7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Insights into a negative triangularity reactor from EUROfusion’s TSVV 2J. Ball1 on behalf of the TSVV 2 team2
1 Ecole Polytechnique Federale de Lausanne (EPFL), Swiss Plasma Center (SPC)2
See wiki.euro-fusion.org/images/6/61/TSVV2proposal.pdf
e-mail (speaker): Justin.Ball@epfl.ch
Experimental observations show that Negative
Triangularity (NT) plasma shaping cansignificantly improve the energy confinement
time of tokamaks. Moreover, relative to thestandard Positive Triangularity (PT) shape, NT
plasmas have much more difficulty accessing
H-mode. Together these two facts may enable
an attractive power plant design – the plasma
can be heated to reactor conditions whileremaining in L-mode (avoiding the material
survivability concerns associated with ELMs),
yet still achieve sufficiently good confinement
for high fusion gain. This potential has
motivated EUROfusion’s Theory, Simulation,Verification, and Validation (TSVV) project on
NT, which is investigating the most important
issues for the feasibility of a NT power plant.
In this talk, we will summarize the results
obtained thus far.
Local and global gyrokinetic
simulations generally find NT has improved
energy confinement relative to PT, though thistypically requires a kinetic treatment ofelectrons. Such simulations are quantitatively
consistent with TCV experiments andreproduce significant trends. They show theheat flux reduction in NT is primarily due to anincrease in the critical gradient, while thestiffness remains similar.
Interestingly, simulations show that
NT is more beneficial at large aspect ratio,
while it can degrade confinement in sphericaltokamaks (for trapped electron modeturbulence) [1]. This indicates that the standard
physical picture for why NT decreases
transport, which is based on trapped electron
stability [2], is incomplete. Further
investigation motivated an expanded physicalpicture based on FLR damping as well as aresonance between the magnetic drift velocity
and the ion (or electron) diamagnetic velocity.
This physical picture holds well at conventional
and large aspect ratio and can be used toexplain previously observed trends [3].
Global gyrokinetic simulations andnovel flux tube simulations with profile
shearing [4] both indicate that the scaling oftransport with device size is similar in NT andPT. Additionally, finite plasma  simulations atconventional aspect ratio indicate thatelectromagnetic turbulence is similar or weakerin NT, as compared to PT. These facts suggestthat the confinement benefits of NT will persist
in reactor-scale devices. However,electromagnetic turbulence does appear
significantly stronger for NT (relative to PT) in
spherical tokamaks [1,5].
Reduced modeling of NT has been
performed using ASTRA-TGLF with SAT2,
displaying good agreement with nonlinear
gyrokinetics [6]. Profile predictions sometimes
display a weak improvement from NT, butmore often see no effect from triangularity.
This observation, as well as analysis of TCV
experiments, suggests that most of theconfinement improvement is coming from tor
> 0.9, which is largely outside the simulation
domain and is challenging to model.
Theoretical and numericalinvestigations into tearing stability suggest
little distinction between PT and NT. Modelingof Alfven eigenmode stability and the resulting
fast ion transport indicate that both aremodestly improved in NT [7], which isconsistent with TCV experiments. Predictive[8] and interpretive [9] simulations of theScrape-Off Layer (SOL) suggest that NT will
have an SOL width that is between that of a PT
L-mode and a PT H-mode [10]. This indicatesthat NT should be beneficial for power exhaust.
References[1] A. Balestri et al. EPS (2023).
[2] A. Marinoni et al. Rev. Mod. Plasma Phys. 5 (2021).
[3] G. Merlo et al. J. Plasma Phys. 89 (2023).
[4] J. Ball et al. PPCF 65 (2023).
[5] R. Davies et al. PPCF 64 (2022).
[6] A. Mariani et al. Nucl. Fusion (submitted).
[7] P. Oyola et al. IAEA (2023).
[8] K. Lim et al. PPCF 65 (2023).
[9] P. Muscente et al. Nucl. Mater. Energy 34 (2023).
[10] D. Silvagni et al. PPCF 62 (2020).