Uncovering Fast Ion Dynamics in Field Reversed Configurations Using Orbital Topology

The C-2W advanced beam-driven field reversed configuration (FRC) device utilizes 15-40 keV neutral beams to create a large population of fast ions, which are critically important for FRC heating, current drive, and stabilization. [1] Fast ion orbits in high-beta FRCs can generally be classified into four types. [2] The majority of fast ion current and pressure comes from one type, betatron orbits, which are characterized by large orbits (ρ ~ a) that cross the magnetic null. Determining the fractions of orbits in each type is critical for understanding the impact of fast ions on the FRC, as well as the dynamics of the fast ions themselves. To aid in that understanding, we have developed an Integrated Data Analysis (IDA) paradigm, which combines our suite of fast ion diagnostics [3] and fast ion modeling [4] to represent their sensitivities in “orbit space.” Orbit space models utilize an action-angle coordinate system to compactly represent the full phase space behavior of an ion with a set of three conserved variables. By projecting fast ion diagnostic sensitivities into this space, we are able to combine all fast ion diagnostics into a cohesive model which maximally leverages the available data. We present the development of this model, and applications to studying fast ion dynamics in experiments on C-2W.

[1] H. Gota et al., Nucl. Fusion 61, 106039 (2021).

[2] A. S. Landsman et al., Physics of Plasmas 11, 947 (2004).

[3] T. Roche et al., Rev. Sci. Instrum. 92, 033548 (2021)

[4] S. Dettrick et al., Winter Workshop on Energetic Particles (2013).