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WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix, n sep , similarly to ASDEX Upgrade (AUG) tokamak and Joint European Torus (JET) tokamak in H-mode (Verdoolaege et al 2021 Nucl. Fusion 61 076006). Reflectometry in the midplane provides n sep , while the temperature at the separatrix, T sep is inferred by the ‘two-point model’ using Langmuir probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations (Eich et al 2020 Nucl. Fusion 60 056016). As expected in the presence of an efficient neutral source due to recycling fluxes, n sep correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density n ˉ , lower n sep correlates with a larger density gradient around the separatrix as well as a larger global density peaking, n ˉ / ⟨ n ⟩ , measured by interferometry. The latter correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes (Angioni et al 2007 Nucl. Fusion 47 1326). The correlations reported allow phrasing the subsequent causality question: what is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature and better core energy confinement quality? Understanding the causality chain is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumped flux with respect to the plasma volume will be lower than presently operating tokamaks.

To improve the Current Drive (CD) capability in long-pulse (up to ∼1000 s) H-mode operation, it has been decided to develop a new Lower Hybrid Current Drive system at 4.6 GHz with an active cooling Passive Active Multijunction (PAM) launcher on EAST. In this paper, both the radio frequency (RF) and the physical properties of this PAM are studied numerically. The same nominal parallel refractive index (N || = k ||c/ω, where k || is the parallel wavenumber, c the velocity of light, and ω the wave angular frequency) of 2.04 as the existing 4.6 GHz Full Active Multijunction (FAM) is chosen. Ray-tracing calculations indicate that good accessibility could be achieved when the LH waves radiate with this nominal N || in typical long-pulse H-mode plasmas. The coupling performance in terms of power reflection coefficient (R C), power spectrum, maximum electric field, power directivity (D P) and global CD capability is evaluated with the ALOHA code based on the linear coupling theory. Good coupling performance with averaged R C ⩽ 1% and D P ∼ 70% could be expected with the density (n e) in front of the PAM close to the cut-off value (n e_co). The simulated R C remains below 6.5% over a wide density range 0.5 ⩽ n e/n e_co ⩽ 10, which is similar to the plasma edge conditions produced by Edge Localized Mode activity. A detailed comparison with the existing 4.6 GHz FAM launcher is also performed.

This paper presents the first results of a passive active multijunction (PAM) launcher at 2.45 GHz during the lower hybrid current drive (LHCD) experiments on EAST. Good coupling performance with a power reflection coefficient ( RC ) ∼3% has been achieved at the plasma–antenna distance up to ∼11 cm in L-mode edge plasmas without local gas puffing near the PAM launcher. Reliable power coupling of this PAM during the edge perturbations induced by type I edge localized modes (ELMs) has been successfully demonstrated. Compared with the old full active multijunction (FAM) launcher, the new PAM can be placed ∼2 cm further away from the plasma in normal operations, which is in good agreement with the previous prediction (Li et al 2019 Fusion Eng. Des. 147 111 250), by the linear wave–plasma coupling code ALOHA (Hillairet et al 2010 Nucl. Fusion 50 125 010). The flexibility of the power spectrum by changing the phase difference between adjacent modules was validated and ray-tracing/Fokker–Planck simulations can reproduce the experimental features. The achievable power handling is as high as 25 MW m −2 , although with a shot pulse length of ∼10 s. The first experiment successfully demonstrated the coupling performance of a PAM launcher at low density and this launcher construction provides helpful engineering experience for the 4.6 GHz PAM development in the near future on EAST.

This paper reports a fishnet hyperbolic metamaterial that mimics the electromagnetic properties of magnetically confined plasma. These electromagnetic properties are strongly anisotropic and different from any conventional material, therefore cannot be mimicked by bulk materials. The structure is made of a stack of thin copper grids spaced by Rohacell foam. We numerically and experimentally show that this kind of structuration matches well the properties of a homogeneous plasma. This solution breaks a long-lasting bottleneck and will accelerate the development of high-frequency heating systems to be used in nuclear fusion.

The Lower Hybrid (LH) wave is widely used in existing tokamaks for tailoring current density profile or extending pulse duration to steady-state regimes. Its high efficiency makes it particularly attractive for a fusion reactor, leading to consider it for this purpose in ITER tokamak. Nevertheless, if basics of the LH wave in tokamak plasma are well known, quantitative modeling of experimental observations based on first principles remains a highly challenging exercise, despite considerable numerical efforts achieved so far. In this context, a rigorous methodology must be carried out in the simulations to identify the minimum number of physical mechanisms that must be considered to reproduce experimental shot to shot observations and also scalings (density, power spectrum). Based on recent simulations carried out for EAST, Alcator C-Mod and Tore Supra tokamaks, the state of the art in LH modeling is reviewed. The capability of fast electron bremsstrahlung, internal inductance li and LH driven current at zero loop voltage to constrain all together LH simulations is discussed, as well as the needs of further improvements (diagnostics, codes, LH model), for robust interpretative and predictive simulations.

Mitigation of Edge localized mode(ELM) has been achieved with different external actuators such as lower hybrid wave (LHW), mixture supersonic molecular beam injection(SMBI), and laser blow-off(LBO) impurity seeding on HL-2A. During these experiments, the pedestal turbulence enhancement is commonly observed, which is found closely related to ELM mitigation. The turbulence enhancement is caused by the externally driven the velocity shear rate without change of the turbulence correlation length, but correlated to its radial wavenumber spectral shift. A common plausible mechanism for the ELM mitigation with different external source input seems to be involved. A modified theoretical model based on the turbulence radial wavenumber spectral shift is used and successfully interprets the experimental observations. The simulation suggests that a critical growth rate of the most unstable mode, also identified by the experimental results, survives in the competition of the velocity shear rate, enhancing the turbulence intensity. An example of the LHW case is used and good agreements have been found between the experimental results and simulation results.

ICRH was extensively used in the 2015-16 JET-ILW (ITER like wall) experimental campaign; bulk heating together with high-Z impurity chase-out from plasma centre importantly contributed to the good DD fusion performance obtained recently in JET. Power up to 6 MW was launched in H-mode deuterium plasmas and 8 MW during the hydrogen campaign. The ILA was re-installed and contributed positively to the availability of ICRH power. The ILA produces slightly less high-Z impurities than the A2's and the PWI measured via Be line emission on limiters is in the same ballpark. Specific experiments were conducted to optimise ICRH scenarios in preparation for DT in particular the dual frequency scheme, (H)D and (He)D were tested. In addition, it was confirmed that the (D)H scenario is accessible in a ILW environment and the novel 3-ions ICRH scheme was validated experimentally.

The JET hybrid scenario has been developed from low plasma current carbon wall discharges to the record-breaking Deuterium-Tritium plasmas obtained in 2021 with the ITER-like Be/W wall. The development started in pure Deuterium with refinement of the plasma current, and toroidal magnetic field choices and succeeded in solving the heat load challenges arising from 37 MW of injected power in the ITER like wall environment, keeping the radiation in the edge and core controlled, avoiding MHD instabilities and reaching high neutron rates. The Deuterium hybrid plasmas have been re-run in Tritium and methods have been found to keep the radiation controlled but not at high fusion performance probably due to time constraints. For the first time this scenario has been run in Deuterium-Tritium (50:50). These plasmas were re-optimised to have a radiation-stable H-mode entry phase, good impurity control through edge T i gradient screening and optimised performance with fusion power exceeding 10 MW for longer than three alpha particle slow down times, 8.3 MW averaged over 5 s and fusion energy of 45.8 MJ.

The mission of WEST (tungsten-W Environment in Steady-state Tokamak) is to explore long pulse operation in a full tungsten (W) environment for preparing next-step fusion devices (ITER and DEMO) with a focus on testing the ITER actively cooled W divertor in tokamak conditions. Following the successful completion of phase 1 (2016-2021), phase 2 started in December 2022 with the lower divertor made entirely of actively cooled ITER-grade tungsten mono-blocks. A boronization prior the first plasma attempt allowed for a smooth startup with the new divertor. Despite the reduced operating window due to tungsten, rapid progress has been made in long pulse operation, resulting in discharges with a pulse length of 100 s and an injected energy of around 300 MJ per discharge. Plasma startup studies were carried out with equatorial boron nitride limiters to compare them with tungsten limiters, while Ion Cyclotron Resonance Heating assisted startup was attempted. High fluence operation in attached regime, which was the main thrust of the first campaigns, already showed the progressive build up of deposits and appearance of dust, impacting the plasma operation as the plasma fluence increased. In total, the cumulated injected energy during the first campaigns reached 43 GJ and the cumulated plasma time exceeded 5 h. Demonstration of controlled X-Point Radiator regime is also reported, opening a promising route for investigating plasma exhaust and plasma-wall interaction issues in more detached regime. This paper summarises the lessons learned from the manufacturing and the first operation of the ITER-grade divertor, describing the progress achieved in optimising operation in a full W environment with a focus on long pulse operation and plasma wall interaction.

We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed ‘three-ion’ scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of 3 He ions to high energies in dedicated hydrogen–deuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast 3 He ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3 He-rich solar flares. Triggering and sustaining fusion reactions — with the goal of overall energy production — in a tokamak plasma requires efficient heating. Radio-frequency heating of a three-ion plasma is now experimentally shown to be a potentially viable technique.