Measuring the Phase Space Transport of Energetic Particles in Magnetic Fusion Experiments

Energetic particles (EP) are nearly collisionless ions or electrons that occur in natural and laboratory plasmas [1]. They can cause damage in both space and experiments and their confinement is essential in a fusion reactor. Unlike thermal particles, their distribution function F is a complicated function, most conveniently expressed in terms of orbital constants of motion in the equilibrium magnetic field. In magnetic fusion experiments, measurements of F rely on reactions and are indirect. Measured signals are most readily understood in terms of the phase-space sensitivity or “weight function” of the measurement, a formalism that enables tomographic inversion to infer F [2]. Waves alter F and cause transport [1]. Alfvén eigenmodes [3] are particularly dangerous. Modulation of the EP source diagnoses this phase-space transport [4-6]. The measured flow of fast ions caused by Alfvén waves agrees well with theoretical predictions [7-9]. Recent work shows that, despite their detrimental effect on fast-ion confinement, Alfvén eigenmodes can improve thermal confinement [10]. Along the way, as I tell this story, I’ll interject historical asides.



[1] W.W. Heidbrink and R.B. White, Phys. Plasmas 27, 030901 (2020).

[2] M. Salewski et al., Nucl. Fusion 52, 103008 (2012).

[3] W.W. Heidbrink, Phys. Plasmas 15, 055501 (2008).

[4] W.W. Heidbrink et al., Nucl. Fusion 56, 112011 (2016).

[5] C.S. Collins et al., Phys. Rev. Lett. 116, 095001 (2016).

[6] W.W. Heidbrink et al., Phys. Plasmas 24, 056109 (2017).

[7] X.D. Du et al., Phys. Rev. Lett. 127, 235002 (2021).

[8] J. Gonzalez-Martin et al., Nucl. Fusion 62, 112003 (2022).

[9] X.D. Du et al., Nucl. Fusion 63, 046020 (2023).

[10] X.D. Du et al, Phys. Rev. Lett. 135 (2025) submitted.