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Reynolds stress of localized toroidal modes

Description: An investigation of the 2D toroidal eigenmode problem reveals the possibility of a new consistent 2D structure, the dissipative BM-II mode. In contrast to the conventional ballooning mode, the new mode is poloidally localized at {pi}/2 (or -{pi}/2), and possesses significant radial asymmetry. The radial asymmetry, in turn, allows the dissipative BM-II to generate considerably larger Reynolds stress as compared to the standard slab drift type modes. It is also shown that a wide class of localized dissipative toroidal modes are likely to be of the dissipative BM-II nature, suggesting that at the tokamak edge, the fluctuation generated Reynolds stress (a possible source of poloidal flow) can be significant.
Date: February 1, 1995
Creator: Zhang, Y.Z. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Escaping radio emission from pulsars: Possible role of velocity shear

Description: It is demonstrated that the velocity shear, intrinsic to the e{sup +}e{sup {minus}} plasma present in the pulsar magnetosphere, can efficiently convert the nonescaping longitudinal Langmuir waves (produced by some kind of a beam or stream instability) into propagating (escaping) electromagnetic waves. It is suggested that this shear induced transformation may be the basic mechanism needed for the eventual generation of the observed pulsar radio emission.
Date: January 1, 1997
Creator: Mahajan, S.M.; Machabeli, G.Z. & Rogava, A.D.
Partner: UNT Libraries Government Documents Department

A photon accelerator -- Large blueshifting of femtosecond pulses in semiconductors

Description: The availability of relatively high intensity (I > 10{sup 9}Wcm{sup {minus}2}) [but moderate ({approximately} nJ) total energy], femtosecond laser pulses with wavelengths ranging from the ultraviolet to the mid-infrared has opened the doors for a serious investigation of the nonlinear optical properties of matter on ultrashort time scales in a new parameter regime. Even small intensity-dependent nonlinearities can begin to play a major role in the overall electrodynamics, and in determining the fate of the propagating pulse. It is shown that a femtosecond pulse propagating near a two-photon transition in a semiconductor waveguide can undergo a large blueshift.
Date: April 1, 1997
Creator: Berezhiani, V.I.; Mahajan, S.M. & Murusidze, I.G.
Partner: UNT Libraries Government Documents Department

Laser wakefield excitation and measurement by femtosecond longitudinal interferometry

Description: Plasma density oscillations (Langmuir waves) in the wake of an intense (I{sub peak} {approximately} 3 {times} 10{sup 17}W/cm{sup 2}) laser pulse (100 fs) are measured with ultrafast time resolution using a longitudinal interferometric technique. Phase shifts consistent with large amplitude ({delta}n{sub e}/n{sub e} {approximately} 1) density waves at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of {approximately} 10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. As this technique utilizes a necessary component of any laser-based plasma accelerator, it promises to be a powerful tool for on-line monitoring and control of future plasma-based particle accelerators.
Date: April 1, 1996
Creator: Siders, C.W.; Le Blanc, S.P.; Fisher, D.; Tajima, T.; Downer, M.C.; Babine, A. et al.
Partner: UNT Libraries Government Documents Department

Ion acceleration and direct ion heating in three-component magnetic reconnection

Description: Ion acceleration and direct ion heating in magnetic reconnection are experimentally observed during counterhelicity merging of two plasma toroids. Plasma ions are accelerated up to order of the Alfen speed through contraction of the reconnected field-lines with three-components. The large increase in ion thermal energy (from 10 eV up to 200 eV) is attributed to the direct conversion of the magnetic energy into the unmagnetized ion population. This observation is consistent with the magnetohydrodynamic and macro-particle simulations.
Date: March 1, 1996
Creator: Ono, Y.; Yamada, M. & Akao, T.
Partner: UNT Libraries Government Documents Department

Spontaneous hole-clump pair creation in weakly unstable plasmas

Description: A numerical simulation of a kinetic instability near threshold shows how a hole and clump spontaneously appear in the particle distribution function. The hole and clump support a pair of Bernstein, Greene, Kruskal (BGK) nonlinear waves that last much longer than the inverse linear damping rate while they are upshifting and downshifting in frequency. The frequency shifting allows a balance between the power nonlinearly extracted from the resonant particles and the power dissipated into the background plasma. These waves eventually decay due to phase space gradient smoothing caused by collisionality.
Date: March 1, 1997
Creator: Berk, H.L.; Breizman, B.N. & Petviashvili, N.V.
Partner: UNT Libraries Government Documents Department

Analysis of the bi-modal nature of solar wind-magnetosphere coupling

Description: It has been shown that the optimal linear prediction filter relating the solar wind electric field and the geomagnetic activity, as measured by the AL index, is both bi-modal and dependent on the level of activity in the magnetosphere. Further studies truncated the prediction filter to a five parameter model containing two low-pass filtered delta functions of arbitrary amplitude and delay time. The present study elaborates on the nature of the bi-modal response by using the five parameter model to quantify the effects of the level of geomagnetic activity on each of the modes of the filter individually. The authors find that at all levels of activity, the second mode, occurring at approximately one hour, is relatively unchanged. The first mode, however, has a one parameter dependence on the level of activity in the magnetosphere. The amplitude of the first mode is shown to have a significant increase with respect to activity.
Date: May 1, 1997
Creator: Smith, J. P. & Horton, W.
Partner: UNT Libraries Government Documents Department

Plasma transport near the separatrix of a magnetic island

Description: The simplest non-trivial model of transport across a magnetic island chain in the presence of collisionless streaming along the magnetic field is solved by a Wiener-Hopf procedure. The solution found is valid provided the boundary layers about the island separatrix is narrow compared to the island width. The result demonstrates that when this assumption is satisfied the flattened profile region is reduced by the boundary layer width. The calculation is similar to the recent work by Fitzpatrick but is carried out in the collisionless, rather than the collisional, limit of parallel transport, and determines the plasma parameters on the separatrix self-consistently.
Date: December 1, 1996
Creator: Hazeltine, R.D.; Helander, P. & Catto, P.J.
Partner: UNT Libraries Government Documents Department

Stabilization of the resistive wall mode using a fake rotating shell

Description: Tokamak plasma performance can, in theory, be greatly improved if the so called resistive wall mode is stabilized. This can be achieved by spinning the plasma rapidly, but such a scheme is not reactor relevant. A more promising approach is to apply external feedback in order to make a resistive shell placed around the plasma act like a perfect conductor. A scheme is outlined by which a network of feedback controlled conductors surrounding the plasma can be made to act like a rotating shell. This fake rotating shell combined with a stationary conventional shell (e.g. the vacuum vessel) can completely stabilize the resistive wall mode. The gain, bandwidth, current, and power requirements of the feedback amplifiers are extremely modest. A previously proposed stabilization scheme (the intelligent shell) is also investigated, and is compared with the fake rotating shell concept. The main disadvantage of the former scheme is that it requires a high gain.
Date: November 1, 1995
Creator: Fitzpatrick, R. & Jensen, T.H.
Partner: UNT Libraries Government Documents Department

Nonlinear m = 1 mode and fast reconnection in collisional plasmas

Description: Time evolution of the m = 1 resistive kink mode is shown to be compromised of two exponential growth phases separated by a transition period during which the growth becomes temporarily algebraic. A modified Sweet-Parker model that takes into account some of the changes in the geometry of the core plasma and the growing island is offered to explain the departure from the algebraic growth of the early nonlinear phase.
Date: February 1, 1997
Creator: Aydemir, A.Y.
Partner: UNT Libraries Government Documents Department

Plasma analog of particle-pair production

Description: It is shown that the plasma axial shear flow instability satisfies the Klein-Gordon equation. The plasma instability is then shown to be analogous to spontaneous particle-pair production when a potential energy is present that is greater than twice the particle rest mass energy. Stability criteria can be inferred based on field theoretical conservation laws.
Date: September 1, 1996
Creator: Tsidulko, Yu.A. & Berk, H.L.
Partner: UNT Libraries Government Documents Department

Space charge tracking code for a synchrotron accelerator

Description: An algorithm has been developed to compute particle tracking, including self-consistent space charge effects for synchrotron accelerators. In low-energy synchrotrons space charge plays a central role in enhancing emittance of the beam. The space charge effects are modeled by mutually interacting (through the Coulombic force) N cylindrical particles (2-{1/2}-dimensional dynamics) whose axis is in the direction of the equilibrium particle flow. On the other hand, their interaction with synchrotron lattice magnets is treated with the thin-lens approximation and in a fully 3-dimensional way. Since the existing method to treat space charge fully self-consistently involved 3-D space charge effect computation, the present method allows far more realistic physical parameters and runs in far shorter time (about 1/20). Some examples on space charge induced instabilities are presented.
Date: June 1, 1997
Creator: Ottinger, M.B.; Tajima, T. & Hiramoto, K.
Partner: UNT Libraries Government Documents Department

{open_quotes}Heavy light bullets{close_quotes} in electron-positron plasma

Description: The nonlinear propagation of circularly polarized electromagnetic waves with relativistically strong amplitudes in an unmagnetized hot electron-positron plasma with a small fraction of ions is investigated. The possibility of finding localized solutions in such a plasma is explored. It is shown that these plasmas support the propagation of {open_quotes}heavy light bullets{close_quotes}; nondiffracting and nondispersive electromagnetic (EM) pulses with large density bunching.
Date: March 1, 1995
Creator: Berezhiani, V.I. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Singular eigenfunctions for shearing fluids I

Description: The authors construct singular eigenfunctions corresponding to the continuous spectrum of eigenvalues for shear flow in a channel. These modes are irregular as a result of a singularity in the eigenvalue problem at the critical layer of each mode. They consider flows with monotonic shear, so there is only a single critical layer for each mode. They then solve the initial-value problem to establish that these continuum modes, together with any discrete, growing/decaying pairs of modes, comprise a complete basis. They also view the problem within the framework of Hamiltonian theory. In that context, the singular solutions can be viewed as the kernel of an integral, canonical transformation that allows us to write the fluid system, an infinite-dimensional Hamiltonian system, in action-angle form. This yields an expression for the energy in terms of the continuum modes and provides a means for attaching a characteristic signature (sign) to the energy associate with each eigenfunction. They follow on to consider shear-flow stability within the Hamiltonian framework. Next, the authors show the equivalence of integral superpositions of the singular eigenfunctions with the solution derived with Laplace transform techniques. In the long-time limit, such superpositions have decaying integral averages across the channel, revealing phase mixing or continuum damping. Under some conditions, this decay is exponential and is then the fluid analogue of Landau damping. Finally, the authors discuss the energetics of continuum damping.
Date: February 1, 1995
Creator: Balmforth, N.J. & Morrison, P.J.
Partner: UNT Libraries Government Documents Department

Studies of instability and transport in tokamak plasmas with very weak magnetic shear

Description: Ion temperature gradient (ITG or {eta}{sub i}) driven microinstabilities are studied, using kinetic theory, for tokamak plasmas with very weak (positive or negative) magnetic shear (VWS). The gradient of magnetic shear as well as the effects of parallel and perpendicular velocity shear (v{prime}{sub {parallel}} and v{prime}{sub E}) are included in the defining equations. Two eigenmodes: the double (D) and the global (G) are found to coexist. Parametric dependence of these instabilities, and of the corresponding quasilinear transport is systematically analyzed. It is shown that, in VWS plasmas, a parallel velocity shear (PVS) may stabilize or destabilize the modes, depending on the individual as well as the relative signs of PVS and of the gradient of magnetic shear. The quasilinear transport induced by the instabilities may be significantly reduced with PVS in VWS plasmas. The v{prime}{sub E} values required to completely suppress the instabilities are much lower in VWS plasmas than they are in normal plasmas. Possible correlations with tokamak experiments are discussed.
Date: April 1, 1997
Creator: Dong, J.Q.; Zhang, Y.Z. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Numerical simulation of bump-on-tail instability with source and sink

Description: This paper presents results of the simulations of the bump-on-tail instability with a weak source and sink. This problem has been posed as a paradigm for the important problem in controlled fusion, that of the unstable excitation of Alfven waves in a tokamak by resonant energetic alpha particles. The source of alpha particles is the controlled fusion reaction produced by the background plasma and the sink is the collisional transport processes that slow down or scatter the energetic particles. The mathematical techniques that are needed to address this applied problem can be demonstrated in the much simpler bump-on-tail problem, which is explained in this paper.
Date: February 1, 1995
Creator: Berk, H.L.; Pekker, M. & Breizman, B.N.
Partner: UNT Libraries Government Documents Department

Microbunching and coherent acceleration of electrons by subcycle laser pulses

Description: The pick up and acceleration of all plasma electrons irradiated by an intense, subcyclic laser pulse is demonstrated via analytical and numerical calculations. It is shown that the initial low emittance of the plasma electrons is conserved during the process of acceleration, leading to an extremely cold, bunched electron beam. Compression of the electron bunch along the longitudinal coordinate is naturally achieved due to the interaction of electrons and laser pulse. In this paper, the authors find the localized solutions to Maxwell`s equations of a subcyclic laser pulse and use these to determine the acceleration of charged particles and they suggest future application for this acceleration mechanism as low energy particle injector and as electron source for coherent x-ray generation.
Date: May 1, 1997
Creator: Rau, B.; Tajima, T. & Hojo, H.
Partner: UNT Libraries Government Documents Department

Stabilization of external kink modes in magnetic fusion experiments using a thin conducting shell

Description: In nearly all magnetic fusion devices the plasma is surrounded by a conducting shell of some description. In most cases this is the vacuum vessel. What effect does a conducting shell have on the stability of external kink modes? Is there any major difference between the effect of a perfectly conducting shell and a shell of finite conductivity? What happens if the shell is incomplete? These, and other, questions are explored in detail in this lecture using simple resistive magnetohydrodynamic (resistive MHD) arguments. Although the lecture concentrates on one particular type of magnetic fusion device, namely, the tokamak, the analysis is fairly general and could also be used to examine the effect of conducting shells on other types of device (e.g. Reversed Field Pinches, Stellerators, etc.).
Date: November 1, 1995
Creator: Fitzpatrick, R.
Partner: UNT Libraries Government Documents Department

Bootstrap current close to magnetic axis in tokamaks

Description: It is shown that the bootstrap current density close to the magnetic axis in tokamaks does not vanish in simple electron-ion plasmas because the fraction of the trapped particles is finite. The magnitude of the current density could be comparable to that in the outer core region. This may reduce or even eliminate the need of the seed current.
Date: December 1, 1996
Creator: Shaing, K.C. & Hazeltine, R.D.
Partner: UNT Libraries Government Documents Department

Orbit width scaling of TAE instability growth rate

Description: The growth rate of Toroidal Alfven Eigenmodes (TAE) driven unstable by resonant coupling of energetic charged particles is evaluated in the ballooning limit over a wide range of parameters. All damping effects are ignored. Variations in orbit width, aspect ratio, and the ratio of alfven velocity to energetic particle birth velocity, are explored. The relative contribution of passing and trapped particles, and finite Larmor radius effects, are also examined. The phase space location of resonant particles with interact strongly with the modes is described. The accuracy of the analytic results with respect to growth rate magnitude and parametric dependence is investigated by comparison with numerical results.
Date: July 1, 1995
Creator: Wong, H.V.; Berk, H.L. & Breizman, B.N.
Partner: UNT Libraries Government Documents Department

Collisionless damping of perpendicular magnetosonic waves in a two-ion-species plasma

Description: Propagation of finite-amplitude magnetosonic waves in a collisionless plasma containing two ion species is studied with a one-dimensional, fully electromagnetic code based on a three-fluid model. It is found that perpendicular magnetosonic waves are damped in a two-ion-species plasma; a magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front, which results in the excitation of a longer wavelength perturbation behind the pulse. The damping due to the energy transfer from the original pulse to the longer wavelength perturbation occurs even if the plasma is collisionless and the pulse amplitude is small. The theoretically obtained damping rate is in agreement with the simulation result.
Date: August 1, 1996
Creator: Dogen, Daiju; Toida, Mieko & Ohsawa, Yukiharu
Partner: UNT Libraries Government Documents Department

Normal modes and continuous spectra

Description: The authors consider stability problems arising in fluids, plasmas and stellar systems that contain singularities resulting from wave-mean flow or wave-particle resonances. Such resonances lead to singularities in the differential equations determining the normal modes at the so-called critical points or layers. The locations of the singularities are determined by the eigenvalue of the problem, and as a result, the spectrum of eigenvalues forms a continuum. They outline a method to construct the singular eigenfunctions comprising the continuum for a variety of problems.
Date: December 1, 1994
Creator: Balmforth, N.J. & Morrison, P.J.
Partner: UNT Libraries Government Documents Department

Experimental plasma astrophysics using a T{sup 3} (Table-top Terawatt) laser

Description: Lasers that can deliver immense power of Terawatt (10{sup 12}W) and can still compactly sit on a Table-Top (T{sup 3} lasers) emerged in the 1990s. The advent of these lasers allows us to access to regimes of astronomical physical conditions that once thought impossible to realize in a terrestrial laboratory. We touch on examples that include superhigh pressure materials that may resemble the interior of giant planets and white dwarfs and of relativistic temperature plasmas that may exist in the early cosmological epoch and in the neighborhood of the blackhole event horizon.
Date: November 1, 1996
Creator: Tajima, T.
Partner: UNT Libraries Government Documents Department