Decay Rate of an Excited Atom in a Magnetized Plasma Medium

The effect of magnetic field on the decay rate of an excited atom in a dispersive and dissipative plasma medium is calculated. For this purpose, two particular cases of parallel and perpendicular propagation with respect to the magnetic field is considered for an homogeneous infinitely extended medium. The difference between these two cases is shown in the index of the refraction of the media. For calculating the decay rate, the approach of the Green’s function is used. It is applicable to evaluate the decay rate of an excited atom that allows us to understand the particular structure of the vacuum state of the electromagnetic field.     

MHD Equilibrium and Kink Stability in Damavand Tokamak

Magnetohydrodynamic (MHD) equilibrium is vulnerable to numerous destabilizing mechanisms. Instabilities introduce distortions to the plasma magnetic surfaces and its boundaries, their driving force being the radial gradient of plasma toroidal current density. For certain modal numbers, internal kink modes may develop, and their study is feasible according to the energy principle, in which the change in total potential energy due to the disturbance is evaluated. In this article, we present a totally new analysis of MHD equilibrium and stability, and apply it to Damavand tokamak which has a large aspect ratio. For this purpose, we combine perturbation and Green’s function methods to solve for the equilibrium configuration. At this stage, plasma profiles are found explicitly in terms of Bessel functions, and we present a simple expression for estimation of total toroidal plasma current. Then the rest of plasma profiles, including poloidal magnetic flux, safety factor, and toroidal current density, are obtained and plotted. In the next step, we turn to the stability calculations and show that Damavand plasma is resistant to most of the disturbances.     

Estimation of the Lyman-α signal of the EFILE diagnostic under static or radiofrequency electric field in vacuum

The electric field induced Lyman-α emission diagnostic aims to provide a non intrusive and precise measurement of the electric field in plasma, using a beam of hydrogen atoms prepared in the metastable 2 s state. The metastable particles are obtained by means of a proton beam extracted from a hydrogen plasma source, and neutralised by interaction with vaporised caesium.When a 2 s atom enters a region where an electric field is present, it undergoes a transition to the2 p state(Stark mixing). It then quickly decays to the ground level, emitting Lyman-α radiation,which is collected by a photomultiplier. The 2 s→2 p transition rate is proportional to the square of the magnitude of the electric field, and depends on the field oscillation frequency(with peaks around 1 GHz). By measuring the intensity of the Lyman-α radiation emitted by the beam it is possible to determine the magnitude of the field in a defined region. In this work, an analysis of the behaviour of the diagnostic under static or radiofrequency electric field is presented. Electric field simulations obtained with a finite element solver of Maxwell equations, combined with theoretical calculations of the Stark mixing transition rate, are used to develop a model for the interpretation of photomultiplier data. This method shows good agreement with experimental results for the static field case, and allows to measure the field magnitude for the oscillating case.     

Greek Fire: Nicholas Christofilos and the Astron Project in America’s Early Fusion Program

The Astron project, conducted from 1956 to1973 at Livermore National Laboratory, was the brainchild of Nicholas Christofilos, a Greek engineer with no formal physics credentials. Astron’s key innovation was the E-layer, a ring of relativistic electrons within a magnetic mirror device. Christofilos predicted that at sufficient E-layer density the net magnetic field inside the chamber would reverse, creating closed field lines necessary for improving plasma confinement. Although Astron never achieved field reversal, it left important legacies. As a cylindrical device designed to contain toroidal plasmas, it was the earliest conception of a compact torus, a class that includes the Spheromak and the FRC. The linear induction accelerator, developed to generate Astron`s E-layer, is now used in many applications. Through examination of internal lab reports and interviews with his colleagues and family, this research charts Christofilos’ career and places Astron in its historical context. This paper was originally prepared in 2004 as an undergraduate Junior Paper for the Princeton University History Department.     

Excited state positronium collisions with the noble gases

Calculations of the discrete positronium (Ps) transition Ps(2s) → Ps(2p) in collisions with the ground state noble gases helium, argon and xenon are presented at total and single differential levels. For collisions where the target atom is not excited or ionized the impulse approximation (IA) is used. For collisions where the target is excited or ionized the first Born approximation (FBA) is used. For this latter case the Hartley-Walters approximation is used to sum over all possible final atom states.     

Observation of an Enhanced Magnetic Helicity Injection Mode by a Rotating Plasma Annulus

Coaxial plasma guns are commonly used to inject magnetic helicity in innovative confinement concepts (ICC’s) for magnetic fusion. One of the key issues in magnetic helicity injection is to maximize the magnetic helicity injection rate. We have identified experimentally an alternative way to increase the magnetic helicity injection rate through rotating plasmas by extending the length of the inner electrode of a coaxial plasma gun so that an additional E × B region interweaves the standard J//B configuration. In the so-called “enhanced helicity injection” mode, the gun voltage is larger compared with the “normal” mode and decays more slowly. Another signature of the enhanced mode is increased edge magnetic field in conjunction with larger edge rotation. The results indicate that tuning plasma rotation is another way to enhance magnetic helicity injection using coaxial plasma guns. An alternative ICC is proposed based on the experimental observations.     

Excited state positronium collisions with helium

Calculations of the discrete positronium (Ps) transition Ps(2s) → Ps(2p) in collisions with ground state helium are reported. Results are presented at total and single differential levels. Calculations are made within the first born approximation (FBA). The Hartley-Walters approximation is used to sum over all possible final excited atom states. In addition, impulse approximation (IA) calculations are reported for Ps(2s) and Ps(2p) fragmentation collisions with ground state helium, where the atom does not change state. Again, the FBA with the HWA is used to account for collisions in which the atom is excited or ionized.     

Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.     

Neutron-Induced Metallic Spike Zones in GaAs

The metallic spike model for neutron damage has been shown to account for the observed anomalous infrared absorption in GaAs. In this paper, the electrical properties of semiconductors containing metallic spikes are explored. The metallic zones are shown to act as deep potential wells which trap carriers from the host semiconductor energy bands. The component of mobility associated with carrier scattering from the depletion region surrounding charged spikes is estimated as a function of temperature, including the temperature dependence of the trapped charge. Hall measurement data taken before and after neutron irradiation of n-type GaAs are compared with theory and good agreement is obtained. It is proposed that the high field trapping and slow release of electrons observed in neutronirradiated Gunn diodes is associated with the presence of metallic spikes. Hot electrons in high field domains penetrate the electrostatic barrier and are trapped within the spikes. When the low field condition is restored, excess electrons return to the host semiconductor matrix. The rate of escape of excess electrons is estimated from considerations of the processes of emission over and tunneling through the electrostatic barrier. Measurements of the temperature - dependent decay rates of the excess charge are obtained from neutron-irradiated Gunn diodes. Two decay rates were obtained at each temperature. The shorter decay time shows a temperature dependence consistent with a quantum tunneling mechanism. The longer decay time shows a stronger temperature dependence which is in qualitative agreement with emission of electrons over the barrier.     

Combining the “Kinetic Tandem” and the “Kinetic Stabilizer” Concepts

A novel means of MHD stabilizing axially symmetric mirror systems was proposed by Ryutov and demonstrated in the Gas Dynamic Trap at Novosibirsk. It relies on the strongly stabilizing effect exerted by low density plasma on the expanding field lines outside the mirrors. The “Kinetic Stabilizer Tandem Mirror” implements Ryutov’s technique by injecting axially directed ion beams into the expander. The ions, stagnated, and reflected by the converging magnetic field, form the stabilizing plasma. MHD stability code calculations show stabilization at beta values of 40%, with stabilizer beam powers that are small compared to the T-M fusion power output. Implicit in the calculations is the assumption that adequate “communication” exists between the plug plasmas and the stabilizer plasmas. This paper examines one means for enhancing communication: Utilize the stabilizer plasma to create a potential peak that, together with the plug potential, forms a potential well that traps and contains a “bridging” plasma.     

Developing and Benchmarking MH4D, A Tetrahedral Mesh MHD Code

Periodic boundary conditions, applicable to parallel and non-parallel planes have been developed. Mode selection is achieved by modeling a “slice” of a domain. For instance, modeling a 1/4-slice of a cylindrical Z-pinch geometry eliminates the m = 1,2, and 3 modes. M = 4 and higher modes will still be simulated. An insulating boundary condition has been developed. External circuits can be modeled by specifying electric and magnetic surface fields. A specified magnetic field simulates a circuit with constant current where Ampere’s law dictates the current flow. A specified electric field simulates a constant applied voltage. A constant current model is applicable to experiments with relatively high-impedance power supplies. To function with the insulating boundary condition, implicit and semi-implicit solvers require inclusion of surface terms. These solvers are being modified. Also, ionization physics capability is being developed. Progress in boundary condition development is presented, and a benchmarking and verification strategy is described.     

The decay of autoionizing states in electric field formed during collisions of multiply charged ions with metal surface

When a multicharged ion approaches the metal surface, a strong electric field occurs due to interaction between the ion and the charge induced in the metal. During ion-surface scattering, double excited autoionizing states can be formed, and their decay occurs in the presence of strong electric field. The Auger transition rates for such states are higher than those in the absence of the electric field; this is connected with the increase in the wave functions overlap and also with mixing of states with different orbital momentums. This effect changes our view on the population scheme of states with the lowest principal quantum numbers for the collisions considered.     

Calculation of Beta-Ray Spectra from Individual and Aggregate Fission Products

The β-ray spectra of individual fission products were calculated by using the β-decay data assuming every β-decay to be allowed transition. For the nuclides without measured decay data the β-feeding function was evaluated with the gross theory of β-decay and the β-ray spectrum was calculated from the function. The measured decay data were also supplemented with the data calculated by the gross theory for the excitation energy range above the highest measured excitation energy level. The β-ray spectra from aggregate fission products after a burst fission were calculated by using the β-ray spectrum and the atom number of each fission product nuclide and they were compared with the ones measured for thermal neutron induced fission of ²³⁵U, ²³⁹Pu and ²⁴¹Pu at Oak Ridge National Laboratory. The spectrum calculations showed excellent agreement with the measured data at shorter cooling times than 10s when many short-lived nuclides without measured decay data contributed considerably to the spectrum.     

A study of a hydrogen atom in a two-dimensional quantum well

A feature of a hydrogen atom confined in a two-dimensional quantum well in the presence of an electric field is investigated. The binding energy of the confined hydrogen atom is calculated as a function of the confined potential radius and as a function of the intensity of an applied electric field. It is shown that the binding energy and the oscillator strength of the confined hydrogen atom are highly dependent on the confined potential radius and the intensity of an applied electric field.     

Progress in Condensed Matter Nuclear Science

The progress of the Condensed Matter Nuclear Science reported during ICCF-12 is summarized with emphasis on reply to the DOE review in 1989 and in 2004. The 18 reviewers might not be aware of the new achievement in the Advanced Technology Research Center, Mitsubishi Heavy Industries; hence, their conclusion should have been more positive towards this research. Arata’s DS-reactor, NASA’s early experiment, and the“heat after death” experiment should change the conclusion about the “excess heat” and its prospect. Various fundamental researches have shown the consistent nature in understanding. A cost effective and comprehensive study is mentioned.     

Design evaluation of decay tank for a pool-type research reactor from the required minimum flow residence time point of view

A decay tank shall be designed to provide enough flow residence time to ensure that the N-16 activity decreases before the coolant leaves the decay tank's shielding room. However, when a proper criterion for the flow residence time in a decay tank is not presented, the tank would be oversized/undersized. In this paper, design evaluation for a decay tank is performed by investigating the effect of the fluid distribution along the residence time on the total dose rate and the required minimum flow residence time. The evaluation is also carried out to resize the predesigned decay tank.     

Regular behaviors in SU（2） Yang—Mills classical mechanics

In order to study regular behaviors in high-energy nucleon-nucleon collisions,a representation of the vector potential Ai^a is defined with respect to the (a,i)-dependence in the SU(2) Yang-Mills classical mechanics,Equations of the classical infraed field as well as effective potentials are derved for the elastic or inelastic collision of two plane waves in a three-mode model and the decay of an excited spherically-symmetric field.     

Scientific Challenges,Opportunities and Priorities for the U.S. Fusion Energy Sciences Program

In October 2003, Dr. Raymond Orbach, Director of the Department of Energy’s Office of Science, issued a charge to the Fusion Energy Sciences Advisory Committee (FESAC) “to identify the major science and technology issues that need to be addressed, recommend how to organize campaigns to address these issues, and recommend the priority order for these campaigns.” The sections in this report document the results of the Panel’s work. The first two sections describe the concepts of the overarching themes, topical scientific questions, and campaigns. The next six sections (Sections 3–8) describe in detail the six scientific campaigns. Section 9 describes some important enabling research activities necessary for the campaigns. Sections 10–12 describe the overarching themes, which provide a crosscutting perspective of the activities in the six campaigns. Finally, the Panel’s recommendations are set forth in Section 13. The charge letter to the panel is provided as Appendix A; the FESAC response letter is provided as Appendix D.     

Particle Densities of the Atmospheric-Pressure Argon Plasmas Generated by the Pulsed Dielectric Barrier Discharges

Atmospheric-pressure argon plasmas have received increasing attention due to their high potential in many industrial and biomedical applications. In this paper, a 1-D fluid model is used for studying the particle density characteristics of the argon plasmas generated by the pulsed dielectric barrier discharges. The temporal evolutions of the axial particle density distributions are illustrated, and the influences of changing the main discharge conditions on the averaged particle densities are researched by independently varying the various discharge conditions. The calculation results show that the electron density and the ion density reach two peaks near the momentary cathodes during the rising and the falling edges of the pulsed voltage. Compared with the charged particle densities, the densities of the resonance state atom Ar~r and the metastable state atom Ar~mhave more uniform axial distributions, reach higher maximums and decay more slowly. During the platform of the pulsed voltage and the time interval between the pulses, the densities of the excited state atom Ar*are far lower than those of the Ar~r or the Ar~m. The averaged particle densities of the different considered particles increase with the increases of the amplitude and the frequency of the pulsed voltage. Narrowing the discharge gap and increasing the relative dielectric constant of the dielectric also contribute to the increase of the averaged particle densities. The effects of reducing the discharge gap distance on the neutral particle densities are more significant than the influences on the charged particle densities.     

Post collision stark-interaction after excitation of fast ions by a solid surface at grazing incidence

The anisotropic distribution of the angular momenta of atoms or ions excited by the ion beam surface interaction at grazing incidence is modified by post collision Stark-interaction due to the electric field at the surface. By studying the effect of the Stark-mixing on the anisotropy in dependence of the time the atom spent in the electric field conclusions can be drawn with respect to the nature of the field and the excitation process.