摘要:

The Ashby–Jephcott interferometer is used to measure the temporal variations in electron densities in plasmas. A double‐ended HeNe laser is used as part of an interferometer to make these measurements. In the present work, the experimental setup of the original method has been modified to prevent the drift in the temporal fringe patterns that are observed. This allows for more precise measurements of electron densities in plasmas. Further, the modification allows for commercially available HeNe lasers to be used to measure electron densities. The fringe patterns observed in aZ‐pinch discharge using the original Ashby–Jephcott method and the modified method are shown. The differences in the temporal variation of the electron density measurements using the two methods are discussed.  

ISSN:0034-6748    

DOI:10.1063/1.1144899

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The effect of field irregularity due to the use of conducting meshes to cover holes of a lower electrode in a parallel‐plate electrostatic analyzer, is experimentally and numerically investigated. Displacement of a focal point and degradation of analyzer characteristics are found in the experiment. Dependence of analyzer characteristics on wire spacing is obtained. Primary results are also confirmed by numerical analysis. Criteria for error estimation are theoretically derived and found to be consistent with the experiment.

ISSN:0034-6748    

DOI:10.1063/1.1144900

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

At the tokamak ASDEX upgrade, an extensive cooling water calorimetry system was installed. This system has measured the toroidal and poloidal distributions of the energy deposition by monitoring the temperature rise of the cooling water in 80 separate cooling units in the divertor plates and the central heat shield.

ISSN:0034-6748    

DOI:10.1063/1.1144901

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A reassessment of the basic equation governing calorimetry shows the crucial importance of temperature stability in the calorimeter environment. Following the principles derived, we have developed a new temperature control method and demonstrated it in a prototype calorimeter over a wide dynamic range (5 &mgr;W–2 W). Temperature control is achieved by using a metal block of high thermal inertia, surrounded by heat flow sensing thermopile arrays, which is embedded in an isothermal environment. Any temperature fluctuations in this environment are detected by the heat flow sensors and can be corrected by conventional feedback control. The advantage of this method is its high sensitivity to temperature changes: whereas conventional thermometers typically provide signals in the order of 1×10−3V/K−1, a correctly selected combination of inertial mass and thermopile arrays can provide signals in the order of 1 V K−1, thus permitting an improvement of temperature control by three orders of magnitude. A commercially available thermoelectric calorimeter in an isothermal environment controlled by the new method permitted measurement of sample powers with ±10 &mgr;W accuracy (long‐term, short‐term noise ∼±1 &mgr;W), whereas in a conventional thermostatic bath (temperature stability ±0.02 K h−1) its resolution was limited to ±3 mW.

ISSN:0034-6748    

DOI:10.1063/1.1144902

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

During lower hybrid current drive on tokamaks, suprathermal electrons are generated that emit hard x‐ray bremsstrahlung. A pinhole camera has been installed on the PBX‐M tokamak that records 128×128 pixel images of the hard x‐ray plasma bremsstrahlung emitted in the 30–200 keV photon energy range. The camera has a temporal resolution of 3 ms and a spatial resolution of 3–4 cm in the plasma. The detector is a 230‐mm‐diam hard x‐ray image intensifier. This instrument has identified hollow radiation profiles on PBX‐M, indicating off‐axis current drive. A detailed account of the construction of the hard x‐ray camera, its operation, and its performance is given.

ISSN:0034-6748    

DOI:10.1063/1.1144850

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A novel spectrograph comprised of a transmission grating, toroidal mirror, and microchannel plate detector is presented here. It is capable of detecting spectra in the wavelength range from 5 to 150 A˚ with a resolution of about 1.5 A˚. Owing it to high efficiency, even an x‐ray spectrum of a low‐energy laser produced plasma, i.e., emission of the order of 10−10J/sr, can be measured in a single shot.

ISSN:0034-6748    

DOI:10.1063/1.1144851

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new type of neutron spectrometer, which is called COTETRA (counter telescope with thick radiator), has been developed for diagnosing DT fusion plasma. It is based on the recoil proton measurement and has such advantages as: (1) direct measurement of the neutron energy without the unfolding procedure, (2) sufficiently fine energy resolution and high detection efficiency for 14 MeV neutrons, (3) wide dynamic range of counting rate, and (4) goodn–&ggr; discrimination. A prototype of COTETRA was constructed and tested using a DT neutron accelerator. The energy resolution of 5.3%±0.9% was obtained for 14 MeV neutron with the detection efficiency of 1.3×10−4[counts/(n/cm2)]. A Monte Carlo simulation code was developed and the performances of COTETRA were examined. The calculation agrees with the results of the experiment within its margin of error and suggests that energy resolution up to 3% with a detection efficiency of 10−5[counts/(n/cm2)] could be achieved if the condition of the radiator could be successfully adjusted.

ISSN:0034-6748    

DOI:10.1063/1.1144852

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An improvement of the pulse‐echo‐overlap (PEO) method is proposed to make it accessible for computer control. The modification makes it possible to avoid the systematic and subjective errors of the original PEO method. A digital oscilloscope and a frequency synthesizer is controlled by a personal computer providing a fast automatic method for determination of ultrasonic wave transit times. Description of the method and the computer program, as well as the technical parameters is given. The Panametrics 5053A ultrasonic time intervalometer has been slightly modified to be applicable in the new configuration with a better precision. The 0.5 ppm precision velocity determination in water requires a 0.0003 °C level temperature control and reproducible wettability of the surface of the transducer. For both problems, a practical solution is proposed. Error analysis shows that the reproducibility of measurements is within 0.5 ppm.

ISSN:0034-6748    

DOI:10.1063/1.1144853

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A nonconductive ferromagnetic core is inserted into the pickup coil of a superconducting flux transformer which is matched to a 19 MHz rf‐superconducting quantum interference device (SQUID). We demonstrate that the flux sensitivity of the SQUID is enhanced by a factor of 2–2.7 with respect to a comparable air core flux transformer for signals up to 80 kHz. The equivalent flux noise of 2×10−3&Fgr;0/&sqrt;Hz is of the same magnitude as for a conventional transformer and associated with external fluctuations from vibrations and from the superconducting shields. We take as a source a very thin spin sheet like that which is formed by spin‐polarized electrons excited in thin semiconducting epitaxial layers in a photomagnetization experiment. Under experimental conditions (changes of the irradiated area, absorption depth, beam deflections) one expects distinct distributions of spins in the sheet and corresponding variations of the flux depicted by the flux transformer. To detect the magnetic moment of a certain number (≳1010) of spins the proportionality between the detected flux &Fgr; and the excited total magnetic momentmof the spins is checked. We compare various configurations of spin sheets of distinct size and locations beginning with an air core flux transformer. The flux threading the pickup loop is then more efficiently coupled by employing a ferromagnetic shell‐shaped core with an axial hole (for the entrance of the light beam in photomagnetization studies). Thus flux line patterns are simulated for various sizes of cores, different permeabilities, and different air slits (where the sample is located). Due to additional degrees of freedom of design parameters a simultaneous increase of sensitivity is achieved together with a response &Fgr;∝m. Simulations and measurements are compared. The ultimate resolution of the magnetic moment is &Dgr;m=10−13A m2/&sqrt;Hz.

ISSN:0034-6748    

DOI:10.1063/1.1144854

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Electric and magnetic characteristics of a Co‐Fe‐Si‐B‐based amorphous thin wire with a diameter of 120 &mgr;m were investigated at low temperatures from an applicative point of view. The electric resistivity decreases with the decrease of temperature and has the minimum point at about 30 K. The maximum decrease of the resistivity is, however, only 2.7% as compared with that at room temperature. The magnetic characteristics in term of the coercive force, the saturation induction, and the maximum permeability are all increased with the decrease of temperature. The coercive force shows the increase of 25% at 4.4 K as compared with that at room temperature; and the saturation induction and the maximum permeability about 15%. The very slight decrease of electric resistivity and the increase of the coercive force with the decrease of temperature promise a high‐frequency application of the wire at low temperature. These results led to the fabrication of a multivibrator‐type magnetometer being able to work at low temperature below 77 K. The magnetometer has a sensitivity of about 1 mV/mG at room temperature in a magnetic field range between +1 and −1 Oe. The sensitivity change is less than 0.5% even at low temperature of 6 K, as compared with that at room temperature.

ISSN:0034-6748    

DOI:10.1063/1.1144855

出版商:AIP    

年代:1994

数据来源: AIP