摘要:

The use of negative hydrogen ions in different accelerators, especially in charge‐exchange injection of protons into orbits of high energy accelerators, is reviewed. The principles of charge exchange for particle acceleration and control and the benefits are discussed after which the performance and characteristics of selected accelerators are summarized. Recent trends in negative ion sources for charge‐exchange accelerators also are reviewed. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147513

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We describe a time‐resolved pulse‐counting system well adapted for the detection of continuous laser induced fluorescence (LIF) signals in repetitive phenomena, when a strong background emission is present. It consists of 256 channels coupled to a first in first out memory and interfaced to a 486 DX 33 PC, for data storage. It accepts time‐averaged count rates up to 450 kcount/s. Time between channels can be set from 12.5 ns to several &mgr;s and the dead time between two consecutive cycles of the physical phenomena is less than 20 ns. In phase with a chopper, which modulates the laser beam, it adds the observed photon signal to the channel memories when the beam is on and substracts it when the beam is stopped, acting like a lock‐in amplifier which detect only the modulated part of the signal. The minimum detectivity on the LIF signal is only limited by the shot noise of the plasma induced emission signal. As an application, we studied the time variation of the Ar+*(2G9/2) metastable ions, detected by LIF, in two types of plasmas. Their radiative lifetime and collisional quenching frequencies were deduced from their decay rate in the afterglow of a pulsed Helicon reactor. We also observed the evolution of their density in a 455 kHz capacitively coupled argon discharge. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147150

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report in this article an experiment of laser‐based continuous‐wave (cw) midinfrared difference‐frequency generation using a synchronous scanning technique for continuous tuning of the complete spectral region from 4.7 to 6.5 &mgr;m (1550 to 2200 cm−1). A wavelength‐tuned, noncritically phase‐matched AgGaS2crystal was used as the nonlinear optical mixing medium. Midinfrared output power of ∼10 &mgr;W was obtained by mixing two tunable lasers with ∼430 mW total pump power. The pump lasers were synchronously scanned during infrared wavelength tuning. The infrared frequency was deduced from the difference between the two laser frequencies which were simultaneously measured by wavemeters with an absolute accuracy of better than 0.007 cm−1. No reference cell was needed for frequency calibration. High‐resolution spectra of carbonyl sulfide (OCS) were recorded for an evaluation of the frequency calibration and high‐resolution spectroscopic characteristics. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147151

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A multiple interferometer using the short coherence length of white light has been constructed and its operation demonstrated. With this apparatus, it is possible to count the number of light reflections between two parallel mirrors. The shift in fringe position resulting from differences in the optical thickness is proportional to the number of reflections between the two mirrors, enabling the resolution and measurement of a step difference of the order of 20 A˚. This technique inherently has higher resolution than Michelson interferometers. As compared with Fabry–Perot interferometers, it offers an independent determination of the density of fringes, and has lower sensitivity to errors in minor parallelism. Further, the method does not require the extreme closeness in mirror spacing as compared to the Tolansky interferometer. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147152

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this article, the phase change of interference in the shadow region of a wedge‐shaped object with partially coherent light is illustrated. It is shown that this phase change can be applied to the detection of the width of adjacent boundaries of small samples with partially coherent light. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147153

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This article treats a class of systematic errors in determining the intensity–intensity correlation functionS(t) with a digital correlator. Errors in determiningS(t) will lead to incorrect computations of apparent diffusion coefficients and size distributions for micelles, vesicles, and other supramolecular assemblies. Systematic errors in interpreting the spectrumSd(tm) measured by real correlators, in terms of the true spectrumS(t), are minimized by optimum interpretations of the delay timetmof each correlator channel. This identification is more complex for multitau correlators than for simple linear correlators. For a multitau correlator with current and delayed sampling timesTand &thgr;, and a delay channel beginning &tgr; after the beginning of the zero‐time sampling channel, the optimum choice istm=&tgr;−T/2+&thgr;/2. The so‐called ‘‘half‐channel’’ correctiontm=&tgr;+&thgr;/2 is erroneous. For the ‘‘zero delay channel’’ seen on some correlators, the most appropriate identification istm=T/3. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147154

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An apparatus and technique are described for the characterization and energy stabilization of a monochromatic x‐ray beam from a synchrotron radiation source that is used for diffraction experiments. The energy of the beam between 6–30 keV can be absolutely measured to an accuracy of 0.3 eV and continuous stability can be guaranteed to 0.2 eV for the duration of an experiment. The particular case of multiple wavelength anomalous diffraction is considered in detail. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147155

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The feasibility of microcantilever‐based optical detection is demonstrated. Microcantilevers may provide a simple means for developing single‐element and multielement infrared sensors that are smaller, more sensitive, and lower in cost than quantum well, thermoelectric, or bolometric sensors. Here we specifically report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers, such as those used in atomic force microscopy. In this work, optical transduction techniques were used to measure microcantilever response to remote sources of thermal energy. The noise equivalent power at 20 Hz for room temperature microcantilevers was found to be approximately 3.5 nW/&sqrt;Hz, with a specific detectivity of 3.6×107cm Hz1/2/W, when an uncoated microcantilever was irradiated by a low‐power diode laser operating at 786 nm. Operation is shown to be possible from dc to kHz frequencies, and the effect of cantilever shape and the role of absorptive coatings are discussed. Finally, spectral response in the midinfrared is evaluated using both coated and uncoated microcantilevers. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147149

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A compact and flexible circuit for operating avalanche photodiodes in Geiger mode was designed, fabricated, and tested. A new voltage driver stage, based on fastn‐channel double‐diffused metal–oxide–semiconductor (DMOS) transistors in a bootstrap configuration, makes it possible to obtain quenching pulses up to 25 V amplitude and fast active reset of the detector. At 20 V excess bias voltage above the photodiode breakdown level, an overall deadtime shorter than 36 ns is attained. The avalanche pulse charge is minimized by means of a mixed passive–active quenching approach, thus reducing self‐heating and afterpulsing effects in the photodiode. A user‐controllable hold‐off time is available for further reducing the afterpulsing effect. The saturated counting rate of the circuit exceeds 25 Mcounts/s, but, by working with avalanche photodiodes with high breakdown voltage (250–400 V) and high avalanche current (10–40 mA), a practical limit is set at about 9 Mcounts/s by thermal effects in the detector. Gated‐detector operation with gate times down to 10 ns is provided. The suitability of the new active‐quenching circuit for the development of compact, all‐solid‐state instruments for high‐performance photon counting was verified in experimental tests. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147156

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik’s coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147157

出版商:AIP    

年代:1996

数据来源: AIP