Bombardon: an apparatus for investigating the influence of 5–20 keV ions on metal targets

An apparatus for ion bombardment in the energy range of 5–20 keV is described. The ion beam, produced by a duoplasmatron ion source, is purified by a magnetic sector field and directed into a target chamber 60 cm in diameter. The chamber is evacuated by sputter ion and titanium sublimation pumps with a total pumping speed of 20000 ls^?1 for H₂. The energy and angular distributions of particles backscattered from metal targets can be measured with an electrostatic spherical condenser which can be pivoted around the target. The whole apparatus is bakeable up to 250°C. With a 100 μA proton beam a pressure of 10^?9 torr is maintained in the target chamber. The black off pressure is a few 10^?11 torr.     

The cryopumping of water vapour in the continuum pressure region

Pumping speed measurements in the continuum pressure region, P>10^?3 torr, have been made for water vapour impinging on copper spheres and coils cooled to liquid nitrogen temperatures. Water vapour flow rates between 0.06 mg s^?1 and 420 mg s^?1 were used. The volumetric pumping speed was constant over the pressure range 2 × 10^?3 torr to 2 × 10^?2 torr and was, as expected, higher than that obtained in the free molecular flow region. Above 2 × 10^?2 torr the pumping speed decreased and possible reasons for this were investigated and are discussed. These included the effects of inadequate heat transfer from the liquid nitrogen refrigerant to the cryopump, a poor thermal conductivity of the cryodeposit, and an impurity, nitrogen gas, in the water vapour.     

1.15 Deposition of thin chromium films on substrates heated between room temperature and 300°C in the uhv chamber of an annular cryopump

A high vacuum evaporation apparatus is descried which includes an annular cryopump. The inner cylindric uhv chamber of the annular cryopump is confined by the cylindric cooling surface, the rotary substrate holder and the shutter and evaporator arrangement. With liquid helium filled in the cryopump a pressure of lower than 10^?9 torr can be attained. The substrate holder is equipped with a temperature-controlled heater for temperatures up to 400°C and with a mask-changing device which can be operated under vacuum.Thin chromium films were deposited on glass substrates above room temperature up to 300°C. The sheet resistance was measured and the film thickness determined using Tolansky's method of multiple beam interferometry.     

The operation of the ISR facility in relation to the UHV environment

At CERN Geneva, two kilometers of UHV at 10⁻¹⁰ torr is in daily use at the Intersecting Storage Ring (ISR) facility. An appreciable fraction of the stainless steel vacuum chamber, some 200 m in fact, is continuously at 10⁻¹¹ torr.     

Studies of photon induced desorption of surface gas within an aluminium vacuum chamber using an X-ray source

The role of photon induced desorption of gases arising from hard X-rays has been investigated within a vacuum chamber of aluminium alloy (Extrudal) as proposed for the new European LEP accelerator. The radiation was derived from a commercial 100 kV X-ray unit, and was introduced into the test chamber of length around 1 m by means of an end window formed of aluminium foil 25 ωm thick. In this manner a photon flux density of up to some 10⁹ photons cm² s^?1 irradiated a total surface area of around 4 × 10³ cm². The mean energy of the radiation was continuously variable in the range 9–30 keV. Measurements of desorption efficiency appropriate to each species of desorbed gas have been carried out with chamber pressures at around 1.5 × 10^?8 and 5 × 10^?11 torr attained, respectively, with evacuation at room temperature and following bakeout at 150°C. Studies have further been extended to assess the contribution of the pretreatment of the chamber by glow discharge cleaning in argon gas. In all cases the desorbed gas species under irradiation are predominantly H₂ and CO₂ which contribute in excess of 80% of the total desorption. The relative preponderance of these species is influenced by the treatments of bakeout and glow discharge cleaning, although the total desorption efficiency at around 10^?1 mol photon^?1 is not strongly affected by these actions. The desorption efficiencies of all species and at all stages of preparation of the chamber are found to decrease with increasing mean energy of the radiation, a factor of reduction of one third being observed over the range of energies available in the test.     

The design and operation of a new extractor gauge for UHV pressure measurements

The design of a new extractor gauge is described for ultra high vacuum pressure measurements. which employs a separate extractor electrode external to the grid. The advantages to be gained by the use of this system, the importance of providing an adequate ion extraction field and also of preventing the flow of electrons into the collector region, are discussed. A method is described of reducing the effect on the residual current of X-rays reflected onto the collector. This allows a residual current of 10^?14 A, at 1 mA electron current, to be achieved. For a gauge sensitivity of 10 torr^?1 this is equivalent to a residual pressure of 1 × 10^?12 torr. The addition of a modulator has been found to give a high modulation factor of 0.95 which should enable the low pressure limit to be extended to below 10^?14 torr.     

2.13 Behaviour of ceramic insulator materials during initiation of trigger discharge in TVG

An effort has been made by the author to design a simple triggered vacuum gap (TVG) in which a plasma discharge over the surface of a ceramic insulator initiates an arc in high vacuum (p ≤ 10^?5 torr). This design is reported elsewhere. A comprehensive study on the performance of different insulator materials is not reported so far.Thermally stable insulator materials like barium titanate (LCC HTD), silicon carbide, lead titanate, rutile, supramica, alumina, sillimanite and fused quartz in the order of decreasing permittivities, were investigated. The plasma discharge is triggered with a positive voltage pulse to the trigger electrode under low voltage conditions ≤220 V, 100 A for the main electrodes. Samples like barium titanate, rutile, sillimanite showed very consistent performance even up to 1000 trials indicating that a low voltage switching device operating in high vacuum is feasible. During these experimental investigations, both external triggering and self triggering modes have been tried, the latter successfully solving the arc-initiation problem in a much simpler way. This paper presents the results obtained with the above samples at 1–5 × 10^?6 torr.     

Optical, electrical and thermoelectric power studies of Al-Sb thin film bilayer structure

The III-V semiconductors are of great importance due to their applications in various electro-optic devices. The Al-Sb thin film was deposited on glass substrate by thermal evaporation method at a pressure of 10^-5 torr. The samples were annealed for 3 h at different constant temperatures in a vacuum chamber at a pressure of 10^-5 torr. The electrical resistance vs temperature studies show phase transformation from metallic to semiconducting. The observed positive thermoelectric power indicates that Al-Sb thin films arep-type in nature. The Rutherford back scattering analysis and optical band gap measurements also indicate that the inter-diffusion concentration varies with temperature.     

Investigation by techniques of electron stimulated desorption of the merits of grow discharge cleaning of the surfaces of vacuum chambers at the CERN intersecting storage rings

To provide surface cleaning of the CERN intersecting storage rings, in which the pressure is below 10^?10 torr over a 2 km length of mostly elliptical pipe (160 X 52 mm), ion bombardment during a glow discharge has been utilized. The discharge in argon at a pressure of about 3 × 10^?2 torr was carried out intermittently in the course of a 15 h bake-out at 300°C. Subsequent electron scrubbing enabled the electron desorption efficiency of the surface of the ISR vacuum chambers to be reduced by two to three orders of magnitude. Information is given on the apparatus used and the procedure adopted, together with a discussion of the diagnostic technique and a consideration of the action of the discharge cleaning.     

Ion beam nitriding of titanium and titanium alloy

Titanium and Ti8A/1Mo1V alloy have been nitrided with an ion beam source of nitrogen or agon and nitrogen, at a total pressure of 2?10×10^?4 torr. The treated surface has been characterized by surface profilometry, X-ray diffractometry, Auger Electron Spectroscopy (AES), and microhardness measurements. It was found that tetragonal Ti₂N phase forms in pure titanium and Ti8A/2Mo1V alloy with traces of AIN in the alloy. Two opposite processes were found to compete during the ion beam nitriding: (a) formation of nitrides in the surface layers; (b) sputtering of the nitrided layers by the ion beam. The highest surface hardness, of about 500 kg mm^?2 in titanium and 800 kg mm^?2 in Ti8A/1Mo1V, was obtained by nitriding with an ion beam of pure nitrogen at 4.2×10^?4 torr, at beam voltage of 1000 V.     

Mass spectrometric determination of oxygen at partial pressures of less than 1.5 μPa (10−8 torr)

During an investigation of chemisorption and photodesorption reactions between oxygen and thin films of zinc oxide, the need arose for the determination of oxygen partial pressures in the range 1 μPa to 100 pPa (10^?8 to 10^?12 torr), in a static system, using a residual gas analyzer. Within this range the pumping actions for various components of the vacuum system were significant and this paper describes these actions and the techniques developed to lower the limit of detection of oxygen by mass spectrometric analysis to about 700 pPa .l (5 × 10^?12 torr .l)     

Kinetics of the initial stage of oxidation in lacunar spinel formation electrical conductivity study of the transformation from γ(Fe3+1−yM3+y)2O2−3 to α(Fe3+1−yM3+y)2O2−3

The kinetics of the oxidation of aluminum or chromium-substituted magnetites to the lacunar phase γ, under very low oxygen pressure (PO₂ between 10^?6 and 10^?9 torr), in equilibrium conditions is found to be governed by the surface reaction. The transformation of the lacunar phase γ to the rhombohedral phase α, in these equilibrium conditions, was investigated by electrical conductivity. The lower the percentage of γ in solid solution, in the unoxidized spinel, the higher the temperature of the γ→α transformation.     

The influence of O2 and N2 on the hall and field effect mobilities in CdSe and PbTe thin films

Thin films of CdSe and PbTe were evaporated onto SiO in a vacuum system in which a pressure of less than 5 × 10⁻¹⁰ torr was available. The Hall and field effect measurements were made directly after producing the sample. Furthermore, these measurements were repeated under various partial pressures of O₂ and N₂ up to nearly 10⁺² torr. A remarkable influence of these gases on the mobilities was observed. One can understand this effect by considering that p-type regions are formed at the surface of the n-type semiconductors and thus a diminution of the measured mobilities occurs.     

Very high vacuum (VHV) electron beam floating zone (EBFZ) furnace

An electron beam floating zone (EBFZ) furnace, capable of melting cyclindrical specimens up to 10 mm in diameter, was designed and constructed. A circular thermionic tungsten-filament emitter enclosed in a tantalum housing preventing a direct line-of-sight relation between the cathode and the concentrical specimen rod was used to minimized mutual contamination of the emitter and specimen materials. Since the furnace was designed for melting reactive materials, it originally employed an ion-pump vacuum system capable of obtaining lower than 10^?8 torr levels. For highly outgassing materials the throughput of this pumping system was not sufficient and a replacement by a liquid nitogen trapped diffusion pump system was introduced without affecting the basic design of the furnace or the vacuum chamber. This interchangeble vacuum system could successfully cope with the problem of high gas loads released during the melting process, while still attaining vacuum levels in the 10^?7 torr range. More than a dozen metals and intermetallic compounds were melted in this furnace but not in all cases were perfect single crystals obtained. This was mostly caused by certain properties of the materials which were found unsuitable for the EBFZ technique rather than by instrumental shortcomings of the system.     

Electrodynamics of a low-pressure electrode microwave discharge

Results are given of self-consistent two-dimensional simulation of self-sustained steady electrode microwave discharge in a chamber at the end of central conductor of a coaxial line. The discharge parameters are calculated in the diffusion mode in hydrogen at pressures of 0.5, 2, and 8 torr and incident power of 30–200 W. The dependence of matching between the discharge chamber and delivery path on the geometric dimensions of the chamber is investigated in the presence of plasma. It is demonstrated that the length of central electrode is the key factor affecting the structure of plasma formation and its matching with the pumping wave. The maxima and minima of matching alternate when the central electrode is elongated by quarter wave. The maxima and minima of matching for the case of low pressure (< 2 torr) are shifted by λ/4 relative to those for high pressure (> 2 torr). The problem of maximal energy input to the discharge region at the end of antenna-type electrode is analyzed. It is demonstrated that a restriction exists on the maximal energy input to such a discharge (and, accordingly, on the size of plasma formation). These restrictions are associated either with the runaway of discharge toward the generator or with the ignition of discharge in the region of entry of antenna into the chamber.     

Empirical determination of adsorption isotherm for nitrogen on glass for various conditions of vacuum and near-ambient temperature

We have measured the adsorption isotherm of nitrogen on glass in the ranges 10^?2–10^?3 torr and ?78–0°C and compared it with Hobson's theory.     

Properties of TiNxfilms reactively sputtered in an argon-nitrogen atmosphere

The dependence of the resistivity and temperature coefficient of resistivity (TCR) of TiN_x films on nitrogen pressure is described. The partial nitrogen pressure was varied from 10^?5 Torr to 2×10^?4 Torr. The maximum value of the resistivity (216 μΩ cm) and the lowest negative value of TCR (?33ppmK^?1) were obtained in the nitrogen pressure range (2?4)×10^?5 Torr. The minimum value of the resistivity (44 μΩ cm) and the highest positive value of the TCR (1160 ppm K^-1 were obtained in the nitrogen pressure range (4?10)×10^?5 Torr. The influence of aging temperature up to 573 K on the resistance changes are shown. X-ray diffraction analysis indicated the presence of oriented or non-oriented TiN in these films.     

Structural and electrical properties of chromium and nickel films evaporated in the presence of oxygen

Electron-optical investigations of Cr films evaporated in the presence of oxygen (1 × 10^-7 to 5 × 10^-7 torr) have shown that the negative temperature coefficient of resistivity (TCR) is caused by small amounts of an amorphous oxide. On annealing these films in a vacuum of better than 5 × 10^-9 torr (e.g. for 140 h at 395°C) the amorphous phase crystallized and could be identified as Cr₂O₃. When Ni is evaporated in the presence of oxygen the TCR changes from positive to negative values at about 5 × 10^-5 torr. Since the negative TCR of oxygen-doped Cr-Ni (60/40) films did not change after annealing these films in a hydrogen atmosphere at 300°C (reduction of Ni oxide) it becomes plausible that the negative TCR of the Cr-Ni films, too, is caused by an amorphous Cr oxide surrounding the metal crystals.     

A reciprocating liquid helium pump used for forced flow of supercritical helium

The performance of a small double acting piston pump for circulating helium in a closed heat transfer loop is described. The pump was manufactured by LINDE AG, Munich, West Germany. The measured flow rate of supercritical helium was about 17 gs^?1 (500 lhr^?1) with a differential pressure of Δρ = 0.5 × 10⁵Nm^?2 at a working pressure of p = 6 × 10⁵Nm^?2. At differential pressures beyond 0.5 × 10⁵Nm^?2 the volumetric efficiency decreases.     

On the cooling of large enthalpy systems by metallic contact in vacuum

The heat transfer across metallic contacts in vacuum is studied in the temperature range from 300 to 6 K. An enthalpy of 35 MJ is removed from two large copper rings by a cooling device built into a levitated superconducting quadrupple plasma physics device. Above 50 K the contact conductance has a value of 3.5 W per K^?1, which is found to be independent of temperature. Between 5.5 and 20 K the conductance varies as shown by the following empirical relation: C (WK^?1) = 4 × 10^?4 × T^2.8 (where T is in K).