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

TL;DR

This paper demonstrates that externally-launched helicon waves can resonantly scatter relativistic electrons in tokamak plasmas, effectively limiting their maximum energy and altering their emission signatures.

Contribution

It provides the first experimental evidence of resonant pitch-angle scattering of relativistic electrons by helicon waves in a fusion plasma.

Findings

Helicon waves limit the growth of high-energy relativistic electrons.

Pitch-angle scattering increases synchrotron and electron-cyclotron emissions.

High-energy electron population decreases below noise floor despite strong electric fields.

Abstract

Helicon 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; and energy-resolved hard x-ray measurements also show a striking decrease in the number of high-energy REs (above the resonance at approximately \SI{8}{MeV}) to below the noise floor. 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.