Theoretical and experimental investigation of magnetic field related helium leak in helium vessel of a large superconducting magnet

The helium vessel of the superconducting cyclotron (SCC) at the Variable Energy Cyclotron centre (VECC), Kolkata shows a gradual loss of insulation vacuum from 10⁻⁷ mbar to 10⁻⁴ mbar with increasing coil current in the magnet. The insulation vacuum restores back to its initial value with the withdrawal of current. The origin of such behavior has been thought to be related to the electromagnetic stress in the magnet. The electromagnetic stress distribution in the median plane of the helium vessel was studied to figure out the possible location of the helium leak. The stress field from the possible location was transferred to a simplified 2D model with different leak geometries to study the changes in conductance with coil current. The leak rate calculated from the changes in the leak geometry was compared with the leak rate calculated from the experimental insulation vacuum degradation behavior to estimate the initial leak shape and size.     

Calibration of ultrahigh vacuum gauge from 10 Pa to 10 Pa by the two-stage flow-dividing system

A new two-stage flow-dividing system has been developed for the calibration of ultrahigh vacuum gauges from 10⁻⁹ Pa to 10⁻⁵ Pa for N₂, Ar, and H₂. This system is designed based on the techniques for our previously developed calibration system in the range from 10⁻⁷ Pa to 10⁻² Pa. Three modifications were performed to extend the calibration pressure to a lower range. The relative standard uncertainty of the generated pressure (k = 1) is in the range from 2.3% to 2.6%, from 10⁻⁹ Pa to 10⁻⁵ Pa. The characteristics of ultrahigh vacuum gauges were also examined by using this system. The stabilities of the pressure reading, the linearity, the temperature dependence, and the long-term stability were examined. These results show that the calibration of ultrahigh vacuum gauges is possible in the range from 10⁻⁹ Pa to 10⁻⁵ Pa for N₂, Ar, and H₂ with the uncertainty of about 6.0% (k = 2) by this new system.     

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10⁻⁴ to 10⁻¹ Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.     

Field-emission electron source for vacuum micropump

In the work a conception of a miniature, orbitron ion vacuum micropump for an integration with vacuum MEMS devices is presented. It is made of silicon and glass using microengineering technology. The main part of the device is a lateral field-emission source of electrons, which has been fabricated on oxidized silicon wafer. Both, cold cathode and anode of the source are made of thin gold layer using only one photolithography process. Fabrication process and the preliminary results of electrical tests of the field-emission electron source are presented. Experimental studies have shown its good emission parameters: a low threshold voltage (over a dozen Volts), a high electron current (from tens to several hundred micro amperes), and field enhancement coefficient from 10⁷ to 10⁸ cm⁻¹. These results are promising and give possibility to fabricate orbitron micropump as an integrated part of vacuum MEMS.     

Morphological and structural analysis of nano-structured gold thin film on silicon by pulsed laser deposition technique

Structural and morphological investigations on optimized nano-structured gold thin film (under vacuum ∼10⁻³ Torr) are reported. The Au optimized thin film was deposited on 4 N polished and analytic grade p-type single crystal (111) Silicon wafer by pulsed laser deposition (PLD) technique, under a vacuum of about 10⁻³ Torr at room temperature. The space resolved dynamics of the plume is studied by analyzing CCD images of plume. Average size of deposited nanoparticles is along the preferred (111) orientation is ∼ 20 nm using PLD technique. The deposited film is non-uniform with particle size within the range of 6.19 nm-19.62 nm. There is decrease in the value of dislocation line density. XRD and SEM investigations support each other.     

Measurement of the gas-flow reduction factor of the KATRIN DPS2-F differential pumping section

The gas-flow reduction factor of the second forward Differential Pumping Section (DPS2-F) for the KATRIN experiment was determined using a dedicated vacuum-measurement setup and by detailed molecular-flow simulation of the DPS2-F beam tube and of the measurement apparatus. In the measurement, non-radioactive test gases deuterium, helium, neon, argon and krypton were used, the input gas flow was provided by a commercial mass-flow controller, and the output flow was measured using a residual gas analyzer, in order to distinguish it from the outgassing background. The measured reduction factor with the empty beam tube at room temperature for gases with mass 4 is 1.8(4) × 10⁴, which is in excellent agreement with the simulated value of 1.6 × 10⁴. The simulated reduction factor for tritium, based on the interpolated value for the capture factor at the turbo-molecular pump inlet flange is 2.5 × 10⁴. The difference with respect to the design value of 1 × 10⁵ is due to the modifications in the beam tube geometry since the initial design, and can be partly recovered by reduction of the effective beam tube diameter.     

Wireless communication using detectors located inside vacuum chambers

Progress in modern vacuum technology requires an increasing number of measurements to be performed in the vacuum environment itself and in many applications data transfer outside the vacuum vessels is both difficult and expensive. This paper explores the potential of wireless technology to transmit data from and into ultra high vacuum (UHV) compatible vacuum chambers using glass viewport windows attached to normal standard flange fittings. The current tests focus in particular on Bluetooth technology, and its characteristics and limitations investigated. Signal propagation has been verified up to about 9 m and the outgassing of components shown to be insignificant in a clean metal-sealed vacuum system down to a pressure of 10⁻⁷ mbar. Some preliminary tests of noise and electromagnetic field immunity have been carried out, confirming the robustness of Bluetooth spread spectrum modulation technique. Signal data from Integrated Circuit Temperature Sensors and Photodiodes have been successfully transmitted from a vacuum chamber, proving the capability of this technology standard to handle both low and high frequency data. The future prospects and the further developments of the approach are also discussed.     

On morphological, structural and electrical properties of vacuum deposited pentacene thin films

Films of different thickness (50, 100, 150 and 200 nm) were deposited by thermal evaporation in vacuum on two types of substrates glass and ITO. The deposition was performed under a pressure of 10⁻⁶ mB with a rate of 0.25 nm/s. Films surface investigations showed morphological and structural changes in function of films thickness and the nature of the substrate. Films optical transmission was analysed in the 280-1600 nm spectral range and the electrical measurements were done in low vacuum (10⁻¹:10⁻² mB) and in dark.     

A novel green emitting phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb

Vacuum ultraviolet (VUV) excitation and emission properties of Tb³⁺ ion doped silico-aluminate phosphor Ca_1.5Y_1.5Al_3.5Si_1.5O₁₂:Tb³⁺ was studied. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphor showed strong green-emission peaked at 545 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 545 nm was found at a content of about 14 mol% Tb³⁺. The 4f–5d transition absorption of Tb³⁺ is in the range from 150 nm to 260 nm, and there is an energy transfer from the host to the rare earth ions. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm.     

Optical functions and time-resolved luminescence of lithium hydride single crystals upon far-ultraviolet excitation

Low-temperature reflection spectra of lithium hydride (LiH) single crystals cleaved in ultrahigh vacuum (3 × 10⁻¹⁰ Torr, T = 10 K), were recorded using synchrotron radiation in vacuum ultraviolet spectral region. Based on the obtained experimental data, the optical functions of LiH in the energy range from 3.7 to 35 eV were analyzed using the Kramers–Krönig relations. Time-resolved photoluminescence excitation spectra were studied in detail for the near edge free exciton-phonon luminescence at 4.67 eV and photoluminescence at 2.4 eV due to the Bi³⁺ impurity centers. The effect of multiplication of electronic excitations due to inelastic scattering of hot photoelectrons and hot photoholes was revealed at photon energies above 15 eV (more than 3E_g). It was found that the radiative lifetime for free excitons in LiH at 4.67 eV is less than 1 ns as low temperatures as at 10 K. The interpretation of the electronic band structure of lithium hydride in the ultraviolet and vacuum ultraviolet spectral regions were carried out on the basis of the present experimental results with the involvement of the available band structure calculations.     

Vacuum sealing using surface activation bonding of Si wafer

Bonding technology of Si wafer for vacuum seal is important in MEMS. We have tried the vacuum seal using surface activation bonding without any binder. It is the ultimate bonding technique and gives the precise dimension due to the direct contact. The technique is, however, not easy. We have investigated the surface conditions in order to achieve the bonding. The surfaces cleaned by Ar ion beam bombardment were measured by XPS and AFM. The natural oxide on the Si surface was removed by Ar ion bombardment. The surface roughness depended on the condition and the irradiation time of the Ar ion beam. The surface bonding at room temperature was achieved for the clean surface of the surface roughness less than Ra = 1 nm, but it was not done with the rough surfaces more than Ra > 2 nm. The vacuum sealing was checked using the cavities made in the Si wafer. The cavity part sealed in vacuum was depressed in the atmosphere, which was measured using a needle-contact profiler and a 3D laser profiler. The gas in the cavity was measured with a mass spectrometer by clashing the seal in vacuum. Any other gas except Ar gas closed in the cavity was not detected. We concluded that the vacuum sealing using surface activation bonding of Si wafer was achieved. The sealing condition has not changed even after one year.     

Study on volatilization rate of silicon in multicrystalline silicon preparation from metallurgical grade silicon

The quantity of silicon lost during evaporation is greater than theoretical expectation during the purification of metallurgical grade silicon by vacuum evaporation. In this paper, silicon volatilization rates were measured for evaporation times of 30, 45 and 60 min at 1723, 1773 and 1823 K, respectively. Results indicate that volatilization rates determined in our experiments are one or two orders of magnitude greater than those from theoretical calculation. The equation for theoretical calculation was revised (ω = (2.23-6.30)× 10⁻¹ p_pa(M/T)^1/2) using silicon evaporation coefficient of 8.5-24. The details of the experimental set-up were found to be important and the mass of silicon evaporated in particular was found to be related to the water-cooling system. The carbon/graphite inserts also and the presence of trace amounts of oxygen in the vacuum furnace could reduce the content of silicon in gaseous phase and support the evaporation of silicon. It was found under certain conditions that there are two principal stages involved: 1) Formation of vapor-liquid equilibrium, 2) Maintenance of the established vapor-liquid equilibrium during the silicon evaporation process. It was found that silicon process losses can be reduced by shortening the time of the first stage.     

Study on electrical conduction of viologen derivatives using scanning tunneling microscopy

The electrical conduction of self-assembled monolayers (SAMs) made from viologen derivatives was measured using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with a focus on the molecular structural effect on the electrical conduction. For viologen derivative SAMs, resistances through the monolayers increased exponentially with increases in molecular length when the decay constants of transconductance β were ca. 0.35 to 0.48 nm^− 1. The estimated monolayer resistances of the viologen derivatives such as N-methyl-N′-(10-mercaptodecyl)-4,4′-bipyridinium (VC₁₀SH), N-methyl-N′-di(8-mercaptooctyl)-4,4′-bipyridinium (HSC₈VC₈SH), and N-methyl-N′-di(10-mercaptodecyl)-4,4′-bipyridinium (HSC₁₀VC₁₀SH) SAMs were 1.3 × 10⁸ Ω, 3.6 × 10⁹ Ω, and 3.8 × 10⁹ Ω, respectively.     

A transparent vacuum window for high-intensity pulsed beams

The HiRadMat (High-Radiation to Materials) facility [1] will allow testing of accelerator components, in particular those of the Large Hadron Collider (LHC) at CERN, under the impact of high-intensity pulsed beams. To reach this intensity range, the beam will be focused on a focal point where the target to be tested is located. A 60 mm aperture vacuum window will separate the vacuum of the beam line which is kept under high vacuum 10⁻⁸ mbar, from the test area which is at atmospheric pressure. This window has to resist collapse due to beam passage. The high-intensity of the beam means that typical materials used for standard vacuum windows (such as stainless steel, aluminium and titanium alloy) cannot endure the energy deposition induced by the beam passage. Therefore, a vacuum window has been designed to maintain the differential pressure whilst resisting collapse due to the beam impact on the window. In this paper, we will present calculations of the energy transfer from beam to window, the design of the window and associated mechanical calculations.     

Measurement of conductance of a sapphire sealed variable leak valve

A variable leak valve (VLV) is used to a great extend to regulate throughput of process gases into the vacuum system. A gas flow through the VLV can be in different flow regimes: viscous, transition or molecular, depending on the upstream gas pressure. In this work the pressure dependence of conductance of a sapphire sealed VLV has been investigated.The conductance of the VLV was determined by measuring gas flow in known volume using the pressure rise method. The pressure rise was measured using a spinning rotor gauge while the upstream pressure was measured by capacitance diaphragm gauges. The VLV conductance was determined with a relative standard uncertainty of 1.2%.The measurements of conductance were done for different VLV settings. To reduce the influence of outgassing on the gas flow measurement by the pressure rise method we have used a non-evaporable getter in the measurement volume. Argon was used as the test gas. As a result, the throughput of VLV for Ar could be measured down to 5 × 10⁻¹¹ Pa m³ s⁻¹. The time stability of the VLV for conductance set at 5 × 10⁻¹⁰ l s⁻¹ and fill pressures of Ar ranging from 0.2 kPa to 5 kPa was also determined.     

Electrostatic collection of tribocharged lunar dust simulants

Levitation and consecutive deposition of naturally charged particles on lunar surface were troublesome in previous NASA explorations. Protecting sensitive surfaces from dust deposition in the limiting condition of the lunar atmosphere is imperative for future space exploration. This study reports experimental investigation of the collection efficiency of an electrostatic lunar dust collector (ELDC). A dual-functional remotely controlled particle charger/dropper was designed for tribocharging 20 μm lunar dust simulants, and a system of Faraday cup connected to an electrometer working in the nC range was used to measure the particle charges. Tribochargeability of two lunar dust simulants was studied, and the process was found to be the most effective with the JSC-1A samples. Aluminum was verified to be a more effective plate material than stainless steel. For the tested range of electrostatic field strength (0.66–2.6 kV/m), the mass-based and charge-based collection efficiencies were in the range of 0.25–1% and 0.45–1.45% for the low vacuum (10⁻¹ Torr), and 8–35% and 12–54% for the high vacuum (10⁻⁵ Torr) conditions. The linear relationship between the applied voltage and ELDC collection efficiency predicted by the theoretical model was confirmed, and the collection pattern of the collected particles over the collection plate was consistent with the previously computed charge distribution on the collection plate. Aside from validating the predictability of the theoretical model, this study offers a novel method of particle charging inside a vacuum chamber with a variety of applications for studying chargeability of particles at different temperatures and pressures.     

Novel load responsive multilayer insulation with high in-atmosphere and on-orbit thermal performance

Aerospace cryogenic systems require lightweight, high performance thermal insulation to preserve cryopropellants both pre-launch and on-orbit. Current technologies have difficulty meeting all requirements, and advances in insulation would benefit cryogenic upper stage launch vehicles, LH₂ fueled aircraft and ground vehicles, and provide capabilities for sub-cooled cryogens for space-borne instruments and orbital fuel depots. This paper reports the further development of load responsive multilayer insulation (LRMLI) that has a lightweight integrated vacuum shell and provides high thermal performance both in-air and on-orbit.LRMLI is being developed by Quest Product Development and Ball Aerospace under NASA contract, with prototypes designed, built, installed and successfully tested. A 3-layer LRMLI blanket (0.63 cm thick, 77 K cold, 295 K hot) had a measured heat leak of 6.6 W/m² in vacuum and 40.6 W/m² in air at one atmosphere. In-air LRMLI has an 18× advantage over Spray On Foam Insulation (SOFI) in heat leak per thickness and a 16× advantage over aerogel. On-orbit LRMLI has a 78× lower heat leak than SOFI per thickness and 6× lower heat leak than aerogel.The Phase II development of LRMLI is reported with a modular, flexible, thin vacuum shell and improved on-orbit performance. Structural and thermal analysis and testing results are presented. LRMLI mass and thermal performance is compared to SOFI, aerogel and MLI over SOFI.     

Surface area, pore size distribution and microstructure of vacuum getter

An effective getter is necessary in vacuum technology. In order to obtain the adsorption mechanism of the getter, its microstructure information must be studied. Surface area, pore size distribution and microstructure of vacuum getter were studied with XRD, SEM and N₂ adsorption technique.Vacuum getter is composed of the different proportions of PdO and Ag₂O. The crystalline size of 3^# vacuum getter that includes W₂AgO = 22% and W_PdO = 78% is the maximum among all the vacuum getters. SEM images showed that vacuum getters expose a large number of nanometer-size pores. The adsorption isotherms of the vacuum getters are typical of type Ⅳ, characteristic of mesoporous material, and a type H₂ hysteresis loop is observed. Langmuir model describes N₂ adsorption at low pressure region. At the medium pressure, N₂ adsorption can be modeled by BET model. The pore size distributions of the vacuum getters are calculated by applying BJH method to the adsorption branch of N₂ isotherms at 77 K. With percentage of Ag₂O elevation, Langmuir surface area, BET specific surface area, the adsorption capacity and the pore volume become larger. But the average pore diameter becomes smaller. However, above Ag₂O content of 22%, an inverse behavior is observed. 3^# vacuum getter has the highest surface area and pore volume among all the vacuum getters. The experimental results and related analysis can be adopted in the later design of the vacuum tank.     

Microstructure and shear strength of Mo-Cu/Cr18-Ni8 brazing joint in vacuum

High density Mo-Cu composite and Cr18-Ni8 stainless steel were brazed with Ag-Cu filler metal in vacuum of 10⁻⁵ Pa. The microstructure characteristics of Mo-Cu/Cr18-Ni8 brazed joint were investigated by field-emission scanning electron microscope (FESEM) with energy dispersive spectrometer (EDS) and shear strength was measured by shearing test. The results indicated that a Mo-Cu/Cr18-Ni8 joint with shear strength of 125 MPa was obtained at 940 °C for 20 min. There were Ag-Cu eutectic and Cu-rich phase without brittle intermetallic compounds formed in the joint. The shear fracture exhibited plastic feature with shear dimple and fracture located at the interface between braze seam and Cr18-Ni8 stainless steel.