摘要:

The mathematical solution for the NMR signal produced by a series of pulses in the presence of time‐dependent gradients is derived for the first time. This is used to discuss the localization properties of the sensitive line method of imaging and the resolution is expressed in terms of the experimental parameters. It is pointed out that an upper limit of sample size to resolution exists.

ISSN:0021-8979    

DOI:10.1063/1.326555

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

This paper describes a refined rf‐SQUID magnetometer in which not only the fabrication of the SQUID sensor becomes easier but also the magnetic flux could be stably measured with sufficient sensitivity at a wider range of temperatures compared with existing SQUID systems. Two frequency components are used in the proposed rf‐SQUID magnetometer, one of which is to excite the SQUID sensor and the other is to detect its response. Furthermore, from both the phenomenological analysis and the experimental results it is shown that the present rf‐SQUID magnetometer performs its function well even if the value ofLIcof the SQUID sensor is much larger than the flux quantum &fgr;0.

ISSN:0021-8979    

DOI:10.1063/1.326556

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The characteristics of LaBr3 : Pr and LaCl3 : Pr infrared quantum counters used for detection near 4.5 &mgr;m have been investigated in detail. Quantum efficiency for infrared‐to‐visible upconversion is in agreement with our estimates based on the spectroscopic properties of these materials. Highest sensitivity was obtained with pulse laser pumping in LaCl3 : Pr at 82 °K. Maximum crystal quantum efficiency and detectivity under these conditions wereQ=1.2×10−3andD*=1.4×109cm Hz1/2/W. The detectivity is the highest reported for any IRQC device. Pump‐induced fluorescence measured without ir signal proved to be the limiting source of noise for both cw and pulsed pumping.

ISSN:0021-8979    

DOI:10.1063/1.326557

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A surface negative‐ionization source was created in order to produce intense negative‐ion beams. The advantages of an ionizer functioning in transmission were shown, which led to the theoretical determination of ionizer‐pore useful dimensions. It was thus shown that, contrary to the case with positive ions, the pore diameter necessary for a satisfactory transmission‐ionization efficiency is highly dependent on the electron affinity of the element to be ionized. A lanthanum hexaboride ionizer was then chosen since it gives a good ionization of halogens and especially of iodine. Experimental study showed that the maximum densities of the extracted‐ion current are comparable to those obtained by the same process for positive Cs+ions ionized on tungsten. Finally, the influence of the ionizer‐pore diameter on ion‐current characteristics according to temperature and flow was highlighted. These characteristics confirm the hypothesis established for the working of transmission ionizers.

ISSN:0021-8979    

DOI:10.1063/1.326558

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A 3×1011‐W modular magnetically insulated ion diode for inertial‐confinement fusion studies is described. Power is supplied to the diode, which is centered in an elongated solenoidalB‐field coil structure, by two self‐magnetically insulated transmission lines. A circular ion beam is emitted preferentially from one surface of the curved anode plate and extracted through a 13‐cm‐diam virtual cathode and exposedB‐field coils. The virtual cathode is formed by electrons which spiral along theB‐field lines that conform to the anode surface. The ion beam converges onto a planar target, located 13‐cm from the anode, through a 1‐Torr air plume which allows space‐charge and partial current neutralization of the beam. The beam is initially primarily composed of protons, but later carbon ions dominate. The proton beam has a divergence of 2.1°±0.3° and peak focused proton‐current density of 60 kA/cm2.

ISSN:0021-8979    

DOI:10.1063/1.326559

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

X‐ray emission was used to diagnose the plasma formation in a gas‐puff pulsed‐discharge device. X‐ray pinhole images were collected simultaneously with x‐ray spectra from Ar and Kr gases. X rays were emitted fromz‐pinched regions (50–100 &mgr;m in diameter) along a collinear track formed by the plasma implosion of a hollow gas jet. Spectral analysis indicated Ar plasma formation with electron densities of ∼2×1021cm−3. The Ar plasma temperature was ∼1 keV, based on a collisional‐radiative transport model. Li‐like satellite lines were classified in Ar XVI with atomic structure calculations. The Kr x‐ray spectrum (6–8‐A˚ region) was dominated by Ne‐like Kr XXVII lines. New spectral transitions were classified in Na‐like Kr XXVI and F‐like Kr XXVIII. The wavelength calibrations in the Kr spectrum were obtained from exploded‐Al‐wire plasma generated in coincidence with the plasma implosion formed in the puff gas.

ISSN:0021-8979    

DOI:10.1063/1.326560

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Several two‐photon transitions leading to various (J,K) rotational sublevels of the &ngr;2=2−state in14NH3have been observed using the high‐resolution technique of Doppler‐free two‐photon spectroscopy. It is shown that these transitions can be both tuned into resonance with the optical Stark effect and saturated with intensities of order 1–10 MW/cm2, so that laser action on many wavelengths in the vicinity of 16 &mgr;m is obtainable. This method of excitation is generally applicable for the excitation of molecular systems in frequency‐conversion processes.

ISSN:0021-8979    

DOI:10.1063/1.326561

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A new sensitive method for the detection of low‐energy ion beams in low‐pressure nearly collisionless unmagnetized plasma is presented. The method makes use of a swept planar Langmuir probe in the electron saturation regime. An ion beam with direction perpendicular to the plane of the probe produces a double knee (’’beam edge’’) on the characteristic at a positive bias with respect to plasma potential. The magnitude of this bias is a measure of the directed energy of the beam. It seems to be possible to deduce the beam temperature from the form of the beam edge. The amplitude of the beam edge is proportional to the beam ion flux. The size of the effect is much greater than can be inferred from ion collection alone. An explanation is suggested based on the presence of a space‐charge layer in front of the probe produced by the reflected beam and that this layer in turn causes enhanced electron collection by the probe.

ISSN:0021-8979    

DOI:10.1063/1.326554

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Employing a conserved‐flux concept, theT‐matrix equations describing boundary‐value problems of potential theory and electromagnetic scattering are obtained without recourse to the Huygens principle or physically fictitious fields. For scattering by dielectric objects, tangential electric and magnetic fields on the surface are both represented in a single expansion, cutting the computation in half. In the low‐frequency limit the dynamical equations are shown to reduce to the static case, and numerical computations then indicate that in comparison with other approaches, the present method can achieve as much as an order of magnitude reduction in the number of equations and unknowns needed for a given accuracy. New exact relations are found between the electrostatic and magnetostatic problems, and analytical results are also obtained from the equations, with and without truncation.

ISSN:0021-8979    

DOI:10.1063/1.326562

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

This paper describes a new type of waveguide mechanism found in oxide‐insulated stripe‐geometry GaAs/Ga1−xAlxAs lasers. The photoelastic waveguide is formed by small variations of the refractive index of the active layer beneath the stripe contact of the laser. The refractive‐index variations are caused by the effect of a strain field in the semiconductor surrounding the stripe window. The large photoelastic coefficients of III‐V semiconductors cause significant changes in refractive index for relatively small strains. The strain field in the semiconductor is caused by the high compressive stress in the oxide film; it builds up to its highest value beneath the edges of the stripe window. The maximum calculated compressive strain in the active layer 2‐&mgr;m beneath a stripe window 20 &mgr;m wide opened in an oxide film 2700 A˚ thick is 3×10−4. This strain reduces the dielectric constant of the active layer by approximately 10−2. The sign of the oxide stress, the stripe width, and the active layer depth determine whether a waveguide or antiwaveguide is formed. Experimental measurements have been carried out on two types of photoelastic waveguide at 1.15&mgr;m wavelength. The results confirm the theoretical calculations. These simply fabricated waveguides may be suitable for integrated optic applications. The effect of the photoelastic waveguide on operating stripe‐geometry lasers has been determined by controlled experiments, which clearly indicate that it is the predominant builtin waveguide mechanism. The results also show that a reasonably strong builtin photoelastic waveguide is essential if well‐behaved (’’kink’’‐free) stripe lasers are to be made with stripe widths of about 20 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.326563

出版商:AIP    

年代:1979

数据来源: AIP