摘要:

The statistics of the atom‐by‐atom dissection of planes, in the atom‐probe field‐ion microscope, have been investigated. Tungsten specimens oriented in the [110] direction, with the probe hole over the center of the plane, were slowly pulsed field‐evaporated on a plane‐by‐plane basis, and statistical analyses were made on the number of tungsten atoms detected per plane; 30 separate slow dissection experiments were performed. Observed fluctuations in the number of atoms per plane are used to infer a range of allowable values for the detection efficiency. We find that, in some cases, the number of atoms per plane can be described as following a binomial distribution. From these results detection efficiencies in the range 0.11 to 0.54 were inferred. This range of efficiencies can be understood with the aid of field‐ion desorption images. In addition, a value for the detection efficiency of 0.2 was obtained under the assumption that all atoms in the area projected by the probe hole, along the specimen radius, were analyzed. Thus, we find that this geometrical procedure, a first‐order approach to the problem, yields a reasonable result. Also, the results of a Monte Carlo simulation of atom‐by‐atom field evaporation of a large number of planes are presented. The Monte Carlo simulation shows that if a binomial distribution is obtained, the uncertainty in concentrations determined by the atom‐probe technique will have only a small component owing to the uncertainty in the number of solvent specimen atoms—this is subject to the caveat that there are no special problems with the field‐evaporation behavior of the solvent atoms. Although the statistical analyses were applied to a specific crystallographic plane and position, the methodology is reasonably general and can be applied to other situations.

ISSN:0034-6748    

DOI:10.1063/1.1137527

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A grating polychromator for electron cyclotron temperature diagnostic experiments is described. Two aluminum gratings are used to allow the instrument to operate for tokamak magnetic fields between 16–25 kG, i.e., for second harmonic electron cyclotron emission frequencies ranging between 90–140 GHz. The instrument utilizes an array of ten cryogenically cooled InSb hot electron bolometers to collect radiation at the focal surface of the spectrometer covering a fractional plasma radial distance of &Dgr;R/R0∼0.3–0.4, whereR0is the major radius of the plasma. The resolving power of the spectrometer has been measured to beR=&ngr;/&dgr;&ngr;≊50. This instrument thus provides high spatial (3 cm) and temporal resolution (∼1 &mgr;s) and large spatial coverage. The detector cryostat has measured cryogen hold times of ≥24 h (liquid nitrogen) and ≊9 days (liquid helium) providing ease of operation during long plasma runs.

ISSN:0034-6748    

DOI:10.1063/1.1137528

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

We have developed a multichannel x‐ray analyzer for monitoring electron temperatures in plasmas. The device consists of a step‐wedge x‐ray filter and a Reticon sensor array. By measuring the x‐ray continuum emission from a laser‐produced plasma with the device, the electron temperature can be determined to within 10%.

ISSN:0034-6748    

DOI:10.1063/1.1137529

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

We report details of an x‐ray sensitive streak camera with an estimated time resolution of less than 10 ps. The streak camera is based on a modified RCA 73435 image tube and features a large photocathode area and high sensitivity. A direct comparison of the relative quantum efficiency of Au and CsI photocathodes for 1–10 keV x rays is also presented.

ISSN:0034-6748    

DOI:10.1063/1.1137530

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A plug‐in module bearing 14–20 tungsten filament emitters has been developed for use with the field‐free neutral beam sources to be employed in the Tara tandem mirror experiments. Eight such modules constitute the total distributed cathode for one source. Each module has its own plug‐in power connector, and may be removed from the source assembly as a unit, independently of the outer modules. The plug‐in approach to construction results in a reduction both in the cost to build the source and in the time required for routine servicing and filament replacement. Plasma profiles are the same as in conventionally engineered sources of the same geometry.

ISSN:0034-6748    

DOI:10.1063/1.1137531

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The effect of magnetic field on the performance of a hollow cathode ion source was experimentally studied. The field strength and the field distribution around the hollow cathode were changed step by step and the source parameters were recorded for each step. The result showed that with the optimum field configuration the discharge was stabilized even at low‐operating gas pressures and the arc efficiency of the source was improved. Examination of the field and the neutral gas density distributions along the hollow cathode axis made it clear that the optimum field configuration was such that electrons emitted from the hot cathode surface might easily go out of the cathode cavity through the orifice being guided by the lines of force. Beam extraction from an intensively water‐cooled small bucket source with the hollow cathode was carried out and hydrogen ion beams of up to 3 A at 50 keV were successfully obtained for the pulse length of up to 10 s.

ISSN:0034-6748    

DOI:10.1063/1.1137532

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A new method is described for simulating charged particle rays in arbitrary electrostatic lens systems of cylindrical symmetry. The method is so efficient (<4 ms/step) that even for complex lens systems the trajectories can be computed and displayed during the lens voltage adjustment, using the same laboratory microcomputer which controls the lens voltages. The time lag between a change of a lens element voltage and the trajectory display is of the order of a few seconds for long, multielement lens systems. This also allows one to optimize the voltage combinations by performing trajectory calculations for many lens voltage settings. The performance is demonstrated by comparing with recent experimental data of Heddle and co‐workers on imaging properties of a three‐element tube lens. As an application of the fast computation time feature, trajectories and voltages are shown for a lens system providing an electron optically adjustable angle resolution as used in a spin polarized photoemission experiment with synchrotron radiation.

ISSN:0034-6748    

DOI:10.1063/1.1137533

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A semiconductor detector is described which responds to the atomic hydrogen in an atomic beam but is insensitive to molecular hydrogen. The hydrogen flux is measured through the change in conductivity of the semiconductor material which occurs when the hydrogen is chemisorbed. The atomic flux from a rf hydrogen discharge is used to determine the sensitivity of the detector. The minimum detectable signal is ∼109hydrogen atoms mm−2 s−1. The detector has a response time of less than 2 ms.

ISSN:0034-6748    

DOI:10.1063/1.1137526

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The determination of accurate subnanosecond fluorescence lifetimes by the time‐correlated single‐photon counting technique is often limited by difficulties in obtaining the correct instrumental response functionf(&lgr;,t) to the excitation pulse. These difficulties are increased when a grating monochromator is used to disperse the emission due to the introduction of an additional temporal broadening off(&lgr;,t). A technique for determining the correctf(&lgr;,t) at the wavelength of sample emission, &lgr;em, is described. The technique consists of using a very short‐lived fluorophore to ‘‘mimic’’ the excitation pulse shape at &lgr;em, yielding the convoluted mimic decay functionCm(&lgr;em,t), and then to computationally extractf (&lgr;em,t) fromCm(&lgr;em,t). The technique is experimentally and computationally simple and yields the desired instrumental response function at &lgr;emwhich eliminates problems due to the sensitivity off(&lgr;,t) to &lgr; (color shift artifact). The photomultiplier tube receives spatially equal illumination when both the sample decay andCm(&lgr;em,t) are determined because both the mimic and sample emission are spectrally broad, eliminating the problem of the variation off(&lgr;,t) with position of incident light on the photomultiplier tube photocathode (targeting artifact). Artifacts due to monochromator temporal broadening are eliminated since both the mimic and sample emission are monitored at constant wavelength.

ISSN:0034-6748    

DOI:10.1063/1.1137534

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Design and construction of a CO2laser‐triggered plasma shutter is described. An inexpensive pyroelectric detector and a novel needle‐resistor spark gap, requiring an applied voltage of less than 500 V, are employed. The total electronic delay can be made to be less than 15 ns with subnanosecond jitter. This plasma shutter finds applications in producing picosecond CO2laser pulses and in optical isolation.

ISSN:0034-6748    

DOI:10.1063/1.1137535

出版商:AIP    

年代:1983

数据来源: AIP