arXiv subjects

L. Votta

Publications and source records attributed to L. Votta.

5 recordsLinked to original sources

arXiv2026

Runaway electron generation in ITER mitigated disruptions with improved physics models

We assess runaway-electron (RE) generation in ITER disruptions mitigated by shattered pellet injection (SPI) using improved physics modelling in the 1D disruption simulation framework Dream. To this end, we extend Dream with four ITER-relevant physics models: (i) a reduced model for RE scrape- off associated with the vertical plasma motion, (ii) a semi-analytical plasmoid- drift model for material deposition, (iii) an adaptive hyper-resistive transport model to suppress unphysical thin-current channels during the current quench (CQ), and (iv) an updated Compton RE generation seed calculated for the new ITER tungsten first-wall design. We simulate full-current 15 MA L-mode (H26, non-nuclear) and H-mode (DTHmode24, nuclear) scenarios, and an intermediate- current 7.5 MA H-mode non-nuclear case, from realistic ITER inputs. Complete avoidance of a multi-MA RE beam is found to require a long pre-thermal quench (TQ) duration to thermalize the hot-tail electrons, high deuterium assimilation with limited neon, and a representative seed current comparable to a single RE in ITER. As previously found with lower fidelity setups [Vallhagen et al, Nucl. Fusion 64 (2024)], these conditions are met by staggered or low-Ne injections in H26, but are typically violated in DT H-mode when nuclear seeds are present. In addition to analyzing the effect of the new models, we investigate the role of the current spike associated with the TQ and importance of radial transport of runaways in the CQ. After incorporating these additional physical effects into a comprehensive disruption model and analyzing their impact, we present a representative ITER DT H-mode SPI scenario which provides a theoretically viable route to tolerable RE currents in ITER fusion power operation.

physics.plasm-ph↗

arXiv2025

Experimental and numerical investigation of suprathermal electron dynamics using vertical electron cyclotron emission

The Tokamak \`a Configuration Variable (TCV) is equipped with an advanced set of diagnostics for studying suprathermal electron dynamics. Among these, the vertical electron cyclotron emission (VECE) diagnostic offers valuable insights into the electron energy distribution by measuring electron cyclotron emission (ECE) along a vertical line-of-sight. However, reconstructing the electron distribution from ECE measurements is inherently challenging due to harmonic overlap and thermal radiation noise. A more practical approach leverages forward modeling of ECE based on kinetic simulations. To this end, we introduce YODA, a novel synthetic ECE diagnostic framework that simulates emission and (re)absorption of electron cyclotron radiation for arbitrary electron distributions and antenna geometries. The framework is validated against the well-established synthetic ECE code SPECE, using an ohmic TCV discharge as a reference case. In this study, the 3D bounce-averaged Fokker-Planck code LUKE is used to model electron distributions in two electron cyclotron current drive (ECCD) experiments. The synthetic spectra generated using the combined LUKE-YODA framework successfully reproduce the main features of the experimental VECE measurements in both simulated discharges. The combination of kinetic and synthetic ECE simulations allow the identification of the features in the electron distribution function which give rise to certain signatures in the VECE signal.

physics.plasm-ph↗

arXiv2025

Simulation of Shattered Pellet Injections with Plasmoid Drifts in ASDEX Upgrade and ITER

Pellet injection is an important means to fuel and control discharges and mitigate disruptions in reactor-scale fusion devices. To accurately assess the efficiency of these applications, it is necessary to account for the drift of the ablated material toward the low-field side. In this study, we have implemented a semi-analytical model for ablation cloud drifts in the numerical disruption modelling tool DREAM. We show that this model is capable of reproducing the density evolution in shattered pellet injection (SPI) experiments in ASDEX Upgrade, for model parameters within the expected range. The model is then used to investigate the prospects for disruption mitigation by staggered SPIs in 15 MA DT H-mode ITER scenarios. We find that the drifts may decrease the assimilation of pure deuterium SPIs by about an order of magnitude, which may be important to consider when designing the disruption mitigation scheme in ITER. The ITER scenarios studied here generally result in similar multi-MA runaway electron (RE) currents, regardless of the drift assumptions, but the effect of the drift is larger in situations with a fast and early thermal quench. The RE current may also be more strongly affected by the drift losses when accounting for RE losses caused by the vertical plasma motion.

physics.plasm-ph↗

arXiv2024

Cross-Calibration and First Vertical ECE Measurement of Electron Energy Distribution in the TCV Tokamak

This paper describes the first Vertical Electron Cyclotron Emission (V-ECE) measurement of non-thermal electron distributions in the \textit{Tokamak \`a Configuration Variable}, TCV. These measurements were conducted in runaway electron scenarios and in the presence of Electron Cyclotron Current Drive. Measured intensities of linearly polarised X- and O-mode radiation from fast electrons allow the analysis of the energy distribution. The measurements were made possible through the creation of an operational regime for the diagnostic that is free of thermal background radiation, in relaxed electron density operations. This operational regime notably enables the cross-calibration of the diagnostic system, relying on thermal plasma measurements and modeling with the ray-tracing code SPECE.

physics.plasm-ph↗

arXiv2024

Expulsion of runaway electrons using ECRH in the TCV tokamak

Runaway electrons (REs) are a concern for tokamak fusion reactors from discharge startup to termination. A sudden localized loss of a multi-megaampere RE beam can inflict severe damage to the first wall. Should a disruption occur, the existence of a RE seed may play a significant role in the formation of a RE beam and the magnitude of its current. The application of central electron cyclotron resonance heating (ECRH) in the Tokamak \`a Configuration Variable (TCV) reduces an existing RE seed population by up to three orders of magnitude within only a few hundred milliseconds. Applying ECRH before a disruption can also prevent the formation of a post-disruption RE beam in TCV where it would otherwise be expected. The RE expulsion rate and consequent RE current reduction are found to increase with applied ECRH power. Whereas central ECRH is effective in expelling REs, off-axis ECRH has a comparatively limited effect. A simple 0-D model for the evolution of the RE population is presented that explains the effective ECRH-induced RE expulsion results from the combined effects of increased electron temperature and enhanced RE transport.

physics.plasm-ph↗