摘要:

A radially movable detector located just below the outer midplane of the Tokamak Fusion Test Reactor (TFTR) [inPlasmaPhysicsandControlledNuclearFusionResearch, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 27] has been used to study the confinement of charged fusion products (CFP’s). The scrape‐off length of escaping CFP flux was measured using perturbative techniques based on shadowing the detector with different obstacles. Experimental results indicate the presence of a diffusive process for trapped particles. Derived diffusion step sizes were found to be in excellent agreement with those expected from toroidal magnetic field (TF) stochastic ripple diffusion.

ISSN:0899-8221    

DOI:10.1063/1.860895

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A simple modification to the heliac coil configuration [A. H. Boozeretal.,PlasmaPhysicsandControlledNuclearFusion(International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] is described that reduces the toroidal perturbation to the magnetic field structure and significantly restores the helical symmetry. This is accomplished by shifting the internal current‐carrying conductor (hardcore) of the heliac radially inward from its normal position at the geometric center of the externall=1 coils. It is shown by computations that a large‐aspect‐ratio toroidal heliac is similar to a helically symmetric linear heliac in which the hardcore has been shifted. This similarity is seen in the analytic form of the magnetic potential, the flux surface shape, and in the harmonic spectra of the field lines. It is then shown that the toroidicity perturbations can be reduced in a toroidal heliac by combining these effects—using a radially inward hardcore shift in the toroidal configuration. In the experiments reported here the toroidal‐like effects are created in the linear High Beta Q Machine (HBQM) heliac [C. M. Greenfieldetal., Phys. Fluids B2, 133 (1990)] by shifting the hardcore off the geometric center of thel=1 coils. The induced toroidal‐like effects are seen from an asymmetry in the axial excluded flux; however, no decrease in global beta is seen as compared to the unshifted symmetric case (G. G. Spanjers, Ph.D. dissertation, University of Washington, 1992) indicating the absence of large‐scale magnetohydrodynamic instabilities at high beta.

ISSN:0899-8221    

DOI:10.1063/1.860896

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A heavy ion‐beam probe has been used to make the first internal measurements of magnetic fluctuations in a hot tokamak. The magnetic vector potential fluctuation,A˜&fgr;s, is measured during magnetohydrodynamic (MHD) activity and is in reasonable agreement with the prediction of a three‐dimensional, resistive, compressible, nonlinear MHD code. Associated density and potential fluctuation measurements and magnetic island widths are also presented.

ISSN:0899-8221    

DOI:10.1063/1.860897

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The gyrokinetic integral equations for the study of the ion temperature gradient driven mode (&eegr;imode) in toroidal geometry, at low plasma pressure, are extended to include equilibrium ion parallelv0∥(r) and perpendicularvE(r) sheared flows, whereris the minor radius of the flux surface. Magnetic gradient and curvature drifts of the ions, as well as finite ion Larmor radius effects, are included. The parallel sheared flow is shown to be destabilizing. The perpendicular sheared flow is a stabilizing mechanism. Mixing length estimates show that the &eegr;i‐mode induced ion thermal transport increases with increasing parallel flow shear, and decreases with perpendicular flow shear. The decrease of the ion transport is due not only to the decrease of the mode growth rate, but also to the shrinking of the mode width. Using the mixing length formulas for the thermal transport associated with the unstable modes, it is shown that the results are consistent with the experimental observations concerning the improvement of confinement in H‐mode discharges coincident with the increase of cross‐field sheared flows.

ISSN:0899-8221    

DOI:10.1063/1.860898

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new formulation of the toroidal, finite beta, resistive tearing stability problem is presented. As in standard tearing mode theory, the mode structure throughout most of the configuration is determined by an ideal, inertia‐free model. Thus, it is very closely related to that obtained from standard ideal magnetohydrodynamic (MHD) numerical stability programs that depend on an energy principle. The effects of inertia, resistivity, and any other plasma properties are important only in thin layers enclosing resonant surfaces. These surfaces are distinguished by the fact that they are composed of closed field lines. Instability growth rates are obtained from the condition of matching between the inner and outer regions. The data needed from the outer region for matching are conventionally reduced to a quantity &Dgr;’, but in toroidally coupled axisymmetric systems the relevant quantity is a matrix. A previous paper [Pletzer and Dewar, J. Plasma Phys.45, 427 (1991)] presented a relation between an extension of the ideal energy and the information from the outer region that is needed in matching to the inner layers. Here, this is used to construct a relation for the tearing mode growth rates directly in terms of an extension of the ideal energy matrix. This demonstrates a convenient way to extend the numerical programs for ideal stability to include stability against tearing modes.

ISSN:0899-8221    

DOI:10.1063/1.860899

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The implosion of a family of reactor‐size targets for inertial confinement fusion (ICF) is studied analytically and numerically. The targets consist of a deuterium–tritium (D–T) shell filled with D–T vapor and they are imploded by a multistep pressure pulse designed in such a way that the final hot spot is formed mainly from the initially gaseous fuel. The formation of the hot spot is described by means of a relatively simple model, and scaling laws for the quantities that characterize the state of the initially gaseous part of the fuel prior to ignition are derived. The results of the model are compared with one‐dimensional fluid simulations, and good agreement is found. A parametric study of the fuel energy gain is then presented; the dependence of the gain and of the hot spot convergence ratio on the pulse parameters and on the filling gas density is analyzed. It is also shown that a substantial increase in the gain (for a given target and pulse energy) can be achieved by replacing the last step of the pulse with an exponential ramp.

ISSN:0899-8221    

DOI:10.1063/1.860900

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An electron plasma oscillation driven unstable by ion streaming is identified with the low‐frequency mode observed inquicksilver[ComputationalPhysics, edited by A. Tenner (World Scientific, Singapore, 1991), pp. 475–482] numerical simulations. This mode heats the electrons along the magnetic field and is ultimately stabilized by the thermal spread. A quasilinear theory determines the saturation level of the fluctuations, the ion divergence, and the ion energy and momentum spread as they exit the diode. The ion divergence is predicted to be independent of the ion mass for fixed diode voltage and scales as the product of the effective gap and the ion beam enhancement factor over Child–Langmuir current.  

ISSN:0899-8221    

DOI:10.1063/1.860901

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Microwave sources based on backward‐wave oscillators (BWO’s) driven by relativistic electron beams are capable of producing high‐power coherent radiation in the cm and mm wavelength region. When the axial magnetic field used in these devices to confine the electron beam satisfies the condition of cyclotron resonance there is a significant modification in the behavior of BWO due to beam coupling to cyclotron modes. In this paper a time‐dependent, self‐consistent theory of BWO’s is developed, taking into account a possible cyclotron interaction. The analysis of the system near the cyclotron resonance yields the power drop due to the cyclotron absorption effect observed in many BWO experiments. In addition, the theory predicts that there exists a region of magnetic field strength, where an increase in power and efficiency is possible. Depending on the value of the magnetic field and beam coupling to cyclotron mode various regimes of BWO operation are possible, including stable single‐frequency oscillations, self‐modulation, and a succession of microwave bursts.

ISSN:0899-8221    

DOI:10.1063/1.860902

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A linear analysis for a gyrotron backward wave oscillator operating at a general transverse electric (TEln) mode of a cylindrical waveguide (slotted or uniform cross section) has been developed using a Maxwell–Vlasov equation under the tenuous beam approximation. The equation is solved by Laplace transformation to allow easy insertion of the boundary values. The start‐oscillations conditions are determined from an interference of the waveguide and beam modes with appropriate amplitudes and phases. The theory is used to determine the design parameters for stable operation of a second cyclotron harmonic gyropeniotron amplifier at 35 GHz in the &pgr; mode of a six vane magnetron‐type circuit with a 70 kV, 3 A beam.  

ISSN:0899-8221    

DOI:10.1063/1.860960

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The flow of plasma into a perfectly absorbing wall, primarily in the sheath and presheath regions, is investigated. Fluidlike moment equations are derived from kinetic theory to include the improved parallel viscosity and heat flux for a transition region between the collisional presheath and the collisionless sheath of the plasma. Both the electron and ion distribution functions are obtained. The ion viscous force, which has usually been considered to be a small term, is deduced from the distribution function, and it is found to be comparable to the electric field force everywhere. The treatment removes the singularity between the presheath and sheath regions in the ion moment equations. One of the results is that, because of the thermal conduction effects in the sheath region, the total heat flux at the wall is larger (∼30%) than in previous work.

ISSN:0899-8221    

DOI:10.1063/1.860797

出版商:AIP    

年代:1993

数据来源: AIP