摘要:

In this article we present some basic considerations in the design of a time detector for high energy (0.2–1.0 MeV) particle detection. Based on these considerations, a novel time detector design is proposed in order to overcome some of the problems in existing designs. Improvement in the timing resolution(<50 ps)and efficiency(>70&percent;for 4.0 MeV &agr; particles) is expected and a compact detector structure with ultrahigh vacuum compatibility is achieved. The new design employs a novel form of crossed electric and magnetic fields to deflect the electrons emitted from a thin foil to form a timing signal. In this design electrons are transported in a grid-free region and the energetic particle needs to pass through the grid once only. The electric field was produced by three metal plates forming a triangular prism. The magnetic field was generated by a coil pair which creates a uniform field in the electron transport region. Computer simulation and numerical analysis were performed to calculate the electric and magnetic field as well as the electron trajectory and flight time. The detector timing resolution is analyzed and the spread in electron initial energy contributes a significant portion. To avoid the complicated numerical details in field and trajectory calculations, we use an empirical approach in this article to illustrate the design principle. Some experimental results are presented to compare with the calculations. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148003

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A two-color laser polarimeter on large tokamaks such as ITER has been proposed for electron density measurement based on the midplane Faraday rotation. The two-color scheme in the polarimeter eliminates the Faraday rotation at vacuum windows from measured polarization angles. TwoCO2lasers which have different wavelengths of 10.6 and 9.27 &mgr;m are favorable as light sources of the two-color laser polarimeter for large tokamaks. A couple of photoelastatic modulators are used for stable polarization measurement even though the intensity and the propagation axis of an incident laser beam are changed. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148377

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A multichannel microwave interferometer has been built for the W7-AS fusion experiment in order to study temporal phenomena in plasma density. For this reason, electron density profile reconstruction methods based on regularization functionals were studied. Simulations showed, that the minimum Fisher-information approach (adapted from tomography) gives better results than the maximum entropy approach. This is due to the fact, that in the minimum Fisher-information method the reconstructed distribution is inherently assumed to be continuous, whereas the maximum entropy method the distribution is assumed to be noncontinuous. The profiles reconstructed from multichannel interferometer data are in good agreement with the profiles measured by other diagnostic systems. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148378

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article describes the use of Langmuir probes to measure plasma parameters in low density, low temperature plasmas with a strong applied magnetic field. The experiment has been performed at the Physics Department of Milan’s University on the steady-state toroidal machine “Thorello.” Results have been analyzed by taking into account instrumental and ionic sheath effects. Finally, experimental results have been compared with direct measurements of the electron distribution function in Thorello. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148346

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Theq-profile measurement presented in this article is based on the observation of pellets injected into the plasma and viewed with a fast-framing camera. The pellets sublimate in the plasma and the ablated material is ionized. The ions move along the magnetic field lines, while an ablation cloud is forming. This ablation cloud contains a small fraction of neutral particles which emit visible light after collisional excitation. It is therefore possible to visualize the magnetic field lines along which the pellet passes. The inclination angle of the magnetic field lines with respect to the torus midplane can be determined from this observation. The results are compared with the inclination angles delivered by an equilibrium code. Further analysis shows that it is not meaningful to determine theq-profile from the data of only the pellet ablation, in the case of an elongated plasma as in ASDEX Upgrade. However, the accuracy of theq-profile determined by an equilibrium code, especially in the plasma center, can be greatly improved by using the pellet measurements as additional input data. ©1997 American Institute of Physics. 

ISSN:0034-6748    

DOI:10.1063/1.1148386

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The technique of cantilever magnetometry is shown to be functional in pulsed magnetic fields. Employing micromachined single crystal silicon cantilevers and capacitance detection, we demonstrated a utilizable sensitivity to magnetic moment of2.5×10−12 Am2in magnetic fields to 36 T, representing an improvement of more than a factor of 10 over competing technologies. Torque magnetization measurements on microcrystals of anisotropic superconductors are presented as evidence of the feasibility of the technique in long pulse magnets of pulse duration∼0.1–1 s.©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148347

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An improved read-out electronics without flux modulation for dc superconducting quantum interference devices (dc SQUIDs) is presented where the preamplifier is directly coupled to the SQUID. Compared to our previous designs, the1/fvoltage and current noise levels of the preamplifier were reduced by about a factor of 4 (rms) by using bipolar matched transistors instead of operational amplifiers at the input. Three types of flux-locked-loop electronics were developed and successfully tested: a low-power (120 mW) version for large multichannel arrays, a high-speed version with a 15 MHz bandwidth and a very fast (300 ns) reset mode, and a low-noise(0.44 nV/Hz)version with a fast and precise bias reversal circuit. The flux-locked-loop electronics are compatible with each other, so that only one type of low-power (180 mW) control unit is required to operate them. All essential specifications of practical SQUID systems (noise, dynamic behavior, and temperature stability) were determined by using liquid-helium and liquid-nitrogen cooled SQUIDs, respectively. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148348

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A completely automated ac susceptometer is constructed to measure phase resolved linear and nonlinear ac susceptibility. Measurements over a wide range of experimental variables, such as ac and superimposed dc fields up to 300 Oe, frequency from 1 Hz to 100 kHz, and temperature from 77 to 300 K are made using only a lock-in amplifier (LIA), a PC, and some simple electronic circuits. A temperature-measurement and control unit is built utilizing the analog–digital input and digital–analog output of the LIA. Using a proportional and integral algorithm, a control accuracy better than 50 mK is achieved. The overall sensitivity of the setup is better than10−7emu and a reproducibility better than 0.1&percent; is achieved in the system. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148349

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This work reports temperature measurement of fine wires using the photothermal radiometry (PTR) technique. In this method, a laser is employed to create a small periodic temperature perturbation on the wire and thus a small modulation of the thermal emission signal from the wire. The temperature of the wire is derived from the ratio of the photothermal emission signals at two different wavelengths. Temperature measurement is performed on an electrically heated metallic wire of 127 &mgr;m in diameter. A calibration procedure is developed to account for the emissivity difference at the two signal wavelengths. The measured temperatures by the PTR technique are in good agreement with the thermocouple readings at different laser modulation frequencies. The possibility of extending the technique to optical fibers is discussed based on Mie scattering theory. Calculations suggest that the PTR technique can be used to measure temperature of optical fibers during the fiber drawing process by choosing an appropriate excitation laser and the signal wavelengths. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148350

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A balanced calorimetric device has been developed to measure the absorption cross section of asinglemacroscopic sized particle (in this case a carbon fiber) at a microwave frequency of 35 GHz, as a function of size and orientation with respect to the electric field. The technique used is based on simple thermodynamic principles that relate the electromagnetic absorption of a particle to its thermal response. A single particle is irradiated in a fixed volume of gas by an approximate plane wave. Absorption by the particle results in a change in the ambient pressure that is proportional to the absorption cross section of the particle. Comparison of measured quantities with recently developed theory agree well.

ISSN:0034-6748    

DOI:10.1063/1.1148388

出版商:AIP    

年代:1997

数据来源: AIP