摘要:

The design and construction of a novel capacitive probe array to directly measure the ionization growth in high‐voltage discharges is presented. By optimizing the design geometry, the probe output is made to be dependent only on the variation of charge development or potential changes at the vicinity of the probe in the interelectrode space. A set of these probes was used in an experiment to allow the time and space development of a virtual anode to be directly observed.

ISSN:0034-6748    

DOI:10.1063/1.1144677

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A broadband heterodyne reflectometer, operating in the frequency range 75–110 GHz in extraordinary mode polarization, has been installed and operated at the Wendelstein 7‐AS stellarator for the study of density fluctuations and density profile determination. At 2.5 T it covers the density range between 5×1018and 5×1019m−3. The system differs from the usual setup as it combines the advantages of heterodyne detection with broadband capability avoiding the limitations of phase‐locked sources and permits the unambiguous determination of the phase delay independent of amplitude fluctuations in the reflected beam. After a first downconversion to an intermediate frequency of about 6 GHz, two additional mixing steps lead to an intermediate frequency of 60 MHz, which carries the phase delay from the plasma as a phase modulation. The phase information is yielded numerically from sin/cos detection at 60 MHz. Due to the balanced detection scheme, the drift as well as the broadband noise of the signal and local oscillators are canceled. The system has a dynamic range of about 60 dB and is within this range almost insensitive to amplitude fluctuations of the reflected beam. Examples are given demonstrating the potential of the system installed.

ISSN:0034-6748    

DOI:10.1063/1.1144678

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A simple technique has been proposed and tested in a dc plasma machine to determine the second derivative of a Langmuir probe characteristic. The principle of current continuity was used to arrange the current sampling network of a group of three probes so that the second‐order difference was achieved without any active arithmetic circuits. In this way, the second derivative signal of the probe characteristic is obtained automatically and accurately. The error remaining in this technique can be monitored easily and then minimized to a tolerable extent.

ISSN:0034-6748    

DOI:10.1063/1.1144679

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An 80‐channel soft x‐ray tomography system has been constructed for diagnosing the RTP (Rijnhuizen Tokamak Project) tokamak plasma. Five pinhole cameras, each with arrays of 16 detectors are distributed more or less homogeneously around a poloidal plasma cross section. The cameras are positioned close to the plasma edge to enable viewing angles which could not be reached otherwise. The system can be used for making tomographic reconstructions of the soft x‐ray emissivity with a relatively high poloidal mode number (up to both components ofm=4), which is an improvement with respect to other plasma tomography systems presented in literature. The temporal resolution of the system is 5 &mgr;s. Apart from a description of the hardware and geometrical layout of the system, the paper will focus on two important issues, namely the calibration of the detectors and the positioning of the cameras in the vacuum vessel of the tokamak. A dedicated x‐ray source has been designed for the relative calibration of the detectors of each individual camera. Misalignment causes several artifacts in tomographic reconstructions obtained with multicamera systems. Therefore, two new methods are proposed which make use of the plasma itself for improving the knowledge about the positions of the cameras.  

ISSN:0034-6748    

DOI:10.1063/1.1144680

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The suitability of laser thermal desorption (LTD) from titanium hydride targets for the production of short intense hydrogen gas pulses for diagnostic applications in fusion devices was investigated. A hydrogen yield of 2.2×1018H2molecules was achieved with a pulse energy of 11 J on a target spot diameter of 4.5 mm. The duration of the gas puff is closely coupled to the laser pulse length of 1 ms. The production of gas pulses by LTD close to the plasma creates the requirements for investigating very fast transport processes, especially at the high‐field side. The hydrogen pulses were contaminated by 2×10−3Ti atoms/H2molecule, primarily as a result of evaporation on overheated surface inhomogeneities. Conditioning the surface with giant laser pulses allowed the contamination to be reduced by a factor of 2.

ISSN:0034-6748    

DOI:10.1063/1.1145226

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new production technique of sheet‐shaped plasma by means of electron cyclotron resonance heating (ECRH) is proposed. The sheet plasma is produced in a vessel with rectangular cross section set in a rectangular magnetic field coil. The maximum density and electron temperature of the sheet plasma are about 2.6×1011cm−3and 7 eV, respectively, in a typical argon gas pressure ofP0=5×10−4Torr. The sheet plasma has several peaks of the density profile across the plasma width. These modes appear according to the change of working gas pressure. Those parameters including plasma density, density profile, and ion energy are controllable. This plasma is expected to be useful for material processing or plasma‐based high‐energy particle accelerators.

ISSN:0034-6748    

DOI:10.1063/1.1144681

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The design, construction, and the testbed results for a novel compact gas gun injector for solid diagnostic pellets of different sizes and materials is reported here. The injector was optimized for the diagnostic requirements of the ASDEX Upgrade tokamak, yielding the possibility of a widely varying deposition profile of ablated material inside the plasma. This allows variation of the pellet velocity and the total number of injected atoms. The use of spherical carbon pellets and different propellant gases (He,N2,H2) results in an accessible velocity range from about 150 m/s to more than 600 m/s and pellet masses from 2×1018to 1020atoms. Both the scattering angle (∼1°) and the maximum propellant gas throughput to the tokamak (less than 1016gas particles) were found to be sufficiently low. The injector provided both high efficiency (≥85%) and high reliability during the whole testbed operation period and also during the first injection experiments performed on ASDEX Upgrade. The pellet velocities achieved for different propellant gas pressures and pellet diameters were analyzed. It was discovered that, although the pellet diameters range from 0.45 to 0.85 times the barrel diameter, the pellet acceleration is mostly caused by gas drag. Pellet velocities in excess of those calculated on the basis of the gas drag model were observed. Additional acceleration that increases with the pellet diameter contrary to the gas drag model may be explained by the influence of the pellet on the gas dynamics in the barrel.

ISSN:0034-6748    

DOI:10.1063/1.1144682

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The Storey–Purcell array is an arrangement of four coaxial coils, composed of a Helmholtz pair nested inside a larger opposing Helmholtz pair whose respective area–turns products are sufficiently closely balanced to cancel the dipole moment of the combination when the magnitude of series current is the same in each coil pair. Ideally, the external field will exhibit 25‐pole character and falloff with distance asr−7. The array has desirable properties as a flux sensor because it is closely coupled to internal oscillating flux yet strongly decoupled to fluctuating external fields arising in distant sources. We have built an array and measured the external axial field component up to ten radii away. Our measurement technique relies on the principle of reciprocity for mutual inductance. Our analysis indicates area–turns unbalance of less than 500 ppm is required for the measured sensitivity profile to approach the calculated shape characteristic of a 25pole. Comparison of measured field values to those calculated from the magnetic multipole expansion show our imperfect construction results in an area–turns unbalance of the order of 2.5% or 25 000 ppm.

ISSN:0034-6748    

DOI:10.1063/1.1144683

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The influence of detector nonidealities on multiphoton timing experiments is investigated. Deviations from ideal detector behavior include dead‐time, afterpulsing, and a smeared detector response. A statistical model of dead‐time effects combined with cumulative afterpulsing is developed which allows the correction of distorted experimental data. The theory is verified with both Monte Carlo simulations and experimentally, whereby particular emphasis is put on fiberoptic optical time‐domain reflectometry measurements. Though the theory is applicable to any kind of detector we discuss the various effects for two examples of commercially available single photon avalanche diode modules. One of the two detectors is the passive quenched EG&G SPCM‐100 while the other module contains an active quenching circuit and is manufactured at the Czech Technical University at Prague.

ISSN:0034-6748    

DOI:10.1063/1.1144684

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A spectrum analysis method for the linear, direct, and self‐consistent measurement of dynamic modulation depth of polarization modulators is presented. This method utilizes the Bessel recurrence relation to determine the modulation depth from the photodetector voltage amplitudes at the fundamental frequency and its next three harmonics. Based on the existingJ1–J4method of dynamic phase‐shift measurement in homodyne interferometry, this method is useful for calibration of polarization modulated ellipsometers. The method is demonstrated through the use of a highly birefringent transparent thin film of piezoelectric polyvinylidene fluoride with indium tin oxide electrodes. The theoretical analysis of the measured noise factor for the particular system configuration predicted a minimum detectable polarization modulation depth of 0.2 rad, and was experimentally verified.

ISSN:0034-6748    

DOI:10.1063/1.1144685

出版商:AIP    

年代:1994

数据来源: AIP