Recent Publications

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.

Max Planck Institute for Plasma Physics, Massachusetts Institute of Technology, Columbia University, Ecole Polytechnique Federale de Lausanne

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.

Characterization of a prototype parallel-plate 238U fission chamber with DD and DT fusion neutron sources

V. Hagenlocker, R.A. Tinguely, X. Wang, J.L. Ball, B. Buschmann, M. Gatu Johnson, R. Gocht, P. Raj

Massachusetts Institute of Technology, Commonwealth Fusion Systems, Coastal Carolina University

Sheared poloidal plasma flows in the island divertor scrape-off layer of Wendelstein 7-X

Sean Bozkurt Ballinger, Seung Gyou Baek, Olaf Grulke, Carsten Killer, Floris Scharmer, Jim L Terry, Adrian von Stechow

Massachusetts Institute of Technology, Max-Planck-Institut fur Plasmaphysik, Technical University of Denmark

Gas puff imaging (GPI) has been routinely operated since the installation of the water-cooled high heat flux divertor in the Wendelstein 7-X (W7-X) stellarator. GPI provides the first systematic, two-dimensional experimental characterization of perpendicular plasma flows in the magnetic island divertor scrape-off layer (SOL) on W7-X. Using spatiotemporal analysis of turbulent fluctuations, we find that, across magnetic configurations and plasma scenarios, poloidal flows ubiquitously dominate over radial flows, with typical poloidal velocities of order km/s. In contrast to tokamaks, radially propagating blob filaments are not observed, consistent with the predominantly normal fluctuation statistics measured in the island SOL. Net radial motion is detected only in the vicinity of an X-point of the magnetic island, which is accessed in a low-iota magnetic configuration by the present GPI field-of-view. In general, the poloidal flow profiles in the island SOL exhibit a complex structure, with multiple counter-propagating flow channels as narrow as 1 cm. The experimental parameters that order the observed poloidal flow structure near island O-points are investigated. Both the magnitude of the poloidal velocity and the number of shear layers, as well as their radial location, depend sensitively on the size and position of the magnetic islands, as well as on the toroidal current, plasma density, and toroidal field direction. These results demonstrate that cross-field transport in the W7-X island divertor SOL is correlated primarily with topology-driven ExB sheared flows, rather than by radial filamentary transport, and provide the first experimental documentation of the complex, shear-layer-dominated flow dynamics in the island divertor over a wide range of plasma conditions.

Gyrokinetic investigations of an I-mode pedestal on Alcator C-Mod

Michael T Kotschenreuther, David R Hatch, Xing Liu, Swadesh M Mahajan, Jerry W Hughes, Amanda E Hubbard

University of Texas at Austin, Exofusion, Massachusetts Institute of Technology

The I-modes are an excellent laboratory for investigating the role of drift micro-instabilities in pedestals since I-mode pedestals are not controlled by Magneto-HydroDynamic (MHD) instabilities. We present here a study based on gyrokinetic simulations (using GENE) to model fluctuations and heat transport in the I-mode pedestals in C-Mod. We find the Weakly Coherent Mode observed on C-Mod I-mode to be an electrostatic Ion Temperature Gradient/Impurity density gradient (ITG/Impurity) driven mode. {\color{red} The ITG/Impurity mode frequency matches the experimentally observed frequencies. Moreover, targeted gyrokinetic simulations reproduce the impurity confinement time observed on the I-mode.} Nonlinear ETG simulations can match experimental heat flux with profile adjustment well within experimental error bars. Simulations, varying impurity level ($Z_{\rm eff}$) and temperature and density profiles (within experimental error bars), are used to probe the sensitivity of fluctuations and transport.

Pre-conceptual design of compact-fusion prototypical neutron source target for optimized fusion materials testing

Jiankai Yu, Nesrin Cetiner, Pei Biorn-Hansen, Brian D Wirth, Jaime Marian, Ethan Peterson, Daniel Winklehner, Lance Snead

Massachusetts Institute of Technology, University of Tennessee, UCLA, Stony Brook University

This work describes a design study of a cyclotron-driven Fusion Prototypic Neutron Source (FPNS) target optimized to meet US specifications for a fusion materials irradiation target. The analysis supports the use of multiple compact deuterium cyclotrons beams at a total current under 40 mA in the range of 35 MeV to 60 MeV converging on flowing annulus of lithium surrounding the sample target. The sample target region is modeled as miniaturized tensile and Charpy bend-bar specimens located within the central region of the annulus. A specific focus of this work is to provide a highly uniform flux trap of fusion-relevant neutrons maximizing the average neutron damage per unit of beam current. This design achieves the desired sample volume (>50 cm3) and damage (>10 dpa/yr) metrics with damage gradient (<40%) within the sample target region. The proposed design is modeled using the neutronic and transmutation analysis codes, PHITS [1] and FISPACT [2], with the assistance of a customized coupling computation platform. Sensitivity analyses are presented investigating the impact of the deuteron beam (height and energy), dimension of the various system elements , and the chosen nuclear data library (deuteron library, neutron library, Damage library) on the ultimate figure of merits including damage dose rate (DPA), gas production. Additionally, a complete transmutation calculation has been carried out on relevant fusion materials: Eurofer97, Silicon Carbide, Alloy 316, V-4Cr-4Ti, and pure tungsten, to ensure relevance of this C-FPNS elemental transmutation spectrum to that of a fusion reactor, especially for high deuterium energies. Implications to materials response of using deuterium energies in the 35-60 MeV range as compared to self-same materials exposed to a true fusion reactor neutron spectrum are presented.

Neutron detector calibration for the KATANA facility: Experimental and computational validation of water activation neutron emission

Julijan Peric, Domen Kotnik, Domen Govekar, Luka Snoj, Zdenek Vykydal, Marco De Pietri, Vladimir Radulović

”Jožef Stefan” Institute, University of Ljubljana, Czech Metrology Institute, MIT

Strong gradient neoclassical transport in the plateau regime

Silvia Trinczek, Felix I. Parra, Peter J. Catto, Iván Calvo

Princeton Plasma Physics Laboratory, Massachusetts Institute of Technology, CIEMAT

Strong gradient regions in tokamaks such as the pedestal or internal transport barriers are regions of reduced turbulence where neoclassical transport can play a dominant role. In pedestals, gradient lengths comparable to the ion poloidal gyroradius have been measured. Standard neoclassical theory can miss important strong gradient effects in these regions because it assumes that the gradient length scales of density, temperature and potential are larger than the ion poloidal gyroradius. We extend plateau regime neoclassical theory into regions of gradients of the order of the ion poloidal gyroradius to capture strong gradient effects on transport processes in the pedestal and internal transport barriers. The fundamental idea behind our new framework is to keep a scale separation between the orbit widths and the gradient length scales by performing a large aspect ratio expansion. In the plateau regime, strong gradients cause poloidal variation that is in–out as well as up–down asymmetric. We study two different test cases assuming either radial force balance or the absence of turbulence and show that strong gradient effects can enhance or reduce standard neoclassical theory predictions in the plateau regime in strong gradient regions.

Criteria for the economic viability of fusion power plants

D. G. Whyte, A. Lo, R. Bielajew, M. Hancock, R. Moeykens, G. Shaw

Rutherford Energy Ventures, Massachusetts Institute of Technology, Santa Fe Institute

Predictions of high field side lower hybrid current drive in positive and negative triangularity DIII-D-class and ARC-class plasmas

Grant Rutherford, Paul T Bonoli, Collin Dunn, Ethan Peterson, Stephen J Wukitch

Massachusetts Institute of Technology

Negative triangularity (NT) offers a potential solution to the high heat fluxes incident on the divertor of reactor-class tokamaks by blocking access to H-mode. An important step in assessing how NT may scale to a reactor is determining the behavior of existing current drive schemes in NT. Lower hybrid current drive (LHCD) is an attractive technology due to its high efficiency. The effect of triangularity on high field side (HFS) LHCD was evaluated with the ray-tracing/Fokker-Planck codes GENRAY/CQL3D using positive triangularity (PT) and NT equilibria with otherwise identical shaping parameters generated by the Grad-Shafranov solver TokaMaker. Two classes of devices were considered: DIII-D-class and ARC-class. Triangularity was found to significantly affect LHCD physics, with strong absorption being achieved for a wider variety of launcher configurations in NT than PT. Larger driven currents were predicted in PT than NT for some DIII-D-class cases and the majority of the ARC-class cases. The LH current density profiles were more narrow on average in NT than PT. Finally, the neutron flux incident on the launcher was considered for the ARC-class plasmas. Overall, HFS LHCD is expected to function well in NT.

Integrated modelling simulations of Ohmic and L-mode JET discharges in H, D and T using JETTO-TGLF

Harry George Dudding, Francis J Casson, David Dickinson, Colin M Roach, Bhavin S Patel, Tom W Bache, Ephrem Delabie, Costanza F Maggi, Pablo Rodriguez-Fernandez, Maria Filomena Ferreira Nave

UKAEA, University of York, Oak Ridge National Laboratory, MIT Plasma Science and Fusion Center, Universidade de Lisboa

The ability to capture the isotope mass scaling of core confinement seen in experiment is validated with JETTO-TGLF for low power JET-ILW discharges across H, D and T. The cases analysed include Ohmic discharges spanning the linear and saturated Ohmic confinement regimes as well as a trio of L-modes. The TGLF saturation rules SAT1-SAT3 are seen to predict a similar isotope scaling across both the ITG- and TEM-dominated discharges simulated, despite for the latter case the inclusion of the TEM branch of SAT3. The models demonstrate good agreement with experiment for the scaling between D and T plasmas, however a discrepancy is observed for H in the ITG-dominated discharges of higher density, as well as a systematic overprediction of the confinement time on the order of 20% in most cases. A retuned version of the SAT3 model, which was fit to better recreate fluxes close to the transport threshold, is seen to improve the magnitude of confinement predictions across all shots owing to an increased transport stiffness. This retuning was not seen to influence the confinement isotope scaling however, and possible transport mechanisms responsible for the continued discrepancy of higher density Ohmic and L-mode discharges in H are discussed.

A coherent structure transport model for scrape-off layer turbulence

Zhichen Feng, James Myra, Junyi Cheng, Calder Haubrich, Yang Chen, Xinxing Ma, Darin R. Ernst, Scott Parker

University of Colorado Boulder, Lodestar Research Corporation, General Atomics, Massachusetts Institute of Technology

Understanding the locality of high-temperature plasma energy deposition on material surfaces in fusion reactors is critical for design. Here, we utilize the gyrokinetic electromagnetic turbulence, including X-points simulation, using ions as tracer particles, together with SOLPS-ITER solutions for the background equilibrium electric field, including drifts, to model the heat flux at the divertor plate and characterize the heat load width using realistic X-point geometry. We use a theory-based blobby transport model called the “Coherent Structure Transport” (CST) model to include the effect of plasma transport in the edge scrape-off layer. The CST model is extremely fast and can be used to quickly analyze any SOLPS-ITER solution. SOLPS-ITER provides the steady state, or equilibrium, on which we superimpose blobby turbulence characterized by blob size, amplitude,e and frequency. We obtain the 1/Bp scaling of the heat load exponential decay width λq, in agreement with the Eich empirical scaling and with the Goldston heuristic theory. When including blobby turbulence in combination with the SOLPS-ITER electric field, we find a secondary peak in the heat flux radial profile, outwardly displaced from the strike point radius, with a relative amplitude that increases with the initial blob density. We describe the CST model in detail and provide initial investigations of the scaling of λq and the secondary heat flux peak with blob size and amplitude.

BABY 1L: first tritium breeding campaign results

Rémi Delaporte-Mathurin, Nikola Goles, Collin Dunn, Emily Edwards, Sara Ferry, Ross MacDonald, Ethan Peterson, Davide Pettinari, Stefano Segantin, Weiyue Zhou, et al.

Massachusetts Institute of Technology, United Kingdom Atomic Energy Authority, Politecnico di Torino

Achieving tritium self-sufficiency is a critical challenge for future fusion power plants. The BABY 1 L experiment, part of the LIBRA project at MIT, aims to benchmark tritium breeding and release in molten salt breeder systems under deuterium–tritium (DT) neutron irradiation. Building on the initial 100 ml campaign, BABY 1 L introduces a tenfold increase in breeder volume, improved thermal and gas handling systems, and enhanced neutron diagnostics, including a proton recoil telescope. We report on results from four irradiation experiments using sealed-tube DT neutron generators, with tritium collected by water bubblers measured via liquid scintillation counting. Experimentally determined tritium breeding ratios (TBRs) were compared to OpenMC neutronics simulations, showing very good agreement. The measured TBR values demonstrate a six-fold improvement over the 100 ml experiments, largely attributed to the increased solid angle and improved measurement fidelity. We also investigate tritium release dynamics and identify diffusion-limited transport as the dominant regime in the salt volume in the temperature range 630 ∘ C–750 ∘ C under an inert atmosphere. Additionally, we observe that the introduction of hydrogen in the helium carrier gas significantly accelerates tritium release, consistent with an isotopic exchange mechanism, thus transitioning out of the diffusion-limited regime. All analysis is conducted through the open-source libra-toolbox (Delaporte-Mathurin 2025 Zenodo ( https://doi.org/10.5281/zenodo.17143485 )), which streamlines simulation, data processing, and validation across experimental campaigns. These results provide critical insights into the design and operation of future liquid breeder systems and demonstrate the maturity of the BABY platform as a testbed for tritium breeding studies.

Improved Particle Confinement with Resonant Magnetic Perturbations in DIII-D Tokamak H-Mode Plasmas

N. C. Logan, Q. Hu, C. Paz-Soldan, R. Nazikian, T. Rhodes, T. Wilks, S. Munaretto, A. Bortolon, F. Laggner, F. Scotti, et al.

Princeton Plasma Physics Laboratory, General Atomics, Lawrence Livermore National Laboratory, University of California, Los Angeles, Massachusetts Institute of Technology

Experiments on the DIII-D tokamak have identified a novel regime in which applied resonant magnetic perturbations (RMPs) increase the particle confinement and overall performance. This Letter details a robust range of counter-current rotation over which RMPs cause this density pump-in effect for high confinement (H mode) plasmas. The pump in is shown to be caused by a reduction of the turbulent transport and to be correlated with a change in the sign of the induced neoclassical transport. This novel reversal of the RMP induced transport has the potential to significantly improve reactor relevant, three-dimensional magnetic confinement scenarios.