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

Magnetic reconnection: From compact fusion to plasma propulsion
1Fatima Ebrahimi 1
Princeton Plasma Physics Laboratory, and Department of Astrophysical Sciences, Princeton
University NJ, 08544
ebrahimi@pppl.gov

Initial plasma-current formation via unconventional
methods is a necessity in compact fusion concepts suchas spherical tokamaks (STs), one of the leading fusion
reactor designs with high magnetic field. A 3D
current-sheet plasmoid instability, which is a common
process both in space, fusion [1-3] and astrophysical [4]plasmas, has been shown to be instrumental during
unconventional current start-up in STs. Multiple
significant contributions have been made inunderstanding of the nonlinear dynamics [5-8] during
helicity injection start-up, including a transition from
Sweet-Parker [8] reconnection to a plasmoid instability
that was for the first time simulated and predicted by
global simulations with a guide field.[5] Fundamental
theoretical and computational studies of fast magnetic
reconnection in NSTX led to the experimental evidenceof plasmoids [5]. Inspired by this research,state-of-the-art large-scale extended MHD simulations
have for the first time been used to demonstrate that a
particular configuration of externally applied static
electric and magnetic fields can both create plasma andaccelerate it by continuously and spontaneously
producing plasmoids, detached plasma enclosed bymagnetic fields.[9] This led to a new type of thruster, amagnetic reconnection plasma thruster, which relies onfast plasmoid-mediated magnetic reconnection [9].
Because of the Alfvénic outflow from the reconnection
site, its thrust is proportional to the square of themagnetic field strength and does not ideally depend on
the mass of the ion species of the plasma. Variable
unidirectional exhaust velocities were directlydemonstrated in the simulations (Figure 1). This plasma
propulsion technology describes a practical device
configuration that produces the required electric andmagnetic fields (no guide field) to induce a fast magnetic
reconnection process in a thruster channel for the firsttime [9]. New MHD simulation results for exhaust
velocity and thrust scalings will also be presented. Asanother application of magnetic reconnection, I willconclude with new results on the rise and nonlinear
relaxation of Peeling-Ballooning ELMs at the tokamakedge. Axisymmetric as well as small-scale poloidally
extending current sheets, are formed as the pressure
driven P-B ELM filaments nonlinearly evolve [3].
References[1] F. Ebrahimi “Dynamo-Driven Plasmoid Formationfrom a Current-Sheet Instability", PoP, 23, 120705(2016).

[2] F. Ebrahimi “Nonlinear Reconnecting Edge Localized
Modes in Current-Carrying Plasmas", PoP, 24, 056119(2017).
[3] F. Ebrahimi, A. Bhattacharjee, “Plasmoid-mediatedreconnection during nonlinear Peeling-Ballooningedge-localized modes, arXiv:2110.09706, submitted toNuclear Fusion (2023).
[4] J. Rosenberg and F. Ebrahimi, "Onset of Plasmoid
Reconnection during Magnetorotational Instability",
ApJL 920 L29 (2021).
[5] F. Ebrahimi and R. Raman "Plasmoids Formation
During Simulations of Coaxial Helicity Injection in theNational Spherical Torus Experiment", PRL 114, 205003(2015).
[6] F. Ebrahimi and R. Raman, "Large-volume flux
closure during plasmoid-mediated reconnection inCoaxial Helicity Injection", Nucl Fusion, 56(4):044002(2016).
[7] F. Ebrahimi “Three-dimensional plasmoid-mediatedreconnection and the effect of toroidal guide field in
simulations of coaxial helicity injection", PoP, 26,092502 (2019).
[8] F. Ebrahimi et al. “Magnetic reconnection
process in transient coaxial helicity injection" PoP 20 ,090702 (2013).
[9] F. Ebrahimi “Alfvenic reconnecting plasmoidthruster", JPP 86 (2020).

Figure 1: Continuous formation of momentum-carrying
plasmoids with exhaust velocities of 400 km/s are
obtained during three-dimensional global MHD
simulations for the Alfvenic reconnecting plasmoid
thruster concept [9].