摘要:

The construction and operation of a novel high temperature pulsed quartz nozzle employed for the generation of rotationally excited supersonic molecular beams are reported herein. At high stagnation pressures (≳ 1 bar) the NO molecules in the beam pulse exhibit a translational and rotational behavior identical to heated continuous nozzles (Trot<50 K;Ekin≊7/2kTN). At reduced stagnation pressures (10 mbar–1 bar) the rotational temperatures of the beam molecules are increased substantially (up toTrot≊700 K) accompanied by a translational behavior, which can be characterized by a small Mach number. The design has produced the rotationally hottest supersonic beam ever prepared in the pulsed mode.

ISSN:0034-6748    

DOI:10.1063/1.1145147

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A cell for examining the density profile of sheared fluids at the solid‐liquid interface by neutron reflectometry is presented. This cell has also proven valuable in examining near‐surface bulk structures in the plane perpendicular to the shear flow using small angle neutron scattering. The shear rates can be controlled by changing the volume flow through the cell over three orders of magnitude. All components of the cell are designed to be chemically inert. A temperature‐controlled environment compatible with neutron studies is also briefly described. Preliminary neutron reflectivity and small angle neutron scattering results using this cell are presented, and potential applications are discussed.

ISSN:0034-6748    

DOI:10.1063/1.1145148

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A small hollow cathode Penning surface‐plasma source (SPS) was developed and studied. The H−yield was proportional to the emission apertures area and increased over a wide range of discharge current. The H−yield, with an intensity of up to 0.95 A and an emission current density of up to 3.6 A/cm2, was obtained in a pulsed mode. With the discharge current of 20 A and a pulse duration of 60 s, an H−yield with current of 0.1 A was obtained. The H−emission current density had approximately the same value for various diameters (0.5–7 mm) and thicknesses (0.3–4.0 mm) of cylindrical emission holes, if the thickness of hole walls did not exceed the hole diameter. The H−yield extracted through the thick conical emission holes had a value 25% higher than that for a thin cylindrical hole with the same permeable diameter. Dependencies of the H−yield versus magnetic field and hydrogen feed were different from that of the standard Penning SPS. The optimal cesium coverage of the electrodes was stable for both high‐current, long‐pulse and low‐current, dc source operation. Noticeable H−production was realized with a low‐discharge voltage of 15 V. The heavy negative ions fraction was less than 1% of total negative ion yield for long‐pulse operation. The yield of D−ions from deuterium discharge had a value 15% lower than that of H−ions from the hydrogen discharge. Most of the extracted H−ions are produced by conversion of fast and superthermal (E≳1 eV) atoms on the cesiated surfaces of the emission hole cones or they are due to the resonant charge exchange of anode‐produced H−ions in the volume adjacent to the emission apertures.

ISSN:0034-6748    

DOI:10.1063/1.1145149

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Different methods are tested for the injection of small (<100 &mgr;m) pellets into tokamak plasmas using laser acceleration. In one type of method the pellets are formed in the random fragmentation process of a foil, while in another series of experiments aluminum granules were accelerated. In the first case pellet velocities up to 4 km/s were achieved, but the reproducibility was found to be poor. In the second case the reproducibility was much better, and pellets with diameters in the ten micron range were accelerated to 200–300 m/s velocity. With some further improvements, this technique may become a simple source of impurities for transport studies in fusion plasmas.  

ISSN:0034-6748    

DOI:10.1063/1.1145150

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The injection of pellets of frozen deuterium into magnetically confined, hot plasmas is one of the most attractive methods for refueling fusion power reactors. High injection speeds for solid deuterium pellets are necessary for improving the experimental analysis of pellet ablation in a hot plasma and the effects of the pellet on the plasma properties, in particular the confinement parameters. The optimization of a two‐stage light gas gun is being pursued; the search for the cryogenic conditions of freezing deuterium with good mechanical properties is carried out. In particular, the impurities concentration in deuterium during the condensation process has been studied. These developments have allowed one to reach velocities up to 3400 m/s for integer bare deuterium pellets (3 or 4 mm in diameter, 1–3×1021particles) with a reliability of 80%.

ISSN:0034-6748    

DOI:10.1063/1.1145151

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The present article describes prevention of electron emission by means of localized magnetic field covering a negative electrode to reduce power loss in an ion beam direct energy converter. An experiment was carried out to recover a 10‐cm‐diam helium ion beam with energy of 50 keV, where the prevention of electron emission was attempted by using a solenoid coil surrounding the electrode and with permanent magnets set on the electrode surface. When the solenoid coil was used, the negative electrode current hardly decreased due to increase in slow ion inflow from gas cell plasma generated by beam‐gas collision. On the other hand, the scheme with permanent magnets reduced the current loss successfully.  

ISSN:0034-6748    

DOI:10.1063/1.1145152

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Instrumentation for making localized, precise relative reflection measurements of metals and superconductors in the terahertz frequency range is demonstrated. The results can be used to determine the surface resistivity of these materials which is of particular importance to development of high‐temperature superconductors. Emission from a commerically available 1000 °C blackbody source was reflected from the materials under test and a reference reflector. The reflected signals were detected by a Schottky diode heterodyne receiver using a CO2‐laser‐pumped 214.5 &mgr;m CH2F2laser as the local oscillator. Double‐sideband bandwidth and receiver noise temperature were 2 GHz and 30 000 K, respectively. The use of a broadband incoherent diagnostic source minimizes instrumentation sensitivity to coherent interference effects. The heterodyne receiver provides for sensitive detection with good spatial and frequency resolution. Unoptimized spatial resolution of four times the diffraction limit was achieved. The potential for relative reflectivity measurement accuracy of better than 0.1% was demonstrated with 30 min measurement times though systematic errors limited actual measurement accuracy to about 0.3%.

ISSN:0034-6748    

DOI:10.1063/1.1145153

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We describe a single‐sideband homodyne reflectometer for sensitive impedance measurements in the 1 GHz range on electronic devices at liquid‐helium temperatures and below. At 50 pW measurement power, the instrument is capable of resolving a fractional change of 10−5in the impedance of a nominally 50 &OHgr; resistive device with a measurement time of one minute. We include details of an application to ac magnetoconductance measurements on mesoscopic devices, and aninsitucalibration method for this application.

ISSN:0034-6748    

DOI:10.1063/1.1145154

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We describe the design and operation of a circuit to combine the output signals of two high‐power radio transmitters into a single load. A working unit is realized using a commonly available ferrite and cheap coaxial cables. Performance tests indicate satisfactory operation from 3 to 50 MHz with less than 1 dB total power loss while guaranteeing adequate isolation between the transmitters. The power combiner can also be constructed specifically to withstand high line voltages for short time periods thus making it suitable for high‐power pulsed applications such as plasma formation and heating.

ISSN:0034-6748    

DOI:10.1063/1.1145155

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new approach of measuring the quality factor of the reflection type microwave cavity resonator for calculating the complex dielectric permittivity of solid and liquid samples in the cavity perturbation technique is proposed. This approach, based on the measurement of reflected power from the cavity at resonance, effectively reduces the measurement procedure and increases the accuracy. As the quality factor can be measured very fast using this approach, this approach can be extended to monitor the conductivity decay of the semiconductor samples.

ISSN:0034-6748    

DOI:10.1063/1.1145156

出版商:AIP    

年代:1994

数据来源: AIP