Fusion Alpha Particles Research Articles

Doppler broadening of gamma ray lines and fast ion distribution in JET plasmas

The Doppler broadening of individual γ-ray lines was measured with a high purity germanium detector in JET plasma experiments. High-resolution γ-ray spectrometry of nuclear reactions between energetic D, 3He and 4He ions accelerated by ion cyclotron resonance heating and main plasma impurities such as carbon and beryllium has been used. The nuclear reactions giving rise to γ-rays have been identified and an effective temperature of the heated ions has been obtained in JET discharges. This technique could be used for fast ion and fusion alpha-particle studies in ITER.

Implementation of the thermonuclear process in D3He-9Be plasma on the basis of a Z pinch with an ultrafast laser ignition

A new concept of inertial-magnetic confinement fusion is proposed. This concept is based on a high-current Z pinch combined with a femtosecond laser. The fusion target is composed of a D3He fuel contained under a high pressure inside a sealed cylindrical capsule made from metallic 9Be. An electric discharge along the capsule preheats the target and transforms it into a state of compressed liner. A subsequent TW femtosecond-laser pulse focused on a target end face causes ultrafast cold ignition of a small portion of the D3He fuel. This laser impact generates energetic electrons and ions, which trigger a nuclear-physics mechanism of a catalytic heating of the fuel and also creates a detonation shock wave capable of propagating along the plasma filament. It is shown that the self-sustaining fusion burn wave can appear in the D3He-9Be plasma, in which case the bulk of the energy release is carried by nonradioactive ions, with the energy gain being in excess of 50. The possibility of probing the fusion process by means of gamma-ray spectroscopy is also discussed. The radiative-capture reactions 3He(d, γ), D(d, γ), and 3He(3He, γ) naturally accompanying the burning of the D3He fuel are shown to serve as a convenient diagnostic tool. A nuclear “marker” of D3He fusion on the basis of the detection of monochromatic gamma rays produced in the reaction 9Be(α, γn), which is induced in the liner beryllium shell by energetic fusion alpha particles, is also examined.

One dimensional simulation of diamond DT methane impact fusion

A new impact fusion concept is presented. Both the projectile (macron) and the fusion fuel are carefully chosen, so that the acceleration of the macron is much easier and the stopping of the fusion alpha particle is much larger. The physics of the impact process is discussed. Other than formerly believed shock compression or global adiabatic compression, we tried a fluidal plus particle diffusion approach to model the impact process. A one-dimensional code is implemented. The simulation shows, with the ion–electron temperature relaxation and bremsstrahlung being considered, that the ignition is much harder than previously believed, 1–2 MJ for a millimetre size diamond macron. Diamond DT methane impact fusion is more similar to volume ignition fusion.

Gamma ray spectroscopy at high energy and high time resolution at JET

In fusion plasmas gamma ray emission is caused by reactions of fast particles, such as fusion alpha particles, with impurities. Gamma ray spectroscopy at JET has provided valuable diagnostic information on fast fuel as well as fusion product ions. Improvements of these measurements are needed to fully exploit the flux increase provided by future high power experiments at JET and ITER. Limiting aspects are, for instance, the count rate capability due to a high neutron/gamma background combined with slow detector response and a modest energy resolution due to the low light yield of the scintillators. This paper describes the solutions developed for achieving higher energy resolution, signal to background, and time resolution. The detector design is described based on the new BrLa3 scintillator crystal. The paper will focus on hardware development, including a photomultiplier tube capable of stable operation at counting rate as high as 1 MHz, the magnetic shielding, and the fast digital data acquisition system.

Fast Ion Collective Thomson Scattering Diagnostic for ITER: Design Elements

The proposed fast ion collective Thomson scattering (CTS) diagnostic system for ITER provides the unique capability of measuring the temporally and spatially resolved velocity distribution of the confined fast ions and fusion alpha particles in a burning ITER plasma. The present paper describes the status of the iteration toward the detailed design of the ITER fast ion CTS diagnostic and explains in detail a number of essential considerations and challenges.The diagnostic consists of two separate receiving systems. One system measures the fast ion velocity component in the direction near perpendicular, and the other measures the component near parallel to the magnetic field. Each system has a high-power probe beam at an operating frequency of 60 GHz and a receiver unit. In order to prevent neutron damage to moveable parts, the geometry of the probes and receivers is fixed. An array of receivers in each receiving unit ensures simultaneous measurements in multiple scattering volumes. The latter receiving system (resolving the parallel component) is located on the high field side (HFS) of the plasma, and this constitutes a significant challenge. This HFS receiving unit has been central in the studies, and new HFS receiver mock-up measurements are presented as well as neutron flux calculations of the influence of the increased slot height.

Suprathermal deuterium ions produced by nuclear elastic scattering of ICRH driven He3 ions in JET plasmas

Measurements of the suprathermal tail of the energy distribution function of deuterium ions, in plasmas containing MeV energy ICRH driven minority He3 ions and majority deuterium ions, revealed that the suprathermal tail ion density exceeded by nearly an order of magnitude that expected due to nuclear elastic scattering (NES) of He3 projectile ions on deuterium target ions. The experiments were performed on the Joint European Torus (JET), measurements of the line-of-sight integrated energy distribution functions of He3 and suprathermal deuterium ions were made using a high energy neutral particle analyzer. The NES or ‘knock-on’ deuterium ion energy distribution function was simulated using the FPP-3D Fokker–Plank code (Zaitsev et al 2002 Nucl. Fusion 42 1340) which solves the 3D trajectory averaged kinetic equations in JET tokamak geometry while taking into account NES of He3 ions on the deuterium ions. The required input energy distribution function of ICRH driven He3 ions was simulated using the SELFO code (Hedin et al 2002 Nucl. Fusion 42 527). The comparison between measurement and simulation in the He3 ICRH experiments is contrasted with an analogous previous comparison between measurements and simulation of JET plasmas in which 3.5 MeV DT fusion alpha-particles were the projectile ions, where measurement and simulation roughly agreed. Possible explanations for the observed excess knock-on deuterium tail in the experiments with He3 minority ICRH are discussed. The importance of D + He3 fusion products as additional drivers of suprathermal fuel ions is underlined.

Nonlinear evolution of localized perturbations in the deceleration-phase Rayleigh-Taylor instability of an inertial confinement fusion capsule

The evolution of axially symmetric localized perturbations in the deceleration-phase Rayleigh-Taylor instability of an inertial confinement fusion shell capsule is studied by two-dimensional simulations. Large amplitude divot-like mass perturbations were tracked into the deeply nonlinear regime. Dense fluid spikes penetrating the hot spot as well as light material bubbles rising into the dense shell have been studied, using both a full physics model and a simplified (classical) model neglecting thermal conductivity and fusion reactions. The stabilizing effect of ablation (due to electron thermal conductivity and fusion alpha-particle transport) is found to be more pronounced for spikes than for bubbles. For small width perturbations, bubbles grow faster than spikes, contrary to classical model results.

A Survey of Thermal Instabilities in Tokamak Plasmas: Theory, Comparison with Experiment, and Predictions for Future Devices

The strong temperature dependence, over certain temperature ranges, of the radiation cooling rate of low-Z impurities, of the atomic physics cooling and particle source rates associated with recycling and fueling neutrals, of the ion-electron recombination particle loss rate, of the turbulent transport loss rate, and of the fusion alpha-particle heating rate have all been identified as “drivers” of thermal instabilities in the coupled plasma particle, momentum, and energy balances. This paper surveys the experimental observations of a number of abrupt transition phenomena in plasma operating conditions - i.e., density-limit disruptions, multifaceted asymmetric radiations from the edge (MARFEs), divertor MARFEs, detachment, in-out divertor heat flux asymmetries, H-L and L-H transitions, confinement, and pedestal deterioration - or anticipated in future reactors - i.e., power excursions - their theoretical interpretations in terms of thermal instabilities driven by the temperature dependence of various radiative and atomic physics cooling mechanisms, and a comparison of theoretical prediction with experimental observations. Also surveyed are theoretical predictions of thermal instabilities in the power balance driven by the strong positive temperature dependence of the fusion heating rate.

Materials for plasma facing components of fusion reactors

Abstract According to current knowledge and understanding, nuclear fusion can be developed to a sustainable energy technology. Fuel is abundant and key points for fusion power production and alpha particle heating have already been demonstrated. The next step device, international thermonuclear experimental reactor (ITER), is designed to demonstrate net fusion power production and to address most of the technological issues on the way to a power reactor. There is, however, a series of materials problems related to the plasma facing components and to the structural materials which cannot be fully addressed by ITER. These developments are covered by long term development of radiation resistant low activation materials, heat sink materials, plasma facing protection materials as well as functional tritium barrier materials.A brief survey of the current status of materials development for plasma facing applications is given in the present paper. To provide materials which can sustain the severe loading conditi...

Electron Acceleration at Reconnecting X‐Points in Solar Flares

The acceleration of electrons in solar flares is simulated using an extended version of a full orbit test-particle code previously used to compute losses of fusion alpha particles from tokamaks and to study flare proton acceleration. The magnetic and electric fields in the model are obtained from solutions of the linearized MHD equations for reconnecting modes at a magnetic X-point with zero equilibrium current. The magnetic field is assumed to have a small finite component perpendicular to the plane of the X-point. The version of the code used in the simulations is fully relativistic and includes Coulomb collisions. It is demonstrated that electrons cross the system boundary at distinct footpoints with the relative numbers and energy distribution at each footpoint sensitive to the longitudinal magnetic field component and collision time. These results are discussed in the context of recent flare observations showing asymmetric hard X-ray emission from the chromospheric footpoints of magnetic loops.

Converging geometry Rayleigh–Taylor instability and central ignition of inertial confinement fusion targets

The Rayleigh–Taylor instability (RTI) of the inner surface of an inertial confinement fusion shell is studied through high-resolution two-dimensional numerical simulations. The instability is seeded by a mass displacement introduced in the simulations at the end of the implosion coasting stage. Analysis of single-mode, small-amplitude perturbations confirms that ablation caused by electron conduction and fusion alpha-particles causes significant growth reduction of all modes and stabilization of high-l modes. Different measures of the instability are discussed and compared with modified Takabe-like expressions. Large-amplitude multi-mode simulations are performed to study the effects of RTI on ignition and burn. RTI perturbations reduce the size of the central hot spot and delay ignition. For a few different perturbation spectra the dependence of fusion yield on the initial perturbation root mean square amplitude is studied.

Engineering design of the neutral particle analyser system on ITER

Engineering aspects of a neutral particle analyser setup for International Experimental Thermonuclear Reactor (ITER) have been considered. The setup consists of two neutral particle analysers, LENPA (for energy range of 10–200 keV) and HENPA (for range of 0.1–4 MeV). LENPA is intended for control of isotope ratio of the plasma hydrogen component by measuring an intensity ratio of hydrogen/deuterium/tritium fluxes. It will be incorporated into a system controlling the plasma isotope composition. HENPA is assumed to analyse He atom fluxes to study the fusion alpha particle distribution function. The analysers are located in the protecting vacuum container and are connected with ITER plasma vessel by a beam line. A suggested design of the beam line is described, a pressure distribution along the beam line is calculated, influence of collisions of neutral atoms with the working gas molecules on spectra of atoms is estimated. The design of the analysers will be described shortly.

Chapter 5: Physics of energetic ions

Physics knowledge (theory and experiment) in energetic particlesrelevant to design of a reactor scale tokamak is reviewed, and projections forITER are provided in this Chapter of the ITER Physics Basis. The reviewincludes single particle effects such as classical alpha particle heating andtoroidal field ripple loss, as well as collective instabilities that might begenerated in ITER plasmas by energetic alpha particles. The overall conclusionis that fusion alpha particles are expected to provide an efficient plasmaheating for ignition and sustained burn in the next step device. The majorconcern is localized heat loads on the plasma facing components produced byalpha particle loss, which might affect their lifetime in a tokamak reactor.

Energetic particle transport and alpha driven instabilities in advanced confinement DT plasmas on TFTR

The article reviews the physics of fusion alpha particles andenergetic neutral beam ions studied in the final phase of TFTRoperation, with an emphasis on observations in reversed magnetic shear(RS) and enhanced reversed shear (ERS) DT plasmas. Energy resolvedmeasurements of the radial profiles of confined, trapped alphas in RSplasmas exhibit reduced core alpha density with increasing alphaenergy, in contrast to plasmas with normal monotonic shear. Themeasured profiles are consistent with predictions of increased alphaloss due to stochastic ripple diffusion and increased first orbit lossin RS plasmas. In experiments in which a short tritium beam pulse isinjected into a deuterium RS plasma, the measured DT neutron emissionis lower than standard predictions assuming first orbit loss andstochastic ripple diffusion of the beam ions. A microwavereflectometer measured the spatial localization of low toroidal modenumber (n), alpha driven toroidal Alfvén eigenmodes (TAEs) in DTRS discharges. Although the observed ballooning character of the n = 4mode is consistent with predictions of a kinetic MHD stability code,the observed antiballooning nature of the n = 2 mode isnot. Furthermore, the modelling does not show the observed strongdependence of mode frequency on n. These alpha driven TAEs do notcause measurable alpha loss in TFTR. Other Alfvén frequency modeswith n = 2-4 seen in both DT and DD ERS and RS discharges arelocalized to the weak magnetic shear region near qmin. In 10-20% ofDT discharges, normal low n MHD activity causes alpha loss at levelsabove the first orbit loss rate.

Neoclassical simulations of fusion alpha particles in pellet charge exchange experiments on the Tokamak Fusion Test Reactor

Neoclassical simulations of alpha particle density profiles in high fusion power plasmas on the Tokamak Fusion Test Reactor [Phys. Plasmas 5, 1577 (1998)] are found to be in good agreement with measurements of the alpha distribution function made with a sensitive active neutral particle diagnostic. The calculations are carried out in Hamiltonian magnetic coordinates with a fast, particle-following Monte Carlo code which includes the neoclassical transport processes, a recent first-principles model for stochastic ripple loss and collisional effects. New calculations show that monotonic shear alpha particles are virtually unaffected by toroidal field ripple. The calculations show that in reversed shear the confinement domain is not empty for trapped alphas at birth and allow an estimate of the actual alpha particle densities measured with the pellet charge exchange diagnostic.

The dissipative Budden problem: Effect of converted-wave damping on primary-wave reflection

The Budden equation represents a double conversion, whereby a primary wave first converts to a localized secondary wave, which then converts to a reflected primary wave. We analyze this process in phase space, to include secondary-wave damping between the two conversions, which reduces the reflection coefficient. The results are then applied to gyroresonant damping of the (secondary) ion-hybrid wave by fusion alpha-particles, suggesting a diagnostic test.

Nonperturbing meaurement of energy distribution function of confined D-T fusion alpha particles in Joint European Torus plasmas using impurity induced neutralization (abstract)

We present measurements of the energy distribution function of fusion α particles in the range 0.8⩽E(MeV)⩽3 in hot-ion H-mode plasmas, using a NPA. Two contributions to the measured atomic flux are distinguished: (i) alpha particles due to direct neutralization effected in double charge–exchange reactions with intrinsic plasma impurities C4+, Be2+, and He0, and (ii) MeV energy deuterons formed by elastic scattering on fusion alpha particles, their neutralization effected by reactions with C5+, Be3+, He1+, and D0. Application of the impurity induced neutralization model to the measurements is presented. Deduced fusion α particle and knock-on deuteron energy distribution functions are compared with Fokker–Planck simulations in which measured time-dependent α-particle source and collisionality are used.

Physics of energetic particles in ITER

The paper summarizes issues and presents results of theITER physics work in the area of energetic particles such as fusionalpha particles, energetic ions produced by plasma auxiliary heating,and runaway electrons which can be generated during plasmadisruption.

Higher-order terms in mode conversion

The development of model equations to describe mode conversion and tunneling in an inhomogeneous plasma usually uses expansions of the Modified Bessel functions in the parameter λ=12k⊥2ρL2, keeping only the lowest possible order to demonstrate tunneling. With the high temperatures of fusion parameters, especially for fusion alpha particles, this parameter may not be small. It is shown here that the effective expansion parameter is really βj=2μ0njkTj/B02, the ratio of plasma pressure to the magnetic pressure.

Deuterium–tritium plasmas in the Joint European Torus (JET): Behavior and implications

This paper reports the first deuterium–tritium (D-T) fusion experiments in the geometry of the International Thermonuclear Experimental Reactor (ITER) [R. Aymar, V. Chuyanov, M. Huguet, R. Parker, Y. Shimamura, and the ITER Joint Central Team and Home Teams, Proceedings of the 16th International Conference on Fusion Energy (International Atomic Energy Agency, Vienna), Fusion Energy 1, 3 (1996)], with long pulse length and an ITER-like divertor. Physics aspects, such as the isotope dependence of confinement, the H-mode (high confinement) threshold, shear optimisation, heating methods, high fusion performance and alpha particle heating, are discussed together with their implications. The technology aspects of tritium wall loading and clean-up, the close coupled tritium plant and the future remote handling divertor target exchange are also mentioned.