Magnetic Fusion Reactor Research Articles

Compact stellarators with modular coils.

Compact stellarator designs with modular coils and only two or three field periods are now available; these designs have both good stability and quasiaxial symmetry providing adequate transport for a magnetic fusion reactor. If the bootstrap current assumes theoretically predicted values a three field period configuration is optimal, but if that net current turns out to be lower, a device with two periods and just 12 modular coils might be better. There are also attractive designs with quasihelical symmetry and four or five periods whose properties depend less on the bootstrap current. Good performance requires that there be a satisfactory magnetic well in the vacuum field, which is a property lacking in a stellarator-tokamak hybrid that has been proposed for a proof of principle experiment. In this paper, we present an analysis of stability for these configurations that is based on a mountain pass theorem asserting that, if two solutions of the problem of magnetohydrodynamic equilibrium can be found, then there has to be an unstable solution. We compare results of our theory of equilibrium, stability, and transport with recently announced measurements from the large LHD experiment in Japan.

Feasibility of SMES devices based on the developed technology of superconducting magnets for tokamak fusion reactors

Achievements in the technology of superconducting magnets for the tokamak fusion devices contribute to the applicability of GJ-range SMES for the utilities by means of using already developed components and constructed production lines. The paper presents the technologies developed by the NIIEFA, VNIIKP and VNIINM, institutes, covering most of the critical items needed to construct the GJ-range SMES devices for the utility needs. That includes software packages for the supporting analysis and SMES optimization, heavy current CICC production lines with the length of pull-through jacketing tooling of up to 1 km, 30 kV 40 kA-range cryogenic current leads and 30 kV high-pressure insulating breaks for cold and warm helium communications, various types of heavy-current 30 kV range commutating equipment, including superconducting switches, and cryogenic test facilities for the acceptance tests of the produced conductors, current leads and insulation breaks.

Simulation of plasma-neutral dynamics for radiation cooling

An advanced heat removal scenario is required to handle the highinput power of magnetic fusion reactors. The concepts of gas target and radiative divertorshave been explored. A set ofself-consistently coupled fluid equations for plasma and neutralsis suitable for this system because of the sophisticated equationsfor neutrals. The transfer of plasma momentum to neutrals, whichis crucial in understanding the plasma-neutral interface, isdescribed through the charge exchange process. Our code resultsshow good agreement with gas target experiments on the PISCES experiment anddemonstrate its superiority to the previous simulation. Thisbenchmarked code and a set of equations have been applied to aslot divertor for DEMO reactor. The slot length for the divertor can bedetermined by producing enough frictional force to overcome thethermal force which creates a backflow of impurities. A smallamount of radiating argon is enough to exhaust 40 MW of power fromthe core plasma in a 40 cm long slot for the inner divertor of DEMO.

Evaluation Parameter of Magnetic Damping Vibration. 1st Report. Experiment on Magnetic Damping Behavior of Thin Plate in Magnetic Field.

Evaluation of magnetic damping effect, which is produced by the coupled effect between deforma tion and induced eddy current, is needed for the mechanical design of structures located in a high magnetic field such as magnetic fusion reactor components. The authors have defined the coupling intensity parameter which represents the intensity of electromagnetic and structural coupling effect. This paper describes the method to obtain the coupling intensity parameter from the results of magnetic damping experiment. Since the vibration profile obtained from the experiment includes the damping by air and the structural damping, these damping effects should be removed from the experimental results to obtain the coupling intensity parameter accurately. The dependence of the parameter on external magnetic field and thickness is obtained experimentally for the bending problem of a cantilever plate under various conditions.

Improved Magnetic Fusion Energy Economics via Massive Resistive Electromagnets

Abandoning superconductors for magnetic fusion reactors and instead using resistive magnet designs based on cheap copper or aluminum conductor material operating at “room temperature” (300°K) can reduce the capital cost per unit fusion power and simplify plant operations.1 By increasing unit size well beyond that of present MFE. conceptual designs using superconducting electromagnets, the recirculating power fraction needed to operate resistive electromagnets can be made as close to zero as needed for economy without requiring superconductors. Other advantages of large fusion plant size, such as very long inductively driven pulses, may also help reduce the cost per unit fusion power.2

Measurement of plasma-electron temperature and density in front of a neutraliser plate by He I line intensity ratios

The plasma-electron temperature T e and density n e in front of a neutraliser plate of a limiter have been obtained from the line intensity ratio of He I line transitions. The emission is due to the interaction of the scrape-off-layer plasma with the helium recycling neutrals. In this region typical plasma parameters are: 5eV < T e < 30eV and 10 18m −3 < n e < 10 19m −3. The same atomic transitions are used as in the helium thermal beam technique used for n e and T e profile measurement in the tokamak plasma boundary. The choice of the correct line of view determines the capacity of this method to obtain valuable information and determines its spatial resolution, since the passive plasma emission is integrated over the whole observing chord. A good correlation has been found between n e and T e estimations by this technique and local Langmuir probe results. A high signal-to-noise ratio are obtained even in deuterium discharges with down to 15% helium. Due to its good time resolution and non-intrusivity this new method would be attractive for basic plasma edge studies in actual devices. In future magnetic fusion reactors, operating with high convective power to the limiters or divertors, the plasma damage would make difficult the use of fixed Langmuir probes. If helium is naturally present as fusion ashes in the plasma the presented non-intrusive method would be an alternative, permitting a continuous monitoring of T e and n e in front of the neutraliser plates over large surfaces.

Nb/sub 3/Al thin films made by RF magnetron sputtering process with a single target

We have studied a high field pulsed superconductor with high elastic modulus fiber reinforcement. We call the conductor Fiber Reinforced Superconductor (FRS). We have succeeded in making a sputtering target which consists of reacted Nb/sub 3/Sn single material by powder metallurgy. Nb/sub 3/Al superconductor is well known as a candidate for superconducting magnets for fusion reactors because of good properties of mechanical stress and irradiation toughness. To overcome the difficulty of fabricating into superconducting wire, we have made a single sputtering target of Nb-Al compound by powder metallurgy. Thin Nb/sub 3/Al films on MgO substrate were made using this target. Experimental results for these films are presented.

Experiments in FTU with different limiter materials

Over the last few years, a great deal of effort has been devoted to solving the problem of power and particle handling in divertors, which has been recognized as a critical issue for the operation of a magnetic fusion reactor. In particular, the choice of materials for plasma facing components has been examined with a view to developing heat and erosion resistant materials for divertor target plates. A large database on the behaviour of low-Z (carbon or beryllium) materials in tokamaks is available, while for high-Z materials there is little experience in the present generation of magnetic fusion devices. Frascati Tokamak Upgrade (FTU), a high field compact tokamak, has devoted part of its experimental campaign to studying the plasma characteristics when its limiter material is changed from the usual Inconel (nickel) to molybdenum and tungsten. Siliconization of the machine has also allowed the comparison of plasma performance when a relatively low-Z (silicon) ion is the dominant impurity. In this article, results are reported concerning the plasma operation, the differences in plasma characteristics and radiation losses, the impurity generation mechanisms and the relative impurity concentrations in the core plasma. A simulation of the experimental results, made with a self-consistent edge-core coupled model is presented, in order to provide evidence of the main physics mechanisms responsible for the observed behaviour

Near Term Commercial Opportunities from Long Range Fusion Research

An alternate approach to the development of safe, clean, and economical fusion energy for the 21st Century is presented. Instead of continuing exclusively on the path of larger and more costly magnetic confinement fusion reactors based on the DT cycle, it is proposed that near term commercial opportunities using fusion plasmas be identified and pursued. Specific examples of such opportunities are given in the areas of the detection of explosives, the production of medical isotopes, and the destruction of long lived fission product isotopes. It is also suggested that a more profitable path to the goal of fusion electricity might be to concentrate on small, simple devices that eventually can burn the more advanced fusion fuels that emit few if any neutrons. Such devices could gain back the public confidence and counter the `fusion is always 50 years away` syndrome. 11 refs., 4 figs., 2 tabs.

Approach to ignition conditions in a two fluid spheromak with direct ion heating

The spheromak, as a compact toroidal magnetic fusion device, offers substantial advantages as a fusion reactor concept over larger, more complicated, and more costly devices such as the tokamak. The very definite advantages associated with the simply closed geometry, minimized external coil requirements and the possibility of ohmic ignition in the spheromak, represent a substantial improvement over conventional magnetic fusion reactor designs. Furthermore, recent successes in improving confinement parameters (Te~400 eV, Ti~1 keV, ne~5*1014 cm-3, B~1 T) have renewed interest in advancing this concept to a proof of principle, reactor prototype stage. Herein, the initial work by Fowler et al. (Comments Plasma Phys. Control. Fusion 16 (1994) 91), indicating the possibility of ohmic ignition in spheromaks, is extended to a two fluid model that includes direct ion heating. Non-ohmic magnetic dissipation, contributing to direct ion heating, and confinement scaling are quantified through comparison with the latest results from the gun driven Compact Torus Experiment (CTX) spheromak (Phys. Fluids B 2 (1990) 1342). Excellent agreement is demonstrated between experimentally measured plasma parameters and our model predictions. Extrapolation to ignition experiments and reactor relevant conditions is discussed, indicating the possibility of reaching these conditions by gun driven ohmic heating alone and illustrating the merits of direct ion heating. Conservative to pessimistic confinement estimates are used throughout so as to ensure that the promise offered by this concept does not presume unrealizable improvements in energy containment

Solutions of uniform, open-channel, liquid metal flow in a strong, oblique magnetic field

The flow of an electrically conducting fluid in an open channel in the presence of a strong magnetic field of oblique incidence to both the channel walls and the force of gravity is explored. This type of flow has possible applications to the protection of high heat flux surfaces in magnetic confinement fusion reactors. The governing equations of fully-developed flow are derived retaining all viscous terms. They are then solved in the strong field limit in an asymptotic, iterative fashion, carrying the first two terms in the expansion with powers of the effective Hartmann number. The asymptotic solutions for the velocity, induced magnetic field and the flow rate are compared with a numerical solution of the complete governing equations. Good agreement is seen between the asymptotic and numerical predictions of velocity and electric current distribution when the core regions are dominated by magnetic forces. One novel feature of open channel flow of this type is the existence, predicted by the asymptotic analysis and confirmed by the numerical integration, of a second-order Hartmann layer on the free surface. Its presence is required to ensure the condition of no shear stress on this boundary. Also seen is the presence of strong discontinuities across free shear layers, which form along the field lines that extend from the free surface/sidewall corner.

Effects of Heavy-Ion Irradiation on Microstructure of V-4Cr-4Ti Alloy at Moderate Temperatures

AbstractV-4 wt.% Cr-4 wt.% Ti alloy is a promising candidate material for first-wall and structural applications in magnetic fusion reactors. In the past, fast neutron sources were used to evaluate postirradiation properties of fusion candidate materials. The recent shutdowns of the Fast Flux Test Facility (FFTF) and the Experimental Breeder Reactor (EBR-II) left U.S. researchers without local experimental facilities for such tests. Under such circumstances, ease of experimental control, availability, and relatively low cost make heavy-ion irradiation an attractive alternative, provided its limitations are appreciated. We selected 3-MeV V+ and 4.5-MeV Ni++ ions to investigate the effects of irradiation on the microstructure of V-4Cr-4Ti alloy in the temperature range of 200 – 420°C. The main interest is in the evaluation of this alloy's dimensional stability and susceptibility to irradiation embrittlement. In this paper, we report results of ion irradiation experiments and compare them with available data on fast-neutron irradiation. From transmission electron microscopy (TEM) analysis of ion-irradiated specimens, we found that the dominant feature of the postirradiation microstructure was a high density of dislocation loops and pointdefect clusters. Density and defect size depended on irradiation dose and temperature. Precipitates and voids/bubbles were not observed, even in specimens that were simultaneously injected with He and exposed to heavy-ion irradiation. Increased transport of point defects to internal interfaces was observed, as manifested by defect denuded zones along grain boundaries, Defect denuded zones along grain boundaries could lead to segregation of impurities and solutes and formation of precipitates on grain boundaries.

室温および液体ヘリウム温度での極低温用オーステナイト・ステンレス鍛造材の引張特性と疲労き裂進展特性におよぼす熱履歴の影響

By using forging of a newly developed austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N), a candidate material for the support structure of superconducting magnets for thermonuclear fusion reactors, the effects of a sensitizing heat treatment (24 hours at 650°C, followed by water quenching) on tensile and fatigue crack growth properties (crack closure behavior) at room and liquid helium temperatures were clarified. As a result, it has been found that although the sensitizing heat treatment has virtually no effect on both tensile and fatigue crack growth properties at room temperature, a decrease in tensile total elongation and an increase in fatigue crack growth rate relative to the effective stress intensity factor range due to such heat treatment are evident at liquid helium temperature. This is believed to be due to the difference in microscopic cracking path. Fracture occurs transgranularly in a solution treated material, whereas fracture occurs mainly along the grain boundaries in a sensitizing heat treated material. Furthermore, it has been ascertained that, at liquid helium temperature, the fracture surface profiles of fatigue cracks are much more serrated and the crack-opening load increases, due to sensitizing heat treatment.

Temperature ramp dependent reaction kinetics of internal tin processed Nb3Sn wires

Multifilamentary Nb3Sn wires will be a critical component of a magnetic fusion reactor, which requires a high magnetic field for confinement of the plasma. The international effort to demonstrate the feasibility of the reactor design (International Toroidal Engineering Reactor, ITER) has very stringent requirements for the hysteresis loss and the critical current density Jc(≤ 600 mJ/cm3 and ≥700 A/mm2, respectively) of the Nb3Sn wires which are to be used for the central toroidal field coil magnet. Although short wire specimens can meet these requirements, consistently fabricating long lengths of wire meeting these specifications is not trivial. A part of this inconsistency is related to the effects of the temperature ramp rate on these critical properties. Thus, to investigate the observed ramp rate dependencies a metallurgical examination was performed. Samples of wires were subjected to two controlled temperature ramp cycles. Wire cross-sections were examined from samples removed at various stages of the two ramp cycles.

A free-electron laser for cyclotron resonant heating in magnetic fusion reactors

A G-band free-electron laser designed for plasma heating is described using a coaxial hybrid iron (CHI) wiggler formed by insertion into a solenoid of a central rod and an outer ring of alternating ferrite and nonferrite spacers positioned so that the central ferrite (nonferrite) spacers are opposite the outer nonferrite (ferrite) spacers. The CHI wiggler provides for enhanced beam focusing and the ability to handle intense beams and high-power continuous wave radiation. Simulations indicate that a power/efficiency of 3.5 MW/13% are possible using a 690 kV/40 A beam. No beam loss was found in simulation.

Design study of a G-band FEL amplifier for application to cyclotron resonant heating in magnetic fusion reactors

A G-band (140–150 GHz) free-electron laser is described using a coaxial hybrid iron (CHI) wiggler. The CHI wiggler is produced by insertion into a solenoid of a central rod and an outer ring composed of alternating ferrite and nonferrite spacers. The position of the spacers is such that the ferrite (nonferrite) spacers on the central rod are opposite the nonferrite (ferrite) spacers on the outer ring. The field is cylindrically symmetric and exhibits minima in the center of the gap providing for enhanced beam focusing. We describe a tapered wiggler amplifier for plasma heating applications. Preliminary design studies using a nonlinear simulation indicates that output powers of 3.5 MW are possible using a 690 kV/40 A electron beam for a total efficiency of 13%. It is important to note that no beam loss was observed even for realistic values of beam energy spread.

Two-dimensional impinging jet cooling of high heat flux surfaces in magnetic confinement fusion reactors

Two-dimensional impinging jet cooling of high heat flux surfaces in magnetic confinement fusion reactors

Two-dimensional impinging jet cooling of high heat flux surfaces in magnetic confinement fusion reactors

The divertor surface of a magnetic confinement fusion reactor is exposed to strong radiation heating by high flux charged particles. According to the standard design for the ITER, the heat flux on the divertor surface averages 15 MW m −2 or more. In this study, cooling by a two-dimensional impinging jet flow is proposed to cool this surface. For an impinging jet flow on a flat heated surface, a high critical heat flux (CHF) is obtained only in the limited surface region where the jet flow hits directly. Outside this region, the CHF decreases abruptly with distance from the center. The main reason is that the pressure decreases abruptly away from the center region and the liquid flow is spread away from the heated surface region by the strong boiling. To overcome these difficulties, we propose that the impinging jet is applied to a heat transfer wall with a concave surface. In this study, the CHFs and the nucleate boiling curves for two-dimensional impinging jet cooling were first obtained as a function of the distance from the center using a thin copper foil heater designed on the plastic sheet. Experiments were done under various conditions of liquid subcooling, flow velocity and surface curvature. Empirical correlations for the CHF including these parameters were obtained. It is clear that impinging jet cooling of the curved surface is useful to keep the CHF in the downstream region high. Finally, application of the two-dimensional jet to the cooling of a fusion divertor surface is assessed. When the jet velocity is equal to 14.6 m −1 , and the liquid subcooling is 80 K, a two-dimensional jet is able to cool a curved surface area 50 mm wide with an average heat flux of 30 MW m −2 .

Studies on magnetohydrodynamic flow characteristics and heat transfer of liquid metal two-phase flow cooling systems for a magnetically confined fusion reactor

Abstract Liquid metal cooling for the first wall and blanket of a magnetic confinement fusion reactor has various advantages. However, it has the disadvantages of large magnetohydrodynamic pressure drops and heat transfer deterioration under a strong magnetic field. Thus, the present authors have proposed cooling with a helium-lithium annular mist flow as well as the cooling with a liquid metal boiling flow, and as fundamental studies, investigated the effect of a magnetic field on the flow characteristics and heat transfer of liquid metal two-phase systems since the 1970s. In the present paper we summarize the important findings obtained from our experimental studies for (i) an air-mercury stratified flow in a horizontal rectangular channel, (ii) a helium-lithium annular mist flow in a horizontal rectangular channel, (iii) the mercury pool boiling on a horizontal surface, and (iv) air-mercury upward flows in a vertical circular tube. Based on the results, the research subjects for the development of the liquid metal two-phase flow cooling systems are discussed.

Dynamic behavior of fusion structural components under strong magnetic fields

Abstract This paper describes dynamic behavior of non-ferromagnetic and ferromagnetic plates, and a non-ferromagnetic cylinder under strong magnetic field. The dynamic behavior of a ferromagnetic cantilevered plate, which is set along the uniform magnetic field, shows that its natural frequency increases with external magnetic induction. This means that a magnetic stiffness effect occurs in a ferromagnetic plate due to magnetization. The behaviors of non-ferromagnetic and ferromagnetic plates show that the magnetic viscous damping effect becomes larger almost proportionally with the squared external magnetic induction. A non-ferromagnetic cylinder also shows the magnetic viscous damping effect in a strong magnetic field. It is necessary to consider these magnetic stiffness and viscous damping effects for the structural design of a magnetic fusion reactor.