摘要:

A 140 GHz high‐power gyrotron has recently been operated in a 14TBitter magnet to characterize emission as a function of magnetic field and beam current. The velocity ratio (or pitch angle) &agr;=⟨v⊥⟩/⟨v∥⟩ of the beam electrons is a critical parameter for high‐efficiency gyrotron operation and was measured using a capacitive probe located in the beam tunnel before the cavity. The observed velocity ratio decreased as the beam current increased while the beam voltage and magnetic fields were held fixed. This decrease in &agr; partially explains the reduced gyrotron efficiency observed at high‐beam currents. The velocity ratio exhibited saturation effects as a function of both the beam current and the control‐anode voltage, at low cathode magnetic field values. Particle code results show a decrease in &agr; as a function of beam current that is consistent in magnitude with the observed values.

ISSN:0021-8979    

DOI:10.1063/1.348479

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Spectroscopic and laser performance data show that the choice of auxiliary light used to optimize the performance of the NaCl F+2:O2−color center laser has little impact on the tuning range or output power of the system. When F‐band auxiliary light is used to process the laser crystal a transient absorption due to F−2centers occurs, but this feature disappears during laser operation. No such transients appear when other auxiliary light sources are used.

ISSN:0021-8979    

DOI:10.1063/1.348480

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A generalized two‐dimensional flow‐radiation coupled model to extract power from a gasdynamic laser is proposed. The model is used for the study of power extraction from a 9.4‐&mgr;m CO2downstream‐mixing gasdynamic laser, where a cold CO2+H2stream is mixed with a vibrationally excited N2stream at the nozzle exits. This model is developed by coupling radiation with the two‐dimensional, unsteady, laminar and viscous flow modeling needed for such systems. The analysis showed that the steady‐state value of 9.4‐&mgr;m intensity as high as 5×107W/m2can be obtained from the system studied. The role of H2relaxant in the power extraction process has also been investigated.

ISSN:0021-8979    

DOI:10.1063/1.348481

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The asymmetric Maker fringe pattern of third‐harmonic generation in an anisotropic molecular crystal is analyzed to evaluate the third‐order optical susceptibility &khgr;(3). Analysis of a diethyl‐amino‐nitro‐stilbene crystal shows that the molecular axes are not oriented parallel to the crystal surface, but are about 45° relative to the surface. The &khgr;(3)value parallel to the orientation of the molecular axis is estimated to be 1.0×10−11esu, corresponding to about ten times the &khgr;(3)value perpendicular to that axis.

ISSN:0021-8979    

DOI:10.1063/1.348482

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Previous work has demonstrated a technique for acoustically detecting localized delamination of metallized patterns on insulating substrates [W. Imaino, L. Crawforth, A. C. Munce, and A. Julaiana, inProceedingsoftheIEEEUltrasonicsSymposium, edited by B. R. McAvoy (IEEE, New York, 1986), p. 1065]. Employing a high‐spatial‐resolution permeable electromagnetic acoustic transducer to preferentially excite the metallization, the acoustic coupling between the metal foil and substrate may be probed. Scanning, then, provides a map of delaminations. To extend and generalize these results, the detailed generation mechanism and acoustic response of the substrate has been studied. A computer program developed previously [W. Imaino, J. Acoust. Soc. Am. Suppl. 180, S7 (1986)] was used to model the acoustic source. In this investigation, a finite‐element calculation is used to provide a more detailed description of the acoustic behavior of the delaminated plate. The effects of source size relative to defect dimensions and acoustic properties of the substrate have been studied. The determination of the localized coupling is complicated by structural resonances of the substrate, which make single frequency measurements unfavorable. However, the analysis shows that the spectral acoustic behavior provides an indication of metallization to substrate coupling. A signal‐processing algorithm based on this analysis has been formulated and will be described.

ISSN:0021-8979    

DOI:10.1063/1.348453

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A detailed analysis of the method of picosecond acoustic interferometry to study attenuation and velocity of longitudinal acoustic phonons in transparent materials in the Brillouin frequency range with picosecond laser pulses is presented. Experimental results for fused quartz from 90 to 300 K show good agreement with previous Brillouin scattering data. Measurements on a borosilicate glass (Corning 7059) and sapphire have also been made. This method makes these measurements possible under conditions where conventional approaches are not applicable.

ISSN:0021-8979    

DOI:10.1063/1.348958

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Spherical and cylindrical many‐particle models are used to simulate rf (radio frequency) discharges in which the rf powered and the grounded electrodes have different areas. This asymmetry determines the magnitude of the average plasma‐to‐electrode voltageVa(the ion bombarding energy) at the smaller powered electrode, which is a critical process parameter. A collisionless uniform ionization discharge model predicts that the voltage ratioVa/Vbscales as the fourth power of the electrode area ratioAb/Aa, whereVbis the potential drop at the other electrode. However, measurements indicate a much weaker dependence ofVa/Vbon the area ratio, which is also observed in our simulations. Over a limited range of area ratios it was found that the power dependence was close to one, in agreement with a local ionization discharge model. The simulation codes used are PDC1 (plasma device cylindrical one‐dimensional) and PDS1 (plasma device spherical one‐dimensional).

ISSN:0021-8979    

DOI:10.1063/1.348436

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A cylindrical surface‐discharge device was developed to study photolytically pumped and photoinitiated gas laser schemes. The discharge plasma plasma produces intense ultraviolet and vaccum‐ultraviolet (vuv) broadband irradiation of a 3‐l active volume. Homogeneous, large‐area (≥103cm2) plasma shells are formed adjacent to the interior surfaces of dielectric cylinders by an electric discharge. Although the device resembles aZ‐pinch, the plasma shells do not collapse to the axis and operation with various gases at high pressure (≤2× 105Pa) is possible. Initial experiments have emphasized electrical, gasdynamic, and radiative characterization of the device. Current wave forms are oscillatory, indicating an impedance mismatch between the surface‐discharge source and capacitor‐bank driver. A radially converging shock wave is launched by the discharge while the expanding plasma shells remain tightly pressed against the dielectric surfaces. Operation at a peak current of 94 kA produced approximately 45 J of radiated energy within the vuv spectral band from 170 to 210 nm in a 1.7‐&mgr;s full‐width‐at‐half‐maximum pulse, which represents a conversion efficiency for stored electrical energy into radiation of 2.7% and an equivalent brightness temperature of &bartil;13 500 K. The effective optical pulse width decreases as one moves from the visible into the vuv spectral regions. vuv radiation is only efficiently produced during the initial current half‐period, i.e., during the gap breakdown process.

ISSN:0021-8979    

DOI:10.1063/1.348437

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Analytical and simple numerical calculations of the stopping power of partially ionized high‐Ztargets are presented. Up to now such calculations are difficult, mainly because of lacking precise average ionization potentials of the target ions. The aim of this paper is both to clarify the importance of a suitable treatment of target ions in different ionization stages and to show by means of general scaling laws and explicit examples, that high‐Zplasmas of high density and low ionization degree may have smaller stopping powers than cold matter. The resulting range lengthening of the projectile ions might be of crucial importance for a proper design of pellets in inertial confinement fusion.

ISSN:0021-8979    

DOI:10.1063/1.348438

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Stopping power calculations of fast ions penetrating a hydrogen plasma target in local thermodynamic equilibrium at arbitrary temperatures are performed. Excited state contributions to the energy loss are included in the framework of the Bethe formalism. Average ionization potentials for the excited ions are given in a quasiclassical approximation. It is shown that the net effect is an enhancement of the stopping power compared to the energy loss when assuming all atoms to be in their ground state.

ISSN:0021-8979    

DOI:10.1063/1.348439

出版商:AIP    

年代:1991

数据来源: AIP