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The ITER Neutral Beam Test Facility (NBTF), called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy and includes two experiments: MITICA, the full-scale prototype of the ITER heating neutral beam injector, and SPIDER, the full-size radio frequency negative-ions source. The NBTF realization and the exploitation of SPIDER and MITICA have been recognized as necessary to make the future operation of the ITER heating neutral beam injectors efficient and reliable, fundamental to the achievement of thermonuclear-relevant plasma parameters in ITER. This paper reports on design and R&D carried out to construct PRIMA, SPIDER and MITICA, and highlights the huge progress made in just a few years, from the signature of the agreement for the NBTF realization in 2011, up to now-when the buildings and relevant infrastructures have been completed, SPIDER is entering the integrated commissioning phase and the procurements of several MITICA components are at a well advanced stage.

On the path to a fusion pilot plant, Thea Energy plans to build Eos, a sub-breakeven, deuterium-deuterium, beam-target fusion, stellarator neutron source facility for producing tritium and other valuable radioisotopes. In this paper, a set of 1D plasma physics models are coupled and used to design the operating point of the facility and predict performance. At this foundational stage of the design, analytic and approximate models are sufficient to capture the leading-order effects, and fast enough to run in the inner loop of an optimizer. Higher-fidelity analyses will follow. Models of 1D profile-dependent neutral beam stopping, ion beam slowing down, beam-target fusion, electron-ion classical heat transfer, energy confinement (ISS04), beam pressure, beam heating of ions and electrons, beam-beam fusion fraction, and neutral beam injection and gyrotron heating electrical efficiencies are included. A numerical optimizer is used to determine the minimum required facility electric power to generate tritium at a given rate. A potentially advantageous regime is described in which modern precisely-quasisymmetric stellarators, new high-temperature superconductors, ITER-derived neutral beam injection, and new high-frequency gyrotrons enable a suitible target plasma with hot electrons, cold ions, peaked density and temperature profiles, and high beam-injected ion density. It appears possible at this time for a facility with a medium-scale and medium-strength stellarator whose required facility electric power is less than 40 MW to produce 2.5×1017 neutrons s−1 for the production of radioisotopes. With the addition of a tritium breeding blanket, such a facility could produce 0.2 grams d−1 or 70 grams yr−1 of tritium.

A promising negative-ion-based neutral beam injection (NNBI) system should achieve high energy, high power and long pulse simultaneously for the plasma heating and current drive in the large-scale fusion devices. A NNBI test facility has been constructed in the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. The installed capability of the whole CRAFT NNBI test facility is the beam energy of 400 keV, the ion beam current of 28 A, and the continuous beam pulse of 1 h. A dual-driver RF negative ion source (beam size: 0.75 × 0.3 m2, design acceleration voltage: 200 kV) has been developed and tested for the first operation of the test facility. Several significant improvements were applied to the test facility or the negative ion source during the system maintenance. In the second experimental campaign of CRAFT NNBI test facility, beam extraction and acceleration have reached 100 s duration and megawatt power levels. The typical values were 135 keV, 10.6 A (≈180 A m−2) and 110 s at the filling pressure of 0.4 Pa. For ten second beam pulses higher energy and current levels were established (173 keV and 12 A). The results demonstrated the negative ion source can reliably operate for long pulse. Some problems about the high-voltage holding, particle and heat flux were revealed in the high-power and long-pulse beam acceleration.

Simulations exploring neutral beam operation in Wendelstein 7-X (W7-X) at reduced magnetic field are performed using a newly implemented gyro orbit model in the BEAMS3D code. Operation at field strengths below the nominal 2.5 T are seen as a path to explore both high beta plasmas and as a means to access magnetic configurations not possible at 2.5 T. As the field strength becomes smaller, the gyro radius for 55 keV fast protons grows from ∼ 1 c m at 2.5 T to ∼ 5 c m at 0.75 T in a device with minor radius ∼ 50 c m bringing into question the applicability of the gyro center approximation. To address this a gyro orbit model was implemented in the BEAMS3D code. Agreement is found between the gyro center and gyro orbit models in a circular cross section tokamak equilibrium at high field. A set of W7-X equilibria are assessed with fixed density and temperature profiles but decreasing magnetic field strength (increasing plasma beta). Neutral beam deposition is found to be mostly unaffected with changes in the core of the plasma associated with the Shafranov-shift. In general good agreement is found between gyro orbit and gyro center simulations at 2.5 T. Both models indicate increasing losses with decreasing magnetic field strength with the gyro orbit losses being higher at all field strengths. Gyro orbit simulations to the first wall of W7-X show a change in loss pattern with decreasing magnetic field strength. A preliminary assessment of losses to fast ion loss detectors are made.

In early 2022 one source of ASDEX Upgrade’s (AUG) neutral beam injector 2 was equipped with a first-of-its-kind beam extraction grid system with in-situ variable extraction/acceleration gap that allows one to choose beam energy and beam power independently in a wide operational space, greatly enhancing experimental flexibility. The gap can be changed from one AUG discharge to the next. The extended operational space makes it possible to reduce beam energy and shine through while maintaining high heating power, or to reduce NBI power at high beam energy, e.g. to stay in L mode. Furthermore, the feature opens the door for advanced control of heat and torque deposition, such as to change torque and ion-to-electron heating ratio at constant power. The prototype system was successfully tested in 2022 and already found first applications in AUG’s physics programme. The remaining three sources of the same injector will also be equipped with variable gaps during the 2022–24 opening of AUG and installation of this new system on all sources of NBI 1 is also under discussion in order to exploit the full potential of the new feature.

We have developed a simulation code for the plasma current driven by the neutral beam in the large helical device (LHD). To compare the results between the experiment and the simulation, not only the beam-driven current, but also the other components, must be estimated. In this paper, we have integrated the codes that solve electromagnetic induction, neoclassical transport (DKES/PENTA), and the fast-ion distribution function (TASK3D-a and TASK/FP) to achieve Neutral beam current drive simulation. The code has been benchmarked against the analytical solution for the evolution of the plasma current. Then the code has been applied to actual LHD discharges. Although the trend and the absolute value of the simulated plasma current became similar to the measured one, the time constant of the current evolution disagreed. By considering the mutual induction between the plasma and the external coils, the simulated time constant of the plasma current evolution became closer to the experimental one.

Spherical Tokamak for Energy Production (STEP) is the UK’s prototype fusion power plant programme aiming to demonstrate net electrical output from a spherical tokamak. The plasma scenarios require a completely non-inductive current drive for the flat-top and the majority of the ramp-up/down phases. Most of the current ( ∼80%) is self-generated by the plasma pressure gradient with the remainder provided by the heating and current drive (HCD) system. The capabilities and limitations of neutral beam injection (NBI) for current drive in relevant STEP scenarios are presented alongside a discussion of integration challenges. It is demonstrated that, in isolation, NBI has excellent current drive efficiency achieving ζ=0.4 at ρ=0 rising to ζ=1.4 at ρ=0.8 for beam energies ⩽1MeV. NBI current drive in STs also demonstrates a strong up-down asymmetry and weak dependence on the effective charge. However, once considered in an integrated design, the poor wall-plug efficiency, large size and consequent high cost make NBI undesirable in STEP compared to microwave based HCD.

The ITER neutral beam system will be equipped with radio-frequency (RF) negative ion sources, based on the IPP Garching prototype source design. Up to 100 kW at 1 MHz is coupled to the RF driver, out of which the plasma expands into the main source chamber. Compared to arc driven sources, RF sources are maintenance free and without evaporation of tungsten. The modularity of the driver concept permits to supply large source volumes. The prototype source (one driver) demonstrated operation in hydrogen and deuterium up to one hour with ITER relevant parameters. The ELISE test facility is operating with a source of half the ITER size (four drivers) in order to validate the modular source concept and to gain early operational experience at ITER relevant dimensions. A large variety of diagnostics allows improving the understanding of the relevant physics and its link to the source performance. Most of the negative ions are produced on a caesiated surface by conversion of hydrogen atoms. Cs conditioning and distribution have been optimized in order to achieve high ion currents which are stable in time. A magnetic filter field is needed to reduce the electron temperature and co-extracted electron current. The influence of different field topologies and strengths on the source performance, plasma and beam properties is being investigated. The results achieved in short pulse operation are close to or even exceed the ITER requirements with respect to the extracted ion currents. However, the extracted negative ion current for long pulse operation (up to 1 h) is limited by the increase of the co-extracted electron current, especially in deuterium operation.

The impact of uncertainties in different parameters characterizing the plasma on the power deposition and the particle sources is analyzed and presented. The analysis focuses on the application of uncertainty quantification and sensitivity analysis (SA) to the neutral beam injection (NBI) by performing parametric modeling of the thermal equilibrium and the ionization process of the plasma. The uncertainties are propagated through the TAPaS code (Toroidal Accelerated PArticle Simulator), obtaining various quantities of interest, such as the birth profile of the NBI and the fraction of energy that is transferred to the ions and electrons of the background plasma. A non-intrusive, black-box approach is considered for the model responses through the utilization of a polynomial chaos expansion (PCE) as surrogate model. The global SA using the analytical computation of the Sobol indices from the estimated coefficients of the PCE allows to assign and rank the individual contribution of each uncertain parameter relative to the variation exhibited in the plasma heating. Further contributions from the interactions of different parameter combinations to the variance are also investigated.

Mechanisms of destabilization of toroidal Alfvén eigenmodes (TAEs) in tokamaks are analyzed with the aim to reveal those leading to modes propagating in the direction opposite to plasma current, i.e. counter-propagating modes (ctr-TAE). Plasmas with fast-ions sources [such as neutral beam injection (NBI), ion cyclotron resonance heating, fusion reactions] and without them (Ohmic discharges) are considered. A particular NSTX-U experiment with NBI, where co- and counter-propagating TAEs were observed simultaneously (Podestà et al 2018 Nucl. Fusion 58 082023), is considered. It is concluded that both types of TAEs occurred because their destabilization was caused by the velocity anisotropy of beam ions, which overrode effects of spatial inhomogeneity of these ions.