Index of content:
Volume 74, Issue 4, April 2003
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
74(2003); http://dx.doi.org/10.1063/1.1536256View Description Hide Description
A delay-line-free, high-speed electro-optic sampling (EOS)system is proposed by employing a delay-time-controlled ultrafast laserdiode as the optical probe. Versatile optoelectronic delay-time controllers (ODTCs) based on modified voltage-controlled phase-locked-loop phase-shifting technologies are designed for the laser. The integration of the ODTC circuit and the pulsed laserdiode has replaced the traditional optomechanical delay-line module used in the conventional EOSsystem. This design essentially prevents sampling distortion from misalignment of the probe beam, and overcomes the difficulty in sampling free-running high-speed transients. The maximum tuning range, error, scanning speed, tuning responsivity, and resolution of the ODTC are 3.9π (700°), <5% deviation, 25–2405 ns/s, 0.557 ps/mV, and ∼1 ps, respectively. Free-running wave forms from the analog, digital, and pulsed microwave signals are sampled and compared with those measured by the commercial apparatus.
74(2003); http://dx.doi.org/10.1063/1.1556948View Description Hide Description
We have measured the intensity profile and transmission of x rays focused by a series of biconcave parabolic unit lensesfabricated in lithium. For specified focal length and photon energy lithium compound refractive lenses (CRL) have a larger transmission, aperture size, and gain compared to aluminum, kapton, and beryllium CRLs. The lithium compound refractive lens was composed of 335 biconcave, parabolic unit lenses each with an on-axis radius of curvature of 0.95 mm. Two-dimensional focusing was obtained at 8.0 keV with a focal length of 95 cm. The effective aperture of the CRL was measured to be 1030 μm with on-axis (peak) transmissions of 27% and an on-axis intensity gain of 18.9.
74(2003); http://dx.doi.org/10.1063/1.1544085View Description Hide Description
A device is described and mathematically analyzed which creates electrostatic multipole potentials at least up to rank 3 in a 2 cm diam spherical region. The fields in the Stark ball are produced by the application of programmed voltages to 24 conducting rods that are inserted symmetrically in an empty 5 cm conducting cavity. Three mutually orthogonal unobstructed paths through the center of the device between the rods are provided for access of light and particle beams, and four other tetrahedrally located openings can be added for beams, detectors, or imaging. The device is intended for application in atom or molecule trapping and cooling, recoil-ion momentum spectroscopy, particulate analysis, reactive scattering, and coherent control. Potential distributions can be established, altered, and rotated on subnanosecond time scales, and frequencies higher than 20 GHz can be introduced via the same electrodes. Uniform fields, i.e., first-rank multipoles, quadrupole, and octupole potentials can be programmed and superposed with arbitrary and independent time profiles, so that the device may be applied to the study of orientational properties of complex anisotropic molecules, pollens, grains of dust, and other particulates. The device is nonmagnetic. An extensive analysis of fields in the center region, fringing fields, and multipole purity up to rank 7 has been carried out, and recipes are given for pure (2,0), and (3,0) multipoles. The principles and methods laid out here and applied to a “truncated octahedron” can be applied to the design of electrostatic environments having fewer or more independent electrodes in any polyhedral geometry.
- PARTICLE SOURCES, OPTICS and ACCELERATION; PARTICLE DETECTORS
74(2003); http://dx.doi.org/10.1063/1.1556952View Description Hide Description
An experimental setup that furnishes slow highly chargedion beams is described. A 14.5 GHz CAPRICE electron cyclotron resonanceion source provides highly charged ions at a kinetic energy of 10 to 20 keV per charge. The kinetic energy is reduced in a deceleration system, which consists of two stages, each of which is built up by two electrostatic zoom lenses. The ion beam can be decelerated down to a kinetic energy of 1 eV/charge. Typical beam currents, measured at the experiment with an emittance of (20 π mm mrad) are about 100 pA, even for the highest measuredcharge states
74(2003); http://dx.doi.org/10.1063/1.1544422View Description Hide Description
The effects of stochastic electron–electron interactions have been studied by a computational technique that attempts to avoid systematic errors, leaving only the random errors that are inherent in a Monte Carlo calculation. The technique has been applied to the energy and angle spreading in a converging round beam. The results are compared with previous analytical results.
74(2003); http://dx.doi.org/10.1063/1.1556944View Description Hide Description
Mini rf-driven ion sources with 1.2 cm and 1.5 cm inner chamber diameter have been developed at Lawrence Berkeley National Laboratory. Several gas species have been tested including argon, krypton, and hydrogen. These mini ion sources operate in inductively coupled mode and are capable of generating high current densityion beams at tens of watts of absorbed rf power. Since the plasma potential is relatively low in the plasma chamber, these mini ion sources can function reliably without any perceptible sputtering damage of the chamber wall. The mini rf-driven ion sources will be combined with electrostatic focusing columns, and are capable of producing nanofocused ion beams for micromachining and semiconductorfabrications.
74(2003); http://dx.doi.org/10.1063/1.1556941View Description Hide Description
A flat-top acceleration system appropriate to minimization of energy spread in an ion beam was investigated for the JAERI AVF cyclotron. A combination of the fundamental- and the fifth-harmonic voltages to obtain a homogeneous energy gain distribution of accelerated particles is ideally suited to a variable-energy multiparticle cyclotron using acceleration harmonics of 1, 2, and 3. The flat topping of the energy gain distribution using the fifth harmonics has the advantages of minimizing amplifier power, reducing power dissipation in a resonator, and increasing the energy gain per turn. The flat-top acceleration system was designed to reduce the energy spread to 0.02%, which fulfills a beam focusing condition for production of a microbeam with a beam spot diameter of 1 μm. Tolerable fluctuations of acceleration voltages, required to achieve the energy spread of 0.02%, were for the fundamental voltage and for the fifth-harmonic voltage. Both fundamental- and fifth-harmonic phases were required to be stabilized within 0.2 rf degrees. The tolerance of the magnetic excitation was In order to enhance compactness of the flat-topping cavity and to make a substantial saving of the amplifier power, optimum geometric parameters of the flat-topping cavity were determined by a cold model test and a calculation using the MAFIA code. A full range of the fifth-harmonic frequency, 55–110 MHz, was covered by the flat-top system, which enables us to apply the flat-top acceleration to a wide range of energy.
- NUCLEAR PHYSICS, FUSION and PLASMAS
High-repetition-rate, hard x-ray radiation from a laser-produced plasma: Photon yield and application considerations74(2003); http://dx.doi.org/10.1063/1.1544054View Description Hide Description
We present an experimental study of hard x rays produced in laser-produced plasmas. The laser used is a 1 kHz system, delivering 0.7 mJ for 25 fs onto a solid target. The x-rayspectrum was measured with calibrated germanium detectors, allowing a very good estimate of the absolute number of photons emitted from the plasma over a wide energy range; from 7 keV to 0.5 MeV. Assuming a bi-Maxwellian electron distribution with temperatures of 4.5 and 63 keV, theoretical calculations support the experimental findings. The imaging characteristics of the x-ray source were investigated experimentally employing image plates and theoretically based on the electron distribution.
74(2003); http://dx.doi.org/10.1063/1.1544080View Description Hide Description
Both one-dimensional (1D) and two-dimensional (2D) tomographic reconstruction of the total radiation power distribution from the large helical device (LHD) was based on the data obtained from absolute extreme ultraviolet silicon photodiodes. Two arrays (16 and 19 channels) installed in the normal LHD cross section (constant toroidal angle) provided simple and reliable 1D poloidally symmetric radiation profile reconstruction. The data obtained from two other arrays (20 and 20 channels) were used for 2D reconstruction of the radiation distribution in a semitangential plasma cross section. Using a 2D peeling away algorithm, improved by a feedback procedure, enabled reconstruction of several Fourier harmonics at each magnetic flux surface. These measurement and analysis techniques have enabled us to visualize asymmetries in plasma emission due to pellet and gas fueling.
74(2003); http://dx.doi.org/10.1063/1.1561599View Description Hide Description
In order to determine the separatrix shapes of field-reversed configurationplasmas with high accuracy, an iterative method that compares measured magnetic fluxes with the solution of the Grad–Shafranov equation is discussed in detail. Several suggestions for successfully treating the iterative method are given using numerical simulation and a mock-up experiment where conductors with three kinds of shape are inserted into the coil instead of the plasma. The iterative method is also applied to the field-reversed configurationplasma, and it is found that the separatrix shape has distinct ends and the axial location of the X point can be determined.
74(2003); http://dx.doi.org/10.1063/1.1544051View Description Hide Description
The reconnection scaling experiment (RSX), a linear device for studying three-dimensional magnetic reconnection in both collisional and collisionless laboratory plasmas, has been constructed at Los Alamos National Laboratory. Advanced experimental features of the RSX that lead to scientific advantages include the use of simple technology (commercial plasma guns) to create plasma and current channels. Physics motivations, design and construction features of the RSX, are presented. Basic plasma parameters that characterize the RSX are shown together with preliminary measurements of visible light emission during the merging of two parallel current channels.
74(2003); http://dx.doi.org/10.1063/1.1556956View Description Hide Description
Passive spectroscopy is used to measure the plasma parameters on the ZaP experiment at the University of Washington. Twenty spectral intensities, which are functions of the plasma’s density, velocity, and temperature along the viewing chord, are recorded on a charged coupled device. The instrument function is different for each viewing chord. A deconvolution technique based on a shell model, which includes the effects of the instrument function, is developed to deduce the local plasma parameters. The error analysis for this technique is also developed. The technique is able to model complicated plasma parameter profiles and is able to deduce the local plasma parameters and position of the plasma.
Neutral particle analyzer/isotope separator for measurement of hydrogen isotope composition of JET plasmas74(2003); http://dx.doi.org/10.1063/1.1542664View Description Hide Description
This article describes a neutral particle analyzer/isotope separator (ISEP) developed for measurement of the relative hydrogen isotope composition of Joint European Torus (JET) plasmas. The ISEP deployed on the JET can be regarded as a prototype of an instrument proposed for measurement of the spatial profile of the ratio of the density of deuterium and tritium ions in the plasma, in the International Thermonuclear Experimental Reactor (ITER). The ISEP makes simultaneous measurements of the energy distribution of efflux of hydrogen isotope atoms (H, D, and T) from the plasma. From such measurements it is possible to deduce the radial profile of the relative hydrogen isotope ion composition of the plasma and radial transport of ions of one isotope across the plasma of another isotope species. The main elements of the ISEP are (a) use of a thin carbon foil for reionization of the incident atoms, thereby eliminating gas stripping cells and gas sources of conventional neutral particle analyzers (NPAs), (b) acceleration of secondary ions in order to access the regime of higher detection efficiency of the NPA and to better separate ion pulses from neutron/γ-ray induced pulses in scintillator detectors, (c) dispersion of the secondary ions in specially designed nonuniform magnetic and electric fields to provide focusing in the detector plane, increased throughput and greater contrast between neighboring isotopes, and (d) counting of energy and mass analyzed secondary ions using detectors consisting of thin CsI(Tl) scintillators deposited directly on miniature thin window photomultiplier tubes mounted in vacuum. The ISEP has high contrast between atoms of neighboring masses for and much greater at higher energies), and high detection efficiency for atoms of ISEP detectors have very low sensitivity to neutrons and γ rays of ion sensitivity), making it feasible to use the ISEP in JET DT experiments without any shielding. Only a modest amount of neutron/γ-ray shielding would be required in the ITER for similar applications of the ISEP. The initial experiments on JET plasmas using the ISEP demonstrate well the capabilities of the instrument for measurement of the hydrogen isotope composition of the plasma and the energy distribution function of isotope ions.
Optimization of the number of soft x-ray arrays and detectors for the SST-1 tokamak by the tomographic method74(2003); http://dx.doi.org/10.1063/1.1544052View Description Hide Description
An x-ray imagingsystem for tomographic reconstruction of two-dimensional emissivity is being developed for the SST-1 tokamak. In this article it is shown that for circular plasma, which will be generated in the first phase of operation of the machine, 7 arrays and 115 detectors are adequate to reconstruct the simulated data. Sawtooth phenomena depicted by both the Wesson model and the Kadomtsev model and the disruption phenomenon associated with m=2 mode are reconstructed well with the help of this imagingsystem.
74(2003); http://dx.doi.org/10.1063/1.1556949View Description Hide Description
Simple and convenient novel manipulators with two dimensions and three dimensions were demonstrated for measuring various parameters directly in a cylindrical vacuum chamber. Examples of experimental data are presented to prove the usefulness of the systems in a plasma device. Three different mechanisms with neither internal driving nor differentially pumped systems for achieving varying degrees of accuracy were proposed: In one of the two three-dimensional driving systems, detectors were introduced along the axial direction with an arbitrary rotation with respect to the axis by the use of two small-diameter bellows. The other system allowed motion along the radial direction with the capability of arbitrary rotation by small-diameter bellows. In the two-dimensional system, detectors could be scanned vertically using a sliding flange in addition to a radial motion mechanism.
Rapid multiplexed data acquisition: Application to three-dimensional magnetic field measurements in a turbulent laboratory plasma74(2003); http://dx.doi.org/10.1063/1.1544417View Description Hide Description
Multiplexingelectronics have been constructed to reduce the cost of high-speed data acquisition at the Swarthmore SpheromakExperiment (SSX) and Redmond Plasma Physics Laboratory. An application of the system is described for a three-dimensional magnetic probe array designed to resolve magnetohydrodynamic time scale and ion inertial spatial scale structure of magnetic reconnection in a laboratory plasma at SSX. Multiplexing at 10 MHz compresses 600 pick-up coil signals in the magnetic probe array into 75 digitizer channels. An external master timing system maintains synchronization of the multiplexers and digitizers. The complete system, calibrated and tested with Helmholtz, line current, and magnetofluid fields, reads out the entire probe array every 800 ns with an absolute accuracy of approximately 20 G, limited mainly by bit error.
74(2003); http://dx.doi.org/10.1063/1.1556953View Description Hide Description
We present a new x-ray spectrometer setup based on a conically curved mica crystal in the von Hámos configuration. An x-ray sensitive charge coupled device or streak camera can be easily coupled in a perpendicular orientation to the spectrally resolved x-ray line focus. Shifting the crystal along its axis of symmetry allows one to change the dispersion and the signal flux density on the detector. A large spectral range at high resolution and adjustable signal intensity is accessible with this setup.
X-ray crystal spectrometer with a charge-coupled-device detector for ion temperature measurements in the Large Helical Device74(2003); http://dx.doi.org/10.1063/1.1556942View Description Hide Description
A Johann-type x-ray crystal spectrometer has been designed using a ray tracing calculation and constructed for ion temperature measurement in the plasma center of a large helical device(LHD) using Doppler broadening of a He-like resonance line of ArXVII, TiXXI, CrXXIII, or FeXXV. Four curved quartz (2020), (2023), (3140), and (2243) crystals are set in a quadrangular rotary crystal holder for measuring the x-ray line from such four different elements. Each of the crystals can be externally selected through a computer network. The curved crystals with a radius of 3 m are fabricated by gluing thin quartz on an accurately ground cylindrical blue glass plate. The curvature of the crystals was checked by scanning a sharp pin along the crystal surface. As a result, an excellent agreement with the designed cylindrical curvature was obtained with an accuracy of 1 μm. A secular change of the curved crystal surface structure was found by measuring the curvature of the curved crystal fabricated 15 years earlier. A charge-coupled-device (CCD)detector is used instead of a usually used proportional counter to increase the count rate and to avoid the influence of magnetic field leakage from the LHD. The influence of magnetic fields on the proportional counter was investigated in a range of The use of the CCD brought us a large increment of more than ten times in the count rate compared with the multiwire proportional counter. The ion temperature thus has been successfully observed in a time interval of 20 ms with small error bars. The lowest measured ion temperature was 0.2–0.3 keV for ArXVII and TiXXI, which indicates an excellent spectral resolution of the crystal spectrometer. Finally, the influence of the chord integration on the measured ion temperature is discussed as a function of central electron temperature and density profile with analysis from impurity transport calculations.
74(2003); http://dx.doi.org/10.1063/1.1533105View Description Hide Description
A family of Johann configuration curved crystal spectrometers has been designed to share the basic engineering features of compactness, modularity, facility of alignment and focus, and incorporation of solid-statecharge coupled devicedetector arrays. These detectors have intrinsically low noise, useful energy resolution, two-dimensional position sensitivity, and readout modes that are programmable. The spectrometers, although relatively compact, with a Rowland circle diameter in the range 0.5–2 m, can still have sufficient resolving power, dispersion, and throughput to be invaluable in high resolution studies of atomic and plasma sources. This article discusses the basic design features and performance of these doubly dispersive spectrometers and illustrates their versatility by applications to studies of a wide range of laboratory x-raysources such as line emission from highly ionized atoms in the extended plasmas of Tokamaks and nearly point plasmas produced by focused laser irradiation of solids and in beam-foil experiments and electron beam ion traps.
74(2003); http://dx.doi.org/10.1063/1.1534388View Description Hide Description
The measurement requirements and expected performance of the International Thermonuclear Experimental Reactor (ITER) magnetics system are summarized. The constraints of a burning plasma experiment (BPX) on tokamak magnetic diagnostic system designs are listed and illustrated using the parameters and design of sensors being developed for the ITER device. Design principles to meet the constraints are discussed, with an emphasis on radiation effects, reliability, and redundancy for sensors and subsystems. The remaining research and development for the ITER system is outlined.