Recent Publications

Beam optics and stripping losses in a full-scale ITER negative ion source: multibeamlet analysis by beam emission spectroscopy

Riccardo Agnello, Marco Barbisan, Roberto Pasqualotto, Antonio Pimazzoni, Emanuele Sartori, Barbara Zaniol, Edgard Zuin

Consorzio RFX (CNR ENEA INFN University of Padova Acciaierie Venete SpA), École Polytechnique Fédérale de Lausanne Swiss Plasma Center, Consiglio Nazionale delle Ricerche, Università degli Studi di Padova

In this work, Beam Emission Spectroscopy (BES) is applied to the investigation of beam divergence and stripping losses in the full-scale ITER negative ion source prototype SPIDER, operating in multibeamlet configuration. A semi-analytical model is developed to simulate the emission spectra produced by overlapping beamlets, accounting for realistic beamlet divergence, aiming, and beam composition along the accelerator and drift regions. The comparison between synthetic and experimental spectra shows that the Doppler broadening measured in multibeamlet operation cannot be interpreted solely in terms of single-beamlet divergence, but results from the combined effects of beamlet overlap, residual magnetic deflections, electrostatic repulsion, and halo contributions. Using beamlet parameters independently obtained from calorimetric diagnostics, the model reproduces a significant fraction of the experimentally measured divergence. Moreover, the possibility of displacing a line-of-sight allows the detection of variations in the populations of particles with different divergences, namely the core and halo components, across the beam. As a phenomenological characterization, stripping losses are systematically quantified over a wide range of operational parameters, showing an approximately linear increase with source pressure and values consistent with previous single-beamlet studies. These results demonstrate the capability of BES, combined with multibeamlet modelling, to support beam optimization and performance assessment in ITER-relevant negative ion sources.

Machine-agnostic Gaussian process-based surrogate model for electron cyclotron heating and current drive

Andrew M. Irvin, Livia Casali, Sebastian De Pascuale, Ehab Hassan

University of Tennessee-Knoxville, Oak Ridge National Laboratory, Ain Shams University

Real-time total ECRH power control for reliable long-pulse operation and density feedback at Wendelstein 7-X

Laurent Krier, Stefan Marsen, Heike Laqua, Heinrich Laqua, Dmitry Moseev, Frank Noke, Hans Oosterbeek, Niklas Simon Polei, Sergiy Ponomarenko, Taurino Reichert, et al.

Max-Planck-Institut für Plasmaphysik

The Electron Cyclotron Resonance Heating (ECRH) system at the Wendelstein 7-X (W7-X) stellarator is equipped with eleven megawatt-class gyrotrons that operate at a frequency of 140 GHz, designed for pulse durations of up to thirty minutes. For long-pulse detached-divertor plasma experiments at W7-X, a stable total ECRH power delivered to the plasma is crucial, because the radiated power at the plasma edge is preferably close to the heating power. Consequently, the unexpected shutdown of only one gyrotron can cause a premature end of the experiment due to radiation collapse of the plasma. This paper introduces a system-level controller that dynamically redistributes power among gyrotrons to maintain total ECRH power, improving reliability and enabling feedback control. The controller maintains a specified total ECRH power output by adjusting the accelerating voltage of all gyrotrons. First implemented in the operational phase (OP) 2.2, the system contributed to two major milestones of W7-X during OP 2.3: highest long-pulse triple product for 43 s and highest energy turnover of 1.8 GJ during 360 s. In addition to reliability improvements, the controller enables the adjustment of the total ECRH power for other control objectives, such as maintaining a desired line-integrated plasma density to counteract the ECRH pump-out effect in high-performance scenarios. This paper also presents the first proof-of-principle experiments demonstrating an ECRH-based density feedback system.

Evaluation of beam properties using a checkerboard masking configuration at the NNBI test stand BATMAN Upgrade

Araceli Navarro, Eleni Nanou, Michael Barnes, Christian Wimmer, Dirk Wünderlich, Ursel Fantz

Max-Plank-Institut für Plasmaphysik

GAM frequency structure and properties in ohmic and powerful ECR-heated plasmas in a tokamak

Alexander V Melnikov, Leonid E Eliseev, Yaroslav Maksimovich Ammosov, Sergey E. Lysenko

Russian Research Centre - Kurchatov Institute

The geodesic acoustic mode (GAM) is a high-frequency branch of zonal flows, considered as a mechanism of the turbulence self-regulation, which affect the radial transport of energy and particles. The GAM studies were performed in the T-10 tokamak, using the heavy ion beam probing (HIBP). The power spectral density of plasma potential has the main GAM peak with frequency f ~ 20 kHz, and two satellite peaks, high-frequency (HF) and low-frequency (LF) ones. In ohmic plasmas and in discharges with moderate electron cyclotron resonance heating (ECRH), both satellites are separated from the main peak by the Δ f ~± 3−4 kHz. Each of three peaks has the character of a global eigenmode of plasma oscillations with the frequency and amplitude of fluctuations almost constant in a wide radial region from the core to the edge. At the edge, the amplitude of the GAM peaks decreases to zero. Thus, the radial dependence of the GAM frequency does not obey the local Winsor formula f GAM ~ C s ( r )/ R , where C s ~ T e 1/2 is the ion-sound velocity. Nevertheless, in ohmic discharges and at the moderate ECRH power P EC <0.5 MW, the frequencies of all peaks depend on C s , taken in their birth points located at the edge. With a further increase in temperature or at the powerful ECRH (0.5 MW < P EC < 2.2 MW) the frequencies of all peaks deviate from the C s dependence and saturate. When temperature increases, the frequency difference between the main GAM peak and the HF-satellite decreases, and these two peaks merge into the single one, reaching the upper limit for f GAM . The bicoherence analysis finds the three-wave coupling of GAM with quasicoherent and stochastic low-frequency turbulent modes. Each peak has its own frequency range of coupling.

R&D of ITER GDC power supply

Zheng Xue, Wang Yingqiao, Wu Yi, Gu Zhengyang, Chen Xin, Li bo, Xiong Zhenming

High-power TCV scenario for conventional and alternative divertor studies

K. Lee, C. Theiler, M. Carpita, M. Zurita, P. Sintre, O. Février, F. Pastore, H. Reimerdes, K. Verhaegh, M. Winkel, et al.

Alternative divertor configurations (ADCs) must be evaluated under boundary plasma conditions approaching reactor-level values to be considered a reliable, physics-based solution for tokamak power exhaust. Most ADC experiments performed to date were at relatively low exhaust power. This work presents a high-power scenario on the TCV tokamak enabling the study of a wide variety of divertor magnetic shapes under an expanded SOL and power exhaust parameter space. The scenario is characterized by high power levels of electron cyclotron resonance heating ($2.5\,\text{MW}$ fully absorbed in a $\sim1\,\text{m}^{3}$ plasma) at high plasma current (edge safety factor $q_{95}\approx 2.5$), and low upstream separatrix densities ($n_{e,\text{u}}\approx1\times10^{19}\,\text{m}^{-3}$, Greenwald fraction $f_{\text{G}}\approx 0.1$). Stationary parallel heat fluxes up to $100\,\text{MW m}^{-2}$ are measured at the divertor target, an order of magnitude above previous TCV power exhaust studies. The obtained SOL collisionality and Lengyel detachment scaling metric lie within range of values expected in future reactors (SPARC, ITER, ARC).

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

Korea Institute of Fusion Energy

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.

Consorzio RFX, Consiglio Nazionale delle Ricerche, ENEA Agenzia Nazionale per Le Nuove Tecnologie l'Energia e lo Sviluppo Economico Sostenibile, EniProgetti SpA, Ecole Polytechnique Federale de Lausanne

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.

Progress of LHCD experiment at 4.6 GHz towards long-pulse and high-power operation on EAST

Miaohui Li, Weiwei Zhang, Junlin Chen, Chenbin Wu, Bojiang Ding, Mao Wang, Liang Liu, Lianmin Zhao, Wendong Ma, Yong Yang, et al.

Chinese Academy of Sciences - Hefei Institutes of Physical Sciences, Institute of Plasma Physics Chinese Academy of Sciences, CEA

Recent achievements of lower hybrid current drive (LHCD) experiment at 4.6 GHz towards long-pulse and high-power operation on EAST tokamak are reported. The duration of long-pulse plasmas has been extended to 1056 s with 1.1 MW LH power in I-mode and 1066 s with 0.92 MW in H-mode. The operational domains in plasma current and line-averaged density for fully non-inductive discharges with LH alone and with the combined LH and electron cyclotron (EC) waves are presented. The dependence of LHCD efficiency on plasma density in both L- and H-mode discharges characterized by residual loop voltage Vloop = 0 is quantified. It is found that the LHCD efficiency is improved significantly by EC heating due to the increase of electron temperature. The dominant issues in long-pulse and high-power operation are summarized and discussed, including the power coupling imbalance, the hot spot and arc events in front of the antenna, and the deteriorated plasma heating effect with high LH power. Finally, prospects with a new 4 MW LHCD system at 4.6 GHz which is under development are given.

The Helios stellarator fusion power plant: Scoping studies, heating, and fueling

S.T.A. Kumar, C.P.S. Swanson, D.A. Gates

Thea Energy, Inc.

Observation of fast ion acceleration under EGAM in EAST pure ECRH plasma with tearing mode

Chaowei Mai, liqing xu, Shiyao Lin, Tianfu Zhou, Tonghui Shi, Kangning Geng, Yifei Jin, Yanmin Duan, Kaiyun Chen, Yan Chao, et al.

Institute of Plasma Physics, Chinese Academy of Sciences, Chinese Academy of Sciences, Guangdong Ocean University, Chinese Academy of Sciences - Hefei Institutes of Physical Sciences

In magnetically confined fusion plasmas heated only by Electron Cyclotron Resonance Heating (ECRH), ion temperature typically saturates in the 1–3 keV range. The EAST tokamak, equipped with an ITER-like tungsten divertor, has operated deuterium H-mode discharges with high-power pure ECRH. In a specific discharge, a continuous ion acceleration process is observed, with ion energy ranging from an off-axis supra-thermal component above 6 keV to an isolated Gaussian peak reaching 1.8 MeV. The acceleration occurs at the location of a large 2/1 magnetic island, and an EGAM-like electrostatic mode at ∼ 23 kHz is detected by microwave diagnostics. The MeV-range cutoff energy, the mode power, and the island width exhibit pairwise strong linear correlations (R2 > 0.9), and the three are spatially coincident. Transfer entropy analysis and a 0D collisional-radiative model suggest a mechanism from island to mode to ion acceleration, excluding a purely electric-field-driven process. These observations provide experimental evidence of MeV-range ion acceleration accompanied by an EGAM-like mode in a pure ECRH plasma without auxiliary ion heating, offering a new data point for understanding ion acceleration under ECRH operation in tungsten-wall devices, particularly with degraded boron wall coating

Model order reduction for fusion components: Dynamic mode decomposition model of the magnetron injection gun

Elia Novarese, Carolina Introini, Aldo Collaku, Laura Savoldi, Antonio Cammi

Politecnico di Torino, Politecnico di Milano, Khalifa University

Validation of hybrid-PIC Simulations for Advanced Beam-Driven FRC Modeling

Roelof Erasmus Groenewald, Scott Karbashewski, Sangeeta Gupta, Jon Drobny, Anton Bondarenko, Shuji Kamio, Marcel Nations, James Titus, Daniel C Barnes, Sean Dettrick

TAE Technologies, Inc, University of California Irvine

TAE Technologies, Inc. has successfully demonstrated maintenance of a stable field-reversed configuration plasmas through the use of external actuators, which include neutral beam injection, core fueling, and electrode biasing. To reproduce this result in simulation, various additions have been made to the open-source particle-in-cell code, WarpX, that allow the effects of these external actuators to be included in simulation. A validation study of the extended WarpX hybrid-PIC model is presented in which simulations are compared against experimental results obtained on the C-2W experiment at TAE. The simulations are shown to accurately reproduce both low amplitude energetic particle driven modes routinely observed during C-2W operation and characteristics of the plasma decay after termination of external actuators. Validation of the simulation infrastructure provides confidence that the model can be used for interpretation of experimental observations.

CFD based heat transfer and thermo-structural analysis of a full-scale SST-1 ion extractor grid under high heat load conditions

Tejendra Patel, Tapan Patel, Mukti Ranjan Jana, Deepak Sharma, Paritosh Chaudhuri

Institute for Plasma Research, Homi Bhabha National Institute (HBNI)

Gas pressure dependence of stray particles in the dual-driver RF negative ion source of CRAFT NNBI

Yuwen Yang, Zhengkun Cao, Na Wang, Yongjian Xu, Wei Liu, Yuming Gu, Chundong Hu, Yahong Xie, Lizhen Liang, Jianglong Wei

Chinese Academy of Sciences Hefei Institutes of Physical Science

The negative-ion-based neutral beam injection (NNBI) system is a reliable plasma heating and current drive method for large-scale magnetic confinement fusion devices. In the negative ion source, the problem of stray particles is a serious and unavoidable issue. These stray particles are mainly produced through particle-gas interaction in the accelerator region and can be subsequently accelerated, leading to high-voltage breakdown and substantial thermal deposition on components. The gas pressure plays a key role in determining the frequency of particle-gas collisions and simultaneously influences plasma and beam parameters, which in turn affect the generation and transport of stray particles. A NNBI test facility has been constructed in the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. To investigate the dependence of stray particles behaviour on gas pressure, three sets of experiments have been conducted on CRAFT NNBI test facility. As the gas pressure increased, the power load on the ground grid (GG) first decreased and then increased, exhibiting an unexpected minimum value in the pressure range of 0.40~0.45 Pa. A similar trend was observed for the vertical beam divergence, whereas the horizontal beam divergence increased monotonically with gas pressure. In contrast, the ejecting electron power, stripping loss, and backstreaming positive ion power were positively correlated with gas pressure. These results provide experimental insight into the complex role of gas pressure in stray particle generation and beam quality in NNBI systems.

BORAY-3D: A ray tracing code for three-dimensional magnetized plasma configurations

Yuxuan Wang, Huasheng Xie

Ray tracing codes are useful tools for studying electromagnetic wave propagation and absorption using the geometrical-optics approximation. Existing codes commonly provide either broad radio-frequency coverage in axisymmetric equilibria or three-dimensional capability specialized for electron-cyclotron (EC) applications. BORAY-3D integrates three desirable features in a single version. First, it has a broad frequency range of validity regime from ion-cyclotron, helicon and lower-hybrid waves to EC waves and emission. Second, it provides a unified treatment of arbitrary two- and three-dimensional magnetic-plasma configurations, including both closed and open field-line regions. Third, it incorporates fully relativistic Maxwellian EC absorption. The code extends the axisymmetric BORAY formulation by solving the ray equations in cylindrical coordinates $(r,φ,z)$ while allowing the toroidal mode number $n_φ$ to vary. Magnetic-field, density and temperature data are directly described in $(r,φ,z)$ coordinates without the restriction of flux functions, so that numerical equilibria and analytic field models can be handled in the same form. The non-relativistic hot-plasma model inherited from BORAY is used for lower-hybrid, ion-cyclotron and helicon absorption, whereas the relativistic model is coupled to reciprocal radiative transfer for electron cyclotron emission (ECE). Practical applications include 13.56 MHz helicon and 50 MHz fast waves, a 3.7 GHz lower-hybrid wave, and 115--220 GHz EC emission. BORAY-3D has been systematically benchmarked against GENRAY for tokamak toroidal-field ripple, Raytrax and TRAVIS for W7-X, as well as public HSX heating and W7-X ECE results.

Integrated modelling of electron cyclotron wave control of core impurity accumulation in HL-3 tokamak

ziqi Fan, Yijun Zhong, Junbo Zhang, Shoulong Xu, Hai Li, Haozhen Gu, xinliang xu, Shuo Wang, Xue-yu Gong, Wulyu Zhong

University of South China, Southwestern Institute of Physics

Core impurity accumulation can severely degrade plasma confinement and may even lead to disruptions; therefore, its mitigation is essential for achieving stable high-performance operation in tokamak devices. In this work, based on the OMFIT integrated modelling platform, the effects of electron cyclotron waves (ECWs) on core tungsten impurity transport are investigated in the HL-3 tokamak under an NBI-heated background plasma. The simulation results show that, in the pure NBI case, the W impurity density profile exhibits pronounced central peaking, whereas ECW injection can effectively mitigate tungsten accumulation, with the control efficiency depending strongly on the power deposition location, injected ECW power, and current-drive scheme. Near-axis ECW deposition at ρ ≈ 0.1 and off-axis deposition at ρ ≈ 0.5 are found to be favorable impurity control schemes, whereas intermediate-radius deposition at ρ ≈ 0.3 provides a relatively weak suppression effect. The two effective deposition regions correspond to different dominant mechanisms: near-axis ECW deposition mainly alleviates W peaking by enhancing core turbulent transport and weakening inward convection, whereas off-axis deposition suppresses W accumulation primarily by flattening the main-ion density profile and thereby reducing the density-gradient-driven neoclassical inward pinch. For near-axis ECW deposition, the current-drive scheme has a significant influence on W transport. In the counter-current drive case, the increase in the local safety factor is accompanied by enhanced neoclassical inward convection, resulting in only limited mitigation of W peaking; by contrast, when ECW is deposited farther off axis, the W transport behaviour is only weakly sensitive to the current-drive scheme. These results provide a physics basis for understanding impurity transport under high-power NBI heating and offer useful guidance for impurity control in the HL-3 tokamak.

WEST long-pulse achievements in support of next-step fusion devices

Remi Dumont, Theo Fonghetti, Patrick Maget, Pierre Manas, Jean-Francois Artaud, Tullio Barbui, Clarisse Bourdelle, Laurent Colas, Guido Ciraolo, Yann Corre, et al.

French Alternative Energies and Atomic Energy Commission, Chinese Academy of Sciences, Princeton University Plasma Physics Laboratory, IUSTI, Koninklijke Militaire School

The WEST tokamak is equipped with a superconducting toroidal magnetic field system, a multi-megawatt radiofrequency auxiliary power system, and an actively cooled ITER-grade tungsten divertor. As such, it is well adapted to explore experimental aspects related to the long pulse operation of next-step devices. Supported by predict-first integrated modeling, bespoke scenario development has allowed zero-loop voltage pulses to be achieved. The resulting discharges, with plasma currents in the range I p ∼0.22-0.28 MA exclusively sustained by the Lower Hybrid Current Drive (LHCD) system as an auxiliary power source, have achieved durations in excess of 22 min and injected/extracted energies up to 2.61 GJ. Plasma performance is characterized by ranges of poloidal beta β p ∼1.6-2.0, normalized toroidal β N ∼0.6-0.9 and confinement factor H 96L ∼1.0-1.3. Mild MHD activity, identified as resulting from the interaction of 3/1 and 4/1 tearing modes, is occasionally present, depending on the LHCD antenna combination used. This article describes the predict-first approach that has been employed in the context of this long-pulse scenario development endeavor. The main achievements and the physics analyses performed are reviewed, including post-experiment integrated modeling aspects. Prospects for further long-pulse developments are drawn.

Efficiency of non-resonant photon trap for future experiments on neutralisation of negative hydrogen ion beam

Magomedrizy Gadjimuradovich Atlukhanov, Alexander Vladimirovich Burdakov, Sergey Sergeevich Popov, Yuri Alexandrovich Trunev, Dmitry Ivanovich Skovorodin, Alexander Alexandrovich Kasatov, Igor Vladimirovich Shikhovtsev, Victor Viktorovich Kurkuchekov

Budker Institute of Nuclear Physics SB RAS, Novosibirsk State University, Novosibirsk State Technical University

This paper presents experimental investigations of a non-resonant photon trap featuring adiabatic radiation confinement, designed for the photoneutralisation of negative hydrogen ( H − / D − ) ion beams in fusion plasma injection heating systems. The trap is formed by two monolithic dielectric mirrors with high reflectivity. The study focuses on the methodology and results of measuring the confinement time and integral optical losses. The diagnostic approach is based on analysing the decay rate of radiation within the optical cavity following abrupt interruption of the pump source, utilising wire attenuators for signal reduction. The influence of the laser beam angular divergence on the accumulation efficiency is examined. The obtained data confirm the adiabatic nature of the photon confinement, demonstrate consistency between the measured losses and the specified mirror coating parameters and validate the feasibility of controlling the density of the photon ensemble within the trap. This work confirms the technical feasibility and potential of this approach for the development of high-efficiency photoneutralisation systems.