摘要:

The effect of magnetic islands on the propagation of lower hybrid waves is investigated analytically for a model slab equilibrium with a single helical perturbation corresponding to anm=1 orm=2 island. From the variation ofk∥inside the island, the parallel current is estimated with the same pitch as the original perturbation. The effect of such a current on the nonlinear evolution of anm=2 island corresponds to a destabilization of the mode if the rf driven current is in the same direction as the global parallel current. A comparison with numerical solutions of the problem using a toroidal ray trajectory code is also presented.

ISSN:0899-8221    

DOI:10.1063/1.858944

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The poloidal coil system of a stellarator can be used to modify the ‖B‖ spectrum and, as a consequence, change the magnitude of the bootstrap current. The addition of a smalll=1 field component to a stellarator field can cancel or reverse the direction of the bootstrap current. It is shown that currentless operation in the collisionless regime is not impaired by bootstrap current, because the effects of bootstrap current can be eliminated by an appropriate external coil system. The advanced toroidal facility (ATF) [Fusion Technol.10, 179 (1986)] can be used to test some of these results.

ISSN:0899-8221    

DOI:10.1063/1.858945

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The role of them=0 modal components on the reversed‐field‐pinch (RFP) dynamo effect is examined in the single fluid magnetohydrodynamic (MHD) model by comparing results with and without various nonlinear convolutions of them=0 spectrum. Results indicate that the dynamo effect comes directly from them=1 instabilities and the effect of other nonlinear convolutions is small. Scaling studies of &dgr;B/Bwith the Lundquist number (S) are also performed. It is found that &dgr;B/Bis independent ofS(diffusionlike scaling), which is consistent with the view that the dynamo effect arises directly from them=1 modes.

ISSN:0899-8221    

DOI:10.1063/1.858946

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Energy and particle transport processes in Alcator C [Phys. Rev. Lett.60, 298, (1988)] discharges heated by directly launched ion Bernstein waves (IBW) are investigated using theonetwotransport code [Nucl. Fusion26, 329 (1986)]. The dependence of the observed ion heating rate (&Dgr;TH/Prf) on plasma density is shown to result mainly from the inherent ion energy confinement that is characteristic of these discharges in the Ohmic phase and not on IBW propagation and absorption characteristics. The inferred value of the Ohmic ion thermal conduction exhibits an increasing anomaly with increasing plasma density. At a plasma density ofn¯e≲1×1020m−3, the inferred ion thermal conduction is nearly equal to the Chang–Hinton neoclassical prediction. However, at a plasma density ofn¯e≳2×1020m−3, the inferred ion thermal conduction increases to about 5–8 times the Chang–Hinton neoclassical value. This increasing anomaly, which may result from the ion temperature gradient driven instability, can essentially account for the observed ion heating rate behavior during IBW heating. The effect of edge turbulence on IBW propagation is modeled using a Monte Carlo direct sampling simulation. The result is mainly a radial broadening of the calculated power deposition profile with increasing plasma density.

ISSN:0899-8221    

DOI:10.1063/1.858947

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

It has been demonstrated in the Phaedrus‐B tandem mirror [Phys. Fluids31, 714 (1988)] that radio‐frequency (rf) waves applied below the ion cyclotron frequency (&ohgr;/&OHgr;i≊0.75) can stabilize a mirror plasma against the interchange instability. Results presented here demonstrate stabilization over a range of frequencies (0.65≤&ohgr;/&OHgr;i≤0.8) with themMHD=−1 instability stabilized and higher‐order (‖mMHD‖≥2) low‐amplitude modes present. Radial profiles of the rf magnetic fields at a number of axial locations as well as parallel wavenumber measurements indicatemrf=−1 slow shear Alfve´n wave excitation over this frequency range. Measurements at frequencies below 0.75&OHgr;iindicate attenuation of rf waves by gas baffles and a gas box.

ISSN:0899-8221    

DOI:10.1063/1.858948

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Model equations have been derived for the analysis of mechanical equilibria of plasma–beam systems. The plasma has a thermal pressure, and the charged particle beam has effective inertial pressure. Self‐consistent force‐balance conditions are formulated by a set of nonlinear elliptic partial differential equations. Examples of explicit solutions have been given by numerical analyses.

ISSN:0899-8221    

DOI:10.1063/1.858949

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A recently introduced theory of applied‐Bion diodes [Phys. Rev. Lett.59, 2295 (1987)] is developed and presented in detail. The theory incorporates the self‐consistent virtual cathode motion to obtain the steady‐state ion current as a function of diode voltage. The existence of a limiting voltage at which the ion current diverges is demonstrated. The voltage–current characteristics of the diode are combined with a simple circuit model of the accelerator to calculate the operating point of the diode. The theoretical results are in good agreement with experimental data at peak power. An important consequence of the theory is a relation between the limiting diode voltage and the insulating magnetic flux, suggesting a late‐time voltage decay driven by flux penetration into the anode plasma.

ISSN:0899-8221    

DOI:10.1063/1.858950

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The stability of sidebands in a free‐electron laser is determined in the regime in which the three‐dimensional structure of the modes is important. It is found that the growth rate of the sidebands in the presence of a fundamental mode that is weakly guided by the beam is reduced. The indications are that for some parameters the spatial growth rate of the sidebands can be made less than that of the fundamental.

ISSN:0899-8221    

DOI:10.1063/1.858951

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The effect of a spread in energy of the emitted beam on the stability of the space‐charge‐limited diode is investigated. It is found that for small thermal spreads the so‐called emittron or transit time instability persists and is correctly described by cold fluid equations. Thermal effects become important when &ohgr;T∼&egr;−1/4T, where &ohgr; is the mode frequency,Tis a typical particle transit time, and &egr;Tis the ratio of the thermal energy of the beam to potential energy associated with the applied voltage. For modes with frequencies above this value, phase mixing of the particles causes a dramatic reduction in growth rate, such that other damping mechanisms will be able to stabilize the high‐frequency modes. The analysis presented here applies specifically to transverse electromagnetic (TEM) modes. However, the results can be expected to qualitatively describe other modes as well.

ISSN:0899-8221    

DOI:10.1063/1.858900

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The Vlasov theory is used to study kinetic corrections to fluid descriptions of Alfve´n wave nonlinearity. The method is to obtain an expression for the second‐order perturbed distribution function produced by a nonlinear Alfve´n wave. From this distribution function a kinetically correct expression is obtained for &dgr;n, the plasma density perturbation associated with an envelope‐modulated Alfve´n wave. This kinetic theory result differs substantially from the fluid expression when the plasma &bgr;≳1, and the electron and ion temperatures are approximately equal. This result is of interest because density fluctuations are an observationally accessible indicator of wave nonlinearity in solar system Alfve´n waves. It also will assist in the determination of properties of Alfve´n waves in the interstellar medium. Finally, this analysis also yields a kinetically correct expression foru, the magnetic field‐aligned component of the plasma fluid velocity. For parallel‐propagating wave packets, this field‐aligned flow is a major or even predominant contributor to the wave nonlinearity. Therefore these results provide a starting point for a general discussion of kinetic effects on Alfve´n wave nonlinearity.

ISSN:0899-8221    

DOI:10.1063/1.858901

出版商:AIP    

年代:1989

数据来源: AIP