摘要:

The large field of flow visualization is briefly reviewed. Definition, purpose, and classification into subfields are outlined, and the reader is directed toward more detailed and more specialized reviews. The basic methods of flow visualization are explained and the long history of flow visualization is sketched by tracing the introduction of major methods and of discoveries made by flow visualization. Limitations and problem areas in the visual evaluation of velocity and vorticity fields are discussed, and a glimpse into the future role of flow visualization is ventured.

ISSN:0034-6748    

DOI:10.1063/1.1144433

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Improvements for two established methods for plasma‐density measurements have been developed: (a) microwave interferometry and (b) single Langmuir‐probe technique. The new microwave interferometer design allows us to avoid a rigid waveguide as the reference line and therefore results in a high flexibility for the use at plasma chambers with different geometries. The system is operated at 35 GHz and has a bandwidth of 10 MHz. The cylindrical single Langmuir‐probe system has been equipped with a new rf‐suppression circuit. This allows the application of probes of nearly arbitrary geometrical dimensions in rf‐driven plasmas. The circuit is simply mounted outside the vacuum chamber between the probe feedthrough and the dc circuit. The performance of these two developments has been demonstrated and comparative measurements of charge carrier densities in a conventional rf‐driven plasma show good agreement between the two methods.

ISSN:0034-6748    

DOI:10.1063/1.1144432

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A double‐crystal x‐ray spectrometer (DCS) has been used to obtain high‐resolution x‐ray spectra of laser‐produced plasmas. The DCS has a high dispersion, and thus can obtain highly resolved spectra in circumstances where the resolution of a conventional single‐crystal Bragg spectrometer at comparable source‐to‐detector distances would be limited by the source size. In addition the DCS also yields unit‐magnification one‐dimensional spatial resolution in the direction perpendicular to the spectral direction. The instrument produces two spectra symmetrically disposed about a central wavelength, which makes it especially useful for measuring spectral line shifts. Analysis of the spectra from hydrogenic aluminium and helium‐like chlorine indicates a resolving power (&lgr;/&Dgr;&lgr;) greater than 2500 and a spatial resolution better than 30 &mgr;m.

ISSN:0034-6748    

DOI:10.1063/1.1144397

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A four‐grid electrostatic energy analyzer for measurements of the ion velocity distribution and the emission of secondary electrons on the electrodes of low‐pressure radio frequency glow‐discharge systems has been conceived. Problems arising from poor analyzer design are discussed and the performance of the presented analyzer is shown for measurements of the ion velocity distribution in pure hydrogen, helium, and argon discharges. Moreover, the secondary electron yields on aluminium, stainless steel, copper, and amorphous silicon exposed to radio frequency argon, helium, and hydrogen plasmas are determinedinsitu, for the first time to our knowledge. In parallel‐plate radio frequency discharges secondary electron emission involves the contributions of ions, fast neutrals, metastables, and photons impinging on the electrode surfaces. Therefore, secondary electron emission must be considered as a global phenomenon. Global secondary electron emission can be up to ten times larger than only ion‐induced secondary electron emission. Typically, the global emission coefficient is of the order of 0.1 (secondary electrons per ion) but can exceed 1 when the contribution of metastables is important. This is a noteworthy result since secondary electron emission is often neglected or underestimated in modeling of electrical discharges.

ISSN:0034-6748    

DOI:10.1063/1.1144398

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A highly sensitive absorption experiment for diagnosing glow discharge plasmas is described. This experiment is applicable from the VUV to the IR. A very stable Xe arc lamp is used as a source of continuum radiation. An echelle spectrometer equipped with a gated, image‐intensified, charge‐coupled device detector array is used to disperse and detect the continuum, with absorption features, after it has traversed the glow discharge. Digital subtraction is used to discriminate against the line emission from the glow discharge and detect only the continuum emission from the arc discharge. Estimates of the relative spectral radiances of glow and arc discharges suggests the subtraction technique is broadly applicable to glow discharge studies. A fractional absorption of 10−3is detectable with a signal‐to‐noise ratio limited primarily by shot noise. A detection limit for excited Hg atoms of 7×109cm−2is demonstrated in a 400‐mA Hg–Ar glow discharge. Further improvements in the experiment are proposed.

ISSN:0034-6748    

DOI:10.1063/1.1144399

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A modified Michelson interferometer was used to probe the electron density in plasma plumes produced by pulsed XeCl laser ablation of Al2O3. With one of the mirrors of the interferometer translating steadily, the phase angle of the sinusoidal interference signal was tracked and the XeCl laser was fired whenever the detected phase angle matched a predefined value. The transient interference waveform produced by the plasma plume was then synchronously captured. This moving‐mirror interferometer features minimal vibration isolation, fast response time (∼10 ns), powerful noise rejection, and a detection limit of a thousandth of a fringe shift, or ∼1015electrons per cm3for mm size plumes.

ISSN:0034-6748    

DOI:10.1063/1.1144400

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The design and operation of a pure electron plasma imaging system is described. The system yields images with high resolution and a large dynamic range. A two‐dimensional image of the plasma is acquired by streaming the plasma directly onto a microchannel plate and phosphor screen. Resolution of the rapidly evolving plasma requires that all the plasma strike the microchannel plate in a single charge pulse lasting less than 1 &mgr;s. The expected performance and actual operational characteristics of the system are detailed, with special attention given to the properties of a microchannel plate used in a pulsed imaging mode.

ISSN:0034-6748    

DOI:10.1063/1.1144401

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The enthalpy or calorimetric probe is a water‐cooled stagnation/sampling probe for studying the temperature, velocity, and composition of hot‐gas flow fields. In order to derive the thermodynamic properties of complex flow fields such as plasma arc jets or high‐velocity oxygen fuel jets, the specie concentration must be known accurately. To this end a differentially pumped quadrapole mass spectrometer has been integrated with a fully automated enthalpy probe system. An inexpensive modular probe is described along with the system and its theory of operation. Calibration and error are also discussed. Typical results are presented for the system operating in an argon/helium plasma arc jet in atmospheric pressure air. The maximum temperature measured is 13434 K on the center line of the jet, 5 mm from the exit, with a corresponding velocity of 1295 m/s. The utility in integrating the mass spectrometer to the enthalpy probe system is not only an accurate measurement of the gas mixture components for obtaining the correct property information, but also valuable information can be obtained about demixing diffusion and chemical reaction taking place in the plasma plume. The relative amount of argon to helium is shown to deviate from the nominal mixture by as much as 40% at the center of the plume.

ISSN:0034-6748    

DOI:10.1063/1.1144402

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A novel small and rugged type of a free running atmospheric pressure nitrogen laser is presented and its behavior is studied in detail. Pulse half‐widths are between 800 ps for 60 mm active channel length, and 1.1 ns with pulse energies of 150 &mgr;J for 170 mm active length, both at 2.4 mm laser gap. The maximum pulse energy is found at a field strength‐to‐pressure ratio around 110 V/(cm Torr), in agreement with what is reported in literature for nitrogen lasers. Preionization by ultraviolet light from the free running spark gap could be ruled out while corona discharge over the capacitor foil surface between the laser electrodes could not be excluded.

ISSN:0034-6748    

DOI:10.1063/1.1144403

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The performance of a pulsed CO2laser using the capacitor transfer circuit is generally known to improve with an optimum peaking‐to‐storage capacitor ratioCp/Cswhich varies from around 0.1 to almost 0.5. There has been no reported attempt to account for precisely the origin of this optimum ratio and the large difference amongst them with different experimental setups. In this paper, the optimum capacitor ratio is shown to depend uniquely on the distributed circuit inductances. This may be understood from a simple consideration of the impedance matching for achieving an optimum power transfer at a quasiconstantE/Nratio into the main discharge.

ISSN:0034-6748    

DOI:10.1063/1.1144404

出版商:AIP    

年代:1993

数据来源: AIP