摘要:

A design for a scanning tunneling microscope that is well suited for electrochemical investigations is presented. The construction of the microscope ensures that only the tunneling tip and the sample participate in electrochemical reactions. The design also allows rapid replacement of the tip or sample, and enables facile introduction of auxiliary electrodes for use in electrochemical experiments. The microscope utilizes stepper motor driven approach mechanics in order to achieve fully remote operation and to allow reproducible coarse control of tip/sample spacings for electrochemical experiments. Highly ordered pyrolytic graphite images at atomic resolution in air and aqueous solutions can be obtained with this microscope.

ISSN:0034-6748    

DOI:10.1063/1.1139957

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A force microscope is described which uses a fiber‐optic interferometer as the cantilever displacement sensor. Low thermal drift and reduced susceptibility to laser frequency variation are achieved due to the small (several micrometer) size of the interferometer cavity. A sensitivity of 1.7×10−4A˚/(Hz)1/2is observed for frequencies above 2 kHz. The drift rate of the sensor is on the order of 3 A˚/min. As an initial demonstration, laser‐written magnetic domains in a thin film sample of TbFeCo were imaged.

ISSN:0034-6748    

DOI:10.1063/1.1139958

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The internal magnetic field in a magnetically confined plasma may be deduced from the analysis of circular polarization of spectral lines emitted by the plasma. The theory of the measurement and a detailed design of a polarimeter constructed to measure the poloidal field profile in the Texas Experimental Tokamak (TEXT) are presented. The instrument measures the difference between left‐hand and right‐hand circularly polarized line profiles, a quantity directly proportional to the magnetic field component in the direction of observation. The high throughput of the Fabry–Perot interferometer employed in this design, combined with efficient light‐collecting optics and lock‐in detection of the polarization signal, allows measurement of the fractional circular polarization of the magnetic dipole line Ti xvii3834 A˚ with an accuracy on the order of 10−3. The line‐of‐sight averaged poloidal field is determined with uncertainty as small as 50 G. The line emission used in the present measurement is not well localized in the plasma, necessitating the use of a spatial inversion procedure to obtain the local values of the field.

ISSN:0034-6748    

DOI:10.1063/1.1139959

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

To measure local electric field in an rf‐plugged sheet plasma in the RFC‐XX‐M open‐ended machine, a spectroscopic system using a combined technique of Li‐beam probing (laser blow‐off method) and laser‐induced fluorescence has been developed. A Li atom in the rf field can be populated to 42F by a stepwise laser pumping (22S→22P→42F) because of the Stark mixing between 42F and 42D. A cascade fluorescence of 610.4 nm (32D→22P) subsequent to the direct transition (42F→32D) was observed to estimate the rf field strength (rms)Erms. To correct the effect of electron collisions in the plasma, we observed a blue fluorescence consisting of two lines: a forbidden line of 460.2 nm (42F→22P) and an allowed line of 460.3 nm (42D→22P) induced by electron collisional transfer (42F→42D). From these intensities, we obtainedErmsand electron densityneby solving numerically the rate equations, including the relevant radiative and collisional processes under the experimental conditions.Ermsandnewere estimated to be 2.4 kV/cm and 3.1×1011cm−3near the center of the sheet plasma, respectively. It was found that the obtainedErmswas enhanced by about 2.5 times with respect to the vacuum field (1 kV/cm). The obtainedneagreed well with ones by other methods. A spatial distribution ofnewas also obtained from that of the Li0resonance line (22P→22S) due to electron impact excitation. The present spectroscopic system is applicable to plasmas ofne≲1013cm−3and static or quasi‐static electric fields ofE≳400 V/cm.

ISSN:0034-6748    

DOI:10.1063/1.1139960

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The U. S. magnetic fusion energy program has developed a single design, long‐pulse neutral beam source for TFTR, MFTF‐B, and DIII‐D. The arc is a very compact axial magnetic line cusp. The accelerator is an actively cooled tetrode with water‐cooled grid tubes of shaped molybdenum forming ‘‘slot’’ beamlets. DIII‐D and MFTF‐B configurations have an 80‐kV accelerator gap, with a 12×18 cm aperture, and a 10‐m ‘‘module’’ focus. TFTR modules are unfocused, with a 120‐kV gap and 12×43‐cm mask. The first CLPS was tested in the TFTR configuration, at 120 kV, 2s. Optimum current was 73 A, or 1.76 &mgr;pervs (deuterium), with 80%–85% atomic fraction. Optimum divergence of ions plus neutrals was 0.4° parallel to the slots, and 0.7° perpendicular to the slots (1/ehalf‐angle). The combination of an axial cusp magnetic bucket and slot accelerator apertures gives the common long pulse source about twice the beam power per unit cross section of other long pulse sources, plus lower divergence in the direction parallel to the slots.

ISSN:0034-6748    

DOI:10.1063/1.1139961

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A compact multichannel scintillator probe is described, which has been used to make spectral measurements of energetic electron end‐loss (E∼400 keV) from an electron cyclotron resonance‐heated mirror plasma. The probe utilizes entrance foils of various thicknesses for energy discrimination, and photomultiplier tubes that are used in current mode with linear amplifiers for fast response (&Dgr;t∼0.1 &mgr;s) or logarithmic amplifiers for wide dynamic range (10−8–10−4A). The probe is used to obtain spectra in the energy range 10–500 keV, and is sensitive to power densities above 0.1 mW/cm2. The energy dependence of the response of a low‐energy channel (sensitive to electrons above 10 keV) has been measured, and agrees very well with a calculation of the foil discrimination. Calibration results and sample data from the mirror plasma experiment are presented. Uses other than the measurement of electron end‐loss in mirror experiments include measurement of runaway electron populations at the edge of tokamak plasmas and direction‐sensitive measurement of high‐energy electron populations in tokamak plasmas with lower hybrid current drive.

ISSN:0034-6748    

DOI:10.1063/1.1139962

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A microscope slide with several transparent slits installed in a streak camera is used to record time‐resolved two‐dimensional information when a curved luminous plasma sheath traverses these slits. Applying this method to the plasma focus experiment, the axial run‐down trajectory and the shapes of the plasma sheath at various moments can be obtained from a single streak photograph.

ISSN:0034-6748    

DOI:10.1063/1.1139963

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have developed a pump‐probe picosecond spectrometer capable of time‐resolved studies spanning nine decades in time, from the picosecond to the millisecond regimes. The system operates at repetition rates up to 2 kHz with ∼40‐ps time resolution. Pump and probe beam average powers are ∼10 mW. The system is capable of studies of molecular dynamics in complex systems such as proteins, where internal motions span a wide range of time scales.

ISSN:0034-6748    

DOI:10.1063/1.1139911

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Details of a 30‐W, 140‐MHz rf amplifier for cw CO2waveguide laser excitation are presented. The amplifier delivers 30 W into a 50‐&OHgr; load while requiring only 40 W of dc power from a 28‐V supply and 100 mW of rf drive power for an overall efficiency of 75%. A coupling‐starting network design theory is given that provides the initiation over voltage for the discharge plasma from an rf power source of limited output voltage capability. The network then matches the drive circuit to the new input impedance of the operating discharge without any adjustments. This design theory applies to the whole class of networks whose losses can be approximated by a loss conductance in parallel with the gas discharge.

ISSN:0034-6748    

DOI:10.1063/1.1139912

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A reliable, fast‐beam alignment method for a thermal‐type laser‐power meter is proposed. The thermopile‐type power meter that has been developed and manufactured consists of a main‐sensor and a pinhole on the target, and a fast‐response sub‐sensor behind it. The alignment can quickly be performed by monitoring the beam‐power density of the incident‐laser beam. The method has been found to be very effective for beam alignment.

ISSN:0034-6748    

DOI:10.1063/1.1139913

出版商:AIP    

年代:1988

数据来源: AIP