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Rf sheaths and impurity generation by ICRF (ion cyclotron range of frequencies) antennas

Description: In general, Faraday screen elements in an ICRF antenna are not aligned precisely along the combined toroidal and poloidal magnetic fields. When plasma of density n > 2epsilon/sub 0/V/eg/sup 2/ /approximately/ 10/sup 9/cm/sup -3/ is present in the gap between elements, electron response to the parallel electric field shorts out the electric field over most of the gap, leaving a narrow sheath of positive space charge and intense electric field. Here V denotes the voltage across the gap and g the gap spacing. This intense electric field accelerates ions up to an appreciable fraction of the gap voltage (/approximately/ 1 kV), sufficient to cause physical sputtering of the screen material. Impurities so generated constitute the principal limitation on power density (kW/cm/sup 2/) for ICRF antennas. ICRF antenna and Faraday screen design principles which minimize sputtering are discussed. 24 refs., 9 figs., 1 tab.
Date: November 1, 1988
Creator: Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Review of plasma heating by waves in the ion-cyclotron range of frequencies

Description: Three aspects of wave heating in the Ion-Cyclotron Range of Frequencies are reviewed. First, the many interesting physics phenomena are outlined which occur in this frequency regime and how they can be profitably utilized in tokamak and other heating experiments is indicated. Second, the key current theoretical problems are enumerated. Thirdly, how the ICRF program for the PLT tokamak can address most of the physics questions is shown.
Date: January 1, 1978
Creator: Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Issues in tokamak/stellarator transport and confinement enhancement mechanisms

Description: At present, the mechanism for anomalous energy transport in low-{beta} toroidal plasmas -- tokamaks and stellarators -- remains unclear, although transport by turbulent E {times} B velocities associated with nonlinear, fine-scale microinstabilities is a leading candidate. This article discusses basic theoretical concepts of various transport and confinement enhancement mechanisms as well as experimental ramifications which would enable one to distinguish among them and hence identify a dominant transport mechanism. While many of the predictions of fine-scale turbulence are born out by experiment, notable contradictions exist. Projections of ignition margin rest both on the scaling properties of the confinement mechanism and on the criteria for entering enhanced confinement regimes. At present, the greatest uncertainties lie with the basis for scaling confinement enhancement criteria. A series of questions, to be answered by new experimental/theoretical work, is posed to resolve these outstanding contradictions (or refute the fine-scale turbulence model) and to establish confinement enhancement criteria. 73 refs., 4 figs., 5 tabs.
Date: August 1, 1990
Creator: Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Confinement scaling and ignition in tokamaks

Description: A drift wave turbulence model is used to compute the scaling and magnitude of central electron temperature and confinement time of tokamak plasmas. The results are in accord with experiment. Application to ignition experiments shows that high density (1 to 2) . 10/sup 15/ cm/sup -3/, high field, B/sub T/ > 10 T, but low temperature T approx. 6 keV constitute the optimum path to ignition.
Date: October 1, 1985
Creator: Perkins, F.W. & Sun, Y.C.
Partner: UNT Libraries Government Documents Department

Murakami density limit in tokamaks and reversed-field pinches

Description: A theoretical upper limit for the density in an ohmically heated tokamak discharge follows from the requirement that the ohmic heating power deposited in the central current-carrying channel exceed the impurity radiative cooling in this critical region. A compact summary of our results gives this limit n/sub M/ for the central density as n/sub M/ = (Z/sub e//(Z/sub e/-1)/sup 1/2/n/sub eo/ (B/sub T//1T)(1m/R) where n/sub eo/ depends strongly on the impurity species and is remarkably independent of the central electron temperature T/sub e/(0). For T/sub e/(0) approx. 1 keV, we have n/sub eo/ = 1.5 x 10/sup 14/ cm/sup -3/ for beryllium, n/sub eo/ = 5 x 10/sup 13/ cm/sup -3/ for oxygen, n/sub eo/ = 1.0 x 10/sup 13/ cm/sup -3/ for iron, and n/sub eo/ = 0.5 x 10/sup 13/ cm/sup -3/ for tungsten. The results agree quantitatively with Murakami's original observations. A similar density limit, known as the I/N limit, exists for reversed-field pinch devices and this limit has also been evaluated for a variety of impurity species.
Date: March 1, 1984
Creator: Perkins, F.W. & Hulse, R.A.
Partner: UNT Libraries Government Documents Department

Heating tokamaks by parametric decay of intense extraordinary mode radiation

Description: Intense electron beam technology has developed coherent, very high power (350 megawatts) microwave sources at frequencies which are a modest fraction of the electron cyclotron frequency in tokamaks. Propagation into a plasma occurs via the extraordinary mode which is subject to parametric decay instabilities in the density range ..omega../sub o//sup 2/ < ..omega../sub pe//sup 2/ < ..omega../sub o/(..omega../sub o/ + ..cap omega../sub e/). For an incident wave focused onto a hot spot by a dish antenna of radius rho, the effective threshold power P/sub o/ required to induced effective parametric heating is P/sub o/ approx. = 10 MW x/rho ..cap omega../sub e//..omega../sub o/ (T/sub e//1 keV)/sup 3/2/ where x denotes the distance to the hot spot.
Date: August 1, 1979
Creator: Elder, G.B. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Resonant-cavity ICRF coupler for large tokamaks

Description: A new resonant-cavity ICRF coupler is proposed for large tokamaks. The design features a novel resonant cavity, an rf magnetic-field orientation that effectively radiates fast Alfven waves, matching to 40 ..cap omega.. transmission lines, and an electric-field orientation so that the strongest rf electric fields are orthogonal to the main toroidal magnetic field thereby benefitting from magnetic insulation. As a result, the power handling capability is excellent. For the case of the Big-Dee Doublet III tokamak, a single 35 cm x 50 cm coupler can launch 20 MW of fast Alfven waves. Extrapolation to fusion reactor parameters is straightforward.
Date: April 1, 1983
Creator: Perkins, F.W. & Kluge, R.F.
Partner: UNT Libraries Government Documents Department

Effects of impurity radiation on reversed-field pinch evolution

Description: The effects of impurity radiation on the evolution of a reversed-field pinch plasma are studied by means of a one-dimensional MHD simulation code that includes both plasma transport and impurity effects, and follows the plasma through a series of equilibrium states. The equations are split into two sets, one that contains plasma transport and another that contains atomic physics effects. Two codes were developed and linked together to solve the full problem. Results are presented for the ZT-S, ZT-40, and RFX experiments with a fixed set of parameters, typical or envisioned, and various concentrations of impurities. Radiation barriers are encountered, and limits are found on the acceptable level of low Z impurities that may be present in these devices. It is shown that the temperatures currently observed in the ZT-S experiment are radiation limited. Next, a criterion for radiation barrier burn-through is derived in terms of appropriate nondimensional parameters and calibrated by comparison to the numerical simulations for the case of oxygen.
Date: January 1, 1980
Creator: Caramana, E.J. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Scattering of lower-hybrid waves by density fluctuations

Description: The investigation of the scattering of lower-hybrid waves by density fluctuations in tokamaks is distinguished by the presence in the wave equation of a large, random, derivative-coupling term. Assuming the fluctuations to be of long wavelength compared to the incident wave the similarity of the wave equation to the Schroedinger equation for a particle in a random magnetic field is used to derive a two-way diffusion equation for the wave energy density. The diffusion constant found disagrees with earlier findings and the source of the discrepancy is pointed out. When the correct boundary conditions are imposed this equation can be solved by separation of variables. However most of the important features of the solution are apparent without detailed algebra.
Date: July 1, 1981
Creator: Andrews, P.L. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Nonambipolarity, orthogonal conductivity, poloidal flow, and torque

Description: Nonambipolar processes, such as neutral injection onto trapped orbits or ripple-diffusion loss of ..cap alpha..-particles, act to charge a plasma. A current j/sub r/ across magnetic surfaces must arise in the bulk plasma to maintain charge neutrality. An axisymmetric, neoclassical model of the bulk plasma shows that these currents are carried by the ions and exert a j/sub r/B/sub theta/R/c torque in the toroidal direction. A driven poloidal flow V/sub theta/ = E/sub r/'c/B must also develop. The average current density <j/sub r/> is related to the radial electric field E/sub r/' = E/sub r/ + v/sub /phi//B/sub theta//c in a frame moving with the plasma via the orthogonal conductivity <j/sub r/> = sigma/sub /perpendicular//E/sub r/', which has the value sigma/sub /perpendicular// = (1.65epsilon/sup 1/2/)(ne/sup 2/..nu../sub ii//M..cap omega../sub theta//sup 2/) in the banana regime. If an ignited plasma loses an appreciable fraction ..delta.. of its thermonuclear ..cap alpha..-particles by banana ripple diffusion, then the torque will spin the plasma to sonic rotation in a time /tau//sub s/ approx. 2/tau//sub E//..delta.., /tau//sub E/ being the energy confinement time. 10 refs., 1 fig.
Date: February 1, 1989
Creator: Hulbert, G.W. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Alfven resonance heating via magnetosonic modes in large tokamaks

Description: The theory of Alfven resonance heating of tokamaks is extended beyond the incompressible MHD model to include finite ..omega../..cap omega../sub i/ effects, which lead to off-diagonal terms in the conductivity tensor, and compressibility, which permits the fast Alfven mode. The finite frequency effects can greatly change the dissipation resulting from the shear Alfven resonance. With an appropriate choice of parameters, the dissipation can vanish allowing high-Q toroidal eigenmodes in large tokamaks such as PLT.
Date: August 1, 1978
Creator: Karney, C.F.F. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Current driven instabilities of the kinetic shear Alfven wave: application to reversed field pinches and spheromaks

Description: The kinetic Alfven wave is studied in a cylindrical force-free plasma with self-consistent magnetic fields. This equilibrium represents a reversed field pinch or a spheromak. The stability of the wave is found to depend on the ratio of the electron drift velocity to the Alfven velocity. This ratio varies inversely with the square root of the plasma line density. The critical line density using the Spitzer-Harm electron distribution function is found for reversed field pinches with deuterium plasmas to be approximately 2 x 10/sup 18/ m/sup -1/ and is 5 x 10/sup 17/ m/sup -1/ in spheromaks with hydrogen plasmas. The critical line density is in reasonable agreement with experimental data for reversed field pinches.
Date: April 1, 1984
Creator: Meyerhofer, D.D. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Double layers without current

Description: The steady-state solution of the nonlinear Vlasov-Poisson equations is reduced to a nonlinear eigenvalue problem for the case of double-layer (potential drop) boundary conditions. Solutions with no relative electron-ion drifts are found. The kinetic stability is discussed. Suggestions for creating these states in experiments and computer simulations are offered.
Date: November 1, 1980
Creator: Perkins, F.W. & Sun, Y.C.
Partner: UNT Libraries Government Documents Department

Parabolic approximation method for fast magnetosonic wave propagation in tokamaks

Description: Fast magnetosonic wave propagation in a cylindrical tokamak model is studied using a parabolic approximation method in which poloidal variations of the wave field are considered weak in comparison to the radial variations. Diffraction effects, which are ignored by ray tracing mthods, are included self-consistently using the parabolic method since continuous representations for the wave electromagnetic fields are computed directly. Numerical results are presented which illustrate the cylindrical convergence of the launched waves into a diffraction-limited focal spot on the cyclotron absorption layer near the magnetic axis for a wide range of plasma confinement parameters.
Date: July 1, 1985
Creator: Phillips, C.K.; Perkins, F.W. & Hwang, D.Q.
Partner: UNT Libraries Government Documents Department

Tokamak fusion reactor start-up simulation

Description: A simulation code TSEC (Time-dependent Spectral Equilibrium Code) has been developed to model the axisymmetric evolution of a tokamak on the resistive (L/R) time scale of the external coils, conductors, or shell. The electromagnetic interaction between the plasma and the external circuit is taken into account in a self-consistent manner. TSEC is Lagrangian and utilizes magnetic flux coordinates with spectral decomposition in the angle variable theta. The plasma is modeled as a finite-size, zero-inertia, finite-pressure fluid which adjusts its position and shape to remain in free-boundary equilibrium consistent with the currents in the external circuits. At the heart of TSEC is a fast method of calculating the self-consistent free-boundary plasma equilibrium at each time step which is based on the minimization of a certain mean-square error. 3 refs., 6 figs., 3 tabs.
Date: February 1, 1986
Creator: Ling, K.M.; Jardin, S.C. & Perkins, F.W.
Partner: UNT Libraries Government Documents Department

Generation of plasma rotation in a tokamak by ion-cyclotron absorption of fast Alfven waves

Description: Control of rotation in tokamak plasmas provides a method for suppressing fine-scale turbulent transport by velocity shear and for stabilizing large-scale magnetohydrodynamic instabilities via a close-fitting conducting shell. The experimental discovery of rotation in a plasma heated by the fast-wave minority ion cyclotron process is important both as a potential control method for a fusion reactor and as a fundamental issue, because rotation arises even though this heating process introduces negligible angular momentum. This paper proposes and evaluates a mechanism which resolves this apparent conflict. First, it is assumed that angular momentum transport in a tokamak is governed by a diffusion equation with a no-slip boundary condition at the plasma surface and with a torque-density source that is a function of radius. When the torque density source consists of two separated regions of positive and negative torque density, a non-zero central rotation velocity results, even when the total angular momentum input vanishes. Secondly, the authors show that localized ion-cyclotron heating can generate regions of positive and negative torque density and consequently central plasma rotation.
Date: June 13, 2000
Creator: Perkins, F.W.; White, R.B. & Bonoli, P.
Partner: UNT Libraries Government Documents Department

On Plasma Rotation Induced by Traveling Fast Alfvin Waves

Description: Absorption of fast Alfven waves by the minority fundamental ion-cyclotron resonance, coupled with finite banana width physics, generates torque distributions and ultimately rotational shear layers in the bulk plasma, even when the toroidal wavenumber k(subscript ''phi'') = n/R of the fast wave vanishes (n=0) and cyclotron absorption introduces no angular momentum nor canonical angular momentum [F.W. Perkins, R.B. White, P.T. Bonoli, and V.S. Chan, Phys. Plasmas 8 (2001) 2181]. The present work extends these results to travelling waves with non-zero n where heating directly introduces angular momentum. Since tokamak fast-wave antennas have approximately one wavelength per toroidal field coil, the toroidal mode number n lies in the range n = 10-20, independent of machine size. A zero-dimensional analysis shows that the rotation rate arising from direct torque is comparable to that of the rotational shear layer and has the same scaling. Nondimensional rotation profiles for n = (-10, 10) show modest changes from the n = 0 case in the expected direction. For a balanced antenna spectrum, the nondimensional rotational profile (averaged over n = -10, 10) lies quite close to the n = 0 profile.
Date: August 9, 2001
Creator: Perkins, F.W.; White, R.B. & Chan, and V.S.
Partner: UNT Libraries Government Documents Department

Generation of high intensity rf pulses in the ionosphere by means of in situ compression

Description: We demonstrate, using a simple model, that high intensity pulses can be generated from a frequency-chirped modifier of much lower intensity by making use of the dispersive properties of the ionosphere. We show that a frequency-chirped pulse can be constructed so that its various components overtake each other at a prescribed height, resulting in large (up to one hundred times) transient intensity enhancements as compared to those achievable from a steady modifier operating at the same power. We examine briefly one possible application: the enhancement of plasma wave amplitudes which occurs as a result of the interaction of such a compressed pulse with pre-generated turbulence.
Date: April 1, 1993
Creator: Cowley, S.C.; Perkins, F.W. & Valeo, E.J.
Partner: UNT Libraries Government Documents Department

Rotation and particle loss in Tore Supra

Description: Although plasma heating with ICRF imparts negligible angular momentum to a tokamak plasma, the high energy particles give significant torque to the plasma through diamagnetic effects. This effect has been directly modeled through guiding center simulations. It is found that heating in Tore Supra, with the location of the resonance surface on the high field side of the magnetic axis, can produce negative central rotation of up to 40 km/sec. Particle loss also contributes to negative rotation, but this is not the dominant effect in most discharges. In this work the authors examine the effect of collisions and strong plasma rotation on the loss of high energy particles. Magnetic field strength variation due to discrete toroidal field coils, or ripple, produces two important loss channels in tokamaks. The trapping of particles in local ripple wells produces super banana orbits and, in the case of strong ripple, direct loss orbits leading to the plasma edge. These particles leave the device in the direction of vertical drift, and are characterized by small values of parallel velocity, or pitch. Ripple also causes high energy particles in banana orbits to diffuse stochastically, leading to banana orbits which impact the wall near the outer midplane. Both these loss processes are modified by the magnitude of the collision rate, and by plasma rotation. In Tore Supra the magnitude of the ripple makes ripple trapping a dominant loss mechanism for the background plasma as well as for ICRF produced non Maxwellian high energy tails. The authors have examined the loss as a function of collisionality and rotation using the Hamiltonian guiding center code ORBIT.
Date: June 13, 2000
Creator: White, R.B.; Perkins, F.W.; Garbet, X.; Bourdelle, C. & al, et
Partner: UNT Libraries Government Documents Department