Vacuum Deterioration Research Articles

Thermodynamic analysis of vapor-cooled shield with para-to-ortho hydrogen conversion in composite multilayer insulation structure for liquid hydrogen tank

Reducing the heat leakage during liquid hydrogen storage (LH2) has been a critical issue for the long-distance transportation and large-scale application of hydrogen. A composite thermal insulation structure for LH2 tanks, which integrated the variable density composite multilayer insulation (FVD-MLI) and vapor-cooled shield (VCS) with para-to-ortho hydrogen conversion (P–O) catalysts, is used. Based on the “Layer by Layer” models, para-hydrogen equilibrium fractions with different VCS temperatures and P–O efficiency with different flow rates are combined. The thermal effects of P–O in VCS are considered. Experimental results prove the insulation performance calculation model's accuracy. The heat leakage and temperature profile after filling the P–O catalysts in single VCS and double VCSs are obtained. The optimal insulation positions of single VCS and double VCSs are determined, and the consequence of the vacuum level on the insulation performance is discussed in detail. Increasing the VCS number from single to double and adding P–O catalysts can effectively improve the LH2 tank's insulation performance. Compared with the single VCS, the heat flux through LH2 tanks with double VCSs is reduced by 25.6%. The heat leakages to the tanks can be further reduced by 5.08% and 7.45%, respectively, after adding P–O catalysts in single VCS and double VCSs. Moreover, the cooling effects of P–O shift the optimal position of VCS towards the cold boundary. The single VCS optimal position is moved from 58% of the insulation structure thickness to 50.6%. The optimal positions of double VCSs are moved from 38.9% to 75%–31.1% and 71.7%, respectively. It is found that adding P–O catalysts and increasing the VCS numbers can effectively increase the insulation effect regardless of the vacuum degree between the thermal insulation structure. However, considering the P–O efficiency, the insulation improvement effects after adding P–O catalysts decrease with interlayer vacuum deterioration.

Numerical simulation of outgassing characteristics of microchannel plate glass

Outgassing of internal materials is one of the main factors that cause vacuum deterioration of low-light-level image intensifiers and shorten their lifetime (especially storage lifetime). To perceive the attenuation mechanism of the vacuum degree of image intensifiers from the microscopic view, the outgassing characteristics of lead silicate glass used for the fabrication of microchannel plates (MCPs), which are the core components of image intensifiers, were numerically simulated by giant canonical Monte Carlo and molecular dynamics methods. A glass structure model of the third-generation MCPs was constructed, and the diffusion coefficients and molecular motion trajectories of hydrogen, carbon dioxide, carbon monoxide, water vapor, oxygen, and methane gases, together with their outgassing rates and cumulative outgassing amounts, were calculated based on the condensed-phase optimized molecular potentials for atomistic simulation studies force field. The calculation results show that the rising temperature of MCP glass augments the diffusion coefficients of these gases, which makes the outgassing rates increase in the initial stage but then decrease relatively quickly. Among these six kinds of gas molecules, hydrogen molecules have the largest skip distance and the highest diffusion coefficient because MCP glass can supply more effective diffusion paths for the gas molecules with a relatively small size. When an MCP glass lies in vacuum, first, the cumulative outgassing amounts of various gases from it increase rapidly and then gradually reach stable values, and the cumulative outgassing amount of hydrogen tends to reach a stable value faster than that of other gases due to its highest diffusion coefficient.

Thermal performance analysis of a new structured-core translucent vacuum insulation panel in comparison to vacuum glazing: Experimental and theoretically validated analyses

The notion at which, nowadays, building sector is being recognized to be nearly zero-energy buildings (NZEBs) relies partly on the thermal performance of its fabric insulation. Vacuum glazing (VG) technology attracted the research interest as an option to reduce heat loss through windows. However, the total glazing thermal transmittance (U-value) for VG increases with the use of smaller glazing area due to the edge-seal effects, due to the thermal short-circuit around the edges and the overall construction cost of VG leading to an unaffordable option to deal with energy conservation of buildings. Therefore, this study aims to propose a new structured core transparent vacuum insulation panel (TVIP) to accomplish insulation for the windows without edge sealing effect, with lower cost and can be easily retrofitted to the conventional windows of the existing buildings. To do this, VG and TVIP were constructed and their thermal conductivity were measured using heat flow meter apparatus. In addition, a 3D finite volume model considering the effect of surface to surface radiation, gas conduction, and thermal bridges through the spacer material and sealing material is developed. The model is validated with the experiments in this work and with the data for VG in the literature. The effect of vacuum pressure increase is simulated to mimic the vacuum deterioration problem and the effect of glazing size on the insulation performance of both VG and TVIP were investigated. The results indicate that for a smaller glazing area of less than 30 cm × 30 cm, the TVIP accomplished lower U-value compared with the VG at vacuum pressure of 0.1 Pa and 1 Pa. While, at a vacuum pressure of 10 Pa, the TVIP attained a lower U-value over the entire range of the investigated glazing sizes. Further, the edge-seal effect in the VG is diminished with the use of TVIP. Furthermore, the material cost per unit area of the TVIP is three times less than the cost of VG at laboratory scale. The results of the current study can guide vacuum window designers and researchers to further enhance the performance of TVIP based window to compete for the VG in the markets.

Analysis on the Calculating of Air Leakage Threshold which Caused the Condenser Vacuum Deterioration

There is an air leakage threshold under certain condenser heat load and air ejector output. When the amount of air leakage is less than the threshold, the condenser pressure is essentially the same, we can maintain a stable condenser vacuum, once exceed the threshold, condenser pressure begin to gradually larger, condenser vacuum will deteriorate. Considered the air leakage influences on the heat transfer coefficient of condenser, this paper had been established the one-dimensional steady-state heat transfer model of condenser to obtain the condenser pressure curve along with the amount of air leakage, the turning point on the curve is the air leakage threshold.

Effect of vacuum deterioration on thermal performance of coaxial evacuated tube solar collector considering single and two phase flow modelling: A numerical study

Boiling and condensation play an important role in heat transfer in evacuated tube solar collectors (ETC), especially for low flow rates. Another factor that influences the efficiency of an ETC is deterioration of vacuum inside the ETC. The present study investigates numerically the effect of deteriorating vacuum during single and two phase flows inside an ETC to analyze its thermal performance. A steady-state one-dimensional model is developed which can simulate the steady-state thermal performance of the Evacuated tube solar collector (ETC) with single and two-phase flows under saturation conditions considering effects of vacuum deterioration to simulate a more real-life situation of solar collectors. The rise in pressure inside the ETC has a significant effect on the suppression of boiling and condensation due to rise in overall heat loss coefficient. The radiative, convective and conductive heat loss coefficients from the collector are studied for gradual pressure rise inside the ETC. The two phase model also indicates that if boiling starts inside the ETC in a particular inlet flow condition, consideration of two phase flow along with the effect of vacuum deterioration shows significant deviation of efficiency and temperature profile from those of single phase flow models and predicts more accurate results. Inducing of two-phase heat transfer in ETC, with all the fluid condensing again within the collector tube, will result in improved efficiency for that range of mass flow rates. It also helps in maintaining uniform temperature and equal heat gain throughout the collector absorber plate. The efficiency of the collector for higher inlet temperatures is seen to be effected more significantly by vacuum deterioration than at lower inlet temperatures in single phase flows. However, in two phase flows for a fixed saturation temperature of the fluid, vacuum deterioration has negligible effect on the efficiency at higher inlet temperatures (Tfi ≥ 85 °C).

Prediction of vacuum deterioration caused by vacuum accident in the beamline

Vacuum deterioration at the most upstream of the front-end was predicted quantitatively under the assumption that a rupture of windows and subsequent inrush of air might occur in the SPring-8 beamline. A 10-m-long shock tube was employed to simulate an inrush of air into a region with rarefied gas dynamics. The shock Mach number (Ma) and pressure increase were measured with the fast-closing shutter (FCS) system both activated and de-activated. Actual front-end components as well as plain pipes having various lengths and another pipe with baffle plates inside were inserted into the shock tube by arranging various combinations in tandem. Although it could be observed that an increase in the number of repetitions of the cross-sectional change decreases Ma, the pressure increase essentially depends on the total internal volume of the shock tube. Besides, for the other internal structure, which has a gently tapered cross-sectional change, the pressure increase are suppressed as well as the decrease of Ma. The effective leak rate of the FCS at the inrush of air could be presented as a function of the total internal volume; thus, the number of gas molecules flowing into the most upstream of the front-end could be estimated.

Intense heavy ion beam-induced material evaporation and the resulting dynamic vacuum deterioration of the beam line

Abstract We studied the explosive evaporation of high energy density matter generated by irradiation of intense heavy ion beams and the resulting effect on the dynamic vacuum of the accelerator beamlines. High Energy Density Physics (HEDP) experiments are proposed at the plasma physics terminal at the Facility of Antiproton and Ion Research (FAIR), which is currently under construction. The target response to irradiation by intense beam was simulated by HEIGHTS (High Energy Interaction with General Heterogeneous Target Systems) simulation package. The vacuum deterioration resulting from the target evaporation was simulated with the Molflow+ Monte-Carlo simulator. Experimentally we measured the pressure wave propagation at the HTD beamline at GSI in order to benchmark the simulation. The pressure wave was generated through ohmic heating of a cold surface.

Accelerator mass spectrometer with ion selection in high-voltage terminal

The folded electrostatic tandem accelerator with ion selection in a high-voltage terminal is the basis of accelerator mass spectrometry (AMS) at the BINP. Additional features of the BINP AMS are the target based on magnesium vapors as a stripper without vacuum deterioration and a time-of-flight telescope with thin films for reliable ion identification. The acceleration complex demonstrates reliable operation in a mode of 1 MV with 50 Hz counting rate of 14C+3 radiocarbon for modern samples (14C/12C ~ 1.2 × 10–12). The current state of the AMS has been considered and the experimental results of the radiocarbon concentration measurements in test samples have been presented.

Operational experience with forced cooled superconducting magnets

Force-cooled concept has been chosen for ITER superconducting magnet to get reliable coil insulation using vacuum-pressure impregnation (VPI) technology. However 17 breakdowns occurred during operation of six magnets of this type or their single coil tests at operating voltage<3kV, while ITER needs 12kV. All the breakdowns started on electric, cryogenic and diagnostic communications (ECDCs) by the high voltage induced at fast current variations in magnets concurrently with vacuum deterioration, but never on the coils, though sometimes the latter were damaged too. It suggests that simple wrap insulation currently employed on ECDCs and planned to be used in ITER is unacceptable. Upgrade of the ECDC insulation to the same level as on the coils is evidently needed. This could be done by covering each one from ECDCs with vacuum-tight grounded stainless steel casings filled up with solid insulator using VPI-technology. Such an insulation will be insensitive to in-cryostat conditions, excluding helium leaks and considerably simplifying the tests thus allowing saving time and cost. However it is not accepted in ITER design yet. So guarantee of breakdown prevention is not available.

Quench in High-Temperature Superconducting Motor Field Coils: Computer Simulations and Comparison With Experiments

For high temperature superconducting (HTS) motor field coils two types of quench initiation are of practical importance. The first type will be referred to as current induced quench (CIQ). It typically occurs during coil or assembled rotor testing, when the current is increased in small increments until the quench is detected. The current at which the quench starts is referred to as the quench current Iq [1]-[7]. The objective of these experiments is to establish actual limits of operational conditions. The second type will be referred to as temperature induced quench (TIQ) and it is caused by failure or malfunctioning of the cooling system. The examples include vacuum deterioration, cryocooler failure etc. Numerous CIQ experiments [1]-[8], [10] have showed a certain pattern of quench event, including a well defined Iq, slow near-linear increase with respect to time of coil voltage and temperature with subsequent transition to fast non-linear voltage and temperature growth. TIQ experiments [10] showed the same behavior when coil cooling system was shut down while the current was maintained constant. The presented here work is a computational 2-D and 3-D Finite Element Analysis (FEA) study of CIQ and TIQ in HTS coils. The effects of magnetic field, non-homogeneity of tape properties, current sharing and amount of stabilizer are studied. It is shown that the formation and propagation of the normal zone is preceded by the process that we chose to call pre-quench instability. The pre-quench instability starts when the current exceeds Iq and demonstrates experimentally observed features mentioned above. It is shown that with an insufficient amount of stabilizer for second generation (2-G) HTS tapes the pre-quench instability may be practically undetectable. The comparison with some of the experiments [8], [10] is presented. We intensively used the concepts developed in articles [1]-[6]. Many (but not all) our conclusions are in agreement with [1]-[6].

Development of remote control and monitoring system for the FE gun in HVEM.

This paper describes a control and monitoring system for the field emission (FE) gun in the high-voltage electron microscope (HVEM), where the whole FE gun system is located inside a pressure vessel and is floated on a high voltage of 1 MV. The operating condition of the FE gun system is controlled and monitored mainly by a bi-directional communication system between ground and high potential through optical fibres. A mechanical driving system by combination of insulator rods and stepping motors is partially used as a control system for FE gun operation. The monitoring system developed here also functions as a safety circuit that detects abnormal situations, such as an abnormal deterioration of vacuum, and protects the FE tip against fatal damage. The fundamental performance of the microscope has been tested for transmission electron microscope (TEM) and scanning TEM operation modes.

Electron diode with a large area ferroelectric plasma cathode

The operation of a planar diode with a large area (64 cm2) ferroelectric plasma cathode based on BaTiO3 ceramics is studied. Diode parameters and parameters of the generated electron beam are investigated under accelerating voltage of 200–350 kV and pulse duration of 250 ns. Parameters of the plasma, which forms on the surface of the ferroelectric cathode due to incomplete discharge, are reported as well. It is shown that the application of this cathode allows one to produce an electron beam simultaneously with application of the accelerating pulse. The generation of an electron beam with current density up to 20 A/cm2 and divergence of less than 1° is demonstrated. It is shown that diode operation is governed by parameters of the cathode plasma which fills the anode–cathode gap prior to application of the accelerating voltage pulse. By proper adjustment of the cathode and diode parameters the operation of the diode with almost constant impedance is achieved. Measurements of the potential distribution in the diode confirm that the diode operates in a plasma-prefilled mode. The cathode is found to be capable of operating for up to 105 pulses at a repetition rate of few Hz without any maintenance and with relatively small vacuum deterioration.

Emission properties of different cathodes at E⩽105 V/cm

We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedance indicated the existence of a quasistationary cathode plasma boundary for electron current density ⩽20 A/cm2. We present the dependencies of the average emitted electron current density and of the time delay in the electron emission on the number of generator shots. We also present data of the vacuum deterioration as a result of the tested cathodes operation. The obtained data are discussed within the framework of plasma formation as a result of cathode surface flashover.

Operational evaluation of ultrahigh-vacuum protection systems for intense-photon-flux wiggler beamlines.

The ultrahigh-vacuum (UHV) protection system at the 2.5 GeV synchrotron radiation source (positron storage ring) at the Photon Factory allows the intense-photon-flux wiggler beamlines to operate safely during synchrotron radiation experiments for a long time. There are six high-power wiggler/undulator beamlines that provide intense photon-flux beams to the experimental hall. In the case of a possible instantaneous vacuum failure at the experimental hall, the intense photon-flux radiation from the wiggler could cause a meltdown of the titanium-alloy fast-closing valve. The authors have developed a UHV protection system. Upon a vacuum failure, the protection system can dump the positron beam by turning off the RF power in the four RF klystrons (150 kW maximum), and then initiate a blade closure of the fast-closing valve. In this paper, the operational performance of the vacuum protection system for the intense-photon-flux wiggler beamlines is evaluated and discussed regarding the actual vacuum deterioration downstream of a beamline at the experimental hall.

Evacuation and deterioration of all-glass tubular solar thermal collectors

Two evacuation and degassing processes for all-glass tubular evacuated solar collectors are described, the total process times being ≈ 50 min and ≈ 10 min. Collectors incorporating a sputtered graded metal—carbon on copper selective surface have been evacuated usther process, then aged using various procedures, at temperatures of 300 to 375°C for up to 5000 h. Deterioration of vacuum in each collector was studied by measuring thermal conduction between the absorber tube and the collector envelope. A significant proportion of the small amounts of gas detected is probably due to permeation of atmospheric helium through the borosilicate glass absorber tubes of the collectors. No deterioration of the properties of the selective surface was observed after ageing.

A Study of Vacuum Requirements for the Acceleration of Heavy Ions in an MP Tandem Van De Graaff

Accelerator vacuum systems, which are adequate for the acceleration of intense beams of light elements, may not be suitable for heavy ions which have large charge changing cross sections. Charge charging collisions with residual gases in the accelerator tubes produce a continuous distribution of ion energies as well as the desired discrete energy output of the accelerator. An MP tandem with a carbon foil terminal stripper was used to accelerate Iodine ions. The attenuation of the unique energy beam components due to unwanted charge changing collisions was measured by energy and magnetic analysis of the accelerator output. The effects of terminal pumping, vacuum deterioration due to added gas, and reduction of the accelerator tube conductance were studied with particular attention to high charge states. Under typical conditions with a 6 MV terminal and pressure ~ 4 × 10-6 Torr in the terminal region, the total beam attenuation was ~20%.

Fast Closing Vacuum Valve of Large Aperture

A guillotine valve is described that is designed to seal off a vacuum system within 12 msec of the beginning of vacuum deterioration. Deterioration of the vacuum is indicated within a few mu sec by a vacuum gage, the remaining time is taken up in the motion of the guillotine slide, which moves 50 mm. The proximate cause of the vacuum loss is, in the case for which the valve is designed, the rupture of a thin Al foil window that separates the vacuum system from a region at atmospheric pressure. (T.F.H.)