摘要:

In a static magnetic field, some well‐known guiding center equations maintain their form when extended to next order in gyroradius. In these cases, it is only necessary to include the next order term in the magnetic moment series. The differential equation for guiding center motion which describes both the parallel and perpendicular velocities correctly through first order in gyroradius is given. The question of how to define the guiding center position through second order arises and is discussed, and second order drifts are derived for one usual definition. The toroidal canonical angular momentum,P&fgr;, of the guiding center in an axisymmetric field is shown to be conserved using the guiding center velocity correct through first order. When second‐order motion is included,P&fgr;is no longer a constant. The above extensions of guiding center theory help to resolve the different tokamak orbits obtained either by using the guiding center equations of motion or by using conservation ofP&fgr;.

ISSN:0031-9171    

DOI:10.1063/1.862226

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A new formulation for determining the two‐fluid equilibrium of an axially symmetric plasma is presented. The inertial force due to plasma motion is taken into account in the form of a modified electrostatic potential (due to the centrifugal force) and a modified magnetic field (due to the Coriolis’ force). A set of partial differential equations which determines the modified flux functions and the self‐consistent electric and magnetic fields is derived under the assumption that the pressure is isotropic and is a function of the density and the flux function from an equation of state. The result is also formulated in the form of a variational principle which contains no constraints and hence is useful for numerical and perturbational analyses.

ISSN:0031-9171    

DOI:10.1063/1.862227

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Backscattered laser radiation has been observed from a CO2laser‐produced and heated underdense plasma in a steady solenoid magnetic field, and is attributed to the stimulated Brillouin scattering process. Spectral measurements and observations of the dependence of the backscattered intensity on laser power are presented to support this identification. Backscattered radiation, amounting to as much as 1% of the incident power, was found to be redshifted by 80–100 A˚, the shift expected for stimulated Brillouin scattering from a plasma at the measured temperatureTe=80±25 eV. Data are also presented on the dependence of backscattered power on the density and convergence angle of incoming radiation.

ISSN:0031-9171    

DOI:10.1063/1.862238

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The influence of electron trapping on a large amplitude plasma oscillation driven by the nonlinear interaction of two electromagnetic waves (stimulated Raman scattering) is studied analytically and by means of numerical simulation. When the plasma oscillation is resonantly excited to sufficiently large amplitude and electron trapping occurs, there ensues considerable modification of the electron velocity distribution function. The stimulated scattering ceases to be a resonant three‐wave process but continues as induced scattering by resonant electrons (stimulated Thomson scattering). The scattering of the electromagnetic waves becomes slower, but does not terminate with the onset of particle trapping in the beat wave.

ISSN:0031-9171    

DOI:10.1063/1.862239

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Theory and experiments are described for the evolution of the energy distribution of a group of test electrons interacting with a background plasma through electron‐electron (e‐e), electron‐ion (e‐i), and elastic electron‐neutral (e‐n) collisions. The experiments were performed on the afterglow of a pulsed microwave discharge (1010<ne<1012cm−3,Te∼0.2 eV). The test electrons (4<E<14 eV) were injected into the plasma as a feeble quasi‐monoenergetic beam. For the electrons, thee‐nmean‐free path &lgr; was much smaller than the length of the plasmaL, while thee‐emean‐free path was much greater. Nonetheless, because of the large neutral‐to‐electron mass ratio, the dynamic friction and smaller diffusion‐in‐speed were determined by Coulomb collisions, while frequente‐ncollisions maintained the distribution nearly isotropic. As this distribution gradually advanced along the discharge, it was shifted downward in mean energy due to dynamic friction, and broadened because of the distribution of friction path lengths, and because of diffusion‐in‐energy. The Boltzmann equation governing the beam electrons was reduced, by taking advantage of the smallness of &lgr;/Landm/M, to a simple partial differential equation for the distribution function which was solved analytically. The remarkably close agreement between experiment and theory, including the lack of sensitivity of the latter to all parameters save plasma density and initial test particle energy, suggests wide applicability for the concepts examined here.

ISSN:0031-9171    

DOI:10.1063/1.862240

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Earlier renormalized theories for collisionless drift wave turbulence were deficient in several respects when &ohgr;<kzviand the principal perpendicular motion is the E×B drift. The earlier theories did not preserve the perpendicularity between the E×B ion current and the electric field E and consequently, did not conserve energy. In addition, earlier theories did not correctly treat the caseni∼e&fgr;/Tin which the nonlinear convective term ‐&fgr;×B⋅‐nishould be very small. These problems are analyzed and corrected in a new version. The new theory predicts that drift wave energy is converted into parallel ion energy, producing a damping rate &ggr;∥=−(kzvi/&ohgr;)2k⊥2D, whereDis a test particle diffusion coefficient. For a fluctuating potential &fgr; due only to drift waves,Dis greater than zero only when the thresholde&fgr;/T∼ (k⊥Ln)−1for ion stochastic motion is exceeded. The nonlinear stabilization will occur slightly above this threshold if &ggr;e/&ohgr;< (kzvi/&ohgr;)2(&ggr;eis the linear growth rate). For this case the density diffusion coefficientDn(which is less thanD) will beDn∼&ggr;e/k⊥2.

ISSN:0031-9171    

DOI:10.1063/1.862241

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Low frequency plasma density fluctuations have been observed in the common flux volume of the dc octopole. For collision frequencies comparable to the fluctuation frequency (which is close to the diamagnetic drift frequency), the fluctuations are identified as dissipative trapped electron modes. In the high collision frequency range the collisional drift wave is observed. A strong quenching of the trapped electron modes is observed when the local safety factor is a low‐order rational number. Very little transport is associated with the observed fluctuations.

ISSN:0031-9171    

DOI:10.1063/1.862242

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The fraction &Dgr; of warm plasma needed to stabilize the drift cone mode is calculated for a loss cone plasma slab in a uniform magnetic field withRp∼aH∼k−1, whereRpis the width of the slab (representing the radial scale length),aHis the hot ion Larmor radius, andkis the wavenumber of the mode. For 2XIIB‐like parameters, a minimum &Dgr; of 14% is needed for stability, which is consistent with experimental observations. This result is an improvement on previous calculations, which used the local approximation (assumingRp≫aH≫k−1) and gave a minimum &Dgr; of 10% for the same parameters. Information is also obtained on the radial location and width of the normal modes, and on the effect of varying the warm plasma radial density profile.

ISSN:0031-9171    

DOI:10.1063/1.862243

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The excitation of the ion cyclotron wave and the harmonic ion cyclotron waves by injection of a neutral beam is studied as a function of beam injection energy and angle. The fundamental mode is found to be stable for parameters similar to the current and future neutrally injected tokamaks. The harmonic cyclotron waves are found to have growth rates of a few percent of the ion cyclotron frequency when the thermal ion temperature is below a critical level. The results of these calculations are compared to several current experiments.

ISSN:0031-9171    

DOI:10.1063/1.862244

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A comparative study is presented of the axial and azimuthal bunching mechanisms of the electromagnetic electron cyclotron instability. Axial bunching can be described with a nonrelativistic treatment, but azimuthal bunching is relativistic in origin. As is well known, the axial bunching mechanism drives the Weibel‐type instability while the azimuthal bunching mechanism drives the electron cyclotron maser instability. For an electron ensemble of cold helical trajectories, a unified physical interpretation of both instabilities is presented. It is shown that the two bunching mechanisms are actually simultaneously present in either instability andcompetewith one another. As a result, the dominant mechanism determines the type of instability. A criterion for distinguishing the two types of instabilities is derived. It is shown that theenergyof the electrons plays an insignificant role in the criterion and, hence, should not be a factor in the justification of a nonrelativistic treatment. Regimes of validity of nonrelativistic models are defined, however. Applications to gyrotron experiments are discussed.

ISSN:0031-9171    

DOI:10.1063/1.862245

出版商:AIP    

年代:1978

数据来源: AIP