摘要:

A three velocity and one‐space dimensional nonrelativistic electromagnetic particle simulation code employing the fast Fourier transform algorithm is described and used to simulate the amplification of electromagnetic radiation by an electron beam passed over a rippled static magnetic field. In the beam frame the rippled magnetic field looks like an intense electromagnetic pump and thus a parametric instability can be produced. In one case, it was observed that 30% of the beam energy was converted to electromagnetic radiation.

ISSN:0031-9171    

DOI:10.1063/1.1694656

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A simple analytical relationship for theQof a resonant spherical cavity with a central droplet is derived assuming superconducting cavity walls. In this caseQ = (&ohgr; × stored energy)/( (power loss)where power loss refers to the energy rate delivered to the central droplet. It is assumed that the skin depth&dgr;is small compared with the droplet radiusa, and that the wavelength,&lgr; = 2&pgr;c/&ohgr;, be small compared with the cavity radiusb, and large compared with the droplet radius. The equation isQ = (bc2) / (a2&dgr;&ohgr;2). It is shown that in the case of conducting cavity walls a short wavelength minimizes the wall losses.

ISSN:0031-9171    

DOI:10.1063/1.1694657

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A sudden anomalous increase in the plasma resistivity at a magnetic neutral point is observed which results in fast energy dissipation and strong plasma heating. It appears earlier and its magnitude increases as (1) the plasma density is decreased, (ii) the discharge plasma current is increased, or (iii) the external quadrupole magnetic field is increased. When the sharp rise in the resistivity appears, the electron and ion temperatures rise abruptly up to the range of several keV within 0.5&mgr;secand about 10% of the initial energy stored in the capacitor (for the plasma current excitation) is converted into the ion thermal energy. This strong and efficient ion heating cannot be explained by a simple classical process and the role of turbulent waves should be important.

ISSN:0031-9171    

DOI:10.1063/1.1694658

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Reversed‐field current layer evolution with magnetic field annihilation is examined numerically using single‐fluid, two‐temperature, magnetohydrodynamic equations with anisotropic, classical transport properties. Self‐pinching occurs with magnetic field reversal in the current layer. Very high local plasma temperatures and density are produced in the pinched current layer and magnetic field diffusion is inhibited. Numerical results are compared with experimental data.

ISSN:0031-9171    

DOI:10.1063/1.1694659

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Variation methods, originally developed for deriving stability criteria from the energy principle, are well suited for obtaining bounds for the maximum growth rate from a modified energy principle. An upper bound is derived which corresponds to a sufficient stability criterion, and a lower bound is derived which corresponds to a necessary criterion. Each of these bounds equals the actual maximum growth rate in special cases.

ISSN:0031-9171    

DOI:10.1063/1.1694660

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An experiment on large amplitude electrostatic waves is described, the results of which illustrate the phase velocity variations during the first bounce period predicted by the single‐wave model. The spatial growth of sidebands, after the initial large amplitude wave has reached saturation due to nonlinear trapping effects, is also described. It is found that the amplitude of the phase velocity variation is proportional to&phgr;1/2(where&phgr;is the wave amplitude) and that the slower phase velocity, higher‐frequency sideband (for instability near the plasma frequency) is always the dominant one.

ISSN:0031-9171    

DOI:10.1063/1.1694661

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Numerical solutions of the nonlinear Vlasov equation show that the long time behavior of an electrostatic plasma wave depends critically on the value of the parameter&ggr;&tgr;(&ggr;is Landau's damping coefficient and&tgr;is the electron bounce time), both for large and small initial wave amplitudes. When the initial wave amplitude is large, the initial nonlinear damping is stronger than Landau damping; for smaller initial amplitudes, the behavior is initially described by Landau's theory. For both types of problems if&ggr;&tgr; > 0.5, the wave damps monotonically; if&ggr;&tgr; < 0.5, the numerical results indicate that the plasma performs amplitude oscillations. When&ggr;&tgr; ≈ 0.5, the plasma displays its critical behavior; here, after some initial damping the wave amplitude levels off at a constant value. The critical behavior observed for mild nonlinear problems is in good agreement with the experimental results of Franklin, Hamberger, and Smith. For a moderately strong nonlinear wave displaying the critical behavior, the wave performs periodic oscillations with a frequency smaller than Landau's linear frequency; this frequency shift is explained because the uniform electron distribution develops a plateau centered at Landau's phase velocity.

ISSN:0031-9171    

DOI:10.1063/1.1694662

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A set of fluid equations is used to determine the stability boundaries for flute instability of a plasma in a nonuniform magnetic field, in the presence of simultaneous density(&dgr;′)and temperature(&lgr;′)gradients. The resulting surfaces in&dgr;′, &lgr;′, P,Pspace (Pproportional to plasma density) show the various conditions leading to instability for either magnetic wells or ordinary mirrors. It is demonstrated that a modified form of the fluid equations given by Varma is most appropriate for high‐energy injection plasma. A temperature gradient in the same direction and of the same magnitude as the magnetic field gradient is always destabilizing.

ISSN:0031-9171    

DOI:10.1063/1.1694663

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

From the collisionless Vlasov‐Poisson equations the renormalization of plasma turbulence is derived by a selective summing of secular terms to all orders in perturbation theory. The resulting renormalized theory agrees with earlier renormalizations of quasilinear theory and the mode coupling equation exhibits the same symmetries known in weak turbulence theory. The present formulation differs from that of Rudakov and Tsytovich by containing a separation of the turbulentkscales and by the associated feature of retaining the form of induced wave scattering in the nonresonant mode coupling terms. The theory is applied to the problem of finding the stationary spectrum for current driven ion‐acoustic turbulence. Balancing the induced wave scattering off screened ions with a quasilinear growth rate, a modified Kadomtsev spectrum having a lowk&lgr;Dcut off is obtained self‐consistently within the collisionless theory. The turbulent energy density and effective collision frequency are computed from the new spectrum.

ISSN:0031-9171    

DOI:10.1063/1.1694664

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

During the past two years computer simulation has been used extensively to obtain a detailed description of pulse trapping and pulse stacking of relativisticElayers in astron. Resistors are essential to good trapping, in agreement with experiment. In the code, pulse trapping can easily be arranged to be 100% efficient—in marked contrast to the experiment. Details of pulse stacking are dependent on resistor configuration, degree of charge neutralization, and external well shape, but the field reversal increase invariably runs into a saturation due to axial expansion of the layer. This process can be described as a phase space exclusion or, alternatively, as nonadiabatic axial heating. The pulse‐stacking process involves a tearing and bunching, and it is also nonadiabatic in the radial motion; as a consequence, radial expansion always occurs, and this can also act as a limitation to field reversal unless the resistor locations allow considerable radial room. Very tightly focused (axially) pulses can result from vacuum pulse trapping if system conditions are optimized correctly. This in turn lessens the axial expansion problem for vacuum layers. In contrast, the radial expansion problem is much more severe for vacuum layers, and this effect causes radial loss at some layer strength well short of field reversal. Large‐current (2500 A) nuetralized pulses result in field reversal with one pulse.

ISSN:0031-9171    

DOI:10.1063/1.1694665

出版商:AIP    

年代:1974

数据来源: AIP