A new preprint details the first successful demonstration of helicon current drive in the core of a tokamak plasma, a significant step toward achieving steady-state fusion reactor operation. The experiments, conducted at the DIII-D National Fusion Facility, utilized a megawatt-level helicon system with a traveling wave antenna to inject power. Analysis of the results shows clear evidence of power deposition in the plasma core and efficient current generation, aligning with theoretical predictions. This validation is critical, as helicon waves are considered a promising candidate for non-inductive current drive in future power plants due to their high predicted efficiency and ability to penetrate dense, high-temperature plasmas. Source: arXiv
The diagnostic approach combined direct measurement with advanced modeling to confirm the mechanism. Researchers observed that the profile of the measured electron temperature response to helicon power injection was in good agreement with time-dependent integrated modeling. This modeling simultaneously incorporated ray tracing to track the wave propagation and the effects of thermal transport within the plasma. The consistency between the empirical data and the computational models provides strong support for the conclusion that the helicon waves were effectively coupling to the core plasma electrons as intended, a foundational requirement for any current drive system. Source: arXiv
The diagnostic approach combined direct measurement with advanced modeling to confirm the mechanism.
To isolate the effect of the helicon waves, the team performed comparative discharges. In shots where helicon power was injected continuously to drive current in the same direction as the main plasma current (co-Ip), the reconstructed safety factor profile flattened significantly faster than in control shots. The control experiments substituted the helicon power with a comparable amount of electron cyclotron heating (ECH). The helicon-driven discharges also triggered sawtooth instabilities earlier, an expected consequence of modifying the central current profile. These distinct dynamic behaviors provide compelling evidence that the helicon system was actively and efficiently driving current in the plasma core. Source: arXiv
Calculations of the non-inductively driven current profile, based on the experimental data, show a profile that is peaked in the plasma core. This result is consistent with both the observed power deposition profile and the predictions from ray-tracing codes, creating a self-consistent picture of the underlying physics. For a tokamak reactor, the ability to precisely tailor the current profile, particularly in the core, is essential for maintaining magnetohydrodynamic stability and optimizing plasma confinement. The peaked profile demonstrated in these DIII-D experiments suggests helicon waves could provide a high degree of control for this purpose. Source: arXiv
These findings, presented in a preprint on arXiv, represent a crucial validation for a technology that has been under theoretical and component-level development for years. The next steps will involve scaling the system to higher power levels and testing its performance across a wider range of plasma conditions, including higher density and temperature regimes relevant to a burning plasma. Further experiments will also aim to quantify the absolute current drive efficiency and benchmark it against established methods like neutral beam injection and other radiofrequency systems. The results will inform the design of current drive systems for next-generation devices, including compact, high-field tokamaks aiming for continuous operation. Source: arXiv