Comparison of secondary electron emission in helium ion microscope with gallium ion and electron microscopes

To evaluate secondary electron (SE) image characteristics in helium ion microscope, Si surfaces with a rod and step structures is scanned by 30 keV He and Ga ion beams and 1 keV electron beam. The topographic sensitivity of He ions is in principle higher than that for scanning electron microscope (SEM) because of the stronger dependency of SE yield versus incident angle for He ions. As shrinking to sub nm patterns, the pseudo-images constructed from line profile of SE intensity by the electron beam lose their sharpness, however, the images for the He and Ga ion beams keep clearness due to darkening the bottom corners of the pattern. Here, the sputter erosion for Ga ions must be considered. Furthermore, trajectories of emitted SEs are simulated for a rectangular Al surface scanned by the beams to study voltage contrast, where positive and negative voltages are applied to the small area of the sample. Both less high energy component in the energy distribution of SEs and dominant contribution of direct SE excitation by a projectile He ion keep a high voltage contrast down to a sub nm sized area positively biased against the zero-potential surroundings.     

Two-phase flow instability in channels with sinusoidal heat supply

This paper deals with the problem of Ledinegg instability for two-phase flow in systems reproducing the operative conditions of the channels between the fuel rods of a boiling nuclear reactor. Particular attention is devoted to the influence of the heat flux distribution. In addition, a numerical method is applied to predict the occurrence of two-phase flow instability.     

Laminar flow instability in multiple channels

Temperature-viscosity-induced laminar flow instability (LFI) in two gaseous heated parallel channels with interchannel heat exchange is purely excursive, rather than oscillatory. Constant total flow always leads to a stable system, while constant pressure drop could have an instability, depending on the sign of (∂ΔP/∂W)_Q. The system was studied numerically with negative heat perturbations yielding bounded excursions, and positive heat perturbations giving unbounded excursions asymptotically approaching zero. A nine-channel system was probed, giving excursive behavior with the ultimate growth rate the same for single and multichannel systems.     

Review of two-phase flow instability

Boiling flow in a water-cooled reactor, an evaporator, or an electronic cooling system is susceptible to thermal-hydrodynamic instabilities, which may cause flow oscillations of constant amplitude or diverging amplitude. These oscillations could induce boiling crisis, disturb control systems, or cause mechanical damage. This paper identifies the causes and mechanisms of these instabilities. Based on their mechanisms, various types of instabilities are classified and tabulated. The parametric effects on flow instability, observed experimentally, are systematically presented. Various analytical techniques for predicting the instability threshold are reviewed in terms of their applicability and accuracy.     

强迫循环下蒸汽发生器流动不稳定性研究

由于两相流不稳定性在实际应用中的重要性,许多学者对其本质进行了大量的研究.本文建立数学模型对强迫循环下蒸汽发生器的流动不稳定性进行了计算.此模型采用均相流假设并认为相间热力学平衡.模型考虑了管壁的蓄热.此模型被用于系统压力、质量流速、进口过冷度、进、出口节流和间隙、不同内、外径之比和内、外管的加热比对不稳定性影响的研究.     

Flow excursion instability in downward flow systems part II: two-phase instability

A mechanistic model is presented in this paper to predict the onset of flow excursion in downward flows at low-pressure conditions. The model is represented in a graphical form on the subcooling number versus the Zuber (phase-change) number plane. The subcooling number and the Zuber number are measures of integral system parameter such as fluid properties, inlet subcooling, heat flux, channel geometry and coolant flow rate (all measurable quantities) resulting in an easy extrapolation of the predictive correlation to the physical system. The model addresses the distinction between the point of significant void and the onset of flow excursion and provides an analytical method for its evaluation. The model is compared with flow excursion data for downward flows and the agreement is satisfactory.     

THz plasma wave instability in field effect transistor with electron diffusion current density

The plasma wave instability in rectangle field effect transistors (FETs) is studied with electron diffusion current density by quantum hydrodynamic model in this paper. General dispersion relation including effects of electrical thermal motion, external friction associated with electron scattering effect, electron exchange-correlation contributions and quantum effects were obtained for rectangle FETs. The electron diffusion current density term is considered for further analysis in this paper. It is found that the quantum effects, the electron diffusion current density and electrical thermal motion enhance the radiation power and frequencies. But the electron exchange- correlation effects and the electron scattering effects reduce the radiation power and frequencies. Results showed that a transistor has advantages for the realization of practical terahertz sources.     

Flow excursion instability in downward flow systems: Part I. Single-phase instability

A procedure for predicting the onset of flow excursion instability in downward flows at low-pressure and low-flow conditions without boiling is presented. It is generally accepted that the onset of significant void in subcooled boiling precedes, and is a precondition to, the occurrence of static flow instability. A detailed analysis of the pressure drop components for a downward flow in a heated channel reveals the possibility of unstable transition from single-phase flow to high-quality two-phase flow, i.e. flow excursion. Low flow rate and high subcooling are the two important conditions for the occurrence of this type of instability. The unstable transition occurs when the resistance to the downward flow caused by local (orifice), frictional, and thermal expansion pressure drops equalizes the driving force of the gravitational pressure drop. The inclusion of the thermal expansion pressure drop is essential to account for this type of transition. Experimental data have still to be produced to verify the prediction of the present analysis.     

The present status of the Shanghai electron beam ion trap

1Construction of the device EBIT(electron beam ion trap)is an ideal device to produce,trap,and study highly charged ions.Basi-cally it can provide any charge states of any element in the periodic table.After the first development at the Lawrence Livermore National Laboratory in1986,[1,2]EBITs have been constructed in Oxford,[3]NIST,[4]Tokyo,[5]Berlin[6]and Freiburg[7](now moved to Heidelberg).Shanghai EBIT project was launched at the beginning of the year of2002.[8,9]The conceptual d…     

Two-phase flow instability in a parallel multichannel system

The two-phase flow instabilities observed in through parallel multichannel can be classified into three types, of which only one is intrinsic to parallel multichannel systems. The intrinsic instabilities observed in parallel multichannel system have been studied experimentally. The stable boundary of the flow in such a parallel-channel system are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results show that parallel multichannel system possess a characteristic oscillation that is quite independent of the magnitude and duration of the initial disturbance, and the stable boundary is influenced by the characteristic frequency of the system as well as by the exit quality when this is low, and upon raising the exit quality and reducing the characteristic frequency, the system increases its instability, and entrance throttling effectively contributes to stabilization of the system.     

Prediction of the onset of flow instability in transient subcooled flow boiling

A new model to predict the onset of flow instability (OFI) in transient subcooled flow boiling has been developed. The model is based upon the influence on vapor bubble departure of the single-phase temperature profile. The steady-state result of the present model was compared to the experimental data of Whittle and Forgan [1] and Dougherty et al. [2], showing an excellent agreement. The model was then employed in a transient analysis of OFI for vertical downwards turbulent flow to predict whether OFI takes place. The condition for OFI to occur in transient flow situations was also predicted by this model. Two modes for pressure gradient change inside the channel are considered in the present study: step change and ramp change. The calculations were made for various combinations of the flow operating condition and the mode of pressure drop change.     

The relaxation of strong electron excitations in solids induced by multicharged ion bombardment

The relaxation of electron excitations arising from irradiation by multicharged ions of metals and insulators is considered. The excitation processes in solids are described. In metals, the relaxation is determined by the electron thermal conductivity, and electron energy transfer to the lattice is small. In insulators, the electron excitation relaxation is dependent on the low temperature ionization wave, the electron heat is localized, and lattice atoms receive enough energy for melting. The electron-phonon interaction at high electron temperatures and lattice relaxation is considered.     

Experimental research on flow instability in vertical narrow annuli

A narrow annular test section of 1.5mm gap and 1800mm length was designed and manufactured, with good tightness and insulation. Experiments were carried out to investigate characteristics of flow instability of forced-convection in vertical narrow annuli. Using distilled water as work fluid, the experiments were conducted at pressures of 1.0～3.0 MPa, mass flow rates of 3.0～25 kg/h, heating power of 3.0～ 6.5kW and inlet fluid temperature of 20 ℃, 40 ℃ or 60℃. It was found that flow instability occured with fixed inlet condition and heating power when mass flow rate was below a special value. Effects of inlet subcooling, system pressure and mass flow rate on the system behavior were studied and the instability region was given.     

Theoretical research on two-phase flow instability in parallel channels

Two-phase flow instability of two-channel system has been theoretically studied in the present study. Based on the homogeneous flow model, the parallel channels model and system control equations are established by using the control volume integrating method. Gear method is used to solve the system control equations. The marginal stability boundary (MSB) at different system pressure conditions is obtained. The typical MSB shape is usually a classical inclination “L” at some operation condition (i.e. the system pressure is low and the inlet resistance coefficient is small). The three-dimensional instability spaces (or instability reefs) with different inlet resistance coefficients are obtained. The three coordinates consists of phase change number (N_pch), subcooling number (N_sub) and nondimensional pressure (P⁺). The lower part of the instability space is larger than the upper one. Increasing the system pressure or inlet resistance coefficient can strengthen the system stability. However, increasing the heating power destabilize the system stability. The influence of inlet subcooling on the system stability is multi-valued.