Energetic Ion Confinement Research Articles

A quasi-isodynamic configuration with good confinement of fast ions at low plasma β

A new quasi-isodynamic (QI) stellarator configuration optimized for the confinement of energetic ions at low plasma β is obtained. The numerical optimization is carried out using the STELLOPT suite of codes. New proxies to measure closeness to quasi-isodynamicity and quality of fast ion confinement have been included. The new configuration has poloidally closed contours of magnetic field strength, low magnetic shear and a rotational transform profile allowing an island divertor. It shows ideal and ballooning magnetohydrodynamic stability up to , reduced effective ripple, with in the plasma core. Even at low β, the configuration approximately satisfies the maximum-J property, and the confinement of fast ions is good at and becomes excellent at reactor values, . An evaluation of the D 31 neoclassical mono-energetic coefficient supports the expectation of a reduced bootstrap current for plasmas confined in QI configurations. A set of filamentary coils that preserve the good confinement of fast ions in the core is presented.

Large Volume and Fast Response Gamma Ray Diagnostic in the Large Helical Device

A large volume and fast response gamma ray diagnostic based on the LaBr3(Ce) scintillator was installed to obtain the gamma ray spectrum in the Large Helical Device (LHD) for understanding energetic ion confinement. The advantages of the LaBr3(Ce) scintillator are relatively sensitive to gamma rays due to its relatively heavy weight density of 5.3 g/cc, high counting operation because of a relatively short pulse width of ∼100 ns, and relatively better energy resolution of ∼3%. The gamma ray diagnostic was installed at the outboard side of LHD. The radiation shielding for the LaBr3(Ce) detector was designed to avoid unwanted signals due to stray neutrons and gamma rays using the three-dimensional radiation transport calculation MCNP6. In-situ energy calibration of the LaBr3(Ce) detector was performed using 60Co and 137Cs gamma ray sources. We surveyed a neutron effect on the LaBr3(Ce) detector in an electron-cyclotron-heated deuterium plasma discharge. The pulse counting rate of LaBr3(Ce) detector under the total neutron emission rate of 2×1011 n/s was 110 kcps. Therefore, the LaBr3(Ce) detector is expected to be utilized in most of ion cyclotron resonance frequency (ICRF) discharges, where the total neutron emission rate of ∼1011 n/s. We plan to measure the gamma ray spectrum in deuterium ICRF discharges.

Fusion product diagnostics based on commercially available chemical vapor deposition diamond detector in large helical device

Fusion product diagnostics based on four commercially available single-crystal chemical vapor deposition (s-CVD) diamond detectors are installed in the Large Helical Device (LHD) in order to understand energetic ion confinement. Characteristics of s-CVD diamonds were surveyed using alpha and D-T neutron sources. It is found that the energy resolutions of s-CVD diamonds for ∼ 5 MeV alpha particles and 14 MeV neutrons are 1%–3% and ∼ 1.7%, respectively. Moreover, the response of four s-CVD diamond detectors to alpha particles and D-T neutrons is almost identical. The installation positions of the diamond detectors in the vacuum vessel are searched for, based on the loss points of charged fusion products reckoned by Lorentz orbit calculations. Energy- and time-resolved measurement of fusion product flux will progress in further understanding of energetic ion confinement in LHD.

Trapped particle resonance effects on the NTM driven losses of energetic particles

The m = 2, n = 1 (where m and n are the poloidal and toroidal mode numbers, respectively) neoclassical tearing mode (NTM) has been proposed as a candidate to explain the larger than expected losses of high energy ions produced by neutral beam injection observed in several experiments. Although the numerical simulations performed so far to study the effect of NTMs on energetic ions have reproduced several features observed in experiments, the agreement is not completely satisfactory. In particular, it has been difficult to reproduce the total amount of losses, which are affected by the details of the perturbation in the edge region. In this work, we study the effect of NTMs on the confinement of energetic ions produced by NBI injection using FOCUS, a full orbit code that runs in graphical processing units. This allows us to follow the evolution of a large number of ions with modest resources. A reconstruction technique that includes the experimental information available is employed to calculate the perturbed magnetic field. The main result of this study is that when the frequency of the NTM matches the toroidal precession frequency ω tp of the trapped ions (, where E is the energy of the ion), the losses increase significantly.

Development of a high-temporal resolution neutron flux measurement system for the HL-2M tokamak

To enhance the understanding of the physics of energetic ions in fusion plasma, a high-temporal resolution neutron flux measurement (HTRNFM) system, which is equipped with a fast-neutron scintillation detector embedded with ZnS:Ag phosphor, has been developed for the HL-2M tokamak. It has a temporal resolution of 10 μs during conventional operations. Its dynamic range is sufficiently wide for neutron flux measurements by adopting the combination use of the scalar mode and the Campbell mode. Based on the Monte Carlo calculations, the applicable count rate ranges of both the scalar mode and the Campbell mode are respectively 0.1–10 Mcps and 10–200 Mcps. The performance validation of the HTRNFM system has been performed by neutron flux measurements in magnetohydrodynamic (MHD) quiet plasmas in the HL-2A tokamak. In another plasma with abundant MHD instabilities, both the continuous neutron flux decreases and the rapid neutron flux decreases caused by different MHD instabilities are observed in a more detailed manner for the first time with the HTRNFM system than with other neutron flux measurement (NFM) systems that have a lower temporal resolution of 1 ms. The HTRNFM system will serve as a powerful diagnostic tool for research on energetic ion confinement quality in the HL-2M tokamak.

Improving the stellarator through advances in plasma theory

Improvements to the stellarator concept can be realized through advancements in theoretical and computational plasma physics. Herein, recent advances are reported in the topical areas of: (1) improved energetic ion confinement, (2) the impact of three-dimensional (3D) shaping on turbulent transport, (3) reducing coil complexity, (4) novel optimization and design methods, and (5) computational magnetohydrodynamic tools. These advances enable the development of new stellarator configurations with improved confinement properties.

A model for the fast evaluation of prompt losses of energetic ions in stellarators

A good understanding of the confinement of energetic ions in non-axisymmetric magnetic fields is key for the design of reactors based on the stellarator concept. In this work, we develop a model that, based on the radially-local bounce-averaged drift-kinetic equation, classifies orbits and succeeds in predicting configuration-dependent aspects of the prompt losses of energetic ions in stellarators. Such a model could in turn be employed in the optimization stage of the design of new devices.

Progress on Integrated Neutron Diagnostics for Deuterium Plasma Experiments and Energetic Particle Confinement Studies in the Large Helical Device During the Campaigns from FY2017 to FY2019

To achieve a steady-state fusion burning plasma, energetic particle confinement studies have been performed in fusion devices. Deuterium plasma experiments in the Large Helical Device starting from March 2017 have expanded the energetic ion confinement study toward a helical-type fusion reactor. To conduct this study, integrated neutron diagnostics, such as a wide dynamic range neutron flux monitor composed of three sets of detectors, a neutron activation system with two irradiation ends, three vertical neutron cameras, four types of scintillating fiber detectors, and a fast time response neutron fluctuation detector, were installed based on the plan and were working stably, as designed. Moreover, an energetic particle confinement study was advanced by utilizing integrated neutron diagnostics. Furthermore, the results of the energetic particle confinement study using neutron diagnostics obtained from FY2017 to FY2019 are reviewed in this paper.

Energetic particle transport and loss induced by helically-trapped energetic-ion-driven resistive interchange modes in the Large Helical Device

In this work, energetic-ion confinement and loss due to energetic-ion driven magnetohydrodynamic modes are studied using comprehensive neutron diagnostics and orbit-following numerical simulations for the Large Helical Device (LHD). The neutron flux monitor is employed in order to obtain global confinement of energetic ions and two installed vertical neutron cameras (VNCs) viewing different poloidal cross-sections are utilized in order to measure the radial profile of energetic ions. A strong helically-trapped energetic-ion-driven resistive interchange mode (EIC) excited in relatively low-density plasma terminated high-temperature state in LHD. Changes in the neutron emission profile due to the EIC excitation are clearly visualized by the VNCs. The reduction in the neutron signal for the helical ripple valley increases with EIC amplitude, which reaches approximately 50%. In addition to the EIC experiment, orbit-following simulations using the DELTA5D code with EIC fluctuations were performed to assess the energetic-ion transport and loss. Two-dimensional temporal evolution results show that the neutron emissivity at the helical ripple decreases significantly due to the EIC. The rapid reduction in neutron emissivity shows that the helically-trapped beam ions immediately escape from the plasma. The reduction in the VNC signals for the helical ripple valley and the total neutron emission rate increase with increasing EIC amplitude, as observed in the experiment. Calculated line-integrated neutron emission results show that the profile measured by VNC1 has one peak, whereas the profile measured by VNC2 has two peaks, as observed in the experiment. Although the neutron emission profile for VNC2 has a relatively wide peak compared with the experimental results, the significant decrease in neutron signal corresponding to the helical ripple valley was successfully reproduced.

Resonant effects on the loss of energetic trapped ions induced by low-n resonant magnetic perturbations

The fast ion dynamics and the associated loss channels under resonant magnetic perturbation (RMP) fields are investigated numerically with newly upgraded Monte-Carlo code ORBIT-RF. Special attention is paid to the effect of low-n () perturbations on the confinement of energetic trapped ions in an equilibrium from EAST experiments. Here, n donates the toroidal mode number. It is found that the dominant loss mechanism under n = 2 RMP is essentially different from that under n = 1 RMP. For n = 2 RMP, the enhanced losses are mainly due to the single resonance with . Here, and are, respectively, the precession and bounce frequencies. For n = 1 RMP, the losses are mainly due to orbital stochasticity near the trapped-passing boundary, which is mainly promoted by the nonlinear resonances. It is found that the loss fraction has a sine-like dependence on the phase difference between the upper and lower coil currents under both n = 1 and n = 2 RMPs, which suggests the possibility of active control of fast ion profile by designing the spectrum. The loss rate under n = 2 RMP shows a linear dependence on the perturbation amplitude, while the loss rate under n = 1 RMP shows a quadratic dependence. The results here highlight the importance of nonlinear resonances in fast ion confinement under RMP, in addition to that of linear resonances frequently emphasized before.

Time dependent neutron emission rate analysis for neutral-beam-heated deuterium plasmas in a helical system and tokamaks

A neutron emission rate analysis code called FBURN, based on the classical energetic ion confinement assumption with radial diffusion, is developed for the time-dependent analysis of the total neutron emission rate (Sn) in neutral beam (NB) heated deuterium plasmas. The time trend of Sn evaluated by FBURN shows good agreement with the Sn measured by the neutron flux monitor on the deuterium operation of the Large Helical Device. The dependence of Sn on line-averaged electron density (ne_avg) has a peak at ne_avg of around 2.5 × 1019 m−3 in both experiment and calculation. Here, the absolute value of Sn evaluated by calculation agrees with that obtained in experiments within a factor of two. A time trend analysis of Sn in an electron cyclotron heated plasma with a short pulse NB injection is performed. The analysis shows that the diffusion coefficient of co-going transit beam ions is 0.2 to 0.3 m2 s−1. In addition, the diffusion coefficient of helically trapped beam ions decreases from 5 to 3 m2 s−1 with the inward shift of the magnetic axis position. Time-resolved analysis of the triton burnup experiment shows that the diffusion coefficient of tritons is around 0.15 m2 s−1. It is found that the diffusion coefficients of the beam and tritons are of a similar value as obtained in JT-60U. The trend of the triton burnup ratio on the ne_avg calculated by the FBURN agrees with the experiments. The results suggest that the decrease of the triton burnup ratio with the increase of ne_avg is due to the shorter slowing down time of tritons by the decrease of the electron temperature, and the increase of the triton burnup ratio with the increase of ne_avg is due to the diffusion of tritons. Time trend analysis of Sn in the Korea Superconducting Tokamak Advanced Research and the Experimental Advanced Superconducting Tokamak plasmas with a short pulse NB injection is performed. The time trend of Sn is successfully reproduced by FBURN.

Scenario development for the observation of alpha-driven instabilities in JET DT plasmas

In DT plasmas, toroidal Alfvén eigenmodes (TAEs) can be made unstable by the alpha particles resulting from fusion reactions, and may induce a significant redistribution of fast ions. Recent experiments have been conducted in JET deuterium plasmas in order to prepare scenarios aimed at observing alpha-driven TAEs in a future JET DT campaign. Discharges at low density, large core temperatures associated with the presence of internal transport barriers and characterised by good energetic ion confinement have been performed. ICRH has been used in the hydrogen minority heating regime to probe the TAE stability. The consequent presence of MeV ions has resulted in the observation of TAEs in many instances. The impact of several key parameters on TAE stability could therefore be studied experimentally. Modeling taking into account NBI and ICRH fast ions shows good agreement with the measured neutron rates, and has allowed predictions for DT plasmas to be performed.

Effect of the helically-trapped energetic-ion-driven resistive interchange modes on energetic ion confinement in the Large Helical Device

The effect of the helically-trapped energetic-ion-driven resistive interchange modes (EICs) on energetic ion confinement is studied in the Large Helical Device deuterium plasmas. Neutron diagnostics such as the neutron flux monitor and the vertical neutron camera (VNC) are used in order to measure neutrons mainly created by beam-plasma reactions. The line-integrated neutron profiles are obtained by VNC in magnetohydrodynamic-quiet plasma with various neutral beam (NB) injection patterns. The profiles are consistent with that expected by the beam ion density calculated using orbit-following simulations. Significant decreases of the total neutron emission rate (Sn) and the neutron counting rate of the VNC (Cn) in central cords are observed to be synchronized with EIC bursts with perpendicular-NB injection. The drop rates of both Sn and Cn increase with EIC amplitude and reach around 50%. The line-integrated neutron profiles before and after EIC burst show that in the central cords, Cn decrease due to EIC burst whereas there is almost no change in the other cords. The experimental results suggests that the effect of EIC on helically-trapped beam ion is substantial, however the effect of passing beam ion is not significant.

Observation of enhanced radial transport of energetic ion due to energetic particle mode destabilized by helically-trapped energetic ion in the Large Helical Device

A deuterium experiment was initiated to achieve higher-temperature and higher-density plasmas in March 2017 in the Large Helical Device (LHD). The central ion temperature notably increases compared with that in hydrogen experiments. However, an energetic particle mode called the helically-trapped energetic-ion-driven resistive interchange (EIC) mode is often excited by intensive perpendicular neutral beam injections on high ion-temperature discharges. The mode leads to significant decrease of the ion temperature or to limiting the sustainment of the high ion-temperature state. To understand the effect of EIC on the energetic ion confinement, the radial transport of energetic ions is studied by means of the neutron flux monitor and vertical neutron camera newly installed on the LHD. Decreases of the line-integrated neutron profile in core channels show that helically-trapped energetic ions are lost from the plasma.

Ways to improve the confinement of fast ions in stellarators by RF waves: general analysis and application to Wendelstein 7-X

A possibility to improve the confinement of energetic ions in stellarators by transforming trapped particles into passing ones using RF waves is investigated. It is concluded that waves with frequencies around the ion gyrofrequency or lower can, in principle, be used for this purpose if certain requirements are satisfied. The suitability of the W7-X ICRH system is considered, and other means for achieving fast-ion confinement are discussed.

Multi-phase hybrid simulation of energetic particle driven magnetohydrodynamic instabilities in tokamak plasmas

Magnetohydrodynamic (MHD) instabilities driven by energetic particles in tokamak plasmas and the energetic particle distribution formed with the instabilities, neutral beam injection, and collisions are investigated with hybrid simulations for energetic particles and an MHD fluid. The multi-phase simulation, which is a combination of classical simulation and hybrid simulation, is applied to examine the distribution formation process in the collisional slowing-down time scale of energetic ions for various beam deposition power () and slowing-down time (). The physical parameters other than and are similar to those of a Tokamak Fusion Test Reactor (TFTR) experiment (Wong et al 1991 Phys. Rev. Lett. 66 1874). For = 10 MW and = 100 ms, which is similar to the TFTR experiment, the bursts of toroidal Alfvén eigenmodes take place with a time interval 2 ms, which is close to that observed in the experiment. The maximum radial velocity amplitude (vr) of the dominant TAE at the bursts in the simulation is where vA is the Alfvén velocity at the plasma center. For = 5 MW and = 20 ms, the amplitude of the dominant TAE is kept at a constant level . The intermittency of TAE rises with increasing and increasing (= decreasing collision frequency). With increasing volume-averaged classical energetic ion pressure, which is well proportional to , the energetic ion confinement degrades monotonically due to the transport by the instabilities. The volume-averaged energetic ion pressure depends only on the volume-averaged classical energetic ion pressure, not independently on or . The energetic ion pressure profile resiliency, where the increase in energetic ion pressure profile is saturated, is found for the cases with the highest where the TAE bursts take place.

The effects of resonant magnetic perturbations on fast ion confinement in the Mega Amp Spherical Tokamak

The effects of resonant magnetic perturbations (RMPs) on the confinement of energetic (neutral beam) ions in the Mega Amp Spherical Tokamak (MAST) are assessed experimentally using measurements of neutrons, fusion protons and fast ion Dα (FIDA) light emission. In single null-diverted (SND) MAST pulses with relatively low plasma current (400 kA), the total neutron emission dropped by approximately a factor of two when RMPs with toroidal mode number n = 3 were applied. The measured neutron rate during RMPs was much lower than that calculated using the TRANSP plasma simulation code, even when non-classical (but axisymmetric) ad hoc fast ion transport was taken into account in the latter. Sharp drops in spatially-resolved neutron rates, fusion proton rates and FIDA emission were also observed. First principles-based simulations of RMP-induced fast ion transport in MAST, using the F3D-OFMC code, show similar losses for two alternative representations of the MAST first wall, with and without full orbit effects taken into account; for n = 6 RMPs in a 600 kA plasma, the additional loss of beam power due to the RMPs was found in the simulations to be approximately 11%.

Fast particle behaviour in the Globus-M spherical tokamak

The behaviour of the fast particle population during 18 keV hydrogen and 26 keV deuterium neutral beam injection in deuterium plasmas is investigated. Experiments reveal large fast ion losses. The experimental results are confirmed using different types of modelling: simulation using the NUBEAM module, solution of the Boltzmann kinetic equation and solution of the 3D fast ion tracking algorithm. The dynamics of the energetic particle redistribution and losses during sawtooth oscillation and toroidal Alfvén eigenmodes are studied. A method to decrease fast ion losses under the current conditions (0.4 T, 0.2 MA) is shown. The influence of the plasma parameters on the energetic ion confinement rate is investigated. Modelling for the Globus-M2 conditions (1 T, 0.5 MA) is performed.

Fast particle confinement with optimized coil currents in the W7-X stellarator

One of the principal goals of the W7-X stellarator is to demonstrate good confinement of energetic ions at finite β. This confinement, however, is sensitive to the magnetic field configuration and is thus vulnerable to design modifications of the coil geometry. The collisionless drift orbit losses for 60 keV protons in W7-X are studied using the ANTS code. Particles in this energy range will be produced by the neutral beam injection (NBI) system being constructed for W7-X, and are particularly important because protons at this energy accurately mimick the behaviour of 3.5 MeV α-particles in a HELIAS reactor.To investigate the possibility of improved fast particle confinement, several approaches to adjust the coil currents (5 main field coil currents +2 auxiliary coil currents) were explored. These strategies include simple rules of thumb as well as computational optimization of various properties of the magnetic field.It is shown that significant improvement of collisionless fast particle confinement can be achieved in W7-X for particle populations similar to α particles produced in fusion reactions. Nevertheless, the experimental goal of demonstrating confinement improvement with rising plasma pressure using an NBI-generated population appears to be difficult based on optimization of the coil currents only. The principal reason for this difficulty is that the NBI deposition profile is broader than the region of good fast-ion confinement around the magnetic axis.

Fusion alpha-particle losses in a high-beta rippled tokamak

In tokamak plasmas, the confinement of energetic ions depends on the magnetic field structure. If the plasma pressure is finite, the equilibrium current (i.e., the Pfirsch-Schlüter current and diamagnetic current) flows in the plasma to maintain the magnetohydrodynamic (MHD) equilibrium. These plasma currents generate poloidal and toroidal magnetic field and alter the field structure. Moreover, if we consider the non-axisymmetry of magnetic field structures such as toroidal field (TF) ripples, the non-axisymmetric component of the equilibrium current can alter TF ripples themselves. When the plasma beta becomes high, the changes in the field structure due to the equilibrium current might affect the confinement of energetic ions significantly. We intend to clarify how these currents alter the field structure and affect the confinement of alpha particles in high-beta plasma. The MHD equilibrium is calculated using VMEC and the orbits of fusion alpha particles are followed by using the fully three-dimensional magnetic field orbit-following Monte Carlo code. In relatively low-beta plasma (e.g., the volume-averaged beta value ⟨β⟩≤2%), the changes in the magnetic field component due to the plasma current negligibly affect the confinement of alpha particles except for the Shafranov shift effect. However, for ⟨β⟩≥3%, the diamagnetic effect reduces the magnetic field strength and significantly increases alpha-particle losses. In these high-beta cases, the non-axisymmetric field component generated by the equilibrium current also increases these losses, but not as effectively as compared to the diamagnetic effect.