Vacuum Integrity Research Articles

Cementitious composite material with vacuum integrity at elevated temperatures: Bright, R.P., Wise, S. and MacKenzie, M.L. (Cemcom Corporation, Lanham, MD, USA) US Pat 4 891 071 (2 January 1990)

Cementitious composite material with vacuum integrity at elevated temperatures: Bright, R.P., Wise, S. and MacKenzie, M.L. (Cemcom Corporation, Lanham, MD, USA) US Pat 4 891 071 (2 January 1990)

How to Prolong the Life of Your Gas Chlorinator

Chlorinators meter and deliver a set dosage of chlorine gas from a chlorine cylinder. To do this, gas chlorinators must be operated and maintained properly. This article provides an overview of the steps to operating and maintaining gas chlorinators. The easiest and most safest way to make the connection from gas cylinder to chlorinator is by direct cylinder mounting; step by step instructions are given. A general discussion of interconnection manifold and isolation valve is given for those situations in which several cylinders are used. The ejector is the heart of the system and must be maintained. The check valve assembly keeps water from getting into the regulator; instructions for preventing that and for removing water from the chlorinator are given. Some troubleshooting tips for detecting problems with the chlorinator's vacuum integrity are given.

Surface micromachined pressure transducers

Typical IC processing is fundamentally two dimensional; sensors are three-dimensional structures. In surface micromachining, two-dimensional IC processing is extended to sensor structures by the addition of one or more sacrificial layers which are removed by lateral etching. The resulting sensor structures involve the substrate and one or more deposited films which form the intended micromechanical component. The concepts of this type of sensor manufacturing are readily demonstrated by considering absolute pressure transducers in some detail. Absolute pressure transducers involve a vacuum-sealed cavity and a deformation sensing technique. The cavity is formed from the substrate and a low-pressure chemical vapor deposited polycrystalline silicon film. The mechanical properties of this film must be controlled well enough to allow the device to be designed. This implies morphological control during processing. Optimized films which do exhibit controlled compressive or tensile strains exclude oxygen or nitrogen and are therefore not modified by extended hydrofluoric acid etches. Their mechanical behavior is monitored by micromechanical test structures which measure Euler buckling and thereby determine the value of the built-in strain. The cavity vacuum is established by reactive sealing. Long-term vacuum integrity is achieved by a low-stress silicon nitride barrier which also acts as a dielectric isolation barrier. Sensing is accomplished via deposited polysilicon resistors. These devices behave like metal resistors in terms of their temperature coefficient of resistance and noise figure. Their piezoresistive behavior is larger than that of typical metal film structures and smaller than that of single-crystal resistors. Pressure sensors with four diaphragms, two active and two inactive, have been constructed and optimized towards manufacturability. The measured performance is excellent and agrees with the predictions of the design algorithm.

An analysis of vacuum effects on ion implanter performance

The generation of an ion beam and its impact into photoresist-masked wafers will have an adverse effect on the vacuum of an ion implanter. This is particularly significant when doping with boron using BF 3 or BCl 3 as the source feed material. As well as affecting high voltage performance, poor vacuum will allow charge exchange and partial neutralization of the ion beam which could cause energy contamination as well as dosimetry errors. The nature of these effects depends not only on the system design but also on the implanted species and its charge state and energy. This paper discusses these effects on ion implanter performance and shows how a matrix-based general vacuum model may be used to investigate vacuum integrity. A time-dependent vacuum model that simulates the effect of pressure on dosimetry is also described.

Identifying type 304 stainless steel flange defects using scanning Auger microscopy (SAM)

AbstractAn unusual number of large stringer defects were found in cross‐forged type 304 austenitic stainless steel ultrahigh vacuum (UHV) flanges. These defects are cosmetically displeasing, they can affect the material's machinability and they can also affect the vacuum integrity of a UHV system. In this study, scanning Auger microscopy was used to identify accurately the defects as ferritic inclusions. This information was used to justify a change in forging and heat‐treating procedures, resulting in the reduction of large ferrite segregants and creating a more homogeneous microstructure. Subsequently, the rejection rate was reduced more than 50‐fold.

Noninvasive vacuum integrity tests on fast warm-up traveling-wave tubes

A method of tube vacuum monitoring that uses the tube's existing internal electrodes as an ion gauge is discussed. This method has been refined using present-day instrumentation and has proved to be a precise, simple, and fast method of tube vacuum measurement. The method is noninvasive due to operation of the cathode at low temperature, which minimizes pumping or outgassing. Because of the low current levels to be measured, anode insulator leakage must be low, and the leads must be properly shielded to minimize charging effects. A description of the method, instrumentation used, limitations and data showing results over a period of 600 days are presented. >

Improved indium seal for LiF windows of resonance lamps for use at low temperatures

A method of attaching crystalline LiF windows to glass lamps has been developed. The resulting seals have good vacuum properties and can be cooled to cryogenic temperatures while maintaining their vacuum integrity. The technique uses dual, indium O rings to provide a vacuum seal for a LiF window on a resonance lamp. It is appropriate for resonance lamps that produce line spectra. LiF crystals must be cooled to transmit Lyman-β radiation that can optically pump atomic hydrogen. This indium seal has other applications such as windows for VUV lasers. Details of the construction, assembly, and performance of the seal are described. A discussion of the merits and applications of the window seal is presented.

Coolant Leakage Through Cracks in Fusion Reactors

The successful operation of a fusion reactor system depends on many factors. One important factor is to maintain the vacuum integrity between the first wall and the plasma. The leakage of the first wall coolant through small pre-existing cracking or cracks developed during operation may destroy the required vacuum boundary and subsequently contaminate the plasma even before the structural failure of the wall. Cracks of few hundred microns in size may be difficult to detect by nondestructive testing in thin complicated structures such as the first wall. Cracks of such sizes may also grow to these dimensions during early phases of reactor operation.The coolant leakage rate through these cracks in fusion reactor first walls are investigated. Critical leakage rates and/or the maximum number of allowable small cracks are estimated as a function of reactor operating conditions based on plasma response to impurity contaminations. A wide range of first wall and coolant operating conditions are considered. The theoretical flow rate predictions and their range of applicability for conditions relevant to fusion reactors are discussed. These theoretical predictions are compared with available experimental data. It is found that a few through wall cracks below the limit of detection by standard techniques in a helium cooled reactor may contaminate the plasma and consequently limit the successful operation of fusion reactor.

Operational efficiency increase in copper vapor laser due to the replacement of vacuum jacket Brewster windows with flat windows

The vacuum integrity of the discharge tube of a copper vapor laser (CVL) is normally protected by Brewster angled windows. In an attempt to increase the operating efficiency of the CVL, flat windows were used to replace the Brewster windows. Experimental data confirm that the overall efficiency of the CVL does increase when such windows are used. The experimental results are discussed in terms of a computer model found in the literature. The cause of the efficiency increase appears due to a double optical cavity set up by the flat windows. However, the variation of the efficiency due to changes in the pulse repetition frequency (PRF) and buffer gas pressure are less well understood.

Using angle resolved ESCA to characterize Winchester disks

Recent improvements in ESCA instrumentation have led to a renewed interest in the use of angle resolved ESCA to study thin (<75 Å) film structures such as the fluocarbon lubricants on Winchester disks. Monitoring disk production with ESCA to assure optimal lubricant thickness is an essential part of disk manufacture. Assumptions are made about the morphology of the disk surface and the vacuum integrity of the lubricant. Angle resolved ESCA reveals the shortcomings of these assumptions and establishes the conditions required for valid lubricant thickness measurements. Angle resolved ESCA also identifies any surface contamination covering the lubricant and suggests the molecular orientation of the lubricant.

Leak Testing of Large Vacuum Vessels

Vacuum integrity of fusion devices has become increasingly important as the size, complexity, and requirements for wall cleanliness have increased. If vacuum leaks occur during device construction or operation, it will take a long time to locate and repair the leaks. In this course of vacuum technology, methods of measuring leak rates and searching for leaks at large vacuum vessels are described. They include the conventional tracer probe (helium spray) method, the detector probe (helium sniffing) method and the in-vessel leak sensor method. Some remarks on detecting very fine leaks are also presented.

Cryogenic leak test procedure for a large superconducting cyclotron helium vessel

Abstract A procedure for testing the vacuum integrity of very large stainless steel weldments used at cryogenic temperatures has been developed at Michigan State University. This development, which uses large quantities of liquid nitrogen, is a modification of a technique commonly applied to small devices and involves cooling the cryostat's liquid helium vessel (bobbin) to liquid nitrogen temperature, and then proceeding immediately with leak testing. This method was applied to the K800 superconducting magnet helium vessel, which seemed leak tight at room temperature, but was found to have an easily detectable helium leak when cooled. After repairing the leak, retesting revealed no leaks, where upon the K800 cryostat construction was completed; i.e. the bobbin was wrapped with superinsulation, a liquid nitrogen radiation shield was added, and the assembly was inserted into the vacuum jacket. The final leak test occurred when the cryostat was cooled to liquid helium temperature and was found to be helium leak tight.

Evaluation of the colliding beam accelerator first string full cell vacuum system

The CBA (colliding beam accelerator, formerly know as ISABELLE) full cell magnet system consisting of six superconducting dipole magnets and two superconducting quadrupole magnets requires two separate vacuum systems. One, known as beam vacuum, operates below 3×10−11 Torr and the other, known as insulating vacuum, operates at less than 10−7 Torr to isolate cryocircuits from atmosphere and from the UHV beam tubes. The UHV bore tube is isolated from the 4.0 K magnet by 36 layers of superinsulation and insulating vacuum. Heat load measurements on the bore tube have been completed and found to agree with data obtained in smaller controlled experiments. Measurements of helium, accumulated on cryogenic pumped charcoal panels over many weeks, have verified sensitive helium mass spectrometer leak detection methods for vacuum integrity, proving sound design of the welded complex. The full cell was assembled and operated under conditions that would exist in the completed machine. Pressures below 2×10−11 Torr beam vacuum requirement and below 1×10−7 Torr insulating vacuum, were routinely achieved during all phases of the full cell operation and support systems testing.

The development of valves, connectors, and traps for vacuum systems during the 20th century

The components used in vacuum systems have undergone profound change during the 20th century. The development has been driven both by the needs of research and of commercial manufacturing. It is, however, remarkable that the sealing wax and string era persisted through the 1940’s even in large research establishments. The present wide range of reliable commercial components became available only with the demand for ultrahigh vacuum capability, and with the development of helium leak detection techniques which permit rapid assessment of vacuum integrity. This paper provides a historical perspective of the evolution of: (a) sealing techniques for feedthroughs and for connections between components, (b) valves for isolation and for control of gas flow, and (c) traps to minimize the backstreaming of pumping fluids.

Design considerations for achieving high vacuum integrity in fusion devices

Achieving high vacuum integrity in fusion devices requires close attention to both the overall system configuration and the design details of joints and seals. This paper describes the factors in selecting the system configuration, from a vacuum standpoint, for the Princeton Plasma Physics Laboratory (PPPL) TFCX-S (toroidal fusion core experiment with all-superconducting toroidal field coils) tokamak device. The TFCX-S driven current tokamak is the eighth design in a series of tokamak concepts defined to cover the magnetic confinement and development gap between the Tokamak Fusion Test Reactor (TFTR) and the Engineering Test Reactor (ETR). The definition of a configuration that minimizes the consequences of leaks is considered a vital activity, as well as the development of leak detection concepts. A major part of the vacuum boundaries of the magnet system and the plasma system is common. For the major penetrations, primary and secondary seals are provided with vacuum control over the region between seals. The intent is to instrument these activities and provide automated recordings of these measurements for leak maintenance.

Polymer Impregnation of an Impervious Cementitious Composite Material

AbstractWell compacted, silica fume-modified ordinary Portland cement pastes are well known for their low permeability. This quality can be maintained to temperatures slightly in excess of 200°C. Above this temperature, however, a continuous pore structure develops, making the material permeable and therefore unsuitable for high temperature applications (250–400°C) where vacuum integrity is necessary. To overcome this obstacle the permeability can be exploited for impregnating the concrete with an organic polymer. Organic polymers traditionally employed for impregnated concretes are not thermally stable enough to be used at high temperatures. Therefore, a high temperature-resistant organic resin has been studied for this application. The optimum conditions for impregnating this viscous resin into a high strength mortar have been investigated.

The leak testing program of the doublet III project

Vacuum integrity of large fusion tokamaks has become increasingly important as the complexity and requirements for cleanliness have increased. Doublet III is a large noncircular tokamak (R = 1.43 m, V = 27 m3) designed for a base pressure of 2×10−8 T and a leak rate of 5×10−6 T⋅l/s. Initial leak testing of the vacuum chamber used conventional techniques. After final assembly, leak testing became more difficult as diagnostic systems, coil systems, and heating blankets enveloped the vacuum vessel. Methods were developed to locate and quantify vacuum leaks in this difficult environment. A residual gas analyzer was used for temporal response to gases flowed at various points outside the vessel. Leaks occurring in the primary vessel wall were measured by pumping on gas cooling channels adjacent to the primary vacuum wall. Air in these channels was also displaced by other gases at a constant rate to give location of leaks to within 50 cm. Vacuum leaks occurred during machine operations due to applied stresses, plasma‐material interactions, and diagnostic equipment failures. Machine vents to fix these leaks involve a significant loss of time, particularly in returning to clean wall conditions. Methods other than venting the vessel have been used, such as helium ’’patches,’’ if these leaks are sufficiently small.

X-ray diffractometer for use with cryogenic fluid targets

An x-ray diffractometer has been constructed so as to allow the study of cyrogenic fluids, in particular liquid helium, at temperatures below 4.2 K. The design and operating characteristics of the diffractometer are presented. Data acquisition is automatic. The x-ray beam is almost exclusively in vacuum and the slits which define the beam are adjustable while the vacuum integrity is maintained. Monochromatization of the x-ray beam is accomplished without use of crystal diffraction.

Experimental Investigation of Heating Phenomena in Linac Mechanical Interfaces from RF Field Penetration

In a high duty-factor, high-current, drift-tube linear accelerator, a critical interface exists between the drift-tube stem and the tank wall. This interface must provide vacuum integrity and RF continuity, while simultaneously allowing alignment flexibility. Because of past difficulties with RF heating of vacuum bellows and RF joints encountered by others, a paucity of available information, and the high reliability requirement for the Fusion Materials Irradiation Test (FMIT) accelerator, a program was initiated to study the problem. Because RF heating is the common failure mode, an attempt was made to find a correlation between the drift-tubestem/linac-tank interface geometry and RF field penetration from the tank into the interface region. Experiments were performed at 80 MHz on an RF structure designed to simulate the conditions to which a drift-tube stem and vacuum bellows are exposed in a drift-tube linac. Additional testing was performed on a 367-MHz model of the FMIT prototype drift-tube linac. Experimental results, and a method to predict excessive RF heating, is presented. An experimentally tested solution to the problem is discussed.

Versatile UHV sample transfer system

A vacuum transfer system has been developed that allows samples to be inserted from air into an Auger analyzer. Following an initial analysis they may then be moved to and from a separate vacuum chamber for other studies. The system is constructed of standard UHV components and can be fabricated to cover any desired transfer distance. All-metal sealed valves isolate the various chambers so that the vacuum integrity of each is maintained. The sample size is limited only by the smallest constriction within the components, which is 2 cm in the system described. The transfer is rapid and is capable of being controlled remotely for use in a hostile environment.