摘要:

The toroidicity‐induced shear Alfve´n eigenmode is found to be destabilized by fusion alpha particles in an ignited tokamak plasma.

ISSN:0899-8221    

DOI:10.1063/1.859057

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The ambiguity in boundary conditions for the Darwin model is considered in depth. While no single boundary condition seems dictated by physical considerations alone, one set of boundary conditions is presented which has several desirable properties. These include a minimum error principle, numerical tractability, a well‐behaved Poynting theorem, and a conservation law for the boundary charge density. The consequences of the boundary conditions are worked out for several simple geometries which are common in computational work.

ISSN:0899-8221    

DOI:10.1063/1.859058

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The behavior of sidebands growing in the presence of a launched large amplitude wave in a beam–plasma system is studied numerically. The large amplitude wave traps the beam and both the upper and the lower sidebands become unstable. The behavior of the instability is found to be dependent on a phase relationship that exists between the two sidebands and the large amplitude wave. The essential features of the instability are well described by a simple model in which the trapped electrons are replaced by a single particle oscillating in each well of the large amplitude wave.

ISSN:0899-8221    

DOI:10.1063/1.859059

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

High‐resolution numerical studies of decaying two‐dimensional magnetohydrodynamic turbulence were performed with up to 10242collocation points in general periodic systems using various initial states, but restricting consideration to weak velocity‐magnetic field correlation &rgr;. The global evolution is self‐similar with constant kinetic to magnetic energy ratioEV/EM, macro‐ and microscale Reynolds numbers, and correlation &rgr;, while the total energy decays asE(t)∝(t+t0)−1. As in three dimensions, dissipative small‐scale turbulence adjusts in such a way as to make the energy dissipation rate &egr; independent of the collisional dissipation coefficients. Normalized energy spectra are also invariant. The spectral index in the inertial range is, in general, close to 3/2 in agreement with Kraichnan’s Alfve´n wave argumentEk=DB1/2&egr;1/2k−3/2,B=(EM)1/2,D&bartil;1.8±0.2, but may be close to 5/3 in transient states, in which turbulence is concentrated in regions of weak magnetic field. In the dissipation range, intermittency gives rise to a modified dissipation scaleleff=(l2&lgr;)1/3, withl=Kolmogorov scale and &lgr;=Taylor microscale. This reflects the intermittency of the dissipation process, which is consistent with the picture of current microsheets of thicknessland width and spacing &lgr;.

ISSN:0899-8221    

DOI:10.1063/1.859060

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In this paper a theory of weak ion temperature gradient‐driven turbulence near the threshold of instability is presented. The model considers kinetic ions and adiabatic electrons in a sheared slab geometry. Linear theory shows that for &eegr;th<&eegr;i≲&eegr;th+(1+Ti/Te)Ln/Ls(where &eegr;th=0.95 is the instability threshold andLn/Ls≪1) then &ggr;≪&ohgr; and a weak turbulence theory applies. The nonlinear wave kinetic equation indicates that ion Compton scattering is the dominant nonlinear saturation process, and it is shown that the energy scatters to the linearly stable lowkymodes. The resulting fluctuation spectrum (peaked aboutk⊥&rgr;i&bartil;1) is much lower than that suggested by naive extrapolation from the strong turbulence regime. The resulting ion thermal conductivity is also extremely low, so that strong ion heating can be expected to drive the ion temperature gradient to a level where this weakly turbulent threshold regime is surpassed. Thus the critical &eegr;irelevant to magnetic confinement experiments is not the linear instability threshold but the point where the diminutions of the weak turbulence regime vanish, and the transport increases to the strong fluid turbulence level.

ISSN:0899-8221    

DOI:10.1063/1.859061

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The theory of ion temperature gradient‐driven turbulence in tokamaks is extended to the flat density regime. The values of ion and electron thermal conductivities, &khgr;iand &khgr;e, momentum diffusivity &khgr;&Jgr;, and particle flux &Ggr;rare also calculated. These formulas extend previous calculations, which were restricted to the regimeLn<(LsLT)1/2[Phys. Fluids29, 3291 (1986); Phys. Fluids31, 1180 (1988)]. This allows an assessment of the role of ion temperature gradient turbulence in H modes, where the density gradient is often observed to be flattened in the plasma core.

ISSN:0899-8221    

DOI:10.1063/1.859062

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A general dispersion equation, which describes the parametric instabilities of some electrostatic waves in a weak/moderate magnetic field, is derived. The dispersion equation is solved numerically to study the effects of a moderately intense magnetic field on the parametric instabilities of relatively large amplitude Langmuir waves. A Langmuir pump wave can couple with various low‐frequency branches such as lower‐hybrid, electron‐acoustic, ion‐cyclotron, and ion‐acoustic branches. The transverse modulational instability is most sensitive to a magnetic field. Even for a Langmuir wave withW¯∼0.1, whereW¯ is the ratio of the time‐averaged wave energy to the electron thermal energy, some portions of the transverse modulational instability in the direction exactly perpendicular to the external magnetic field are quite easily suppressed. However, some oblique instabilities may be enhanced by a magnetic field. In general, a moderate magnetic field suppresses most of the oblique instabilities, and the parametric instabilities become essentially one dimensional.

ISSN:0899-8221    

DOI:10.1063/1.859063

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Three electron‐cyclotron‐heating techniques, which preferentially couple microwave power to different energy segments of the electron distribution function, have been experimentally investigated in the AMPHED facility [C. Bodeldijk, Special Supplement, Nucl. Fusion26, 184 (1986)]. Whistler waves launched from the high‐field mirror throat are strongly absorbed in a single pass across the resonant interaction layer, producing highly overdense cold background plasma but no relativistic hot particles. On the other hand, ordinary waves launched from the system side wall are only weakly damped, giving rise to local cylindrical cavity modes and preferential coupling to hot electrons in the 100 keV region. Low levels (≤5%) of upper‐off‐resonance heating power were shown to be most effective for preferential hot‐electron plasma formation, with ∼100% of the injected power being absorbed by the energetic electrons.

ISSN:0899-8221    

DOI:10.1063/1.859064

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The conditions are determined under which the damping of the fast Alfve´n wave, used in the heating of tokamak plasmas, can be described by the geometrical optics approximations. Also, for these conditions, an analytic formula is derived, which explicitly determines the amount of energy that is damped onto the plasma particles by the fast wave. The conditions for the validity of the geometrical optics approximations cover a wide range ofk∥’s (the component of the wave vector parallel to the total magnetic field). There is a narrow range ofk∥’s over which this is not valid; this is the regime where mode conversion is dominant. The results from this theory are in good agreement with those obtained from numerical solutions of fourth‐ and sixth‐order differential equations that are commonly used to describe this type of heating. However, this theory has a distinct advantage of lending itself to detailed scaling studies as explicit expressions for the damping of the fast Alfve´n wave are obtained.

ISSN:0899-8221    

DOI:10.1063/1.859065

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Using the technique first described by Robinson and Rusbridge [Plasma Phys.11, 73 (1969)] electron temperature fluctuations have been measured in the scrape‐off layer of the TEXT tokamak [Nucl. Technol. Fusion1, 479 (1981)] to beT˜e/Te&bartil;0.4n˜e/ne. Particle and energy transport were estimated from ⟨n˜e E˜&thgr;⟩ and ⟨T˜e E˜&thgr;⟩ correlations. The fluctuation‐driven particle transport was consistent with earlier measurements and with measurements of global particle confinement. A high correlation of temperature and electric field fluctuations and a low but positive correlation between density and temperature fluctuations were observed.

ISSN:0899-8221    

DOI:10.1063/1.859066

出版商:AIP    

年代:1989

数据来源: AIP