Internal Vacuum Research Articles

Geometry optimization, energetics and solvation studies on four- and five-membered cyclic and disulfide-bridged peptides, using the programs quanta 3.3 and charmm 22

Force field parameters for peptides and proteins in the charmm 22 force field have been refined using the results of MP2/6-311G ∗∗ calculations on N-formyl alanine amide (Frey et al., J. Am. Chem. Soc., 114 (1992) 5369) and other data (Remek et al., J. Mol. Struct. (Theochem), 235 (1991) 1). Significant changes in the peptide backbone torsional energy terms in charmm 22 were made in order to be consistent with the MP2 results relative to previously fitted HF results. To test the new force field parameters against experimental data, molecular mechanics calculations were carried out on small four- and five-membered cyclic and disulfide-bridged peptides. It was found that in general these molecules are in their calculated minimum energy conformation or a low energy conformation in the X-ray structure. The average deviation of the dihedral angles calculated for the seven energy minimized vacuum structures from the X-ray values is 10°. The dihedral angle deviations of the minimized and dynamically averaged conformations are similar for studies which included explicit TIP3P solvent molecules. Upon solvation and minimization, internal vacuum energies increase by 4–7 kcal mol −1. In addition to calculated structural results, we report some of the force field parameters used for the calculations, energy decompositions, and harmonic vibrational free energies.

A Method for Compensating Bellows Pressure Loads while Accommodating Thermal Deformations

Many metal bellows are used on storage ring vacuum chambers. They allow the ring to accommodate deformations associated with alignment, mechanical assembly and thermal expansion. The NSLS has two such electron storage rings, the vuv ring and the x-ray ring. Both rings utilize a number of welded metal bellows within the ring and at every beam port. There are provisions for 16 beam ports on the vuv and 28 ports in the x-ray ring. At each of these locations the bellows are acted on by an external pressure of 1 atmosphere, which causes a 520 lb reaction at the vacuum chamber beam port and at the beamline flange downstream of the bellows. The use of rigid tie rods across the bellows flanges to support this load is troublesome because most storage ring vacuum chambers are baked in situ to achieve high internal vacuum. Significant forces can develop on components if thermal deformation is restrained and damage could occur.

Vacuum Measurements on the Tandem Mirror Experiments Upgrade (TMX-U) Fusion Experimenta

The gas inventory of the Tandem Mirror Experiment Upgrade (TMX-U) must be carefully controlled, if it is to successfully create various plasma configurations for thermal barrier experiments designed to provide an improved performance for tandem mirror experiments. This paper is a progress report on the calibration methods and pressure measurements of machine conditions deriving from recently improved neutral-beam gas control, and changes to the internal baffling geometry and the gettering system. Various recorded pressure vs time histories produced by the machine's internal vacuum systems and their pumping mechanisms are presented. An upgraded computer code for calculating dynamic pressures is described. Our concluding remarks deal with the problems of establishing and maintaining a pressure measuring system for a large fusion experiment, where the quality and reliability of components must be carefully considered.

Macroscopic plasma behavior in a high-voltage toroidal theta pinch

The behavior of the implosion and post-implosion phases of two different toroidal high-beta plasma configurations has been investigated. The plasma is produced in a nominally axisymmetric toroidal theta pinch in which a tokamak-like low-beta plasma is heated and compressed to high beta by a fast-rising toroidal field. The two plasma cases differed from each other by the direction of their initial toroidal magnetic field relative to the direction of the main implosion magnetic field. The purpose of this investigation was to determine what factors govern the development and lifetime of the magnetic configuration during the early time post-implosion phase. It was determined from the q profiles for the parallel field case that the plasma should be unstable to the m=1 kink mode, i.e., q<1, but the normal outward shift of the plasma with respect to the external conducting wall carried it into the internal vacuum chamber wall before the kink could grow to an observable level. The magnetic fields in the case with the initial toroidal field antiparallel to the main implosion magnetic field exhibited nonaxisymmetric behavior.

Generation of the inside of a thin wall

Care is required to generate the inside surface of a thin-walled dome. When the dome is held in an internal vacuum chuck and the atmospheric pressure pushes against the outside a distortion of the surface results. However, when an adjustable swiveling center support is used as described most of the distortion is eliminated.

PDX Divertor operation

PDX was brought into operation in January 1980 as a diverted tokamak with typical parameters of B T = 15–20 kG, a = 38 cm, R 0 = 123–159 cm, I P = 180–300 kA, q ~ 3.7, n ̄ e = 1–3.8 × 10 13 cm −3, Z ̄ = 1.1–3, t es ~ 25 ms , and pulse lengths up to 0.7s. Internal vacuum components that were exposed to the plasma (such as limiters, shields, microwave horns, etc.) were fabricated from 99% pure titanium. Glow discharge cleaning with 3 × 10 −2 Torr H 2 and pulse discharge cleaning were used to condition the vessel for high power discharges. For the divertor studies, work has concentrated on obtaining long, high current stable discharges. Radial position, plasma current, and gas injection control systems have been used to facilitate this effort. Discharges of inside-D, square, and inverse-D cross-section have been produced. Microwave interferometers, spectroscopy, an X-ray pulse height analyzer system, scanning and fixed bolometers, and thermocouple array have been used to determine plasma and impurity densities, temperature, radiation, and power loss to the divertor. A comparison of diverted and undiverted discharges is presented.

A Quick Change Axial Cold Cathode Ion Source

An axial cold cathode heavy ion source has been built and successfully operated on the Michigan State University Cyclotron. This ion source was designed to minimize the time to change cathodes and thereby increase the cyclotron efficiency. The ion source change time (i.e. the time to change the cathode and restrike the arc) can be less than 15 minutes. In particular, all source parts that need to be cleaned at the end of a source lifetime are easily removed and clean pieces substituted. No internal ion source vacuum and water seals are broken. The ion source operates at gas flows less than 1 cc/min. and typically at 1/2 cc/min. This reduced gas usage results in a good accelerating chamber pressure and small attenuation of the accelerated beam to the external beam stop. The external ion beam intensity for a given particle and charge state are comparable to the intensity obtained from Oak Ridge and Berkeley.

Comparison of internal mixing and vacuum deposition procedures for calibrating ESCA [electron spectroscopy for chemical analysis] spectra

Comparison of internal mixing and vacuum deposition procedures for calibrating ESCA [electron spectroscopy for chemical analysis] spectra

The development and evaluation of a cryosorption pump capable of pumping all gases including helium: Ronald Dawbarn and J D Haygood, ARO, Inc, Aerospace Environmental Facility, Arnold Engineering Development Centre, Arnold Air Force Station, Tennessee, USA

New absorption cells designed specifically to achieve stable temperatures down to 66 K inside the sample compartment of an evacuated Bruker IFS-125HR Fourier transform spectrometer (FTS) were developed at Connecticut College and tested at the Jet Propulsion Laboratory (JPL). The temperature stabilized cryogenic cells with path lengths of 24.29 and 20.38 cm were constructed of oxygen free high conductivity (OFHC) copper and fitted with wedged ZnSe windows using vacuum tight indium seals. In operation, the temperature-controlled cooling by a closed-cycle helium refrigerator achieved stability of ±0.01 K. The unwanted absorption features arising from cryodeposits on the cell windows at low temperatures were eliminated by building an internal vacuum shroud box around the cell which significantly minimized the growth of cryodeposits. The effects of vibrations from the closed-cycle helium refrigerator on the FTS spectra were characterized. Using this set up, several high-resolution spectra of methane isotopologues broadened with nitrogen were recorded in the 1200–1800 cm−1 spectral region at various sample temperatures between 79.5 and 296 K. Such data are needed to characterize the temperature dependence of spectral line shapes at low temperatures for remote sensing of outer planets and their moons. Initial analysis of a limited number of spectra in the region of the R(2) manifold of the ν4 fundamental band of 13CH4 indicated that an empirical power law used for the temperature dependence of the N2-broadened line widths would fail to fit the observed data in the entire temperature range from 80 to 296 K; instead, it follows a temperature-dependence similar to that reported by Mondelain et al. [17], [18]. The initial test was very successful proving that a high precision Fourier transform spectrometer with a completely evacuated optical path can be configured for spectroscopic studies at low temperatures relevant to the planetary atmospheres.

451. Factors influencing ultrahigh vacuum in microwave electron devices: S Du Buske, Rep ECOM-2571, AD-615449, Army Electronics Labs, For Monmouth, NJ, Jan 1965

Abstract : In microwave electronic device design, the residual gases problem is usually given secondary attention though its effects on device performance and reliability can be of primary importance. The initial effects of residual gases in complex microwave electronic devices can adversely affect the cathode emission, beam focusing, output power, noise level, and internal vacuum almost instantaneously. In addition, irreparable damage can be done to internal precision components which can result in the premature failure of the device. Recent advances in the state-of-the-art in partial pressure mass spectrometer systems make a residual gases analysis of the materials and methods of microwave device fabrication possible on a rapid and scientifically accurate basis. To attain and to maintain the ultra-high vacuum needed for optimum performance, long life, and high reliability demanded by modern technology of microwave electronic devices, a knowledge of the interactions of gases with solid surfaces is required. The problem of residual gases in microwave electronic devices is discussed with respect to adsorption and desorption and the composition, sources and types of residual gases. (Author)

Influence of Internal Vacua and Ionization on the Life of Paper Insulated High-Tension Cables

At the end of the year 1926 the High-Voltage Laboratory of the Electrotechnical Institute in Leningrad undertook an experimental research in order to study the influence of internal vacua and ionization on the life duration of paper insulated high-voltage cables, as well as to clear up the conditions under which such vacua may occur. W. A. Del Mar3 drew attention to the importance of internal vacua in high-voltage cables and pointed out the three probable causes of their appearance, namely: 1. Temperature changes after installation of the cable, which produce a change of volume of air and oil in the cable in view of the different thermal expansion coefficient of lead and insulation of the cable. 2. Residual deformations, occurring when the cable is put on a reel and taken off it afterwards. S. Changes in chemical structure in the impregnating compound under the influence of ionized air, which produce a decrease of volume of the compound. The aim of the research was to clear up the part of the above mentioned causes in the formation of internal vacua, then to determine the values of those vacua and the decrease of life duration of the cable under the influence of the latter. The research is not yet finished and only its preliminary results are reported on these pages.

The effect of internal vacua

The effect of internal vacua

The effect of internal vacua upon the operation of high-voltage cables

The effect of internal vacua upon the operation of high-voltage cables

The Effect of Internal Vacua Upon the Operation of High-Voltage Cables

It has been known for some time that the internal mechanical pressure of cables is a factor in the quality of the insulation, and that the oil used in impregnating paper for cable insulation has a high thermal expansion which may alter the internal pressure of cables with varying temperature. This paper considers the rate of pressure equalization in cables and the relative effects of slow equalization upon long and short cables, indicating that there may be considerable difference in dielectric properties depending on the length and past history in regard to temperature cycles. Means of overcoming these differences are suggested. The paper is suggestive rather than demonstrative and it is proposed to publish a sequel giving experimental proofs of the facts stated.