Spherical Torus Research Articles

Numerical investigation of electron cyclotron and electron Bernstein wave current drive in EXL-50U spherical torus.

Numerical investigation of electron cyclotron and electron Bernstein wave current drive in EXL-50U spherical torus.

Poloidal field system and advanced divertor equilibrium configuration design of the EHL-2 spherical torus

Abstract The EHL-2 (ENN He-Long 2) spherical torus (ST) project focuses on advancing spherical torus technology to address the unique challenges of p-11b fusion, which demands significantly higher ion temperature and heat flux to the divertor plate compared to traditional deuterium-tritium fusion. With a major radius of 1.05 m and a plasma current of 3 MA, the project aims to evaluate and optimize advanced divertor configurations, specifically the Super-X and X-point target divertors (XPT). The design incorporates an up-down double null configuration featuring a conventional inner divertor and an XPT outer divertor to reduce the heat flux effectively. The poloidal field (PF) coil system is meticulously optimized to balance engineering constraints with the flexibility in equilibrium configurations. This design is expected to provide a reference equilibrium configuration for other physics design issues and offer critical insights into heat load management.

Physics design of current drive and strategy of heating system for EHL-2 spherical torus

Abstract ENN He Long-2 (EHL-2) is the next-generation large Mega-Ampere (MA) spherical torus (ST) proposed and funded by ENN company. The design parameters are: Ti0 > 30 keV, ne0~1×1020 m-3, Ip ~ 3 MA, Bt ~ 3 T. How to achieve several MA current flat-top with limited voltage-second (Vs) of center solenoid (CS) coils is one of the most challenges issue of EHL-2. In order to minimize the consumption of Vs, fully non-inductive start-up by ECRH will be applied in EHL-2. The ramp-up phase will be accomplished with the synergetic mode between CS and non-inductive methods. The strategy of non-inductive start-up and ramp-up with synergetic mode has been verified on EXL-50U’s experiments. Based on this strategy, numerical simulations give the feasibility of EHL-2 to achieve 3 MA plasma current. A high-performance steady-state scenario with Ip ~ 1.5 MA is also designed. In this scenario, the bootstrap current fraction fBS > 70%, the safety factor q at the magnetic axis q0 > 2, the minimum safety factor qmin > 1, the poloidal beta β p > 3 and normalized beta β N > 2.3. Each design iteration integrates the validation of physical models with the constraints of engineering implementation, gradually optimizing the performance of the Heating and Current Drive (H&CD) systems. Numerical simulation results for general auxiliary H&CD systems such as neutral beam injection, electron cyclotron wave, ion cyclotron wave, and lower hybrid wave are presented. These simulation results ensure that the 31 MW H&CD systems comprehensively cover all scenarios while maintaining engineering feasibility

Preliminary considerations and challenges of proton-boron fusion energy extraction on the EHL-2 spherical torus

Abstract EHL-2 spherical torus (ST) is one of the key steps of p-11B (proton-boron or hydrogen-boron) fusion energy research in ENN. The fusion produced energy is carried mainly by alpha particles of average energy 3 MeV, which ideally can be converted to electricity with high efficiency (> 80%). However, there exist serious difficulties to realize such conversion in a fusion device, due to the high energy density and high voltage required. To comprehensively describe the progress of the EHL-2 physics design, this work presents preliminary considerations of approaches for achieving energy conversion, highlighting critical issues for further investigation. Specifically, we provide an initial simulation of alpha particle extraction in the EHL-2 ST configuration as a starting point for p-11B fusion energy conversion.

Divertor heat flux challenge and mitigation in the EHL-2 spherical torus

Divertor heat flux challenge and mitigation in the EHL-2 spherical torus

Predictions of gyrokinetic turbulent transport in hydrogen-boron plasmas on EHL-2 spherical torus

Abstract The EHL-2 spherical torus at ENN is the next-generation experimental platform under conceptual design, aiming at realizing hydrogen-boron (H-B11) thermonuclear fusion which is an attractive pathway towards neutron-free fusion. In order to achieve high performance steady-state plasma, it is extremely necessary to study the turbulence transport characteristics with high boron content in the plasma core. This work investigates the transport properties in the core internal transport barrier (ITB) region of hydrogen-boron plasma utilizing the gyrokinetic code GENE in view of the high ion temperature scenario of EHL-2, specifically focusing on the impact of boron fractions and plasma β on the microinstabilities and corresponding transport features. Numerical findings indicate that the inclusion of boron species effectively suppresses the trapped electron modes (TEMs) as well as promoting a transition from electromagnetic to electrostatic turbulence with increased boron fraction, which is a result of the suppression of microinstabilities by effective charge and mass. Moreover, it has been identified that the external E×B rotational shear has a notable inhibitory influence on transport which can reduce the transport level by 2-3 orders of magnitude especially at medium boron contents. The suppressive effects of E×B on turbulence is weakened once the kinetic ballooning mode (KBM) is excited and the transport shows a rapid increase with β together with a reduction in zonal flow amplitude which is consistent with previous findings. Therefore, it is strongly suggested that exploring advanced strategies for mitigating turbulent transport at high β regimes are necessary for the active control of plasma behavior regarding H-B11 plasma based fusion devices such as EHL-2.

Discovery III: Missions to the Outer Planets Using Inertial Confinement Fusion Propulsion

In the future, space missions to the outer planets within our solar system that carry a human crew may be possible. This work revisits a NASA crewed spacecraft design concept Discovery II, which proposed to use a spherical torus magnetic confinement fusion reactor. The revised spacecraft Discovery III is modeled using a numerical spacecraft propulsion and mission profile code called HeliosX written in Fortran 95, but using an inertial confinement fusion engine for the same 172-ton payload mass, matching mass flow rates and exhaust velocities, achieving a mission time of 118 days for Jupiter and 212 days for Saturn. Both the inertial confinement fusion (ICF) and magnetic confinement fusion vehicles utilize expellant propellant for mass flow rate augmentation by the thermalization of injected hydrogen and thermonuclear fuel in the exhaust. Calculations are performed and the models are assessed for deviation from optimality. It is concluded that, while the ICF version is more massive, less mature technologically, and the worst performer in terms of jet efficiency, preliminary results indicate a lower fuel requirement, which may imply a technological advantage.

Predictions of H-mode access and edge pedestal instability in the EHL-2 spherical torus

Predictions of H-mode access and edge pedestal instability in the EHL-2 spherical torus

Evaluation of thermal and beam-thermal p-11B fusion reactions in the EHL-2 spherical torus

Abstract This paper presents the first comprehensive simulation study of p-11B fusion reactions in a spherical torus. We developed relevant program modules for fusion reactions based on energetic particle simulation frameworks and analyzed the two main fusion channels: thermal and beam-thermal. Using EHL-2 design parameters with n_boron=0.07n_ion and a hydrogen beam at 200 keV and 1MW, our simulation indicates that p-11B reactions produce approximately 1.5×10^15 α particles per second (~0.7 kW) from the thermal channel, and 5.3×10^14 (~0.25 kW) from the beam-thermal channel. We conducted parameter scans to establish a solid physics foundation for the high ion temperature conditions (T_i>26 keV) designed for EHL-2. This work also laid the groundwork for studying various operation modes to explore different reaction channels. The simulation results suggest that the conditions in EHL-2 could be sufficient for investigating p-11B thermonuclear reactions. In addition, we found that EHL-2 offers good confinement for energetic particles, allowing us to research the interactions between these ions and plasmas. This research enhances our understanding of burning plasma physics.

Forming a database to study reversed magnetic shear from the National Spherical Torus eXperiment using machine learning

Achieving a long-lived reversed magnetic shear (RMS) target plasma in the National Spherical Torus eXperiment Upgrade will require developing various sustainment scenarios. To help with the ongoing plasma control efforts, the development of a new analysis for the motional Stark effect (MSE) diagnostic using a machine learning algorithm, namely, MSE-ML, is described. MSE-ML will be used to identify patterns during RMS discharges, some of which suffer magnetohydrodynamic (MHD) events resulting in current redistribution and monotonic q-profiles. A database consisting of q and magnetic shear profiles is being constructed primarily based on the existing National Spherical Torus eXperiment data with equilibrium reconstructions constrained by the magnetic field pitch angle profile measured using the multi-channel MSE diagnostic. An unsupervised k-means clustering of the data is developed to study the RMS formation as a function of time. The initial clustering from the q-profiles shows significant differences in both amplitude and the duration of the RMS period. As a goal, the clustering results that detect and distinguish shots with substantial and sustained RMS are to be used as a preprocessing step in a supervised algorithm to identify the underlying conditions that lead to long-lasting improved confinement with RMS. Another aim of the MSE-ML study is to identify precursors of RMS-destroying MHD events in either derived data such as the q-profile or directly measured data such as the magnetic field pitch angle profile.

Experimental investigation of kinetic instabilities driven by runaway electrons in the EXL-50 spherical torus

Abstract In this study, high-frequency instabilities driven by runaway electrons in the EXL-50 spherical torus have been reported using a high-frequency magnetic pickup coil. The frequency of these instabilities is found to be power function dependent on the plasma density, similar to the dispersion relation of the whistler wave. The observed instability seems to exhibit a fluctuating pattern, resembling frequency chirping behavior, which appears to align with the expected outcomes predicted by the Berk-Breizman model. Theoretically, the excitation threshold of the instability driven by runaway electrons is related to the ratio of the runaway electron density to the background plasma density, and the stability criterion is first demonstrated qualitatively in this work. The instability can be stabilized by the spontaneous rise of plasma density, consistent with the wave-particle resonance mechanism. This investigation demonstrates the excitation characteristic of chirping instabilities in a tokamak plasma and reveals new features of these instabilities, thereby advancing the understanding of the mechanisms for controlling and mitigating runaway electrons.

Nonlinear simulations of GAEs in NSTX-U

A set of nonlinear simulations has been performed in order to study the nonlinear evolution of unstable global Alfvén eigenmodes in the National Spherical Torus Experiment-Upgrade (NSTX-U). Results of the single toroidal mode number, n, simulations are compared with a full nonlinear simulation (all toroidal harmonics included). In single-n simulations, the conservation of two integrals of motion of a particle in a cyclotron resonance with a monochromatic wave is demonstrated, resulting in a one-dimensional evolution of the particle distribution in (E,μ,pϕ) phase-space. Nonlinear simulations (both single-n and full nonlinear) show a significant redistribution of the resonant fast ions, especially in the pitch parameter. Thus, the changes in the resonant particle's parallel and perpendicular energies can be several times larger than the total particle energy change, with only a small fraction transferring into the excitation of the mode itself. This implies that even a relatively small amplitude mode can significantly modify the beam distribution in the resonant region. For the NSTX-U case considered, the single-n simulation results are close to full nonlinear simulation only for the most unstable mode, in which case the saturation amplitudes and changes in the fast ion distribution are comparable. In contrast, peak amplitudes of subdominant modes in all-n simulations are smaller by a factor of 3–10 compared to single-n runs due to the flattening of the beam ion distribution by the fastest growing mode.

Development of a multi-channel visible bremsstrahlung measurement system for the effective charge in the versatile experiment spherical torus (VEST).

A diagnostic system for measuring the effective charge in the versatile experiment spherical torus (VEST) has been developed. The system utilizes a toroidal array to observe the plasma radius on the low magnetic field side, providing a spatially resolved Zeff. The target wavelength of visible bremsstrahlung (VB) was carefully selected to avoid contamination by line emissions. The detector signal was calibrated using a halogen light source and an integrating sphere to obtain an absolute value of the radiative power from each chord. The local emissivity profile was reconstructed from the line-integrated VB emission using the Abel inversion method. Reconstruction tests were performed on various shapes of phantom profiles to effectively reconstruct the local emissivity from the measurements. We found that the initial measurements of the multi-channel VB system were consistent with the results of other independent measurements, supporting the validity of the new measurements. Finally, we obtained the initial result of Zeff in the VEST.

High speed fast-ion D-alpha spectrometer for the NSTX-U tokamak.

Here, we present the design and first calibration results of a new single-channel Fast-Ion D-Alpha (FIDA) spectrometer to be employed at the National Spherical Torus Experiment Upgrade (NSTX-U). The Czerny-Turner-type spectrometer uses a custom-designed aspherical lens setup instead of mirrors and achieves excellent spectral resolution, with high photon throughput through a round-to-linear fiber bundle, and camera frame rates around 8.4kHz. The spectrometer uses a blocking bar to avoid saturation effects of the cold D-alpha emission line and will allow for detailed studies of the fast-ion confinement in NSTX-U. Expected synthetic spectra predicted with the TRANSP and FIDASIM codes show that the spectral range from 648.5 to 658nm will sufficiently cover halo, the red-shifted beam emission, and the blue-shifted portion of FIDA emission in NSTX-U, which is sufficient for fast-ion transport studies of co-rotating fast ions.

Applicability of alkali beam emission spectroscopy on NSTX-U.

Understanding fast pedestal dynamics and turbulent transport in the edge and scrape-off layer (SOL) plasma of spherical tokamaks is crucial for the design and operation of future fusion reactors. The alkali beam emission spectroscopy diagnostic technique offers a means to measure the absolute electron density radial profile and fluctuation amplitude in these regions. In this study, we demonstrate that injecting a sodium neutral beam radially into the plasma and analyzing the light emission from its 3p-3s atomic transition using near-orthogonal viewing angles allows for accurate measurement of the electron density profile and fluctuations in the National Spherical Torus Experiment (NSTX) Upgrade spherical tokamak. Our findings indicate a peak signal-to-noise ratio of 118 in the pedestal and 12 in the SOL under typical NSTX plasma conditions. The spatial resolution for the electron density profile is estimated to be between 2 and 8mm, while for fluctuation measurements, it ranges from 12 to 15mm.

Toroidal Alfvén mode instability driven by plasma current in low-density Ohmic plasmas of the spherical tori

Due to intrinsically low magnetic fields, at low density the drift speed of the current-carrying electrons in spherical tokamaks can exceed fraction of the Alfvén speed sufficient for the excitation of the Alfvén gap modes. A particular case of the toroidal mode, observed during minor disruptions in Ohmic shots on SUNIST [Liu et al., Phys. Plasmas 23, 120706 (2016)], is considered in the present communication. Due to the negligible effect of the electron pressure gradient, the growth rate scales linearly with the drift speed, with slope inversely proportional to electron thermal velocity. In the absence of continuum damping, the threshold value of the drift speed for TAE excitation is independent of electron temperature.

Response to “Comment on ‘ENN’s roadmap for proton-boron fusion based on spherical torus'” [Phys. Plasmas 31, 062507 (2024)]

Response to “Comment on ‘ENN’s roadmap for proton-boron fusion based on spherical torus'” [Phys. Plasmas <b>31</b>, 062507 (2024)]

Comment on “ENN's roadmap for proton-boron fusion based on spherical torus” [Phys. Plasmas 31, 062507 (2024)

This comment discusses the feasibility of hot ion mode Ti/Te=4 for proton–boron fusion, which is critical for the roadmap proposed in Liu et al. [Phys. Plasmas 31, 062507 (2024)]. The hot ion mode Ti/Te=4 has been calculated to be far from accessible (Ti/Te&amp;lt;1.5 for Ti=150 keV) under the most optimal conditions if fusion provides the heating [Xie, Introduction to Fusion Ignition Principles: Zeroth Order Factors of Fusion Energy Research (USTC Press, Hefei, 2023)], i.e., that all fusion power serves to heat the ions and that electrons acquire energy only through interactions with ions. We also discuss if hot ion mode of Ti/Te=4 could be achieved by an ideal heating method, which is much more efficient than fusion itself (near 20 times fusion power for Ti=150 keV) and only heats the ions, whether it makes sense economically.

New millimeter-wave diagnostics to locally probe internal density and magnetic field fluctuations in National Spherical Torus Experiment-Upgrade (invited).

A set of new millimeter-wave diagnostics will deliver unique measurement capabilities for National Spherical Torus Experiment-Upgrade to address a variety of plasma instabilities believed to be important in determining thermal and particle transport, such as micro-tearing, global Alfvén eigenmodes, kinetic ballooning, trapped electron, and electron temperature gradient modes. These diagnostics include a new integrated intermediate-k Doppler backscattering (DBS) and cross-polarization scattering (CPS) system (four channels, 82.5-87GHz) to measure density and magnetic fluctuations, respectively. The system can access reasonably large normalized wavenumbers kθρs ranging from ≤0.5 to 15 (where ion sound gyroradius ρs = 1cm and kθ is the binormal density turbulence wavenumber). The system addresses the challenges for making useful DBS/CPS measurements with a remote control of launch polarization (X- or O-mode), probed wavenumber, polarization match of the launch beam with the edge magnetic field pitch angle, and beam steering of the launched beam for wave-vector alignment. In addition, a low-k DBS system consisting of eight fixed frequencies (34-52GHz) and four tunable frequencies (55-75GHz) for low-k density turbulence and fast ion physics will be located at a nearby port location. The combined systems cover the near LCFS and pedestal regions (34-52GHz), the pedestal or mid-radius (50-75GHz), and core plasmas (82.5-87GHz).

Design of radial interferometer-polarimeter for internal magnetic and density fluctuation measurements at multiple space-time scales in the National Spherical Torus Experiment-Upgrade (NSTX-U).

A Faraday-effect radial interferometer-polarimeter is designed for the National Spherical Torus Experiment-Upgrade (NSTX-U) to measure multiscale magnetic and density fluctuations critical to understanding fusion plasma confinement and stability, including those originating from magnetohydrodynamic instabilities, energetic particle-driven modes, and turbulence. The diagnostic will utilize the three-wave technique with 5MHz bandwidth to simultaneously measure line-integrated magnetic and density fluctuations up to the ion-cyclotron frequency. Probe beams will be launched radially from the low-field side at the NSTX-U midplane, where the measured Faraday fluctuations mainly correspond to radial magnetic fluctuations that directly link to magnetic transport. A correlation technique will be employed to reduce the measurement noise to below 0.01° enabling detection of small amplitude fluctuations. Two toroidally displaced chords with 7° separation will be installed to measure toroidal mode numbers up to n = 25 for mode identification. Solid-state microwave sources operating at 321μm (935GHz) will be used to minimize the impact of the Cotton-Mouton effect.