203033
|url=https://doi.org/10.1063/5.0203033
}}</ref>

These diagnostics are especially important because the inboard–outboard asymmetry discussed in divertor studies is often diagnosed through strongly asymmetric Lyman-<math>\alpha</math> emission patterns.<ref name="richards2025" /><ref name="llamaUpgrade2024" />

==Combined drift and rotation effects==
The central result of recent work is that drift effects and plasma rotation interact '''nonlinearly'''. Drifts reshape cross-field transport and divertor balance, while rotation modifies the parallel momentum budget and therefore the poloidal projection of SOL flow.<ref name="richards2025" />

When both effects are retained in coupled fluid-kinetic simulations, the inboard divertor receives a substantially larger fraction of the deuterium particle flux than in cases with drift-only or rotation-only physics. This resolves much of the mismatch between earlier simulations and DIII-D measurements.<ref name="richards2025" /><ref name="rognlien2017" />

==Physical interpretation==
The asymmetry is best understood as a consequence of the full edge flow structure rather than as a simple local drift effect. Important ingredients include

* drift-driven redistribution across the SOL
* momentum coupling between core and edge
* modified parallel flows into the divertor
* changes in recycling-neutral source localization

Under favorable ion <math>\mathbf{B}\times\nabla B</math> direction, these processes reinforce one another and enhance transport toward the inner divertor target.<ref name="richards2025" /><ref name="asakura2006" />

==Significance for fusion reactors==
Tokamak reactors require reliable control of divertor heat loads, particle exhaust, impurity screening, and plasma-wall interaction. Since all of these depend sensitively on SOL and divertor physics, realistic reactor modeling must include not only geometry and atomic physics, but also drifts, kinetic corrections, and momentum transport effects.<ref name="krieger2025" /><ref name="ricci2025" />

For this reason, edge asymmetries are not merely a diagnostic curiosity: they are central to divertor design, detachment control, wall lifetime, and the predictive capability needed for ITER-class and DEMO-class fusion devices.<ref name="krieger2025" /><ref name="rognlien2010" />

==See also==
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

==References==
{{reflist|3}}
{{Author|Harold Foppele}}

{{Sourceattribution|Tokamak edge physics and recycling asymmetries|1}}

Physics:Quantum Tokamak edge physics and recycling asymmetries - Revision history

imported>WikiHarold: Repair Quantum Collection B backlink template

imported>WikiHarold: Repair Quantum Collection B backlink template