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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.

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.

In tokamak plasmas, tangential neutral beam injection (NBI) produces (a large fraction of) circulating energetic ions (CEIs) and induces plasma toroidal rotation, both of which play an important role in the stability of tearing mode (TM). In this study, the effect of NBI on TM is systematically investigated using kinetic-magnetohydrodynamic hybrid code M3D-K. Here, the effect of NBI is modeled as the combined effects of CEI and toroidal rotation. The analysis focuses on the dependency of NBI’s effect on key physical parameters, including magnetic shear, total beta and plasma shape. The modification of rotation on equilibrium is self-consistently included in simulation, which can enhance the destabilizing effect of counter-CEI on TM and has a negligible contribution to the effect of co-CEI. Furthermore, the simulation results reveal that the co-NBI always reduces TM’s growth rate due to the dominant stabilizing effect of rotation and the weak net effect of co-CEI, agreeing well with most experimental results. Whether the counter-NBI stabilizes or destabilizes TM depends on the competition between the stabilizing contribution from rotation and the destabilizing contribution from counter-CEI. Specifically, the counter-NBI tends to stabilize TM when elongation and triangularity decrease, while magnetic shear, total beta and aspect ratio increase.

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a national large research infrastructure in China. CRAFT aims to develop the key technologies for future fusion reactor and the negative ion source neutral beam injection (NNBI) is one of the test facilities or prototypes. CRAFT NNBI test facility mainly includes two test stands and several shared supporting systems. One is the Hefei Open-facility for Negative-ion Source Research, which is devoted to the experimental study of the negative ion source. Other one is the Chinese Fusion Engineering Testing Reactor Advance Neutral Beam Equipment, which acts as the prototype of the neutral beam injector for Chinese Fusion Engineering Test Reactor. Under the CRAFT project, the single-drive, the dual-drive, and the quad-drive RF negative ion sources have been developed and tested step by step. The associated CRAFT NNBI Physics Activity was established to carry out synergistic study of key issues of NNBI technology. Since the experimental research starting in 2021, the achieved parameters of the CRAFT NNBI, such as the negative ion beam energy, beam power and pulse duration, have been steadily improved. At present, based on the dual-drive RF negative ion source with single-stage accelerator, the negative hydrogen ion beams with the beam energy of >200 keV, beam power of >2 MW, and pulse duration of >100 s have been attained repeatedly. Simultaneously, several accompanying experimental research can give support to the future improvements of the CRAFT NNBI in terms of the RF coupling frequency, plasma uniformity, Cs injection, beam divergence, high voltage holding, neutralization efficiency.

Neutral beam injection has been shown to create field-reversed configuration plasmas on C-2W. Recent advances in optimization and control of eight neutral beam injectors help deliver customized current drive for a plethora of plasma regimes. The optimization of neutral beam injection (NBI) relies on in situ beam-characterization measurements and carefully executed experiments. During optimization, beam parameters are finely tuned to maximize the beam current injected into the plasma while minimizing gas bleed into the main plasma chamber. Precisely controlled power supplies and fast measurements enable feed-forward waveforms and real-time control of beam current and energy, beyond initial requirements. This includes beam energy modulations up to +/−7.5 keV (25 % mean energy) at rates greater than 3 kHz and stable beam current ( +/−3 %). Subsequently, unique combinations of in phase and out of phase tuning of NBI modulations, in conjunction with the axisymmetric positioning of the tunable energy beams, are shown to enhance fast-ion effects or reduce energetic particle modes within the plasma. These methods are described herein and represent significant advances in NBI operation.

The Joint European Torus recently carried out an experimental campaign using a plasma consisting of both deuterium (D) and tritium (T). We observed a high-frequency mode using a reflectometer and an interferometer in a D-T plasma heated with low power neutral beam injection, P N B I = 11.6 MW . This mode was observed at a frequency f = 156 kHz and was located at major radii 3.1 ⩽ R ( m ) ⩽ 3.3 . The observed mode was identified as a toroidal Alfvén eigenmode (TAE) using the linear MHD code, MISHKA. Beam ions and fusion-born alpha particles were modelled using the full orbit particle tracking code LOCUST, which produces smooth distribution functions suitable for stability calculations without analytical fits or the use of moments. We calculated the stability of the 21 candidate modes using the HALO code. These calculations revealed that beam ions can drive TAEs with toroidal mode numbers n ⩾ 8 with linear growth rates γ b / ω ∼ 1 % , while TAEs with n  < 8 are damped by the beam ion population. Alpha particles drive modes with significantly smaller linear growth rates, γ α / ω ≲ 0.1 % due to the low alpha power generated almost exclusively by beam-thermal fusion reactions. Non-ideal effects were calculated using complex resistivity in the CASTOR code, leading to an assessment of radiative, collisional, and continuum damping for all 21 candidate modes. Ion Landau damping was modelled using Maxwellian distribution functions for bulk D and T ions in HALO. Radiative damping, the dominant bulk damping mechanism, suppresses modes with high toroidal mode numbers. Comparing the drive from energetic particles with damping from thermal particles, we find all but one of the candidate modes are damped. The single net-driven n  = 9 TAE with a net growth rate γ n e t / ω = 0.02 % matches experimental observations with a lab frequency f = 163 kHz and location R = 3.3 m . The TAE was driven by co-passing particles through the v ∥ = v A / 5 resonance. Both co- and counter-passing alpha particles drive the TAE through the v ∥ = v A / 3 resonance. Additional sideband resonances contribute significant drive for both beam and alpha particles.

An ad hoc calculation is presented to evaluate the growth rate of ion cyclotron emission (ICE) driven by ionized atoms from neutral beam injection (NBI). The distribution function of ionized NBI atoms is modeled with a power-law dependence on particle speed ( v ) with a power-law index α and a Gaussian distribution in pitch angle ( μ ) with a standard deviation. The growth rate of nearly perpendicular propagating fast magnetosonic waves is then calculated in uniform plasma to study the effects of the velocity gradient and the pitch-angle anisotropy on the excitation of ICE. In the limit of μ s → 0, the contribution to the growth rate ( γ ) arising from the velocity gradient satisfies γ ∝ α and the contribution due to the pitch-angle anisotropy satisfies γ ∝ μ s − 1 . The dependence of γ on the injection direction of NBI and the speed of NBI atoms is examined in the limit of μ s → 0. The growth rate of ICE on EAST and ASDEX Upgrade is analyzed using TRANSP-calculated distribution of ionized NBI atoms. The fast decrease in the signals of core ICE in discharge #100 126 on EAST is attributed to the increase of μ s and the decrease of α of the distribution of NBI ions at the core. The core ICE found on ASDEX Upgrade is, however, primarily caused by the velocity gradient, since ionized NBI atoms at the core of ASDEX Upgrade have a much softer distribution in v than ionized NBI atoms at the core of EAST. The possibility of pitch-angle scattering of NBI ions on self-generated fast magnetosonic waves through quasi-linear interaction is examined in uniform plasma, assuming that the excited waves only propagate at a specified angle. The results, though idealized and heuristic, indicate that nearly perpendicular propagating ICE at the second and third harmonic frequencies may be more efficient in scattering resonant particles in pitch-angle space.

In magnetic confinement fusion experiments, ion temperature is one of the key parameters affecting the fulfillment of Lawson’s criterion, the rate of reaction generation and the quality of confinement. Currently, the commonly used methods to diagnose ion temperature in magnetic confinement fusion devices include Langmuir probe, charge exchange recombination spectroscopy (CXRS) method, laser-induced fluorescence method, neutron yield and energy spectrum method, etc. However, these methods are generally characterized by the shortcomings of not being able to measure the ion temperature of the core, or needing to occupy the optical diagnostic window, or not being able to measure the temperature in real time, or only being able to measure the temperature of the plasma in the thermally-equilibrium state. How to diagnose the ion temperature of the core in future closed fusion reactors without optical diagnostic windows has become an important issue in Tokamak operation. By utilizing the characteristics of neutral beams mainly deposited in the plasma core and strong penetration ability of fusion neutrons, we establish a method for diagnosing core ion temperature using fusion neutron yield coupling under neutral beam injection conditions. This method can self consistently distinguish the contributions of thermonuclear neutrons and beam target neutrons, thereby achieving real-time measurement of ion temperature. First, the evaluated nuclear data file (ENDF) database is used as the baseline to perform fine velocity-group calculations. A temperature-dependent correction is then applied to the D–D fusion reaction cross section. This process yields a multi-temperature differential cross section database. Then, the ionization and slowing down process of deuterium atoms is calculated, which leads to the formation of a database of the neutron yields for the thermonuclear and the beam-target reaction. Finally, an iterative algorithm is designed to calculate the respective yields of the two types of neutrons and the ion temperatures in real time. The effects of yield and density input parameters on the applicability of the method were evaluated. Their influence on uncertainty amplification was also assessed. This method provides data support for the operation and parameter optimization of the Tokamak device. The method is demonstrated and validated using experimental data from the HL-3 tokamak.