摘要:

The characteristics of the beam line used for surface EXAFS (extended x‐ray absorption fine structure) studies at the Daresbury Synchrotron Radiation Source (SRS) are described. Monochromatic photons are available in the energy range 60≤h&ngr;≤11 100 eV. This has been achieved using a monochromator with a combination of crystal and grating optics. The radiation is focused through the monochromator by a toroidal premirror at 0.5° grazing angle. Three pairs of crystals and a plane grating with focusing mirror are available which can be interchanged under ultrahigh vacuum (UHV). The plane grating/mirror monochromator has two ranges of zero‐order angles, 6.4° and 2°, giving photon energy ranges of 60–550 eV and 600–2000 eV, respectively, with a 1200 l mm−1grating. The crystals in use are InSb(111), Ge(111), and Ge(220) having photon energy ranges of 1745–7360 eV, 2000–8430 eV, and 3260–11 100 eV. The performance of the gratings and crystals in respect to intensity, resolution, scattered light, and higher orders is discussed, and some representative results from the beam line are given. Detailed consideration has been given to thermal effects on the crystal and grating optics. The grating can be exposed to a maximum power density of 9 W cm−2and has so far withstood the intense radiation. The Ge crystals, which can see power densities of up to 150 W cm−2, have survived, but exit beam movement and energy scale shifts are observed. Efforts to reduce these beam movement effects have been successful inasmuch as the instrument is usable. InSb crystals suffered some radiation damage, and eventually surface melted. Beryl and quartz crystals have been tried but did not survive. The crystal part of the monochromator is in routine use, but the grating part has yet to be proved successful for surface EXAFS experiments.

ISSN:0034-6748    

DOI:10.1063/1.1139077

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A new soft x‐ray spectrometer designed for use with photon excitation from synchrotron light sources is described and characterized. Special design features, including a close‐spaced input slit, large toroidal gratings, and a two‐dimensional charge‐coupled‐device array based detector system, provide exceptional measuring efficiency in a 5‐m Rowland circle design. Descriptions are given of the spectrometer’s mechanical and detector design, and of calibration and alignment procedures. The beam line providing photon excitation from a synchrotron light source is described. Typical electron beam and/or photon excited emission spectra of Al, Si, and LiF are presented and compared with those produced by other instruments.

ISSN:0034-6748    

DOI:10.1063/1.1139078

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The rotor electrometer is a new instrument which we hope will make possible searches for rare fractionally charged impurities in very large quantities of matter. The ultimate goal of the project is to be able to measure the net charge of 10 mg samples of any material to an accuracy of 0.05qein a few minutes (qeis the electron’s charge). This paper reports the achievement of subelectron (0.3qe) charge resolution with the new device. We discuss effects which limit the resolution and consider prospects for improving the performance to the point where a fractional charge search may be attempted.

ISSN:0034-6748    

DOI:10.1063/1.1139079

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A pulsed hot cathode hydrogen discharge of several milliseconds duration is used to produce a dense (<1014cm−3), uniform plasma target for atomic collision studies. This plasma, whose cross section is determined by the cathode shape, is rectangular, since it is produced by a discharge (1500 V, ∼100 A) from a 2×11‐cm2rectangular LaB6slab cathode along a 0.1‐T magnetic field to a gas‐fed anode. Background hydrogen (∼1 Pa) and contaminant gas (<10−2Pa) are kept low by injecting H2during the discharge into an evacuated (∼10−4Pa) chamber. One drawback of this discharge for atomic physics applications is that at high plasma density (ne>2×1013cm−3), sufficient fluxes of >1‐keV x rays are produced to flood our solid‐state detectors with background counts.

ISSN:0034-6748    

DOI:10.1063/1.1139080

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Users of fusion devices have identified heating requirements for positive ion based neutral beams to include energies of 80 or 120 kV with pulse length up to 30 s. Additional requirements are low beam divergence (0.3°×1.0°; 1/ehalf angles), low impurity (less than 1%), high species (over 80% atomic), and cathode lifetime exceeding 5 h of beam operation. Accelerator design remains as an engineering problem, whereas most of the performance goals have required development of the plasma generator. Problems of concern which relate to the performance goals are the heat dissipation, magnetic field configuration, and cathode placement. The plasma generator was tested on TS IIA (the plasma generator testing facility) which does not have beam extraction capability but is used to evaluate efficiency, operating conditions, arc notching characteristics, species, plasma uniformity, and cathode conditioning. The source, consisting of the plasma generator mounted on the long pulse accelerator was mounted on NBETF (Neutral Beam Engineering Test Facility) for beam testing. During beam operation the back‐streaming electrons add power to the source and affect the arc operation. Source durability and stability were studied at 80 kV and 40 A of accelerator current (deuterium). The arc efficiency was higher than the value used for the design. Power loading from back‐streaming electrons was much less than the design level. With feedback control, plasma density and accel current were constant to ±2% during 30‐s shots. The beam atomic fraction of 84%–88% (deuterium) was slightly higher than measured on TS IIA. Cathode durability was tested by operating over 500, 30‐s full shots at 80 kV and 40 A of deuterium. Arc conditioning was found to be an important phase to avoid filament damage.

ISSN:0034-6748    

DOI:10.1063/1.1139081

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The 10×40‐cm long pulse neutral beam source developed at Lawrence Berkeley Laboratory has been selected as the prototype for the design of the U. S. common long pulse source to be used on tokamak fusion test reactor (TFTR), Doublet‐III, and mirror fusion test facility (MFTF‐B). The long pulse source consists of a magnetic line cusp bucket with an actively cooled backplate electron dump and a four‐grid accelerator using slot apertures and water‐cooled molybdenum rails. Deuterium testing of the long pulse source at 120 kV and 53 A and with a 2‐s pulse length has been completed on the neutral beam engineering test facility, including an 8‐h 100‐shot test to demonstrate the durability and reliability of the source. Maximum 2‐s beam parameters obtained (not simultaneously) are 125 kV, 58 A (deuterium), and 7.1 MW. Beam pulse length has been extended to 5 s at 120 kV. Atomic species fractions in excess of 80% have been obtained, with a beam divergence (full energy, 1/ehalf‐angle) of approximately 0.8×0.4°. Calorimetric measurements on the accelerator grids and backplate electron dump indicate heat loads consistent with predictions and within the design limits of the source. This testing has verified the design of the source and its ability to exceed the basic performance requirements.

ISSN:0034-6748    

DOI:10.1063/1.1139082

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The construction and operation of a novel magnetized plasma device are described. The device combines characteristics of both linear mirror and toroidal confinement. It opens up new areas of basic plasma physics research. Examples of experiments on potential formation are presented.

ISSN:0034-6748    

DOI:10.1063/1.1139031

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The plasma background light in Thomson scattering measurements is often far higher than expected for a pure hydrogen plasma. The spectral distribution of light from three plasmas (duration: 1 ms to steady state; electron density: below 1012to over 1014cm−3; temperature: below 20 to over 1000 eV) is studied. Signal‐to‐noise and intensity data from the Thomson scattering systems used on them are compared with analytic estimates to show that in two of these plasmas molecular light dominates the spectrum, while in the other, molecular light is present, but bremsstrahlung is usually more intense. Knowledge of the mechanism for background light can aid in designing detection systems for Thomson scattering and provide information on the neutral species composition and effective charge of the plasma.

ISSN:0034-6748    

DOI:10.1063/1.1139032

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

We report on the construction and testing of a 100‐ps framing camera based on a gated microchannel plate. The gating is obtained by applying a fast voltage pulse directly across the microchannel plate detector. An infinite ground‐plane microstrip transmission‐line configuration is used. We have measured the transit time for electrons through the microchannel plate to be 150±35 ps and the rise and fall times of the detector to be less than 100 ps. The time resolution computed using the experimentally measured edge response times is less than 100 ps.

ISSN:0034-6748    

DOI:10.1063/1.1139033

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A totally automated picosecond transient absorption spectrometer is described. This experimental apparatus requires two photodiodes arrays as detectors and a single spectrograph. It takes advantage of the high dynamic range of such detectors. Typical transient spectra are presented. They show the reproducibility and accuracy of this method of time‐resolved spectroscopy.

ISSN:0034-6748    

DOI:10.1063/1.1139034

出版商:AIP    

年代:1986

数据来源: AIP