摘要:

A current drive scheme for tokamaks, involving oscillating the Ohmic heating (OH) circuit, is proposed. This scheme is based upon the experimentally observed anomalous skin penetration during the start‐up phase of tokamaks. Analytic and numerical results indicating feasibility are shown and scaling of steady‐state current and power dissipated with frequency and plasma resistivity is presented. An explanation is given of why this scheme should work, in spite of the fact that the external circuit supplies no magnetic helicity. This scheme is potentially more efficient in terms of current generated per unit power dissipated than other current drive schemes. It is shown that a resistive magnetohydrodynamic (MHD) activity responsible for anomalous current penetration, and therefore required for the success of this current drive scheme, must be fully three dimensional. Finally, a comparison of the effectiveness of this scheme with that of F‐&THgr; pumping in reversed‐field pinches (RFP’s) is presented.

ISSN:0031-9171    

DOI:10.1063/1.866083

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

In this work, a nonlinear coupling mechanism, in which the mode‐converted kinetic Alfve´n waves beat with each other to resonantly modify the resistive tearing mode characteristics, has been investigated. Using the variational as well as the asymptotic methods, it is shown that newm=1 andm=2 tearing instabilities are excited with significantly enhanced growth rates varying as fractional powers of the pump amplitude. For rather moderate Alfve´n wave intensity and a typical choice of tokamak parameters, the calculated growth rates turn out to be large and fall in the range 104–106sec−1.

ISSN:0031-9171    

DOI:10.1063/1.866084

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The spontaneous formation of magnetohydrodynamic (MHD) equilibrium profiles in toroidal current‐carrying plasmas has been discussed for tokamaks, reversed‐field pinches (RFP’s), and also for their intermediate states ultra‐lowq’s (ULQ’s). The significance ofm(poloidal mode number)=1 fluctuations causing a shift of the magnetic axis is stressed. The characteristics of MHD relaxation have been studied in comparison with classical diffusion. The dynamics of ULQ’s is shown to illustrate the characteristics of both dissipative processes. A global‐mode stable ULQ is observed to be produced through MHD relaxation, and then to be deformed, in a longer time scale, by classical diffusion which destabilizes the stable profile.

ISSN:0031-9171    

DOI:10.1063/1.866085

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The thermal stability of a resistive, force‐free plasma in a two‐dimensional, slab geometry is investigated numerically. Linearly unstable modes that arise as an energetic consequence of Ohmic heating are examined in relation to the dynamic magnetic‐tearing instability. Plasma compression is found to give rise to a set of unstable thermal modes not seen in previous incompressible studies. Such modes can occur in parameter regimes where no purely growing tearing mode exists.

ISSN:0031-9171    

DOI:10.1063/1.866086

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Kinetic stability properties of an intense relativistic electron ring in modified and conventional betatron configurations are investigated using the linearized Vlasov–Maxwell equations. Included is the important influence of intense equilibrium self‐fields. It is assumed that the ring is thin, and that &ngr;/&ggr;b≪1, where &ngr; is Budker’s parameter and &ggr;bmc2is the characteristic electron energy. The stability analysis is carried out for eigenfrequency &ohgr; close to harmonics of the cyclotron frequency &ohgr;czin the vertical betatron field. Also included in the analysis is the influence of transverse electromagnetic effects and surface‐wave perturbations. Dispersion relations for longitudinal perturbations are obtained, where it is assumed that the ring is located inside a perfectly conducting toroidal shell. There are several noteworthy points. First, transverse electromagnetic effects can completely stabilize the negative‐mass instability for sufficiently high‐current rings when betatronfocusing forces exceed defocusing self‐field forces (&ohgr;2cz>&ohgr;2pe/&ggr;2b). Second, for &ohgr;2cz>&ohgr;2pe/&ggr;2bwith no charge neutralization or stabilizing spread in canonical angular momentum ( f=0 and &Dgr;=0), surface‐wave instabilities can be completely stabilized at a sufficiently low transverse beam temperature. Third, for &ohgr;2cz<&ohgr;2pe/&ggr;2b, sufficiently low transverse temperature together with surface effects combine to drive a radial kink instability. Finally, the dispersion relation is analyzed numerically for parameters representative of the Naval Research Laboratory’s modified betatron (National Technical Information Service Document No. AD‐A108359/1) and the Los Alamos National Laboratory’s Liner Driven Ring Accelerator (National Technical Information Service Document No. DE84007994) and Phermex Injected Conventional Betatron (National Technical Information Service Document No. DE86002441/XAB). Detailed stability results are presented for the projected operating regimes of these devices, including the effects of canonical angular momentum spread, inverse aspect ratio, slowly varying accelerating fields, location of the conducting wall, and radial elongation of the minor ring cross section.

ISSN:0031-9171    

DOI:10.1063/1.866087

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A 3‐D theory is presented for a free electron laser that employs an electron beam of a thickness comparable to both the wiggler wavelength and the waveguide radius. The time‐independent and the linearized time‐dependent cold fluid and Maxwell equations are expanded in a small parameter, which is the ratio of the perpendicular to parallel electron momentum. The stability problem is reduced to a nonlinear eigenvalue problem of a fourth‐order system of linear ordinary differential equations. A perturbation method is justified and used to solve these equations. A dispersion relation is derived which results from the solvability condition for the first‐order equations in the perturbation. The orders of magnitude of the beam density and wave frequency, for which the growth rate of the instability scales as in the strong‐pump regime of the 1‐D analysis, are determined. An equation, which the beam energy radial profile has to satisfy, is also derived.

ISSN:0031-9171    

DOI:10.1063/1.866088

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the interaction of collisionless shocks. The interaction between pairs of sub‐ and supercritical shocks are studied for a wide range of parameters relevant to both the solar and interplanetary plasma. These parameters include the shock Mach number, the electron and ion plasma betas, and the angle between the shock normal and the magnetic field. In the collision of two subcritical quasiperpendicular shocks, the shock electromagnetic field structures pass through each other, but do not carry the plasma associated with them. When unequal shocks collide, there is no evidence of the contact discontinuity predicted by fluid theory. In the collision of supercritical quasiperpendicular shocks, significant acceleration of ions occurs. This acceleration arises when ions reflected at one shock encounter the electric fields of the other shock and takes place in a number of distinct phases. For a wide range of parameters, the maximum ion energy is roughly an order of magnitude larger than the ram kinetic energy of the colliding shocks. This ion energization will subsequently manifest itself as a high ion to electron temperature ratio in the doubly shocked plasma. Finally, it was found that the values of the Mach number and angle between the shock normal and magnetic field play a significant role in the collisions. For shock geometries significantly different from perpendicular, ions from each oncoming shock can escape upstream and undergo considerable acceleration.

ISSN:0031-9171    

DOI:10.1063/1.866089

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

When injected across an ambient magnetic field, a stream of collisionless plasma polarizes, and the resulting polarization electric field may allow the stream to penetrate the magnetic field. The polarization charge spreads from the stream sides along the magnetic‐field lines and any ambient plasma that is magnetically connected to the stream will be picked up to form Alfve´n wings of co‐moving plasma. As the stream and wing plasmas move across the magnetic field, their outer edges are continually stripped off. Because of these processes, the stream must expend charge, momentum, and kinetic energy in order to continue its cross‐field motion. These expenditures derive from five requirements of charge from the stream: charge is required (1) to polarize the stream plasma, (2) to replace the charge stripped from the stream’s edges as it propagates, (3) to set up the Alfve´n wings on the stream, (4) to replace the charge stripped from the sides of the wings as they propagate, and (5) to supply any conduction currents in the ambient plasma. By using idealized stream configurations, approximate limits on the propagation distance are obtained.

ISSN:0031-9171    

DOI:10.1063/1.866090

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The formation and equilibrium of a high &bgr; tokamak in the Columbia High Beta Tokamak (HBT) (Plasma Physics and Controlled Nuclear Fusion Research 1986, in press) is documented using magnetic probes as the principle diagnostic. The formation from a toriodal field stabilized Z pinch is simulated with 2‐D resistive magnetohydrodynamic (MHD) equations. From the measurement of the magnetic field by high‐impedence internal magnetic probes, the poloidal flux, toroidal current, and safety factor are calculated. In addition, the plasma position and cross‐sectional shape are determined. The tokamak has a major radius of about 0.24 m, a minor radius of about 0.05 m, and a toroidal current density of about 106A/m2. The high &bgr; tokamak plasma is observed to have an outward shift in major radius of both the magnetic center and the peak of the toroidal current density. The magnetic center moves inward in major radius after 20–30 &mgr;sec, presumably because the plasma maintains major radial equilibrium as its pressure decreases because of the radiation of impurity atoms.

ISSN:0031-9171    

DOI:10.1063/1.866091

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Axial and radial plasma dynamics in the CO2laser‐heated solenoid have been studied experimentally and numerically. The axial behavior is found to be well described by a self‐regulated bleaching wave model. The radial expansion is found to be strongly dependent on the focusing ratio of the input laser beam. With a fast focus ( f/5), the early radial expansion rate is twice that found with a slower focusing arrangement ( f/15). The faster focusing ratio also results in a significantly wider plasma column. On the other hand, no significant dependence off/♯ on the axial propagation was found. A finite ionization time and the rapid formation of a density minimum on axis are observed and verify earlier experimental results. Detailed comparisons are made with a 2‐D magnetohydrodynamic (MHD) and laser propagation code. The axial and radial plasma behavior and, in particular, the dependence of the radial behavior on the focal ratio of the laser are reasonably well supported by the simulation results. Computational results are also in good agreement with experimental measurements of temperature and density using stimulated scattering (Brillouin, Raman) and interferometry diagnostic techniques.

ISSN:0031-9171    

DOI:10.1063/1.866092

出版商:AIP    

年代:1987

数据来源: AIP