Recent Publications

Development and high-power validation of a 476 MHz twin-disk RF window for the KSTAR helicon current drive system

Jeehyun Kim, Hyunho Wi, Sonjong Wang

Precision Machining of M72 Inconel 718 Studs for ITER Vacuum Vessel Gravity Support

G.H. Kim, Y.J. Jung, M.S. Ha, H.S. Kim, Y.G. Jeong, S.G. Han, H.K. Kim

Mechanical characterization of fine-grain dispersion-strengthened tungsten as a plasma facing material

Gabriel D. Parker, Zachariah Koyn, Tim Graening, Hsin Wang, Xiang (Frank) Chen, Patrick E. Albert, Xiao-Ying Yu

Reconstruction methods for compressed sampling data in the CUP-VISAR system

Lei Zhang, Feng Wang, Zanyang Guan, Yu Long Li, Xin Yulong, Xi Wang, Miao Li, Xinru Zhang, Huaquan Gan

The temporal evolution of shock wave velocity encodes critical physical information in inertial confinement fusion (ICF) experiments. Integrating a line-imaging velocity interferometer system for any reflector (VISAR) with compressed ultrafast photography (CUP), the resulting CUP-VISAR technique enables promising high spatiotemporal resolution two-dimensional (2D) measurement of continuous shock waves. However, reconstructing time-varying 2D fringe patterns from single-shot, highly underdetermined compressed sampling remains a formidable challenge. To improve reconstruction accuracy of continuous 2D shock wave velocity fields, this work proposes two technical enhancement strategies. First, a deep unfolding network reconstruction scheme for conventional CUP: adopting a hybrid data-and-model-driven approach, it deploys a trainable deep network as a denoiser to capture inherent spatiotemporal correlations in fringe patterns, markedly improving both fringe reconstruction efficiency and velocity field accuracy. Second, a novel integral-camera-based CUP architecture: unlike the conventional coded-aperture/streak-camera configuration, it acquires signals directly via an integral camera, with fringe reconstruction implemented by mixed integer programming guided by discrete phase-space constraints and interference fringe priors, simplifying system structure and enhancing noise robustness. Both schemes effectively improve compressed fringe reconstruction quality and reduce velocity field reconstruction errors, providing more accurate and reliable data support for ICF diagnostic analysis.

Characteristics of core beta-induced Alfvén eigenmodes with shear flow in EAST NBI heating plasmas

Lizhe Guo, Jian Bao, Ming Xu, Hailin Zhao, Yingying Li, Chao Dong, Jintao Cao, Zhiyong Qiu, Wenlu Zhang, Zhihong Lin, et al.

The basic features and relationship with zonal flows of multiple branches of beta-induced Alfv'en eigenmodes (BAEs) are investigated in the EAST tokamak. Those branches of BAEs, excited by tangentially injected NBI, are located in the central region ($0.2 \leq \rho \leq 0.4$), with frequencies in the range of 50 $\leq f \leq$ 70~kHz and low toroidal mode numbers $1 \leq n \leq 5$. The excitation of BAEs is clearly characterized by three typical features: an increase in the $\omega_{EXB}$ shear locally, accompanied by simultaneous rises in both central rotation velocity and electron temperature. Radial profiles of the mode phase angle from electron cyclotron emission (ECE) diagnostics vary significantly with $n$. Global gyrokinetic simulations of core BAEs are performed in both linear and nonlinear regimes. In the linear regime, the unstable BAE branches are consistent with experiment on toroidal mode number, real frequency and the mode structure phase angle. The radial variation of BAE phase angle is primarily caused by bulk plasma kinetic effects rather than the energetic particle (EP) non-perturbative drive. In the nonlinear regime, single-$n$ BAE without zonal fields saturates at a large amplitude that triggers the non-adiabatic frequency chirping on a fast timescale of sub-millisecond, while the self-organized zonal flow beat-driven by BAE greatly reduces BAE saturation amplitude below the chirping threshold, which is in consistency with experimental observation. Moreover, it is confirmed that the criterion of zonal flow regulation on dissipative-type instability (such as AEs) becomes $\omega_{E\times B} > \omega_r$ rather than traditional $\omega_{E\times B} > \gamma_L$ for reactive-type instability. In general, the self-organized zonal flow can improve plasma confinement through regulating both BAEs and drift-wave turbulence.

Analysis of background plasma behavior under external fields in the low energy beam transport section of LIPAC

Tomonobu Itagaki, Tomoya Akagi, Kai Masuda, Nicolas Chauvin, Luca Bellan, Fabio Cismondi, Hervé Dzitko, Yann Carin

Background plasma behaviour in the low energy transport section of the LIPAc is analysed with 3-dimensional particle-in-cell simulation. The simulation showed some characteristic effect under external field elements in the LEBT: a positive biased chopper absorbed the electron plasma in the surrounding drift region. Such absorption by the chopper is limited spatially by the solenoid lenses. However, some extent of electron plasma flows through the solenoid. Analytically estimated flux of the electron leakages through solenoid is compared to the simulation results. Results of experiments still show mysterious transient motion of beam pulse shape that is not shown in the simulations, while saturated state is roughly reproduced in it. Additionally, we developed a one-dimensional electron fluid simulation to investigate effects by thermalization of the background electron plasma, but this also could not reproduce the transient motion observed in the experiment. As a secondary outcome, the fluid simulation showed qualitatively close results to that of the particle in cell simulation in saturated states with much smaller computational resource.

ELM suppression and confinement in negativetriangularity with stronger shaping in ASDEXUpgrade

Branka Vanovac, Joerg Hobirk, Andrew Oakleigh Nelson, Olivier Sauter, Mike G Dunne, Michael Faitsch, Thomas Pütterich, Rainer Fischer, Dirk Stieglitz, Erika Strumberger, et al.

Recent experiments conducted during the 2025 campaign in ASDEX Upgrade have pushed the limits of negative triangularity shaping, achieving top triangularity values of δ top ∼-0.55 while keeping bottom triangularity around 0 in diverted plasmas. Guided by ideal-MHD stability calculations using the linear MHD BALOO solver and supported by TCV results, these experiments have demonstrated a transition to an edge that is more stable against peeling-ballooning modes, leading to ELM-free plasmas. Those ELM-free plasmas also exhibit a dithering behavior similar to limit-cycle oscillations. Linear MHD modeling showed that the achieved shapes remained marginal with respect to the second stability region for ballooning modes suggesting that with minor further shaping, a robust NT edge could be achieved. The associated energy confinement is generally reduced, with clear signs of strong power degradation. In addition, a high fraction of fast-ion energy is observed, particularly in high-power/low-density discharges. An approximately 15% improvement in normalized energy confinement is obtained when seeding nitrogen. As this effect comes from the edge, it highlights a path towards pedestal tailoring for further confinement optimization and supporting negative triangularity as a potentially viable reactor- relevant operational scenario in full-metal-wall devices.

The FLARE Facility

Hantao Ji, Jongsoo Yoo, Peiyun Shi, Euichan Jung, Kush Maheshwari, Adam Robbins, Sunghyun Son, Adam Stanier, Yang Ren, Sayak Bose, et al.

The Facility for Laboratory Reconnection Experiments (FLARE) has been constructed to study magnetic reconnection in multiple X-line regimes relevant to space, astrophysical, and fusion plasmas. Building upon the successful design of the Magnetic Reconnection Experiment (MRX), FLARE features a larger physical volume, stronger magnetic fields, and an independent ohmic heating drive to significantly extend the accessible parameter space, targeting Lundquist numbers up to S ~ 10^5 and normalized system sizes up to λ~ 10^3. This paper details the facility's core engineering components, including the primary vacuum vessel, internal flux cores, highly segmented external coil systems, modular capacitor banks, and the safety interlock and control architecture. An initial diagnostic suite is presented, comprising high-resolution 2D magnetic probe arrays, triple Langmuir probes, a fully fiber-coupled interferometer, ion Doppler spectroscopy, and fast camera imaging. Initial operations demonstrate the device's experimental flexibility and reliability, successfully executing symmetric push-pull reconnection, spheromak merging, and asymmetric downstream configurations. Currently operating within "Stage 2.5" with S ~ 2,500 and λ~ 60 for anti-parallel reconnection, FLARE provides immediate access to the multiple X-line regimes. Planned hardware upgrades, advanced diagnostic additions, and integration with fully kinetic simulations will further expand its capabilities as it transitions into a collaborative user facility for the broader plasma science community.

Sub-Second Collisionless Gyrokinetic Eigenvalue Solutions via Orbit-Invariant Decomposition

Anrui Luo, Jingyi Yu, Huasheng Xie, Jian Bao

Fast analysis of microscopic drift-wave instabilities based on linear gyrokinetic simulation is desirable for modeling anomalous transport in fusion device. In this work, we present an orbit-invariant decomposition method for solving collisionless gyrokinetic eigenvalue problems. By discretizing velocity space along orbit invariants using particle energy and magnetic moment, the full eigenvalue matrix is separated into independent orbit blocks that couple with each other through the field equation, greatly reducing both matrix dimension and computational cost without sacrificing physics. Based on this method, we extend the MGK code [Phys.\ Plasmas 24, 072106 (2017)] with both CPU and GPU implementations, supporting collisionless electrostatic linear simulations in $s$--$α$ and Miller equilibrium model with kinetic. For kinetic ion temperature gradient (ITG) and trapped electron mode (TEM) eigenvalue problems, the solver reduces single-solution times to the 0.01--0.1s range---more than three orders of magnitude faster than CGYRO on the same hardware---enabling efficient large-scale parameter scans. The eigenfrequencies and mode structures are verified by comparing with CGYRO results. The method is generally adapt to to all collisionless gyrokinetic eigenvalue formulations and can be extended to fully electromagnetic simulations.

Bright dual-pulse betatron X-ray generation from a laser wakefield accelerator

Bo Guo, Yang Wan, Shuang Liu, Xiaonan Ning, Jianfei Hua, Wei Lu

Pump-probe experiments using dual ultrashort X-ray pulses provide unique opportunities for resolving non-equilibrium dynamics initiated by intense X-ray excitation. Betatron radiation from laser wakefield accelerators offers femtosecond duration, micrometer-scale source size, and intrinsic synchronization with the driving laser, making it a promising candidate for compact ultrafast X-ray sources. Here, we experimentally demonstrate a high-flux, dual-pulse betatron X-ray source based on a density-tailored gas-mixture target. Two electron bunches are generated within a single plasma wakefield through ionization-induced and shock-front-triggered injection, subsequently producing twin X-ray pulses. The measured electron spectra and dual-component X-ray angular profiles, together with particle-in-cell simulations, identify the contributions of the two electron populations to the radiation. The total X-ray photon yield reaches the level of 10^{10} photons per shot with a 40-TW laser system. These results establish a compact, single-stage route toward high-flux dual-pulse betatron sources for laboratory-scale ultrafast X-ray spectroscopy.

Tunable high-charge relativistic electron beams via direct laser acceleration in hohlraum-preheated foam targets

Ziyao Wang, Jieru Ren, Zhigang Deng, Wenqing Wei, Wei Qi, Olga N. Rosmej, Nikolay E. Andreev, Sergey Yu. Gus'kov, Rafael Yakhin, Yifang Gao, et al.

Direct laser acceleration (DLA) in near-critical-density (NCD) plasmas can efficiently generate high-charge relativistic electron beams, yet beam parameters depend critically on precise plasma state manipulation. Solid-ablation NCD plasmas evolve rapidly, posing severe controllability challenges. We produce NCD plasma via indirectly heating foam targets with ns laser driven hohlraum soft X-ray. Electrons are generated through irradiating the plasma with another picosecond laser. Tuning the laser pulse delay $τ$ enables control of plasma profiles and beam parameters. Experiments show that when the foam is heated ($τ$ = 6 ns, 9 ns), the beam exhibits $T \sim 13$ MeV effective temperature, $E_k \sim 80$ MeV cutoff energy, and hundreds of nC/sr charge for $E_k > 7.5$ MeV. These values are significantly higher than those from solid-foil ($T$ $\sim$ 2.7 MeV, $E_k$ $\sim$ 20 MeV, $Q$ $\sim$ 9 nC/sr) and cold-foam ($T$ $\sim$ 12 MeV, $E_k$ $\sim$ 50 MeV, $Q$ $\sim$ 5 nC/sr) interactions. At a longer delay of $τ$ = 15 ns, the charge increases further while the temperature decreases, and at a shorter delay of $τ$ = 3 ns, both temperature and charge are lower. 3D PIC simulations link these observations to the interplay between the microstructure of the cold foam and the evolving plasma density profile at different delay times, which together determine the beam charge, effective temperature, and divergence. The finding provides a routine to generate and tailor the relativistic electron beams, which is essential for designing laser-driven electron sources for high energy density physics and photonuclear reaction applications.

Control of Magnetic Reconnection in High Energy Density Plasmas

J. L. Latham, B. K. Russell, C. Dong, C. A. Walsh, K. G. Miller, P. T. Campbell, L. Willingale, P. Nilson, K. Krushelnick

Magnetic reconnection governs the explosive release of magnetic energy in systems from the solar corona to fusion plasmas, yet controlling it in the laboratory has remained out of reach. Here we demonstrate active control of reconnection in high-power laser-driven plasmas using a third, relativistic-intensity laser pulse that injects filaments of electron current into the reconnecting system. Two moderate-intensity lasers drive colliding magnetized plumes that reconnect, forming plasmoids in the current sheet as seen in proton deflectometry. The relativistic laser generates magnetic fields matching the polarity on either side of the layer, and, depending on its arrival time, either accelerates the breakup of the current sheet or suppresses reconnection. Arriving early, before the plumes strongly interact, it builds a pocket of magnetic pressure that repels them via flux pileup; arriving after the current sheet forms, it accelerates electrons that extend current filamentation instabilities into the upstream, causing rapid dissipation of the reconnecting magnetic field. This approach opens a route to steering magnetic energy flow in fusion plasmas and broadens the range of systems accessible to laboratory astrophysics.

Bayesian Optimization of Molybdenum-99 Production by Laser Wakefield Acceleration Using Coupled PIC and Monte Carlo Simulations

Bruno Silveira Nunes, Nilson Dias Vieira Junior, Mirko Salomón Alva Sánchez, Alexandre Bonatto, Ricardo Elgul Samad

This work applies Bayesian optimization to a loop composed of PIC simulations of laser electron acceleration and Monte Carlo (MC) simulations of bremsstrahlung-induced nuclear reactions, to maximize the production of molybdenum-99, the precursor of the most used radiopharmaceutical in nuclear medicine, metastable technetium-99. PIC and MC simulations are computationally intensive, and besides reducing the time spent, the Bayesian optimization coupling both simulations resulted in an improvement of an order of magnitude in the $^\text{99}$Mo yield over a previous work, in which the output of an optimization loop based solely on PIC simulations was used a posteriori to estimate $^{99}\mathrm{Mo}$ production through a MC simulation.

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

Hefei University of Technology, Institute of Plasma Physics, CAS

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

Korea Institute of Fusion Energy, University of Science and Technology

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

University of Wisconsin-Madison, Max Planck Institute of Plasma Physics Greifswald Branch, DIFFER

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

Max-Planck-Institut für Plasmaphysik

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

Max-Planck-Institut für Plasmaphysik

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.

Consorzio RFX, Culham Centre for Fusion Energy, United Kingdom Atomic Energy Authority, Max-Planck-Institut fuer Plasmaphysik, PPPL

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.