The Improvement of Confinement by the Use of RF Waves

It is suggested that the ponderomotive force induced by radio frequency (rf) waves in the range of the Alfven frequency can create a transport barrier in a tokamak. The linear and nonlinear behaviour of the drift-like perturbation with a parallel velocity shear is studied in the presence of rf waves. It is shown if the radial profile of the rf field energy is properly chosen the linear mode is stabilised and turbulent momentum transport reduces. The rf power required for this stabilisation is found to be rather modest and hence should be easily obtained in actual experiments.     

Utilizing RF Electric Field for Injection-Trapping High Energy Molecular Ions in Mirror Field, (I)

A method of using rf electric field in a device for high-energy injection is described, and the behavior of the molecular ions injected into a system combining rf and DC magnetic fields is investigated by calculation and observation.

In this method, some of the injected molecular ions are decelerated by the rf field, and trapped in a small region between the mirror fields, where they are dissociated by collision with the background gas or with the plasma particles, and are thus trapped in the form of atomic ion. The relation between the rf frequency ω and the Larmor frequencies ΩN⁺ ₂, ΩN⁺ ₁ is given by ω=N/2· ΩN⁺ ₂=N/4· ΩN⁺ ₁, where N is the number of rf electrodes. The number N is chosen so that N/2 is an integer and N/4 a non-integer.

Calculation shows that the ions injected in a certain favorable phase are stably decelerated despite their initially possessing a precessional motion. This is proven by observation of actual electron behavior.     

Tokamak Plasma Flows Induced by Local RF Forces

The tokamak plasma flows induced by the local radio frequency(RF) forces in the core region are analyzed. The effective components of local RF forces are composed of the momentum absorption term and the resonant parallel momentum transport term(i.e. the parallel component of the resonant ponderomotive forces). Different momentum balance relations are employed to calculate the plasma flows depending on different assumptions of momentum transport.With the RF fields solved from RF simulation codes, the toroidal and poloidal flows by these forces under the lower hybrid current drive and the mode conversion ion cyclotron resonance heating on EAST-like plasmas are evaluated.     

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (III)—Consideration of the Effect of Microbubbles on Pressure Wave Propagation through a Water Test—

The effects of microbubbles dispersed in a liquid on a high-rising-rate pressure wave were experimentally investigated with water. Intense, high-rising-rate pressure waves with a rise time of about 1.5 ms were produced by a spark discharge in water, and gas microbubbles were produced by two different bubble generators. Particular attention was focused on the attenuation effect of microbubbles on propagating pressure waves. The dependence of the attenuation effect on the radius and void fraction of the microbubbles was carefully examined. It was found that when the microbubbles are sufficiently small (e.g., about 50 μm in peak radius), the amplitude of wall vibration induced by the spark-induced pressure wave is dramatically decreased with an increase in void fraction. The present study provides strong experimental evidence that microbubbles can act as a strong absorber for high-rising-rate pressure waves as recently predicted numerically.     

非晶态合金的射频场崩塌效应的研究

在射频场中非晶合金的穆斯堡尔谱显示崩塌效应。通过谱线热红移的测量，确定样品在射频场中发生崩塌时的温度远低于居里温度，说明崩塌效应并不是射频场的热效应可能引起的铁磁－顺磁转变。较强射频场作用的后效应的观察结果无法用相干翻转模型来解释，而同磁性原子团簇随机起伏的假设一致，说明射频场引起的磁性簇的随机起伏导致了穆斯堡尔谱的崩塌。     

Theoretical Approach to Pseudo-Mode Neutron Waves in a Moderator

An analysis is presented on neutron wave propagation in moderators in the frequency region above the limiting frequency for discrete eigenvalues, and the distribution of the modulated neutrons in the medium is described by integrals over the continua.

When a discrete eigenvalue reaches the edge of a continuum, a singularity appears in the angular distribution of the waves and causes the discrete mode waves to disappear. Further pursuit of the root of the dispersion law making use of the analytic continuation in the angular integral locates the root on a branch of the complex eigenvalue planes that adjoins the principal branch. This root on the adjacent branch can in certain cases bring about a sharp resonance. i.e. a “pseudo-discrete wave eigenvalue” in the continuum. The conditions for the effective excitation of this kind of pseudo-mode wave were examined, which revealed that for this it is essential that the neutrons causing the limiting frequency have a weak coupling and that their injection be light. It was also demonstrated that the distance from source affects the pseudo-discrete eigenvalue.

An application of this analysis to the case of graphite is also presented.     

Cross-focusing of two chirped Gaussian laser beams and THz wave generation in plasmas of multi-ion species under a weakly relativistic and ponderomotive regime

The generation of terahertz (THz) waves via the beating of two high-intensity chirped Gaussian lasers in a multi-ion-species plasma is numerically studied by taking into account the weak relativistic and ponderomotive regime of interaction. The coupled differential equations for beamwidth parameters are extracted by introducing the dielectric function of such plasma and using WKB and paraxial ray approximations. The amplitude of THz radiation at beat frequency resulting from the nonlinear current density induced by the beat ponderomotive force of the cross-focusing of beams was obtained. The impacts of the chirp frequency parameter, initial laser intensity and initial ionic species density (specifically, the presence of singly and doubly charged ions) in the plasma on THz generation were discussed. Our numerical results reveal that THz radiation generation strongly depends on the chirp frequency parameter. A specific range of chirp frequencies exists for self-focusing as well as THz generation with a 'turning point', where the THz emission reaches its maximum value. The results show that the strength of self-focusing and consequently the generated THz radiation are reduced by increasing the density of doubly charged ionic species in the plasma due to the suppression of the nonlinear effects.     

Two fluid model for two-phase turbulent jets

Eulerian two-fluid models are widely used in nuclear reactor safety and CFD. In these models turbulent diffusion of a dispersed phase must be formulated in terms of the fluctuating interfacial force and the Reynolds stresses. The interfacial force is obtained using the probability distribution function approach by Reeks (1992). This paper is the first application of this force to a case of engineering interest outside homogeneous turbulence. An Eulerian multidimensional two-fluid model for a cylindrical two-phase dispersed particle jet is proposed and compared with experimental data. The averaged conservation equations of mass and momentum are solved for each phase and the turbulent kinetic energy equation is solved for the continuous phase. The turbulent diffusion force and the Reynolds stresses are constituted within the context of the k- model of turbulence. A dissipation term has been added to the k- model for the turbulence modulation by the particles. Once the constitutive relations have been defined, the two-fluid model is implemented in a computational fluid dynamics code. It is shown that when the particles are very small the model is consistent with a convection-diffusion equation for particle transport where the diffusivity is defined according to Taylor's model (Taylor, G.I., 1921. Diffusion by continuous movements. Proc. London Math. Society, A20, pp. 196–211). The two-fluid model is also compared against two experimental data sets. Good agreement between the model and the data is obtained. The sensitivity of the results to various turbulent mechanisms is discussed.     

Evaluation of nuclide release scenarios for a hypothetical LILW repository

A program for the safety assessment and performance evaluation of a low- and intermediate-level radioactive waste (LILW) repository system has been developed. Utilizing GoldSim (2006), the program evaluates nuclide release and transport into the geosphere and biosphere under various disruptive natural and manmade events and scenarios that can occur after a waste package failure. We envisaged and illustrated these events and scenarios as occurring after the closure of a hypothetical LILW repository, and they included the degradation of various manmade barriers, pumping well drilling, and natural disruptions such as the sudden formation of a preferential flow pathway in the far-field area of the repository. Possible enhancement of nuclide transport facilitated by colloids or chelating agents is also dealt with. We used the newly-developed GoldSim template program, which is capable of various nuclide release scenarios and is greatly suited for simulating a potential repository given the geological circumstances in Korea, to create the detailed source-term and near-field release scheme, various nuclide transport modes in the far-field geosphere area, and the biosphere transfer. Even though all parameter values applied to the hypothetical repository were assumed, the illustrative results, particularly the probabilistic calculations and sensitivity studies, may be informative under various scenarios. (GoldSim, LILW, Nuclide transport, Safety assessment, Scenario).     

On normal impact of an infinite elastic-plastic thin-walled plate by a finite elastic rod

The problem of the transverse collision of a finite circular cylinder (flat-ended metal projectile) of length L and radius R₀, moving with the initial velocity V₀, with an immobile thin infinite plate, is considered. It is assumed that the plate is in a pure shear state. The one-dimensional waves both in the plate (cylindrical elastic-plastic waves) and in cylinder (plane longitudinal elastic waves) are investigated. Graphs of the peak dimensionless shear stress at the point of impact versus four non-dimensional quantities representing the material and geometrical properties of the plate and the projectile, are presented. These graphs allow the projectile speed just sufficient to initiate ejection of the slug to be estimated.     

A classical relativistic hydrodynamical model for strong EM wave-spin plasma interaction

Based on the covariant Lagrangian function and Euler–Lagrange equation, a set of classical fluid equations for strong EM wave-spin plasma interaction is derived. Analysis shows that the relativistic effects may affect the interaction processes by three factors: the relativistic factor, the time component of four-spin, and the velocity-field coupling. This set of equations can be used to discuss the collective spin effects of relativistic electrons in classical regime, such as astrophysics, high-energy laser-plasma systems and so on. As an example, the spin induced ponderomotive force in the interaction of strong EM wave and magnetized plasma is investigated. Results show that the time component of four-spin, which approaches to zero in nonrelativistic situations, can increase the spin-ponderomotive force obviously in relativistic situation.     

The propagation dynamics and stability of an intense laser beam in a radial power-law plasma channel

By containing ponderomotive self-channeling,the propagation behavior of an intense laser beam and the physical conditions are obtained theoretically in a radial power-law plasma channel.It is found that ponderomotive self-channeling results in the emergence of a solitary wave and catastrophic focusing,which apparently decreases the region for stable propagation in a parameter space of laser power and the ratio of the initial laser spot radius to the channel radius(RLC).Direct numerical simulation confirms the theory of constant propagation,periodic defocusing and focusing oscillations in the parameter space,and reveals a radial instability which prevents the formation of bright and dark solitary waves.The corresponding unstable critical curve is added in the parameter space numerically and the induced unstable region above the unstable critical curve covers that of catastrophic focusing,which shrinks the stable region for laser beams.For the expected constant propagation,the results reveal the need for a low RLC.Further study illustrates that the channel power-law exponent has an obvious effect on the final stable region and laser propagation,for example increasing this exponent can enlarge the stable region significantly,which is beneficial for guiding of the laser and increases the lowest RLC for constant propagation.Our results also show that the initial laser amplitude has an apparent influence on the propagation behavior.     

RFQ accelerators for ion implantation

Radiofrequency quadrupoles (RFQs) are low-energy rf accelerator structures which can efficiently transport and accelerate high-current ion beams. For a wide range of applications, RFQs can be designed that, like all rf accelerators, have a fixed output energy per amu, that can be varied by independently phased postaccelerator cavities. An attractive new development is an RFQ structure with variable energy which is achieved by frequency variation. The status of the work on fixed- and variable-energy RFQ applications is presented.     

Ion cyclotron resonance in dense plasmas

The possibility of heating a plasma by means of the ion cyclotron resonance is investigated. It is shown that in plasmas with charged-particle densities of 10⁷–10¹¹ cm^–3, the use of short (compared with the length of the pinch) heating sections mades it possible to weaken the effect of transverse ionic polarization fields by virtue of the motion of electrons along the lines of force of the external magnetic field. In a low-ionization plasma the efficiency of transfer of energy from the rf field to the ions is reduced as the ion velocity increases; this reduction is due to cooling of the ions by neutral atoms.The authors are indebted to K. D. Sinel'nikov for his interest in this work and for valuable discussions.     

Analysis of High-Power Microwave Propagation in a Magnetized Plasma Filled Waveguide

Plasma filling can dramatically improve the performance of high power microwave devices.The characteristics of high-power microwave propagation along plasma filled waveguides in an axial magnetic field are analyzed in this paper,and the ponderomotive force effect of high power microwave is taken into consideration.Theoretical analysis and preliminary numerical calculations are performed.The analyses show that the ponderomotive effect would change the plasma density,distribution of microwave field intensity,and dispersion of wave propagation.The higher the microwave power,the stronger the ponderomotive effect.In different magnetic fields,the ponderomotive effect is different.     

Machine learning of turbulent transport in fusion plasmas with neural network

Turbulent transport resulting from drift waves,typically,the ion temperature gradient(ITG)mode and trapped electron mode(TEM),is of great significance in magnetic confinement fusion.It is also well known that turbulence simulation is a challenging issue in both the complex physical model and huge CPU cost as well as long computation time.In this work,a credible turbulence transport prediction model,extended fluid code(ExFC-NN),based on a neural network(NN)approach is established using simulation data by performing an ExFC,in which multi-scale multi-mode fluctuations,such as ITG and TEM turbulence are involved.Results show that the characteristics of turbulent transport can be successfully predicted including the type of dominant turbulence and the radial averaged fluxes under any set of local gradient parameters.Furthermore,a global NN model can well reproduce the radial profiles of turbulence perturbation intensities and fluxes much faster than existing codes.A large number of comparative predictions show that the newly constructed NN model can realize rapid experimental analysis and provide reference data for experimental parameter design in the future.     

Mode filtering based on ponderomotive force nonlinearity in a plasma filled rectangular waveguide

Here a new scheme for mode filtering is proposed.Based on the ponderomotive force effect,propagation of the microwave dual-mode through a plasma-filled metallic rectangular waveguide is investigated.To excite the TE20 mode in a rectangular waveguide,the existence of fundamental modes is unavoidable.To filter the destructive mode (TE10),the waveguide is filled with a collisional plasma.Based on the coupling effect,the energy of this destructive TE10 mode is transferred to the TE20 mode.The proposed structure acts like a mode convertor.The TE10 mode become more attenuated and instead the TE20 mode is amplified.The plasma filled rectangular waveguide acts as a mode filtering tool.     

Simulation of two-phase flows in vertical tubes with the CTFD code FLUBOX

The computational two-fluid dynamics (CTFD) code FLUBOX is developed at GRS for the multidimensional simulation of two-phase flows. The single-pressure two-fluid model is used as basis of the simulation. A basic mathematical property of the two-fluid model of FLUBOX is the hyperbolic character of the advection. The numerical solution methods of FLUBOX make explicit use of the hyperbolic structure of the coefficient matrices. The simulation of two-phase flow phenomena needs, apart from the conservation equations for each phase, an additional transport equation for the interfacial area concentration. The concentration of the interfacial area is one of the key parameters for the modeling of interfacial friction forces and interfacial transfer terms. A new transport equation for the interfacial area concentration is in development. It describes the dynamic change of the interfacial area concentration due to mass exchange and a force balance at the phase boundary. Results from FLUBOX calculations for different experiments of two-phase flows in vertical tubes are presented as part of the validation.     

Practical judgment of exemption by applying isotope specific exemption levels for surface contamination

Abstract

In terms of applying the concept of exemption to safety standards for surface contamination, we have derived isotope specific exemption levels for surface contamination (Bq cm^–2) by developing an original dose assessment model for surface contamination to develop reasonable radiation protection systems that reflect the radiological properties of nuclides and avoid excessive regulatory procedures. These exemption levels can be applied to the radiation, waste and transport safety fields by assuming a universal scenario and by applying radiation protection systems consistent with the current International Atomic Energy Agency (IAEA) Basic Safety Standards, safety series no. 115 and safety guide RS-G-1·7. In the case of materials containing a mixture of nuclides, the exemption can generally be judged on the basis of whether the condition ΣD/C<1 is satisfied (where D represents an actual measurement result and C represents the exemption level), and the estimation of the surface contamination density of key nuclides such as ⁶⁰Co, which are easily measured and dominant nuclide components, can be practically applied to the judgment of exemption. In this study, the contribution of nuclides to the summation defined in terms of relative importance was assessed using the exemption levels for surface contamination derived in previous Central Research Institute of Electric Power Industry (CRIEPI) studies and the technical data of nuclide composition ratios contributing to contamination assumed to occur in Japanese nuclear power plants. Important nuclides, whose relative importance was >0·1, were extracted following the 10% summation rule described by the European Commission in its document Radiation Protection 134. It was found that ⁶⁰Co was the only important nuclide in most of the cases and other nuclides can be ignored according to the reasonable exemption levels for surface contamination derived in CRIEPI's previous study, where the α emitter was the most important nuclide in the case of estimating fuel damage by applying the exemption levels for surface contamination derived from the dose conversion factors given in IAEA TECDOC-1449. It is expected that the resulting reasonable exemption levels for surface contamination will be applied practically to future regulation and that consistent radiation protection systems will then be realised throughout the radiation, waste and transport safety fields.     

Reversed-field pinch studies in the Madison Symmetric Torus

Studies of large-size (R=1.5 m,a=0.5 m), moderate current (I <750 kA) reversed-field pinch (RFP) plasmas are carried out in the Madison Symmetric Torus in order to evaluate and improve RFP confinement, study general toroidal plasma MHD issues, determine the mechanism of the RFP dynamo, and measure fluctuation-induced transport and anomalous ion heating. MST confinement scaling falls short of the RFP scaling trends observed in smaller RFPs, although the plasma resistance is classical. MHD tearing modes with poloidal mode numberm=1 and toroidal mode numbersn=5–7 are prevalent and nonlinearly couple to produce sudden relaxations akin to tokamak sawteeth. Edge fluctuation-induced transport has been measured with a variety of insertable probes. Ions exhibit anomalous heating, with increases of ion temperature occurring during strong MHD relaxation. The anomalous heating fraction decreases with increasing density, such that ion temperatures approach the lower limit given by electron-ion friction. The RFP dynamo has been studied with attention to various possible mechanisms, including motion-EMF drive, the Hall effect, and superthermal electrons. The toroidal field capacity of MST will be upgraded during Summer 1993 to allow low-current tokamak operation as well as improved RFP operation.