Volume 70, Issue 3, March 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
Progress in multipass tandem Fabry–Perot interferometry: I. A fully automated, easy to use, self-aligning spectrometer with increased stability and flexibility70(1999); http://dx.doi.org/10.1063/1.1149637View Description Hide Description
A computer control for a Sandercock-type multipath tandem Fabry–Perot interferometer is described, which offers many advantages over conventionally used analog control: The range of stability is increased due to active control of the laser light intensity and the mirror dither amplitude. The alignment is fully automated enabling start of a measurement within a minute after start of align, including optionally finding the optimum focus on the sample. The software control enables a programmable series of measurements with control of, e.g., the position and rotation of the sample, the angle of light incidence, the sample temperature, or the strength and direction of an applied magnetic field. Built-in fitting routines allow for a precise determination of frequency positions of excitation peaks combined with increased frequency accuracy due to a correction of a residual nonlinearity of the mirror stage drive.
70(1999); http://dx.doi.org/10.1063/1.1149638View Description Hide Description
An optical pulse autocorrelator for single or multiple pulses is described that has the novel feature of being essentially uniaxial and offers a number of distinct benefits over traditional designs. The uniaxial nature of the device, coupled with a small number of degrees of freedom means that there is an inherent simplicity of alignment. A fixed and equal group delay in each arm guarantees the necessary temporal overlap of the optical pulses. Calibration is fixed and easily determined and it is extremely compact and transportable. We have verified its performance over three time ranges and have demonstrated that it is capable of measuring pulse lengths ranging from 100 fs to tens of picoseconds.
70(1999); http://dx.doi.org/10.1063/1.1149639View Description Hide Description
A fiber-optic attenuator calibrationsystem based on a high frequency substitution method has been developed using a 30 MHz precision piston attenuator equipped with a laser interferometer as a standard. The uncertainty of attenuation calibration for a typical 10 dB attenuator is 0.025 dB at two standard deviation levels. The resolution of the system is 0.001 dB and the dynamic range is 25 dB in a single step.
70(1999); http://dx.doi.org/10.1063/1.1149640View Description Hide Description
ELETTRA is a third generation synchrotron radiationsource. The energy spectrum allows the design of beamlines suitable for x-raylithography from soft to hard x-ray wavelengths. An appropriate lithographic window for micro- and nanofabrication can be obtained by a combination of selected filters and mirrors. As the beamline is interfaced to a vertical x-ray stepper, a uniformity in the beam intensity better than 3% (3σ) in the horizontal direction has to be reached. The present beamline is designed by taking into account the main factors which can affect the beam quality, namely, thermal loading on mirrors and filters, slope errors, and surface roughness of the mirrors. The resulting lithographic resolution at soft x-ray wavelengths is better than 100 nm.
Collimation of plasma-produced x-rays by spherical crystals: Ray-tracing simulations and experimental results70(1999); http://dx.doi.org/10.1063/1.1149641View Description Hide Description
Ray-tracing simulations, validated by experimental results, demonstrate that high intensity collimated x-ray beams can be produced from an isotropic x-ray source. A spherically bent mica crystal was used to collimate and monochromatize x rays emitted by a femtosecond laser-produced plasma. The result is a short pulse x-ray beam with a high degree of collimation (less than 1 mrad divergence), good spectral resolution and tunability over a wide spectral range. The role of the experimental parameters in the resulting beam divergence is thoroughly analyzed by ray-tracing modeling. These simulations are validated by test experiments. The ray-tracing calculations define a set of boundaries in the experimental parameters, which will guarantee the achievement of collimated beams better than 1 mrad in further experiments.
Preliminary experiment of total reflection x-ray fluorescence using two glancing x-ray beams excitation70(1999); http://dx.doi.org/10.1063/1.1149687View Description Hide Description
In conventional total reflection x-ray fluorescence (TXRF), one glancing x-ray beam irradiates the sample carrier for excitation of x-ray fluorescence. However, it is also possible to excite x-ray fluorescence by multiple x-ray beams. We performed a preliminary TXRF experiment excited by two independent glancing x-ray beams. The two x-ray beams excitation would be effective to enhance the intensity of x-ray fluorescence and to improve the detection limit in TXRF. X rays from both a rotating Moanodex-ray tube and a glow dischargex-ray tube with an Fe target irradiated a Cr thin film sample on a quartz glass. The x-ray fluorescence of from the sample was slightly enhanced compared with the intensity measured by one x-ray beam excitation.
70(1999); http://dx.doi.org/10.1063/1.1149642View Description Hide Description
An instrument is described which can obtain x-rayspectral power measurements utilizing the diffraction pattern produced when x rays pass through a slit. Traditionally, these types of measurements yielding low to moderate spectral resolution have been made with filtered x-ray diodes or with a transmission grating. The instrument described below has several advantages over filtered x-ray diodes in determining the spectral power profile such as an insensitivity to surface contamination. In addition, this instrument does not require the use of filters which can be destroyed during a shot making absolute measurements difficult and very time consuming. The advantages over a transmission grating system include cost, mechanical robustness, and fewer components which require spectralcalibration.
A method for measuring the population in a fast beam by means of laser atomic absorption spectroscopy70(1999); http://dx.doi.org/10.1063/1.1149643View Description Hide Description
We test the laser atomic absorption spectroscopy as a method to measure the population density of the level in an energetic beam. The projectiles are produced by quasiresonant charge transfer collisions of in lithium vapor. The population density is measured with a tunable distributed Bragg reflectordiode laser operating at 1083 nm. The absorption of the transitions yields an absolute value for the line-integrated density, and from the knowledge of the radial beam particle density profile the density of the state itself is obtained.
70(1999); http://dx.doi.org/10.1063/1.1149644View Description Hide Description
A source of highly dissociated, intense, cold hydrogen and deuterium atomic beams to be used as a dissociator in polarized gas targets for storage ring experiments has been developed. This dissociator is based on a plasma source which couples a 2.45 GHz surface wave to the discharge. At the end of the discharge tube a cold atomic beam is formed by means of a cooled nozzle. The entire system has been optimized to obtain a beam with a high atomic fraction and long-term stable reproducible operation. We measured a high degree of dissociation of more than 70% at gas throughputs up to 3 mbar l/s for both hydrogen and deuterium, corresponding to an atomic flow rate of Excellent long-term stability has been achieved. We also studied quantitatively the effect of recombination of atoms on the nozzle surface for various materials.
Refractory graphite skimmers for supersonic free-jet, supersonic arc-jet, and plasma discharge applications70(1999); http://dx.doi.org/10.1063/1.1149645View Description Hide Description
The fabrication of molecular beam “skimmers” from electrical discharge machining (EDM) graphite is reported. EDM graphite is highly refractory and is easily machined using conventional cutting and grinding techniques. In its most fine-grained form, EDM graphite can be machined free-standing to a knife-edge lip radius of ∼1 μm, providing excellent skimmer aerodynamics. Being refractory, such EDM skimmers are of particular interest in sampling or collimating high-temperature plasma discharges. Our explicit application is in skimming an electrical discharge supersonic free-jet of molecular nitrogen, forming a molecular beam of metastable to be used in the heteroepitaxialgrowth of III–N wide-band-gapsemiconductors. In view of the their economy, ease of manufacture, and excellent aerodynamics, the skimmers may also find use in skimming conventional, nondischarge supersonic free-jets. The performance of the EDM skimmers was tested in a conventional helium supersonic free-jet expansion, measuring the time-of-flight distribution and beam intensity as a function of nozzle pressure and nozzle-skimmer separation. Direct comparison with commercial nickel and copper skimmers showed the EDM graphite skimmers to perform nearly as well as the best commercial metal skimmers. The refractory properties of the skimmers were tested in a high-temperature arc-discharge supersonic free-jet expansion of 10% nitrogen in argon. Exposure to a plasma plume of ∼7500 K for over 6 h produced only relatively minor deterioration of the EDM graphite skimmer.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
70(1999); http://dx.doi.org/10.1063/1.1149646View Description Hide Description
The fabrication of self-supported ultrathin metallic wires is described. The initial material is a Pt/Rh (10%) wire embedded inside a 50 μm diameter Ag sheath. The thinning procedure which includes mechanical, chemical, annealing, and ion milling processes, leads to diameters down to 60 nm. Each wire is curved then fixed on a dedicated support suited to all the successive preparation steps. This mechanical attachment permits a precise positioning of such a wire at micrometric distances from a flat surface and therefore, its use as an electron biprism in low-energy electron interferometry experiments. The wire thinning is limited by the creation of cusps on the wire shape.
70(1999); http://dx.doi.org/10.1063/1.1149647View Description Hide Description
An innovative electron detector geometry for measuring electron multiplicities and absolute total electron yields from particle-induced electron emission is described. This setup has been successfully applied in coincidence measurements of slow singly and multiply charged ion-induced electron emission from a clean monocrystalline gold surface at grazing incidence of projectile ions. We describe characteristic features and operation of the electron detector which may also prove useful in various other experimental situations.
70(1999); http://dx.doi.org/10.1063/1.1149648View Description Hide Description
A high-resolution x-ray spectrometer with a spherically bent quartz crystal and an x-ray sensitive charge coupled device(CCD) have been applied to the observation of highly charged ions produced and trapped in the Tokyo electron beamion trap (EBIT). The spectrometer made it possible to measure the spatial distribution and wavelength of the radiation at the same time. A simple, but lower energy resolution method was also used, by taking advantage of the intrinsic energy resolution of the CCD. The possibility to apply such techniques to diagnostics of an EBIT is discussed.
70(1999); http://dx.doi.org/10.1063/1.1149649View Description Hide Description
A custom-designed magnet/cryostat system is described which has demonstrated remarkably improved field stability over previous designs. To shield from external magnetic noise, a custom-fabricated flux-gate device remotely senses the changes in magnetic field and cancels them out at the site of the magnet/cryostat via a 1.7-m-diam Helmholtz coil. To provide further shielding, the basic superconducting solenoid includes a passive flux-stabilizing coil. To stabilize internal field shifts, the temperature of the materials in the immediate vicinity of the solenoid (which have a temperature-dependent susceptibility) is stabilized via the new cryostat geometry and by controlling the pressure of the evaporating liquid helium to a few parts per million. As a result, the total system now has a composite shielding factor of approximately and an overall temporal stability on the order of 17(2) parts in per hour. This instrument, the heart of our new Penning trap mass spectrometer, has recently been used to determine a preliminary value of the proton’s atomic mass to an accuracy of 1.4 parts in With the new magnet/cryostat system, this spectrometer now has a potential resolution which exceeds 2 parts in with 100 hours of data using a single carbon 4+ ion.
Characterizing the emittance contribution due to rotated quadrupoles and canonical angular momentum using the quadrupole scan technique in electron accelerators70(1999); http://dx.doi.org/10.1063/1.1149650View Description Hide Description
Standard emittance measurement techniques for bright electron beams are either insensitive to the emittance due to correlated effects like misaligned quadrupoles or a nonzero axial magnetic field at the cathode, or are unable to differentiate that emittance from the beam’s intrinsic emittance corresponding to the beam’s actual entropy. We describe a technique based on the standard quadrupole scan, in which the beam correlations leading to emittance growth can be directly measured, and the emittance from these correlations can be found for special cases.
70(1999); http://dx.doi.org/10.1063/1.1149651View Description Hide Description
The TGV32, a 32-channel preamplifier–multiplicity discriminator chip for the multiplicity vertex detector (MVD) at PHENIX, is a unique silicon preamplifier in that it provides both an analog output for storage in an analog memory and a weighted summed-current output for conversion to a channel multiplicity count. The architecture and test results of the chip are presented. Details about the design of the preamplifier, discriminator, and programmable digital–analog converters performance as well as the process variations are presented. The chip is fabricated in a 1.2 μm, n-well, complementary metal–oxide–semiconductor process.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Quantitative measurement of x-ray images with a gated microchannel plate system in a -pinch plasma experiment70(1999); http://dx.doi.org/10.1063/1.1149652View Description Hide Description
A multiframe, gated pinhole system capable of quantitative acquisition for pulsed soft x rays is described and tested. The system based on a gated microchannel plate (MCP) is employed to observe the evolution of z-pinch plasma in a plasma focus facility with a time resolution of ∼220 ps. The quantitative relationship between x-ray source intensity and the recorded images has been investigated. To make a quantitative measurement, the phosphor screen current was measured to calculate the total electrons output from the MCP, which is proportional to both the incident x-ray intensity and the intensity of the recorded images. Furthermore, by taking into account the pinhole geometry, MCP gain and system spectral response, a quantitative calibration of the x-rayimages has been established. We have employed the system to observe the plasma evolution in a plasma focus facility. An order of soft x-ray emission within 6–14 Å was observed in a neon puffed experiment with time resolved four successive frames.
70(1999); http://dx.doi.org/10.1063/1.1149653View Description Hide Description
In this article we describe a new slit gratingspectrograph which is based on an e-beam written 10 000 linepairs/mm freestanding transmission diffraction grating. In combination with a thinned, back-illuminated charge coupled device(CCD), the spectrograph allows for real-time spectroscopy of laser-produced plasmax-raysources within the wavelength region λ=1–20 nm. Calibration of grating and CCD allow for the possibility to measure absolute photon fluxes, currently within the wavelength region λ=1–6 nm. The compact spectrograph is easy to align and flexible in its use. Absolutely calibrated spectra were obtained from a liquid-jet laser-plasma source in the water window, with a spectral resolution λ/Δλ⩾330 at λ=3.37 nm. A simple change in experimental geometry allowed single-shot spectra to be recorded with λ/Δλ⩾60 at the same wavelength. In addition, spectra from this laser-plasma source were measured within the range λ=9–20 nm.
70(1999); http://dx.doi.org/10.1063/1.1149654View Description Hide Description
A three-wave heterodyne O-mode reflectometer was developed and tested in experiments on the T-10 tokamak. Three launched waves are obtained by frequency splitting due to amplitude modulation with a diode. Source frequencies from 36 to 78 GHz were used in the experiments. The ability of the reflectometer to measure simultaneously density profile characteristics, such as the time evolution of the relative phase and the unambiguous time delay of the reflected wave together with small-scale density fluctuation characteristics such as the radial and poloidal correlation lengths, was demonstrated with T-10 experimental data. The reflectometer can provide important information about fine changes in the density profile during sawteeth and low-m magnetohydrodynamics oscillations. Poloidal correlation measurements make it possible to measure transverse turbulencevelocities and velocity shear. It was shown that both types of correlation measurements enable one to distinguish different types of turbulence according to the values of their correlation lengths, and that probing the plasma from several poloidal directions simultaneously greatly enhances the potential of reflectometry for the investigation of turbulence physics. In particular, it was found that different turbulence types may be either poloidally symmetrical or have much higher amplitude on the low field side of the discharge. Probing with several poloidal directions also enables one to make toroidal correlation measurements over long distances at certain resonant q values after one or two turns of magnetic field line around the major tokamak axis. In fact, correlations near 40% were observed experimentally. Such measurements may give additional information about turbulencecharacteristics on the one hand and allow radial q profile measurements on the other hand. In addition, the capabilities of reflectometry may be enhanced with the application of a proposed holography approach.
70(1999); http://dx.doi.org/10.1063/1.1149655View Description Hide Description
High-density microwaveplasma created and sustained by evanescent waves emitted from a holey plate has been studied. This source is called a holey-plate plasma source.Microwave power at 2.45 GHz is supplied from a coaxial cavity and then converted into an evanescent mode through the use of a holey plate. The plate is made from a 0.5 mm thick, 54 mm diam stainless steel sheet with 2.5 mm diam, 3.6 mm pitch holes, thus allowing high-density plasma to be generated near the surface of the plate. An evanescent electric field is produced from the cavity resonator to a plasma production chamber that is 54 mm in diameter. A detailed description of the plasma source and the plasma characteristics are presented in this article. The plasma density is with an electron temperature of 3 eV at an argon gas pressure of 2 Pa.