At JET, Deuterium-Helium-3 (D-3He) experiments using ion cyclotron resonance heating (ICRH) with the three-ion scheme were conducted, involving thermal Deuterium (D), thermal Helium-3 (3He), and a fast D beam injected via neutral beam injection (NBI) and further heated using ICRH. In these plasmas, intense neutron emission at the level of approximate to 1016 n s-1 was observed, and a non-thermal neutron spectrum was measured with the JET time of flight neutron spectrometer. Furthermore, the spatial profile of approximate to 16.4 MeV gamma rays originating from the 3He(D,gamma)5Li reaction was observed. This is a weak branch ( approximate to 10-5 relative probability) of the more common 3He(D,p)alpha reaction and it can be used to assess the production of alpha particles in these experiments. In this work we analyse a set of data from nuclear diagnostics in D-DNBI-3He experiments at JET and compare them with synthetic diagnostics based on TRANSP simulations of the fast deuterium distribution function, using the so-called 'kick operator' to model power transfer from the wave to the beam fast deuterons. The neutron rate predicted by the TRANSP simulations broadly agrees with data measured with fission chambers. The shape of the neutron emission spectrum is also well understood, once finite Larmor radius effects are introduced into calculations. Data from the gamma ray camera are used to infer the alpha birth profile through tomographic reconstructions based on the TREVISO code. These were compared with corresponding calculations starting from TRANSP results and showing an overall good agreement with the experimental tomographies. The validated TRANSP results are finally used to discuss the modifications of the fast deuterium distribution function and of the alpha birth profile in a set of discharges.

To diagnose energetic and bulk ions in fusion plasmas, we have developed a millimeter-wave collective Thomson scattering (CTS) diagnostic system utilizing a megawatt-class gyrotron, originally designed for electron cyclotron heating (ECH), in the Large Helical Device (LHD). This paper presents CTS measurements of energetic and anisotropic ions produced by neutral beam injection (NBI), along with a method for analyzing CTS spectra. In addition, we report on the development of a 303 GHz gyrotron intended for CTS diagnostics, which enables lower background radiation levels and improved signal-to-noise ratio (SNR) in the detection of scattered signals.which is expected to reduce background radiation and improve the signal-to-noise ratio in CTS measurements. Furthermore, a high-performance notch filter has been developed to block stray radiation from the gyrotron beam within the vacuum vessel, preventing it from entering the receiver as noise. This type of notch filter is employed not only in CTS measurements but also in electron cyclotron emission (ECE) and correlation ECE diagnostics. This paper outlines these developments and their application to energetic ion diagnostics in LHD plasmas.

This study introduces the use of a deep convolutional neural network for reconstructing fast-ion velocity distributions from fast-ion loss detectors and imaging neutral particle analyzers (INPAs), automatically integrating uncertainty quantification through Monte Carlo dropout. The network-based reconstructions reveal pitch-angle splitting in high-energy features of lost fast-ion velocity distributions at ASDEX Upgrade during active neutral beam injection, a previously observed phenomenon now confirmed through neural networks. Moreover, contrary to common theories attributing these high-energy features to edge localized mode (ELM)-driven acceleration, we provide experimental evidence that they also occur in type-I ELM-quiescent phases. Additionally, we demonstrate improved reconstructions from INPA measurements, both synthetic and from an ASDEX Upgrade commissioning discharge, with the reconstructions closely matching TRANSP simulations. These findings suggest that neural networks can provide robust reconstructions with well-defined uncertainties, improving the reliability of interpretations of fast-ion behavior in magnetically confined plasmas.

Comprehending and optimizing fast ion behaviors is critical for the enhancement of performance in Experimental Advanced Superconducting Tokamak (EAST). This study explores the potential benefits of several factors that can improve the fast ion confinement. First, experiments show the change in the direction of the NBI2 from counter-Ip to co-Ip leads to a significant reduction in fast ion losses. TRANSP/NUBEAM simulation and tomography results based on fast-ion D-alpha measurements reveal that after the neutral beam injection (NBI) upgrade, the beam ion prompt loss is reduced by approximately 50%. Second, the upgraded ion cyclotron resonant frequency (ICRF) antenna at the N-port features twice the coupling resistance of the original antennas at EAST. This improved ICRF power coupling has enhanced the synergistic heating effect of NBI + ICRF, where the ICRF wave field accelerates beam ions at the harmonics. Experiments demonstrate that NBI + ICRF synergistic not only enhances plasma neutron yield and beta P, but also accelerates beam ions to hundreds of keV. Further, the electron density and the neutral beam voltage have been optimized to reduce the fast ion slowing-down time and beam ion losses. Experimental and simulation results indicate that increasing the electron density reduces beam ion losses and enhances the bootstrap current fraction. While higher beam voltage results in a slight decrease in beam power absorption, it can increase the fraction of bootstrap current. With the understanding of these optimization of fast ion confinement, experiments have demonstrated fully non-inductive operation at high density (ne/nG similar to 0.67, beta P similar to 3.1, beta N similar to 2.1, H98,y2 similar to 1.2) even without the support of co-Ip beam NBI2. This investigation presents a potential regime to enhance fast ion confinement and extend performance in the high beta P plasma for future experiments.

An important step on the way to future fusion power plants was the 2021 deuterium-tritium experimental campaign (DTE2) at the Joint European Torus (JET), in which crucial DT physics was investigated. In this study, we have reconstructed the fast-ion deuterium distribution function in JET discharge 99971 which broke the former fusion energy record. It is the first time that the fast-ion distribution has been reconstructed from experimental data in a DT discharge. The reconstruction shows that the fast-ion deuterium distribution is anisotropic, with a bias towards co-going ions (p > 0). The fast-ion deuterium distribution likely peaks in energy (E) at around E similar to 60-70 keV and has a marginal high-energy tail ( E greater than or similar to 180 keV). Furthermore, an orbit analysis shows that the fast-ion distribution is composed of mostly co-passing orbits ( 50%), trapped orbits ( 21%) and counter-passing orbits ( 27%), as well as a small population of potato orbits ( 1.7%) and counter-stagnation orbits ( 0.3%). The orbit-type constituents of the neutron measurements are distributed in similar fractions.

We present a fully analytical model for calculating energy spectra of neutrons generated by fusion reactions involving a fast ion, or beam, and a stationary ion, or target, in magnetic fusion plasmas. For neutrons moving along the line-of-sight of a detector, the neutron spectrum is given by an analytical expression and the usual differential cross section. This makes the model several orders of magnitude faster than ordinary Monte Carlo simulations and free of any related statistical noise. Additionally, the analytical description of the reaction physics provides much more insight into the formation of the spectrum. An example of this is the bias of beam-target spectra towards high-energy neutron counts, which corresponds to forward-emission events. On the other hand, the fast-ion uniform gyro-angle distribution has an opposite effect, but is ultimately weaker than the preferential forward emission of neutrons. The model is validated against numerical calculations from the forward model code GENESIS to verify its validity and it is furthermore derived from a probabilistic viewpoint, adding further insight.

We investigate parametric decay instabilities (PDIs) of electron cyclotron waves due to a rotating neoclassical tearing mode (NTM) in ASDEX Upgrade. Strong scattering characteristic of PDIs is observed in a discharge where a (2,1) NTM has been identified. By mapping out the structure of the NTM toroidally, it is possible to determine the phases of the NTM which enable the decay to occur. Signatures of PDIs are seen when the edges of the magnetic island intersect the gyrotron beam path, but not when the O-point of the magnetic island is located directly in front of the gyrotron launcher. We propose an explanation using a simplified model, which reproduces features of the scattering in 1D particle-in-cell (PIC) simulations. The simulations show that a density perturbation caused by an NTM can give rise to a lowered PDI power threshold. The threshold is lowest in an intermediate density perturbation region where certain waves excited in PDIs can become trapped. The PIC simulations show that several decay and combination events involving approximately half frequency waves produce waves slightly downshifted from the main pump frequency of 140 GHz. This is the first time PIC simulations based on experimental profiles reproduce signals close to the pump frequency, which result from interactions with half frequency waves. The numerical results support previous observations from the tokamak TEXTOR.

In this paper we demonstrate how the inversion, in energy and major radius (E, R) coordinates, of imaging neutral particle analyser (INPA) measurements can be used to obtain the fast-ion distribution. The INPA is most sensitive to passing ions with energies in the range (20-150) keV and pitches near 0.5 in the core and 0.7 near the plasma edge. Inversion of synthetic signals, via 0th-order Tikhonov and Elastic Net regularization, were performed to demonstrate the capability of recovering the ground truth fast-ion 2D phase-space distribution resolved in major radius and energy, even in the presence of moderate noise levels (10%). Finally, we apply our method to measure the 2D phase-space distribution in an MHD quiescent plasma at ASDEX Upgrade and find good agreement with the slowing down fast-ion distribution predicted by TRANSP.

Tokamak a configuration variable (TCV), recently celebrating 30 years of near-continual operation, continues in its missions to advance outstanding key physics and operational scenario issues for ITER and the design of future power plants such as DEMO. The main machine heating systems and operational changes are first described. Then follow five sections: plasma scenarios. ITER Base-Line (IBL) discharges, triangularity studies together with X3 heating and N2 seeding. Edge localised mode suppression, with a high radiation region near the X-point is reported with N-2 injection with and without divertor baffles in a snowflake configuration. Negative triangularity (NT) discharges attained record, albeit transient, beta(N) similar to 3 with lower turbulence, higher low-Z impurity transport, vertical stability and density limits and core transport better than the IBL. Positive triangularity L-Mode linear and saturated ohmic confinement confinement saturation, often-correlated with intrinsic toroidal rotation reversals, was probed for D, H and He working gases. H-mode confinement and pedestal studies were extended to low collisionality with electron cyclotron heating obtaining steady state electron iternal transport barrier with neutral beam heating (NBH), and NBH driven H-mode configurations with off-axis co-electron cyclotron current drive. Fast particle physics. The physics of disruptions, runaway electrons and fast ions (FIs) was developed using near-full current conversion at disruption with recombination thresholds characterised for impurity species (Ne, Ar, Kr). Different flushing gases (D2, H2) and pathways to trigger a benign disruption were explored. The 55 kV NBH II generated a rich Alfvenic spectrum modulating the FI fas ion loss detector signal. NT configurations showed less toroidal Alfven excitation activity preferentially affecting higher FI pitch angles. Scrape-off layer and edge physics. gas puff imaging systems characterised turbulent plasma ejection for several advanced divertor configurations, including NT. Combined diagnostic array divertor state analysis in detachment conditions was compared to modelling revealing an importance for molecular processes. Divertor physics. Internal gas baffles diversified to include shorter/longer structures on the high and/or low field side to probe compressive efficiency. Divertor studies concentrated upon mitigating target power, facilitating detachment and increasing the radiated power fraction employing alternative divertor geometries, optimised X-point radiator regimes and long-legged configurations. Smaller-than-expected improvements with total flux expansion were better modelled when including parallel flows. Peak outer target heat flux reduction was achieved (>50%) for high flux-expansion geometries, maintaining core performance (H-98 > 1). A reduction in target heat loads and facilitated detachment access at lower core densities is reported. Real-time control. TCV's real-time control upgrades employed MIMO gas injector control of stable, robust, partial detachment and plasma beta feedback control avoiding neoclassical tearing modes with plasma confinement changes. Machine-learning enhancements include trajectory tracking disruption proximity and avoidance as well as a first-of-its-kind reinforcement learning-based controller for the plasma equilibrium trained entirely on a free-boundary simulator. Finally, a short description of TCV's immediate future plans will be given.

After a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement.

Collective Thomson Scattering (CTS) diagnostics measure the scattering spectrum of monochromatic incident radiation off collective fluctuations in the plasma. In this contribution, we present the first results from the upgraded CTS diagnostic at Wendelstein 7-X (W7-X) operating in the frequency range between 172 and 176 GHz. This frequency range allows for minimization of noise originating from the electron cyclotron emission in the plasma. Consequently, the good signal-to-noise ratio allows measurements of fast ions or bulk plasma parameters with higher temporal resolution compared with the previously used 140 GHz system.

The Doppler-shifted resonance condition for high frequency Alfv & eacute;nic eigenmodes has been extensively studied on ASDEX Upgrade in the presence of one or a combination of two neutral beam injected (NBI) fast ion populations. In general, only centrally deposited NBI sources drive these modes, while off-axis sources globally stabilize the mode activity. For the case of a single central NBI source, the observed trend is: the highest frequency modes are driven by the lowest energy and lowest pitch angle NBI sources, in line with the expectation from the Doppler-shifted resonance condition. The expected mode frequencies are determined analytically from the two-fluid cold plasma dispersion relation and the most unstable frequency relation, while the mode growth rates are estimated using the fast ion slowing down distribution functions from the ASCOT code. The overall mode frequency trend in a source-to-source variation is tracked, although a systematic overestimate of similar to 1 MHz is observed. Possible causes of this overestimate include the finite size of the resonant fast ion drift orbit and non-linear effects such as mode sideband formation. Alternatively, the expected mode frequencies are determined by tracking the growth rate maxima trajectories, this method improves the agreement with the experimentally measured values. A combination of two central mode-driving NBI sources results in the suppression of the mode driven by the lowest energy and the lowest pitch angle NBI source. Computing the analytically expected mode frequency following the method outlined above, again, generally tracks the experimentally observed trend. The mode's Alfv & eacute;nic nature allows for a practical application to track the core hydrogen fraction by following the mode frequency changes in response to a varying ion mass density. Such application is demonstrated in a discharge where the average ion mass is varied from similar to 2m(p) to similar to 1.5m(p) (where m(p) is the proton mass) via a hydrogen puff in a deuterium plasma, in the presence of a strong mode activity. The expected mode frequency changes are computed from the existence of the resonance condition, and the values track the measured results with an offset of similar to 0.5 MHz. Overall, the results suggest an intriguing possibility to monitor and control the D-T ion fraction in the core of a fusion reactor in real time using a non-invasive diagnostic.

The fast-ion phase-space distribution function in the magnetic fusion devices is always underdiagnosed, and every new fast-ion diagnostic should be carefully assessed before installation to minimize redundancies in measurements and maximize the information from the yet undiagnosed part of the fast-ion phase space distribution function. Here, we present a novel method of assessing the added value of a considered fast-ion diagnostic, taking actual geometry and an existing set of fast-ion diagnostics into account. The new method is based on a reformulation of the diagnostic weight functions in constants of motion (COM). We compare the proposed method with the previous approach using Monte Carlo simulations.

In 2021 JET exploited its unique capabilities to operate with T and D-T fuel with an ITER-like Be/W wall (JET-ILW). This second major JET D-T campaign (DTE2), after DTE1 in 1997, represented the culmination of a series of JET enhancements-new fusion diagnostics, new T injection capabilities, refurbishment of the T plant, increased auxiliary heating, in-vessel calibration of 14 MeV neutron yield monitors-as well as significant advances in plasma theory and modelling in the fusion community. DTE2 was complemented by a sequence of isotope physics campaigns encompassing operation in pure tritium at high T-NBI power. Carefully conducted for safe operation with tritium, the new T and D-T experiments used 1 kg of T (vs 100 g in DTE1), yielding the most fusion reactor relevant D-T plasmas to date and expanding our understanding of isotopes and D-T mixture physics. Furthermore, since the JET T and DTE2 campaigns occurred almost 25 years after the last major D-T tokamak experiment, it was also a strategic goal of the European fusion programme to refresh operational experience of a nuclear tokamak to prepare staff for ITER operation. The key physics results of the JET T and DTE2 experiments, carried out within the EUROfusion JET1 work package, are reported in this paper. Progress in the technological exploitation of JET D-T operations, development and validation of nuclear codes, neutronic tools and techniques for ITER operations carried out by EUROfusion (started within the Horizon 2020 Framework Programme and continuing under the Horizon Europe FP) are reported in (Litaudon et al Nucl. Fusion accepted), while JET experience on T and D-T operations is presented in (King et al Nucl. Fusion submitted).

We introduce an anisotropic regularization framework for the reconstruction of distribution functions from measurements, utilizing an approach that applies distinct regularization techniques such as non-negative constrained Tikhonov, total variation, and Besov-space priors, either penalizing the one-norm or the two-norm, in each dimension to reflect the anisotropic characteristics of the multidimensional data. This method, applied to fast-ion loss detector (FILD) measurements, demonstrates a significant improvement over conventional nonnegative-constrained zeroth-order Tikhonov regularization because the prior information of the form of the distribution allows better reconstructions. The validity of the approach is corroborated through FILD measurements of prompt fast-ion losses in an ASDEX Upgrade discharge, where the reconstructed distribution function agrees well with the prompt-loss distribution predicted by ASCOT simulations. Moreover, we develop a composite quality metric, Q , that combines the mean squared error and the Jaccard index for a comprehensive evaluation of reconstruction accuracy and spatial fidelity. Finally, anisotropic regularization is applied to FILD measurements at ASDEX Upgrade to study fast-ion acceleration by edge-localized modes. The refined analysis resolves fine structure in the pitch of the accelerated ions and clearly shows that some ions are accelerated to over twice the injection energy.

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (similar to 1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high beta(P) has been significantly extended (H-98y2 > 1.3, beta(P) similar to 4.0, beta(N) similar to 2.4 and n(e)/n(GW) similar to 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q(95) approximate to 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m(-1)) with EC pre-ionization is developed. A beneficial role on the lower hybrid wave injection to control the tungsten concentration in the NBI discharge is observed for the first time in EAST suggesting a potential way toward steady-state H-mode NBI operation.

Electron and impurity ion density profiles have been controlled by using tangential and perpendicular neutral beams for plasma heating in a stellarator/heliotron for the first time. Reduced anisotropy of stored energy for energetic ion En⊥/Enǁ has resulted in an inward electron and impurity transport, forming a core electron density peaking. Increased anisotropy leads to a flat or hollowed electron density profile with an impurity exhaust in a core region [Yoshinuma et al., Nucl. Fusion 49, 062002 (2009)]. A high confinement state of particles in LHD has yet to be achieved, except for a temporal state of an electron density peaking created by a pellet injection. As a pioneering and crucial research result, the operation of energetic ion anisotropy by neutral beams has newly demonstrated that the direction of the radial transport of bulk and impurity ions can be controlled. At the same time, the overall plasma performance rises in neutron flux and stored energy. On the other hand, the increase in the anisotropy flattens the density profile. This new finding holds promise for a control knob of nuclear fusion reactors to enhance fusion power output.

Ion cyclotron emission (ICE) at deuterium ion cyclotron harmonics, driven by sub-Alfvénic beam-injected deuterium ions, has been observed by the high-frequency B-dot probe in the EAST tokamak. The origin of ICE shifts from the plasma core to the plasma edge soon after an L-H mode transition, where the beam-injected deuterium ions have a relatively peak bump-on tail structure in the energy direction and a very intense pitch angle anisotropy. Based on the fast ion distribution function obtained from the TRANSP/NUBEAM code, together with a linear analysis theory of magnetoacoustic cyclotron instability (MCI), the growth rates of MCI could be calculated. It is shown that MCI, resulting in the generation of obliquely propagating fast Alfvén waves at deuterium ion cyclotron harmonics, can occur under such conditions. And the temporal evolution of the MCI growth rate closely follows that of the observed ICE amplitude in the EAST.

Within the 9th European Framework programme, since 2021 EUROfusion is operating five tokamaks under the auspices of a single Task Force called ‘Tokamak Exploitation’. The goal is to benefit from the complementary capabilities of each machine in a coordinated way and help in developing a scientific output scalable to future largre machines. The programme of this Task Force ensures that ASDEX Upgrade, MAST-U, TCV, WEST and JET (since 2022) work together to achieve the objectives of Missions 1 and 2 of the EUROfusion Roadmap: i) demonstrate plasma scenarios that increase the success margin of ITER and satisfy the requirements of DEMO and, ii) demonstrate an integrated approach that can handle the large power leaving ITER and DEMO plasmas. The Tokamak Exploitation task force has therefore organized experiments on these two missions with the goal to strengthen the physics and operational basis for the ITER baseline scenario and for exploiting the recent plasma exhaust enhancements in all four devices (PEX: Plasma EXhaust) for exploring the solution for handling heat and particle exhaust in ITER and develop the conceptual solutions for DEMO. The ITER Baseline scenario has been developed in a similar way in ASDEX Upgrade, TCV and JET. Key risks for ITER such as disruptions and run-aways have been also investigated in TCV, ASDEX Upgrade and JET. Experiments have explored successfully different divertor configurations (standard, super-X, snowflakes) in MAST-U and TCV and studied tungsten melting in WEST and ASDEX Upgrade. The input from the smaller devices to JET has also been proven successful to set-up novel control schemes on disruption avoidance and detachment.

In this paper, we present the design and commissioning results of the upgraded collective Thomson scattering diagnostic at the Wendelstein 7-X stellarator. The diagnostic has a new radiometer designed to operate between the second and third harmonics of the electron cyclotron emission from the plasma at 171–177 GHz, where the emission background has a minimum and is of order 10–100 eV. It allows us to receive the scattered electromagnetic field with a significantly improved signal-to-noise ratio and extends the set of possible scattering geometries compared to the case of the original instrument operated at 140 GHz. The elements of the diagnostic are a narrowband notch filter and a frequency stabilized probing gyrotron that will allow measuring scattered radiation spectra very close to the probing frequency. Here, we characterize the microwave components applied to the radiometer and demonstrate the performance of the complete system that was achieved during the latest experimental campaign, OP2.1.