共查询到20条相似文献,搜索用时 31 毫秒
1.
Masumi Sato 《Physics letters. A》1978,65(3):233-234
The upper critical current for the occurrence of ionization waves in an axial magnetic field is found in the positive column of rare-gas discharges. As the field is increased, the critical current slightly increases from the Pupp value, and after passing the maximum, finally becomes very small. 相似文献
2.
The dynamics of back cathode bombardment (BCB) instability in a magnetron diode (a coaxial diode in a magnetic field, B ≡ B
0z ≡ B
0) is numerically simulated. The quasi-stationary regime of electron leakage across the high magnetic field (B
0/B
cr > 1.1, where B
cr is the insulation critical field) is realized. An electron beam in the electrode gap is split into a series of bunches in
the azimuthal direction and generates the electric field component E
θ(r, θ, t), which accelerates some of the electrons. Having gained an extra energy, these electrons bombard the cathode, causing secondary
electron emission. The rest of the electrons lose kinetic energy and move toward the anode. Instability is sustained if the
primary emission from the cathode is low and the secondary emission coefficient k
se=I
se/I
e, BCB is greater than unity. The results of numerical simulation are shown to agree well with experimental data. A physical model
of back-bombardment instability is suggested. Collective oscillations of charged flows take place in the gap with crossed
electric and magnetic fields (E × B field) when the electrons and E × B field exchange momentum and energy. The self-generation and self-organization of flows are due to secondary electron emission
from the cathode. 相似文献
3.
Large volume of the experimental data on the ionization cross sections of atoms and molecules by electron impact obtained by various authors using different methods is analyzed. The dependence of the ionization cross section Q on the energy of the incident electron E is described by a curve with a maximum. For E I, where I is the ionization potential, the cross section naturally vanishes. For E > I, it first increases fast, passes through a high maximum, and then monotonically decreases with increasing E. For comparatively large energies E > 100 I, the Bethe formula describes the experimental data almost exactly, but in the region of maximum I < E < 10 I, it deviates significantly from the measurement data. In the present paper it has been established that the experimental dependence Q(E) in the region of maximum is well described by a resonant curve similar to Lorentz distribution. It is assumed that the main contribution to the atomic ionization by a slow electron comes from the resonant excitation and the decay of the Fermi electron-hole system. An empirical formula for the cross section of atom ionization by electron impact Q(E) is suggested which takes into account resonance for incident electrons of small energies and is transformed into the Bethe formula for large E. The parameters of the formula for the ionization cross section are calculated by the least-squares method for H, He, Ne, Ar, C, N, O, Li, Na, H2, N2, O2, K-shells of C, N, Ne, Ar, K, Ca, Rb, and Sr atoms and molecules. A comparison of the experimental dependences of the ionization cross section on the energy of the incident electron with the Bethe theoretical formula and empirical formulas suggested by Lotz, Alkhazov, Kim and Rudd, and Povyshev et al. demonstrates that the formula suggested in the present paper describes the available experimental data better than others. 相似文献
4.
We have studied the effect of the internal circumferential magnetic field HB created by the dc bias current IB on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current IB it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with IB of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field HE and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field. 相似文献
5.
This paper reports on an experimental study of the effect of a magnetic field, B≤70 G, and an electric field, E=120 MV/m, on the critical current I c and I–V curves of DyBa2Cu3?x Oy HTSC ceramics (x=0 and 0.2), both undoped and doped with 1 wt % Pt. It has been established that, in stoichiometric ceramics (x=0) at 77 K, I c drops sharply (by more than an order of magnitude) already at very low B<1 G. In copper-deficient ceramics (x=0.2), I c decreases with increasing B slowly, with Pt-doped samples exhibiting [on the dropping I c (B) dependence] a peak effect, i.e., an increase rather than decrease of I c at B≈10 G. As for the effect of an electric field on I c and the I–V curves (the E effect), it is not observed in ceramics of a stoichiometric composition. DyBa2Cu2.8O y samples acted upon by an electric field reveal a substantial increase in I c and a decrease in the resistance R for I>I c . In the case of DyBa2Cu2.8Oy/Pt, the electric field practically does not affect I c but R decreases for I>I c . In a sample placed in a magnetic field, the magnitude of the E effect is observed to correlate with the I c (B) dependence. In particular, in Pt-doped samples, the E effect decreases with increasing magnetic field B not gradually but with a maximum appearing at B ≈10 G, i.e., in the region of the peak effect in the I c (B) dependence. The data obtained suggest the conclusion that the electric-field effect in ceramics exhibiting weak links of the superconductor-insulator-superconductor (SIS) type correlates with magnetic vortex pinning. 相似文献
6.
The electron-hole plasma in intrinsic InSb at 300 K can be compressed by electric and magnetic fields. In the theta-pinch
configuration one applies, by a capacitor discharge over a one-winding coil around the sample, a magnetic field (0, 0,B
z), which induces an electric field (0,E
φ, 0); both fields cause the ambipolar motion (v
r, 0, 0) of the plasma. We describe the electron-hole plasma by means of a magnetohydrodynamical two-fluid model. The resulting
system of differential equations is solved numerically under the assumptions of scalar hydrodynamic pressure and field-independent
mobilities. The influence of the plasma properties on the theta pinch is studied as well as the dependence on the experimental
conditions like radius of the sample, maximum value and risetime of the magnetic field. 相似文献
7.
T.P. Kiviniemi T. Kurki-Suonio S.K. Sipilä J.A. Heikkinen A.G. Peeters 《Czechoslovak Journal of Physics》2001,51(10):1053-1064
The E
r×B flow shear is simulated with a fully kinetic five-dimensional neoclassical Monte Carlo simulation for JET and ASDEX Upgrade plasma edge conditions. Here, E
r is the radial electric field and B is the magnetic field. It is shown that high enough shear for turbulence suppression can be driven at the Low (L) to High (H) transition conditions by pure neoclassical effects including ion orbit losses. Simulations indicate higher threshold shear for ASDEX Upgrade than for JET. Shear values are compared to different models and experimental results of critical shear, and isotope effect on shear is discussed. Formation of an internal transport barrier in FT-2 in the presence of lower hybrid waves is also studied. 相似文献
8.
P. Mandelbaum J. L. Schwob 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,49(2):173-184
Detailed level-by-level calculations of cross sections and rate coefficients for electron impact direct and indirect ionization
of ions belonging to the GaI isoelectronic sequence (ground 3d
104s
24p) have been performed. The cross sections are presented in the energy range near the threshold for the five ions Kr5+, Mo11+, Xe23+, Pr28+ and Dy35+. The rate coefficients are given for ions from Kr5+ to U61+ in the GaI sequence at seven electron temperatures (kT
e = 0.1E
I
, 0.3E
I
, 0.5E
I
, 0.7E
I
,E
I
, 2E
I
and 10E
I
, where E
I
is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated using the Lotz formula approximation and the contributions of excitation-autoionization (EA) computed in the
framework of the distorted wave (DW) approximation for the 4s-nl, 3d-nl and 3p-nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is presented as a function of Z along
the GaI isoelectronic sequence. The present results show the great importance of the EA processes; an ionization enhancement factor of up to 10 is predicted for instance for La26+ (Z = 57) at electron temperature of coronal equilibrium maximum abundance. 相似文献
9.
The influence of a transverse magnetic field on a contracted argon discharge column (p = 23 torrs; Idischarge = 100…?200 ma) is investigated. The experiments show oscillatory behaviour of the column similar to rope waves, which becomes irregular with higher B-field intensities. The results of spectral and correlation measurements are discussed in relation to the corresponding non-linear wave equation qualitatively. 相似文献
10.
S. Ohya S. Muto M. Katsurayama K. Heiguchi N. Mutsuro N. J. Stone J. Rikovska 《Hyperfine Interactions》1992,75(1-4):269-274
The magnetic hyperfine splitting frequencies of123INi,124INi and131INi in a zero external magnetic field have been determined by the NMR-ON method as 258.9(1), 165.9(1) and 179.5(2) MHz, respectively.
With the known values of the magnetic moments, the magnetic hyperfine fields have been deduced:B
HF(123INi)=30.17(5) T,B
HF(124INi)=30.14(9) T,B
HF(131INi)=30.06(4) T; the weighted average isB
HF(INi)=30.11(4) T. The small difference of theB
HF(131INi) with those of123INi and124INi is discussed comparing with results of the hyperfine splitting frequency of iodine in iron host. 相似文献
11.
Masumi Sato 《等离子体物理论文集》1973,13(1):9-25
Evolution from the linear growth to the nonlinearly saturated state of ionization waves (moving striations) is investigated from a viewpoint of an instability which appears in a plasma. The experiments were performed observing backward waves excited below the upper critical current Ic in xenon, argon, and argon-mercury gases at pressures of the order of Torr. It is found that (1) the behavior of the evolution obeys the Landau amplitude equation; (2) the saturated amplitude does not depend on an initial value, but only on plasma parameters; (3) the linear growth and squared saturation level are proportional to the excess over Ic, and when frequency-controlled, they have a parabolic dependence on the frequency; and (4) nonuniform axial changes in the dc state due to the nonlinear effects appear in a form of enhanced ionization in the plasma where the electron temperature and density are increased. The way of these behavior can be applied to a large number of spatially or temporally unstable modes. 相似文献
12.
The E33 and E44 optical spectra of semiconducting single-walled carbon nanotubes under axial magnetic field (B) are studied using the tight-binding model, which also takes into account the exciton effect. It is found that the E33 and E44 splitting, induced by the axial magnetic field, is line increased with magnetic field, which can be described by the splitting rate. Also by investigation of the dependence of splitting rate, we found that it shows a clear (2n+m) family behavior besides the diameter dependence, which can be used as a supplemental tool to identify the tubes used in the experiment, and is expected to be detected by the future experiment. 相似文献
13.
H.-B. Valentini 《等离子体物理论文集》1970,10(3):223-236
The phase velocity, the amplification rate and the critical Hall parameter are theoretically determined for ionization waves in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both the magnetic field and the stream velocity. The investigations hold for seeded rare gases at any degree of seed ionization. The critical Hall parameter βc depends on the degree of ionization, the ionization energy and the temperatures of electron gas T0 and neutral gas Tg · βc is always greater than one, if 0 < T0 — Tg ? T0 holds. The three-dimensional treatment indicates the existence of waves with a nonvanishing wave vector component in the direction of the magnetic field. The influence of ionization waves on mean current density, mean Hall field intensity and mean electron temperature is determined up to second order terms in the relative fluctuations of the electron temperature. The amplification of ionization waves reduces the effective electric conductivity, the effective Hall parameter and the mean electron temperature compared to the undisturbed state. Similar results are also obtained for steady state homogeneous isotropic turbulence and a special case of axially symmetric turbulence. Furthermore, a component of the electric field in direction or in opposite direction to the magnetic field vector may be generated by non isotropic and non homogeneous turbulence. 相似文献
14.
M. A. Zelikman 《Physics of the Solid State》1997,39(11):1751-1755
A system of pancake vortices formed near the boundary of a sample in a monotonically increasing external magnetic field is
calculated with allowance for pinning due to the cellular structure of the medium for various values of the pinning parameter
I, which is proportional to the critical current of the junction and the cell diameter. The shortest distance from the outermost
vortex to the nearest neighbor is proportional to I
−11. It is shown that the pinning parameter has a critical value I
c separating two regimes with different types of critical states. For I<I
c the external magnetic field has a threshold value H
t(I), above which the field immediately penetrates the interior of the junction to an infinite distance. For I>I
c the magnetic field decays linearly from the boundary into the interior of the junction. The value obtained in the study,
I
c=3.369, differs from the value of 0.9716 postulated by other authors. The dependence of the slope of the magnetic field profile
near the boundary on I is determined. It is shown that the slope is independent of I in intervals 2πk<I<2πk+π.
Fiz. Tverd. Tela (St. Petersburg) 39, 1958–1963 (November 1997) 相似文献
15.
Summary A previous investigation by one of us, concerning the self-consistent equilibria of a two-region (plasma+gas) cylindrical
Tokamak, is extended to the similar equilibria of a Reversed-Field Pinch, where a significant current density is driven by
a dynamo electric field due to turbulence. The previous model has been generalized under the following basic assumptions:a) to the lowest order, the turbulent dynamo electric fieldE
t
is expressed as a homogeneous function of degree 1 of the magnetic fieldB, sayE
t
=α·B, with α being a 2nd-rank tensor, homogeneous of degree 0 inB, and generally depending on the plasma state;b)E
t
does not appear in the plasma power balance, as if it were produced by a Maxwell demon able to extract the needed power from
the plasma internal energy. In particular we show that, in the simplest case when both α and the plasma resistivity η are
isotropic and constant, the magnetic field turns out force-free with constant abnormality αμ0/η for vanishing axial electric fieldE
z
. This case has also been solved analytically, for whateverE
z
, under circular, besides cylindrical, symmetry. 相似文献
16.
A. S. Kyuregyan 《Journal of Experimental and Theoretical Physics》2009,109(5):833-851
A numerical simulation of the origination and evolution of streamers in semiconductors has been performed using the diffusion-drift
approximation including the impact and tunnel ionization. It is assumed that an external electric field E
0 is static and uniform, an avalanche and a streamer are axisymmetric, background electrons and holes are absent, and all their
kinetic coefficients are identical. The linear evolution of an electron-hole avalanche, an avalanche-to-streamer transition,
and two successive stages of the evolution of the streamer—intermediate “diffusion” and main exponentially self-similar—have
been examined in detail. It has been shown that a streamer is similar to a dumbbell with conical weights. The bases of these
cones, streamer fronts, are thin shells, which contain almost the entire streamer charge and are close in shape to the halves
of ellipsoids of revolution. A front propagates so that its shape and the shape of the weight of the dumbbell, the maximum
field on the front, and the electron-hole plasma density in weights remain unchanged. The field strength behind the front
is much smaller than E
0, but increases with approaching the bar of the dumbbell whose diameter increases with the time t owing to the transverse diffusion. The electron and hole densities in the bar increase due to the impact ionization in an
almost uniform field, which is only slightly lower than E
0. At the diffusion stage, the length of the streamer and the curvature radius of its front increase with constant rates, which
are determined not only by the impact ionization and drift, but also by diffusion. In relatively low fields (E
0 ≲ 0.4 MV/cm for silicon) this stage ends due to the appearance of the instability of the front. In higher fields, the tunnel
ionization is manifested before the appearance of instability and gives rise to the appearance of a new-type streamer. Its
main feature is the stable exponential increase in all spatial scales with the same response time t
R
, so that the charge-carrier density and field strength at large times t depend only on one vector variable $
\hat R
$
\hat R
= Rexp(−t/t
R
). This means that the solution of a Cauchy problem describing the evolution of the streamer in the uniform field is asymptotically
exponentially self-similar. 相似文献
17.
The necessity of simulations in design of superconducting dipole magnets is due to the following circumstances. First, the critical current as a function of the magnetic field I c(B) for the multicore superconducting cable which drops strongly requires the knowledge of the value of maximum magnetic field “felt” by its coils for estimation of the working current of the magnet. Second, for choosing the optimal number of coils of the winding (1 or 2) and the working current of the magnet, the ratio of B max for the inner and outer layers of the dipole magnet winding should be known. Since usually the length of the dipole magnet exceeds many times its transverse size, in this work all calculations of B(x, y) are performed in the transverse plane crossing the center of the magnet. The field at the central point is chosen to be B(0, 0) = 2 T (this is the characteristic working value close to the maximum value in the dipole magnet of this type). In this work, the results of calculation of B(x, y) for single-and double-layered windings with 8 and 16 coils from circular hollow cable are presented. 相似文献
18.
M. A. Zelikman 《Technical Physics》2009,54(2):197-203
A new approach to magnetic field profiling inside a Josephson contact is suggested. Its essence consists in analyzing continuous
variation of a current configuration leading to a decrease in the Gibbs potential. With this approach, one can find a configuration
into which the Meissner state turns when an external field slightly exceeds the upper boundary of the Meissner regime and
trace the evolution of this configuration with increasing field. Calculations show that there exists critical value I
c of the pinning parameter in the range 0.95–1.00. This critical value separates two possible conditions of magnetic field
penetration into the contact. At I > I
c, a near-boundary current configuration completely compensating for the external field inside the contact arises irrespective
of the external field strength. At I < I
c, such a situation is observed only until the external field strength exceeds certain value H
max. Higher fields penetrate into the contact indefinitely deep. In nearboundary configurations, the magnetic field drops with
increasing depth almost linearly. Its slope k has rational values, which remain constant within finite intervals of I. As I goes beyond a given interval, k rises stepwise and takes on another rational value. When an external magnetic field is switched on adiabatically, configurations
with a maximal growth rate of the magnetic field are observed. 相似文献
19.
Experimental observations on mode transition in a positive column of Ne gas under non-uniform axial magnetic field are reported. The self-excited ionization wave is damped and a different type of ionization wave is grown in the region of the strong magnetic field near the magnetic coils. The mode transitions appear on both sides of the magnetic coils where the magnetic field crosses a critical value. 相似文献
20.
An ion cyclotron drift instability is excited by a cross field current due to Er × B drift of electrons and ion heating is observed in a plasma column in a magnetic field when a dc radial electric field Er is applied. 相似文献