摘要:

The effects of scattering by density fluctuations on the single pass lower‐hybrid propagation in tokamaks for current drive are analyzed in a cylindrical plasma model with sheared magnetic field. Even for the launchedN∥spectrum peaked around 2, the turbulent scattering and propagation in the sheared field significantly modifies theN∥spectrum so that the waves can effectively couple to slower electrons. At low densities, a small amount of the incident energy experiences a largeN∥upshift and creates a high‐energy electron tail which interacts with the main part of the wave energy. At higher densities, the bulk of the wave energy is shifted inN∥before damping resulting in lower current drive efficiencies.

ISSN:0031-9171    

DOI:10.1063/1.865038

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Solutions of the coupled steady‐state equations, Ohm’s law, and curl B=&lgr;(r)B, are obtained in slab and cylindrical geometries with two ignorable coordinates. Aside from the stark feature of small toroidal‐field reversal near the plasma edge, the numerically obtained field profiles in cylindrical geometry mimic the observed experimental reversed‐field‐pinch profiles; e.g.,Bzis a monotonically decreasing function of radius and, if the parallel resistivity is spatially independent, then also &lgr; (i.e.,j/B) is a monotonically decreasing function of radius. It is also shown that toroidal‐field reversal is impossible in a toroidally symmetric, force‐free, steady‐state, Ohmic plasma having nested flux surfaces. It is suggested that the more sophisticated dynamics required to sustain the observed field reversal may be able to be relegated to the outer annular region containing the reversal surface. Understanding of this dynamics then may be facilitated by the fact that only one field component, the poloidal one, is dominant throughout this region.

ISSN:0031-9171    

DOI:10.1063/1.865039

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A field‐aligned current couples the accelerated and decelerated Alfve´n modes together. When the coupling is strong enough, instability results. The coupling term is proportional to the deviation from Taylor’s state. Characteristics of this instability resemble experimental observations on reversed field pinches. In the appropriate limit, these instabilities reduce to the magnetohydrodynamic kink mode.

ISSN:0031-9171    

DOI:10.1063/1.865040

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Various toroidal reversed‐field pinch (RFP) equilibria stable to ideal Mercier and ballooning modes are subjected to resistive ballooning analysis. The stability properties of these configurations are thus investigated as a function of quantities that are experimentally measurable. Resistive ballooning is seen as a likely candidate for the interpretation of a certain class of density fluctuation phenomena observed in these devices.

ISSN:0031-9171    

DOI:10.1063/1.865041

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The cold‐fluid extraordinary‐mode eigenvalue equation is solved numerically to determine the influence of equilibrium profile shape on the detailed stability properties of relativistic non‐neutral electron flow in a planar diode with cathode located atx=0 and anode atx=d. Stability properties are investigated for the class of equilibrium energy profiles &ggr;b(x) specified by &ggr;b(x)=&lgr; cosh &agr;1x+(1−&lgr;) {[1−&agr;22(b2−x2)]1/2/[1−&agr;22b2]1/2} over the interval 0≤x≤b. Here &agr;1and &agr;2are constants (with &agr;22b2<1),x=bis the outer edge of the electron layer, and &lgr; is a constant parameter in the range of 0≤&lgr;≤1. The corresponding equilibrium profiles forBz(x),nb(x), andEx(x) are determined self‐consistently from the steady‐state (∂/∂t=0) cold‐fluid‐Maxwell equations. As the parameter &lgr; is varied from unity to zero there is a large change in the equilibrium profile fornb(x)/&ggr;b(x), ranging fromnb(x)/&ggr;b(x)=const over the interval 0≤x<bwhen &lgr;=1, to monotonic decreasing profiles fornb(x)/&ggr;b(x) when &lgr;<1. The numerical analysis of the extraordinary‐mode eigenvalue equation shows that the detailed stability properties are very sensitive to the shape of the equilibrium profiles. As &lgr; is reduced from unity, and the profile fornb(x)/&ggr;b(x) becomes monotonic decreasing, it is found that the instability growth rate Im &ohgr; is reduced. Moreover, the more rapid the decrease innb(x)/&ggr;b(x) (i.e., the smaller the value of &lgr;), the more the growth rate is reduced. Indeed, in some parameter regimes, the instability growth rate can be reduced to zero over the range of wavenumberkexamined numerically.

ISSN:0031-9171    

DOI:10.1063/1.865042

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A mechanism for producing suprathermal electrons having energies comparable to ion beam energy is studied by a numerical simulation. The system consists of background ions and electrons and a beam flowing across the magnetic field. The time scale considered is less than the ion gyro period, so the ion Larmor motion is ignored. Suprathermal electrons are produced when the ion beam density is smaller than the background plasma density, through resonant interactions with the excited waves propagating obliquely to the magnetic field. The work is motivated by Alfve´n’s suggestion of the critical ionization phenomenon.

ISSN:0031-9171    

DOI:10.1063/1.865043

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Calculations show that modes resembling the free‐electron laser (FEL) instability are excited when electrons move in quasicircular orbits under the combined action of a uniform axial magnetic field and an azimuthally periodic wiggler magnetic field. In the model, a thin annular ring of rotating electrons is confined in a hollow cylindrical waveguide, or between concentric cylinders comprising a coaxial waveguide, and the dispersion equations for the transverse magnetic (TMlm) modes are derived and analyzed. Coherent radiation occurs near frequencies &ohgr; corresponding to the crossing points of the electromagnetic modes &ohgr;=&ohgr;c(l,m) and the beam modes &ohgr;=(l+N)&OHgr;∥, where &ohgr;cand &OHgr;∥are the waveguide cutoff frequency and the electron cyclotron frequency, respectively, andNis the number of wiggler periods.

ISSN:0031-9171    

DOI:10.1063/1.864993

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Stimulated Raman scattering from a magnetized plasma has been examined using the set of Vlasov–Maxwell equations. The incident laser light, propagating in the form of an extraordinary mode, decays into a scattered extraordinary mode together with electron‐Bernstein waves. For sufficiently strong magnetic fields and high electron temperatures, the growth rates of these waves become comparable to the field‐free values. At relatively low plasma densities the magnetic field can result in scattered light at &ohgr;0/2 (where &ohgr;0is the laser frequency), a frequency associated with radiation emitted from the quarter‐critical density region of unmagnetized plasmas. We have compared the results from the kinetic theory analysis with observations from numerical experiments using a 1 1/2 ‐D electromagnetic particle code and found them to be in good agreement.

ISSN:0031-9171    

DOI:10.1063/1.864994

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An analytic hydrodynamic model for the formation and decay of strong shock waves (0.1–10 TPa) is derived in this paper by using self‐consistent and well‐defined assumptions. A trapezoidal pressure profile as a function of time applied to the target surface is used as the initial condition. This model permits the calculation of the pressure, velocity, and acceleration of the shock front, as well as the calculation of the pressure gradient immediately behind the shock front, as functions of time and position. This model is applied to laser‐generated shock waves with maximum intensities of 5×1013and 5×1014W/cm2. Good agreement with previous computer simulations of solid–laser interaction is obtained.

ISSN:0031-9171    

DOI:10.1063/1.864995

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The thickness of the coating on a strip produced by modern hot‐dip galvanizing lines is controlled with air jets which generate a ‘‘barrier’’ to the molten coating material as it is drawn from its bath by the emerging strip. It is shown that an improved understanding of the process and more accurate predictions may be obtained by modifying the existing theory to include the jet‐induced surface shear acting in conjunction with the jet stagnation pressure gradient effect.

ISSN:0031-9171    

DOI:10.1063/1.864996

出版商:AIP    

年代:1985

数据来源: AIP