Abstract Details
Abstracts
Author: Neeraj Kumar
Requested Type: Consider for Invited
Submitted: 2025-02-18 12:41:11
Co-authors: G. Avdeeva, J. Candy, M. Reynolds, E. Belli, A. Tancetti, C. P. McNally
Contact Info:
General Fusion Inc.
6020 Russ Baker Way
Richmond, BC V7B 1B4
Canada
Abstract Text:
In the Magnetized Target Fusion (MTF) technology developed at General Fusion a spherical tokamak target plasma is formed and then heated by compression to reach fusion conditions. Success relies on a favourable ratio of plasma transport and compression timescales. It is thus crucial to understand transport in pre-compression plasmas. This work presents a detailed gyrokinetic analysis of turbulent transport and micro-instabilities in small aspect ratio plasmas in the PI3 device. These plasmas are characterized by low temperatures and high collisionality compared to standard tokamaks. Linear and ion-scale nonlinear gyrokinetic flux tube simulations are performed for PI3 discharge 18669 at radial positions r/a = 0.60, 0.65, 0.70, and 0.75 using the gyrokinetic code CGYRO [1]. Compared to other spherical tokamaks, PI3 linear stability analysis reveals notable differences: ITG modes dominate at ion scales, ETG modes at electron scales, while in NSTX transport is driven by TEM and KBM. High collisionality in PI3 suppresses TEM modes, whereas in NSTX[2] ITG modes are suppressed by strong E × B shear generated by NBI. In PI3, increasing plasma beta stabilizes linear growth rates of dominant unstable modes below a critical threshold for KBM, but destabilizes nonlinear turbulence, especially at the outer radius r/a = 0.75. High collisionality significantly reduces nonlinear turbulent energy fluxes, and decreasing collisionality leads to larger flux increases than equivalent increases in plasma beta do [3]. These trends for beta and collisionality are the same at T_i/T_e above one. Interestingly, increasing T_i/T_e stabilizes ITG modes in the linear regime while destabilizing long-wavelength micro-tearing modes (MTM). This MTM destabilization may be significant in scenarios where MTMs become the dominant nonlinear mode.
[1] J. Candy et al., JCP 324 (2016)
[2] C. F. Clauser et al., POP 29 (2022)
[3] N. Kumar et al., submitted to Nuclear Fusion (2025)
Characterization: 1.0
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