Fusion Reactor Plasma Research Articles

Properties of neutron irradiated carbon-based materials for fusion reactor application

Abstract Carbon-based materials are widely used for fusion reactor plasma facing components. They are characterized by a low atomic number, fair thermophysical characteristics thermal shock resistance and low neutron activation. Ignition machines such as ITER will produce high neutron fluxes and influence of neutron damage on different properties of graphites and carbon-carbon composites must be evaluated. Irradiation was performed in the Kurchatov Institute IR-8 reactor, where a 0.18 Mev neutron flux intensity reacted 1–3•1013 1/cm2•s. A peak neutron fluence of 2.6•1020 1/cm2, at temperature up to 650°C. The recrystallized graphites and four- and five- directional carbon-carbon composites were irradiated. Thermal conductivity, dimensional changes, strength and coefficient of thermal expansion are reported for the materials examined.

An efficient physics-based Monte Carlo algorithm for simulating alpha-particle orbits in tokamak fusion-reactor plasmas with toroidal-field ripple

A Monte Carlo code ALPHANET, designed to simulate the energetic alpha-particle behavior in tokamak fusion-reactor plasmas with toroidal-field ripple, has been developed. ALPHANET employs a physically well-justified approach to speed up the particle simulation: far from the bounce points, the trajectory is approximated by assuming only the axisymmetric magnetic-field term in the guiding-center equations. Since the ripple term is only dropped after a careful procedure, the physical trajectory is well reproduced. In this paper we give the rationale for our approach, we describe the physics embedded in the code, we point out some of the pitfalls of the algorithm, and, we evaluate the final results from both physical and numerical standpoints.

Status and prospects of controlled thermonuclear fusion

Of all approaches to controlled thermonuclear fusion the tokamak experiments have been most successful. Over the last decade particularly three large devices have achieved plasma density,n, temperature,T, and energy confinement time,τ E, in ranges necessary for a fusion reactor plasma. Such maximum values have, however, been obtained not yet simultaneously but only in separate pulses, although the crucial triple product,nTτ E, has also been improved by several orders of magnitude. The high temperatures sufficient in a fusion reactor can be produced by injection of neutral atoms or by absorption of radio frequency waves in the ion cyclotron frequency range. The plasma confinement (τ E≈1s) is still not understood and is handled through empirical “scaling laws”. Particle densities have usually been on the low side (n≤5×1019 m−3) because increased fuelling rates can easily lead to violent current disruptions. Progress in obtaining peaked density profiles with pellet injection has led to high density plasmas without disruptions. Serious unsolved problems concern the spoiling of the fusion rates by (nonhydrogenic) impurities, the plasma parameter control over longer periods of time and indeed the plasma heating by fusion alpha-particles (“ignition, burning”). The most urgent technological question refers to the lifetime of the first wall which is in direct contact with the plasma. An important step towards ignition has been made by the recent JET/DT experiments in which, for the first time, the actual reactor fuel component tritium has been used to produce neutrons. The “next generation” tokamak ITER is, at present, being planned and designed in a world-wide collaborative effort. It should be operating before the year 2010 and is intended to investigate an ignited plasma burning for several minutes.

Dynamics of coupled temperature and density in a fusion reactor plasma for centrally pronounced heating and particle injection

The coupled reaction-diffusion equations for temperature and density in a fusion plasma are studied for a particular case, where an increased value of the exponent of the temperature in the expression for the heat source term is used as a model for describing a centrally pronounced heating. For the case considered no equilibrium is obtained, not even an unstable equilibrium. The temperature evolution becomes either a decrease of the temperature profile towards zero or an explosion, depending on a measure (the maximum) of the initial temperature. The effects of temperature and density pinch effects are considered.

Nonlinear Dynamics of Reaction-Diffusion Systems: Analysis and Computations

In this paper, equilibria and dynamics of reaction-diffusion systems are studied by means of analysis and computations based on a central expansion method for radially symmetric as well as angularly asymmetric distributions. Effects of boundary conditions are included. The interplay between the different processes in the evolution of the system is considered. The investigation provides a unified description in one, two, and three dimensions. A particular application concerns the time evolution of temperature profiles in a fusion reactor plasma.

The effect of neutron irradiation on the structure and properties of carbon-carbon composite materials

Carbon-based materials are an attractive choice for fusion reactor plasma facing components (PFCs) because of their low atomic number, superior thermal shock resistance, and low neutron activation. Next generation plasma fusion reactors, such as the international thermonuclear experimental reactor (ITER), will require advanced carbon-carbon composite materials possessing extremely high thermal conductivity to manage the anticipated severe heat loads. Moreover, ignition machines such as ITER wilt produce high neutron fluxes. Consequently, the influence of neutron damage on the structure and properties of carbon-carbon composite materials must be evaluated. Data from an irradiation experiment are reported and discussed here. Fusion relevant graphite and carbon-carbon composites were irradiated in a target capsule in the high flux isotope reactor (HFIR) at Oak Ridge National Laboratory (ORNL). A peak damage dose of 1.58 dpa (displacements per atom) at 600°C was attained. The carbon materials irradiated included nuclear graphite grade H-451 and one-, two-, and three-directional carbon-carbon composite materials. Dimensional changes and strength are reported for the materials examined. The influence of fiber type, architecture, and heat treatment temperature on properties and irradiation behavior are reported. Carbon-carbon composite dimensional changes are interpreted in terms of simple microstructural models.

Design-Oriented Methods for One-Dimensional Analysis of Fusion Reactor Plasma Performance

Novel and efficient methods for evaluating fusion reactor plasma performance are discussed. The approach, which is based on spectral collocation techniques for solving the particle and power balanc...

Evolution of temperature profiles in a fusion reactor plasma

The dynamics of plasma temperature profiles depends on the simultaneous processes of diffusion and creation, e.g. heating by alpha particles, as well as losses, e.g. bremstrahlung, and furthermore on the effects of boundaries, which play an essential role in the formation of equilibria. The temperature evolution is governed by a partial differential equation of the reaction-diffusion type. The heating by alpha particles is represented by a source term which depends on temperature to a certain power. The present contribution summarizes the concepts and recent results of a new technique of analysis based on a central expansion in the spatial variable. The problem of studying the nonlinear partial differential equation governing the evolution of a temperature profile is transformed to a description in terms of three characteristic dynamic variables (amplitude, width and shape) which depend only on time and which are governed by three coupled first order differential equations. Such equations allow for dynamic description of selforganisation in open as well as bounded systems. The technique simplifies the descripion of the dynamic properties of the temperature evolution of a burning fusion plasma. It provides physical insight in the dynamics of the system by analysis of the interplay between the various simultaneous processes. In particular, the problems of existence, accessibility and approach to possible equilibria can be studied by means of the new technique.

Simultaneous diffusion, reaction and radiative loss processes in plasmas: numerical analysis with application to the dynamics of a fusion reactor plasma

Results are presented accounting for a selection of extensive numerical calculations of the evolution in space and time for a dynamic system, governed by a nonlinear diffusion equation with source and loss terms (which could also be nonlinear). The system is assumed to be circularly symmetric and limited by an external boundary where the value of the depending variable (e.g. temperature) is supposed to be zero.

Transition to thermonuclear burn in fusion plasmas

An analytical investigation is made of the time evolution of the 1-D temperature profile in a fusion reactor plasma where the nonlinear energy balance equation is dominated by alpha-particle heating and thermal conduction losses. Special emphasis is given to the problem of establishing sufficient conditions for the transition to thermonuclear burn for given initial temperature profiles. In particular, it is demonstrated that for strongly nonlinear alpha-particle heating, temperature profiles initially peaked on-axis are more easily ignited than profiles similar in form to the equilibrium profile of the energy balance equation. Simple analytical criteria for ignition are established and are shown to compare favourably with results of numerical calculations.

Approximate analysis of the energy balance equation of a burning fusion plasma

An approximate analysis is made of the 1-D thermal balance equation determining the plasma temperature in a fusion reactor plasma. This highly nonlinear transport equation is investigated for equilibrium and stability. Simple approximate equilibrium solutions are found using variational optimization techniques. Comparisons with numerically obtained solutions show good agreement.

Diffusion, creation and decay processes in plasma dynamics:: Evolution towards equilibria and the role of bifurcated states

Model descriptions of the behaviour of a fusion reactor plasma are based on systems of partial-differential equations, accounting for the processes occurring in the plasma as well as for the effects of boundaries. It is the purpose of the present contribution to introduce a new technique of analysis based on a central expansion in the spatial variable for a bounded state of a fusion plasma. The technique considerably simplifies the description of the dynamics as well as the determination of equilibria. For particular cases of practical interest, combining effects of alpha particle heating, external heating, ohmic heating and bremsstrahlung, explicit analytic results are obtained for equilibrium distributions. The analytic results are complementary to those of extensive direct computer calculations and useful as checks or for interpretation. The problem of studying nonlinear partial differential equations of a reaction-diffusion type is transformed to a description in terms of three characteristic dynamic variables which depend only on time and which are governed by three coupled nonlinear first-order differential equations. The three variables are: (i) a central value A, (ii) a width l , and (iii) a form function η. These quantities define at any moment a certain spatial temperature or density profile. A unified description is given in one, two and three spatial dimensions and assumes radial symmetry for 2D and 3D cases. The technique, which implies a central expansion in the spatial variable, allows for an analytic description which includes the influence of an external boundary. As a result, profiles of equilibria can be calculated in a simple way with high accuracy. It is described how such equilibria may be approached from initial states, which even deviate appreciably from the equilibrium state. It is furthermore demonstrated how, in the presence of diffusion and, provided an external boundary exists, the competition between the effects of a source term and a loss term may lead to situations where several equilibria are possible, i.e. to bifurcated states. Applications of the new technique are found in many fields of science, notably in plasma physics. One specific application is to the problem of thermal stability and control of a burning fusion plasma.

Evolution and self formation in some nonlinear systems

The evolution of plasma temperature profiles and the formation of equilibria for such profiles, governed by reaction-diffusion equations, depend on the simultaneous processes of diffusion and creation, e.g. heating, as well as losses, e.g. bremsstrahlung, and furthermore on the effects of boundaries, which play an essential role in the formation of equilibria. A nonlinear diffusion equation including source and loss terms (which are also nonlinear) is studied by means of : (i) a time-dependent central approximation technique, and (ii) direct numerical integration of the nonlinear partial differential equation. The two methods are compared and are found to be in excellent agreement numerically. It is concluded that the methods may be regarded as complementary for the understanding and description of the results. Applications are found, e.g. in the field of fusion reactor plasmas, for which one notices the interesting fact that several different equilibria may occur for the same set of parameter values in the original equation.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. IV. Inhomogeneous plasma.

The thermal thermonuclear instability of a deuterium plasma with inhomogeneous temperature and number density in a strong magnetic field is studied theoretically. The distribution of the number density is supposed to be proportional to that of the temperature. By solving the basic equation which governs the reactions, the growth rates in a pure DD fusion reactor and a catalyzed DD fusion reactor are obtained. It is shown that the plasmas with inhomogeneous temperature and number density in the above reactors are unstable to the thermal instability.

New ideas in dosimetry and damage calculations for fusion materials irradiations

Nuclear data and techniques are being developed to improve our ability to characterize fusion materials irradiations. The production of long-lived isotopes have been measured near 14 MeV, including reactions from AI, Fe, and Mo to 26A1, 53Mn, and 94Nb, respectively. These data can be used for fusion reactor activation, dosimetry, and plasma diagnostics. Dosimetry cross sections can be adjusted using measurements at Be(d,n) sources ( E d = 7–40 MeV ) and mono-energetic neutron sources from 1 to 14 MeV. Helium production cross sections are being tested at all fusion irradiation facilities. We have recently discovered a new thermal helium production in copper, similar to the well-known effect in nickel. Recommended cross sections and procedures are included. A new program SPECOMP has been developed to calculate displacement damage for compounds. This program uses the SPECTER recoil atom distributions and integrates over secondary displacement functions for each combination of incident ion and matrix material to determine displacement cross sections for alloys, insulators, and breeder materials.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. III Inhomogeneous temperature.

A thermal thermonuclear instability of a deuterium plasma with an inhomogeneous temperature in a strong magnetic field is studied theoretically. The growth rate of the instability is obtained. The plasmas with an inhomogeneous temperature in a DD fusion reactor and in a catalyzed DD fusion reactor are unstable for the thermal instability.

Vacuum pumping by the halo plasma

An estimate is made of the effective vacuum pumping speed of the halo plasma in a tandem mirror fusion reactor, and it is shown that, if the electron temperature and line density are great enough, the halo can be a very good vacuum pump. One can probably obtain the required density by recycling the ions at the halo dumps. An array of small venting ports in the dump plates allows local variation of the recycle fraction and local removal of the gas at a conveniently high pressure. This vented-port concept could introduce more flexibility in the design of pumped limiters for tokamaks.

Helium trapping in aluminium near the critical dose on blister formation

Blistering and flaking caused by energetic He ions emitted from the plasma in fusion reactors possibly contribute to first-wall erosion. In order to study their characteristics, the numbers of He atoms trapped in He-ion-irradiated Al samples have been measured by a He atom measurement system and every sample has been observed by a scanning electron microscope. The samples have been prepared from a polycrystalline plate and irradiated with 20 keV He ions at room temperature. The saw-tooth like variation of the trapped He atoms with the dose has three edges corresponding to the blistering, flaking and double flaking, respectively. The critical doses for the three events are found to be 4 × 10 21 , 7 × 10 21 , 12 × 10 21 He atoms m −2, respectively. The average number of He atoms included in an event is 5.4 × 10 10 He atoms in the case of the blistering and 2.1 × 10 11 He atoms in the case of flaking.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. II Catalyzed DD fusion reactor.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. II Catalyzed DD fusion reactor.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. I.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. I.