Kinetic simulation of laser plasma instabilities including collisional effects on sub-nanosecond timescalesLi, Lei; Weng, Suming; Ma, Hanghang; Chen, Xingyu; Pan, Kaiqiang; Cheng, Xiantao; Yan, Ji; Sheng, Zhengming
doi: 10.1088/1361-6587/ae51c0pmid: N/A
Efficient laser-target energy coupling is of critical importance in inertial confinement fusion, where collisional absorption and laser-plasma instabilities (LPIs) compete to determine overall energy deposition. However, few studies have simultaneously captured both collisional absorption and LPIs, particularly in the kinetic regime. In this work, we develop a code capable of performing unified kinetic simulations of these two kinds of competing processes on sub-nanosecond timescales. The code builds upon a previous one-dimensional particle-mesh code PM1D, which was originally designed to simulate only LPIs, by self-consistently incorporating various collisional effects. It integrates collisional absorption and collisional damping terms into the electromagnetic wave equation and the motion equations for electrons and ions, coupled with the temperature evolution equations that describe energy deposition and thermal equilibration. Numerical tests verify that the upgraded PM1D code accurately simulates collisional effects across a broad range of laser-plasma parameters while maintaining robust numerical stability and high computational efficiency. Our simulations further reveal that collisional damping can effectively suppress stimulated Brillouin scattering, while its direct impact on stimulated Raman scattering (SRS) is negligible. Notably, collisional effects not only contribute directly to collisional absorption but can also enhance the anomalous absorption driven by various LPI processes. Moreover, the increase in plasma temperature resulting from collisional absorption elevates the electron plasma wave frequency, which may manifest as an experimentally observable redshift of the SRS scattered light. This work therefore establishes a robust numerical framework for investigating the complex interplay between collisional processes and LPIs in laser-target coupling.
First field test of a novel optical gas analyser in the exhaust of Wendelstein 7-XSchlisio, G; Brindley, J; Law, M; Klepper, C C; Delabie, E; Bräuer, T; Pölöskei, P Zs; Boumendjel, Y M; Siddiki, F B T; Krychowiak, M; ,
doi: 10.1088/1361-6587/ae52b0pmid: N/A
A novel optical gas analyser, designed for isotope-resolved exhaust composition measurement, was field-tested at Wendelstein 7-X (W7-X) to validate its laboratory-proven concept under operational fusion experiment conditions. The system, Optix, comprises a cold cathode remote plasma generator and a high-resolution Fabry–Perot spectrometer and was deployed in the exhaust line of W7-X during the OP2.3 campaign. The injection of 3He and 4He for minority ion-cyclotron heating provided a test case for helium isotope discrimination. Despite limitations due to background gas and low partial pressures of the target species, isotope-resolved spectral signatures were successfully observed, demonstrating the fundamental viability of the Optix approach. Additionally, the spectrometer was evaluated for plasma emission measurements from both core and edge sightlines. While helium line emission was detectable, interpretation was hindered by complex background signals, highlighting the benefits of controlled remote plasma generators for spectroscopy. This first deployment provides critical insight into pressure requirements, spectral resolution, and operational constraints, informing future applications of optical exhaust diagnostics in fusion devices.
Diagnostic x-ray source using electrons produced by a 100 J-class picosecond laser*Sinclair, Mitchell; Pagano, Isabella; Lemos, Nuno; Arrowsmith, Charles D; Shaw, Jessica L; Miller, Kyle G; King, Paul M; Aghedo, Adeola; Marsh, Kenneth A; Gregori, Gianluca; Albert, Félicie; Joshi, Chan
doi: 10.1088/1361-6587/ae41e4pmid: N/A
Many laser-based high-energy-density science (HEDS) facilities have one or more short-pulse (sub- to few-picosecond) laser beams for diagnostics. For the past decade, we have been developing a novel x-ray probing capability using such picosecond lasers interacting with an underdense plasma to produce relativistic electrons. The ultimate goal of these experiments is to demonstrate a new type of x-ray backlighter using the short-pulse ARC laser at the National Ignition Facility (NIF). Before this diagnostic is fielded at the NIF, it is critical to demonstrate the viability and reproducibility of the x-ray source on comparable high-power short-pulse laser systems. We present experiments that were carried out with the OMEGA EP laser at the University of Rochester’s laboratory for laser energetics. In these experiments, high-energy electrons are produced through a combination of the self-modulation instability and direct laser acceleration in an underdense gas jet. These electrons generate directional x-rays with characteristic energies up to several tens of keV as they execute betatron motion in the ion channel, and the inverse Compton scattering process generates even harder x-rays, with characteristic photon energies of 60–240 keV. When implemented on the OMEGA EP laser(s), this x-ray source yields results that are comparable to those obtained recently on the short-pulse Titan laser at the Jupiter Laser Facility at Lawrence Livermore National Laboratory, after accounting for differences in laser energy, peak intensity, focusing f/#, and plasma source. Applications of such an x-ray source for HEDS experiments are discussed.
Accelerating the GENeuSIS design phase through machine learning: a neutron test bed facility for ITERDamiano, M; Rossi, R; Colangeli, A; Flammini, D; Fonnesu, N; Gaudio, P; Lungaroni, M; Moro, F; Noce, S; Previti, A; Villari, R
doi: 10.1088/1361-6587/ae51c2pmid: N/A
In fusion reactors, large numbers of high-energy neutrons are generated, creating a harsh and demanding environment for reactor materials and components. In particular the ITER radiation environment will be characterised by harsh conditions in terms of neutron and gamma fields: consequently it is crucial to test its sensitive components, such as electronics, in dedicated facilities. To address this need, we propose the development of the GENeuSIS (General Experimental Neutron Systems Irradiation Station) project. GENeuSIS is an innovative test facility designed to assess and characterise the behaviour of diagnostics, electronics, and other ITER critical components when exposed to 14 MeV neutron irradiation from the Frascati neutron generator. The GENeuSIS assembly consists of a set of neutron-moderating materials slabs enclosing an inner cavity where the neutron and gamma spectra foreseen in specific ITER locations are reproduced. In this context, a machine learning (ML) model automatises the selection of materials required to achieve the desired neutron and photon spectra. This work focuses on the development of a supervised ML model, specifically a neural network, trained on a database generated from previous neutron transport simulations using the MCNP code. These simulations have already demonstrated the feasibility of GENeuSIS by replicating the neutron spectrum at specific ITER locations, such as the Port Interspace (GENeuSIS-I assembly) and Port Cell (GENeuSIS-II assembly). However, the design of each GENeuSIS assembly using Monte Carlo methods generally demands significant computational resources and depends on extensive ‘trial-and-error’ transport simulations, often resulting in a slow process. The proposed ML model aims to accelerate the optimisation phase of GENeuSIS assemblies by rapidly identifying promising configurations, which are subsequently validated through full Monte Carlo simulations.
A numerical investigation of stimulated Brillouin scattering driven by broadband lasers in high-Z plasmasLi, Xiaoran; Qiu, Jie; Hao, Liang; Zou, Shiyang
doi: 10.1088/1361-6587/ae46dcpmid: N/A
The evolution of stimulated Brillouin scattering (SBS) driven by broadband lasers in high-Z plasmas is investigated using one-dimensional collisional particle-in-cell simulations. The temporal incoherence of broadband lasers modulates the pump intensity, generating stochastic intensity pulses that intermittently drive SBS. The shortened coherence time weakens the three-wave coupling and continuously reduces the temporal growth rate, while the saturated reflectivity remains nearly unchanged until the bandwidth exceeds a critical threshold. Simulations with varying laser intensities and bandwidths reveal a consistent empirical scaling behavior, indicating that effective SBS suppression occurs only when the laser bandwidth exceeds the SBS growth rate of the corresponding monochromatic case by approximately two orders of magnitude under the present simulation conditions. Comparative simulations in Au and AuB plasmas exhibit similar suppression trends, with AuB showing reduced SBS growth rate and reflectivity, and the onset of suppression occurring at a lower bandwidth. These findings elucidate the coupled dependence of SBS mitigation on laser bandwidth and intensity in high-Z plasmas, and may serve as a useful reference for evaluating broadband mitigation strategies in inertial confinement fusion.
Surrogate modeling of capillary-discharge on-axis density profiles for laser guiding: forward prediction and inverse designSciuto, A; Arjmand, S; Mauro, G S; Cirrone, G A P
doi: 10.1088/1361-6587/ae49fapmid: N/A
Laser wakefield acceleration (LWFA) are promising sources of high-energy electron beams, but their performance is highly sensitive to the plasma target properties, making optimization challenging. In this work we present and validate the first module of a reverse-design workflow: a neural-network surrogate of a capillary-discharge plasma source trained on COMSOL Multiphysics hydrodynamic simulations. The model predicts the on-axis electron density profile along the capillary from the discharge control parameters (gas pressure and applied voltage) for a fixed capillary geometry. We assess generalization on held-out data and demonstrate a differentiable, gradient-based inverse design procedure that retrieves pressure and voltage values consistent with target density profiles within the surrogate model. Extensions to an LWFA-stage surrogate and downstream coupling to particle-transport simulations (e.g. via ONNX) are outlined as future work.
Numerical study of plasma shape on stability of ideal quasi-interchange mode in weakly magnetic shear tokamaksLi, R; Xu, H; Ma, J; Guo, W
doi: 10.1088/1361-6587/ae4f21pmid: N/A
In this paper, the stability of n=1 and high-n quasi-interchange mode (QI mode) are numerically investigated in shaped tokamak plasmas under the weakly magnetic shear (both low magnetic shear and weakly reversed magnetic shear) configurations, where n is the toroidal mode number. The results reveal that the inverse aspect ratio (ϵ=a/R0, where a is the minor radius and R0 is the large radius) generally destabilizes the n=1 QI mode but stabilizes high-n modes, irrespective of the shear configuration. This is attributed to the competition between the stabilization from the weakened ballooning drive α and the intrinsic destabilization from the more compact geometry. Elongation (κ) and triangularity (δ) individually destabilize QI mode regardless of magnetic shear configuration, whereas their combined effect can stabilize QI mode in positive triangularity side and destabilize QI mode in negative triangularity side. These complex dependencies highlight the critical need to incorporate realistic tokamak geometry when interpreting related experimental observations.