摘要:

A simple and compact x‐ray luminosity time‐of‐flight spectrometer for measurements of the velocity distribution of highly charged ions in laser‐produced plasmas is presented. The spectrometer is based on making time‐ and space‐resolved measurements of x‐ray line emission from these ions in the expansion zone. Design, operation, and performance of this system are described. The spectrometer is useful in studying processes of interaction between highly charged ions and buffer gases, and nonthermal behavior of laser‐produced plasma expansion.

ISSN:0034-6748    

DOI:10.1063/1.1139729

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We present a technique for rapidly acquiring time‐resolved, ensemble‐averaged Langmuir probe characteristics. Fifty probe characteristics are acquired using a digital storage oscilloscope in the time it would take to acquire a one‐probe characteristic using a single‐channel boxcar averager. A single Langmuir probe is used, and the probe bias is swept quite slowly, so that the probe is always in equilibrium with the plasma. A method for the automatic extraction of electron temperature, electron density, and the plasma potential from the acquired probe characteristics is described. This technique for acquisition and analysis is applied to the study of plasma decay and the effects of rf excitation in a pulsed, strongly magnetized plasma.

ISSN:0034-6748    

DOI:10.1063/1.1139730

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A nondestructive method for measuring the mass of high‐speed, frozen hydrogen pellets is described. The measurement technique is an improvement on previous methods based on the perturbation to a resonant cavity caused by a dielectric pellet passing through the cavity. In the new method, an oscillator circuit is formed with a resonant cavity in the positive feedback loop of a microwave power amplifier. An injected pellet perturbs the resonance characteristics of the cavity causing a shift in the operating frequency of the oscillator proportional to the ratio of the pellet volume to the volume of the cavity. Through digital measurement of the frequency shift the size of the pellet is determined automatically. In a ‘‘proof‐of‐principle’’ experiment it was shown that polyethylene pellets, with an average volume of 0.26 mm3and velocity ≊500 m/s, caused a shift in the operating frequency of the oscillator circuit that was within 10% of the predicted value. The hydrogen pellets (of interest to us) will have 4–10× the volume of the plastic pellets. Thus with a dielectric constant of about 1.26 for frozen hydrogen, the signals in the actual experiment would be 1–2× those described here.

ISSN:0034-6748    

DOI:10.1063/1.1139731

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A control and averaging system for microwave Fourier transform spectroscopy is described, in which data acquisition and accumulation are performed in parallel to obtain a maximum repetition rate of 75 kHz. Details of circuitry as well as examples of performance are given.

ISSN:0034-6748    

DOI:10.1063/1.1139732

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A photolithographic technique for fabrication of very smooth and highly uniform metallic, free‐standing, inductive meshes, is described. The fabrication process was developed in order to produce highly reflective, low‐loss Fabry–Perot e´talon elements for use in the infrared spectral region. The measured rms smoothness of the grids is typically ≤30 A˚. Because of their nickel core, the mechanical strength of the grids is such that they can be easily stretched flat on stainless‐steel rings.

ISSN:0034-6748    

DOI:10.1063/1.1139733

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have built a new apparatus to time resolve ultrafast fluorescence following ultraviolet excitation. A synchronously pumped dye laser produces optical pulses of 1‐ps or 70‐fs full‐width half‐maximum, depending upon dyes and optical configuration. These pulses are amplified at a 8.2‐kHz repetition rate using a copper vapor laser‐pumped multipass amplifier. The resulting amplified laser pulses are frequency doubled to obtain ultrafast pulses in the ultraviolet. This ultraviolet light is used to electronically excite a sample; the resulting fluorescence is time resolved using fluorescence upconversion as the optical gating technique. A minimum 300‐fs full‐width half‐maximum instrument response function is obtained. After a brief introduction, we discuss the principles involved in this method of time resolving fluorescence. We review special considerations for femtosecond laser experimentation, and discuss the construction of our apparatus. Finally, as an example, we show how this system can be used to study fundamental physical processes in solution.

ISSN:0034-6748    

DOI:10.1063/1.1139734

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A CO2laser interferometer and fringe counter, made of commercially available equipment, which can resolve displacements of 5 &mgr;m and accurately track velocities of up to 50 m/s in either a positive or negative sense, is described. Coupled to a heterodyne interferometer, this simple fringe‐counting circuit does not require exact quadrature to determine the direction of motion, is wideband and capable of high count rates (dc—10 MHz) and will not introduce false fringe counts at phase reversal or due to dithering of the target. A prototype has been built and employed to characterize the operation of a pulsed gas valve.

ISSN:0034-6748    

DOI:10.1063/1.1139735

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Avalanche photodiodes biased above the breakdown voltage are an interesting alternative to photomultiplier tubes in time‐correlated single‐photon counting. The characteristics and performance of a commercially available device (RCA C30921S) have been investigated. The time resolution is found to improve as the excess bias above the breakdown voltage is increased. Full width at half‐maximum values down to 400 ps have been measured with the detector cooled at −40 °C, and down to 460 ps at room temperature. The best results were obtained with an active quenching circuit, suitable for operation with excess bias voltage up to 40 V; at room temperature, fast gated operation was used for attaining optimum performance. Experimental data on the statistical behavior of the avalanche current pulses in these devices are reported and discussed.

ISSN:0034-6748    

DOI:10.1063/1.1139736

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

This paper characterizes an imaging x‐ray detector formed by coupling a gadolinium oxysulphide phosphor to the input of an optical imaging photon detector. The device is small, light, easy to use, and features a direct digital readout. It exhibits fairly high efficiency (40%–80%) and high resolution (160 &mgr;m‐width point spread function) near the center of its 40‐mm active area. There is a small amount of pincushion distortion which seems to be associated with a loss in resolution toward the edge of the active area. The device has very low noise and can be used at x‐ray fluxes down to about 0.1 x‐ray/mm2/s without loss of accuracy but it is count rate limited at 105x rays/s over the active area and so is not usable in high‐flux situations such as are often found at synchrotrons. We have used the device to record good diffraction patterns from striated rabbit muscle in 30 min on a rotating anode x‐ray generator: less than one tenth the time needed under similar conditions when using film.

ISSN:0034-6748    

DOI:10.1063/1.1139737

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A double‐crystal x‐ray spectrometer designed for in‐laboratory EXAFS (extended x‐ray absorption fine structure) study is described. By the use of two curved crystals with different structure factors such as Ge(220) and Ge(111), reflections other than the desired order are virtually eliminated and monochromatic as well as intense x‐ray flux can be obtained by applying higher voltage to the x‐ray generator. As a result, data of very high quality can be collected in a short period by measuring the incident and the transmitted beam intensity simultaneously. It can cover a wide spectral range (7–26 keV) and is especially useful in the high‐energy region, where the laboratory EXAFS system previously reported has some difficulties.

ISSN:0034-6748    

DOI:10.1063/1.1139738

出版商:AIP    

年代:1988

数据来源: AIP