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S. Akbas, B. Bienkowska, A. Cubi, N. Hajnal, M. Cavinato, M. Di Giacomo, S. Soare, A. Belpane, M. Valisa, A. Fassina, et al.
Amir Jafari
Dynamic alignment in magnetohydrodynamic turbulence is commonly inferred from the decrease of an amplitude-weighted average of the mutual angle between Elsässer increments toward smaller separations. That decrease, however, need not imply that the increments themselves rotate toward alignment: large-angle fluctuations can simply lose more amplitude than small-angle ones. We therefore study the joint evolution of increment amplitude and mutual angle using finite-step conditional transition probabilities. In forced incompressible full MHD and in balanced strong-guide-field reduced MHD, we find that, at fixed initial angle, large-amplitude Elsässer-increment pairs undergo smaller angular changes than small-amplitude pairs, including when they begin at large angles. We call this amplitude-dependent angular persistence. Independently, normalized amplitude moments increase toward smaller separation, while stronger amplitude weighting produces progressively smaller angular averages and stronger scale dependence, linking apparent alignment to the intermittent large-amplitude tail. In RMHD, a source-state decomposition of the Politano-Pouquet third-order moment shows that initially large-angle, high-amplitude populations contribute with the sign associated with transfer toward smaller perpendicular scales despite their comparatively small angular changes. Time-resolved full-MHD data reproduce the same amplitude ordering, and the local Elsässer advective term closely tracks the conditional dependence of both angular and amplitude changes on the initial state. These results show that the conventional dynamic-alignment diagnostic reflects the joint statistical evolution of amplitude and angle and, by itself, is not evidence for a population-wide dynamical rotation toward alignment.
Nobuyuki Nakamura, Ryohko Ishikawa, Motoshi Goto
We present the linear polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ excited by electron collisions, measured at electron energies between 85 and 1000~eV. The measured polarizations are compared with theoretical values calculated using the Flexible Atomic Code (FAC). Although the overall trend that the degree of polarization decreases with increasing electron energy is reproduced, the theoretical values are systematically larger in magnitude than the experimental values. After examining possible experimental sources of depolarization, we suggest that the discrepancy likely originates from an overestimation in the theoretical model. Further theoretical and experimental investigations along the isoelectronic sequence are needed to resolve this discrepancy.
Giridharan Paramasivam, Özgür D. Gürcan
The nature of turbulent energy cascade of simple two-dimensional fluid models of ion temperature gradient driven turbulence is studied in detail. Notably, it is observed that a minimal two-field model of toroidal ITG, behaves qualitatively differently with or without the diamagnetic nonlinearity. In its absence, the zonal flows always dominate and the system never reaches a high-transport state. In contrast, when this term is included, zonal flows dominate only near marginality, while away from it, an inverse cascade with high levels of transport is observed, requiring large-scale dissipation (hypoviscosity) to saturate and hyperviscosity to regularize small scale instability associated with this nonlinear term. However introducing such a term, together with the existence of the curvature term, breaks potential vorticity conservation, which is one of the key symmetries of drift-wave turbulence. This can be remedied by considering a more complete model that retains higher-order terms in the pressure equation. This form of the model, with four nonlinearities, conserves potential vorticity and behaves similarly to the original model, requiring hyperviscosity to saturate since the added nonlinearity generates small scale instability also for the pressure equation. To characterize the roles of potential vorticity conservation and higher-order terms in the pressure equation, the behavior of both the standard ITG system, and the potential vorticity conserving system, is studied by analyzing their spectra, turbulent cascades, and sensitivity to viscosity. Finally, the direction of turbulent cascade due to the different nonlinearities (i.e. the diamagnetic nonlinearity in particular) is investigated by examining their contributions to the spectral energy transfer and by considering the triadic instability assumption for each of these nonlinearities separately.
Rui Ding, Jin Guo, Lei Mu, Guoliang Xu, Yaowei Yu, Yuming Liu, Rong Yan, Hai Xie, Dahuan Zhu, Junling Chen, et al.
Charge-exchange neutrals (CXNs), in particular of hydrogen isotopes deuterium and tritium, are expected to contribute notably to first wall erosion in future fusion reactors. To understand the CXN-induced first wall erosion under different discharge conditions in deuterium, dedicated experiments with a set of new diagnostics have been performed on EAST. Measurements of CXN energy spectrum by the low-energy neutral particle analyzer (LENPA) shows that the integrated CXN flux at the first wall positively correlated with the heating power and line-averaged electron density (n_"e" ), and increased by more than one magnitude from ohmic to high power discharges in the database. Deeper plasma fueling by supersonic molecular beam injection (SMBI) leads to a lower edge neutral pressure and thereby a ~50% lower CXN flux. The CXN flux in the intra-ELM phase is ~2 times higher than that in the inter-ELM phase. Measurements of material erosion rate by the quartz crystal microbalance (QMB) show that higher heating power can lead to stronger material erosion by CXNs. The erosion rate increases with n_"e" at first due to the higher CXN flux and then saturates due to the lower incident energy. The 3D-GAPS code is applied to model the CXN-induced erosion based on the LENPA-measured CXN energy spectrums, which shows good agreement with post-mortem analysis of exposed samples and QMB measurements.
Agata Chomiczewska, Yevgen Kazakov, Wojciech Gromelski, Irena Ivanova-Stanik, Agnieszka Jardin, Axel Jardin, Ewa Kowalska-Strzęciwilk, Kerry Lawson, Evie Litherland-Smith, Andy Meigs, et al.
This study investigates impurity behaviour during ion cyclotron resonance heating (ICRF) experiments, focusing on the application of two different three-ion heating schemes in H-D and D-T plasmas at JET. In the D-(³He)-H scenario, the phasing of the ICRF antenna straps was varied to modify the launched parallel wave number k||, enabling a systematic study of its effect on fast-ion generation, plasma dynamics and impurity transport. The results indicate dependence of impurity behaviour on antenna phasing, particularly for the nickel (Ni) in the main plasmas and for the beryllium (Be) source. Analysis of sawtooth oscillations using the symmetrised dot pattern method reveals correlations between sawtooth frequency and crash intensity, the applied ICRF power and antenna phasing. The lowest impurity levels are obtained with +90° antenna phasing, corresponding to maximised fast-ion generation. A comparative study of impurity behaviour in D-T plasmas is also presented for the three-ion T-(⁹Be)-D and hydrogen minority heating scenarios. The three-ion scheme produces the largest increases in ion temperature Ti, while hydrogen minority heating yields higher electron temperatures Te and slightly reduced impurity levels. These results suggest that impurity behaviour in three-ion ICRF scenarios depends on the chosen heating optimisation (fast-ion generation versus ion heating) and can be further controlled through appropriate selection of ICRF antenna phasing.
Hua Yang, Wei Zhang, Lunan Liu, Pengjun Sun, tao JIN, Hui-Hui Wang, Liqing Xu, Zhengshuyan Wang, Tonghui Shi, Hailin Zhao, et al.
Neoclassical tearing modes (NTMs) in high-beta plasmas can degrade confinement and trigger disruptions. Experiments were conducted on EAST to investigate the effects of ion cyclotron range of frequency (ICRF) heating on NTMs through controlled variation of the power deposition location and fast-ion distribution. Using hydrogen minority heating, on-axis and off-axis ICRF heating scenarios were achieved by varying the toroidal magnetic field, together with additional ICRF power modulation. The results show that on-axis ICRF heating effectively suppresses the m/n = 3/2 tearing mode, while off-axis heating tends to enhance the m/n = 4/3 mode. On-axis heating also improves plasma confinement and increases the neutron yield. TROIC-TRANSP simulations confirm the distinct power deposition locations, while ASCOT calculations indicate that the fast-ion energy reaches up to 800 keV during on-axis heating, which is substantially higher than that in the off-axis cases. A modified Rutherford equation incorporating fast-ion effects suggests that the fast-ion-driven uncompensated cross-field current term is responsible for the observed NTM behaviors. These results demonstrate that controlling the ICRF resonance position is a feasible approach for NTM suppression and improved plasma performance.
Mike Kryjak, Cyd Cowley, David Moulton, Ryoko Tatsumi Osawa, Stuart Scott Henderson, Omkar Myatra, Benjamin Dudson, Peter Alec Hill, Liam Pattinson, Christopher Paul P Ridgers
Alternative Divertor Configurations (ADCs) often make use of high total flux expansion (fR) and connection length (L∥) to improve the access to and the controllability of detachment, a regime vitally important for reactor-class tokamaks such as the Spherical Tokamak for Energy Production (STEP). Achieving detachment on such high-power devices is enabled through radiation from seeded impurity species such as argon and neon, which can lead to radiative losses throughout the flux tube, altering the impact of magnetic geometry on detachment. The Detachment Location Sensitivity (DLS) model analytically predicts detachment access and sensitivity based on upstream conditions, the magnetic geometry and seeded impurity radiation. It assumes the radiation region to have no spatial extent. In this work, we relax this assumption and introduce DLS-Extended, a new reduced 1D model which can capture the effects of the spatial distribution of impurity radiation along the field line. The prediction of radiation extent was verified against SOLPS-ITER simulations of an initial STEP design with a good match. DLS-Extended predicts that the radiation extent found in STEP weakens the detachment access benefit of fR and strengthens the benefit of L∥, while the detachment stability of the inner and outer legs is increased and decreased, respectively. This is due to a number of novel radiaton-driven effects and has implications for the optimum strike point position in reactor scale devices. These findings highlight the need for more experimental studies in the presence of broad-radiating impurities. DLS-Extended is distributed under the LGPL-3 open source license and is publicly available on GitHub.
Shota Sugiyama, Takeo Nishitani, Hideaki Matsuura, Shuhei Sumida, Kouji Shinohara, Vitaly Krasilnikov, Bruno Coriton
We have investigated neutron emission anisotropy caused by deuterium beam injection in ITER deuterium plasmas. We evaluate the double-differential emission spectrum and emission anisotropy of neutrons produced by the D(d,n)3He reaction, using the energetic deuteron velocity distribution function obtained by following guiding-centre orbits of test particles. We clarify the dependence of neutron emission anisotropy on the electron density and temperature. Anisotropy increases with decreasing electron density and temperature. We examine the effect of neutron emission anisotropy on the neutron incident flux distribution on the first wall. We show that neutron emission anisotropy can affect the measurements of the neutron emission rate and emission profile based on the incident flux distribution, the distribution of neutron emission anisotropy, and the correspondence relationship between the poloidal angular positions and the neutron detectors that are planned to be installed in ITER. It is inevitable to consider neutron emission anisotropy for plasma diagnostics and monitoring of the neutron generation rate in beam-injected deuterium plasmas.
Zhenchao Sun, Yu Guo, Xiujie Zhang, Teng Zhang, Wei Qian, Yao Zhao, Lei Wang, Xinting Lv, Yiming Wang, Zhengdong Li
The compatibility of RAFM steel with liquid Pb-17Li is a critical concern for its application as a structural material in liquid metal blankets of fusion reactors. This study investigated the stress corrosion mechanism of CLF-1 RAFM steel under a tensile stress of 250 MPa for 5000 hours in flowing Pb-17Li at 450 °C. The results indicate that stress-induced strain disrupts the continuity of the protective oxide layer, shortening the incubation period and increasing corrosion initiation sites, thereby intensifying the corrosion severity and susceptibility. The corrosion morphology is jointly influenced by the flow velocity of Pb-17Li and the steel’s microstructure. At high flow velocities, the martensitic lath structure is eroded, resulting in an etched surface. Conversely, lower velocities preserve the more corrosion-resistant laths, producing lamellar corrosion structures. Furthermore, shear stress promotes microstructural coarsening, which eliminates certain grain and sub-grain boundaries. Although this locally improves corrosion resistance, the resulting long and straight grain boundaries instead facilitates the penetration of liquid metal, thereby accelerating corrosion failure. These findings clarify the stress corrosion mechanism of RAFM steel in Pb-17Li and provide new insights for the modeling of liquid Pb-17Li corrosion and design of liquid metal blankets.
Jose Ramon Martin Solis, Jose Angel Mier, F J Artola, Alberto Loarte
A simplified model analysis is used for the investigation of the survival of runaway electrons following the break-up of the magnetic field lines and the reformation of the flux surfaces during the current-quench phase of tokamak disruptions in vertically unstable plasmas. It is found that even low levels of the runaway current (~ a few tens of kAs) when the plasma touches the wall might lead to runaway damage due to the strong runaway avalanche during scraping-off. Low enough runaway deconfinement times (τd < 1 ms) in a sufficiently long stochastic phase (τ/τd > 5) would be required to avoid a large energy transfer to the runaway electrons and damage of the plasma facing components for low temperatures (a few eVs) of the residual ohmic plasma. The role played by the primary runaway generation mechanisms in regenerating the runaway beam after the magnetic surfaces have healed is also discussed.
Jie Zhang, Florian Koechl, Alexei R Polevoi, Clarisse Bourdelle, Sunhee Kim, Alberto Loarte, Simon D Pinches, Ge Zhuang
The enhanced confinement of tokamak plasmas (H-mode) makes it a preferred regime for achieving fusion power production goals in future devices such as ITER. Nevertheless, low confinement mode (L-mode) remains worthy to investigate in reactor relevant conditions, primarily due to no/reduced requirements for ELM and divertor heat load control. In this regard, this study aims at exploring a new potential approach to maximise the achievable fusion gain Qfus. This approach attempts to increase the core density with enhanced pellet fuelling and then investigates the feasibility of high Qfus L-mode operation in ITER. The JINTRAC integrated modelling suite has been employed for core-edge transport and source modelling, using the HPI2 module for pellet fuelling. In some of the scenarios considered, the core density reaches up to ∼185% of the Greenwald density, nGW, with edge densities approaching nGW, motivated by recent re-evaluations of the density limit that suggest a power-dependent threshold. We compare core transport modelling results obtained by applying the semi-empirical Bohm-gyro-Bohm (BgB) or the quasi-linear gyrokinetic TGLF-SAT2 anomalous transport models, with interpretive vs. predictive impurity transport modelling, and pellet fuelling describing continuous vs. discrete particle sources. The core plasma confinement of high-density L-mode operation in ITER predicted by the TGLF-SAT2 model is significantly better than that predicted by the BgB model, resulting in a significantly improved Qfus. Fusion performance metrics, including Pfus and Qfus, exhibit only minor changes when switching from the interpretive impurity model to the predictive SANCO model, and/or from the continuous ad-hoc pellet model to the discrete HPI2 pellet model. The highest Qfus value predicted in the ITER high-density L-mode simulations is ~ 4, with indications that further improvement may be limited by increased transport associated with electro-magnetic turbulence at elevated plasma beta. This integrated modelling prediction demonstrates the potential of improved Qfus L-mode operation in ITER and future fusion devices, while exploring its boundary.
Ting Wang, Arkadi Kreter, Peng Bi, Hanqing Wang, Y. Mao, Hao Wang, Yue Yuan, Long Cheng, Li-Qun Shi, Jun Tang, et al.
Controlling hydrogen isotope retention is a critical challenge for plasma-facing materials (PFMs) in fusion reactors. Chemical vapor deposited tungsten (CVD-W), featuring columnar grain structures, has attracted increasing attention as a candidate PFM. However, its surface blistering and deuterium (D) retention behavior under fusion-relevant, complex irradiation conditions remain insufficiently understood. In this work, CVD-W and ITER-like forged W both with grains elongated normal to the exposed surface were irradiated with pure D and mixed D+5% He plasma, with and without prior W self-ion irradiation at damage levels of 0.2 and 2 dpa. Compared with ITER-like W, CVD-W exhibits substantially enhanced tolerance to D-induced surface blister formation and markedly reduced D retention under pure D plasma exposure. Regardless of displacement damage, He seeding, or their combination, CVD-W consistently retains less D than ITER-like W across all irradiation sequences studied. This persistently low D retention in CVD-W is primarily associated with its lower defect density (e.g. fewer grain boundaries) and a blister-resistant <001> surface texture that limits blister-related D trapping. Furthermore, displacement damage and He seeding effectively suppress surface blistering, but exert opposing individual effects on D retention in both materials. Their combined effect shows a clear dependence on D fluence and material microstructure. This work highlights the advantages of CVD-W in suppressing hydrogen isotope retention under complex irradiation environments and provides valuable insights for the selection and microstructural design of advanced PFMs with improved resistance to hydrogen-induced blistering and reduced retention.
Andong Xu, Mingyuan Xu, Yunhe Li, Tao Yu, Jiayi Zhang, Yubo Zhang, Yongqiang Zhang, Chenyu Pan, Baolong Hao, Pan Li, et al.
This paper reports the analysis of neutron emission characteristics in high poloidal beta (β p ) discharges on the EAST tokamak, where a record fusion neutron rate of S n = 3.9×10 14 s -1 was achieved with β p ∼2.8, β N ∼2.2, and H 98,y2 ∼1.3. Statistical analysis reveals that while ion cyclotron range of frequencies (ICRF) heating significantly boosts both the neutron rate and plasma stored energy, the neutron rate scales sub-linearly with neutral beam injection (NBI) power (S n ∝P NBI 0.92 ). Interpretive TRANSP simulations demonstrate that the NBI--ICRF synergistic effect directly contributes approximately 30% to the total neutron rate through the formation of a high-energy fast-ion tail. However, this enhancement is partially offset by NBI-induced profile degradation, including fuel dilution, impurity accumulation, and core electron temperature reduction. The generation of the fast-ion tail in velocity space is validated by multi-sightline neutron emission spectroscopy. Furthermore, orbit topology analysis using the ORBIT code reveals that the synergistic effect drives suprathermal fast ions into smaller orbits, such as stagnation orbits, leading to a spatial redistribution of fast ions and a consequent peaking of the neutron emissivity profile. These findings provide critical insights into the complex interplay between auxiliary heating, fast-ion behavior, and neutron emission, offering valuable insights for achieving higher beam--thermal fusion rates in future deuterium--deuterium and deuterium--tritium experiments.
A. Diaw, J. D. Lore, S. Smolentsev
A lithium droplet transport and evaporation model has been developed within the direct simulation Monte Carlo code OpenEdge. This model integrates gravity, collisional ion drag, orbital-motion-limited charging, energy-balance evaporation, and an anisotropic rocket recoil force using a Strang-split integrator. Validation against analytical drag-gravity solutions and independent RK45 evaporation integration demonstrates relative errors below 10−5 for droplet radii of 1.5, 2.5, and 3.5 mm. Simulations of ensembles containing 105 droplets, launched from inner and outer divertor surfaces in SOLPS-ITER plasma background for the CAT tokamak reactor concept, indicate that transport outcomes are determined by initial size, velocity, and launch location. Outer-divertor droplets predominantly redeposit locally, whereas inner-divertor droplets reach the low-field-sidewall. Smaller droplets lose most of their mass to evaporation before reaching the core, while larger droplets retain their mass and redeposit on nearby tiles. Both one-way and iterative two-way coupling frameworks map the evaporated lithium onto the SOLPS-ITER mesh as volumetric sources, facilitating self-consistent evaluation of lithium droplet impacts on edge-plasma performance.
C. D. Decker, C. J. Bruulsema, W. A. Farmer, J. Harte, D. P. Higginson, A. J. Kemp, W. Riedel, J. S. Ross, D. J. Strozzi, G. Zimmerman
We examine the validity of a multi-species 13-moment fluid approach to modeling hohlraum wall expansion occurring in inertial confinement fusion targets used at the National Ignition Facility. We compare our simulation to experiments conducted at the Omega laser facility [Pape et al., Phys. Rev. Lett. 124, 025003 (2020)], which produced counter-propagating gold–carbon plasmas, a phenomenon arising during hohlraum wall expansion. The two experiments we use to benchmark this fluid model produced counter-propagating gold–carbon plasmas in (1) vacuum and (2) a helium atmosphere. We find that simulations using this advanced fluid model replicate certain aspects of the vacuum experiment, such as material interpenetration, and distinct anisotropic gold and carbon ion temperatures are not captured with single-species lower-moment fluid models. However, our simulations underpredict the temperature of scarce gold ions that penetrate deep into the carbon—a phenomenon potentially requiring a kinetic treatment to model. When applied to the helium gas filled experiment, this advanced fluid model matches the amount of material mixing and gold stagnation point but overpredicts ion temperatures in the gold. Finally, we examine the applicability of single fluid models (as well as lower-order multi-species models) by comparing simulated plasma quantities vs the helium gas-fill density. As expected, we find the models converge as the helium gas density is increased. We find that for gas-fill densities larger than 0.6 mg/cm3 the single fluid model gives similar bulk plasma densities and temperatures as the multi-fluid 13-moment model. However, for gas-fill densities lower than 1.0 mg/cm3, simulations show substantial low-z material mixing in the gold—requiring multi-species modeling. Moreover, accurately modeling the low-z mixing to within 30% requires the full multi-species 13-moment fluid model.
V. Yu. Zaslavsky, I. V. Zheleznov, M. N. Vilkov, A. S. Sergeev, A. N. Kuftin, N. S. Ginzburg
At present, the novel compact fusion reactors with strong magnetic fields, including the DEMO project, require the development of 220–240 GHz megawatt continuous-wave heating sources. In this paper, gyrotrons of planar geometry of interaction space with a sheet polyhelical electron beam and transverse energy extraction are considered as such sources. An advantage of this design in comparison with the conventional cylindrical gyrotron configuration is the possibility to ensure effective mode selection over the open transverse coordinate in combination with radiation outcoupling. The theoretical analysis and numerical 3D PIC simulations of a 230 GHz megawatt power planar gyrotron operating at the first harmonic of the cyclotron frequency are performed. The paper considers the feasibility of single-mode generation with output power exceeding 1 MW, efficiency of 30%, and Ohmic loads less than 1.5 kW/cm2, which is compatible with the continuous-wave operation regime.
Eiji Hoashi, Daiki Fukunaga, Takafumi Okita, Shunsuke Kenjo, Makoto Oyaizu, Kentaro Ochiai
Seong Dae Park, Suk-Kwon Kim, Jae-Sung Yoon, Hyoseong Gwon
Yung-Kun Liu, Pisin Chen, Ching-En Lin, Spencer Gessner, Bernhard Hidding
It is known that only a narrow plasma wakefield sliver in the electron-beam-driven blowout regime suits positron acceleration. Using 3D simulations, we show that matching the cyclotron frequency $ω_{c}$ with the plasma frequency $ω_{p}$ forms a stable electron column on axis, expanding the suitable phase space for positron acceleration sizably. For a plasma density $n_p = 10^{16}\text{ cm}^{-3}$ in a 35 T field, the interval expands 4.3 times, and a witness positron beam gains 100--150 MeV over 6 cm ($1.6$--$2.5$~GeV/m) with a 92\% capture rate.
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