摘要:

Ion Bernstein wave heating (IBWH) utilizes the ion Bernstein wave (IBW), a hot plasma wave, to carry the radio frequency (rf) power to heat the tokamak reactor core. Earlier wave accessibility studies have shown that this finite‐Larmor‐radius (FLR) mode should penetrate into a hot dense reactor plasma core without significant attenuation. Moreover, the IBW’s low perpendicular phase velocity (&ohgr;/k⊥≊VTi≪V&agr;) greatly reduces the otherwise serious wave absorption by the 3.5 MeV fusion &agr; particles. In addition, the property of IBW’s thatk⊥&rgr;i≊1 makes localized bulk ion heating possible at the ion cyclotron harmonic layers. Such bulk ion heating can prove useful in optimizing fusion reactivity.In another vein, with proper selection of parameters, IBW’s can be made subject to strong localized electron Landau damping near the major ion cyclotron harmonic resonance layers. This property can be useful, for example, for rf current drive in the reactor plasma core. IBW’s can be excited with loop antennas or with a lower‐hybrid‐like waveguide launcher at the plasma edge, the latter structure being one that is especially compatible with reactor application. In either case, the mode at the plasma edge is an electron plasma wave (EPW). Deeper in the plasma, the EPW is mode transformed into an IBW. Such launching and mode transformation of IBW’s were first demonstrated in experiments in the Advanced Concepts Torus‐1 (ACT‐1) [Phys. Rev. Lett.45, 1105 (1980)] plasma torus and in particle simulation calculations. These and other aspects of IBW heating physics have been investigated through a number of experiments performed on ACT‐1, the Japanese Institute of Plasma Physics Tokamak II‐Upgrade (JIPPTII‐U) [Phys. Rev. Lett.54, 2339 (1985)], the Tokyo University Non‐Circular Tokamak (TNT) [Nucl. Fusion26, 1097 (1986)], the Princeton Large Tokamak (PLT) [Phys. Rev. Lett.60, 294 (1988)], and Alcator‐C [Phys. Rev. Lett.60, 298 (1988)]. In these experiments both linear and nonlinear heating processes have been observed. Interestingly, improvement of plasma confinement was also observed in the PLT and Alcator‐C experiments, opening up the possible use of IBW’s for the active control of plasma transport.Two theoretical explanations have been proposed: one based on four‐wave mixing of IBW with low‐frequency turbulence, the other on the nonlinear generation of a velocity‐shear layer. Both models are consistent with the observed threshold power level of a few hundred kW in the experiments. Experiments on lower field plasmas on JFTII‐M [EighthTopicalConferenceonRadio‐FrequencyPowerinPlasmas, Irvine, CA, 1989 (American Institute of Physics, New York, 1989), p. 350] and DIII‐D [EighthTopicalConferenceonRadio‐FrequencyPowerinPlasmas, Irvine, CA, 1989 (American Institute of Physics, New York, 1989), p. 314] have raised some concern with the IBW wave‐launching process. The experiments showed serious impurity release from the walls but little or no core heating, a combination of circumstances strongly suggestive of edge heating. Possible parasitic channels could include the excitation of short wavelength modes by the Faraday shield’s fringing fields, antenna‐sheath‐wave excitation, an axial‐convective loss channel, and nonlinear processes such as parametric instability and ponderomotive effects.Suggested remedies include changes in the antenna phasing, the use of low‐Zinsulators, operating at higher frequencies, positioning the plasma differently with respect to the antenna, eliminating the Faraday shields, and using a waveguide launcher. The recent JIPPTII‐U experiment, employing a 0‐&pgr; phased antenna array with a higher frequency 130 MHz source, demonstrated that those remedies can indeed work. Looking to the future, one seeks additional ways in which IBWH can improve tokamak performance. The strong ponderomotive potential of the IBWH antenna may be used to stabilize external kinks and, acting as an rf limiter, to control the plasma edge. Control of the plasma pressure profile with local IBWH heating is already an important part of the Princeton Beta Experiment‐Modified (PBX‐M) [NinthTopicalConferenceonRadio‐FrequencyPowerinPlasmas, Charleston, SC, 1991 (American Institute of Physics, New York, 1991), p. 129] program in its exploration of the second‐stability regime. Application of IBWH may also improve the performance of neutral beam heating and the efficiency and localization of lower‐hybrid current drive for current profile control. Used with pellet injection, IBWH may also prolong the period of good confinement.The three planned high‐power IBWH experiments covering vastly different parameters:f=40–80 MHz for PBX‐M;f=130 MHz for JIPPT‐II‐U; andf=430 MHz for the Frascati Tokamak‐Upgrade (FT‐U) [16thEuropeanPhysicalSocietyConferenceonControlledFusionandPlasmaPhysics, Venice, Italy, 1989 (European Physical Society, Amsterdam, 1989), Vol. III, p. 1069] appear to be well positioned to explore these possibilities and to clarify other issues including the physics of wave launching and associated nonlinear processes.

ISSN:0899-8221    

DOI:10.1063/1.860569

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This paper establishes some general properties of a two‐dimensional (z‐invariant) model describing smooth equilibria of a magnetized quasineutral plasma enclosed in a finite cylindrical box of arbitrary cross section &OHgr; and submitted to the following constraints: (i) the normal component of the magnetic field is fixed on the box boundary; (ii) the plasma is in contact with a heatbath at temperatureT; (iii) the plasma carries in thezdirection a current whose total intensityIis externally imposed. It is shown in particular that: (i) the free energy of the system is bounded from below if and only if the dimensionless control parameter &lgr;≤1 (&lgr;∝IT−1/2); (ii) a minimum free energy state (thus thermodynamically absolutely stable) exists for &lgr;≤1 and is actually the unique equilibrium that can be achieved if &lgr; is small enough; (iii) this state is absolutely stable with respect to all ideal magnetohydrodynamic perturbations; (iv) no equilibrium can exist if &lgr; exceeds a critical value &lgr;c≥1 (at least in the case where &OHgr; is ‘‘star shaped’’). The nonexistence of a minimum free energy state and even of any available regular equilibria with high current may have some consequences for understanding eruptive processes in plasmas, which are briefly discussed.

ISSN:0899-8221    

DOI:10.1063/1.860570

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

When a single‐species plasma is confined in a harmonic Penning trap at cryogenic temperature, the thermal equilibrium is approximately a uniform density spheroid (ellipsoid of revolution). Normal modes corresponding to quadrupole excitations of this plasma have recently been measured. In this paper, nonlinear equations of motion are derived for these quadrupole oscillations. For large amplitudes, the oscillations deform a spheroidal plasma into a triaxial ellipsoid with time‐dependent shape and orientation. The integrals of the motion are found and the cylindrically symmetric finite‐amplitude oscillations of a spheroid are studied. An analysis of all possible ellipsoidal equilibria is also carried out. New equilibria are discovered which correspond to finite‐amplitude versions of the noncylindrically symmetric linear quadrupole oscillations. The equilibria are shown to fall into two classes in which the ellipsoids are either tilted or aligned with respect to the magnetic field. Some of these equilibria have densities well above the Brillouin limit.

ISSN:0899-8221    

DOI:10.1063/1.860571

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Time‐varying plasma currents associated with low‐frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary‐free interior of a large laboratory plasma by means of insulated magnetic antennas. The time‐varying magnetic field is measured in three dimensions and the current density is calculated from R∇×B(r,t)=&mgr;0J, whereJincludes the displacement current density. Typical fieldsB(r,t) andJ(r,t) induced by a magnetic loop antenna show three‐dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed alongB0is higher than oblique toB0. The wave vector is highly oblique toB0while the energy flow is mainly alongB0. The electric field in the wave packet contains both inductive and space‐charge contributions, the latter arising from the different dynamics of electrons and ions as explained by physical arguments. The dominant electric field in a whistler packet is a radial space‐charge field. Neither the field topology nor the propagation characteristics are sensitive to the induced magnetic field amplitude up toBwave≲B0. The results are relevant to both the basic properties of whistlers and to applications such as large loop antennas and electrodynamic tethers in space plasmas.

ISSN:0899-8221    

DOI:10.1063/1.860517

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The transverse stability of a relativistic electron beam focused by an ion channel in the presence of a diffuse background plasma is investigated. The linear behavior of the ion‐hose and electron two‐stream instabilities is treated analytically using a spread‐mass model for the beam and ion channel and a cold‐fluid model for the plasma. The electron two‐stream instability is found to be quite weak. As the plasma neutralization radius approaches the beam radius, the ion‐hose growth rate is reduced up to 50% before the model’s assumptions break down. Particle‐in‐cell simulations confirm the linear analytic theory and show that the electron two‐stream instability can saturate nonlinearly with little beam emittance growth.

ISSN:0899-8221    

DOI:10.1063/1.860518

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Ion orbits and equilibrium ion flow in crossed electric and magnetic fields are examined for the case of a strongly nonuniform electric field such as found in edge plasmas of tokamak fusion experiments and in space plasmas. It is shown that theE×Bdrift approximation no longer applies, either to the motion of a single ion or to the collective response of the ion species when the absolute value of the shear parameter, ‖&zgr;‖, defined as the absolute value of the ratio of the gradient ofE×Bspeed to ion gyrofrequency, is order one. It is also found that the ion velocity is strongly dependent on the electric field geometry. The results suggest that the existence of a strongly sheared electric field does not necessarily indicate the existence of strongly sheared plasma flow, and that the spatial shape of the electric field, when ‖&zgr;‖ is order one, may be a dominant factor in determining the resulting plasma flow speed.

ISSN:0899-8221    

DOI:10.1063/1.860519

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Starting from the dynamical plasma equations with the help of relaxed Zakharov simplification assumptions, and through taking use of the time‐averaged two‐time‐scale two‐fluid plasma description, the Zakharov equations (ZE) are generalized to contain the self‐generated magnetic field. The generalized Zakharov equations (GZE) are a set of three coupled equations and of seventh‐order nonlinearity with an amplification factor in almost all nonlinear terms of GZE. The conclusion that electron nonlinearities other than those appearing in ZE are not important and that the ion nonlinearities may be important when Langmuir turbulence level is fairly high, which was obtained through using both particle‐in‐cell (PIC) simulations and numerically integrating ZE by Newmanetal. [Phys. Fluids B2, 2600 (1990)] and Clarketal. [Phys. Fluids B4, 708 (1992)], is demonstrated by the theoretical analysis given in this paper.

ISSN:0899-8221    

DOI:10.1063/1.860520

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The energy conversion process as well as the plasma heating mechanism in the starting‐up phase of driven reconnection is investigated in detail using two‐dimensional magnetohydrodynamic (MHD) simulation. It is revealed that the compressional heating effect is predominant in this phase rather than the Ohmic and the viscous heating. The predominance of the compressional heating means that the plasma flow energy is considerably converted to the thermal energy. The strong compression takes place in the process during which the fast shock is generated in the downstream region of the reconnection point. The plasma heating due to nonsteady reconnection is briefly discussed as a promising mechanism for the anomalous ion heating, which is observed in reversed‐field pinch (RFP) experiments.

ISSN:0899-8221    

DOI:10.1063/1.860521

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Results of the kinematic fast dynamo problem for two classes of steady incompressible flows are presented. These flows are the ABC flow and the spatially aperiodic flow of Lau and Finn [Physica D57, 238 (1992)]. In a range of parameters, these three‐dimensional flows have stagnation points (A and B type) and there are chaotic streamlines. The chaos is associated with the intermingled stable and unstable manifolds of the stagnation points. In the aperiodic flow the chaos takes the form ofchaoticscattering. The growth rate of the dynamos for the aperiodic flow is found to obey a certain scaling law with resistivity &eegr; (as &eegr;→0), from which the results are extrapolated to the limit &eegr;→0 (infinite magnetic Reynolds number). Numerical results are presented indicating that fast kinematic dynamos can exist in these flows and that chaotic flow is a necessary condition. The structure of the dynamo magnetic fields is also shown, in particular, the relationship between the regions of maximal field strength and the invariant dynamical structures of the aperiodic flow. For the aperiodic flow, the unstable mode has a real frequency and these regions consist of two fingers of oppositely directed field. These regions rotate about a streamline (the one‐dimensional unstable manifold) coming out of the type A stagnation point. For the ABC flow withA=B=C, it is found that there are two dynamo modes: an oscillating mode and a purely growing mode. The mode crossing occurs at magnetic Reynolds number between 300 and 350, with the purely growing mode dominating for larger magnetic Reynolds numbers. For the oscillating mode, the region of large ‖B‖ is similar to that for the aperiodic flow. For the purely growing mode, the region of large ‖B‖ is localized in single fingers about the one‐dimensional unstable manifolds. The distribution function of ln‖B‖ is observed to be approximately Gaussian for both modes of the ABC flow. The distribution function for the mode found for the aperiodic flow has a much more complex structure, apparently associated with the escape of streamlines in chaotic scattering.

ISSN:0899-8221    

DOI:10.1063/1.860522

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The nonlinear evolution of the tearing mode instability with equilibrium shear flow is investigated via numerical solutions of the resistive magnetohydrodynamic (MHD) equations. The two‐dimensional simulations are in slab geometry, are periodic in thexdirection, and are initiated with solutions of the linearized MHD equations. The magnetic Reynolds numberSwas varied from 102to 105, a parameterVthat measures the strength of the flow in units of the average Alfve´n speed was varied from 0 to 0.5, and the viscosity as measured by the Reynolds numberS&ngr;satisfiedS&ngr;≥103. When the shear flow is small (V≤0.3) the tearing mode saturates within one resistive time, while for larger flows the nonlinear saturation develops on a longer time scale. The two‐dimensional spatial structure of both the flux function and the streamfunction distort in the direction of the equilibrium flow. The magnetic energy release decreases and the saturation time increases withVfor both small and large resistivity. Shear flow decreases the saturated magnetic island width, and generates currents far from the tearing layer. The validity of the numerical solutions was tested by verifying that the total energy and the magnetic helicity are conserved. The results of the present study suggest that equilibrium shear flow may improve the confinment of tokamak plasma.

ISSN:0899-8221    

DOI:10.1063/1.860523

出版商:AIP    

年代:1993

数据来源: AIP