First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally Launched Helicon Waves

H. Choudhury; A. Battey; C. Paz-Soldan; J. Lestz; N. Leuthold; A. Lvovskiy; C. Marini; J. Barr; W. Heidbrink; D. Spong; S. Tang; B. Van Compernolle; Q. Zhang; Y. Zhang; X. Tang

doi:10.1103/jz8f-ck22

First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally Launched Helicon Waves

H. Choudhury ¹, A. Battey ², C. Paz-Soldan ^1,*, J. Lestz ³, N. Leuthold ¹, A. Lvovskiy ³, C. Marini ⁴, J. Barr ³, W. Heidbrink ⁵ et al.

D. Spong ⁶, S. Tang³, B. Van Compernolle ³, Q. Zhang ⁷, Y. Zhang ⁷, and X. Tang ⁷

¹Columbia University, New York, New York 10027, USA
²EPFL, Swiss Plasma Center (SPC), Lausanne CH-1015, Switzerland
³General Atomics, San Diego, California 92186, USA
⁴University of California San Diego, La Jolla, California 92093, USA
⁵University of California, Irvine, California 92697, USA
⁶Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁷Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

^*Contact author: carlos.pazsoldan@columbia.edu

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Phys. Rev. Lett. 136, 025101 – Published 14 January, 2026

DOI: https://doi.org/10.1103/jz8f-ck22

Abstract

Helicon waves (also known as whistler waves) satisfying the normal wave-particle cyclotron resonance are observed to limit the growth and maximum energy of relativistic electrons (REs) in low-density Ohmic DIII-D tokamak plasmas. Following the application of helicon waves, pitch-angle scattering of high-energy REs causes an increase in both synchrotron and electron-cyclotron emissions. The hard x-ray emission, a proxy for the RE population, ceases to grow. Energy-resolved hard x-ray measurements also show a striking decrease in the number of high-energy REs (above the resonance at approximately 8 MeV) to below the noise floor and an increase in low-energy ( $\sim 4 MeV$ ) REs. This occurs despite the toroidal electric field remaining high enough to drive exponential RE growth in the absence of helicon waves. These results open new directions for limiting the maximum energy of RE populations in laboratory and fusion plasmas.

Physical Review Letters

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First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally Launched Helicon Waves

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First Demonstration of Resonant Pitch-Angle Scattering of Relativistic Electrons by Externally Launched Helicon Waves

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