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SUMMARY:Electromagnetic schemes in the global gyrokinetic PIC code XGC for
higher-fidelity simulation of long-wavelength modes in the edge
DTSTART;VALUE=DATE-TIME:20210513T162500Z
DTEND;VALUE=DATE-TIME:20210513T164500Z
DTSTAMP;VALUE=DATE-TIME:20211016T202616Z
UID:indico-contribution-17547@conferences.iaea.org
DESCRIPTION:Speakers: Amil Y. Sharma ()\nIn addition to the existing hybri
d electromagnetic (EM) scheme that uses gyrokinetic ions and fluid electro
ns (Ref. 1)\, two new EM schemes\, which use kinetic electrons\, have been
implemented in the whole-volume particle-in-cell (PIC) code XGC (Ref. 2).
In particular\, these new EM schemes in XGC will allow higher-fidelity gl
obal simulation of long-wavelength modes\, which are important for studyin
g the onset of edge localized modes (ELMs) and their interaction with micr
o-turbulence and neoclassical dynamics\, and also ELM control\, via resona
nt magnetic perturbations (RMPs) and other means. Additionally\, EM effect
s for XGC-focused ITER problems\, such as divertor heat-load width\, L-H a
nd H-L transitions\, pedestal height and shape\, and RMP penetration\, sho
uld include long-wavelength modes self-consistently with other physics eff
ects. Traditionally\, the PIC approach exhibited higher numerical sensitiv
ity to the so-called "cancellation problem" (Ref. 3) at long wavelengths t
han the continuum approach.\n\nThe first new kinetic-electron EM scheme th
at has been implemented in XGC is a fully implicit numerical scheme (Ref.
4) that uses the symplectic formulation of gyrokinetics. The second new sc
heme that has been implemented is an explicit numerical scheme (Ref. 5) th
at uses the Hamiltonian formulation of gyrokinetics. Both the implicit and
explicit numerical schemes have significant advantages over each other th
at make both viable for consideration\, depending on the particular physic
s of interest and computational architecture. For example\, the implicit s
cheme completely avoids the numerical cancellation problem that has to be
mitigated in the explicit scheme (Ref. 5)\, whereas\, the explicit scheme
has a simpler numerical implementation and better performance in general.
Our ability to rigorously evaluate these two numerical schemes is\, theref
ore\, an important aspect of this work.\n\nThe verification of the hybrid
EM scheme in XGC was presented in Ref. 1. The hybrid EM scheme\, which doe
s not suffer from the cancellation problem\, showed that kinetic balloonin
g modes (KBMs) in NSTX are generally stable in the pedestal region\, but c
ould be unstable at the pedestal top\, and that the low-$n$ peeling modes
generally exist in the steep gradient region of DIII-D and KSTAR pedestals
\, without a sharp instability boundary\, which are contrary to the usual
MHD results. We see gyrokinetic peeling modes at low $n<15$ and electromag
netic ITG modes at higher $n>15$\, as shown in Figure [1]. It appears that
the gyrokinetic peeling modes require interaction with mean $E\\times B$
shearing and microturbulence to be nonlinearly unstable.\n\n![Growth rate
as a function of toroidal mode number $n$ in a high current DIII-D dischar
ge (144981) with the hybrid scheme (Ref. 1).][1]\n\n\n\nThe verification o
f the new implicit and explicit numerical schemes has been performed for s
hear-Alfvén modes\, as well as the ion temperature gradient (ITG) to KBM
transition at finite plasma $\\beta$ (Ref. 6). Verification results for sh
ear-Alfvén modes with the implicit scheme are shown in Figure [2]. Perfor
ming such verification simulations with the explicit scheme would be parti
cularly challenging because of the cancellation problem. Verification resu
lts for the ITG-KBM transition for the explicit Hamiltonian scheme are sho
wn in Figure [3]\, with the cancellation problem successfully mitigated. T
he stabilization of the ITG instability from 0% to 1.5% plasma $\\beta$ is
reproduced\, and there is good quantitative agreement with the GENE code
(Ref. 6). The ITG to KBM transition is also reproduced\, and there is agai
n good quantitative agreement with the GENE code for the destabilization o
f the KBM above 1.5% plasma $\\beta$.\n\n![Analytic and simulated shear-Al
fvén wave dispersion relations for the kinetic implicit scheme.][2]\n\n![
Growth rates (a) and real frequencies (b) as a function of plasma $\\beta$
for the explicit version of XGC compared against that for the GENE code (
Ref. 6).][3]\n\nSome results from electromagnetic studies of critical ITER
edge physics\, such as divertor heat-load width\, L-H transition\, pedest
al structure\, and ELM onset and RMP control\, will be presented.\n\nRefer
ences:\n\n 1. R. Hager\, J. Lang\, C. S. Chang\, S. Ku\, Y. Chen\, S. E. P
arker\, and M. F. Adams\, Phys. Plasmas **24**\, 054508 (2017)\;\n 2. S.
Ku\, C. S. Chang\, R. Hager\, R. M. Churchill\, G. R. Tynan\, I. Cziegler
\, M. Greenwald\, J. Hughes\, S. E. Parker\, M. F. Adams\, E. D’Azevedo\
, and P. Worley\, Phys. Plasmas **25**\, 056107 (2018)\;\n 3. Y. Chen and
S. Parker\, Phys. Plasmas **8**\, 441 (2001)\;\n 4. G. Chen and L. Chacón
\, Comput. Phys. Commun. 197 (2015) 73–87\;\n 5. A. Mishchenko\, R. Hatz
ky\, and A. Könies\, Phys. Plasmas **11**\, 5480 (2004)\;\n 6. T. Görler
\, N. Tronko\, W. A. Hornsby\, A. Bottino\, R. Kleiber\, C. Norscini\, V.
Grandgirard\, F. Jenko\, and E. Sonnendrücker\, Phys. Plasmas **23**\, 07
2503 (2016).\n\n[1]:https://nstx.pppl.gov/DragNDrop/Scientific_Conferences
/IAEA/IAEA_2020/Synopses/Figures/sharma1.png\n[2]:https://nstx.pppl.gov/Dr
agNDrop/Scientific_Conferences/IAEA/IAEA_2020/Synopses/Figures/sharma2.png
\n[3]:https://nstx.pppl.gov/DragNDrop/Scientific_Conferences/IAEA/IAEA_202
0/Synopses/Figures/sharma3.png\n[4]:https://nstx.pppl.gov/DragNDrop/Scient
ific_Conferences/IAEA/IAEA_2020/Synopses/Figures/sharma4.png\n\nhttps://co
nferences.iaea.org/event/214/contributions/17547/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17547/
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