Recent Publications

Real-time multi-harmonic signal processing system based on FPGA+ARM architecture for MSE diagnostic on the EAST tokamak

Dongmei Liu, Biao Xiang, Jia Fu, Zhen Cheng, Ziyi Chang, Qian Liu

Overview of the helios design: A practical planar coil stellarator fusion power plant

C.P.S. Swanson, S.T.A. Kumar, D.W. Dudt, E.R. Flom, W.B. Kalb, T.G. Kruger, M.F. Martin, J.R. Olatunji, S. Pasmann, L.Z. Tang, et al.

FEM-based plasma equilibrium reconstruction for transient and off-normal phases in KSTAR

Jeongwon Lee, Jayhyun Kim, Jun-Gyo Bak, Sang-hee Hahn, Heung-Su Kim, Chanyoung Lee, Jeongyeon Nam, Hyunsun Han, YongUn Nam

Comparing HSX Reflectometer Measurements with a Full-Wave Synthetic Diagnostic

Henrique Oliveira Miller Hillebrecht, Michael Gerard, Michael Richardson, Gavin W Held, Gavin McCabe Weir, Benjamin J Faber, M J Pueschel, Xiang Han, Benedikt Geiger

A synthetic reflectometer diagnostic has been developed to model the reflectometer used at the Helically Symmetric Experiment (HSX) stellarator. The main purpose of the HSX reflectometer is to measure density fluctuations in order to study turbulence and coherent modes in HSX. A full-wave 2-D synthetic diagnostic modeling the HSX reflectometer has been used to investigate its radial localization, characterize its sensitivities, and relate gyrokinetic simulations to physical measurements. The synthetic diagnostic results show that signal localization peaks slightly outboard of the cutoff surface, with the main localization lobe spanning between 15% and 30% of the minor radius. The poloidal and radial wavenumber sensitivities are k θ < 1.7 cm -1 and k r < 6 cm -1 , respectively, with minimal variation in k r sensitivity across probing regions and density profiles. Conversely, k θ sensitivity increases to > 2 cm -1 in the core of the device, in accordance with an increase in local flux-surface curvature. Synthetic reflectometer signals scale linearly with density fluctuation amplitude over a large range of fluctuation levels. Multiple density fluctuation level estimators are tested to identify the optimal choice for use in HSX, with an equation from G.D. Conway Plasma Phys. Control. Fusion 1999 consistently performing best overall. Synthetic measurements of rescaled density fluctuations from a gyrokinetic simulation are then compared against experimental measurements, with coherent mode activity at 13 -15 kHz and 22 kHz observed in both experimental and synthetic frequency spectra. Synthetic spectra overestimated experimental spectra at low frequencies and underestimated at high frequency.

Coupling time-dependent equilibrium evolution to 1D current diffusion equation towards integrated modeling of stellarators

Emiliano Fable, Fabian Solfronk, Elisa Buglione-Ceresa, David Kulla

It is presented a framework in which time-dependent simulations of a 3D toroidal magnetized plasma (stellarator device) are self-consistently taking into account the variation of the magnetic equilibrium together with the plasma current and a background varying pressure inside a given plasma boundary shape (prescribed boundary simulations). The key result is to solve the issue of prescribing the degree of freedom present when performing prescribed boundary simulations with a 3D equilibrium solver, that is the anchor vacuum field not usually considered in this kind of framework.

Uncertainties of magnetic equilibrium reconstructions

Rainer Fischer, Michael G Dunne, Joerg Hobirk, Tilmann Lunt, Wolfgang Suttrop

Uncertainties of quantities of magnetic equilibrium reconstructions are essential for the validation and quantification of estimated plasma parameters and their uncertainties. The uncertainties of a few equilibrium quantities can be evaluated analytically. A Monte-Carlo method is proposed to estimate the uncertainty of any equilibrium quantity. The Monte-Carlo method was verified using analytic formulas for the uncertainty of some equilibrium quantities and applied to various scalar, profile, flux-surface averaged and integral plasma quantities. Additionally, the uncertainty of the category of negative triangularity plasmas and alternative divertor configuration plasmas were evaluated.

Identifying non-performing or Dud Plasmas for Burning Plasma Control: Insights from JET and TFTR Deuterium-Tritium Campaigns

Lidia Piron, Nicolò Ferron, Eric Fredrickson, Morten Lennholm, Alessandro Pau, Timo Ravensbergen, Olivier Sauter, Fulvio Auriemma, Matteo Baruzzo, Krassimir K. K Kirov, et al.

Among the burning plasma controllers for research fusion reactors, the dud detector will be of primary importance as it determines whether the plasma is performing well or if it is a dud. In the latter case, the discharge needs to be terminated to remain within tritium and neutron activation limits. To this scope, monitors which track the plasma performance will be integrated in the plasma control system. In this work, we present a novel dud detector that has been empirically identified based on Deuterium-Tritium campaigns carried out at JET and TFTR. This controller estimates a proxy of the neutron rate using a combination of the diamagnetic energy and the density peakedness. If the predicted neutron rate deviates from the real-time measurement, then the dud detector will trigger an alarm leading to a safe plasma termination if plasma recovery is not expected or, to actuator requests when adjustment is possible. This monitoring function can also be coupled with equilibrium solver and control-oriented models, such as RAPDENS, as proposed in the 15 MA plasma current, 5.3 T toroidal magnetic field baseline Deuterium-Tritium ITER scenario.

Change of the Tungsten neoclassical particle transport characteristic due to the coexisting of the lower-Z impurity in the tokamak plasmas

Chengkang Pan

The high-Z impurity Tungsten neoclassical particle transport in the tokamak plasmas with the coexisting of the lower-Z impurity is investigated. The lower-Z impurity could reduce or enhance the Tungsten neoclassical particle transport. The Tungsten neoclassical transport characteristic will be changed with the lower-Z impurity content large enough. The dependence of the Tungsten neoclassical transport on the bulk ion radial gradients predicted by the existing impurity neoclassical transport theory will be broken. The existing impurity transport theory will over-predict (under-predict) the Tungsten neoclassical particle transport for the bulk ion radial gradients less (larger) than the critical value. The effect of the lower-Z impurity on the high-Z impurity neoclassical transport is through the parallel friction force coupling and it should be included in the Tungsten neoclassical particle transport calculation.

Mesh-based multiphysics coupling acceleration for fusion neutronics clustering for fusion blanket applications

Jin Whan Bae, Arpan Sircar, Katarzyna Borowiec, Vittorio Badalassi, Cami Collins

Accurate modeling of particle transport within fusion blankets is essential for predicting performance metrics such as heat deposition and the tritium breeding ratio (TBR). However, high-fidelity coupling of thermal fluids from computational fluid dynamics (CFD) to neutronics simulations often incurs significant computational costs due to the complexity of surface intersection calculations in Monte Carlo codes. This paper presents an accelerated multiphysics coupling method for neutronics that utilizes hierarchical agglomerative clustering to map complex material property distributions to a neutronics model. Implemented within the Fusion Reactor Design and Assessment (FREDA) framework, the method leverages existing Python packages to automate the creation of clustered geometries for OpenMC. The approach is demonstrated on a sector model of an ARC-class tokamak with an immersion molten salt blanket, and an simple geometry with varying isotopic concentrations. Results show that the clustering method significantly reduces computational burden without compromising fidelity, providing a foundation for agile iteration of neutronics simulations involving multiple coupled material properties.

The status and design challenges of the heating and current drive systems for DTT

Gustavo Granucci, Silvio Ceccuzzi, Afra Romano, Andrea Murari, Gian Luca Ravera, Piero Agostinetti, Sofia Bertolami, Falk Braunmüller, Alessandro Bruschi, Daniele Busi, et al.

This paper reports the main design keys and the challenging issues of the Heating and Current Drive System (HCD) of the Divert Tokamak Test (DTT) facility that is under construction at ENEA site in Frascati with the aim to perform studies on the power exhaust in a flexible and easily modifiable environment. The selected HCD systems for DTT are those with the most consolidated technology and expected to be relevant for the future reactor. The status of each system is reported, both in terms of design and procurement, which are well advanced for the system required in the first phase of the DTT exploitation: electron and ion cyclotron resonant heating. The third system is neutral beam injector, based on negative ion acceleration, which will be installed in DTT in a second phase, after the first five years of operation. The full heating power will be reached with the doubling of the radiofrequency power to reach the 45 MW at plasma needed to test the divertor with a power density at reactor level.

Assessing the Projector Augmented-Wave Method for Stopping Power Calculations

Bryn Lloyd, Dirk O. Gericke, Gilles Rodway-Gant, Gianluca Gregori

The stopping power of charged particles is investigated using time-dependent density functional theory (TDDFT). Such simulations are made possible by recent advances in computational resources and numerical implementations of this first-principles method. In practice, DFT simulations widely employ the projector augmented-wave (PAW) method to approximate all-electron behaviour, but the implications of the PAW approximation for non-adiabatic stopping simulations remain insufficiently explored. Here, the suitability of the PAW method for stopping power simulations is evaluated. A workflow for generating and selecting PAW datasets tailored to these simulations is developed, enabling systematic optimisation of augmentation radii and projector constructions. The approach is applied to proton stopping in FCC aluminium, demonstrating how dataset design influences stopping predictions, and enabling an investigation of crystal channelling effects on charged-particle transport.

Proton Radiography with Isotrajectory Optics

Dmitry Varentsov, Maksym Miski-Oglu, Zs. Major, Paul Neumayer, Martin Schanz, John Schmidt

We consider a compact isotrajectory-optics scheme for proton radiography. The calculation uses a 30-60 MeV proton interval as a reference example, typical of high-energy short-pulse laser acceleration in the TNSA regime. The idea is to use synchronized time-dependent fields such that, for one selected energy-arrival-time branch, the proton energy changes mainly the arrival time and not the transverse image position. The reference lattice consists of four pulsed electric quadrupoles with horizontal sign pattern + - + -, magnification Mx = My = -3.0 at a detector plane of 2.57 m, a common Fourier plane at zF = 0.962 m, and equal outer and equal inner lens lengths. The ideal closure residual is calculated from second central moments. Fourth central moments are kept separately as a diagnostic of non-Gaussian tails. The quoted residual widths are numerical closure tests of the ideal map, not experimental resolution predictions. Real electrode fields, waveform errors, scattering, detector blur, and space charge define the next level of the problem.

Microcoulomb-level electron beam and multi-Joule hard X-rays driven by a high-efficiency laser-plasma accelerator

B. Mahieu, L. Ribotte, W. Cayzac, G. Boutoux, R. Parreault, J. Gastineau, E. Lamoine, F. Audo, R. Babjak, D. Batani, et al.

We report on the production of ultrahigh-charge relativistic electron beams and the development of a laser-wakefield acceleration platform at the LMJ facility. Making use of the kilojoule-class, sub-picosecond PETAL laser pulse focused onto a supersonic helium gas jet, electron beams carrying a total charge beyond 1 $μ$C were generated, with energies up to $\sim$500 MeV. Given the ps-scale laser pulse duration, an on-target intensity approaching $10^{19}~\mathrm{W/cm^2}$, and a plasma density reaching 2% of the critical density, electron energisation arises from a combination of self-modulated laser wakefield acceleration (SMLWFA) and direct laser acceleration (DLA). The resulting electron spectrum exhibits a Maxwellian-like distribution, characteristic of this mixed SMLWFA/DLA regime. The total energy carried by the electron beam is estimated to be up to 17 J, within a sub-ps duration. A broadband Joule-level photon beam was also produced by Bremsstrahlung, demonstrating the potential for future applications. Experimental results are supported by start-to-end numerical simulations, including 3-D particle-in-cell and Monte-Carlo particle transport calculations. These findings pave the way for applications requiring high-charge electron beams, including the generation of high-power secondary radiation or particle sources. The use of these beams to probe matter in high-energy density states driven by the nanosecond-duration LMJ beams represents another promising avenue.

Data-Driven Reconstruction of Spatially Resolved Electron and Ion Energy Distributions from Macroscopic Plasma Quantities with Deep Neural Networks

Libin Varghese, Kaushik Prajapati, Bhaskar Chaudhury

Spatially resolved EEDFs/IEDFs provide essential kinetic information about low-temperature plasmas (LTPs) and play a central role in determining transport, chemical reaction rates, and plasma surface interactions. While kinetic simulations directly resolve these distributions, experimental measurements remain challenging and are often invasive, spatially limited, or require assumptions regarding the distribution shape such as a Maxwellian. However, several macroscopic plasma observables can be measured non-invasively using advanced diagnostic techniques, providing spatially resolved information about the plasma state. An important inverse problem is therefore whether readily measurable macroscopic plasma quantities contain sufficient information to reconstruct the underlying kinetic state. In this work, we investigate this problem by learning a nonlinear mapping from spatially resolved macroscopic plasma observables to the corresponding spatially resolved EEDFs/IEDFs using a deep learning framework. Paired datasets comprising 2D macroscopic observables and spatially resolved EDFs are generated using 2D-3V PIC-MCC simulations. Three representative learning paradigms, a U-Net, a FNO, and a MeshGraphNet, are employed in this study to learn this inverse mapping. The predicted EDFs reproduce both bulk plasma and sheath characteristics with good agreement to the PIC-MCC reference data, with the FNO providing the best overall performance. Beyond conventional metrics, physics-based validation demonstrates that the reconstructed EDFs accurately recover the corresponding density and temperature, and rate coefficients. These results demonstrate that macroscopic plasma observables encode sufficient information to infer important kinetic properties in LTPs, providing a potential foundation for surrogate kinetic modeling and next-generation plasma diagnostics.

Modelling of Flowing Plasma in the Magnetic Field of the Small Volume Plasma System Experiment

Subhasish Bag, Ashish Ganguli, Vikrant Saxena, Ramesh Narayanan, Debaprasad Sahu

A flow model for a magnetized plasma has been developed to investigate the flow dynamics in the small volume plasma system (SVPS) experiment. The SVPS experimental conditions require the model to describe a stationary, collisional, quasineutral, axisymmetric plasma. Also, the ions are cold while the electrons are isothermal and in thermal equilibrium, obeying the Boltzmann relation. In a plasma flowing along a magnetic field, the velocity of ions along the magnetic field lines is much greater than the velocity perpendicular to the field. The latter feature permits a unique ordering of the relevant variables, when the flow equations are transformed to the magnetic coordinate system (MCS), where the coordinate axes are parallel and perpendicular to the field lines. The ordering of the flow variables in the MCS allows a further simplification of the flow equations, by permitting their splitting into set of reduced, simplified equations. The SVPS experimental data are used to provide the requisite boundary conditions for initializing and solving the reduced flow equations on a magnetic coordinate grid along the different lines of the MCS. An important aspect of the present work is the validation of the splitting scheme used to derive the simplified and reduced flow equations. This is achieved by an in-depth comparison of the predictions from the model equations with the experimental data. The obtained numerical results compare favourably with the SVPS observations and have been discussed rigorously. The model developed here provides a framework for exploring magnetized plasma dynamics in the given cylindrically symmetric magnetic field configuration and can be further extended to more complex configurations.

Compatibility of Type 304H Stainless Steel in Static Molten FLiBe for Inertial Confinement Fusion Reactors: Role of Impurities and Redox Control

Rishi Pillai, Debashish Sur, Severine Cambier, Adam Willoughby, Tracie Lowe, Youyang Zhao, Amanda Bye, Susana Reyes

Modeling of divertor heat flux limits and lithium vapor shielding in NSTX-U using UEDGE code

MD Shahinul Islam, Maxim V Umansky, Vlad Soukhanovskii

In this work, UEDGE simulations coupled self-consistently with a two-dimensional wall transport solver (Wall-Li) are used to investigate two closely related issues important for NSTX-U operation: (1) plasma-surface interactions of graphite plasma-facing components (PFCs) under increasing input power for standard and snowflake divertors and (2) lithium PFC vapor shielding and its dependence on upstream plasma conditions for a standard lower single null divertor with a 5~mm layer of lithium on the graphite tiles. Simulations with graphite target PFCs show a monotonic increase in surface temperature with increasing heat flux incident on the target plates. For a broad range of operating conditions, surface temperatures exceed the graphite sublimation temperature (>1200°C, when graphite starts massively sublimating) once peak heat fluxes exceed 7~MW/m2. This occurs despite enhanced carbon radiation. A snowflake-minus divertor configuration is analyzed to assess divertor heat handling on the secondary strike point (SP\#2) under conditions where both magnetic field-line incidence and cross-field transport are uncertain. Simulation results indicate that the heat flux to SP\#2 is dominated by enhanced CM transport rather than parallel conduction, leading to increased deposited heat flux with increasing CM strength and incidence angle. Lithium vapor shielding is investigated using a self-consistent model in which lithium sourcing depends on local plasma and surface conditions. The model demonstrates that lithium vapor shielding provides passive thermal cooling, maintaining surface temperatures below 700°C even as the core power increases. However, lithium accumulation upstream increases once surface temperatures exceed 600°C. Increasing the core density augments the drag force on impurity ions due to the parallel plasma flow in the scrape-off layer, confining lithium closer to the target and thereby extending the operational window for effective vapor shielding. Overall, these results highlight the necessity of (1) thick lithium layers to prevent strike-point depletion, (2) active cooling mechanisms (such as fast-flowing liquid lithium) to suppress excessive surface temperatures and evaporation, as lithium operation is constrained by surface temperature, and/or (3) deuterium gas puffing near the divertor to further increase SOL flow and density

On Phase Transition of ITG Turbulence in the Dimits shift

L. N. Marquant, P. Morel, Ö. D. Gürcan

The transition between turbulent and zonal flow dominated states is investigated by varying the ion temperature gradient in nonlinear gyrokinetic simulations. Independent gradient scans reveal three distinct regimes: a turbulent regime at high gradients, a zonal flow dominated regime with strongly reduced heat transport at low gradients, and an intermediate regime characterized by intermittent switching between these two states. These different regimes can be classified using an order parameter, defined as the fraction of the zonal to total free energy in the system. To assess the memory effects, the temperature gradient is first lowered gradually from high values that result in fully developed turbulence, to lower values in the Dimits shift region that form strong zonal flows, and then is slowly increased back. Once the zonal flows form, and efficiently suppress turbulence, they can persist at higher gradients, leading to an asymmetric response implying a hysteresis loop. It is observed that, in the zonal flow dominated state, the free energy is mostly condensated in the largest radial scale, with a steep slope of the $k_x$ spectrum, while in the turbulent state, it exhibits a wider spectrum with two distinct slopes.

DKEKAN: A single-parameterized KAN surrogate for Drift Kinetic Equation Toward Fast Neoclassical Toroidal Viscosity Torque Modeling in Tokamaks

Jinpeng Huang, Xingting Yan, Mingyu Zhang, Nana Bao, Zixuan Song, Yuetao Meng, Weiyong Zhou, Youwen Sun

The neoclassical toroidal viscosity (NTV) torque is a critical driver of toroidal rotation in tokamaks, profoundly influencing plasma stability and performance. Consequently, incorporating NTV effects is essential for modern integrated modeling frameworks that aim to self-consistently unify multiple physical processes. However, the high computational cost of NTV modeling precludes its self-consistent integration within such frameworks. This bottleneck arises because NTV calculation requires solving its governing equation--the drift kinetic equation (DKE)--in high-dimensional phase space. To address this issue, this study develops DKEKAN, a single-parameterized Kolmogorov-Arnold Network (SKAN) surrogate for solving DKE, to realize fast NTV modeling in tokamaks. The research process consists of the following steps: Firstly, a large dataset mapping DKE equation parameters to solutions is generated based on first-principle simulations under plasma parameters of the Experimental Advanced Superconducting Tokamak (EAST); Secondly, a surrogate model for solving DKE is developed based on the SKAN framework, which also incorporates a modular expert network design; Finally, the DKEKAN surrogate model is integrated with the NTV modeling framework to realize fast NTV calculation. With its physics-grouped expert layer and SKAN backbone, DKEKAN outperforms the tested MLP, KAN, and neural-operator baselines in overall prediction accuracy, while reducing the standalone DKE-solving time from 35.85s to 3.74s, corresponding to a speedup of approximately 9.6x, and reducing the total coupled NTVTOK runtime from 38.24s to 5.58s, corresponding to an overall speedup of approximately 6.9x. This work effectively overcomes the computational bottleneck in NTV simulations, thus supporting further integrated modeling that incorporates NTV effects.

Minimum-q induced alternation between infernal modes and EP-driven modes in advanced tokamak configurations

Shiwei Xue, Ping Zhu, Haolong Li

For an advanced tokamak configuration in the presence of energetic particles (EPs), the dominant instability is found to alternate between infernal modes and Alfvén eigenmodes with the variation of the minimum safety factor $q_{\min}$. For relatively high $q_{\min}$, the mode is identified as a reversed-shear Alfvén eigenmode (RSAE), characterized by its finite Alfvénic frequency and radial localization near the minimum of safety-factor profile. As $q_{\min}$ is further reduced, the dominant branch sequentially transitions through a low-frequency infernal-mode interval, then an energetic-particle-mode (EPM) regime, and finally another low-frequency infernal-mode interval. Increasing the EP beta fraction $β_h$ tends to destabilize the RSAE and EPM branches but to stabilize the infernal modes. Phase-space diagnostics further indicate that the destabilizing effects of EPs on the RSAE and EPM branches are mainly associated with trapped-particle drive, whereas the stabilizing effects of EPs on the infernal modes is dominated by passing particles.