Isotope effects and Alfven eigenmode stability in JET H, D, T, DT, and He plasmas

TL;DR

This study analyzes Alfven eigenmode stability across various ion species in JET, revealing isotope-dependent damping trends and demonstrating a novel method for plasma isotope analysis, crucial for future fusion reactors.

Contribution

It provides a comprehensive database of AE stability measurements across multiple ion species and introduces a new active MHD spectroscopy technique for plasma isotope inference.

Findings

Damping decreases with increasing hydrogenic mass.

Helium plasmas show increased damping, consistent with radiative damping.

Active MHD spectroscopy can infer effective ion mass, aiding isotope analysis.

Abstract

While much about Alfven eigenmode (AE) stability has been explored in previous and current tokamaks, open questions remain for future burning plasma experiments, especially regarding exact stability threshold conditions and related isotope effects; the latter, of course, requiring good knowledge of the plasma ion composition. In the JET tokamak, eight in-vessel antennas actively excite stable AEs, from which their frequencies, toroidal mode numbers, and net damping rates are assessed. The effective ion mass can also be inferred using measurements of the plasma density and magnetic geometry. Thousands of AE stability measurements have been collected by the Alfven Eigenmode Active Diagnostic in hundreds of JET plasmas during the recent Hydrogen, Deuterium, Tritium, DT, and Helium-4 campaigns. In this novel AE stability database, spanning all four main ion species, damping is observed to decrease with increasing Hydrogenic mass, but increase for Helium, a trend consistent with radiative damping as the dominant damping mechanism. These data are important for confident predictions of AE stability in both non-nuclear (H/He) and nuclear (D/T) operations in future devices. In particular, if radiative damping plays a significant role in overall stability, some AEs could be more easily destabilized in D/T plasmas than their H/He reference pulses, even before considering fast ion and alpha particle drive. Active MHD spectroscopy is also employed on select HD, HT, and DT plasmas to infer the effective ion mass, thereby closing the loop on isotope analysis and demonstrating a complementary method to typical diagnosis of the isotope ratio.