摘要:

A calculation is presented that accounts for rapid nonlinear growth of them=1 kink‐tearing instability. The equilibrium analysis contained in the Rutherford theory [Phys. Fluids16, 1903 (1973)] of nonlinear tearing‐mode growth is generalized to islands for which the constant‐&psgr; approximation is not valid. Applying the helicity‐conservation assumption introduced by Kadomtsev [PlasmaPhysicsandControlledNuclearFusionResearch(IAEA, Vienna, 1977), Vol. I, p. 555], the presence of a current‐sheet singularity is shown that gives rise to a narrow tearing layer and rapid reconnection. This rapid reconnection, in turn, justifies the use of the helicity conservation assumption. The existence of a family of self‐similarm=1 equilibrium islands is demonstrated. The formalism introduced here is shown to apply both to the case of them=1 kink‐tearing mode and to the case of forced reconnection. These two cases are compared and contrasted.

ISSN:0899-8221    

DOI:10.1063/1.859172

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The semicollisional layer equations governing the linear stability of small mode number tearing modes in a low beta, large aspect ratio, tokamak equilibrium are derived from an expansion of the gyrokinetic equation. In this analysis only the cases where the ion Larmor radius is either much less than, or much greater than, the layer width are considered. Both the electrons and the ions are assumed to lie in the banana collisionality regime. One interesting feature of the derived layer equations, in the limit of small ion Larmor radius, is a substantial reduction in the effective collisionality of the system due to neoclassical ion dynamics. Next, using a shooting code, a dispersion relation is obtained from the layer equations in the limits of small ion Larmor radius and a vanishingly small fraction of trapped particles. As expected, strong semicollisional stabilization of the mode is found, but, in addition, a somewhat weaker destabilizing effect is obtained in the transition region between the collisional and semicollisional regimes.

ISSN:0899-8221    

DOI:10.1063/1.859173

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In the present paper, the parametric decay instability of an ion‐Bernstein wave (&ohgr;ci<&ohgr;0≲2&ohgr;ci) into a scattered ion‐Bernstein wave and a low‐frequency wave (either an ion‐acoustic wave or a kinetic‐Alfve´n wave) has been examined. It has been demonstrated that this parametric process may be one of the possible sources of ion heating in tokamaks. In addition to this, the possibility of oscillating two‐stream instability (OTSI) of an ion‐Bernstein wave (&ohgr;ci<&ohgr;0≲2&ohgr;ci) or a cold plasma ion‐Bernstein wave (&ohgr;0≲&ohgr;ci) has also been discussed. The explicit expressions for threshold and convective threshold of the decay processes are given. For OTSI, the expressions for maximum growth rate have also been given explicitly.

ISSN:0899-8221    

DOI:10.1063/1.859174

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The stability of global Alfve´n eigenmodes is investigated in the presence of super‐Alfve´nic energetic particles, such as fusion‐product alpha particles in an ignited deuterium–tritium tokamak plasma. Alpha particles tend to destabilize these modes when &ohgr;@B|&agr;>&ohgr;A, where &ohgr;Ais the shear‐Alfve´n modal frequency and &ohgr;*&agr;is the alpha particle diamagnetic drift frequency. This destabilization due to alpha particles is found to be significantly enhanced when the alpha particles are modeled with a slowing‐down distribution function rather than with a Maxwellian distribution. However, previously neglected electron damping due to the magnetic curvature drift is found to be comparable in magnitude to the destabilizing alpha particle term. Furthermore, the effects of toroidicity are also found to be stabilizing, since the intrinsic toroidicity induces poloidal mode coupling, which enhances the parallel electron damping from the sideband shear‐Alfve´n Landau resonance. In particular, for typical ignition tokamak parameters, global Alfve´n eigenmodes are found to be completely stabilized by either the electron damping that enters through the magnetic curvature drift or the damping introduced by finite toroidicity.

ISSN:0899-8221    

DOI:10.1063/1.859175

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

If the confining potential of electrostatically trapped electrons fluctuates, then the number of trapped electrons also fluctuates. The linear relationship between potential fluctuations and the number of trapped electrons is investigated, considering two loss mechanisms for a nearly collisionless plasma: (1) small‐angle collisions (diffusion), and (2) large‐angle collisions. This nearly collisionless model predicts the line tying of interchange modes in a mirror‐trapped plasma is orders of magnitude larger than previously thought for a fusion plasma, yet still not strong enough to completely line tie an axisymmetric mirror. Also, a nonlinearity in the response can occur at low amplitude.

ISSN:0899-8221    

DOI:10.1063/1.859176

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Electrical resistivity at the magnetic axis &eegr;(R) and at the separatrix &eegr;(rs) of a field‐reversed configuration (FRC) plasma produced by theta pinch machines are estimated from detailed interferometric measurements, assuming that the temperature is uniform within the separatrix. The ratiof0[=&eegr;(R)/&eegr;(rs)] increases with the beta value at the separatrix &bgr;s, which is consistent with the fact thatf0decreases whenxs(separatrix radius normalized by the inner radius of a theta pinch coil) increases [Phys. Fluids28, 888 (1985)]. This tendency is a natural consequence of the transport properties of the FRC plasma when the particle confinement time is nearly equal to the decay time of the trapped reversed magnetic flux, as is normally the case. Theoretical expectations of the anomaly of &eegr;(rs) over the classical resistivity &eegr;0increases and decreases slightly with &bgr;sfor the case of the lower hybrid drift and the low‐frequency drift instability, respectively. On the other hand, the observed &eegr;(rs)/&eegr;0decreases substantially with &bgr;s. [The value of &eegr;(rs)/&eegr;0changes from about 15 to 5 as &bgr;schanges from 0.45 to 0.6.]

ISSN:0899-8221    

DOI:10.1063/1.859177

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Using exact analytical solutions, it is shown that two recent models of nonlocal electron heat transport are beset with mathematical anomalies leading to unphysical results. First, heat flow with a nonlocal heat flux does not smooth out steep temperature gradients in any finite time. Second, a computation of the thermoelectric field from a vanishing nonlocal current flux is an ill‐posed problem leading to instabilities that violate the very assumption on which the model is based. It is verified that these anomalies can lead to a negative entropy production rate, which implies local thermodynamic instability. In particular, it is proved that a temperature distribution that is initially positive can later become negative for certain nonuniform electron density distributions. These results provide a basic understanding of the various difficulties encountered in numerical implementations of these models.

ISSN:0899-8221    

DOI:10.1063/1.859178

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The effect of induced spatial incoherence (ISI) on laser light self‐focusing is investigated using a two‐dimensional Eulerian hydrodynamic plasma simulation code. In homogeneous low‐density plasmas (one‐tenth of the critical density for radiation of wavelength &lgr;L=0.25 &mgr;m) it is found that ISI can effectively eliminate thermal and ponderomotive self‐focusing for a wide range of intensities. In plasma containing an initial linear gradient in density, strong self‐focusing occurs when the maximum intensity in the incident lightI0(whereI0represents the intensity that would be achieved for a perfectly coherent beam) is allowed to approach 1017W cm−2. At lower intensities,I0∼1016W cm−2, thermal self‐focusing is eliminated and ponderomotive self‐focusing is significantly reduced. The dwell time of the filamented light varies from the laser light coherence time to a few tens of picoseconds.

ISSN:0899-8221    

DOI:10.1063/1.859179

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Resonance absorption ofp‐polarized light, incident at angle &thgr; on a flowing, stratified plasma, is analyzed; profile steepening within (i) a layer around the turning point, and (ii) a thinner, embedded sublayer at the critical surface is taken into account self‐consistently. The entire steepened region is taken as collisionless and isothermal. The structure of the main layer shows a variety of regimes, depending on how the flow crosses a sonic point. The structure of the sublayer is also determined; it isentirelysubsonic(with no wave breaking) for a well‐defined, broad parameter range. Density changes across both layer and sublayer, and fractional absorption, are given in terms of [(wavelength)2×intensity/temperature], &thgr;, and (temperature/mec2). The flow outside the double structure is also analyzed for particular conditions.

ISSN:0899-8221    

DOI:10.1063/1.859180

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Modeling results are presented for a hydrogenlike fluorine soft x‐ray amplifier experiment performed at Rutherford Appleton Laboratory [Plasma Phys. Controlled Fusion30, 35 (1988)] in which a gain coefficient of 4.4 cm−1was observed. Simulations using various values of the flux limiter,f(used to inhibit heat transport), and the amount of resonance absorption, RA, are compared to the experimental results. The maximum calculated gain coefficient is 2 cm−1forf=0.1 and RA=1%. For this choice of hydrodynamic modeling parameters the dependence of gain on intensity is in qualitative agreement with the experimental data. It is shown that the experiment can be modeled adequately in one dimension by comparing to a two‐dimensional calculation. The use of escape probabilities in an expanding cylindrical medium is discussed. General modeling techniques useful in modeling cylindrically symmetric systems are presented.

ISSN:0899-8221    

DOI:10.1063/1.859181

出版商:AIP    

年代:1989

数据来源: AIP