Volume 76, Issue 1, January 2005
- optics; atoms and molecules; spectroscopy; photon detectors
- particle sources, optics and acceleration; particle detectors
- nuclear physics, fusion and plasmas
- microscopy and imaging
- condensed matter; materials
- biology and medicine
- gravity; geophysics; astronomy and astrophysics
- electronics; electromagnetic technology; microwaves
- thermometry; thermal diffusivity; acoustic; photothermal and photoacoustic
- general instruments
Index of content:
- OPTICS; ATOMS AND MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
76(2005); http://dx.doi.org/10.1063/1.1823655View Description Hide Description
An approach to bolometry has been developed. It is based on the measurements of optical reflectivity change of a thin metal layer deposited on a transparent substrate. The reflectivity change results from the temperature rise due to absorption of energetic particles or x rays. The sensor of the bolometer has no ohmic contact with the measuring unit, making this method well suited for an environment with strong electromagnetic noise interference. The technique was applied to characterize a method for the generation of intense electron beams in a dense gas. Very high efficiency for the e-beam generation (up to 95%) was measured.
76(2005); http://dx.doi.org/10.1063/1.1834699View Description Hide Description
We present a simple design for a Mach–Zehnder interferometric autocorrelator that is aimed at real-time laser pulse diagnostics. The device is based around a symmetric and balanced mechanical oscillator that confers a simple and compact design, while still allowing the measurement of a broad range of pulse durations with high speed and high resolution. The device requires a very low drive level and possesses a very weak mechanical coupling to the environment. The apparatus provides two outputs: one output is proportional to the instantaneous velocity of the interferometer arms, which can be integrated to determine the arm position. This gives the means to calibrate the arm position accurately and thereby account for any nonuniformity of the motion. The second output, from a two-photon photodetector, is used to determine the pulse duration. The present device can measure pulses from the femtosecond through to the picosecond domain.
76(2005); http://dx.doi.org/10.1063/1.1830013View Description Hide Description
The development of a multiple-channel lock-in optical spectrometer (LIOS) is presented, which enables parallel phase-sensitive detection at the output of an optical spectrometer. The light intensity from a spectrally broad source is modulated at the reference frequency, and focused into a high-resolution imaging spectrometer. The height at which the light enters the spectrometer is controlled by an acousto-optic deflector, and the height information is preserved at the output focal plane. A two-dimensional InGaAs focal plane array collects light that has been dispersed in wavelength along the horizontal direction, and in time along the vertical direction. The data is demodulated using a high performance computer-based digital signal processor. This parallel approach greatly enhances (by more than 100x) the speed at which spectrally resolved lock-in data can be acquired. The noise performance of a working system optimized for the 1.3 μm wavelength range is analyzed using a laser diode light source. Time-resolved absorption traces are obtained for InAsquantum dots embedded in a GaAs matrix, and for dispersed films of PbSenanocrystals.
76(2005); http://dx.doi.org/10.1063/1.1832192View Description Hide Description
We describe a system combining an ion beam trap and a low energy electron target in which the interaction between electrons and vibrationally cold molecular ions and clusters can be studied. The entire system uses only electrostatic fields for both trapping and focusing, thus being able to store particles without a mass limit. Preliminary results for the electron impact neutralization of ions and aluminum clusters are presented.
76(2005); http://dx.doi.org/10.1063/1.1832411View Description Hide Description
Time-of-flight (TOF) momentum imaging systems utilize the , , information from charged particles striking a position-sensitive detector to infer the , , and components of the particles’ initial momenta. This measurement capability can lead to the complete experimental determination of multi-ionization/fragmentation dynamics. In the case of electron detection, the addition of a magnetic field leads to a significantly increased operational energy range. This study shows that the TOF system has to be carefully designed in order to optimize the magnetic confinement effect. Expressions for the optimal dimensions of a single electric field TOF system are derived and factors contributing to the resolution are discussed, along with their application to an existing imaging system.
76(2005); http://dx.doi.org/10.1063/1.1833331View Description Hide Description
Optical pyrometry is widely used in industry and research laboratories to perform surface temperature measurements of sample materials. These shock physics experiments are normally conducted at powder or gas gun facilities or at facilities where high explosives can be used as a shock wave source, and using high-speed pyrometers that are usually calibrated by using a blackbody source. But, electrical power, time, and space can be limiting factors in such facilities, and blackbody calibration can be difficult. Crucial parts of the experimental setup (fibers, lenses, and/or mirrors) are destroyed in such experiments, and the pyrometry system must be recalibrated before each experiment. We have developed a calibration technique using integrating-sphere sources that allows us to calibrate pyrometers more rapidly and easily than with blackbodies. Two different integrating-sphere systems are described to cover the wavelength range generally used in pyrometry studies. The characterization of these systems is fully detailed including measurements of their spectral radiances. A discussion of the advantages and drawbacks of both calibrationsources is given.
-band electron paramagnetic resonance spectrometer for use in vivo and in studies of aqueous biological samples76(2005); http://dx.doi.org/10.1063/1.1832271View Description Hide Description
The development of -band frequency EPR spectrometers has made feasible many in vivo studies in laboratory animals and, recently, in human volunteers. The lower dielectric and eddy current losses that occur at -band balance the lower Zeeman splitting so useful measurements can be made in conductive aqueous samples. We describe typical resonators used in such studies and provide details on the construction of the spectrometer, including the bridge, the automatic frequency control subsystem, the low-noise high-stability tunable -band frequency source, as well as the low-frequency components—the signal receiver and the modulation unit. The application of EPR spectroscopy to larger subjects requires special care in the design of an appropriate magnet with sufficient homogeneity and stability, yet with dimensions that allow operation with a wide range of subject sizes. We describe our solution, which involves a permanent magnet, air-core scan coils to provide the field sweep and offset, and field stabilization by means of a field-frequency lock. We also describe the magnetic field modulation system, which operates at 25 kHz to avoid distortion in spectra from materials with narrow lines (such as lithium phthalocyanine). We refer to recent reviews to illustrate the range of in vivo studies and the clinical applications of the type of spectrometer described here.
- PARTICLE SOURCES, OPTICS AND ACCELERATION; PARTICLE DETECTORS
76(2005); http://dx.doi.org/10.1063/1.1823653View Description Hide Description
The origin of the low transmission through the low energy beam line between the electron cyclotron resonancesource and the AGOR cyclotron has been investigated. Measurements of beam size and emittance, determined with the “varying quadrupole method,” are compared with calculations including fringe fields up to third order with the code COSY INFINITY. Calculations and measurements qualitatively agree; the calculations exhibit the large beam losses observed. On the basis of the calculations new settings have been determined, resulting in a significant increase of the transmission. To achieve full transmission a complete redesign of the beam line, in particular of the bending magnets, is needed.
76(2005); http://dx.doi.org/10.1063/1.1823779View Description Hide Description
In coil-dominated superconductingdipole magnets the coil size and hence the cost of the magnet can be reduced by optimizing the shape of the coil. We have used the variational calculus and a random search technique to show that the coil shape markedly deviates from the conventional design when one reduces the coil size while holding the field and field quality to specified values. A block-coil dipole giving a field of has been designed on the basis of the optimization. With iron yoke this can give a field of .
76(2005); http://dx.doi.org/10.1063/1.1829996View Description Hide Description
A plasma based electron beamsource apparatus is described which creates a plasma with two distinct density regions separated by a transition which is shorter than the plasmaskin depth of either region. This sharp density modulation is achieved by using a perforated stainless steel screen to filter half of a diffusing plasma column. A simple physical model predicts that the length of the plasma density transition will vary with the distance from the screen. For a weakly magnetized plasma, the transition length will be twice the distance, on a line normal to the screen plane, from the screen edge to the location where the transition is measured. The plasma column is generated using an argon discharge plasma source. It has a peak density of approximately and a full width half maximum width of 5 cm. The discharge source utilizes a 7.5 cm diameter disk cathode heated to 1300 °C using a graphite heater. The plasma column is filtered with a thick stainless steel sheet with holes and 21% open area. Plasma density transitions with lengths between 0.74 and were measured.
76(2005); http://dx.doi.org/10.1063/1.1829975View Description Hide Description
A delay-line position-sensitive detector with improved performance is presented. In this device, timing is carried out by means of fast digitizer boards. The use of dedicated signal processing procedures leads to a timing accuracy of 70 ps and a dead-time below 1.5 ns. As a result, the spatial resolving power of this detector is close to 1 mm leading to a high multihit capability. A temporary detector has been designed in which the delay-line anode is combined with a phosphor screen allowing additional positioning to be made via a charge-coupled devicecamera. This additional positioning is used to unambiguously quantify performances in terms of spatial resolution and multihit capabilities. A three-dimensional atom probe analysis of a material containing low evaporation field phases is used to demonstrate the capabilities of this detector.
76(2005); http://dx.doi.org/10.1063/1.1831251View Description Hide Description
The properties of a detuned nondispersive Si(111) double crystal monochromator were studied by using the ray tracing method. The program used was the PC version of shadow V2.3.3 with gui V1.2. This article concentrates on detuning of the second crystal since detuning of the first crystal causes a larger shift of the energy center of the transmitted beam than detuning of the second one does. The results indicate that for a large detuning angle of the second crystal, the transmitted spectrum splits into two bands with equal bandwidth, which is delayed at a large vertical divergence of the incident beam. With increasing detuning angle of the second crystal, both the fundamental bandwidth (for a well collimated beam) and the harmonic contamination decrease until they reach the minimum at a detuning angle of roughly half the full width at half maximum of the rocking curve, where the left fundamental flux is about 50%. The capability of harmonic suppression by detuning the second crystal is limited, with the harmonic contamination reduced by about two orders of magnitude at most, which implies that other methods, such as double mirror system, must be adopted when smaller harmonic contamination is required.
76(2005); http://dx.doi.org/10.1063/1.1834708View Description Hide Description
A linac consisting of six rf cavities has been constructed as an energy booster of the RIKEN heavy-ion linac. Various heavy ion beams have been accelerated to with intensities of more than (particle microampere) in the continuous-wave mode. The booster cavities are based on a quarter-wavelength resonator of a coaxial structure. The first two cavities are frequency-variable in a range from , whereas the last four are operated at a fixed frequency of . The design study started in 1997 and the booster was commissioned in 2001. The total voltage gain of the booster ever achieved is , which is 70% of the designed value of . The high-intensity beams from the booster have been successfully applied to a systematic study on the synthesis of superheavy elements. The beam energy from the ring cyclotron has also been increased, owing to the additional velocity gain provided by the booster.
- NUCLEAR PHYSICS, FUSION AND PLASMAS
76(2005); http://dx.doi.org/10.1063/1.1823611View Description Hide Description
An actively cooled plasmaelectrode has been developed for long pulse operation in a cesium-seeded negative ion source. To keep the electrodetemperature at about , which is the optimum range of temperature to enhance cesium effects, the electrode cooling structure has been designed using three-dimensional numerical simulation assuming that the heat flux from the sourceplasma was . Water cooling tubes were brazed to the plasmaelectrode substrate with spacers made of stainless steel, which acts as a thermal resistance. The fabricated plasmaelectrode has been tested in a cesium-seeded volume negative ion source called Kamaboko source. The temperature of the electrode reached for the arc power of , which is the operating condition required for producing beams with current densities exceeding . It was demonstrated that the actively cooled plasmaelectrode is applicable to long pulse operations, meeting the temperature requirement for optimizing the surface-production process of negative ions in the cesium-seeded ion source.
76(2005); http://dx.doi.org/10.1063/1.1831671View Description Hide Description
Hydrogen is the main constitute of plasmas in HANBIT magnetic mirrordevice, therefore, measurement of the emission from excited levels of hydrogen atoms is an important diagnostic tool. From the emissivity of radiation one can derive quantities such as the neutral hydrogen density and the source rate, i.e., the rate at which the plasma protons are replenished by ionization of neutral atoms diffusing into the plasma. The reconstruction of emissivity profiles from radiation measurement is a highly undetermined and ill-posed inversion problem, due to the restricted viewing access, number of chords and the increased noise level. An unbiased and consistent probability theory based approach within the framework of Bayesian inference is provided by the maximum entropy method which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in HANBIT magnetic mirrordevice.
New technique deducing plasma potential by a capacitive coupling method in spraying dielectric barrier discharge plasmas76(2005); http://dx.doi.org/10.1063/1.1832172View Description Hide Description
A new method to measure the plasma potential in an atmospheric dielectric barrier discharge (DBD) plasmas is developed for a new spraying DBD plasma source, which is sustained by electric fields generated by flowing plasmas at the outer region of the electrodes, since conventional electric probe can not be applied due to arcing. The new technique is to measure the spatially averaged plasma potential by using a capacitive coupling method with calculation of collisional sheath thickness.
Two-dimensional diagnostic of edge plasma structure using a lithium beam probe in a compact helical system76(2005); http://dx.doi.org/10.1063/1.1809284View Description Hide Description
A neutral lithium beam probe for two-dimensional diagnosis of edge plasmas has been designed and installed on the compact helical system. A lithium beam with an energy of and a current of is used. The spatial resolution is about , and the time response is about . The beam penetration depth is expressed in terms of the line integral density , which is about . The beam injection angle can be varied and the observation point covers the edge and separatrix region of the helical diverter configuration. Two-dimensional electron density profiles for electron cyclotron heating and neutral beam injection (NBI) heated plasmas are obtained near and outside the last closed flux surface (LCFS). Analysis for two-dimensional density profile reconstruction indicates that significant amounts of surface plasma are confined outside the LCFS for NBI plasmas even though the ergodic layer is cut by the vacuum chamber wall (inboard limiter configuration). The usefullness of this new two-dimensional diagnostic in the edge region is demonstrated.
76(2005); http://dx.doi.org/10.1063/1.1834705View Description Hide Description
A compact Gundestrup–Langmuir probe diagnostics system capable of data acquisition as well as data analysis was conceived at INRS-EMT, Canada, and used at IPP-FZJ, Germany. Data acquisition and analysis can be done with this system using several types of probes (Langmuir, double-Langmuir, Mach, Gundestrup,…). The versatility as to the different types of probe that one can use and the relative small size of the whole system makes it advantageous. Using a laptop computer makes the system small size and highly portable. The system acquires data at at resolution on two probe systems simultaneously, using a total of 12 input channels. Bias is done by a DAC-ADC card and is amplified to give a sweeping range. Measured temperatures ranged from with densities observed as low as up to , which are the range of conditions to be found in our experimental device. This system is also easy to reproduce since the hardware is commercially available and the scripts can be duplicated and modified according to the specifics of the hardware.
Time dependent neutrons measurement using a polycrystalline chemical vapor deposited diamond detector at the JET tokamak76(2005); http://dx.doi.org/10.1063/1.1834691View Description Hide Description
A polycrystalline chemical vapor deposited (CVD) diamond detector was installed on a JET tokamak in order to monitor the time dependent neutron emission produced by D–T plasma pulses during the Trace Tritium Experiment (TTE) performed in October 2003. This was the first tentative ever attempted to use a CVD diamond detector as neutron monitor in a tokamak environment. Despite its small active volume, the detector was able to detect the neutron emission with good reliability and stability during the experimental campaign that lasted five weeks. The comparison with standard silicon detectors presently used at JET as neutron monitors is reported, showing excellent correlation between the measurements. The results prove that CVD diamond detectors can be reliably used in a tokamak environment and therefore confirm the potential of this technology for next step machines like ITER.
76(2005); http://dx.doi.org/10.1063/1.1824371View Description Hide Description
A high energy electron spectrometer has been designed and tested using imaging plate (IP). The measurable energy range extends from or even higher. The IP response in this energy range is calibrated using electrons from L-band and S-band LINAC accelerator at energies 11.5, 30, and . The calibration has been extended to using an existing data and Monte Carlo simulation Electron Gamma Shower code. The calibration results cover the energy from and show almost a constant sensitivity for electrons over energy. The temperature fading of the IP shows a 40% reduction after of the data taken at . Since the fading is not significant after this time we set the waiting time to be . The oblique incidence effect has been studied to show that there is a relation when the incidence angle is .