Volume 70, Issue 6, June 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
70(1999); http://dx.doi.org/10.1063/1.1149815View Description Hide Description
We demonstrate the feasibility of transmission grating spectroscopy in the 10 keV range. The diffraction efficiency of a 5000 l/mm free-standing gold transmission grating was measured at the three photon energies of 1.49, 5.4, and 8.04 keV ( of Al, Cr, and Cu). At 1.49 keV, the measured efficiencies agree well with efficiencies calculated by standard grating theory. At 5.4 and 8.04 keV, the measured efficiencies significantly exceed the theoretical ones. For the absolute efficiency into first order is still as high as 3.3%. We present spectra of fs laser plasmas generated by irradiating Al, Cr, and Cu targets.
Construction of the multilayered-mirror monochromator beam line for the study of synchrotron radiation stimulated process70(1999); http://dx.doi.org/10.1063/1.1149816View Description Hide Description
A multilayered-mirror (MLM) monochromator beam line designed specially for synchrotron radiation (SR) stimulated process experiments has been constructed for the first time. The beam line was designed by the criteria; a beam spot size on the sample surface a density of total irradiated photons (for an irradiation time of a few tens of minutes to a few hours), and low-energy background ⩽1% of the output. The performance of the beam line was evaluated by measuring the transmitted photon flux of an Al filter around the absorption edge and by measuring the photoemissionspectra of Ta using the output beam as an excitation light source. The Al thin film deposition was successfully demonstrated by using the monochromatized output beam. We conclude that this MLM monochromator performs sufficiently well to study the excitation energy dependence in SR-stimulated processes.
Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection70(1999); http://dx.doi.org/10.1063/1.1149817View Description Hide Description
A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beamsources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the products from the crossed beamreaction is carried out using the pulsed 157 nm radiation from a excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beamsources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for and these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.
High resolution pulsed field ionization photoelectron spectroscopy using multibunch synchrotron radiation: Time-of-flight selection scheme70(1999); http://dx.doi.org/10.1063/1.1149818View Description Hide Description
We have developed an efficient electron time-of-flight(TOF) selection scheme for high resolution pulsed field ionization (PFI) photoelectron (PFI-PE) measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOFspectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the autoionizing Rydberg peak, which is adjacent to the PFI-PE band. The rotational-resolved PFI-PE band for measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 full width at half maximum (FWHM) and 1.9 FWHM in the PFI-PE bands for and at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.
70(1999); http://dx.doi.org/10.1063/1.1149819View Description Hide Description
We have experimented with a radio-frequency-based streak camera. The basic idea behind the method consists of magnetic deflection of the electron beam imparted by a radio-frequency cavity. All the components of the camera were tested and the time resolution was measured to lie within the sub-ps domain. Further advantages of the method are simplification of triggering, weaker dependence on space-charge effect within the electron beam, high modularity, and ease of implementation.
Three-dimensional optical trapping and evanescent wave light scattering for direct measurement of long range forces between a colloidal particle and a surface70(1999); http://dx.doi.org/10.1063/1.1149820View Description Hide Description
A novel technique is described to measure weaker long range forces between a single colloidal particle and a flat surface as a function of separation distance. The technique uses a three-dimensional optical trap as a force transducer and evanescent wavelight scattering to measure the particle-surface separation distance. The three-dimensional optical trap allows direct manipulation of the particle near the surface, and the measured displacement of the particle from the center of the optical trap determines the force of interaction between the particle and the surface. The particle position is determined by measuring the scattered light from the particle in an evanescent wave created by total internal reflection of a laser beam on the surface. This combination of optical techniques allows forces to be directly measured for smaller particles (∼0.25–5.0 μm) and over a range of forces N) than has been previously reported for similar force measurement techniques such as atomic force microscopy and total internal reflectionmicroscopy. The experimental results have been compared with DLVO theory predictions for 1.5 μm silica microspheres interacting with glass in NaCl solutions of known ionic strength.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
70(1999); http://dx.doi.org/10.1063/1.1149821View Description Hide Description
A compact electron cyclotron resonanceplasmax-raysource, which has a potential use for medical imaging is presented in this article. In this article, the experimental system and the characterization studies on plasma and x ray are presented. Using a Langmuir probe, the plasma parameters are measured for different magnetic field profiles and gas pressures. The x-rayspectrum is obtained for various gas pressures and magnetic field profiles. In the x-rayspectrum, the Bremstrahlung radiation, peaking at 20–60 keV is observed and the final energy of the x ray is extended up to ∼200 keV. Thermo luminescence dosimeter sample) is used for estimating the dose at the port and these results are presented for typical x-ray spectra. Using a teletector, the dose at the port for various coil current is measured and these are compared with the estimated dose obtained from the x-rayspectrum.
70(1999); http://dx.doi.org/10.1063/1.1149822View Description Hide Description
A compact warm electron beamion trap (WEBIT) is described and investigated experimentally. The trap design is based on permanent magnets, an electron gun with a cathodeemissivity of 25 A/cm2 and a gun perveance of 0.1 μA/V3/2 as well as a vacuum vessel allowing trap operation at pressures of The electron beam diameter measured with a pinhole camera was 160 μm corresponding to an electron beam density of 248 A cm−2. X-ray spectra measured with a Si(Li) semiconductordetector indicate the production of and
70(1999); http://dx.doi.org/10.1063/1.1149823View Description Hide Description
The emittance of the intense protonbeam extracted by the source SILHI at Commisariat à l’Energie Atomique (CEA)-Saclay is a key parameter for the design of the IPHI Project RFQ. This parameter has a relevant role even for the design of an intense protonsource for the TRASCO project of Istituto Nazionale di Fisica Nucleare (INFN). The tests performed in the framework of CEA-INFN collaboration have been mainly devoted to a 75 mA beam emittance investigation injecting different gases in the beam line. The results show that the rms normalized emittance decreases up to a factor 3 while the beam losses induced by recombination are contained within 5%. Normalized emittance in plane of about 0.1 π min mrad have been obtained using Ar and Kr.
70(1999); http://dx.doi.org/10.1063/1.1149824View Description Hide Description
Combined function magnets are often used in synchrotrons and beam lines. The field quality of such magnets depends mainly on the pole profile. A new method is described for determining the finite pole shapes of such magnets for obtaining field qualities of or better. Profiles of the typical quadrupole-sextupole and dipole-quadrupole magnets are presented.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Characteristics of 63.6° cylindrical energy analyzer used as charge exchange neutral particle analyzer70(1999); http://dx.doi.org/10.1063/1.1149825View Description Hide Description
A charge exchange neutral particle analyzer including a newly designed multichannel 63.6° cylindrical electrostaticenergy analyzer was constructed. Since the energy analyzer can simultaneously adopt a small radius of cylindrical electrodes and a large width of an entrance slit, high incident current can be analyzed while the size of the analyzer remain compact. Characteristics of the analyzer were investigated using hydrogen beam with energies from about 0.5 to 5 keV. The experimental results are compared well with the numerical analysis.
70(1999); http://dx.doi.org/10.1063/1.1149826View Description Hide Description
Ion cyclotron range of frequency (ICRF) heating on the large helical device(LHD) is characterized by high power (up to 12 MW) and steady state operation (30 min). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6 m) with superconducting coil windings ( ). The main purpose of physical research is to investigate currentless and disruption-free plasma. Research and development for steady state ICRF heating has been carried out in recent years: A high rfpower transmission system consisting of stub tuners, a ceramic feedthrough, and an ion cyclotron heating loop antenna has been developed. In addition, steady state operation of a rfoscillator has been achieved at a power higher than 1 MW. A liquid stub tuner has been proposed as an innovation. The liquid stub tuner makes use of the difference between the rf wavelengths in liquid and in gas due to the different relative dielectric constants. The liquid stub tuner has been experimentally proved to be a reliable rf component for high power transmission systems. Test results have quantitatively demonstrated that it can be used at high rf voltage: 61 kV for 10 s and 50 kV for 30 min. Furthermore, the liquid surface can be shifted under high rf voltage without breakdown, which suggests that it can be employed as a feedback control impedance matching tool to keep reflected rf power at a low level with regard to a temporal variation of plasma loading resistance.
70(1999); http://dx.doi.org/10.1063/1.1149827View Description Hide Description
Spatially resolved visible spectra emitted from a plasma formed by focusing the output of a flashlamp pumped dye laser onto solid targets of lithium have been recorded. The effects of laser wavelength on plasma formation have been analyzed by tuning the laser to the first resonance transition in lithium at 6708 Å and comparing the results with those obtained at an off resonance wavelength of 6728 Å. Laser energies varied from 430 to 645 mJ and were focused to a spot diameter of 300 μm with a pulse length of 850 ns. Light from the plasma was focused onto the slit of a Hilger spectrograph, which incorporated a Pellin–Broca prism whose output was focused onto a charge-coupled devicecamera. Electron densities of were deduced from Stark width and shift measurements of the Li II line at 5485 Å and from the Li I line at 4603 Å. In this way the electron density could be mapped as a function of distance from the target. By observing emission at 5485 Å the electron density was observed to increase when the laser was tuned on resonance compared to off. This was not the case when the neutral Li I line was used as a diagnostic. The anomaly is accounted for by referring to both the ionization rates involved and the temporal profile of the emission.
70(1999); http://dx.doi.org/10.1063/1.1149828View Description Hide Description
A novel, fast-sweep Langmuir probe has been constructed and successfully operated on “Thorello.” It is based on a novel, dual channel circuit that compensates for stray capacitance and permits sweep speeds up to 400 kHz. The circuit response has been tested by measuring the known current–voltage characteristics of resistors and diodes. In addition, the probe has been used to measure the electron temperature and density as well as the plasma potential of plasmas generated in Thorello. A method of three-parameter curve fitting is used to analyze the time-dependent data. The measurements compare favorably with those derived from other standard probe techniques.
Compact floating ion energy analyzer for measuring energy distributions of ions bombarding radio-frequency biased electrode surfaces70(1999); http://dx.doi.org/10.1063/1.1149829View Description Hide Description
A compact floating retarding-field ion energy analyzer and the accompanying electronics have been designed and built to measure the energy distribution of ions bombarding radio-frequency (rf) biased electrodes in high-density plasma reactors. The design consists of two main components, a compact retarding field vacuum probe and an integrated stack of floating electronics for providing output voltages, measuring currents and voltages and transmitting data to a computer. The operation and capabilities of the energy analyzer are demonstrated through ion energy distribution measurementsconducted on a 4 MHz rf-biased electrostatic chuck in a 13.56 MHz high-density transformer coupled plasma (TCP) reactor. The analyzer is capable of operating while floating on several hundreds of volts of rf bias and at pressures up to 30 mTorr without differential pumping. The effects of pressure (2–30 mTorr), TCP power (500–1500 W), rf-bias power (0–800 W), gas composition, and ion mass on the ion energy distributions are demonstrated through Ar, Ne, and Ar/Ne discharges.
- BASIC PHENOMENA
70(1999); http://dx.doi.org/10.1063/1.1149830View Description Hide Description
A new method is proposed to obtain hysteresis loops in ferroelectric materials in which the sample is submitted to a constant electric current. This technique was applied on poly(vinylidene fluoride), PVDF, biaxially stretched samples and the hysteresis loops of the electric displacement and electric polarization versus the electric field compared well with those obtained with the Sawyer–Tower method. The constant current method is advantageous in that no measuringamplifiers are needed and it is insensitive to prebreakdown noise.
70(1999); http://dx.doi.org/10.1063/1.1149831View Description Hide Description
A silicon-based temperature stabilized axial Hall probe with an absolute accuracy of ±40 ppm in the full range between 0 and 6 T is presented. The absolute calibration of the probe is performed against a nuclear magnetic resonance(NMR) probe. The application of the probe to a magnetic field profile of a 5 T superconducting magnet system, with field gradients as high as 30 T/m, reaching an overall accuracy of better than ±100 ppm, is demonstrated. In addition to the high absolute accuracy, this Hall probe allows high spatial resolution measurements of inhomogeneous fields in configurations where present NMR probes are not usable.
70(1999); http://dx.doi.org/10.1063/1.1149832View Description Hide Description
The relation between the voltage induced in an open sensor and the magnetic moment of a sample during pulsed field magnetization measurements has been calculated. The calculation takes into account the size of a sample, its internal flux distribution, and its position with respect to the center of the sensor. It has been found that the behavior of the response signal with respect to the sample position is the same for homogeneously magnetized Ni samples and high temperature superconductors. Because of this universality, it is possible to calibrate the open sensor even in the case of an unfavorable sample geometry. The calculations have been experimentally verified by using Ni samples with different geometries. Our results can easily be extended to samples with an arbitrary local field distribution.
Superconducting quantum interference device based resistance bridge for shot noise measurement on low impedance samples70(1999); http://dx.doi.org/10.1063/1.1149833View Description Hide Description
We present a resistance bridge which uses a superconducting quantum interference device(SQUID) to measure the shot noise in low impedance samples (<1 Ω). The experimental requirements are high direct current (dc) bias currents (≲10 mA), in order to obtain sufficiently high bias voltages, together with high alternating current (ac) sensitivity to detect small changes of the noise away from the thermal noise ground level. These conditions are fulfilled by changing continuously the overall gain of the SQUID electronics and separating the dc and ac feedbacks. This system is used to investigate the shot noise in mesoscopic samples made with superconducting (S) and normal (N) metals This value of source resistance is out of the range of classical noise measurement schemes. Because of its low intrinsic noise our system is dominated by the thermal noise of the resistors composing the bridge ( at 4.2 K); therefore, this method greatly simplifies the analysis of shot noise results.
70(1999); http://dx.doi.org/10.1063/1.1149834View Description Hide Description
Cross-shaped Hall sensors with high sensitivity and excellent temperature stability were fabricated from quantum wells based on an heterostructure. The layers were grown on semi-insulating GaAs substrates by molecular beam epitaxy. Maximum Hall mobilities of 215 000 cm2/V s with sheet carrier concentrations of were measured at 4.2 K for an undoped quantum well structure. These transport properties result in sensitivities as high as 3 T−1 (for voltage drive) and 650 Ω/T (for current drive). Additional Si δ doping in the middle of the InAs quantum well leads to a highly improved temperature stability of the sensitivities.