摘要:

A calculation of the charge state distribution of the asymptotic evolution of a plasma produced by an ultraviolet laser has been carried out by coupling the results of a detailed two‐dimensional hydrodynamic simulation of the creation and heating of the plasma to a simpler one‐dimensional hydrodynamic code. The latter incorporates time‐dependent ionization and recombination physics and is used to model the subsequent expansion and cooling of the plasma. The simulation results are compared to experimental charge state distributions obtained by using a single shot electrodynamic charge analyzer. The sensitivity of the results to the rate coefficients used for the recombination calculations was tested and the calculated distributions were found to be most sensitive to the values of the three‐body recombination rate and the amount of heat returned to the plasma. Reasonable agreement was found between the measured and calculated charge state distributions.

ISSN:0899-8221    

DOI:10.1063/1.860580

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A hydromagnetic equation system for the interplanetary collisionless solar wind is used to derive a set of conservation laws for that medium. It is found that every equation of the original system, including the closure relation, is related to one conservation law. The set that has been derived does not only include the traditional laws, but also a new one for the magnetic moment of the electrons. The conservation set is then used to obtain the space constants for the solar coronal expansion. The new law yields a constant that has not been predicted by other models.

ISSN:0899-8221    

DOI:10.1063/1.860581

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Modification of the magnetic force by a developed small‐scale magnetohydrodynamic (MHD) turbulence can result in the sign reversal of theeffectivemagnetic pressure. It is due to negative contribution of the MHD turbulence, to the large‐scale magnetic force. It can significantly lower the elasticity of the large‐scale magnetic field [ Sov. Phys. JETP70, 878 (1990)]. This effect excites instabilities of the large‐scale magnetic field due to the energy transfer from the turbulent pulsations to the latter. The nonturbulent stability criteria are modified due to theeffectivenegative magnetic pressure. These instabilities may provide a mechanism of the large‐scale magnetic ropes formation in the solar convective zone and spiral galaxies. In addition, the instabilities can excite the short‐period solar oscillations.

ISSN:0899-8221    

DOI:10.1063/1.860582

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A detailed experimental study is presented of a pulsed megawatt gyrotron oscillator operating in the 200–300 GHz range, whose design is consistent with continuous operation for electron cyclotron resonance heating (ECRH) of fusion plasmas. Two different radii beams produced by magnetron injection guns (MIG’s) were used to excite the cylindrical waveguide cavity. The emission was found experimentally to be single mode, single frequency with a single rotation, which can be mode converted for transmission. The highest power reached with the larger radius electron beam was 1.2 MW at 230 GHz in the TE34,6mode with an efficiency of 20% and beam parameters of 59 A and 100 kV. The highest power reached with the smaller radius electron beam was 0.78 MW at 280 GHz in the TE25,13mode with an efficiency of 17% and beam parameters of 51 A and 92 kV. The smaller radius beam gave a peak efficiency of 18% at 0.72 MW, 290 GHz in the TE25,14mode. Efficiencies obtained in this experiment are about half that of less highly overmoded gyrotrons. Analysis of the experiment suggests that the low efficiency is primarily caused by azimuthal mode competition, in agreement with multimode theory for a tapered cavity. These experimental results show that megawatt power levels can be generated in continuous wave (cw) gyrotron oscillators at 200–300 GHz with efficiencies approaching 20%.

ISSN:0899-8221    

DOI:10.1063/1.860583

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The 13 TW pulsed electron beam generated by Hermes III [J. J. Ramirezetal.,DigestofTechnicalPapers, 6th IEEE Pulsed Power Conference (IEEE, New York, 1987), pp. 294] is transported 10.8 m in a low‐pressure gas with 79±1.5±[5]% energy transport efficiency. The uncertainties refer to the rms shot‐to‐shot variation and estimated systematic error, respectively. The high efficiency obtained is accomplished by removing the inward momentum of the beam at injection via an active magnetic lens. The reduced transverse momentum permits self‐magnetic field confinement at low pressures with low inductive and collisional loss. The region of efficient transport lies between regions of instability. Consistent with experiment, the analytic and numerical models developed here predict that them=1 resistive hose instability degrades transport above 100 Torr and plasma return‐current instabilities disrupt the beam below 1 Torr. Within this pressure ‘‘window’’ of stable propagation, the models explain the mechanisms responsible for maximum transport.

ISSN:0899-8221    

DOI:10.1063/1.860584

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The small signal stability of crossed‐field devices fed by a thin electron beam is analyzed. The situation differs from diocotron modes in that the interaction cavity supports slow wave eigenmodes in vacuum. The rippling of the beam causes a modification of the vacuum dispersion relation and mode profiles. The growth rate is found by equating the rate of change of the power flux with the fast scale averaged wave–particle energy exchange rate. The radio frequency (rf) power flow including the energy circulating in the anode structure is related to the wave amplitude via the interaction impedance. The singularities at resonance, the trademark of any linear theory, are avoided by following the particle guiding center (GC) orbits in reference frame with the wave synchronous. The small signal gain is found by expansion in powers of the rf amplitude. A finite linear growth results, even for symmetric particle excursions, due to the self‐field of the rippled beam. Near resonance the growth rate is independent of the detuning between the phase and drift velocities. Higher‐order contributions to the instability are caused by the nonlinear bunching of the GC distribution in space and aresymmetricrelative to resonance. Symmetric frequency response is a unique feature of crossed‐field devices (CFD’s) opposed to theantisymmetricgrowth characterizing other ‘‘unbound electron’’ devices [gyrotrons, free‐electron lasers (FEL’s), traveling‐wave tubes (TWT’s)]. The growth rate goes over to the diocotron growth when all the impedance comes from the rippled beam (i.e., smooth anode at infinite distance). The perturbed mode profiles are nonsingular; the profile singularities of standard linear theories reflect the singularities of the particle orbits at resonance.

ISSN:0899-8221    

DOI:10.1063/1.860585

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A nonlinear theory is developed for the two‐stream interaction of two relativistic annular electron beams propagating through a grounded cylindrical conducting tube. The theory is based on the assumption that the beams experience energy modulation at a cavity before they enter the drift tube. Two coupled integrodifferential equations are derived which describe beam current modulation in terms of timetand propagation distancez. The evolution of the fundamental mode in the current modulation is investigated analytically by making use of these coupled equations. The amplitude of the fundamental mode, which is a function of the propagation distance, is expressed explicitly in terms of the initial energy modulation, the growth rate of the instability, and the beam intensity. It is found that self‐field effects dominate two‐stream effects at the beginning of the propagation. As the beams propagate further, the two‐stream effects start to dominate and then the perturbations grow exponentially. The saturation mechanism is identified as multimode coupling. A theoretical model describing the mode coupling in the current modulation is also developed. The theory is verified by detailed comparisons between analytical and particle‐in‐cell simulation results.

ISSN:0899-8221    

DOI:10.1063/1.860963

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

For the simulation of laser‐created plasmas, hydrodynamic codes need an atomic physics package, for both the equation of state and the optical properties, which does not use the hypothesis of local thermodynamical equilibrium (LTE). However, in x‐ray laser studies, as well as in indirect drive inertial confinement fusion studies, high‐Zmaterials can be found where radiation trapping can induce a significant departure from the optically thin description. A method is presented in which an existing LTE code can be changed into a non‐LTE code with radiation‐dependent ionization. This method is numerically simple and its cost, in terms of computing time, is low enough to be used in two‐dimensional simulations.

ISSN:0899-8221    

DOI:10.1063/1.860586

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Relativistic plasma mode conversion‐tunneling equations at the second and third electron cyclotron harmonics are derived. A finitek∥is introduced which keeps the coupling between the O‐mode, the X‐mode and the Bernstein wave in the mode conversion problem for the first time. The solutions for these mode conversion problems without absorption are obtained, and the connection formulas between different wave branches are established. The corresponding transmission, reflection and conversion coefficients are also given. A comparison between the coupled equation and the uncoupled equations is also made.    

ISSN:0899-8221    

DOI:10.1063/1.860587

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The mode conversion‐tunneling equation with weakly relativistic absorption at 3&ohgr;cehas been solved by using the Green function method. The numerical results show that the reflection and conversion coefficients are dramatically reduced when relativistic absorption is included, while the transmission coefficients and the conversion coefficient from the X‐mode on the high field side to the O‐mode on the low field side are independent of absorption. The reduction of these coefficients leads to the reduction of the asymmetry in the absorption between different wave branches. The numerical results also show that the relativistic effect is important even for a relatively low temperature plasma. An important relationship between solutions is expressed by the reciprocity relations which have been proved for any harmonic number. These analyses modify the interpretation of electron cyclotron emission (ECE).

ISSN:0899-8221    

DOI:10.1063/1.860588

出版商:AIP    

年代:1993

数据来源: AIP