摘要:

A nonlinear saturation mechanism of electrostatic drift waves in a sheared magnetic field is presented. The basic physical mechanism is the coupling of the unstable, lowest radial mode to higher modes which are damped because of their short radial wavelengths. Stationary spectra are obtained analytically and applied to the current‐driven drift wave instability.

ISSN:0031-9171    

DOI:10.1063/1.863950

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

An electromagnetic ion‐cyclotron wave, near the second harmonic cyclotron frequency of majority ions in the central region of a tokamak, is seen to decay efficiently into two Bernstein modes. The coupling coefficient for this decay vanishes for a dipole pump, hence a rigorous kinetic theory for decay instabilities of a finite wavenumber pump is developed. For parameters characterizing the Princeton Large Torus (PLT) experiment, the uniform medium growth rate at 200 kW turns out to be 3 &mgr;sec, whereas, the threshold due to plasma inhomogeneity is 100 kW. The nonresonant decay into ion acoustic waves is also significant, though relatively less important. For a pump wave near the cyclotron frequency of majority ions, nonlinear Landau damping and oscillating two stream instability turn out to be very important.

ISSN:0031-9171    

DOI:10.1063/1.863951

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The nonrelativistic hot plasma resonances, which occur for the O mode propagating at right angles to the equilibrium magnetic field are explained in terms of a mode conversion mechanism. The hot plasma introduces additional wave branches—the transverse cyclotron harmonic waves. At an electron cyclotron resonance, an incident O mode is partially converted to a cyclotron harmonic wave and partially transmitted as an O wave. The energy converted to the cyclotron harmonic wave is shown to be identical to the energy absorbed when relativistic effects are included. A simple argument is given showing why earlier nonrelativistic calculations of the absorption gave the same result as the relativistic theory.

ISSN:0031-9171    

DOI:10.1063/1.863952

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A practical numerical method for finding the magnetohydrodynamic equilibria of minimum magnetic energy subject to certain constraints is described. The constraints may be those of ideal magnetohydrodynamics or the constraint of constant local &mgr; suggested by Taylor. No assumptions of symmetries are necessary to use the method. Examples of the method for finding Taylor states are described, and the potential of the method for simulation of plasma dynamics is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863953

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Analytic three‐dimensional mirror equilibria are calculated from the tensor‐pressure fluid equations by means of an asymptotic expansion procedure. The class of equilibria described here is of interest to the minimum‐Bend cells of current tandem mirror experiments. It is shown that in the shallow‐well approximation, a small plasma pressure can produce a relative change of order unity in the vacuum magnetic well, thereby leading to interchange instability. In addition, if the axial dependence of the quadrupole field is not chosen in a specific way, ’’long thin’’ corrections set an upper limit on the maximum allowable plasma radius. The proper choice of quadrupole field to avoid this limitation is discussed, and an analytic expression is derived for the interchange beta limit.

ISSN:0031-9171    

DOI:10.1063/1.863954

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The effect of a magnetic field on the transition layer between a plasma and an absorbing wall is studied. A numerical model is used which simulates the motion of plasma particles in the electric and magnetic fields for a prescribed particle influx at the plasma boundary. Bohm’s condition for the existence of a monotonic profile of the layer is generalized. The transition layer proves to have a double structure comprising a quasineutral magnetic presheath preceding the electrostatic Debye sheath. The magnetic presheath scales with the ion gyroradius at the sound speed and with the angle of the magnetic field. The total electric potential drop between plasma and wall proves to be fairly insensitive to the magnitude and angle of the magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.863955

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The mass ablation rate from spherical targets irradiated at wavelengths of 1.05 and 0.53 &mgr;m has been measured with particle and with x‐ray diagnostics. The specific mass ablation rate is found to be proportional toI0.7awith little wavelength dependence for a given value ofIa, theabsorbedirradiance. Comparisons with simulations shows that heat fluxes up to and in excess of 10% of the free streaming value are occurring. Values of the ablation pressure are derived.

ISSN:0031-9171    

DOI:10.1063/1.863929

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Steady‐state planar ablative flow in a laser produced plasma is studied. The calculations relate all steady‐state fluid quantities to only three parameters, the material, absorbed irradiance, and laser wavelength. The fluid is divided into three regions; the subcritical expanding plasma, the steady‐state ablation front, and the accelerated slab. Boundary conditions at the interfaces of these regions are given. If the absorbed irradiance is nonuniform, the nonuniformity in ablation pressure is calculated. Results are compared with experiment and fluid simulation for both uniform and nonuniform illumination.

ISSN:0031-9171    

DOI:10.1063/1.863956

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Two‐dimensional numerical simulations of ablatively accelerated thin‐shell fusion targets, susceptible to rupture and failure by Rayleigh–Taylor instability, are presented. The results show that nonlinear effects of Rayleigh–Taylor instability are manifested in the dynamics of the ’’bubble’’ (head of the nonlinear fluid perturbation) rather than in the dynamics of the spike (tail of the perturbation). The role of multiwavelength perturbations on the shell is clarified, and rules are presented to predict the dominant nonlinear mode‐mode interactions which limit shell performance. It is also shown that the essential dynamics of strongly driven flows are governed by the classical Rayleigh–Taylor instability of an ideal, incompressible, thin fluid layer.

ISSN:0031-9171    

DOI:10.1063/1.863925

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Experimental and theoretical results on the properties of CO2laser‐induced carbon and polyethylene (CH2) plasmas at laser intensities of ∼1015W/cm2are presented. The Thomson parabola technique is used to measure the ion velocity distribution in the underdense expanding plasma which is collisionless and isothermal. A model which treats the problem of the collisionless expansion of an isothermal electrostatic multi‐cold‐ion quasineutral plasma will be used to interpret the experimental results. Experiments and theory show that the effect of hydrogen in the CH2target induces a cutoff in the carbon ion velocity distribution. Theory suggests that acceleration field attenuation effects modify the behavior of these plasmas. Data suggest that the space–time evolution of the ion velocity distribution is completed before an ionization‐recombination equilibrium is reached. Experimental results from the underdense region are used to estimate plasma parameters near the critical surface, which show that the presence of hydrogen in the target apparently greatly reduces the thermal temperature near the critical surface, probably due to enhanced lateral energy transport.

ISSN:0031-9171    

DOI:10.1063/1.863926

出版商:AIP    

年代:1982

数据来源: AIP