Effect of 14 MeV Neutrons on Space-Charge-Limited Current of Electrons in High Purity Silicon

Alloyed and symmetrical n+?n+ devices made of nominally 75k?cm ?-type Si are analyzed before and after irradiation with 14MeV neutrons at room temperature and doses of 1.2×1011, 5.5×1011 and 4.0×1012n/cm2. Immediately after the application of a large turn-on voltage step at t ? O, the flow of electrons through these devices is by pure, trap free, space-charge-limited current (sclc). From an analysis of this sclc, it is established that the drift velocity-field relationship of electrons in Si is affected by the radiation only at low temperatures in the low field range. For t>0, the current after irradiation decays below its initial trap free sclc value, thus revealing the presence of traps. Two categories are identified: fast traps with energy levels at about ?Etf ? 0.13eV below the conduction band and slow traps with energy levels at roughly ?Ets?005eV. Cross sections are also obtained. Introduction rates are about equal for both (llcm-l for fast traps and 8cm-1 for slow traps). These results demonstrate the sensitivity of sclc as a tool to detect traps and changes in the drift velocityfield relationship caused by radiation. Some implications of these results are discussed and additional experiments are suggested.     

Linear Connected Array of Non-Adiabatic Traps

An innovative confinement concept of a linearly connected array of non-adiabatic traps is examined. A non-adiabatic trap unit consists of a cylindrical vacuum chamber with external mirror coils and a pair of inner anti-parallel coils. Positions and currents on these coils are adjusted to achieve zero magnetic fields at the center. The plasma is trapped stably in this configuration due to the “Absolute Minimum B” magnetic configuration that is created. However, like a cusp field, energy confinement of plasma in a trap unit is poor, since particles suffer random pitch-angle scattering near the central zero field region and eventually fall into the loss-cone. However, once these non-adiabatic traps are linearly connected, plasma particles escaping a unit will be effectively re-trapped within the neighboring trap due to collision-free pitch-angle scattering in the zero field region. Since the transition of a charged plasma particle from unit to a next unit is stochastic, a connected array of non-adiabatic fusion core units and similar “leak suppressor” array units with low fusion rate at both ends of the core array improves the plasma confinement sufficient to achieve reactor-grade plasmas.     

Electron beam ion traps and their applications

A brief introduction to the historical background and current status of electron beam ion traps (EBITs) is presented. The structure and principles of an EBIT for producing highly charged ions are described. Finally, EBITs as a potential tool in hot-plasma diagnostics and in studying frontier problems of highly charged ion physics are discussed.     

电荷耦合器件的~(60)Co γ射线辐照损伤退火效应

基于60Coγ射线辐照后的商用电荷耦合器件(CCD),对室温退火和100℃高温退火实验进行研究。考察了CCD的功耗电流、输出信号电压波形及光响应灵敏度等参数的变化。结果表明,CCD辐照过程中产生的氧化物电荷和界面态导致了CCD参数在室温和高温退火中的不同表现。     

Electrical characteristics of neutron irradiation induced defects in n-GaAs

Palladium Schottky barrier diodes (SBDs) on epitaxially grown n-GaAs were irradiated with neutrons from a reactor and a p(66)/Be (40) clinical source. From current-voltage (I–V) and capacitance-voltage (C–V) measurements it was found that neutron irradiation caused generation-recombination currents and resulted in a reduction in the free carrier concentrations of the epitaxial layers. A linear relation was found between the irradiation fluence, the free carrier removal and the reverse leakage current of neutron irradiated SBDs. Deep level transient spectroscopy (DLTS) indicated that five electron traps, Enl-En5, were introduced during neutron irradiation. These defects are shown to be responsible for the degradation of neutron irradiated SBDs.     

聚变脉冲中子源特性的近距离测量方法

介绍在近距离条件下,测量聚变脉冲中子源的时间谱、能谱和产额的一种方法-两测点解谱法,对该方法的局限性作了简要地讨论。     

《辐射防护规定》的剂量限制体系

《辐射防护规定》(GB8703-88)结合我国具体情况采纳了国际放射防护委员会(ICRP)第26号出版物所推荐的剂量限制体系。剂量限制体系主要适用于所有可控制源的防护,它规定了辐射防护的目的和基本原则,各种剂量限值及对公众照射的限制原则。本文着重阐述《辐射防护规定》中剂量限制体系的制定依据和主要特点。文中还对与实施剂量限制体系有关的一些问题作了说明。     

Numerical modeling of 3D halo current path in ITER structures

Disruptions represent one of the main concerns for Tokamak operation, especially in view of fusion reactors, or experimental test reactors, due to the electro-mechanical loads induced by halo and eddy currents. The development of a predictive tool which allows to estimate the magnitude and spatial distribution of the halo current forces is of paramount importance in order to ensure robust vessel and in-vessel component design. With this aim, two numerical codes (CARIDDI, CAFE) have been developed, which allow to calculate the halo current path (resistive distribution) in the passive structures surrounding the plasma. The former is based on an integral formulation for the eddy currents problem particularized to the static case; the latter implements a pair of 3D FEM complementary formulations for the solution of the steady-state current conduction problem. A simplified plasma model is adopted to provide the inputs (halo current injected into the first wall). Two representative test cases (ITER symmetric and asymmetric VDEs) have been selected to cross check the results of the proposed approaches.     

Solving the Stand-off Problem for Magnetized Target Fusion: Plasma Streams as Disposable Electrodes, Together with a Local Spherical Blanket

In a fusion reactor based on the magnetized target fusion approach, the permanent power supply has to deliver currents up to a few mega-amperes to the target dropped into the reaction chamber. All the structures situated around the target will be destroyed after every pulse and have to be replaced at a frequency of 1–10 Hz. In this paper, an approach based on the use of spherical blanket surrounding the target, and pulsed plasma electrodes connecting the target to the power supply, is discussed. A brief analysis of the processes associated with creation of plasma electrodes is presented.     

Heat conduction analyses on rewetting front propagation during transients beyond anticipated operational occurrences for BWRs

Our previous study investigated the rewetting behavior of dryout fuel surface during transients beyond anticipated operational occurrences for BWRs, which indicated the rewetting velocity was significantly affected by the precursory cooling defined as cooling immediately before rewetting. This study further investigated the previous experiments by conducting additional experimental and numerical heat conduction analyses to characterize the precursory cooling. For the characterization, the precursory cooling was first defined quantitatively based on evaluated heat transfer rates; the rewetting velocity was investigated as a function of the cladding temperature immediately before the onset of the precursory cooling. The results indicated that the propagation velocity appeared to be limited by the maximum heat transfer rate near the rewetting front. This limitation was consistent with results of the heat conduction analysis using heat transfer models for the precursory cooling expressed as a function of distance from the rewetting front, the maximum wetting temperature, and the heat transfer coefficients in the wetted region. This paper also discusses uncertainties in the evaluation of transient heat flux from the measured surface temperature, and technical issues requiring further investigation.     

Core performance of new concept passive-safety reactor “kamado” - safety, burn-up and uranium resource problem -

New concept of a passive-safety reactor “KAMADO” has a negligible possibility of core melting and flexibility of total reactor power. The reactor core of KAMADO consists of fuel elements of graphite blocks, which have UO₂ fuel rods and cooling water holes. These fuel elements are located in a reactor water pool of atmospheric pressure (1 atm) and low temperature (< 60°C). In case of LOCA, decay heat from fuel rods is removed by conduction heat transfer to the reactor water pool. Since the cooling water does not contact a fuel rod directly, core design has much flexibility without considering dry-out limitation and Minimum Critical Power Ratio (MCPR). Additionally an effective use of spent fuel is expected.     

Investigation on discharge characteristics of a coaxial dielectric barrier discharge reactor driven by AC and ns power sources

A coaxial dielectric barrier discharge(DBD) reactor with double layer dielectric barriers has been developed for exhaust gas treatment and excited either by AC power or nanosecond(ns)pulse to generate atmospheric pressure plasma. The comparative study on the discharge characteristics of the discharge uniformity, power deposition, energy efficiency, and operation temperature between AC and ns pulsed coaxial DBD is carried out in terms of optical and electrical characteristics and operation temperature for optimizing the coaxial DBD reactor performance. The voltages across the air gap and dielectric layer and the conduction and displacement currents are extracted from the applied voltages and measured currents of AC and ns pulsed coaxial DBDs for the calculation of the power depositions and energy efficiencies through an equivalent electrical model. The discharge uniformity and operating temperature of the coaxial DBD reactor are monitored and analyzed by optical images and infrared camera. A heat conduction model is used to calculate the temperature of the internal quartz tube. It is found that the ns pulsed coaxial DBD has a much higher instantaneous power deposition in plasma, a lower total power consumption, and a higher energy efficiency compared with that excited by AC power and is more homogeneous and stable. The temperature of the outside wall of the AC and ns pulse excited coaxial DBD reaches 158 ℃ and 64.3 ℃ after 900 s operation, respectively.The experimental results on the comparison of the discharge characteristics of coaxial DBDs excited by different powers are significant for understanding of the mechanism of DBDs,reducing energy loss, and optimizing the performance of coaxial DBD in industrial applications.     

MHD flow past a vertical oscillating plate

The motion of a semi-infinite incompressible, viscous electrically conducting fluid, caused by the oscillation of a plane vertical plate under the action of a transverse magnetic field, has been studied on taking into account the presence of free convection currents. Closed form solutions to velocity and penetration distance through which the leading edge effect propagates have been derived on neglecting the transient part. In case of air, it has been observed that the velocity profiles lie on left or right hand side of η-axis according as G < 0 (heating of the plate) or G > 0 (cooling of the plate). There may occur instability in case of air for ωt = 0. With increasing magnetic field, the transition from conduction to convection is enhanced.     

Thermal conduction via the transport equation with applications to laser-fusion

We study the problem of heat conduction in a pellet of deuterium and tritium heated by a laser. In particular, we examine the heat flux between the critical surface and the pellet surface. Because of the smallness of this region compared with an electron mean free path, the problem is treated by transport theory using the Boltzmann equation. Two distinct problems are considered. In the first, only ion-electron collisions are allowed and the resulting linear transport equation is solved analytically. Numerical evaluation of the results illustrates the shortcomings of continuum theory and shows the general structure of the temperature profile across the conduction region. A review of previous work in this area is given.

To account for the electron-electron collisions the full non-linear Boltzmann equation is employed with the BGK scattering model. In this approach we allow both electron-electron and ion-electron scattering and by various transformations are able to cast the problem into the form of three coupled, non-linear integral equations. These are solved numerically and values of the heat flux as a function of optical thickness of the conduction zone are obtained. The results show the importance of a rigorous transport treatment of conduction and highlight the limitation of previous attempts to solve this problem.

It is not claimed that the theory presented here can be directly applied to the laser-fusion problem because the effects of non-thermal electron distributions and strong electric and magnetic fields have been neglected. Nevertheless, the general conclusions are believed to be true regardless of the physical application.     

On Approximating the EMP Response of Small Cavities

A straightforward (pedestrian) approximation scheme is applied to a transient electromagnetic field problem associated with a finite-length cylindrical cavity bounded by perfectly conducting walls. The cavity is filled with a homogeneous lossy dielectric material. Analytic expressions for the zeroth-and first-order terms of the perturbation solution for each of the relevant components of the electric and magnetic fields generated by an axially propagating current pulse are presented. Results obtained for various sample problems are examined and comparisons with the exact solutions are highlighted. The adequacy and limitations of such an approximation scheme are discussed, and the errors associated with ignoring the displacement and conduction currents are made evident.     

Process Dependent Build-Up of Interface States in Irradiated N-Channel MOSFETs

The build-up of fast-states (?Dit) continues to occur in the Si/SiO2 interface region even after the irradiation has stopped. For the devices described in this paper, the process of conversion of trapped positive charges (?Qot) into ?Dit appears to be responsible for the build-up. The presence of ambient hydrogen is necessary for the conversion to occur in well annealed MOS devices without SiN-Caps. The decrease in the highest temperature after the source/drain implant, dictated by the demand for shallow junctions, leaves residual neutral traps in the interface region which cause increase in the rate of conversion of ?Qot into ?Dit. The low-field effective mobility (?eff) of the inversion layer electrons has been characterized as a function of the gamma-radiation dose. The reduction of ?eff due to gamma rays is predominantly attributed to the decrease in the number of carriers available for conduction because of the fast-states. A universal model of ?eff is presented; it is a very powerful tool in studying the Si/SiO2 interface properties.     

Design and Characterization of a Low Power Consumption, HPGe Spectrometer

A low power consumption, high resolution germanium spectrometer is described. The spectrometer consists of an HPGe detector (either conventional or reverse electrode), a charge loop amplifier, a high count rate indicator, detector bias shutdown electronics, and a portable cryostat. A brief analysis of the charge loop amplifier is presented. Data for FET noise at reduced drain currents is also presented. The effects of various circuit components on temperature coefficient of gain are described along with measured temperature coefficient. Finally, the resolution and peak shift versus count rate are presented along with the spectrometer configuration.     

An axisymmetric rectangular element used in the stress analysis and heat conduction problem

The application of an axisymmetric element of rectangular cross section (AXSHER) to stress analysis and the steady-state heat conduction problem is considered. The shape functions are continuous at the inter-element boundaries up to the first derivatives. Several examples are presented to demonstrate the efficiency of the element for both thick and thin axisymmetric structures. The comparison between the results of different axisymmetric elements and the exact results confirms the accuracy and convergence of the element. Economic limitation in data preparation and computing time is achieved by the used formulation.     

Control over spectral characteristics of electromagnetic pulses

A theory was elaborated on transition radiation (TR) from electron bunches that pass through diaphragms of various geometries. At the base of this theory are induced charges and currents in the conductive diaphragms that are generated by electron bunches. An important role of the earth points of diaphragms was proven experimentally and theoretically. A feasibility study was made on controlling the spectra of electromagnetic pulses (EMP) and their directivity patterns. A new configuration for the diaphragms was proposed: twin diaphragms. Calculated and experimentally measured were spectra of such EMPs that were formed by the induced conduction current during the flight of the electron bunch through the twin diaphragms.     

Diffusive transport through a medium of static traps

The diffusion coefficient and equilibrium concentrations of particles in solution and in traps are considered for hopping in and out of traps. The spherical traps are static, randomly distributed, allowed to overlap and of arbitrary size and concentration. The author's recent calculation of the trapping rate is used to relate the observed fraction of time spent in solution to the microscopic dwell time within a trap. The observed diffusion coefficient D_net depends on more than just the relative times spent in traps and in solution, contrary to simple theory. If particle is stuck at the surface until it escapes, D_net is reduced because of correlation; if it is free to wander within the trap before escaping, D_net is substantially enhanced. Simulation data are presented in support of the semi-quantitative physical arguments.