Magnetic Fusion Reactor Research Articles

Influence of torsional strain on the Bi-2212 tapes and round wires under background field

As the only high-temperature superconductor (HTS) that can be made into round wire without anisotropy, Bi-2212 round wire holds significant potential for applications as CICC (cable in conduit conductor) for superconducting magnets in fusion reactors. However, Bi-2212 is brittle and sensitive to strain which leads to a low mechanical performance. The effort on studying the impact of strain on the transport properties is mainly focus on the axial strain. However, the conductor will be inevitably subjected to torsional strain in the CICC. The torsional strain of Bi-2212 under background magnetic field has been rarely reported. In this paper, the critical current (Ic) of Bi-2212 tape and round wire under torsion and background magnetic field is investigated and compared with each other. The measurement is performed at 77K and 0–20mT. The result demonstrates that the Ic of Bi-2212 tape is strongly dependent on the magnetic field angle, while magnetic field angle has little influence on round wires. Although the external magnetic field and torsional angle have impact on Bi-2212 tape and round wire, the Ic degradation of tested samples is more dependent on external magnetic field intensity than that of torsional angle.

Radio-Frequency design of a Lower Hybrid Slotted Waveguide Antenna

This paper presents a new type of slow-wave Lower Hybrid Current Drive launcher, based on the Slotted Waveguide Antenna (SWA) concept, which is particularly attractive for the use in future magnetic fusion reactors. A low power prototype SWA has been designed for the COMPASS tokamak at IPP-Prague. The Radio-Frequency (RF) design of this low power prototype is based on hybrid electromagnetic calculations, combining Finite-Element-Method, Mode Matching and RF network computations. Two SWA prototype samples have been manufactured with different slot geometries, and they have been first measured in air. The RF design of the low power SWA prototype and its feeding circuit is discussed in this manuscript; measurements in air are also illustrated.

Bi-directional hydrogen isotopes permeation through a reduced activation ferritic steel F82H

The first wall of a magnetic fusion reactor serves to separate the edge plasma from breeding blankets, which will be subjected to bi-directional hydrogen isotopes permeation: in one direction by plasma-driven permeation, and in the other direction by gas-driven permeation. In this work, hydrogen isotopes transport through a reduced activation ferritic steel F82H has been investigated in the temperature range of 150–500°C. The transport parameters including permeability, diffusivity, solubility and surface recombination coefficient have been measured and the isotopic mass effect has been discussed. Bi-directional hydrogen isotopes gas- and plasma-driven permeation has been demonstrated for the first time under controlled experimental conditions.

Electric currents in REBaCuO superconducting tapes

Magnetic induction and current transport techniques were employed to test the electromagnetic performance of 22 samples of superconducting REBaCuO tapes of several manufacturers with the aim to select the best candidates for wiring new generation superconducting magnets for fusion reactors. An extraordinary shape of pinning force density as a function of magnetic field was observed in ‘advanced pinning, AP’ SuperPower tapes. The induced currents enabled us to classify the tapes and show the best ones, at low temperatures and high magnetic fields on the one hand and at high temperatures and low magnetic fields on the other hand. Transport current experiments confirmed the results. Moreover, they provided information on current anisotropy, which appeared to be specific for each tape type. The present results formed a basis for the ongoing study of neutron irradiation impact on the tapes’ properties. The first experiment showed that neutron irradiation by the fluence of 2.1 E22 m−2 either enhances, reduces, or has no effect on the tape’s properties, according to the tape pinning structure.

Experimental investigation on mass flow rate measurements using fibre Bragg grating sensors

Flow measurement and control of cryogens is one of the major requirements of systems such as superconductor magnets for fusion reactors, MRI magnets etc. They can act as an early diagnostic tool for detection of any faults and ensure correct distribution of cooling load while also accessing thermal performance of the devices. Fibre Bragg Grating (FBG) sensors provide compact and accurate measurement systems which have added advantages such as immunity towards electrical and magnetic interference, low attenuation losses and remote sensing. This paper summarizes the initial experimental investigations and calibration of a novel FBG based mass flow meter. This design utilizes the viscous drag due to the flow to induce a bending strain on the fibre. The strain experienced by the fibre will be proportional to the flowrate and can be measured in terms of Bragg wavelength shift. The flowmeter is initially tested at atmospheric conditions using helium. The results are summarized and the performance parameters of the sensor are estimated. The results were also compared to a numerical model and further results for liquid helium is also reported. An overall sensitivity of 29 pm.(g.s-1)-1 was obtained for a helium flow, with a resolution of 0.2 g.s-1. A hysteresis error of 8 pm was also observed during load-unload cycles. The sensor is suitable for further tests using cryogens.

Investigation of Geometry and Current Density Effects in a Pulsed Electrothermal Plasma Source Using a 2-D Simulation Model

Electrothermal (ET) plasma discharges have application to mass acceleration technologies relevant to military ballistics and magnetic confinement fusion reactor operation. ET plasma discharges are initiated in capillary geometries by passing large currents (order of tens of kiloamperes) along the capillary axis. A partially ionized plasma then forms and radiates heat to the capillary walls inducing ablation. Ablated particles enter the capillary plasma source and cause a pressure surge that can propel pellets to velocities exceeding 2 km/s. These devices present several advantages over other mass accelerator technologies due to their simple design and ability to achieve high projectile launch frequencies. In order to investigate the operation of ET plasma discharges in more detail than previously possible, a 2-D, multifluid model has been developed to simulate the plasma-fluid dynamics that develop in these devices during operation. In this paper, the 2-D simulation model is used to investigate the effect of source geometry and current density on discharge characteristics. Peak pressure and electric field magnitudes for pulsed discharge operation are shown to scale well with theoretical and empirical scaling laws for steady-state discharge operation. The pulse shape of the source internal pressure is shown to change significantly with increasing source radius. The behavior of other plasma parameters is investigated. In addition, observations of the departure from the ablation-controlled arc regime are presented. This analysis suggests that, for the current pulse length investigated, source radii higher than 4 mm require significantly more current density in order to produce sufficient ablation to stabilize the plasma discharge.

Suppressed ion-scale turbulence in a hot high-β plasma.

An economic magnetic fusion reactor favours a high ratio of plasma kinetic pressure to magnetic pressure in a well-confined, hot plasma with low thermal losses across the confining magnetic field. Field-reversed configuration (FRC) plasmas are potentially attractive as a reactor concept, achieving high plasma pressure in a simple axisymmetric geometry. Here, we show that FRC plasmas have unique, beneficial microstability properties that differ from typical regimes in toroidal confinement devices. Ion-scale fluctuations are found to be absent or strongly suppressed in the plasma core, mainly due to the large FRC ion orbits, resulting in near-classical thermal ion confinement. In the surrounding boundary layer plasma, ion- and electron-scale turbulence is observed once a critical pressure gradient is exceeded. The critical gradient increases in the presence of sheared plasma flow induced via electrostatic biasing, opening the prospect of active boundary and transport control in view of reactor requirements.

Wetting properties of liquid lithium on lithium compounds

Liquid metal plasma facing components (LMPFC) have shown a potential to supplant solid plasma facing components materials in the high heat flux regions of magnetic confinement fusion reactors due to the reduction or elimination of concerns over melting, wall damage, and erosion. To design a workable LMPFC, one must understand how liquid metal interacts with solid underlying structures. Wetting is an important factor in such interaction, several designs of LMPFC require liquid metal to wet the underlying solid structures. The wetting of lithium compounds (lithium nitride, oxide, and carbonate) by 200°C liquid lithium at various surface temperature from 230 to 330°C was studied by means of contact angle measurements. Wetting temperatures, defined as the temperature above which the contact angle is less than 90°, were measured. The wetting temperature was 257°C for nitride, 259°C for oxide, and 323°C for carbonate. Surface tensions of solid lithium compounds were calculated from the contact angle measurements.

Radial Profile of Plasma Flow Parameters Following Evolution of Electrothermal Polycarbonate Plasma into a Large Chamber Simulating Impurity Expansion in Fusion Reactor Vacuum Vessel

The 2-D evolution of the plasma as it expands outside the capillary source, which simulates ablation of plasma-facing materials due to high-heat flux deposition, is of importance to investigate the plasma flow scaling laws with inclusion of radial variations. The flow continues from the source exit through a converging–diverging transition region and expands into a large expansion volume that simulates the vacuum chamber of a tokamak fusion reactor. This 2-D modeling resolves the problem of the 1-D steady state analysis of the plasma bulk flow that assumes the flow parameters to vary only in the axial direction with a uniform radial distribution. As the plasma expands towards the sidewalls of the diverging section in the transition region, the bulk plasma has a finite chance to lose energy primarily in the form of heat loss, which might cause readjustment in the profile for plasma bulk temperature giving rise to a decreasing trend in the radial direction. This change in bulk plasma temperature triggers further changes in other bulk flow parameters along the radial directions, away from the axis of the flow. In this work, different mathematical models have been proposed for radial correction in the bulk flow parameter profiles within the system size as a simulation for expansion into the vacuum vessel of a magnetic fusion reactor. Symmetry of the radial profiles over a cross-sectional area has been assumed to facilitate ease of computation and analysis.

Estimation of total fusion reactivity and contribution from supra-thermal tail using 3-parameter Dagum ion speed distribution

Thermonuclear fusion reactivity is a pivotal quantity in the studies pertaining to fusion energy production, fusion ignition and energy break-even analysis in both inertially and magnetically confined systems. Although nuclear fusion reactivity and thereafter the power density of a magnetic confinement fusion reactor and the fulfillment of the ignition criterion are quantitatively determined by assuming the ion speed distribution to be Maxwellian, a significant population of suprathermal ions, with energy greater than the quasi-Maxwellian background plasma temperature, is generated by the fusion reactions and auxiliary heating in the fusion devices. In the current work 3-parameter Dagum speed distribution has been introduced to include the effect of suprathermal ion population in the calculation of total fusion reactivity. The extent of enhancement in the fusion reactivity, at different back-ground temperatures of the fusion fuel plasma, due to the suprathermal ion population has also been discussed.

Generic Magnetic Fusion Reactor Revisited

The original generic magnetic fusion reactor paper was published in 1986 for deuterium-tritium reactors. This update describes what has changed in 30 years. Notably, the construction of ITER is providing important benchmark numbers for technologies and costs. In addition, we use a more conservative neutron wall flux and fluence. But, these cost-increasing factors are offset by greater optimism on the thermal-electric conversion efficiency and potential availability. In addition, today’s inflation and interest rates are low, leading to a cost of money well below that used in the original study. The main examples show the cost of electricity (COE) as a function of aspect ratio and neutron flux to the first wall. The dependence of the COE on availability, thermoelectric efficiency, electrical power output, and the present day’s low interest rates is also discussed. Interestingly, at fixed aspect ratio there is a shallow minimum in the COE at neutron flux of ~2.5 MW/m2. The possibility of operating with only a small COE penalty at even lower wall loadings (to 1.0 MW/m2 at larger plant size) and the possible use of niobium-titanium coils are also investigated. It should be emphasized that the variation in the COEs is important rather than their absolute values.

Catalyzed D-D Stellarator Reactor

The advantages of using the catalyzed deuterium-deuterium (D-D) approach for a fusion reactor–lower and less energetic neutron flux and no need for a tritium breeding blanket–have been evaluated in previous papers, giving examples of both tokamak and stellarator reactors. This paper presents an update for the stellarator example, taking account of more recent empirical transport scaling results and design studies of lower-aspect-ratio stellarators. We use a modified version of the Generic Magnetic Fusion Reactor model to cost a stellarator-type reactor. Recently, this model has been updated to reflect the improved science and technology base and costs in the magnetic fusion program. It is shown that an interesting catalyzed D-D, stellarator power plant might be possible if the following parameters could be achieved: R/<a> ≈ 4, required improvement factor to ISS04 scaling, FR = 0.9 to 1.15, <β= ≈ 8.0% to 11.5%, Zeff ≈ 1.45 plus a relativistic temperature correction, fraction of fast ions lost ≈ 0.07, Bm ≈ 14 to 16 T, and R ≈ 18 to 24 m.

Structure and Magnetic Field Dependences of Joint Resistance in a Mechanical Joint of REBCO Tapes

Mechanical joints of rare-earth barium copper oxide (REBCO) tapes and stacked tape conductors have been investigated for segmented fabrication of high-temperature superconducting (HTS) magnets for future fusion reactors. To design the optimized joint, we need to predict joint resistance depending on operating conditions and the conductor's structure. The prediction method was proposed in our previous study, taking into account the model of contact resistance, resistance of constituent materials, and interfaces. However, that was just based on data of mechanical lap joints for one kind of REBCO tape and a magnetic field perpendicular to the tape. Therefore, in this paper, we first evaluated joint resistances in mechanical lap joints of Fujikura's and SuperPower's REBCO tapes as a function of temperature and magnetic field with different orientations. The results showed that the joint resistance significantly increased with an increase in magnetic field parallel to the tape at temperatures of 10 K and lower. In addition, the reduction rate of joint resistance for the SuperPower's tape with a decrease in temperature was lower than that for the Fujikura's tape. Based on experimental results for Fujikura's REBCO tapes with different field orientations, we modified the model of contact resistance. Further improvement of the prediction method is still needed for different stabilizer structures and a temperature range around 10 K.

Assessment of tritiated activities in the radwaste generated from ITER Chinese helium cooled ceramic breeding test blanket module system

Chinese Helium Cooled Ceramic Breeding Test blanket Module (CNHCCB TBM) will be tested in the ITER machine for the feasibility of in pile tritium production for a future magnetic confinement fusion reactor. The tritium inventories/retentions in the material/components were evaluated and updated mainly based on the tritium diffusion/permeation theory and the analysis of some reported data. Tritiated activities rank from less than 10Bqg−1 to 109Bqg−1 for the different materials or components, which are generally higher than those from the previous neutron transmutation calculation. The amounts of tritiated radwaste were also estimated according to the operation, decommission, maintenance and replacement strategies, which vary from several tens of kilograms to tons in the different operation phases. The data can be used both for the tritium radiological safety evaluation and radwaste management of CNHCCB TBM set and its ancillary systems.

Synthetic H-Alpha Diagnostics for ITER: Inverse Problems and Error Estimations for Strong Non-Maxwellian Effects and Intense Divertor Stray Light

The use of an all-metal first wall in future magnetic fusion reactors equipped with a divertor may impose severe limitations on the capabilities of optical diagnostics in the main chamber because of the divertor stray light (DSL) produced by reflections of the intense light emitted in the divertor. Here, we introduce a synthetic H-alpha diagnostics to estimate the errors of solutions of the inverse problems aimed at recovering the neutral hydrogen parameters (density and isotope ratio) in the scrape-off layer (SOL) with allowance for (a) strong DSL on the observation chords in the main chamber, (b) substantial deviation of the neutral atom velocity distribution function from a Maxwellian in the SOL, and (c) the data from the direct observation of the divertor. The results of recovering the relative contributions of all three sources to the signal along an observation chord in the main chamber (namely, from the high-field-side and low-field-side SOL sections of the observation chord, and the DSL), together with the isotope ratios in the SOL, are presented for the flattop stage of Q = 10 inductive operation of ITER.

RETRACTED ARTICLE: Low Dissipation with Normalized Flux Surfaces in 2-Dimensional Coordinate for IR-T1 Tokamak Using Grad–Shafranov Equation Solution

Together with the problem of confinement, plasma–wall interactions present the major constraints toward a magnetic fusion reactor. The solutions of Grad–Shafranov equation (GSE) analytically can be used for theoretical studies of plasma equilibrium, transport and magneto-hydrodynamic stability. Here we introduce specific choices for source functions, kinetic pressure and poloidal plasma current, to be quadratic in poloidal magnetic flux and derive an analytical solution for GS equation. With applying this solution to IR-T1 tokamak, we have calculated the poloidal magnetic flux, toroidal current density and normalized pressure profiles for this tokamak. Toroidal and poloidal flows can considerably change the equilibrium parameters of tokamak. These effects on the equilibrium of tokamak plasmas are numerically investigated using a code FLOW. As a comparative approach to equilibrium problem, the code is used to model equilibrium of IR-T1 tokamak for case pure toroidal flow.

Existing and new applications of micropellet injection (MPI) in magnetic fusion

The intense heat and energetic particle fluxes expected in ITER and future magnetic fusion reactors pose prohibitive problems to the design, selection and maintenance of the first wall and divertor. Micropellet injection (MPI) technologies can offer some innovative solutions to the material and extreme heat challenges. Basic physics of micropellet motion, ablation and interactions with high-temperature plasmas and energetic particles are presented first. We then discuss MPI technology options and applications. In addition to plasma diagnostic applications, controlled injection of micropellets of different sizes, velocities and injection frequencies will offer several possibilities: (1) better assessment of the core plasma cooling due to dust produced in situ; (2) better understanding of the plasma–material interaction physics near the wall; (3) new methods for plasma fuelling and impurity control; and (4) techniques for edge cooling with minimal impact on the plasma core. Dedicated small-scale laboratory experiments will complement major fusion experiments in development and applications of MPI.

Radiation Heat Transfer and Vapor Shielding in a Two-Dimensional Model of an Electrothermal Plasma Source

Electrothermal (ET) plasma discharges are emerging as valuable mechanisms for pellet injection in magnetic confinement fusion reactors. They have been shown to be capable of achieving the required pellet velocities and pellet launch frequencies required for edge localized mode control. Another advantage of ET plasma discharges is their ability to simulate fusion disruption events by depositing large heat fluxes on exposed materials. A deeper understanding of the heat transfer processes occurring in ET plasma discharges will aid in this particular application. ET plasma discharges involve the passage of high currents (order of tens of kA) along the axis of a narrow, cylindrical channel. As the current passes through the channel, radiant heat is transferred from the plasma core to the capillary wall. Ablated particles eventually fill the plasma channel and the partially ionized plasma is ejected. It is well known that the ablated material separating the plasma core from the ablating surface can act as a vapor shield and limit the radiation heat flux reaching the ablating surface. In this work, the results from a two-dimensional simulation model for ET plasma discharges are presented. The simulation of the plasma in a two-dimensional domain combined with the diffusion approximation for radiation heat transfer is shown to successfully simulate the effects of the vapor shield layer that develops inside these devices.

Physics of Alfvén waves and energetic particles in burning plasmas

In magnetic fusion reactors relying on the burning of deuterium and tritium, sufficient confinement of the alpha particles produced in the nuclear reactions is crucial to sustaining the burning plasma. In this article the interactions of these energetic particles with linear and nonlinear Alfve'n waves generated in the magnetized plasma are reviewed.

Coupling between a multi-physics workflow engine and an optimization framework

A generic coupling method between a multi-physics workflow engine and an optimization framework is presented in this paper. The coupling architecture has been developed in order to preserve the integrity of the two frameworks. The objective is to provide the possibility to replace a framework, a workflow or an optimizer by another one without changing the whole coupling procedure or modifying the main content in each framework. The coupling is achieved by using a socket-based communication library for exchanging data between the two frameworks. Among a number of algorithms provided by optimization frameworks, Genetic Algorithms (GAs) have demonstrated their efficiency on single and multiple criteria optimization. Additionally to their robustness, GAs can handle non-valid data which may appear during the optimization. Consequently GAs work on most general cases. A parallelized framework has been developed to reduce the time spent for optimizations and evaluation of large samples. A test has shown a good scaling efficiency of this parallelized framework. This coupling method has been applied to the case of SYCOMORE (SYstem COde for MOdeling tokamak REactor) which is a system code developed in form of a modular workflow for designing magnetic fusion reactors. The coupling of SYCOMORE with the optimization platform URANIE enables design optimization along various figures of merit and constraints.