摘要:

In this Letter, a high‐frequency, second‐harmonic gyrotron using a 12 T superconducting magnet is described. It has achieved a maximum frequency of 636 GHz (corresponding to a wavelength of 472 &mgr;m). Operating at both the fundamental and the second harmonic of the electron cyclotron frequency enables the gyrotron to act as a high power (several 100 W), step tunable, millimeter to submillimeter wave source in the wide frequency range from 150 to 630 GHz.

ISSN:0899-8221    

DOI:10.1063/1.860928

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An equilibrium under which ions’ escape from a generated plasma due to their own electric field occurs in the presence of an external uniform magnetic field is studied analytically. It is found that for an arbitrary source of ionization there is a limiting value (threshold) of the magnetic field after which no balance between birth and collisionless escape of ions is possible and, therefore, no stationary process of ion generation can exist. An experimental setup allowing the observation of the threshold is discussed.

ISSN:0899-8221    

DOI:10.1063/1.860876

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

When a medium is dissipative, the classic expression for the group velocity,d&ohgr;/dk, is complex with an imaginary part often being far from negligible. To clarify the role of this imaginary term, the motion of a wave packet in a dissipative, homogeneous medium is examined. The integral representation of the packet is analyzed by means of a saddle‐point method. It is shown that in a moving frame attached to its maximum the packet looks self‐similar. A Gaussian packet keeps its Gaussian identity, as is familiar for the case of a nondissipative medium. However, the central wave number of the packet slowly changes because of a differential damping among the Fourier components: Im(d&ohgr;/dk)=d&ggr;/dk≠0, where &ohgr;≡&ohgr;r+i&ggr;. The packet height can be computed self‐consistently as integrated damping (or growth). The real group velocity becomes a time‐dependent combination of Re(d&ohgr;/dk) and Im(d&ohgr;/dk). Only where the medium is both homogeneous and loss free, does the group velocity remain constant. Simple ‘‘ray‐tracing equations’’ are derived to follow the packet centers in coordinate and Fourier spaces. The analysis is illustrated with a comparison to geometric optics, and by two applications: the case of a medium with some resonant damping (or growth) and the propagation of whistler waves in a collisional plasma.

ISSN:0899-8221    

DOI:10.1063/1.860877

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A technique is presented for constructing a self‐consistent Vlasov equilibrium for a bounded single species plasma confined by a strong magnetic field. The equilibrium incorporates particle losses to the surrounding wall due to large gyroradii, a feature not previously considered. Specific examples of the density and potential profiles including particle losses are given.

ISSN:0899-8221    

DOI:10.1063/1.860878

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Necessary and sufficient stability criteria for flute modes localized at the edge of toroidal plasmas are obtained from the Braginskii’s two‐fluid equations without taking into account the collisional effects. It is assumed that the plasma pressure tends to vanish, but its gradient remains finite at the edge of the plasma. The results show that the free‐boundary edge flute modes (namely, the peeling modes) are more dangerous than the fixed‐boundary modes (namely, the Mercier modes). Numerical investigation of the criterion for peeling modes shows that the finite ion‐gyroradius effect can substantially stabilize the modes, especially for the case &Dgr;≥0, where the equilibrium quantity &Dgr;≡1/2+S−1<j⋅B‖∇v‖−2≳, withSdenoting the global shear,Bthe magnetic field,jthe current density,vthe volume inside the reference magnetic surface, and <...≳ denoting the average over the magnetic surface. Equilibria with &Dgr;≥0 are shown to be more stable to the peeling modes than those with &Dgr;≤0.

ISSN:0899-8221    

DOI:10.1063/1.860879

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The calculation of the growth rate of tearing modes is extended to short scale lengths by including the Hall field. A unified dispersion relation is found that describes usual tearing modes at one limit and the Hall tearing modes with the enhanced growth rate at the opposite limit. The dispersion relation is valid for both collisional and collisionless plasmas.

ISSN:0899-8221    

DOI:10.1063/1.860880

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Tokamak neoclassical theory is extended to the plasma edge region, where the gradient scale lengths of density, temperature, and rotation speed are comparable to the ion poloidal gyroradius, and wherev&thgr;B&fgr;/vthB&thgr;∼O(1). A new drift due to viscosity is found to be important. A self‐consistent set of equations for determining particle flows within and across flux surfaces, asymmetries over the flux surface in densities and in the electrostatic potential, and the radial electric field in the plasma edge of tokamaks is presented.

ISSN:0899-8221    

DOI:10.1063/1.860881

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A class of nonaxisymmetric (∂/∂&thgr;≠0) rotating equilibria is investigated theoretically for strongly magnetized, low‐density (&ohgr;pe2/&ohgr;ce2≪1) pure electron plasma confined radially by a uniform axial magnetic fieldB0ezbetween concentric, perfectly conducting, cylindrical walls located at radiir=rwandr=rI≤rw. The analysis is based on a nonrelativistic, guiding‐center model in the cold‐fluid limit that treats the electrons as a massless fluid (me→0) withE×Bflow velocityVe=−(c/B0)∇&fgr;×ez. Assuming two‐dimensional spatial variations (∂/∂z=0), the continuity‐Poisson equations are analyzed for rotating coherent structures that are stationary (time independent) in a frame of reference rotating with angular velocity &ohgr;r=const about the cylinder axis (r=0). The equilibrium Poisson equation ∇2&psgr;=−4&pgr;e2ne(&psgr;)+2&ohgr;reB0/cis solved exactly for the particular case where the electron densityne(&psgr;) is alinearfunction of the streamfunction &psgr;=−e&fgr;+&ohgr;r(eB0/2c)r2, and the plasma fills the region between the conducting walls, withne=0 atr=rIandr=rw. It is found that this class of rotating equilibria can exhibit large‐amplitude, nonaxisymmetric, vortex structures characterized by strong azimuthal density bunching and circulating electron flow within the density bunches. Nonlinear stability properties are investigated using the Lyapunov method, and the vortex equilibria with azimuthal mode numberl=1 are shown to be stable.

ISSN:0899-8221    

DOI:10.1063/1.860882

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The linearized ion Fokker–Planck equation is solved as an eigenvalue problem under the condition of collisionless electrons in the quasineutral limit (&fgr;=0) for ionization‐temperature ratios,ZTe/Ti=2, 4, and 8 for entropy waves and ionization‐temperature ratios,ZTe/Ti=4, 8, 16, 32, 48, 64, and 80 for ion‐acoustic waves. The perturbed ion distribution function for the ion‐acoustic and entropy waves is formed from a Legendre polynomial expansion of eigenvectors and can be used to calculate collisionally dependent macroscopic quantities in the plasma such as gamma (&Ggr;=Cp/Cv), the ratio of specific heats, and the ion thermal conductivity (&kgr;i).

ISSN:0899-8221    

DOI:10.1063/1.860883

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The propagation and self‐focusing of short, intense laser pulses in a tenuous plasma is studied both analytically and numerically. Specifically, pulses of length of the order of a few plasma wavelengths and of intensity, which is large enough for relativistic self‐focusing to occur, are considered. Such pulses are of interest in various laser plasma acceleration schemes. It is found that these pulses are likely to be strongly affected by Raman instabilities. Two different regimes of instability, corresponding to large and small scattering angles, are found to be important. Small‐angle scattering is perhaps the most severe since it couples strongly with relativistic self‐focusing, leading the pulses to acquire significant axial and transverse structure in a time of the order of the self‐focusing time. Thus it will be difficult to propagate smooth self‐focused pulses through tenuous plasmas for distances longer than the Rayleigh length, except for pulse duration of the order of the plasma period.

ISSN:0899-8221    

DOI:10.1063/1.860884

出版商:AIP    

年代:1993

数据来源: AIP