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Current generation by minority species heating

Description: It is proposed that electric currents be generated from the preferential heating of ions traveling in one direction but with no net momentum injected into the system. This can be accomplished with, for example, traveling waves in a two-ion-species plasma. The current can be generated efficiently enough for the scheme to be of interest in maintaining steady-state toroidal currents in a reactor.
Date: July 1, 1980
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Methods of driving current by heating a toroidal plasma

Description: In addition to the usual mechanism which utilizes the Ohmic transformer current, which is necessarily pulsed, there exist several steady-state mechanisms. Heating mechanisms which can lend themselves efficiently to continuous current generation include neutral beams, Alfven waves, ion-cyclotron waves, lower-hybrid waves and electron-cyclotron waves.
Date: August 1, 1980
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Operating tokamaks with steady-state toroidal current

Description: Continuous operation of a tokamak requires, among other things, a means of continuously providing the toroidal current. Various methods have been proposed to provide this current including methods which utilize radio-frequency waves in any of several frequency regimes. Here we elaborate on the prospects of incorporating these current-drive techniques in tokamak reactors, concentrating on the theoretical minimization of the power requirements.
Date: April 1, 1981
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Transport in driven plasmas

Description: A plasma in contact with an external source of power, especially a source that interacts specifically with high-velocity electrons, exhibits transport properties, such as conductivity, different from those of an isolated plasma near thermal equilibrium. This is true even when the bulk of the particles in the driven plasma are near thermal equilibrium. To describe the driven plasma we derive an adjoint equation to the inhomogeneous, linearized, dynamic Boltzmann equation. The Green's functions for a variety of plasma responses can then be generated. It is possible to modify the Chapman-Enskog expansion in order to incorporate the response functions derived here.
Date: March 1, 1985
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Inverse problem for incremental synchrotron radiation

Description: Significantly more information is available from synchrotron emission from a plasma when the plasma is purposefully disturbed. An inverse problem, to deduce properties of the disturbance given time-dependent radiation data, is proposed. The fast time response of radiation detectors is fully exploited by this approach. A special case of interest, perpendicular observation of a steady-state plasma, lends itself to an analytic inversion.
Date: June 1, 1987
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Current generation by phased injection of pellets

Description: By phasing the injection of frozen pellets into a tokamak plasma, it is possible to generate current. The current occurs when the electron flux to individual members of an array of pellets is asymmetric with respect to the magnetic field. The utility of this method for tokamak reactors, however, is unclear; the current, even though free in a pellet-fueled reactor, may not be large enough to be worth the trouble. Uncertainty as to the utility of this method is, in part, due to uncertainty as to proper modeling of the one-pellet problem.
Date: August 1, 1983
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Theory of current-drive in plasmas

Description: The continuous operation of a tokamak fusion reactor requires, among other things, a means of providing continuous toroidal current. Such operation is preferred to the conventional pulsed operation, where the plasma current is induced by a time-varying magnetic field. A variety of methods has been proposed to provide continuous current, including methods which utilize particle beams or radio frequency waves in any of several frequency regimes. Currents as large as half a mega-amp have now been produced in the laboratory by such means, and experimentation in these techniques has now involved major tokamak facilities worldwide.
Date: December 1, 1986
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Current generation in a relativistic plasma

Description: The relativistic dynamics of fast current carriers are rich in intriguing phenomena not appearing in the Newtonian limit. It is shown that because of these dynamics there is a bound to the efficiency of driving current in a tokamak either by waves or other means. Analytic techniques uncover the current-drive scheme which yields the maximum attainable efficiency.
Date: January 1, 1981
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Conductivity of rf-heated plasma

Description: The electron velocity distribution of rf-heated plasma may be so far from Maxwellian that Spitzer conductivity no longer holds. A new conductivity for such plasmas is derived and the result can be put in a remarkably general form. The new expression should be of great practical value in examining schemes for current ramp-up in tokamaks by means of lower-hybrid or other waves.
Date: May 1, 1984
Creator: Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Asymptotic analysis of rf-heated collisional plasma

Description: It is shown that a distribution of electrons in resonance with traveling waves, but colliding with background distributions of electrons and ions, evolves to a steady state. Details of the steady state are given analytically in the asymptotic limit of high electron energy and are compared with numerical solutions. The asymptotic analytic solution may be useful for quickly relating emission data to likely excitations and is more reliable than conventional numerical solutions at high energy. A method of improving numerics at high energy is suggested.
Date: March 1, 1985
Creator: Fisch, N.J. & Karney, C.F.F.
Partner: UNT Libraries Government Documents Department

Current in wave driven plasmas

Description: A theory for the generation of current in a toroidal plasma by radio-frequency waves is presented. The effect of an opposing electric field is included, allowing the case of time varying currents to be studied. The key quantities that characterize this regime are identified and numerically calculated. Circuit equations suitable for use in ray-tracing and transport codes are given.
Date: June 1, 1985
Creator: Karney, C.F.F. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Creating an anisotropic plasma resistivity with waves

Description: An anisotropic plasma resistivity may be created by preferential heating of electrons traveling in one direction. This can result in a steady-state toroidal current in a tokamak even in the absence of net wave momentum. In fact, at high wave phase velocities, the current associated with the change in resistivity is greater than that associated with net momentum input. An immediate implication is that other waves, such as electron cyclotron waves, may be competitive with lower-hybrid waves as a means for generating current. An analytical expression is derived for the current generated per power dissipated which agrees remarkably well with numerical calculations.
Date: May 1, 1980
Creator: Fisch, N.J. & Boozer, A.H.
Partner: UNT Libraries Government Documents Department

Current generation with low-frequency waves

Description: Various types of traveling waves may be injected into a tokamak to continuously sustain the toroidal current. Interest in this problem arises from the possibility of operating tokamak reactors in the steady state. The low-frequency waves most suitable for this task are identified in terms of the power cost for deployment in a reactor. Means of exciting these waves and tradeoffs with design criteria are discussed. A comparison is made with the alternative attractive regime of high-frequency waves. Conclusions are based, in part, on the numerical solution of the two-dimensional Fokker-Planck equation with an added quasilinear term due to the waves.
Date: December 1, 1979
Creator: Fisch, N.J. & Karney, C.F.F.
Partner: UNT Libraries Government Documents Department

Separating variables in two-way diffusion equations

Description: It is shown that solutions to a class of diffusion equations of the two-way type may be found by a method akin to separation of variables. The difficulty with such equations is that the boundary data must be specified partly as initial and partly as final conditions. In contrast to the one-way diffusion equation, where the solution separates only into decaying eigenfunctions, the two-way equations separate into both decaying and growing eigenfunctions. Criteria are set forth for the existence of linear eigenfunctions, which may not be found directly by separating variables. A speculation with interesting ramifications is that the growing and decaying eigenfunctions are separately complete in an appropriate half of the problem domain.
Date: October 1, 1979
Creator: Fisch, N.J. & Kruskal, M.D.
Partner: UNT Libraries Government Documents Department

Current ramp-up with rf waves in a tokamak

Description: The circuit equations for current-drive in a start-up or ramp-up plasma are derived by finding appropriate response functions in the presence of an electric field. The effect of arbitrary wave-induced fluxes on runaway production and current generation can then be determined. An interpretation of the rather remarkable PLT ramp-up efficiencies, difficult to explain using the steady-state efficiency, is now possible. A parameter regime, available also on reactor-grade devices, is identified wherein quick ramp-up by lower-hybrid waves may be efficient.
Date: August 1, 1984
Creator: Fisch, N.J. & Karney, C.F.F.
Partner: UNT Libraries Government Documents Department

Information content of transient synchrotron radiation in tokamak plasmas

Description: A brief, deliberate, perturbation of hot tokamak electrons produces a transient, synchrotron radiation signal, in frequency-time space, with impressive informative potential on plasma parameters; for example, the dc toroidal electric field, not available by other means, may be measurably. Very fast algorithms have been developed, making tractable a statistical analysis that compares essentially all parameter sets that might possibly explain the transient signal. By simulating data numerically, we can estimate the informative worth of data prior to obtaining it. 20 refs., 2 figs.
Date: April 1, 1989
Creator: Fisch, N.J. & Kritz, A.H.
Partner: UNT Libraries Government Documents Department

Currents driven by electron cyclotron waves

Description: Certain aspects of the generation of steady-state currents by electron cyclotron waves are explored. A numerical solution of the Fokker-Planck equation is used to verify the theory of Fisch and Boozer and to extend their results into the nonlinear regime. Relativistic effects on the current generated are discussed. Applications to steady-state tokamak reactors are considered.
Date: July 1, 1981
Creator: Karney, C.F.F. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Efficiency of current drive by fast waves

Description: The Rosenbluth form for the collision operator for a weakly relativistic plasma is derived. The formalism adopted by Antonsen and Chu can then be used to calculate the efficiency of current drive by fast waves in a relativistic plasma. Accurate numerical results and analytic asymptotic limits for the efficiencies are given.
Date: August 1, 1984
Creator: Karney, C.F.F. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Numerical studies of current generation by radio-frequency traveling waves

Description: By injecting radio-frequency traveling waves into a tokamak, continuous toroidal electron currents may be generated. This process is studied by numerically solving the two-dimensional Fokker-Planck equation with an added quasilinear term. The results are compared with the one-dimensional analytic treatment of Fisch, which predicted a reduced plasma resistivity when high-phase-velocity waves are employed. It is shown here that two-dimensional velocity space effects, while retaining the predicted scaling, further reduce the ratio of power dissipated to current generated by about 40%. These effects enhance the attractiveness of steady-state tokamak reactors utilizing this method of current generation.
Date: January 1, 1979
Creator: Karney, C.F.F. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Sensitivity of transient synchrotron radiation to tokamak plasma parameters

Description: Synchrotron radiation from a hot plasma can inform on certain plasma parameters. The dependence on plasma parameters is particularly sensitive for the transient radiation response to a brief, deliberate, perturbation of hot plasma electrons. We investigate how such a radiation response can be used to diagnose a variety of plasma parameters in a tokamak. 18 refs., 13 figs.
Date: December 1, 1988
Creator: Fisch, N.J. & Kritz, A.H.
Partner: UNT Libraries Government Documents Department

Alpha Channeling in Mirror Machines

Description: Because of their engineering simplicity, high-β, and steady-state operation, mirror machines and related open-trap machines such as gas dynamic traps, are an attractive concept for achieving controlled nuclear fusion. In these open-trap machines, the confinement occurs by means of magnetic mirroring, without the magnetic field lines closing upon themselves within the region of particle confinement. Unfortunately, these concepts have not achieved to date very spectacular laboratory results, and their reactor prospects are dimmed by the prospect of a low Q-factor, the ratio of fusion power produced to auxiliary power. Nonetheless, because of its engineering promise, over the years numerous improvements have been proposed to enhance the reactor prospects of mirror fusion, such as tandem designs, end-plugging, and electric potential barriers.
Date: October 19, 2005
Creator: Fisch, N. J.
Partner: UNT Libraries Government Documents Department

Alpha power channeling with two waves

Description: The complete channeling of energy from alpha particles is likely to be realized only through the excitation of a variety of waves, rather than by one wave alone. While one wave constrains more firmly the direction of the energy transfer, the necessary wave characteristics are far more easily achieved through a combination of waves, even at the expense of less restrictive motion of the {alpha}-particles.
Date: September 1, 1995
Creator: Fisch, N.J. & Herrmann, M.C.
Partner: UNT Libraries Government Documents Department

Plasma discreteness effects in the presence of an intense, ultrashort laser pulse

Description: Discrete effects of the plasma irradiated by an ultrashort, intense laser pulse are investigated. Although, for most plasmas of interest, the damping of the laser pulse is due to collective plasma effects, in certain regimes the energy absorbed in the plasma microfields can be important. A scattering matrix is derived for an electron scattering off an ion in the presence of an intense laser field.
Date: March 1, 1996
Creator: Savchenko, V.I. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department