Helium Vapor Research Articles

Direct Current Plasma Electron Source for Electric Propulsion Applications Using Atomic and Molecular Propellants

The design and performance of a novel direct current (dc) neutralizer for electric propulsion applications are presented. The neutralizer exploits an $\text {E}\times \text {B}$ discharge to enhance ionization via electron-neutral collisions. Tests are performed with helium, argon, xenon, air, and water vapor as working gases. The $I$ – $V$ characteristics and extraction parameters are measured for both atomic and molecular gases. The maximum partial power efficiency is 4.2 mA/W in argon, 2.7 mA/W in air, and 2 mA/W in water vapor. The typical utilization factor is below 1 and the power consumption is less than 120 W. A semiempirical model is derived to predict the performance of dc plasma cathodes using atomic gas. A comparison with existing plasma cathodes and conventional LaB6 cathodes is presented, and design optimizations aimed at improving the performance are proposed.

Hyperfine structure constants of atomic bromine (Br I)

The absorption spectrum of the neutral bromine (Br I), lying in the region from 11,300cm−1 to 12,600cm−1 has been investigated using concentration modulation absorption spectroscopy with a tunable Ti:Sapphire laser. The bromine atoms were excited by discharging the mixture of Helium and bromine vapour in a hollow discharge glass tube. The hyperfine structure spectra of 45 lines were analyzed. Hyperfine structure constants A and B were derived for 20 even and 27 odd levels amongst which constants for 12 even and 21 odd levels were newly reported.

Heat transfer resistances in the measurements of cold helium vapour temperature in a subatmospheric process line

The superfluid helium technology, which is essentially used in particle accelerators, requires complex cryogenic systems that include long lines transferring cold helium vapours at a subathmospheric pressure below 50 mbar. Usually in large systems the subatmospheric pressure is generated by a set of warm and cold compressors. In consequence, the heat loads to the line and especially the helium temperature in the inlet to the cold compressors are crucial parameters. In order to measure the helium temperature the temperature sensors are usually fixed to the external surface of the process lines. However, this technique can lead to unwanted measurement errors and affect the temperature measurement dynamics mainly due to low thermal conductivity of the pipe wall material, large pipe diameters and low helium density. Assembling a temperature sensor in a well (cold finger) reaching the centerline of the flowing helium is a technique that can improve the measurement quality and dynamics (response time). The paper presents the numerical simulations of heat transfers occurring in the both measurement techniques and discusses the impacts of the heat transfer resistances on the temperature measurement dynamics.

Numerical analysis of temperature stratification in a subatmospheric cold helium line

In last decades the technology of superfluid helium has become very advantageous for large-scale scientific facilities dedicated to high-energy physics. Superconducting cavities or magnets are usually immersed in helium baths at a temperature of 1.8 to 2.0 K. This temperature is produced by a JT cycle where the return line works at a subatmospheric pressure of 16 to 31 mbar. The subatmospheric line lengths can reach even several thousand meters. The helium flows in the subatmospheric lines are driven by cold compressors or vacuum pumps. Due to a limited pressure drop requirement the line diameters can exceed even 300 mm. Since the thermal conductivity of the line material is very small and the flow rate at some operation conditions can be much lower than at the normal operating conditions, a thermal stratification in the helium flow can appear together with a significant temperature gradient along the pipe circumference. These phenomena can affect the thermo-hydraulic behaviour of the line as well as the operation of the entire cryogenic system.The paper presents the numerical simulations of cold helium vapour flows in a long straight line. The stratification phenomenon is discussed and the potential temperature gradient in the pipe wall is evaluated.

A 1.8K refrigeration cryostat with 100 hours continuous cooling

A refrigeration cryostat has been developed to produce continuous cooling to a sample below 1.8 K over 100 hours by using a cryocooler. A two-stage 4K G-M cryocooler is used to liquefy helium gas from evacuated vapor and cylinder helium bottle which can be replaced during the cooling process. The liquid helium transfer into superfluid helium in a Joule-Thomson valve in connection with a 1000 m3/h pumping unit. The pressure of evacuated helium vapor is controlled by air bag and valves. A copper decompression chamber, which is designed as a cooling station to control the superfluid helium, is used to cool the sample attached on it uniformly. The sample connects to the copper chamber in cryostat with screw thread. The cryostat can reach the temperature of 1.7 K without load and the continuous working time is more than 100 hours.

Design, fabrication and test of superconducting magnet for 1.5 T dedicated extremity magnetic resonance imaging system

Magnetic resonance imaging (MRI) has been a primary diagnostic technique due to its high imaging quality, noninvasion and non-radiation capacity. However, the application of conventional whole body MRI is restricted by its massive size, high installation and management cost. Dedicated MRI overcomes the shortcomings of whole body MRI and has great importance in medical diagnosis. The challenge is that the design of superconducting magnet for extremity MRI is largely constrained by physiological structure of human body. As a result, a limited longitudinal length with high field homogeneity in a 160 mm diameter sphere volume (DSV) is required for superconducting magnet of extremity MRI. In this article, a non-shielded 1.5 T extremity dedicated superconducting magnet is designed by using both 0-1 integer programming and genetic algorithm and fabricated with a comprehensive consideration of superconductivity wire consumption, central magnetic field intensity and imaging region homogeneity. The NbTi superconducting wire is chosen for coil winding, and copper-to-superconducting ratio of the wire is 1.3. The sizes of cross-section of the bare wire and the insulated wire are 0.75 mm×1.20 mm and 0.83 mm×1.28 mm respectively, and the critical currents at 4.2 K and 5 T are both about 935 A.According to the size constraint of the magnet, we first calculate the current carrying zone of the superconducting coils and divide it into grid elements with parallel current. The size of each grid element is equal to that of the superconducting wire, and the distribution of non-rectangular coils is obtained by using 0-1 integer programming. In order to obtain a higher homogeneity of magnetic field, a reverse current zone is manually created in the wide blank area of the feasible current carrying zone. Using the results above, we then optimize the distribution of coils and build a rectangular model which facilitates the fabrication by using genetic algorithm. The inductance of the magnet is 1.8094 H, the operating current is 402.09 A, the stored energy is 146.27 kJ and the peak magnetic field of current carrying zone is 5.48 T. The calculated peak-to-peak homogeneity in 160 mm DSV is about 22 ppm. Taking into consideration the factors such as mechanical error and cold shrinkage, the estimated homogeneity would reach 60 ppm (peak-to-peak) with passive shimming.The 1.5 T extremity dedicated superconducting magnet is successfully fabricated through a series of processes such as winding, curing, assembly and welding. The prototype magnet has a room temperature bore of 280 mm in diameter and a total length of 520 mm, and the volume of liquid helium vessel is about 30 liters. To reduce the evaporation of liquid helium, a 1.5 Watt two-stage Gifford-McMahon refrigerator is employed to cool the system and maintain the evaporation rate of Helium at zero level. The range of 5 Gauss line of the magnet is 3.2 m in the radial direction and 2.6 m in the axial direction. Moreover, the magnet is magnetized to 1.5 T after being conditioned three times and the measured homogeneity in 160 mm DSV achieves 55 ppm (peak-to-peak) and 3.4 ppm (Vrms) after passive shimming using silicon steel pieces.

Horst Meyer and Quantum Evaporation

With their 1963 article in Cryogenics Horst Meyer and his collaborators triggered intense research activity on the evaporation of superfluid helium. Discussing this subject with him in 1975 was enlightening. Fifty years later, the analogy between the photoelectric effect and the evaporation of superfluid helium in the low temperature limit is not yet clear, although remarkable progress has been made in its observation and its understanding. This special issue of the Journal of Low Temperature Physics is an opportunity to recall the history of quantum evaporation, and to express my gratitude to Horst Meyer. It describes quickly most of the experimental and theoretical works which have been published on quantum evaporation during the last 50 years, but it is not a comprehensive review of this fascinating subject.

Development of a compact HTS lead unit for the SC correction coils of the SuperKEKB final focusing magnet system

Forty-three superconducting (SC) correction coils with maximum currents of about 60A are installed in the SuperKEKB final focusing magnet system. Current leads to energize the SC correction coils should have an affordable heat load and fit the spatial constraints in the service cryostat where the current leads are installed. To address the requirements, design optimization of individual lead was performed with vapor cooled current lead made of a brass material, and a compact unit was designed to accommodate eight current leads together in order to be installed with one port in the service cryostat. The 2nd generation high temperature SC (HTS) tape was adopted and soldered at the cold end of the brass current lead to form a hybrid HTS lead structure. A prototype of the compact lead unit with HTS tape was constructed and tested with liquid helium (LHe) environment. This paper presents a cryogenic measurement system to simulate the real operation conditions in the service cryostat, and analysis of the experimental results. The measured results showed excellent agreement with the theoretical analysis and numerical simulation. In total, 11 sets of the compact HTS lead units were constructed for the 43 SC correction coils at KEK. One set from the mass production was tested in cryogenic conditions, and exhibited the same performance as the prototype. The compact HTS lead unit can feed currents to four SC correction coils simultaneously with the simple requirement of controlling and monitoring helium vapor flow, and has a heat load of about 0.762L/h in terms of LHe consumption.

Conceptual design of the superconducting magnet for the 250MeV proton cyclotron.

BackgroundThe superconducting cyclotron is of great importance to treat cancer parts of the body. To reduce the operation costs, a superconducting magnet system for the 250 MeV proton cyclotron was designed to confirm the feasibility of the superconducting cyclotron.FindingsThe superconducting magnet system consists of a pair of split coils, the cryostat and a pair of binary high temperature superconductor current leads. The superconducting magnet can reach a central magnetic field of about 1.155 T at 160 A. The three GM cryocooler with cooling capacities of 1.5 W at 4.5 K and 35 W at 50 K and one GM cryocooler of 100 W at 50 K were adopted to cool the superconducting magnet system through the thermosiphon technology.ConclusionThe four GM cryocoolers were used to cool the superconducting magnet to realize zero evaporation of the liquid helium.

Compact, ultra-low vibration, closed-cycle helium recycler for uninterrupted operation of MEG with SQUID magnetometers

A closed-cycle helium recycler was developed for continuous uninterrupted operation for magnetometer-based whole-head magnetoencephalography (MEG) systems. The recycler consists of a two stage 4K pulse-tube cryocooler and is mounted on the roof of a magnetically shielded room (MSR). A flexible liquid helium (LHe) return line on the recycler is inserted into the fill port of the MEG system in the MSR through a slotted opening in the ceiling. The helium vapor is captured through a line that returns the gas to the top of the recycler assembly. A high-purity helium gas cylinder connected to the recycler assembly supplies the gas, which, after it is liquefied, increases the level of LHe in the MEG system during the start-up phase. No storage tank for evaporated helium gas nor a helium gas purifier is used. The recycler is capable of liquefying helium with a rate of ∼17L/d after precooling the MEG system. It has provided a fully maintenance-free operation under computer control for 7months without refill of helium. Although the recycler is used for single-orientation operation at this initial testing site, it is designed to operate at ±20° orientations, allowing the MEG system to be tilted for supine and reclining positions. Vibration of the recycler is dampened to an ultra-low level by using several vibration isolation methods, which enables uninterrupted operation during MEG measurements. Recyclers similar to this system may be quite useful even for MEG systems with 100% magnetometers.

Numerical analysis of H2/He gas separation experiments performed with a MFI-type tubular zeolite membrane

Zeolite membranes were identified to be used in the future fusion reactors to separate traces of tritiated species from large helium flows. Due to the lack of data in the literature, separation experiments with helium, hydrogen and water vapor have been performed in the Tritium Laboratory Karlsruhe (Germany). The obtained results shall be used to predict performances at reactor scale. Therefore, a simulation tool of a permeator has been developed to perform calculations ab-initio of its performance at several experimental conditions. In the present work, numerical results for the binary mixture hydrogen/helium at different concentrations are compared with the experimental results obtained with a zeolite MFI-ZSM5 tubular membrane, where separation factors below 2 were measured for different cuts. Very good agreement between experiments and simulations was observed. This agreement gives confidence to integrate this numerical tool into a membrane cascade simulator, in development in the TLK, toward the modeling of the zeolite membranes’ performances at reactor scale.

Super-Maxwellian helium evaporation from pure and salty water

Helium atoms evaporate from pure water and salty solutions in super-Maxwellian speed distributions, as observed experimentally and modeled theoretically. The experiments are performed by monitoring the velocities of dissolved He atoms that evaporate from microjets of pure water at 252 K and 4-8.5 molal LiCl and LiBr at 232-252 K. The average He atom energies exceed the flux-weighted Maxwell-Boltzmann average of 2RT by 30% for pure water and 70% for 8.5m LiBr. Classical molecular dynamics simulations closely reproduce the observed speed distributions and provide microscopic insight into the forces that eject the He atoms from solution. Comparisons of the density profile and He kinetic energies across the water-vacuum interface indicate that the He atoms are accelerated by He-water collisions within the top 1-2 layers of the liquid. We also find that the average He atom kinetic energy scales with the free energy of solvation of this sparingly soluble gas. This free-energy difference reflects the steeply decreasing potential of mean force on the He atoms in the interfacial region, whose gradient is the repulsive force that tends to expel the atoms. The accompanying sharp decrease in water density suppresses the He-water collisions that would otherwise maintain a Maxwell-Boltzmann distribution, allowing the He atom to escape at high energies. Helium is especially affected by this reduction in collisions because its weak interactions make energy transfer inefficient.

Superfluid Helium Cryostat for Sensitive Elements Cooling

The objective of the research is super fluid helium cryostat development for navigational sensitive elements (SE) cooling at the temperature of 2K. The peculiarities of the given cryostat are the long-term independent operating mode, the high stability of the SE unit and the high accuracy and stability of super fluid helium temperature maintenance. The SE unit high stability is achieved by the construction. The long-term independent operating mode is carried out by means of helium vapors liquefier-recondensers. High accuracy and stability of superfluid helium temperature maintenance is realized with the help of the modern and advanced thermometry and proportional-integral-differential control.

Time-dependent phase shift of a retrieved pulse in off-resonant electromagnetically-induced-transparency–based light storage

We report measurements of the time-dependent phases of the leak and retrieved pulses obtained in EIT storage experiments with metastable helium vapor at room temperature. In particular, we investigate the influence of the optical detuning at two-photon resonance, and provide numerical simulations of the full dynamical Maxwell-Bloch equations, which allow us to account for the experimental results.

The thermal design, characterization, and performance of the Spider long-duration balloon cryostat

We describe the Spider flight cryostat, which is designed to cool six millimeter-wavelength telescopes during an Antarctic long-duration balloon flight. The cryostat, one of the largest to have flown on a stratospheric payload, uses liquid 4He to deliver cooling power to stages at 4.2 and 1.6K. Stainless steel capillaries facilitate a high flow impedance connection between the main liquid helium tank and a smaller superfluid tank, allowing the latter to operate at 1.6K as long as there is liquid in the 4.2K main tank. Each telescope houses a closed cycle 3He adsorption refrigerator that further cools the focal planes down to 300mK. Liquid helium vapor from the main tank is routed through heat exchangers that cool radiation shields, providing negative thermal feedback. The system performed successfully during a 17day flight in the 2014–2015 Antarctic summer. The cryostat had a total hold time of 16.8days, with 15.9days occurring during flight.

Cost Optimization in Designing Modern Cryogenic Complexes of Large Superconductor Systems

An integrated method for enhancing energy efficiency of modern large-scale cryogenic complexes (annual energy consumption more than 100 GWh) that allow operation of superconducting magnets of particle accelerators is analyzed. Two conventional loops, namely, “cold” and “warm” (compressor), with subsequent separate optimization of each loop, are marked out in the cryogenic complex cycle. Separate optimization helps simplify the choice of optimal cycle pressure as well as the choice of the most efficient helium vapor evacuation for providing the required cryogenic temperature of 2 K.

Hyperfine structure near infrared spectrum of atomic iodine

The hyperfine structure of spectral lines of neutral iodine (II) in the wavenumber region from 11300cm−1 to 13000cm−1 was studied using optical heterodyne concentration modulation absorption spectroscopy with a tunable single-mode cw Ti:Sapphire laser. The iodine atoms were produced and excited by a discharge operated in a mixture of helium and iodine vapor. A total of 89 lines were observed and the hyperfine structure of 45 of these lines was studied. For 40 lines, the hyperfine structure was not previously known. The magnetic dipole and the electric quadrupole hyperfine structure constants of 18 even and 28 odd levels were extracted. The constants of one even level and of 16 odd levels are presented for the first time.

Zeolite Membrane Cascade for Tritium Extraction and Recovery Systems

Membrane separation by zeolite membranes has been proposed as a pre-concentration stage for the tritium extraction from the purge helium of the breeding blanket combined with a final recovery by the catalytic membrane reactor PERMCAT. This fully continuous operation improves the tritium management in fusion machines, minimizing the tritium inventory. In this work permeation measurements for mixtures of hydrogen, helium and water vapor at different compositions are presented. In parallel the first results of a simulation work comprising a model for multistage separation, allowing scaling up towards DEMO application, are discussed.

Probing Gas–Liquid Interfacial Dynamics by Helium Evaporation from Hydrocarbon Liquids and Jet Fuels

We have monitored the speeds of evaporating helium atoms dissolved in liquid octane, isooctane, 1-methylnaphthalene, dodecane, squalane, ethylene glycol, and two jet fuels. In all cases, the average kinetic energies of the evaporating He atoms exceed the Maxwellian value of 2RT. The energies roughly track solvent surface tensions; this correlation may reflect the tighter packing and attractions of interfacial solvent molecules that restrict the gaps through which He atoms escape. Mixtures of dodecane, squalane, and 1-methylnaphthalene generate He evaporation energies that lie between the pure liquid values. We find, however, that He atoms evaporate from pure 1-methylnaphthalene with kinetic energies lower than expected based on its high surface tension, perhaps because the sideways packing of the aromatic rings provides more direct channels for the escaping He atoms. Additionally, He evaporates from two complex fuel mixtures, Jet A and JP-8, with nearly identical energies, implying that the extra additive...

First Operational Results with the SST-1 Superconducting Magnet & its Cryogenics

The Steady State Superconducting Tokamak (SST-1) at the Institute for Plasma Research was commissioned in 2013 with the successful experimental validations of its magnets and cryogenic systems. Subsequently, the first plasma in SST-1 has been obtained on June 20, 2013. Since then, the cryo-magnetic systems of SST-1 have been operating reliably, in successive plasma campaigns. Unlike other contemporary cable-in-conduit-conductor (CICC) based superconducting Tokamak magnets, SST-1 CICC in Toroidal Field (TF) winding packs are cooled with two phase helium from a dedicated 1.3kW helium refrigerator liquefier plant. The TF magnets in SST-1 together with other magnets have been supporting creditable plasma operational scenarios since then, with the TF magnets being charged up to a maximum of 2.0 T on the plasma major radius so far. The vapour cooled current leads of SST-1 are also uniquely operated with cooled helium vapour all along, instead of liquid helium being stored in the lower superconducting sections. The operational experiences with SST-1 TF magnets, PF magnets, and the supporting 5K and 80K cryogenic systems contributing towards successful operations of SST-1 plasma experiments are elaborated in this paper.