Minimum Stored Energy High-Field MRI Superconducting Magnets

A globally optimum minimum stored energy optimization strategy is implemented to design actively shielded superconducting magnet configurations used in high-field applications. The current density map is first obtained and used as a foundation for the magnet configurations by placing coils at current density local extremities. Optimized current density maps based on the stored energy formulation along with final magnet arrangements are provided to illustrate the findings. In this work, the focus was on compact superconducting magnets as measured by physical size and system footprint for given magnetic field properties inside the imaging region. The process of obtaining the current density maps proposed here over the given magnet domain, where superconducting coils are laid out, suggests that peak current densities occur around the perimeter of the domain, where in the most compact designs, with the domain length less than 1 m, the current direction alternates amongst adjacent coils. To reduce the peak magnetic field to acceptable levels on the superconductors in high-field designs, the size of the magnet domain is made larger, to the extent that the current densities no longer alternate between coils.      

Mobility degradation and transferred electron effect in gallium arsenide and indium gallium arsenide

The effect of mobility degradation on the intervalley transfer of electrons in gallium arsenide and indium gallium arsenide is studied. A considerable degradation of the mobility at high electric fields takes place for valleys with high low-field ohmic mobility. The momentum relaxation time τ = m^*µ/e is found to degrade with high electric field, giving the impression of high collision broadening at high electric fields. Similar degradation is expected for the mean free path. The relationship of the high-field transport in terms of ohmic transport parameters is elaborated to explain the transferred electron effect.     

A prototype mechanism for three-dimensional levitated movement of a small magnet

This paper deals with basic technologies for noncontact manipulation of small objects using magnets in micromachine applications. A small permanent magnet is manipulated by controlling the magnetic field applied to it. We propose a prototype mechanism to control the magnetic field for 3D levitated movement of an object. The prototype consists of eight electromagnets, pole-pieces to connect the magnetic poles, a soft iron yoke, and permanent magnets embedded in the yoke to compensate for the gravitational force of an object (a small permanent magnet). Regulation of a magnetic field and, as a result, motion of the object are strongly related to the geometric design of the pole-piece. A strategy for the pole-piece design is proposed and tested experimentally. Motion of the object in the vertical direction is realized by controlling the sum of the electromagnet currents for regulation of magnetic field gradient. Motion in the horizontal plane is realized by controlling the ratio of each electromagnet current for regulating the location of the maximum magnetic field. Results of several experiments show that the proposed prototype is effective.     

A nonlinear speed observer for permanent-magnet synchronous motors

The application of vector control techniques in AC motor drives demands accurate position and velocity feedback information for the current control and servo control loops. The authors describe a speed observer system suitable for use with permanent magnet synchronous motors as a software transducer. The observer is developed from the dq model of the machine. Design considerations for the observer are discussed. The nonlinearities in the machine model present a problem to the observer design, so a state detection technique is used to achieve global stability and consistent convergence of the observer system. The simulations show that the performance of the observer is robust against noise and parameter uncertainties     

On the form and stability of electric-field profiles within a negative differential mobility semiconductor

Using a fixed cathode boundary field model we examine analytically the form and stability of inhomogeneous prethreshold electric-field profiles in long bulk negative differential mobility (NDM) semiconductors. We show that the electric-field profiles and their associated current-voltage relationships depend in a detailed way on the characteristics of the bulk material (e.g., the velocity-electric-field relation) and on the boundary conditions imposed on the bulk. We investigate situations leading to the nucleation of a high-field domain at the cathode and find that for its occurrence: 1) the electric field at the cathode boundary must lie within the NDM region; and 2) space-charge neutrality must be approximated within the vicinity of the cathode. When these conditions are satisfied the current instability at threshold is accompanied by bulk electric-field values that are below the threshold field for NDM. The stability conclusions are drawn from a small-signal analysis that avoids many of the approximations of earlier studies. Supplementary numerical computations are included to demonstrate that the subsequent time development of the current instability depends in a detailed way on: 1) conditions at the cathode boundary; and 2) the external circuit. It is shown that the recycling Gunn effect cathode-to-anode transit-time mode in a resistive circuit is not a necessary consequence of the bulk having a region of NDM.     

Ultrasonic studies of solids

Ultrasonics is being used in many areas in solid-state physics. One is Fermi surface mapping of normal metals by the magnetoacoustic technique. The detailed knowledge gleaned is valuable in theoretical calculations of many of the electrical and optical properties of metals. Another area is that of ``hard' superconductors. Measurements of ultrasonic attenuation in the study of the metallurgical microstructure of high-field superconducting alloys (such as the Nb-Zr alloys) could have technological significance, especially in the field of power transmission. A new ultrasonic tool has been developed to aid investigations?the depletion layer transducer. It has prominent advantages over a conventional quartz piezoelectric transducer, including a higher fundamental frequency and the ability to change the resonant frequency by varying the dc bias level. Along with the new transducer, a new piezoelectric semiconductor amplifier produces substantial amplification of ultrasonic waves in photoconductive CdS by applying a dc electric field in the direction of wave propagation. This feature could find use in acoustic delay line technology.     

High-field effects in silicon nitride passivated GaN MODFETs

This paper presents a detailed study of high-field effects in GaN MODFETs. Degradation of DC characteristics and change of flicker noise due to hot electron and high-reverse current stresses in Si/sub 3/N/sub 4/ passivated GaN MODFETs have been investigated. The authors observe that during hot electron stress, electron trapping in the barrier layer and interface state creation occur. These cause a positive shift of V/sub t/, reduce I/sub D/, skew the transfer characteristics, and degrade g/sub m/. Flicker noise (1/f) measurements show that after hot electron stress, the scaled drain current noise spectrum (S/sub I(D)//I/sub D//sup 2/) decreases in depletion, but increases only slightly in strong accumulation, corroborating the creation of interface states but only a small creation of transition-layer tunnel traps that contribute to 1/f noise. During high-reverse current stress, electron trapping dominates for the first 50-60 s and then hole trapping and trap creation begin to manifest. However, there still is net electron trapping under the gate after one hour of stress. The degradation processes bring about a positive shift of V/sub t/, degrade I/sub D/ and g/sub m/, and increase reverse leakage. After high-reverse current stress, S/sub I(D)//I/sub D//sup 2/ increases substantially in strong accumulation, indicating the creation of transition layer tunnel traps.     

Diffused resistors characteristics at high current densitylevels-analysis and applications

Both heavily (source-drain) and lightly (well) doped diffused resistors used in modern CMOS integrated-circuit technologies are studied under high current density levels. The effects of high-field mobility degradation, space-charge limited-current, and charge-depletion narrowing (pinchoff) are discussed, and the related analytical solutions are derived and characterized. The applicability of the study to electrostatic discharge (ESD) protection networks, latchup protection circuitry, and output noise suppression resistors is addressed. Velocity saturation due to mobility degradation is found to be the prevailing mechanism, while space-charge limiting current and depletion narrowing have only limited effect on the lightly doped ones. The resulting saturation current densities allow proper resistor designs which can effectively limit ESD and latchup current into highly susceptible nodes and thus-improve overall circuit reliability, potential suppression of I/O switching noise is also discussed and shown to be more sensitive to the ratio between peak noise and steady-state currents. The experimental data confirm the theoretically predicted saturation velocity and critical field for the lightly doped case but show substantial discrepancy for the heavily doped layer     

Simultaneous ultrasound and MRI system for breast biopsy: compatibility assessment and demonstration in a dual modality phantom

Simultaneous capturing of ultrasound (US) and magnetic resonance (MR) images allows fusion of information obtained from both modalities. We propose an MR-compatible US system where MR images are acquired in a known orientation with respect to the US imaging plane and concurrent real-time imaging can be achieved. Compatibility of the two imaging devices is a major issue in the physical setup. Tests were performed to quantify the radio frequency (RF) noise introduced in MR and US images, with the US system used in conjunction with MRI scanner of different field strengths (0.5 T and 3 T). Furthermore, simultaneous imaging was performed on a dual modality breast phantom in the 0.5 T open bore and 3 T close bore MRI systems to aid needle-guided breast biopsy. Fiducial based passive tracking and electromagnetic based active tracking were used in 3 T and 0.5 T, respectively, to establish the location and orientation of the US probe inside the magnet bore. Our results indicate that simultaneous US and MR imaging are feasible with properly-designed shielding, resulting in negligible broadband noise and minimal periodic RF noise in both modalities. US can be used for real time display of the needle trajectory, while MRI can be used to confirm needle placement.     

Optimization of transistor structure for transistor-stabilizedfield emitter arrays

Reliability of the metal-oxide-semiconductor field-effect transistor (MOSFET)-stabilized field emitters at high-field operation has been assessed by comparing two different MOSFET structures. Electrical characteristics and behavior of carriers in the device structure have been investigated by means of device simulation. One structure, which is referred to as the externally connected-MOSFET emitter, exhibits an anomalous increase in drain current, which is induced by impact ionization at the drain edge. Upon evaluating the emission characteristics, it was clarified that the anomalous current increase induced by the impact ionization degraded stability and controllability of the emission current significantly. The other structure, which is referred to as the MOSFET-structured emitter, shows higher reliability with negligible effect of impact ionization     

环境一号C 卫星合成孔径雷达相干性分析

合成孔径雷达(Synthetic Aperture Radar, SAR)是一种相干成像雷达,相干性是保证SAR 成像的关键。对相干系统而言,诸多因素会破坏其相干性。该文以环境一号C 卫星SAR 分系统方案和指标为基础,主要分析了频率源稳定度和脉间定时抖动对系统相干性的影响。环境一号C 卫星SAR 采用短期稳定度为101.010 / 5 ms 的晶振,经过直接倍频与分频提供系统所需的基准频率信号,由短期稳定度引入的方位向随机相位误差对积分旁瓣的影响可忽略。环境一号C 卫星SAR 定时脉冲前沿抖动优于2 ns (rms),合成孔径时间内的随机相位误差对方位脉冲压缩旁瓣略有影响。该文通过公式推导和仿真分析证明了HJ-1-C 卫星SAR 系统相干性设计满足合成孔径雷达成像要求。      

Short-Wave Infrared HgCdTe Avalanche Photodiodes

Short-wave infrared (SWIR) HgCdTe avalanche photodiodes (APDs) have been developed to address low-flux applications at low operating temperature and for laser detection at higher temperatures. Stable multiplication gains in excess of 200 have been observed in homojunction APDs with cutoff wavelengths down to 2.8???m and operating temperatures up to 300?K, associated with low excess noise F?<?1.3 and low 1/f noise. The measured dark current density at 200?K of 6.2???A/cm² is low enough to enable high-sensitivity single-element light detection and ranging (lidar) applications and time-of-flight imaging. Corresponding APD arrays have been hybridized on a readout integrated circuit (ROIC) designed for low-flux low-SNR imaging with low noise and frame rates higher than 1500?frames/s. Preliminary focal-plane array characterization has confirmed the nominal ROIC performance and showed pixel operability above 99.5% (pixels within ±50% of average gain). The bias dependence of the multiplication gain has been characterized as a function of temperature, cadmium composition, and junction geometry. A qualitative change in the bias dependence of the gain compared with mid-wave infrared (MWIR) HgCdTe has motivated the development of a modified local electric field model for the electron impaction ionization coefficient and multiplication gain. This model gives a close fit to the gain curves in both SWIR and MWIR APDs at temperatures between 80?K and 300?K, using two parameters that scale as a function of the energy gap and temperature. This property opens the path to quantitative predictive device simulations and to estimations of the junction geometry of APDs from the bias dependence of the gain.     

Nanoscale structural characteristics and electron field emission properties of transition metal–fullerene compound TiC60 films

Transition-metal compound TiC₆₀ thin films were grown by co-deposition from two separated sources of fullerene C₆₀ powder and titanium. Study of structural properties of the films, by Raman spectroscopy, atomic force microscopy, and scanning tunneling spectroscopy reveals that the films have a deformed C₆₀ structure with certain amount of sp³ bonds and a rough surface with a large number of nanoclusters. z–V tunnelling spectroscopic measurements suggest that several charge transport mechanisms are involved in as the tip penetrates into the thin film. Conventional field electron emission (FEE) measurements show a high emission current density of 10 mA/cm² and a low turn-on field less than 8 V/μm, with the field enhancement factors being 659 and 1947 for low-field region and high-field region, respectively. By exploiting STM tunneling spectroscopy, local FEE on nanometer scale has also been characterized in comparison with the conventional FEE. The respective field enhancement factors are estimated to be 99–355 for a gap varying from 36 to 6 nm. The enhanced FEE of TiC₆₀ thin films can be ascribed to structural variation of C₆₀ in the films and the electrical conducting paths formed by titanium nanocrystallites embedded in C₆₀ matrix.     

Pulse discrimination by Gunn-effect switching

Starting with simple equations for the domain dynamics the switching behaviour of a Gunn device with an ohmic load resistance is investigated, in a way similar to tunnel diode switching. An appropriate load line shall define three bias states on the current voltage characteristic, i.e. a homogeneous low-field state I (E_I < E_peak), a homogeneous high-field state III (E_III > E_valley) and a steady-state domain state IV. Then triggering the device from the only d.c. stable state I by (rectangular and triangular) input pulses of various heights and lengths leads to three types of switching processes namely direct return to I, return via state III or via state IV, these transitions yielding three typical output pulses. Discrimination curves for the input-pulse parameters are presented. Simple limiting cases for the three switching processes are treated in some detail yielding the switching times typical for each transition. Finally, the long-time stability of the bias states I, III and IV is discussed.     

An in-probe receiver amplifier with 3 dB noise figure and 50 dB dynamic range for medical ultrasound imaging using CMUTs

This paper presents the design and measurements of an in-probe receiver amplifier for ultrasound imaging applications using a capacitive micromachined ultrasonic transducer (CMUT). In such applications, the noise and the dynamic range play very important roles, as the former dictates the minimum input signal level and the latter defines the maximum input signal level that can be applied to a system. This work concentrates on both of these specifications. The amplifier consists of a transimpedance amplifier followed by a voltage gain stage implemented using a current feedback amplifier. It is designed and fabricated using a 180 nm CMOS process. A noise figure of 3 dB is measured for a CMUT model with 10–30 MHz frequency range. The amplifier shows a dynamic range of 50 dB with 0.8 % total harmonic distortion for the full scale input current of 7 µA peak-to-peak.     

Response of a Twisted Pair Excited by the Nonuniform Electromagnetic Fields of Nearby Loops

This paper derives equations for the noise induced in a twisted pair when the pair is embedded in a nonuniform electromagnetic field of a small current loop. The derived equations are applied to calculate the noise induced in a typical 135-? twisted pair. In the frequency range of 10 kHz-100 MHz, the noise induced in this twisted pair is shown to be less by about 70-135 dB when compared to a similar parallel-conductor line without twist that is embedded in the same field. Included is a simplified method of applying the derived equations to the problem of finding the response of the twisted pair to distinct uncorrelated noise sources, assuming that the sources can be approximated by small current loops.     

Low-dark-current low-voltage 1.3?1.6 ?m avalanche photodiode with high-low electric field profile and separate absorption and multiplication regions by molecular beam epitaxy

The operation of the recently disclosed heterojunction avalanche photodiode with high-low electric field profile and separate absorption and multiplication regions (HI-LO SAM APD) is demonstrated. This new structure which features a doping spike in the wide gap layer offers several advantages over conventional SAM APDs (lower dark current and excess noise factor, greater gain stability). Low dark currents (?1 nA), low voltage operation (?26 V) and gains as high as 50 at 1.60 ?m are demonstrated in an Al0.48In0.52As/Ga0 47In0 53As prototype grown by molecular beam epitaxy.     

Superconducting magnets and their applications

Since the discovery of superconductivity almost a century ago, there has been a steady increase in the variety of superconducting magnet applications. Progress in superconducting magnet technology has resulted in applications in areas of basic science, medicine, separation, and levitation. Performance improvements in a variety of materials, from low-temperature to high-temperature superconductors, are the foundation of recent rapid development. In addition, large increases in affordable computing power, along with steady refinement of three-dimensional analytical tools and improved materials characterization, have allowed many more advanced magnet concepts to be realized directly in hardware without scale prototype testing than was previously possible. This in turn has broadened opportunities for new science and technology results in many fields including the basic sciences, medical imaging, fusion, environmental remediation, and transportation. In this paper, a few examples of these applications will be discussed, representing a range in magnetic field, current density, and overall size, from the practical to the developmental.     

可调谐半导体激光器的精密控制系统设计

为了满足高精密测量领域对半导体激光器高稳定度的要求,设计了一种高稳定度、低噪声的半导体激光器控制系统。该控制系统由电流驱动和温度控制两部分组成,电流控制部分采用负反馈控制保持电流稳定,温度控制部分采用高度集成的MAX1978作为主控芯片,驱动半导体制冷器进行温度补偿。经过实验验证,电流在200mA范围内连续可调,电流控制精度高达1A,在3kHz~100kHz带宽内交流噪声有效值小于300nA,长期温度漂移小于2m℃。结果表明,该系统可用于驱动分布式反馈外腔半导体激光器和分布式布喇格反射半导体激光器。     

Maximum entropy image reconstruction from sparsely sampled coherentfield data

There are many practical problems in which it is required to detect and characterize hidden structures or remote objects by virtue of the scattered acoustic or electromagnetic fields they generate. It remains an open question, however, as to which reconstruction algorithms offer the most informative images for a given set of field measurements. Commonly used time-domain beamforming techniques, and their equivalent frequency-domain implementations, are conceptually simple and stable in the presence of noise, however, large proportions of missing measurements can quickly degrade the image quality. We apply a new algorithm based on the maximum entropy method (MEM) to the reconstruction of images from sparsely sampled coherent field data. The general principles and limitations of the new method are discussed in the framework of regularization theory, and the results of monostatic imaging experiments confirm that superior resolution and artifact suppression are obtained relative to a commonly used linear inverse filtering approach