摘要:

Calculations based on the rate equations for an idealized 4‐level system indicate that efficient repetitive cavity dumping of Nd:YAlG lasers is possible at repetition rates above about 100 kHz. Analytical forms for the field and inversion variables are derived. A numerical example illustrates the resulting operation.

ISSN:0021-8979    

DOI:10.1063/1.1660136

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Experiments inQ‐switching and cavity dumping of a Nd:YAlG laser have been carried out. The system in either case is composed of a Nd:YAlG folded cavity laser and an intracavity acousto‐optic modulator made of fused silica and Brewster cut.Q‐switching is achieved by first feeding the modulator with an acoustic pulse of sufficient duration and intensity to keep the laser below threshold (hold off pulse). Termination of the acoustic pulse allows the power inside the laser cavity to build up. A second short acoustic pulse, introduced to the modulator at the time when the internal energy reaches its maximum, couples the energy out of the cavity. This pulse may also be the beginning of the hold off pulse. Cavity dumping is accomplished by feeding the modulator with short acoustic pulses at fixed intervals. In theQswitching mode the system can operate with a maximum repetition rate of about 30 kHz. In the cavity dumping mode of operation the repetition rate of the light pulses can be controlled from 125 kHz to several MHz. The minimum pulse duration obtained in both modes of operation was about 80 nsec. The average power can reach 2 W, which is the maximum cw power capability of the laser used.

ISSN:0021-8979    

DOI:10.1063/1.1660137

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The heating of a small particle of deuterium‐tritium by laser radiation is considered and conditions for energy gain determined. The size of the D‐T particle for a balance of energy input and neutron energy output is computed as a function of initial plasma temperature. The electrical energy required for this breakeven condition is shown and increases as the inverse fourth power of laser efficiency. Because electrons are first heated by the laser radiation and then must transfer their energy to ions by collisions, a computation of electron‐ion thermalization is presented. These results show that electron‐ion thermalization through collisions applies a severe limitation on most cases for producing net fusion energy and must be considered in a consistent manner for smaller particles.

ISSN:0021-8979    

DOI:10.1063/1.1660138

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The threshold currents of a variety of multifilament, composite Nb&sngbnd;Ti superconductors, both twisted and untwisted, have been measured and the results analyzed. The threshold current is defined as a minimum level of transport current which results in a sample quench following a flux jump. The twisted samples are able to maintain much higher levels of threshold current than the untwisted samples. Flux‐jump‐induced quenches existed in all samples, but analysis of the results reveals that different heat‐transfer regimes were operative in the twisted and untwisted materials. For the case of twisted samples, the magnetic energy released in a flux jump is relatively small, and heat transfer is by nucleate boiling; in untwisted samples the energy released is much greater, and heat transfer is by film boiling. The threshold current levels computed for both the twisted and untwisted samples agree satisfactorily with the experimental results.

ISSN:0021-8979    

DOI:10.1063/1.1660139

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A finite difference scheme is described which yields solutions of the Josephson phase equation. This simulation technique is used to calculate the self‐resonant current peaks of a Josephson junction for the two cases,R≫ZJandR≪ZJ, whereRis the current drive source resistance andZJis the junction impedance. The simulation current peaks are compared with ones derived by a perturbation technique for the same two regimes, and also with experiment. In terms of approximating experimental results, it is found that the perturbation solutions with constant damping, &sgr;, are better than those with constantQ; although at small &sgr; the magnitude and shape of the current peaks do not agree well with observations. However, simulation results at small &sgr; are reasonably consistent with experiment. Simulation results for the caseR≪ZJindicate that self‐limiting will affect the magnetic field dependence of the resonances whenL/&lgr;J≥2, whereLis the length of the junction and &lgr;Jis the Josephson penetration depth.

ISSN:0021-8979    

DOI:10.1063/1.1660140

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A calorimetric method for the determination of the heat dissipation of small superconducting magnets operated in a pulsed or cycling mode in the range from 1.5° to 2.15°K is reported. Preliminary results are given for a NbTi magnet and the accuracy of the technique is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1660141

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Electroluminescence in GaSb liquid‐phase epitaxial diodes has been studied. Optical and electrical measurements were done between 65° and 300°K. Two emission bands are observed, which are both due to transitions between the conduction band and different acceptor levels. The peak photon energy of band I is stationary with applied voltage and is due to a thermal injection mechanism. The low‐energy band II shifts with applied voltage and is described by theories of photon‐assisted tunneling. It is demonstrated that for both emission bands the intensities and peak photon energies are directly related to the applied voltage.

ISSN:0021-8979    

DOI:10.1063/1.1660142

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A wall‐pinning model is applied to the motion of a single domain wall. The model assumes that hysteresis is due to localized pinning of walls at defects and their subsequent snapping free. A distribution function is introduced to describe the differing strengths of defects in a real sample. The general theory is applied to the symmetric minor loop, the virgin curve, and to drifting minor loops. The theory makes two interesting predictions about the relation between symmetric minor loops and the virgin curve. They are both properties of the Rayleigh loop. One of these is that the virgin curve is the locus of apexes of centered minor loops. A method for measuring the distribution function from a minor loop is given. The theory is then specialized to the case of a particular distribution function which has been found to hold experimentally. The theory is applied to the cases described above. Special attention is given to the Rayleigh region. It is found that there is a relation between the Rayleigh constants &mgr; andaand the coercive forceHcsimilar to that given by Ne´el, namely,&mgr;/a=1.58Hc.For a sinusoidal variation of flux, the first three Fourier components of mmf are calculated.

ISSN:0021-8979    

DOI:10.1063/1.1660143

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Theory is compared with experiment. The applicability of the theory to a sample which is magnetically sufficiently simple is demonstrated by comparing certain qualitative predictions of the theory with experiment. In particular it is shown that the ascending branches of all minor loops coincide if they are all started at the same point. The ascending branch of a minor loop coincides with the ascending virgin curve if they both start at the same point and if the gain of the flux and mmf channels of the minor loop are reduced by a factor of 2. The virgin curve is the locus of apexes of centered minor loops. The relation between height and width of minor loops is shown to agree quantitatively with theory. The shape of a branch of a single minor loop is also found to agree. The first three terms of the Fourier analysis of mmf for sinusoidal flux variation are found to be in good agreement with theory except for the third harmonic which at least has the correct order‐of‐magnitude. Theory and experiment are shown to be in good agreement in the Rayleigh region. However, the applicability of the present theory is not confined to the Rayleigh region, but applies right up to saturation for a specimen meeting the criteria of simplicity. The predicted relation between the Rayleigh constants and the coercive force is shown to hold. Finally the distribution function is calculated from a minor loop and shown to be an exponential with reasonable accuracy.

ISSN:0021-8979    

DOI:10.1063/1.1660144

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A sensitive pulsed NMR spectrometer for solids based on a multiple‐pulse technique is described. In the conventional pulse method transient signals are accumulated for a certain time when signal‐to‐noise ratio must be improved; however most of the time spent for measurement is wasted becauseT1≫T2. Since the multiple‐pulse technique prolongs the decay time of transverse magnetization, one can accumulate extra signals and enhance the sensitivity. Under the optimum condition the enhancement factor is (T1/T2)1/2for a given measuring time. One can take full advantage of low temperatures to increase the sensitivity. Typical spectra taken for aluminum metal show sideband peaks. The line width is made narrow if a high‐resolution multiple‐pulse technique is used. The signals were accumulated in a modified boxcar integrator. Several applications are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1660145

出版商:AIP    

年代:1971

数据来源: AIP