BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//CERN//INDICO//EN
BEGIN:VEVENT
SUMMARY:Fluid\, kinetic and hybrid approaches for edge transport modelling
in fusion devices
DTSTART;VALUE=DATE-TIME:20210511T162500Z
DTEND;VALUE=DATE-TIME:20210511T164500Z
DTSTAMP;VALUE=DATE-TIME:20211027T061138Z
UID:indico-contribution-17224@conferences.iaea.org
DESCRIPTION:Speakers: Dmitriy Borodin (Forschungszentrum Jülich GmbH\, In
stitut für Energie- und Klimaforschung – Plasmaphysik\, 52425 Jülich\,
Germany)\n**Introduction and motivation.** \n\nNumerical simulations with
the EIRENE [1] code are indispensable for both understanding and predicti
ng the fuel and impurity transport in the edge and divertor areas of fusio
n devices including ITER. The transport determines impurity penetration to
wards the core\, plasma exhaust and plasma-surface interaction (PSI) issue
s. The insight into the interplay of transport and atomic-molecular (A&M)
processes provided by modelling is key for understanding of the detachment
phenomenon [2]\, critical for many exhaust regimes envisaged for ITER and
DEMO.\n\nEIRENE is a multi-purpose Boltzmann-equation Monte-Carlo (MC) so
lver typically employed in an interactive scheme with a computational flui
d dynamics (CFD) code. A number of CFD-EIRENE kinetic-fluid code packages
such as 2D SOLPS-ITER [3]\, EDGE2D-EIRENE\, SOLEDGE2D-EIRENE [4] and 3D EM
C3-EIRENE [3]\, TOKAM3X-EIRENE [10] are extensively employed and actively
developd by the fusion community\; 3D ones are more CPU and memory demandi
ng. They provide self-consistently generated plasma distributions (2D or 3
D)\, heat and particle fluxes to the wall\, synthetic spectroscopy and rad
iative energy losses. An essential part of the neutral MC tracing procedur
e is the database of A&M processes for main-plasma species and intrinsic/e
xtrinsic impurities. This includes ionization-dissociation-recombination o
f A&M species\, molecular break-up chains in plasma and elastic processes.
\n\nIn fusion-relevant plasmas\, the mean free path $\\lambda$ for neutral
s is large compared to the gradient lengths L and the flow is in the large
Knudsen $K_n=\\lambda/L$ number regime\, for which no accurate fluid clos
ure is available. This is why a kinetic approach is generally used for neu
trals. The MC approach allows solving the kinetic problem on a 3D grid\, a
t the cost of introducing statistical noise. It also provides flexibility
in terms of geometry and A&M processes. However\, especially in large mach
ines such as ITER\, high collisionality regions (HCRs) may appear\, where
the coupling of the neutrals with the background plasma becomes very stron
g\, leading to quasi-Maxwellian distributions for neutrals. This situation
is computationally demanding in the frame of the MC method\, because of t
he high number of collisions to be calculated before the particle is ioniz
ed or absorbed at the surface. HCRs could be addressed more efficiently by
using a hybrid kinetic/fluid approach.\n\nThe CFD side of the packages ty
pically provides sufficient performance allowing calculations for ITER and
other large devices on a realistic time scale. It also helps to impose th
e respective magnetic configuration of the plasma discharge. Often\, plasm
a ions are simulated by a fluid approach\, and just the neutrals including
molecular species are treated kinetically. However\, in some cases\, for
instance when velocity distributions of plasma species are complex and dyn
amic (e.g. thermalisation isotropy) or\, another example\, if A&M processe
s significantly impact the particle trajectories and energy distributions
on time scales shorter than the ion relaxation time\, only a kinetic appro
ach can provide sufficient detail and precision. Therefore\, a kinetic app
roach for plasma ions is developed inside the EIRENE code together with va
rious application schemes described below. Treating ions on the kinetic si
de of CFD-EIRENE packages in addition to a fluid approximation for neutral
s provides more flexible and seamless coupling between the codes as well a
s an internal benchmarking mechanism.\n\nThe paper gives an overview over
the effort on introducing the hybrid kinetic-fluid approach for EIRENE ins
ide EUROfusion. This effort includes the optimization of the code parallel
ization by providing an OpenMP-MPI hybrid scheme to the already available
MPI approach and some general code refactoring. The shared memory (OpenMP)
parallelization is currently being implemented [10] and aims at alleviati
ng memory issues for large 3D grids and improving resource usage when coup
led to other OpenMP-MPI codes (e.g. TOKAM3X).\n \n**Spatial and micro-macr
o methods for fluid-kinetic hybridization (FKH)**\n\nTwo methods for hybri
d tracking of atoms (in future applicable also for ions and molecular spec
ies) in both fluid and kinetic parts of the CFD-EIRENE packages are under
development. \n\nIn the ***spatial hybridisation (SpH)*** approach [7\,11]
\, the whole simulation volume is segregated into kinetic and fluid domain
s with an immersed boundary. Trajectory tracking of kinetic atoms are stop
ped when entering a fluid region and contribute to the source of fluid ato
ms there. The implementation of these models is straightforward and does n
ot require modifications of the kinetic solver\, however the transition be
tween fluid and kinetic regions/boundaries is based on ad-hoc criteria\, w
hose choice affects the accuracy of the scheme. The [7\,10] approach used
e.g. in Soledge2D is inspired by [9]\, where additional reaction channels
are introduced to represent evaporation/condensation between the kinetic a
nd fluid phases. In [11]\, the decision of whether to treat an atom as flu
id or kinetically is based on its birth/recycling location\, after which i
t retains its identity until the next ionization or recycling event.\n\nIn
the ***micro-macro hybridisation (mMH)*** [6]\, the neutral distribution
function is split into a “fluid” part and a “kinetic correction” s
eamlessly in the entire simulation domain. A consistent equation is derive
d in such a way that the sum of the fluid part and the kinetic correction
gives exactly the same result as the solution of the original fully kineti
c equation. The benefit of mMH is that the solution is (in principle) equi
valent to the solution of the full kinetic equation and independent of the
recycling regime. However\, for a significant computational benefit the f
luid model should already capture a large part of the full kinetic distrib
ution (hence\, the kinetic correction part should be small)\, which is exp
ected only for high recycling or detached conditions. The method requires
substantial development efforts for the kinetic correction term and fluid
solver acting for the whole volume right down to the first wall.\nIn addit
ion\, fundamental physical enhancements of the kinetic ion transport part
of EIRENE were performed [5]\, by adding first-order drift effects\, cross
-field diffusion\, and magnetic mirror force. These additions\, which are
relevant for thoroughly investigating the full three-dimensional influence
of impurities on actual fusion devices\, have been cross-checked on analy
tical properties of passing and trapped (banana) particle orbits\, as well
as checking on the introduction of numerical diffusion by our integration
scheme. \n\nFirst applications of the hybrid FKH and ***kinetic ion traci
ng (KIT)*** for ITER and ITER-scale devices with advanced divertor concept
s are ongoing\, which allows testing the code performance\, determination
of the key parameters\, investigating merits and synergies of the hybridiz
ation options.\n\n**Summary and conclusion**\n\nA FKH is developed (both S
pH and mMH) for the CFD-EIRENE packages [6-11]. It combines acceptable com
puting performance with model accuracy approaching full kinetic simulation
s. In addition\, the KIT option is improved [5]. The advantages of hybridi
sation methods are compared based on experience from the first application
s to ITER scale devices [5-8]. Currently\, the main effort is on 1) basic
development of the approaches 2) validation with full-kinetic simulations
to determine the gain in computational speedup and optimal parameters 3) i
mpact demonstration of new physics included on example of ITER-relevant ap
plications.\n\nIn future\, the advantages of various FKH approaches should
be combined. The hybrid OpenMP-MPI code parallelization goes mostly in pa
rallel\, however\, its optimization can depend on the final selection of t
he FHK scheme.\n\n\n[1] D. Reiter\, M. Baelmans\, P. Börner\, Fusion Sci
. Technol. 2005\, 47(2)\, 172. \n\n[2] H. Frerichs et al. NME 18 (2019) 6
2–66\n\n[3] S. Wiesen et al.\, JNM 463 (2015) 480–484\n\n[4] H. Buff
erand et al.\, NF 55\, 053025 (2015) \n\n[5] F. Schluck\, CPP\, https://d
oi.org/10.1002/ctpp.201900144\n\n[6] N. Horsten et al.\, NME 18 (2019) 20
1-207. \n\n[7] M. Valentinuzzi et al. NME 18 (2019) 41–45\n\n[8] F. Ne
spoli et al.\, NME 18 (2019) 29–34\n\n[9] Karney et al.\, CPP\, 38\, 3
19\, 1998\n\n[10] Y. Marandet et al.\, this conference\n\n[11] M.Blommae
rt\, et al. NME 19 (2019) 28–33\n\nhttps://conferences.iaea.org/event/21
4/contributions/17224/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17224/
END:VEVENT
END:VCALENDAR