arXiv subjects

J. Lestz

Publications and source records attributed to J. Lestz.

3 recordsLinked to original sources

arXiv2026

Robust Control of ECH Deposition Profiles on DIII-D

Electron Cyclotron Heating (ECH) is a key actuator in DIII-D and future tokamaks that provides auxiliary heating, localized current drive for scenario development and MHD stability, and even impurity pump-out. Due to its control flexibility and applications, a gyrotron optimization algorithm was developed to multitask and fine-tune the deposition location and heating power of each gyrotron while providing robustness to hardware failure. The ECH Optimization (ECHO) algorithm finds the optimal gyrotron mirror angle and power to achieve a target ECH radial deposition profile. This optimization is accomplished in real-time using a parallelized neural network surrogate of the TORBEAM code combined with a genetic optimizer. This has been deployed in a DIII-D experiment and has been validated with experimental ECE measurements and post-experiment offline ray tracing, showing the algorithm's reliability despite gyrotron failures and significant changes to plasma parameters.

physics.plasm-ph↗

arXiv2026

Effects of Tungsten Radiative Cooling on Impurity, Heat and Momentum Transport in DIII-D Plasmas

A first-of-its-kind experiment was conducted in the DIII-D tokamak under WEST similarity constraints on plasma shape and core parameters. This work presents a detailed transport study comparing a reference regime dominated by intrinsic carbon radiation and a high-radiation regime resulting from controlled tungsten (W) injection using the Laser Blow-Off system, with a core tungsten concentration $n_{\mathrm{W}}/n_e \sim 3\times 10^{-4}$ and a radiated-power fraction $f_\mathrm{rad}>0.5$. The W-induced radiative cooling lowered the electron temperature, thereby decreasing $T_e/T_i$ and stabilizing trapped-electron-mode (TEM) turbulence. This transition in turbulence regime reduced momentum and ion thermal diffusivities, yielding ion temperature peaking and a factor-of-two increase in toroidal rotation. At the outer plasma region, enhanced $E\timesB$ shear and increased collisionality further suppressed ion-scale turbulence, causing a sharp drop in ion heat flux. Consequently, impurity transport, predominantly turbulent in the low-radiation regime, acquired a strong neoclassical inward W convection during radiative cooling, bootstrapping the cooling cycle. Despite $f_\mathrm{rad}>0.5$, radiative collapse was not observed, likely owing to collisional ion-to-electron energy exchange acting as an electron-energy reservoir, together with $1/1$ MHD activity modulating the radiated power through core impurity neoclassical $T_i$-screening. These results support preparation for a tungsten wall change in DIII-D by elucidating tungsten-induced turbulence stabilization. They also provide key insights for interpreting plasma performance in WEST and are relevant to future reactors expected to operate with radiating tungsten-walled plasmas.

physics.plasm-ph↗

arXiv2025

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

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.

physics.plasm-ph↗