摘要:

Stability properties of the free‐streaming mode (space‐charge wave) in a relativistic annular electron beam with radiusR0propagating through a dielectric loaded waveguide is investigated, in connection with the Cherenkov radiation. The stability analysis is carried out within the framework of the linearized Vlasov‐Maxwell equations for an electron distribution function, in which all electrons have a Lorentzian distribution in the axial canonical momentum. One of the most important features of the analysis is that, for some ranges of physical parameters, a strong mode coupling between the vacuum dielectric waveguide and free streaming modes occurs, exhibiting possibilities of a Cherenkov radiation. The typical maximum growth rate of instability is a few percent ofc/R0, wherecis the speed of lightinvacuo. However, the growth rate and bandwidth of instability are substantially reduced by increasing the axial momentum spread.

ISSN:0021-8979    

DOI:10.1063/1.328603

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The interaction of a spontaneous dc magnetic field and nonlinear force‐induced striated motion of plasma caused by an obliquely incidentp‐polarized electromagnetic plane wave upon an inhomogeneous plasma is investigated and consequent additional deconfining pressure is evaluated.

ISSN:0021-8979    

DOI:10.1063/1.328604

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An experimental study of unipolar arcs was conducted in a low‐pressure mercury rf‐generated plasma. The results were found to be consistent with the concept of a sheath mechanism for driving the unipolar arcs. Floating double‐probe measurements yielded electron temperatures of ∼2 eV and electron‐number densities of ∼1×1011cm−3. The variation of the unipolar arc current with (1) the electron temperature and density and (2) the metal surface area exposed to the plasma verified the predicted dependence of the arc current on the plasma parameters and the metal surface area. Finally, alternative mechanisms for sustaining the observed arcs by high‐frequency rectification were ruled out on the basis of the recorded current waveforms of the unipolar arcs.

ISSN:0021-8979    

DOI:10.1063/1.328605

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Electromagnetic implosions of 20‐cm radius, 2‐cm tall, few centimeter thick gas ’’shells’’ using a 2‐MA, 80‐kV, 0.8‐&mgr;sec capacitor bank have been conducted. Implosion velocities ≳100 cm/&mgr;sec and implosion kinetic energies as high as ∼12 kJ, which is ∼17% of the 70‐kJ stored energy in the capacitor bank, have been achieved. Photon yields ≳ 1 kJ above 150 eV and neutron yields ∼107have been obtained. Gas injection and preionization, current and voltage data, and fast photography data are discussed. Comparison with higher energy foil implosion experiments is briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.328672

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A dark‐field, laser‐probing diagnostic has produced the first high‐resolution photographs of density structure on the rear side of laser‐accelerated foils. This diagnostic allows the preferential sampling of the steep‐gradient region of an expanding plasma and permits two‐dimensional, multiple‐time recordings on a single photograph. The studies are aimed at understanding the early‐time physics of target implosions for inertial confinement fusion. Both long (500‐ps) and short (150‐ps) probe pulses were used to study the rear‐side plasmas of thin foils accelerated by the rocketlike reaction to a hot plasma ablated from the front side by the laser radiation. The longer pulse results, both for angular scatter and the lifetime of small, transverse structure, imply a relatively cold (∼1 eV) rear‐side plasma. The short pulses provide high‐resolution photographs of the complete structure. These observations are relevant to two of the basic requirements of inertial confinement fusion: cold fuel isentrope and implosion symmetry.

ISSN:0021-8979    

DOI:10.1063/1.328606

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An argument is presented which justifies the simultaneous use of both the Langmuir Mott‐Smith orbit‐limited, and the diode space‐charge‐limited equations for current collection by electrostatic probes. From these two equations the sheath size can be eliminated and the current to a probe obtained for a large range of values of probe potential and Debye length. Numerical solutions are presented for both the spherical and cylindrical cases. Asymptotic properties are considered. Comparison with results of previous probe theoretical treatments are made.

ISSN:0021-8979    

DOI:10.1063/1.328607

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The Faraday effect should provide a direct method for the measurement of the current density profile in a tokamak fusion device. Measuring the angle of rotation of linearly polarized far‐infrared beams propagating along several vertical chords in the poloidal plane and then applying proper inversion techniques, will provide information about the current density profile. Assuming the Voigt effect is small, a solution is found using the WKB approximation. Due to the large Ohmic heating currents and plasma densities in Alcator C, sufficiently large angles of rotation are produced with a relatively short wavelength sufficient to make the Voigt and refractive effects negligible. For Alcator A the peak rotation is found to be the order of 4°, for Alcator C it is 12°. These values were calculated assuming a wavelength of 120 &mgr;m and ’’typical’’ plasma parameters. In Alcator C, in particular, the large rotation angles should allow measurement of the poloidal field over most of the plasma cross sections.

ISSN:0021-8979    

DOI:10.1063/1.328608

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The usual model for a sheath formed adjacent to a cathode immersed in a high pressure plasma is modified for ionization and secondary emission effects. It is found at high voltages, when the product of the ionization coefficient within the sheath and the sheath thickness is of order unity, that the sheath can collapse catastrophically with an accompanying decrease in its impedance of many orders of magnitude. The calculations, which are numerical, are particularly well suited to the usual description of the ionization coefficient in terms of two empirical constants, appropriate to a given gas. As a result, it is possible to give a simple curve of sheath breakdown voltage versus the quantityji0/&mgr;iP3(whereji0is the ion current density, &mgr;ithe ion mobility andpthe pressure). The breakdown curve for air is given as an example.

ISSN:0021-8979    

DOI:10.1063/1.328609

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Optical emission spectroscopy was used to study the behavior of the excited states of argon and oxygen atoms which occur in glow discharges generated during the rf diode sputtering of a zinc oxide target. The results indicate that there is an interaction between the two gases when the sputtering gas contains less than approximately 15% oxygen. A quasi‐resonant transfer of excitation from a metastable argon atom to an excited, repulsive, neutral state of the oxygen molecule, which subsequently dissociates, is proposed as the interaction mechanism. Correlation with mass spectrometry data indicates that the dissociation products increase the relative rate of zinc oxide production at the target surface and therefore promote single (0001) crystallographic orientation zinc oxide film growth.

ISSN:0021-8979    

DOI:10.1063/1.328610

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The properties of streamer or electron avalanche pulses have been investigated for point‐plane, and positive and negative dc corona discharges in gaseous SF6over the pressure range of 50–500 kPa. Measurements were made of the voltage and pressure dependence of electrically detected corona pulse‐height distributions, pulse shapes, and repetition rates. Positive corona pulses appear at onset as low‐level electron avalanches which, at higher voltages, develop into large streamers usually followed by a burst of many smaller pulses. The burst characteristics of positive corona show a definite dependence on pressure and voltage which is evident in the pulse‐height distribution data. Negative corona displays characteristics which are sensitive to cathode surface conditions. For a point electrode that is either exposed to UV radiation or has been conditioned by prior discharges, the negative corona appears predominantly in a quasiglow mode consisting of relatively low‐level pulses of high repetition rate.

ISSN:0021-8979    

DOI:10.1063/1.328611

出版商:AIP    

年代:1981

数据来源: AIP