摘要:

The Mercier stability criterion is evaluated for various axisymmetric magnetohydrodynamic equilibria in a systematic study of the effects of shaping on the plasma critical &bgr; (volume averaged pressure over the vacuum toroidal field pressure at the center of the plasma chamber). Pressure profiles that are similar to those observed in tokamak experiments are used. Equilibrium parameters of interest are elongation, &sgr;, and triangularity, &dgr;, of D‐shaped cross sections, aspect ratio,A, poloidal beta, &bgr;p, and pressure profile width,wp. Doublet plasmas with various &sgr; are also included for completeness. It is shown that, for fixed plasma shapes, the critical &bgr; values increase strongly with increasingwp(typically by a factor of roughly 4), while for fixed values ofwpthe critical &bgr; values have a weaker dependence on plasma shaping (typically by a factor of roughly 2). The effectiveness of shaping (to increase &bgr;) decreases with decreasingwp, and for experimental values ofwp(≲0.5) the best critical &bgr; values so determined is less than 6%. For fixedwpin D‐shaped plasmas,it is shown that the highest critical &bgr; occurs around &sgr;=1.65 and &dgr;=0.5. The critical &bgr; values increase strongly with decreasingA. However, if the toroidal field is fixed at the coil surface at the distance between the coils and the plasma edge is greater than half the minor radius, the highest critical pressure occurs whenA≳3–4. The doublet plasmas considered here with &sgr;=3 give critical &bgr; values that are similar to those of the optimal D‐shaped plasmas, when identical values ofwpare used.

ISSN:0031-9171    

DOI:10.1063/1.862246

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A new class of particle simulation models has been developed which eliminates the high‐frequency space charge oscillations while keeping the low‐frequency ion‐density fluctuations unmodified. Physically, the quasi‐neutrality is maintained as the adiabatic electrons follow the ion‐density fluctuations so as to establish the Debye shielding. It is, therefore, possible to use an integration time step comparable to the characteristic time scale of the low‐frequency oscillations so that realistic plasma parameters can be used in the present simulation model. Applications to ion sound wave propagation, ion‐ion two‐stream instability, and the low‐frequency fluctuation spectrum in a magnetic field are given. The way in which to include the nonadiabatic electrons in the model is also discussed.

ISSN:0031-9171    

DOI:10.1063/1.862247

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Experimental results are presented for the heating of a 4 m long plasma confined by a uniform magnetic field of 4–5 kG by an intense relativistic electron beam. Beam parameters were 0.5–1 MeV, 25–80 kA, 60–70 nsec pulse duration, and electron density of 2–5×1011/cm3. The initial plasma density ranged from 5×1013/cm3to 4×1015/cm3and the electron temperature was 1–3 eV. The lower density cases were partially ionized withTe≫Ti, and the higher density cases were highly ionized withTe≈Ti. In all cases, the energy coupled from the beam to the plasma was greater than can be explained by binary collisions between beam electrons and the plasma particles. Beam energy transferred to the plasma ranged from 2–7%/m, and was uniform over the 4 m length of the plasma. Over most of the density range tested, 5×1013/cm3to 1.5×1015/cm3, the plasma heating cannot be explained by classical processes. These results are found to be explained quantitatively by the use of a full nonlinear treatment of the electron‐electron two‐stream instability in the kinetic regime.

ISSN:0031-9171    

DOI:10.1063/1.862248

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Single shot photographic spectra of hydrogen‐like aluminum Lyman series lines are obtained from a low inductance vacuum spark. Corresponding pinhole pictures yield information on the source size. The observed line profiles are found to increase in width along the series and the higher series membersL&ggr;andL&dgr;can be matched with convolved Stark and Gaussian profiles. The Gaussian profile includes the effects of source size, thermal Doppler broadening, and crystal resolution. The space and time‐averaged electron density determined from the profile analysis is 2.5×1021–1×1022cm−3.

ISSN:0031-9171    

DOI:10.1063/1.862225

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The magnitude of the skewness of the temperature derivative in a turbulent flow downstream of a heated grid is small and can be reduced to zero by perturbing the flow with a second ambient temperature grid.

ISSN:0031-9171    

DOI:10.1063/1.862249

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The problem of turbulent entrance flow is calculated using a two‐equation model of turbulence and the results are compared with experimental data available in the literature. The main features of the flow are properly predicted; however, important differences between theory and experiment exist.

ISSN:0031-9171    

DOI:10.1063/1.862250

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Two assumptions, a similarity in the turbulent field and an invariance of the large scale motions, are made and a renormalization technique is used for the Burgers turbulence. It turns out that the power index of the energy decay ranges continuously from −2/3 to −2, depending on the cases.

ISSN:0031-9171    

DOI:10.1063/1.862251

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of electron‐electron collisions on the Landau damping of a single wave in a magnetized or unmagnetized plasma is calculated using a model Fokker–Planck operator. The resulting form of Landau damping could play a role in the saturation of some parametric and trapped‐particle instabilities.

ISSN:0031-9171    

DOI:10.1063/1.862252

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The predicted damping of Alfve´n waves in a nonuniform plasma is observed in a magnetohydrodynamic computer calculation. The results are interpreted in terms of a simple model of independent oscillators.

ISSN:0031-9171    

DOI:10.1063/1.862253

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The surface envelopes and rotation velocities of relativistic electron beams propagating through a background gas are observed to vary in a spatially‐periodic manner. In the fast mode of rotation it is found that when the beam radius decreases, the beam rotation velocity increases andviceversa.

ISSN:0031-9171    

DOI:10.1063/1.862223

出版商:AIP    

年代:1978

数据来源: AIP