摘要:

Plasma current ramp‐up by lower‐hybrid waves in a high‐density regime (n¯e≳1×1019m−3) is investigated on the Alcator C tokamak [Nucl. Fusion25, 1127 (1985)]. A time‐averaged conversion efficiency (W˙−Pext)/Prfof over 15% has been obtained at a density ofn¯e&bartil;1×1019m−3, whereWis the poloidal field energy andPextis the external inductive power. The maximum conversion efficiency obtainable is lower at higher densities. Ramp‐up rates of over 1 MA/sec were obtained when rf current ramp‐up is applied to an Ohmically maintained plasma. Lower‐hybrid current drive can also be used to maintain the plasma while recharging the Ohmic‐heating transformer. The conversion efficiency of 10% obtained during recharging is found to be comparable to that obtained during ramp‐up under similar conditions.

ISSN:0031-9171    

DOI:10.1063/1.866316

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A simple model is presented that uses the concept of helicity balance to predict the toroidal loop voltage of reversed field pinches (RFP’s). Data from the RFP’s at Culham [Plasma Phys. Controlled Fusion27, 1307 (1985)] are used to calibrate and verify the model. The model indicates that most of the helicity dissipation occurs in edge regions that are outside the limiters or in regions where field lines contact the walls. The value of this new interpretation to future RFP and spheromak experiments is discussed.

ISSN:0031-9171    

DOI:10.1063/1.866317

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Stimulated Raman scattering of light waves by an underdense plasma is affected by the presence of a density ripple caused by a simultaneously occurring stimulated Brillouin instability. The problem is treated kinetically for the particularly interesting case where the ripple has nearly the same wavelength as the plasma wave. The ripple is found to reduce the growth rate of the usual Raman instability but allows other decay modes to occur. Numerical results for the frequencies, growth rates, andkspectra of these modes are obtained. A physical explanation is given for a baffling result of the calculation. The physical picture is also of interest to particle acceleration by plasma waves.

ISSN:0031-9171    

DOI:10.1063/1.866318

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An experimental study is made of magnetically insulated electron flow in a cylindrical thermionic diode. Emphasis is laid on measurements of plasma waves and leakage current in the insulated state. It is found that over a wide range of conditions the leakage current can be represented byI=[(0.84×10−6&xgr;AcV32)/deff] exp(4015x@B|/&xgr;) Amps, whereAcis the cathode area,Vthe anode–cathode voltage,deffis an effective interelectrode gap (m), andx*is the thickness of the Brillouin layer in contact with the cathode (m). Here &xgr; is a geometrical factor equal to unity in the present nearly planar geometry, increasing by 3 as the cathode radius becomes much less than the anode radius, as is the case in prior magnetron data also fitted by this formula. Unstable electrostatic, transverse magnetic (TM) waves are observed at frequencies predominantly below the electron‐cyclotron frequency with wavelengths equal to the &pgr;d/n, whered=diode diameter,n=1,2,3... . The waves are believed to be driven unstable by a resistive wall instability. The correlation between the leakage current and the wave amplitude is discussed.

ISSN:0031-9171    

DOI:10.1063/1.866319

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

This paper contains a theoretical treatment of the start oscillation condition for backward wave oscillators with rippled wall slow wave structures, driven by relativistic, low current, annular electron beams. In the low current, or Compton, limit of device operation, this condition amounts to setting a minimum value on the product of the beam current and the cube of the structure length; above the minimum value, the device will spontaneously break into oscillations, even in the absence of an input signal. Also computed is the small signal growth rate of the oscillations, which is shown to be less than the single mode growth rate for an infinitely long structure. Approximate closed form expressions are found in the limit of shallow structure ripples, while numerical results are presented for deeper wall ripples.

ISSN:0031-9171    

DOI:10.1063/1.866269

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An extension of G. I. Taylor’s [G.I.Taylor:ScientificPapers,edited by G. K. Batchelor (Cambridge U.P., Cambridge, 1960), Vol. 4, p. 410] scraper problem (i.e., the displacement of a viscous fluid on a flat solid surface by a flat solid scraper) is investigated. Consider an infinite strip of viscous fluid of uniform width and height on a horizontal surface with a scraper that spans the full width. If the speed of the scraperUis given, what should the scraper’s orientation be to the strip to minimize the component of force in the direction of motion that must be applied to the scraper?

ISSN:0031-9171    

DOI:10.1063/1.866519

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

By mounting a delta‐wing‐like passive device on the front part of the junction between a vertical bluff body cylinder and the flat ground, the horseshoe vortex flow structure has been modified significantly. Model test results of a flow visualization study are described to show these changes. The results will have an important and favorable influence on the local scour prevention around river bridge piers

ISSN:0031-9171    

DOI:10.1063/1.866270

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Two kinds of nonlinear stability theories are examined, Herbert’s method [J. Fluid Mech.126, 167 (1983)] and that of Watson [J. Fluid Mech.9, 371 (1960)]. They are compared by calculating the first Landau constant of plane Poiseuille flow numerically according to their definitions. It is found that ‖&lgr;(H)1r−&lgr;(W)1r‖/&lgr;(H)1r∝&lgr;0is satisfied near the neutral state, where &lgr;(H)1rand &lgr;(W)1rare the real parts of the first Landau constant defined by the methods of Herbert and Watson, respectively, and &lgr;0is the linear growth rate. This conclusion is consistent with the error estimation by Herbert, but is more accurate.

ISSN:0031-9171    

DOI:10.1063/1.866271

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The stochasticization of the magnetic separatrix caused by the presence of a helical perturbation in a poloidal divertor tokamak is illustrated by a numerical computation that traces magnetic field lines.

ISSN:0031-9171    

DOI:10.1063/1.866272

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The fully relaxed magnetohydrodynamic (MHD) equilibrium is obtained by imposing nonideal MHD perturbations constrained by the total plasma current. The minimum energy state governed by &Dgr;*&psgr;+CF exp(&psgr;/&lgr;)+CpR2 exp(5&psgr;/4&lgr;) =0 reveals properties of profile consistency, bifurcated solutions, and a loss of equilibrium condition. It predicts the transport coefficient peaked at the plasma edge and a unique current profile for givenq0andql.

ISSN:0031-9171    

DOI:10.1063/1.866273

出版商:AIP    

年代:1987

数据来源: AIP