摘要:

The density limit observed in tokamak experiments is thought to be due to a radiative collapse of the current channel. A transport code coupled with a magnetohydrodynamic (MHD) equilibrium routine is used to determine the detailed, self‐consistent evolution of the plasma profiles in tokamak discharges with radiated power close to or equaling the input power. The present work is confined to Ohmic discharges in steady state. It is found that the shape of the density profile can have a significant impact on the variation of the maximum electron density with plasma current. Analytic calculations confirm this result.

ISSN:0031-9171    

DOI:10.1063/1.866890

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Hot alpha particles and thermalized helium ash particles in tokamaks can have significant effects on high toroidal mode number instabilities such as the trapped‐electron drift mode and the kinetically calculated magnetohydrodynamic ballooning mode. In particular, the effects can be stabilizing, destabilizing, or negligible, depending on the parameters involved. In high‐temperature tokamaks capable of producing significant numbers of hot alpha particles, the predominant interaction of the mode with the alpha particles is through resonances of various sorts. In turn, the modes can cause significant anomalous transport of the alpha particles and the helium ash. Here, results of comprehensive linear eigenfrequency–eigenfunction calculations are presented for relevant realistic cases to show these effects.

ISSN:0031-9171    

DOI:10.1063/1.866891

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

HereZeffprofiles have been obtained across the entire minor radius of the DIII‐D [Fusion Technol.8, 441 (1985);PlasmaPhysicsandControlledNuclearFusionResearch,1986(IAEA, Vienna, 1987), Vol. 1, p. 159] tokamak from measurements of visible bremsstrahlung continuum radiation. TheZeffprofiles are presented for Ohmic divertor and limiter plasmas as well as for neutral beam heated H‐mode and L‐mode plasmas. The measuredZeffprofiles are found to have small gradients over the entire plasma cross section. This implies that resistivity gradient driven electrostatic modes are unlikely to influence the confinement properties in the edge region of these discharges. The H‐mode plasma has a centrally peakedZeffprofile and a relatively flat electron density profile that suggests the existence of a negative value for the parameter &eegr;i.

ISSN:0031-9171    

DOI:10.1063/1.866892

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion22, 549 (1982);PlasmaPhysicsandControlledNuclearFusionResearch,1986,Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio‐frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak &bgr;⊥of 3%, density of 4×1012cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and microstability.

ISSN:0031-9171    

DOI:10.1063/1.866893

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Recent data from the FRX‐C [Proceedingsofthe7thSymposiumonthePhysicsandTechnologyofCompactToroids(Los Alamos National Laboratory, Los Alamos, NM, 1986), p.142] and FRX‐C/LSM [Proceedingsofthe14thEuropeanConferenceonControlledFusionandPlasmaPhysics(European Physical Society, Madrid, 1987), p. 485] field‐reversed theta pinches are presented concerning heating, internal flux, and particle retention after formation. These results extend the data base of field‐reversed configuration plasmas to larger dimensions. Good agreement is found for collisionless plasmas with some existing formation laws [Fusion Technol.9, 48 (1986)] and the main result is confirmation of improved internal flux retention for plasmas of larger radius. Some simple semiempirical laws are proposed, which allow a global analytical formation model. This model should provide reasonable estimates of the equilibrium parameters of field‐reversed configurations formed in theta pinches.

ISSN:0031-9171    

DOI:10.1063/1.866894

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A numerical study, employing the particle coderinga[Plasma Phys.23, 521 (1981)], of two ion rings that merge with each other due to their mutual magnetic attraction is presented. It is shown that an ion ring with closed magnetic field lines (field reversed) can be formed by merging two weaker non‐field‐reversed rings. A rough calculation shows that there is an optimal plasma conductivity &sgr;≊c2/4&pgr;aR&OHgr;&zgr;1/2(aandRare the respective axial width and major radius of the ring, &OHgr; is the ion cyclotron frequency in the external magnetic field, and &zgr; is the field reversal factor of the weaker ring) for which most of the energy of mutual magnetic attraction between the rings is dissipated during the process of merging.

ISSN:0031-9171    

DOI:10.1063/1.866895

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Electron acceleration by an intense electromagnetic wave incident obliquely to the ambient magnetic field in a plasma is considered. It is shown that the wave amplitude has a stochasticity inducing threshold. Although substantial acceleration can be achieved below this threshold, it is limited to the trapping width of a single resonance. Above threshold, the electrons can accelerate from resonance to resonance. It is also shown that when the parallel phase velocity of the electromagnetic waves is supraluminous, the Hamiltonian surfaces describing the wave–particle interaction are topologically open. In this case, electrons gain large amounts of energy. The mechanism is quite robust, being weakly sensitive to changes in the wave parameters and the electron’s initial energy; as a result, it is an ideal mechanism for space applications. As an application, the conditions are obtained for accelerating electrons to energies of several MeV using ground‐based transmitters.

ISSN:0031-9171    

DOI:10.1063/1.866896

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The propagation of plasmoids (neutralized ion beams) in a vacuum transverse‐magnetic field has been studied in the University of California, Irvine laboratory for several years [Phys. Fluids24, 739 (1981);25, 730, 2353 (1982);26, 2276 (1983); J. Appl. Phys.64, 73 (1988)]. These experiments have confirmed that the plasmoid propagates by theE×Bdrift in a low beta and high beta plasmoid beam (0.01<&bgr;<300), where &bgr; is the ratio of beam kinetic energy to magnetic field energy. The polarization electric fieldEarises from the opposite deflection of the plasmoid ions and electrons, because of the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments, plasmoids (150 keV, 5 kA, 50–100 A/cm2, 1 &mgr; sec) were injected into transverse fields ofBt=0–400 G. Anomalously fast penetration of the transverse magnetic field has been observed as in the ‘‘Porcupine’’ experiments [J. Geophys. Res.91, 10,183 (1987)]. The most recent experiments are aimed at studying the plasmoid propagation dynamics and losses in the presence of a background, magnetized plasma which is intended to short the induced polarization electric field and stop the beam. Background plasma was generated by TiH4plasma guns fired alongBtto produce a plasma density,np=1012−1014cm−3. Preliminary results indicate that the beam propagation losses increase with the background plasma density; compared to vacuum propagation, roughly a 50% reduction in ion current density was noted 70 cm downstream from the anode fornp∼1013cm−3. Principal diagnostics include magnetically insulated Faraday cups, floating potential probes, calorimeters, microwave interferometer, and thermal‐witness paper.

ISSN:0031-9171    

DOI:10.1063/1.866897

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The effect of an external magnetic field on the various possible parametric instabilities of the longitudinal beat wave at the difference frequency of two incident laser beams in a hot plasma has been theoretically investigated. The kinetic equation is employed to obtain the nonlinear response of the magnetized electrons caused by the nonlinear coupling of the beat wave with the low‐frequency electrostatic plasma modes. It is noted that the growth rates of the three‐wave and the four‐wave parametric instabilities can be influenced by the external transverse magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.866845

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A previous model [Phys. Fluids31, 658 (1988)] has been generalized and modified to describe ion beam interaction with planar foils in which the energy absorption region is composed of two materials. The model provides a unified description of foils with thicknesses larger and smaller than the ion range (super‐range and subrange foils). The hydrodynamic efficiency of the unheated part of the foil, the payload, is studied for different foil structures and a set of parameters that determine the dynamics of the interaction is found.

ISSN:0031-9171    

DOI:10.1063/1.866846

出版商:AIP    

年代:1988

数据来源: AIP