Fusion-born Alpha Particles Research Articles

Nonlinear burn control in ITER using adaptive allocation of actuators with uncertain dynamics

ITER will be the first tokamak to sustain a fusion-producing, or burning, plasma. If the plasma temperature were to inadvertently rise in this burning regime, the positive correlation between temperature and the fusion reaction rate would establish a destabilizing positive feedback loop. Careful regulation of the plasma’s temperature and density, or burn control, is required to prevent these potentially reactor-damaging thermal excursions, neutralize disturbances and improve performance. In this work, a Lyapunov-based burn controller is designed using a full zero-dimensional nonlinear model. An adaptive estimator manages destabilizing uncertainties in the plasma confinement properties and the particle recycling conditions (caused by plasma–wall interactions). The controller regulates the plasma density with requests for deuterium and tritium particle injections. In ITER-like plasmas, the fusion-born alpha particles will primarily heat the plasma electrons, resulting in different electron and ion temperatures in the core. By considering separate response models for the electron and ion energies, the proposed controller can independently regulate the electron and ion temperatures by requesting that different amounts of auxiliary power be delivered to the electrons and ions. These two commands for a specific control effort (electron and ion heating) are sent to an actuator allocation module that optimally maps them to the heating actuators available to ITER: an electron cyclotron heating system (20 MW), an ion cyclotron heating system (20 MW), and two neutral beam injectors (16.5 MW each). Two different actuator allocators are presented in this work. The first actuator allocator finds the optimal mapping by solving a convex quadratic program that includes actuator saturation and rate limits. It is nonadaptive and assumes that the mapping between the commanded control efforts and the allocated actuators (i.e. the effector model) contains no uncertainties. The second actuator allocation module has an adaptive estimator to handle uncertainties in the effector model. This uncertainty includes actuator efficiencies, the fractions of neutral beam heating that are deposited into the plasma electrons and ions, and the tritium concentration of the fueling pellets. Furthermore, the adaptive allocator considers actuator dynamics (actuation lag) that contain uncertainty. This adaptive allocation algorithm is more computationally efficient than the aforementioned nonadaptive allocator because it is computed using dynamic update laws so that finding the solution to a static optimization problem is not required at every time step. A simulation study assesses the performance of the proposed adaptive burn controller augmented with each of the actuator allocation modules.

Assessment of shutdown dose rates in the ITER Collective Thomson Scattering system and in equatorial port plug 12

The ITER Collective Thomson Scattering (CTS) system will be the main diagnostic responsible for measuring the velocity distribution function of fusion-born alpha particles in the plasma. As the CTS diagnostic is integrated in the equatorial port plug 12 (drawer 3), with direct apertures to the port interspace where maintenance hands-on operation will be carried out, it is essential to assess the shutdown dose rates (SDDR) in these maintenance areas. In this work, the D1S-UNED3.1.4 Monte-Carlo transport code, based on the implementation of the direct-one-step methodology in MCNP5 v1.60, was used to estimate the dose rate level 12 days (106 s) after shutdown in the port interspace. The results show that the CTS system does not contribute significantly to the SDDR in the area where hands-on maintenance is foreseen with contribution to dose rates less than 1 µSv/h. This is consistent with previous estimates, although with the most recent model of the CTS design there is a slight increase of the SDDR values. This deviation can be attributed to design changes and improved shielding modelling and/or most importantly, to statistical fluctuations of the D1S simulations. From a neutronics point of view, the increase in the SDDR falls within the range of the statistical fluctuations, and the design is still compliant with the radiation safety ALARA principle aiming at minimizing radiation doses, and there is no requirement for further design optimizations.

Simulation of heating and current drive sources for scenarios of the ITER research plan

Predicting the impact of heating and current drive (H&CD) sources is essential to evaluate the performance of ITER plasmas and to subsequently optimise the scenarios for the four stages of the ITER research plan. This should be done in the context of global transport calculations of complete plasma discharges. For this purpose, a dedicated workflow has been developed in the ITER integrated modelling and analysis suite as a modular component to be used together with transport solvers to quantify the dynamics of H&CD sources for the different phases of a plasma discharge, including possible synergetic effects between the heating sources. This paper presents the results of the combined modelling of H&CD sources for the ITER DT baseline 15 MA/5.3 T scenario including the synergy between neutral beam injection (NBI) of deuterium, fusion-born alpha particles and ion cyclotron resonance heating (ICRH) at the fundamental frequency of deuterium, showing modest synergetic effects. The results of the combined H&CD sources for an ITER 7.5 MA/2.65 T helium plasma of the second pre-fusion power operation phase (PFPO-2) are also shown, exhibiting more significant synergetic effects between the fundamental ICRH minority hydrogen heating and NBI hydrogen beams. Finally, a study of electron cyclotron heating absorption for an ITER helium PFPO scenario at 7.5 MA/2.65 T is also presented with a discussion on the edge parasitic absorption that arises under specific conditions.

Simulating fusion alpha heating in a stellarator reactor

Gyrocenter following simulations of fusion born alpha particles in a stellarator reactor are preformed using the BEAMS3D code. The Wendelstein 7-X high mirror configuration is scaled in geometry and magnetic field to reactor relevant parameters. A m−3 density plasma with 20 keV core temperatures is assumed and fusion birth rates calculated for various fusion products assuming a 50/50 deuterium-tritium mixture. It is found that energetic He4 ions comprise the vast majority of the energetic particle inventory. Slowing down simulations of the He4 population suggest plasma heating consistent with scaled energy confinement times for a stellarator reactor. Losses for this configuration appear large suggesting optimization beyond the scope of the W7-X device is key to a future fusion reactor. These first simulations are designed to demonstrate the capability of the BEAMS3D code to provide fusion alpha birth and heating profiles for stellarator reactor designs.

Evidence for Alfvén eigenmodes driven by alpha particles in D-3He fusion experiments on JET

Alfvén eigenmodes (AEs) driven by energetic alpha particles can lead to enhanced fast ion transport and losses, thereby degrading the plasma performance in ITER and future magnetic confinement fusion reactors. Unexpectedly, AEs with negative toroidal mode numbers, which are currently not considered for ITER, were observed in dedicated experiments with fusion-born alpha particles on the tokamak Joint European Torus (JET). The paper provides evidence for a complex interplay between fast ions, monster sawtooth crashes and AEs. Our results highlight the need for an improved description of the synergies between different fast ion phenomena in future burning plasmas.

Kinetic simulation of electron cyclotron resonance assisted gas breakdown in split-biased waveguides for ITER collective Thomson scattering diagnostic

For the measurement of the dynamics of fusion-born alpha particles Eα≤3.5 MeV in ITER using collective Thomson scattering (CTS), safe transmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing the electron cyclotron resonance (ECR) in the in-vessel tokamak “port plug” vacuum is a prerequisite. Depending on neutral gas pressure and composition, ECR-assisted gas breakdown may occur at the location of the resonance, which must be mitigated for diagnostic performance and safety reasons. The concept of a split electrically biased waveguide (SBWG) has been previously demonstrated in C.P. Moeller, U.S. patent 4,687,616 (1987). The waveguide is longitudinally split and a kV bias voltage is applied between the two halves. Electrons are rapidly removed from the central region of high radio frequency electric field strength, mitigating breakdown. As a full scale experimental investigation of gas and electromagnetic field conditions inside the ITER equatorial port plugs is currently unattainable, a corresponding Monte Carlo simulation study is presented. Validity of the Monte Carlo electron model is demonstrated with a prediction of ECR breakdown and the mitigation pressure limits for the above-quoted reference case with 1H2 (and pollutant high Z elements). For the proposed ITER CTS design with a 88.9 mm inner diameter SBWG, ECR breakdown is predicted to occur down to a pure 1H2 pressure of 0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using a bias voltage of 1 kV. The analysis is complemented by results for relevant electric/magnetic field arrangements and limitations of the SBWG mitigation concept are addressed.

Nonresonant Diffusion in Alpha Channeling.

The gradient of fusion-born alpha particles that arises in a fusion reactor can be exploited to amplify waves, which cool the alpha particles while diffusively extracting them from the reactor. The corresponding extraction of the resonant alpha particle charge has been suggested as a mechanism to drive rotation. By deriving a coupled linear-quasilinear theory of alpha channeling, we show that, for a time-growing wave with a purely poloidal wave vector, a current in the nonresonant ions cancels the resonant alpha particle current, preventing the rotation drive but fueling the fusion reaction.

Alpha particle driven Alfvénic instabilities in ITER post-disruption plasmas

Fusion-born alpha particles in ITER disruption simulations are investigated as a possible drive of Alfvénic instabilities. The ability of these waves to expel runaway electron (RE) seed particles is explored in the pursuit of a passive, inherent RE mitigation scenario. The spatiotemporal evolution of the alpha particle distribution during the disruption is calculated using the linearized Fokker–Planck solver CODION coupled to a fluid disruption simulation. These simulations are done in the limit of no alpha particle transport during the thermal quench, which can be seen as a most pessimistic situation where there is also no RE seed transport. Under these assumptions, the radial anisotropy of the resulting alpha population provides free energy to drive Alfvénic modes during the quench phase of the disruption. We use the linear gyrokinetic magnetohydrodynamic code LIGKA to calculate the Alfvén spectrum and find that the equilibrium is capable of sustaining a wide range of modes. The self-consistent evolution of the mode amplitudes and the alpha distribution is calculated utilizing the wave-particle interaction tool HAGIS. Intermediate mode number (n = 7–15, 22–26) toroidal Alfvén eigenmodes are shown to saturate at an amplitude of up to δB/B ≈ 0.1% in the spatial regimes crucial for RE seed formation. We find that the mode amplitudes are predicted to be sufficiently large to permit the possibility of significant radial transport of REs.

Effects of energetic particles on the density-gradient-driven collisionless trapped electron mode instability in tokamak plasmas

We investigate analytically the effects of energetic particles (EPs) on the instability of the density-gradient-driven collisionless trapped electron mode (CTEM) through linear gyrokinetic theory and bounce kinetic theory in tokamak plasmas. The effects of EPs, including fusion-born alpha particles and neutral-beam-injection-driven beam ions, on the CTEM instability are compared for the dynamic model with slowing-down (SD) and equivalent Maxwellian (EM) equilibrium EP distribution functions and dilution model. It is found that the density-gradient-driven CTEM instability in the long wavelength regime can be further destabilized by EPs mainly due to the downshift in the real frequency of the mode by dilution effects. This is attributed to more resonant electrons around the smaller phase velocity of the drift wave and the consequent stronger excitation of CTEM instability. The growth rate is slightly higher for the dilution model as compared to that for the dynamic model since the Landau damping effects of EPs are neglected in the dilution model. Moreover, there is no significant difference in the growth rate between the cases of SD and EM equilibrium EP distribution functions, except for the case of alpha particles and with relatively higher electron temperature.

Physics and applications of three-ion ICRF scenarios for fusion research

This paper summarizes the physical principles behind the novel three-ion scenarios using radio frequency waves in the ion cyclotron range of frequencies (ICRF). We discuss how to transform mode conversion electron heating into a new flexible ICRF technique for ion cyclotron heating and fast-ion generation in multi-ion species plasmas. The theoretical section provides practical recipes for selecting the plasma composition to realize three-ion ICRF scenarios, including two equivalent possibilities for the choice of resonant absorbers that have been identified. The theoretical findings have been convincingly confirmed by the proof-of-principle experiments in mixed H–D plasmas on the Alcator C-Mod and JET tokamaks, using thermal 3He and fast D ions from neutral beam injection as resonant absorbers. Since 2018, significant progress has been made on the ASDEX Upgrade and JET tokamaks in H–4He and H–D plasmas, guided by the ITER needs. Furthermore, the scenario was also successfully applied in JET D–3He plasmas as a technique to generate fusion-born alpha particles and study effects of fast ions on plasma confinement under ITER-relevant plasma heating conditions. Tuned for the central deposition of ICRF power in a small region in the plasma core of large devices such as JET, three-ion ICRF scenarios are efficient in generating large populations of passing fast ions and modifying the q-profile. Recent experimental and modeling developments have expanded the use of three-ion scenarios from dedicated ICRF studies to a flexible tool with a broad range of different applications in fusion research.

Numerical simulation of synergistic effect of neoclassical tearing mode and toroidal field ripple on alpha particle loss in China Fusion Engineering Testing Reactor

Confinement of fusion born alpha particles in tokamak is the key issue to burning plasma. Apart from toroidal field ripple, instabilities can induce energetic particles to lose and be redistributed. Based on the parameters of China Fusion Engineering Testing Reactor (CFETT) hybrid scenario, alpha particle distribution and neoclassical tearing mode structure, the alpha particle loss induced under perturbation of ripple and neoclassical tearing mode (NTM) is calculated with the guiding center code ORBIT. The inputs have the initial distribution of alpha particles which is obtained with the TRANSP/NUBEAM code, the static NTM perturbation with different amplitudes which is obtained from TM1 code, and the ripple field from engineering design. The results show that the heat load on last closed flux surface is about 0.1 MW/m<sup>2</sup>, with ripple and collision included. The collisionless stochastic ripple diffusion is the main loss channel of initial alpha particle distribution in the CFETR, and the ripple perturbation has no influence on passing particles. The loss fraction does not increase with the NTM perturbation amplitude increasing, the synergistic effect is negligible. The scanning of ripple amplitude shows that the synergistic effect is slight. The monoenergetic initial distribution of alpha particles can give different types of orbits in the plane of (<inline-formula><tex-math id="M1">\begin{document}$ {P_\zeta },\mu $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20201972_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20201972_M1.png"/></alternatives></inline-formula>), such as the domains of trapped particle and passing particle, lost particle and confined particle. The trapped fraction of initial alpha particles is about 27%, ripple loss region in phase space is narrow and away from the main trapped particle distribution. The increasing of ripple perturbation in simulation does enlarge the ripple loss domain in the phase space (<inline-formula><tex-math id="M2">\begin{document}$ {P_\zeta },\mu $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20201972_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20201972_M2.png"/></alternatives></inline-formula>), which is corresponding to a lager ripple loss fraction and has more trapped-passing boundaries. The NTM perturbation does enlarge the orbit excursions of trapped particles, and thus increasing the trapped passing transition near the boundary. The slight synergistic effect in calculation with larger ripple amplitude is explained by ripple loss region having more trapped-passing boundaries, not by the profile flattening of trapped particles. The NTM perturbation and finite collision can transit the passing particle to trapped particle near the boundary. With the help of kinetic Poincare plot, neither direct particle loss nor profile flattening of trapped particles is observed. The loss fraction enhancement can happen only when the profile flattening of trapped particles takes place within the ripple loss region, which is not the case in CFETR. The conclusion of this work contributes a lot to the design of CFETR and the study of alpha particle physics.

Generation and observation of fast deuterium ions and fusion-born alpha particles in JET plasmas with the 3-ion radio-frequency heating scenario

Dedicated experiments to generate energetic D ions and fusion-born alpha particles were performed at the Joint European Torus (JET) with the ITER-like wall (ILW). Using the 3-ion radio frequency (RF) heating scenario, deuterium ions from neutral beam injection (NBI) were accelerated in the core of mixed plasmas to higher energies with ion cyclotron resonance frequency (ICRF) waves, in turn leading to a core-localized source of alpha particles. The fast-ion distribution of RF-accelerated D-NBI ions was controlled by varying the ICRF and NBI power ( 4–6 MW, 3–20 MW), resulting in rather high D-D neutron (≈ 1 × 1016 s−1) and alpha rates (≈ 2 × 1016 s−1) at moderate input heating power. Theory and TRANSP analysis shows that large populations of co-passing MeV-range D ions were generated using the 3-ion ICRF scenario. This important result is corroborated by several experimental observations, in particular gamma-ray measurements. The developed experimental scenario at JET provides unique conditions for probing several aspects of future burning plasmas, such as the contribution from MeV range ions to global confinement, but without introducing tritium. Dominant fast-ion core electron heating with and a rich variety of fast-ion driven Alfvén eigenmodes (AEs) were observed in these plasmas. The observed AE activities do not have a detrimental effect on the thermal confinement and, in some cases, may be driven by the fusion born alpha particles. A strong continuous increase in neutron rate was observed during long-period sawteeth (1 s), accompanied by the observation of reversed shear AEs, which implies that a non monotonic q profile was systematically developed in these plasmas, sustained by the large fast-ion populations generated by the 3-ion ICRF scenario.

IBIC analysis of SiC detectors developed for fusion applications

In this work, we consider a 4H-SiC detector as a plasma diagnostic system for the detection of fusion-born alpha particles in future nuclear fusion reactors. A nuclear microprobe was used to locally irradiate micrometer-sized regions of the detector with 3.5 MeV He ions to fluences from 5 × 109 to 5 × 1011 cm-2. Ion Beam Induced Charge (IBIC) microscopy was employed to study its degradation in Charge Collection Efficiency (CCE) and energy resolution after irradiation. At high reverse-bias voltages, both parameters remain practically unaffected for fluences up to 1 × 1011 cm-2, while a significant deterioration of the spectroscopic performance was observed above 3 × 1011 cm-2. A theoretical drift-diffusion model, in combination with Shockley-Read-Hall recombination statistics, was used to obtain the holes lifetime from the fitting of the experimental CCE values measured at different reverse voltages. Holes lifetime was found to strongly decrease with increasing particle fluence, changing from 57 ns in pristine detectors to 0.2 ns after irradiation with a fluence of 1 × 1011 cm-2.

Prediction of Alfvén eigenmode energetic particle transport in ITER scenarios with a critical gradient model

A reduced 1D, local, critical-gradient model of energetic particle (EP) transport by Alfvén eigenmodes (AEs)—the TGLF-EP+Alpha model—is applied to a much-studied ITER base case and variations with lower plasma current and lower current penetration. The TGLF-EP+Alpha model is a highly reduced and computationally inexpensive model of EP transport. Such a reduced critical-gradient model, while inapplicable to transport driven by strongly nonlinear or non-local abrupt events, is a valuable tool for scoping studies needed in scenario optimization for ITER and beyond. It relies on the assumption of critical-gradient AE transport with the critical EP density gradient determined by linear AE stability calculations in the TGLF gyro-Landau fluid code automated with the parallel-processed TGLF-EP wrapper. EP transport is treated with simultaneous drive of AEs by fusion-born alpha particles and fast ions born from a MeV neutral beam injection heating. The effect of simultaneous drive creates about 50% increased particle transport in both EP channels. High magnetic safety factor and low shear are generally destabilizing to AEs, but low shear tends to be more important. A tailored q-profile, steady-state-relevant scenario can reduce AE-induced EP redistribution by more than 25% over the ITER base case despite having half the total current.

Design and development of the ITER CTS diagnostic

The Collective Thomson Scattering (CTS) diagnostic will be a primary diagnostic for measuring the dynamics of the confined fusion born alpha particles in ITER and will be the only diagnostic for alphas below 1.7 MeV [1]. The probe beam of the CTS diagnostic comes from a 60 GHz 1 MW gyrotron operated in a ~100 Hz modulation sequence. In the plasma, the probing beam will be scattered off fluctuations primarily due to the dynamics of the ions. Seven fixed receiver mirrors will pick up scattered radiation (the CTS signal) from seven measurement volumes along the probe beam covering the cross section of the plasma. The diagnostic is planned to provide a temporal resolution of ~100 ms and a spatial resolution of ~a/4 in the core and ~a/20 near the plasma edge where a = 2.0 m is the nominal minor radius of ITER. The front-end quasi-optics will be installed in an equatorial port plug (EPP#12). A particular challenge will be to pass the probing beam through the fundamental electron cyclotron resonance, which is located in the port plug (R=10.3 m) for the nominal magnetic field Bt = 5.3 T. Hence, particular mitigation actions against arcing have to be applied. The status of the design and specific challenges will be discussed.

Core plasma ion cyclotron emission driven by fusion-born ions

Ion cyclotron emission (ICE) signals whose spectral peaks match the fundamental cyclotron frequencies of hydrogen and tritium in the plasma core, near the magnetic axis, are observed in ASDEX Upgrade deuterium plasmas. In these cases the only source of energetic (⩾1 MeV) hydrogen and tritium ions is D–D fusion reactions between neutral beam injected deuterium ions and bulk deuterium ions. Hydrogen-matched core ICE is observed in a wide variety of ASDEX Upgrade plasmas, while tritium-matched core ICE is (so far) only observed in so-called H-mode density limit plasmas. In all cases ICE signals are detected directly using B-dot probes, which provide information on the emission frequency, the amplitude, and, in principle, the parallel wavenumber values. These observations support the idea of using an ICE-based diagnostic to monitor the presence of fusion-born alpha particles in tritium-burning fusion plasmas on devices such as JET, ITER, CFETR, or DEMO.

Particle orbits in a force-balanced, wave-driven, rotating torus

The wave-driven rotating torus (WDRT) is a recently proposed fusion concept where the rotational transform is provided by the E x B drift resulting from a minor radial electric field. This field can be produced, for instance, by the RF-wave-mediated extraction of fusion-born alpha particles. In this paper, we discuss how macroscopic force balance, i.e. balance of the thermal hoop force, can be achieved in such a device. We show that this requires the inclusion of a small plasma current and vertical magnetic field, and identify the desirable reactor regime through free energy considerations. We then analyze particle orbits in this desirable regime, identifying velocity-space anisotropies in trapped (banana) orbits, resulting from the cancellation of rotational transforms due to the radial electric and poloidal magnetic fields. The potential neoclassical effects of these orbits on the perpendicular conductivity, current drive, and transport are discussed.

Quantifying Fusion Born Ion Populations in Magnetically Confined Plasmas using Ion Cyclotron Emission.

Ion cyclotron emission (ICE) offers a unique promise as a diagnostic of the fusion born alpha-particle population in magnetically confined plasmas. Pioneering observations from JET and TFTR found that ICE intensity P_{ICE} scales approximately linearly with the measured neutron flux from fusion reactions, and with the inferred concentration, n_{α}/n_{i}, of fusion born alpha particles confined within the plasma. We present fully nonlinear self-consistent kinetic simulations that reproduce this scaling for the first time. This resolves a long-standing question in the physics of fusion alpha-particle confinement and stability in magnetic confinement fusion plasmas. It confirms the magnetoacoustic cyclotron instability as the likely emission mechanism and greatly strengthens the basis for diagnostic exploitation of ICE in future burning plasmas.

Turbulent transport of MeV range cyclotron heated minorities as compared to alpha particles

We study the turbulent transport of an ion cyclotron resonance heated (ICRH), MeV range minority ion species in tokamak plasmas. Such highly energetic minorities, which can be produced in the three ion minority heating scheme (Kazakov et al (2015) Nucl. Fusion 55 032001), have been proposed to be used to experimentally study the confinement properties of fast ions without the generation of fusion alphas. We compare the turbulent transport properties of ICRH ions with that of fusion born alpha particles. Our theoretical predictions indicate that care must be taken when conclusions are drawn from experimental results: while the effect of turbulence on these particles is similar in terms of transport coefficients, differences in their distribution functions—ultimately their generation processes—make the resulting turbulent fluxes different.

Comprehensive evaluation of the linear stability of Alfvén eigenmodes driven by alpha particles in an ITER baseline scenario

The linear stability of Alfvén eigenmodes in the presence of fusion-born alpha particles is thoroughly assessed for two variants of an ITER baseline scenario, which differ significantly in their core and pedestal temperatures. A systematic approach based on CASTOR-K (Borba and Kerner 1999 J. Comput. Phys. 153 101; Nabais et al 2015 Plasma Sci. Technol. 17 89) is used that considers all possible eigenmodes for a given magnetic equilibrium and determines their growth rates due to alpha-particle drive and Landau damping on fuel ions, helium ashes and electrons. It is found that the fastest growing instabilities in the aforementioned ITER scenario are core-localized, low-shear toroidal Alfvén eigenmodes. The largest growth-rates occur in the scenario variant with higher core temperatures, which has the highest alpha-particle density and density gradient, for eigenmodes with toroidal mode numbers . Although these eigenmodes suffer significant radiative damping, which is also evaluated, their growth rates remain larger than those of the most unstable eigenmodes found in the variant of the ITER baseline scenario with lower core temperatures, which have and are not affected by radiative damping.