摘要:

A novel scanning tunneling microscope (STM) is described which is constructed almost entirely of aluminum and uses bimorph piezoelectric disks as thex,y,zdrive elements. The design uses a simple, rugged tripod configuration for the fine motion drive arms. Coarse motion of the sample, which is mounted on an aluminum holder, is achieved by pushing or pulling, with a piezoelectric louse. Differential thermal expansion effects are avoided by design, to first order, and the resulting drift is ≤0.5 A˚/min after only a short warm‐up period. It is easy to build and operate, and has good immunity to mechanical vibrations. The frequency response is flat to 5 kHz. Thex,y,zdrive sensitivity is 8 A˚/V and the range is ≥12000 A˚. Some preliminary experimental results are presented, including atomic resolution images in air, of graphite and NbSe2.

ISSN:0034-6748    

DOI:10.1063/1.1139658

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A scanning tunneling microscope that uses a micrometer coarse approach mechanism is described. The approach mechanism can be decoupled from the rest of the microscope to result in a thermally compensated instrument. Thirty minutes after establishing tunneling, lateral thermal drift in our instrument is down to 0.5 A˚/min. Construction details of the microscope and high resolution images of pyrolytic graphite and 2H‐NbSe2samples are presented.

ISSN:0034-6748    

DOI:10.1063/1.1139414

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

We describe an experimental setup in which resonance‐enhanced multiphoton ionization (REMPI) is used for diagnosis of neutral particles (atoms or molecules) effusing from a plasma. Quantum‐state‐specific detection, achieved by selective ionization with the help of REMPI, yields the distribution over their internal degrees of freedom. Discrimination against nonlaser‐produced ions is performed by means of the combination of an electric and a magnetic field. The applicability of the setup is demonstrated by the determination of the rotational temperature of ground‐state hydrogen molecules in a multicusp ion source. Under nondischarge conditions a detection limit is achieved of ≊2×1010state‐selected hydrogen molecules/cm3, corresponding to a value of ≊7×108/cm3at the analysis region. Under discharge conditions, this detection limit is deteriorated by the presence of metastable hydrogen molecules, which are ionized in a one‐photon process.

ISSN:0034-6748    

DOI:10.1063/1.1139415

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A Doppler‐shift spectroscopy system has been installed on the TFTR neutral beam injection system to measure species composition during both conditioning and injection pulses using short and long pulse ion sources. Two intensified vidicon detectors and two spectrometers are utilized in a system capable of resolving data from up to 12 ion sources simultaneously. By imaging the light from six ion sources onto one detector, a cost‐effective system has been achieved. Fiber optics are used to locate the diagnostic in an area remote from the hazards of the tokamak test cell allowing continuous access, and eliminating the need for radiation shielding of electronic components. Automatic hardware arming and interactive data analysis allow beam composition to be computed between tokamak shots for use in analyzing plasma heating experiments. Measurements have been made using lines of sight into both the neutralizer and the drift duct. Analysis of the data from the drift duct is both simpler and more accurate since only neutral particles are present in the beam at this location. Comparison of the data taken at these two locations reveals the presence of partially accelerated particles possessing an estimated 1/ehalf‐angle divergence of 15° and accounting for up to 30% of the extracted power from the short pulse ion sources. Operation with long pulse ion sources indicates a higher atomic composition, fewer partially accelerated particles, and somewhat reduced neutralization line densities.

ISSN:0034-6748    

DOI:10.1063/1.1139416

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Proton yields in magnetic multipole sources can be increased by the creation of a magnetic ‘‘filter’’ within the discharge volume, although this may introduce nonuniformity in the plasma. We have investigated the properties of a filter which can satisfy the requirements of plasma uniformity and discharge electrical efficiency and creates a proton yield of 87% when a 60‐A beam is extracted at 80 keV. A model describing the behavior of the filter is also described.

ISSN:0034-6748    

DOI:10.1063/1.1139417

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The design and construction of a high‐performance differentiating pick‐up coil for a 500‐kA/ 4‐&mgr;s pulsed current is described. Based on the Rogowski principle, the coil was made by using the printed‐circuit technique. The coil is 1 mm thick; inclusive of electrostatic shielding and can be reproduced from one master print at nearly any size. Measurements up to 5 MHz showed no significant change of self‐inductance. We believe that the coil is a versatile tool to measure and monitor current pulsed in the megampere/megahertz range.

ISSN:0034-6748    

DOI:10.1063/1.1139418

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An acousto‐optic rf spectrometer (AOS) has been designed and constructed for use on the CONSTANCE B and TARA mirror experiments at MIT. The AOS is an analog preprocessor of data that can come from a variety of sources, including rf probes and microwave/laser scattering experiments. The output of the AOS is the Fourier transform amplitude of the input in a parallel format suitable for computer acquisition. The spectrometer, having a bandwidth of 500 MHz and a resolution of 1 MHz, together with its receiver, is capable of looking at fluctuations anywhere in the frequency range of a few MHz up to 4 GHz. Power levels of a few nanowatts can be detected. Complete 500‐MHz‐wide spectra can be processed and acquired at the rate of 1 every 10 &mgr;s. The optical components, receiver, and data‐acquisition interface will be discussed, and initial data taken on the CONSTANCE B experiment will be shown.

ISSN:0034-6748    

DOI:10.1063/1.1139419

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Local impurity particle diffusion coefficients have been measured in a low temperature plasma by the injection of test particles at the center of the plasma. The injection is accomplished by a high‐voltage discharge between two small graphite electrodes on a probe. The probe can be located anywhere in the plasma. The diffusion is observed spectroscopically. An analysis of the spatial and temporal evolution of the C iiradiation from the carbon discharge can determine the parallel and perpendicular diffusion of the impurity ions. Results with the diagnostic have been obtained in the Proto S‐1/C spheromak. The measured value of the diffusion coefficient in the afterglow plasma is in good agreement with classical predictions.

ISSN:0034-6748    

DOI:10.1063/1.1139420

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The four barrel pneumatic pellet injector originally used on the Alcator C tokamak at MIT has been reconfigured for use on the ZT‐40M reversed field pinch (RFP) at Los Alamos National Laboratory. The lower temperature and density in this device require operation at velocities significantly lower than on Alcator. Allowing a 2‐s warmup prior to firing the injector has enabled operation with pressures as low as 40 psig and velocities ranging as low as 200 m/s with H2pellets, without major cryostat modification. Initial injection experiments in the ZT‐40M plasma have verified that pellet refueling is effective for density control, but the net particle inventory increase is less than might be expected based on simple models. Indications of nonoptimal penetration and curvature of the pellet trajectory, in addition to comparable time scales for particle confinement and pellet transit, may account for this.

ISSN:0034-6748    

DOI:10.1063/1.1139421

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

We present a solution to the lock‐in detection method which is valid for arbitrary values of the modulation frequency. Our solutions agree with the results of the quasistatic theory, in the limit of small modulation frequency compared to the resonance linewidth.

ISSN:0034-6748    

DOI:10.1063/1.1139422

出版商:AIP    

年代:1987

数据来源: AIP