Results from one- and two-dimensional particle-in-cell simulations – both of nonlinear electron plasma waves (EPWs) and of self-consistent stimulated Raman scattering (SRS) – are shown that explore effects of small magnetic fields ( omega Subscript c Baseline divided by omega Subscript p Baseline much less than 1 ω c / ω p ≪ 1 $\omega _c / \omega _p \ll 1$ ) oriented perpendicular to the direction of laser propagation on SRS across a range of laser intensities of relevance to inertial fusion energy. The magnetic field effect is strongly intensity dependent. The magnetic field raises the threshold for kinetic inflation and can suppress SRS entirely at intensities just above the unmagnetised threshold. At intermediate intensities, it reduces time-averaged reflectivity by factors of approximately two to three in one dimension. Far above threshold the suppressive effect largely disappears, and the reflectivity becomes comparable to, or modestly greater than, its unmagnetised value. Nevertheless, the instability retains distinct signatures in the presence of the magnetic field. The magnetic field promotes the detrapping of resonant electrons, enhances EPW damping, limits distribution-function flattening and the associated nonlinear frequency shift, and thereby produces a more regular recurrence of SRS bursts. Multi-dimensional effects weaken the contrast in EPW damping through transverse localisation and wavefront effects, but preserve the same qualitative intensity dependence and changes in recurrence behaviour. These results delimit the regime in which small transverse magnetic fields mitigate kinetic SRS and show that time-averaged reflectivity alone does not fully characterise their influence on the instability.
Pablo Rodriguez-Fernandez, Nathan T Howard, Jo Hall, Audrey Saltzman, Adrian Martin Sanabria, Aaron Ho, Garud Snoep, Javier Pimentel Aldaz, Christopher Holland, Marco Muraca, et al.
This paper introduces the MAESTRO workflow, that enables the coupling of the PORTALS framework [P. Rodriguez-Fernandez et al, Nucl. Fusion 2024] with external solvers for the plasma equilibrium, pedestal physics, divertor constraints and heating. The surrogate-based optimization nature of the transport solver is ideally suited for external coupling, allowing efficient steady-state predictions of plasma profiles with full physics models. Improvements in the surrogate modeling of quasilinear transport models with PORTALS are presented, which enable the efficient handling of discontinuities in the transport fluxes that can arise from numerical issues or physical instabilities with extreme stiffness. The combination of physics-informed methods and advanced numerical techniques allows the MAESTRO workflow to provide accurate and efficient predictions of steady-state plasma profiles, which are critical for fusion reactor design and optimization.
Experimental inferences of cross-field particle flux at the separatrix, Γ ⊥ sep , show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in Γ ⊥ sep correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. Γ ⊥ sep grows as the L-mode density limit and the H-L-mode back transition boundaries are approached, consistent with expectations of plasma instability-driven turbulence suggested by theory, confirming the power dependence of density limits. Γ ⊥ sep is well-organized by the characteristic wavenumber for resistive ballooning mode turbulence, k RBM , from interchange-drift-Alfvén fluid turbulence theory, with additional dependence on the cylindrical safety factor, q cyl , yielding an empirical limit to plasma operation of k RBM 2 q cyl = 1. This limit corresponds to the point where the perpendicular heat flux, Q ⊥ , reaches the level of the parallel heat flux, Q ∥ , i.e. Q ⊥ ≈ Q ∥ , beyond which point thermal equilibrium is not satisfied, resulting in a fold catastrophe.
The linear Breit-Wheeler (LBW) process ($γ+γ\rightarrow e^{-}+e^{+}$) is a fundamental prediction of quantum electrodynamics, but yet to be observed under laboratory conditions using real photons. In recent years, a few experimental schemes utilizing high-intense ($\sim10^{22}$W/cm$^2$) laser-plasma interactions to observe the LBW process have been proposed. However, a high level of signal-to-noise-ratio are expected in these schemes, hindering the first-ever experimental detection of the LBW process by real photons. In this paper, we present a simple experimental setup which could enhance the expected positron signals by 2-3 orders of magnitude compared to previously proposed schemes, reaching the level of $10^{6}$MeV$^{-1}$str$^{-1}$. Moreover, such high positron signal is achieve in the direction opposite to the laser propagation, where a significantly quieter background is expected compared to the previously focused direction of laser propagation. The key to achieve this result is a newly discovered self-organized positron reorienting and pinching mechanism, enabled by the in-situ strong plasma fields from the laser-plasma interaction.
Using an approximate quasilinear formulation of gyrokinetic theory, we derive a simple formula for the impurity particle transport in the limit of large charge za and mass ma. When za and ma are large and comparable, the flux decomposes into four separate contributions from curvature drift, parallel motion, density, and temperature gradients. In the extreme limit that ma≫za2, the flux obeys a simple 2-term scaling law proportional to za/ma. The results are limited to electrostatic gyrokinetic theory and are confirmed by direct nonlinear gyrokinetic simulations. The new findings contrast with historical emphases on charge-number scalings alone and have direct implications for tungsten transport in fusion plasmas. In particular, they indicate that most tungsten charge states, which share the same mass but differ in ionization, experience a mass-dependent suppression of impurity particle transport arising from finite-Larmor-radius effects in ion-scale turbulence.
Alessandro Marinoni, Colin Chrystal, Stefano Coda, Reinart Coosemans, Claudio Marini, Mario Podesta, Olivier Sauter, Matteo Agostini, Max E Austin, Emily A Belli, et al.
Similarity experiments were performed on the DIII-D and TCV tokamaks to explore the scaling of energy confinement in negative triangularity plasmas using non-dimensional variables. Near up-down symmetric plasmas with large top-bottom averaged negative triangularity were created in a lower single null configuration, with the shape of the separatrix being closely matched between the two devices. The normalized energy confinement is found to weakly improve at increasing collisionality and, between the two devices, shows a machine size scaling behavior between Bohm and gyro-Bohm. Engineering scaling on a large DIII-D dataset is in agreement with the non-dimensional experiment.
bstract This paper investigates how edge turbulence spreading and blob transport broaden the heat flux width in Ohmic-plasma approaching the operational density limit of the J-TEXT tokamak. At the plasma edge, E_r×B shear flow collapses while turbulent transport and spreading are significantly enhanced when approaching the density limit. The heat flux widths correlate positively with edge radial flux of turbulence internal energy through the LCFS as well as the energy production ratio (the ratio of turbulence spreading from the edge into the SOL to the net local production of turbulence in the SOL). The energy production ratio model combining experimental data shows that turbulence spreading at the LCFS is likely the origin of the SOL turbulence. The mechanism of the heat flux width broadening in high density operation may be the stronger edge turbulence spreading across the LCFS to increase the SOL turbulence as the plasma approaches the density limit. How blob transport (turbulent particle flux and/or radial flux of turbulence internal energy) broadens the heat flux width is investigated in detail. The blob-induced turbulent particle flux fraction (Γ_blob⁄Γ_total ) is 0.2–0.4 while blob-induced turbulence spreading fraction (〖Sp〗_blob/〖Sp〗_total) is 0.5–0.9, suggesting that blob-induced spreading is more important than blob-induced turbulent particle flux. Blobs with larger radial scales induce stronger edge spreading into the SOL, thus dominating the SOL turbulence. These results suggest that edge turbulence spreading and blob transport play crucial roles in broadening the heat flux width as the plasma approaches the density limit.
Warm dense matter remains a challenging regime to characterize experimentally and to model with predictive accuracy. Recent experimental platforms have been developed to generate, characterize, and diagnose uniform warm dense matter, enabling detailed comparisons with models. Here, we present experiments conducted at the OMEGA laser facility that compress and heat a buried layer target to warm dense matter conditions, where the targets are heated to temperatures of approximately 20 eV and compressed to densities of 25 g/cm^3. We probe the warm dense plasma using x-ray absorption spectroscopy, using the K-edge and bound-bound absorption features to constrain the temperature and charge state distribution of the plasma. We compare these measurements with two types of models: collisional-radiative models with detailed electronic structure and ad-hoc density effects, and a multi-ion model based on density functional theory in combination with excited-state projector augmented-wave potentials. Neither approach fully reproduces the observed data, We show that the broad structure and position of the K-edge region can be modeled using density functional theory in combination with excited-state projector augmented-wave potentials. The density functional theory results are contrasted with a collisional-radiative model approach that incorporates ad-hoc density effects, which show incomplete agreement with the experimental observations, highlighting a need for improved density-dependent atomic modeling in warm dense plasmas.
A database of 192 L-H transitions in DIII-D is used to assess the effects of applied three-dimensional (3D) magnetic fields on the H-mode power threshold and the stability of zero-dimensional (0-D) empirical regressions. The dataset includes nominally axisymmetric discharges and discharges with resonant or non-resonant magnetic perturbations. Filtering criteria reduce uncertainties associated with absorbed power, neutral-beam modulation, and fast-ion losses, while applied-field components are quantified using equilibrium reconstruction and spectral analysis. Measured threshold powers show substantial scatter and systematic deviations from the 2008 ITPA multi-machine scaling, including for discharges without applied perturbations. TRANSP modeling indicates that empirical estimates can substantially underpredict fast-ion losses, particularly at low plasma current and with non-axisymmetric fields. Adding global 3D-field metrics does not robustly isolate the effects of applied perturbations. A fully unconstrained regression retains a 70% residual root-mean-square error and produces nonphysical parameter dependencies, including a plasma surface-area exponent of 2.79. Extrapolations to ITER-relevant conditions consequently have broad confidence intervals. These results show that hidden-variable dependencies can strongly affect empirical threshold parameterizations even in a restricted single-machine dataset. Machine-specific conditions, local edge physics, and power-accounting uncertainties limit the predictive capability of purely 0-D scalings. Improved predictions for ITER and future devices will require better fast-ion-loss treatment and physics-based, edge-localized quantities.
The toroidal distribution of runaway electrons (REs) striking the centre post in DIII-D post-disruption RE final loss events is simulated utilizing the MARS-F code combined with the REORBIT module, by tracing the guiding-centre drift orbits of test REs in the presence of external (applied) plus internal (intrinsic instability) 3D fields. To better recover the experimental results, three different equilibria with different safety factor profiles are adopted. Dominant resistive magnetohydrodynamic instabilities are found to be m / n = 2/1, 3/2 and 1/1 modes (where m and n are the poloidal and toroidal mode numbers). Externally applied n = 1 resonant magnetic perturbations (RMPs), with both even and odd parity configuration, are considered in each equilibrium. Locking of MHD instability to the applied RMPs is simulated by minimizing the total perturbed magnetic energy in the plasma. The simulated toroidal locations, where most REs strike in the presence of an edge m / n = 2/1 instability and the n = 1 RMPs in even parity, are found to agree well with experiments. Modelling also captures the measured n = 2 RE loss pattern on the wall for experiments where an edge m / n = 3/2 instability is dominant instead of the m / n = 2/1 mode. The key overall findings include ( a ) the measured global RE toroidal impact pattern in experiments can be explained by a locked resistive kink instability, ( b ) the toroidal phase of the locked mode is well predicted by minimizing the total perturbed magnetic energy, and ( c ) this edge locked mode determines the toroidal phase of the peak RE wall impact.
