Bakeout Temperature Research Articles

Features of electron-beam processing of metal-ceramic powders in the forevacuum

We present the results of sintering of a metal-ceramic sample consisting of alumina ceramics and titanium. Sintering has been conducted via the surface scanning by the electron beam generated by a plasma electron source. It has been shown that the bake-out temperature affects the formation of the layer with a uniform distribution of elements. We have determined the sintering regimes that enable fabrication of samples with uniformly distributed elements over volume.

Heating and Loading Process Improvement for Indium Inserted Mechanical Lap Joint of REBCO Tapes

A mechanical lap joint of REBCO high-temperature superconducting (HTS) tapes with an indium foil inserted between joint surfaces has been proposed for segment-fabrication of HTS magnet of an advanced fusion reactor and various HTS applications. In a previous study, we successfully achieved a joint resistivity of about 3.5 pΩm 2 by heat treatment during fabrication of the joint. In this study, improved heat treatment method was developed by controlling contact pressure and combining bake-out process. We evaluated joint resistance and critical current of the joint depending on the heating condition and bake-out condition. The joint resistivity after heat treatment was 2.5 pΩm 2 without decreasing critical current with conditions of heating temperatures of 90-140 °C, a heating time of 30 min, a contact pressure of 100 MPa, a bake-out time of 30 min, and a bake-out temperature of 150 °C. Owing to applying the contact pressure when the indium becomes softer, the true area of contact increased and the thickness of the indium decreased greater than those in the previous method. In addition, it was shown that voids appearing by heating were removed by bake-out process from thermal desorption spectroscopy test.

Deuterium retention and out-gassing from beryllium oxide on beryllium

The desorption of D implanted into Be with a superficial oxide layer is studied. The different oxide thicknesses and implantation at different energies resulted in a strong variation of the fraction stopped within the oxide layer. Thermal desorption of D was subsequently performed, intermitted by nuclear reaction analysis for assessment of the D depth distributions and total retained amounts. For the conditions, where part of the D was deposited in the Be substrate, a sharp decrease of the retained amount of D occurs around 200°C. This is attributed to the release from metallic Be. Correspondingly, the D and O depth profiles show that above 200°C the remaining D is only retained in the BeO layer. Apparently, the superficial BeO layer does not act as a diffusion barrier for D that is released from the metallic substrate. The retained amount of D deposited within the BeO layer decreases steadily and is not completely released at 350°C, the foreseen bake-out temperature in ITER.

Apparatus to measure adsorption of condensable solvents on technical surfaces by photothermal deflection

This article describes an instrument for the measurement of the mirage effect as a tool to determine the molar adsorption per unit surface area Y(1) of condensable solvents in the presence of a non-condensable carrier gas. The present apparatus is a much improved version of previous prototypes developed in our laboratory and elsewhere with a higher surface bake-out temperature (150 °C rather than 40 °C), lower residual vacuum (3 Pa versus 100 Pa), greater sample surface (40 mm diameter instead of 10 mm), more powerful optical pump beam (150 W cf. 50 W), and larger saturated vapour preparation volume (4 L instead of 1 L). The new set-up also includes the in situ monitoring of the surface via a reflected HeNe laser beam for the real-time detection of the onset of condensation. Here, we give a detailed description of the various components, outline the experimental procedure, show typical results, and suggest some straightforward improvements.

Formaldehyde Emissions and Moisture Content Change of Wood Composites during Bake-out

Wood composites are a hygroscopic material and have ability to exchange its moisture content with air. This study investigated the formaldehyde emission and moisture content change of four wood composites (particleboard (PB), medium density fiberboard (MDF), high density fiberboard (HDF), laminated HDF (L-HDF)) as a function of bake-out temperature and time. The composites were baked out for 1, 3, 5, 7, 10, 14, 21, and 28 days at temperatures of <TEX>$20{\pm}2$</TEX>, <TEX>$35{\pm}2$</TEX>, and <TEX>$50{\pm}2^{\circ}C$</TEX> in a dry oven. The moisture content change was used to determine the emission bake-out of the composites. Best bake-out time results were obtained with after 7 days all composites. Formaldehyde emission values of composites decreased with decreasing moisture content for both temperatures. The formaldehyde emission results of bake-out temperature 35 and <TEX>$50^{\circ}C$</TEX> showed a similar tendency.

Novel light sputter ion pump with neodymium iron boron magnets and the low outgassing body

A light sputter ion pump for ultra high vacuum was fabricated using 5-mm-thick neodymium iron boron (NdFeB) magnets. The weight of the pump was reduced by 30%, compared with conventional sputter ion pumps using ferrite magnets. For the sake of uniformity of magnetic field, three pieces of magnet were combined into a 100 mm × 150 mm magnet block for the 30 ℓ/s pumping element. The authors coped with the low working temperature (below 150 °C) of the NdFeB magnets by forming a pure and dense chromium oxide film on the inner surface of the stainless steel pump body. With this vacuum thermal oxidation, the outgassing rate of water and hydrogen was considerably reduced so that a high temperature bakeout was unnecessary. The ultimate pressure achieved by the developed pump was much lower than 10−10 mbar even with a low bakeout temperature at 100 °C.

Characterization of a linear device developed for research on advanced plasma imaging and dynamics

Within the scope of long term research on imaging diagnostics for steady-state plasmas and understanding of edge plasma physics through diagnostics with conventional spectroscopic methods, we have constructed a linear electron cyclotron resonance (ECR) plasma device named Research on Advanced Plasma Imaging and Dynamics (RAPID). It has a variety of axial magnetic field profiles provided by eight water-cooled magnetic coils and two dc power supplies. The positions of the magnetic coils are freely adjustable along the axial direction and the power supplies can be operated with many combinations of electrical wiring to the coils. Here, a 6 kW 2.45 GHz magnetron is used to produce steady-state hydrogen, helium, and argon plasmas with central magnetic fields of 875 and/or 437.5 G (second harmonic). In order to achieve the highest possible plasma performance within the limited input parameters, wall conditioning experiments were carried out. Chamber bake-out was achieved with heating coils that were wound covering the vessel, and long-pulse electron cyclotron heating discharge cleaning was also followed after 4 days of bake-out. A uniform bake-out temperature (150 °C) was achieved by wrapping the vessel in high temperature thermal insulation textile and by controlling the heating coil current using a digital control system. The partial pressure changes were observed using a residual gas analyzer, and a total system pressure of 5×10(-8) Torr was finally reached. Diagnostic systems including a millimeter-wave interferometer, a high resolution survey spectrometer, a Langmuir probe, and an ultrasoft x-ray detector were used to provide the evidence that the plasma performance was improved as we desired. In this work, we present characterization of the RAPID device for various system conditions and configurations.

Preliminary Thermal and Hydraulic Analysis on the ITER Upper Diagnostic Port Plug During Normal Operation and Baking

A preliminary thermo-hydraulic analysis was performed on the ITER diagnostic upper port plug. Relevant thermal and hydraulic parameters, such as coolant pressure drop, maximum structure temperature and bake-out time, were calculated for normal operation and baking. The upper port plug considered is based on the preliminary generic structure design of Princeton Plasma Physics Laboratory and the Blanket Shield Module (BSM) developed in Europe. The diagnostic shield modules are modeled so that the Korean diagnostic procurement package, which includes Vacuum Ultra-Violet (VUV) spectrometer and neutron activation system, can be integrated. The analysis provides design inputs to optimize flow in the cooling channels of the plug. The conjugated heat transfer analysis for the port plug confirms that it is important to secure accurate nuclear heat and accurate electro-magnetic (EM) force for the design of the joining flange between the BSM and the main body. Thermal analysis shows that it will take ten hours for the port plug to reach the bake-out temperature (240°C), if the window plate is heated additionally from the rear side.

MPACVD-diamond windows for high-power and long-pulse millimeter wave transmission

To satisfy the electrical, thermomechanical and nuclear requirements for continuous wave (CW) millimeter (mm)-wave vacuum windows for electron cyclotron heating in thermonuclear plasma fusion energy research, water-edge-cooled window assemblies using large synthesized diamond disks have been developed in Europe, Japan, Russia and the USA. The diamond employed in these studies has been manufactured predominantly at DeBeers Industrial Diamond Division (UK) Ltd. by microwave plasma assisted chemical vapor deposition (MPACVD) and consists of polycrystalline diamond disks with diameter and thickness up to 119 mm and 2.23 mm, respectively. Due to low mm-wave losses (tan δ=2–3×10 −5), outstanding thermal conductivity (≈1800 W/mK), good ultimate bending strength (400 MPa) and radiation insensitivity up to a neutron fluence of 10 20 n/m 2 (10 −5 dpa), water-edge-cooled MPACVD-diamond windows promise to provide a practical technical solution for the transmission of CW millimeter waves at power levels in excess of 1 MW. MPACVD-diamond gyrotron windows with a bakeout temperature of 450°C (Al-braze) have been tested up to 0.55 MW, 10 s at 110 GHz (CPI, Palo Alto, USA) and 0.45 MW, 8 s at 170 GHz (JAERI/FZK, Naka, Japan). A thermally matched ceramic graded seal prototype 75-mm diameter window mounting which is bakeable to 550°C was developed successfully by DeBeers. The present paper reviews the development status and future prospects of this novel application of MPACVD diamond.

Compact collimated vacuum ultraviolet diagnostics for localized impurity measurements in fusion boundary plasmas

Compact vacuum ultraviolet diagnostics for impurity emission measurements in boundary plasmas of nuclear fusion plasma devices are described. The instruments are designed for monitoring intensities of resonant impurity lines between 300 and 2000 Å. The intensities are used to infer basic yet important plasma parameters, such as density of impurity charge states, radiated power, or electron temperature and density estimates. All utilized components and materials satisfy ultrahigh vacuum and high bake-out temperature requirements, enabling the instruments to qualify for vacuum port or in-vessel placement, in close proximity to emitting plasmas. The instruments have high spatial (Δl⩽1 cm) and temporal (Δτ⩽100 μs) resolution. The spectral resolution is Δλ⩽20 Å. Planar diffraction gratings at near-normal incidence are used for dispersion of incident radiation, collimated by high precision mechanical stacked grid collimators. Highly localized field of view, adequate throughput, and compactness distinguish these diagnostics from conventional slit instruments. A prototype monochromator for λ=1550 Å has been built, evaluated, radiometrically calibrated, and used on the CDX-U spherical torus for monitoring C IV emission in ohmic and high harmonic fast wave heated plasmas with Te(0)⩽100 eV. An attractive mechanical collimator based Wadsworth mount spectrometer concept is presented and its application to impurity content and transport measurements in tokamaks is discussed.

Mechanism of ultralow outgassing rates in pure copper and chromium–copper alloy vacuum chambers: Reexamination by the pressure-rise method

Mechanisms influencing the ultimate outgassing rate of hydrogen from a metal chamber into a vacuum have been clarified by results from a reexamination using the pressure-rise method. The condition of the oxide surface of the metal plays a central role in determining the ultimate outgassing rate. Measurements were made for three materials, oxygen-free high-conductivity (pure) copper, chromium(0.6%)–copper(99.4%) alloy, and stainless steel 304, and are compared with previously published work by the throughput method. In the case of pure copper, the ultimate outgassing rate is dependent on in situ bakeout temperature. When the bakeout temperature is at least 250 °C, the oxide film on the copper surface is completely deoxidized and the ultimate outgassing rate is determined only by the concentration of hydrogen remaining in the bulk. As a result, this pure copper requires over a week in vacuum to attain its ultimate outgassing rate. In the case of chromium–copper alloy, however, the bulk content of hydrogen is mostly degassed during prebakeout at 400 °C, because the surface is not yet covered by a Cr2O3 film and the major copper–oxide film on the surface is easily deoxidized at this prebakeout temperature. During the prebakeout, Cr atoms in the alloy are out-diffused to the surface. The out-diffused Cr atoms on the surface are then oxidized to Cr2O3 when the chamber is subsequently exposed to atmospheric air. Because hydrogen permeation of a Cr2O3 film is over an order of magnitude lower than that for the bulk alloy, the film limits both out-diffusion of hydrogen from the bulk and hydrogen solution into the bulk through permeation when the surface is exposed to atmospheric air. Thus the outgassing rate of the CrCu chamber does not change with changes in bakeout temperature. The key to attaining ultralow outgassing rates in a pure copper chamber is in situ bakeout with extended, continued pumping; but the key for a CrCu alloy chamber is simply prebakeout. By this we can obtain an ultralow outgassing rate at the 10−12 Pa m/s (hydrogen equivalent) level. On the other hand, in the case of stainless steel, the surface is immediately covered by a mixed Cr2O3+Fe2O3 film after the initial electropolishing step, because the content of Cr in stainless steel is over an order of magnitude higher than in the CrCu used. Therefore hydrogen in the bulk is not effectively degassed during 400 °C prebakeout and in situ bakeout. The outgassing rate from the stainless steel chamber was over an order of magnitude higher than from the pure copper and CrCu alloy chambers.

Development of CVD diamond r.f. windows for ECRH

The recent development of CVD diamond windows for high power gyrotrons is summarized. Highlights of this work include the manufacture of a window 100 mm in diameter and 1.6 mm thick having a uniform loss tangent of <1×10 −5 at 145 GHz, and a window 120 mm in diameter and 2.25 mm thick with losses below 3×10 −5 at 145 GHz. Moreover, the dielectric loss and permittivity have been found to be relatively stable over a wide temperature range from 100 to 700 K compared to other materials such as sapphire or gold doped silicon. More recently, development work has concentrated on achieving a robust and cost effective manufacturing process able to produce windows matching the requirements of 1 MW CW gyrotrons for ECRH applications. Typical dielectric properties for this new grade of material are presented using a 110 GHz gyrotron window 114 mm in diameter and 1.715 mm thick as an example. Data show a loss tangent in the region of 2×10 −5 at 145 GHz across the 80 mm clear aperture region of the disc. A numerical model is used to calculate the temperature and stress profile of the window when subjected to high power beam conditions. It is shown that these dielectric properties, in conjunction with a thermal conductivity in the region of 2000 W m −1K −1 at room temperature and a fracture strength in excess of 250 MPa, make this grade of diamond suitable for use as an output window on 1 MW CW gyrotrons. A new method of mounting CVD diamond windows to metal flanges has been developed to allow vacuum bakeout to be performed at 550°C compared to the previous limit of 450°C. This increase in bakeout temperature promises dramatic reductions in the time taken to achieve optimum vacuum conditions in the gyrotron. The new mounting utilizes a novel graded seal arrangement to achieve low thermal mismatch stresses and good dimensional stability during thermal cycling. A 75 mm diameter window mounted using this method has been shown to survive five thermal cycles up to 550°C for 1 h without degradation of its sealing performance.

Outgassing in thin wall stainless steel cells

Outgassing of hydrogen determines the ultimate pressure of a metal ultrahigh vacuum system, but also limits the long-term performance of sealed-off metal devices, where a thermal insulating grade of vacuum is required for a period of several years. Hydrogen concentration in stainless steel is related to equilibrium pressure by means of Sievert’s law, while the outgassing rate qout and equilibrium pressure are not related, particularly in the high vacuum range. Thin wall stainless steel cells (wall thickness 0.15 mm, AISI304 and AISI316) were used to test the relation between thermal treatment and the resulting room temperature outgassing rate qout. They were cleaned and baked in situ at ∼205 °C for ∼16 h. The pressure rise was recorded by a spinning rotor gauge after sealoff. The initial pressure rise slope at a H2 equivalent pressure of ∼10−5 mbar was in the order of qout≅1×10−14 mbar l H2/s cm2. During the measurements, which were performed at least 750, but even in excess of 3000 h, the outgassing rate started to decrease, which may be explained by its approaching equilibrium pressure. From the difference, the recombination rate coefficient KL was determined on the basis of a simple model. It lay in the range of KL (296 K)≅10−25–10−24 molecules H2 cm4/(atoms2 H s). The cells inner surfaces were analyzed by Auger electron spectroscopy. Their composition was rather typical for technical grade stainless steel covered by a native oxide layer. The corresponding KL values, which were not measured previously at room temperature, are matched with extrapolated reported values for an electropolished stainless steel surface. In addition, hydrogen annealing at 1030 °C was applied as a pretreatment of two of the cells without any noticeable effect on the final outgassing rate even if the inner surface was covered by a thick oxide layer. By increasing of the bakeout temperature, it was possible to prepare cells where qout was in the order of ≅1×10−15 mbar l H2/s cm2.

A comparison between the outgassing process of a titanium chamber and a stainless steel chamber

A vacuum chamber made from TiN coated titanium and a vacuum chamber made from electropolished stainless steel were pumped down with the bakeout process at temperatures of 150 °C, 200 °C, 250 °C and 300 °C. The time (t) dependence of the total pressure (P) at each bakeout temperature was described as P = A · t −0.2 for the electropolished stainless steel chamber, and P = A′ · t −1.0 for the TiN coated titanium chamber. The residual gases in the test chambers were also measured in the bakeout process. During the bakeout process, the rate of hydrogen outgassing remained steady while other residual gases decreased continuously. The rate of hydrogen outgassing in the stainless steel chamber was more than 10 times greater than in the titanium chamber at a bakeout process of 250 °C. The reason the pressure in the titanium chamber decreased more rapidly than in the stainless steel chamber during the bakeout process can be explained by the difference in the rate of hydrogen outgassing. Using a turbomolecular pump, a lower pressure was obtained in the titanium chamber than in the stainless steel chamber after cooling down the chamber from a 193 h baking at 250 °C to room temperature.

Measurements of outgassing rate from copper and copper alloy chambers

Using a newly-developed sealing system of copper alloy (CrCu) Conflat-flange with an oxygen-free copper gasket, the outgassing rate from an all chromium-copper (Cr 0.5%, Cu 99.5%) alloy chamber has been measured using the throughput method. After 10 h evacuation at 20 °C, the outgassing rate became 1.02 × 10 −7 Pa m 3 s −1 m −2 nitrogen equivalency. After a low temperature bakeout at 100 °C for 24 h the outgassing rate reached the ultralow level of 3.75 × 10 −12 Pa m 3 s −1 m −2 nitrogen equivalency. This value is about the same as that from oxygen-free high-conductivity (OFHC) copper after bakeout at 250 °C. Increasing the chromium copper bakeout temperature to 250 °C does not change the result.

Latest developments on valve seat-seal assembly

The Helicoflex® metallic seal initially designed for static assemblies is presently used as a valve primary seal for specific applications where the elastomeric seal commonly used in such a configuration is totally unsuitable owing to low resistance to radiation and high temperature, and incompatibility with ultrahigh vacuum conditions. The hydrogen injection quick valves of the JET fusion reactor, as well as the aluminium ultrahigh vacuum valves of the TRISTAN accelerator already operate using this technology over a rather narrow range of working conditions. In order to widen the field of applications and establish the basic principles of the sealing function applied to a valve seat-seal assembly, tests were carried out in the Sealing Techniques Laboratory of the French Atomic Energy Commission (CEA). The seat configuration and seal holding device have been reconsidered. Seals have been tested using different grades of sealing lining material, i.e. successively aluminium, silver and copper. In this respect, such an assembly configuration was found to be in full compliance with the requirements of the new types of valve, those which are being considered for applications where ultrahigh vacuum is combined with a leak rate requirement of 10 −10 Pa m 3 s −1 and a high bake-out temperature.

Effect of bake-out on the adsorption kinetics of gases in a vacuum chamber

Adsorption and desorption of gases in a vacuum chamber can be characterized by sticking probability, s, mean sojourn time, τ, and amount, σ, of adsorption on the surface in the chamber. The values of s and τ can be obtained through the analysis of pressure changes in the chamber under nonequilibrium conditions. These conditions were realized by means of a pressure pulse generated by pulsed-laser-beam irradiation of the stainless steel surface in the chamber. After laser beam irradiation, ion currents of a quadrupole mass spectrometer were measured for m/q = 2, 18, and 28. The time constant of pressure decrement for water vapor desorbed was much smaller than the value calculated. The difference can be explained by a small s and a large τ on the chamber surface. For a new stainless steel chamber, s of water vapor was constant, at about 9 × 10 −4, when bake-out temperature was lower than 150°C, and increased markedly after bake-out at 250°C.

Discharge cleaning on Tokamak Fusion Test Reactor after boronization

At the beginning of the 1990 Tokamak Fusion Test Reactor (TFTR) experimental run, after replacement of POCO-AXF-5Q graphite tiles on the midplane of the bumper limiter by carbon fiber composite (CFC) tiles and prior to any pulse discharge cleaning (PDC), boronization was performed. Boronization is the deposition of a layer of boron and carbon on the vacuum vessel inner surface by a glow discharge in a diborane, methane, and helium mixture. The amount of discharge cleaning required after boronization was substantially reduced compared to that which was needed after previous openings when boronization was not done. Previously, after a major shutdown, about 105 low current (∼20 kA) Taylor discharge cleaning (TDC) pulses were required before high current (∼400 kA) aggressive pulse discharge cleaning (PDC) pulses could be performed successfully. Aggressive PDC is used to heat the limiters from the vessel bakeout temperature of 150 to 250 °C for a period of several hours. Heating the limiters is important to increase the rate at which water is removed from the carbon limiter tiles. After boronization, the number of required TDC pulses was reduced to &amp;lt;5000. The number of aggressive PDC pulses required was approximately unchanged.

Thermal effect of periodical bakeout on tritium inventory in first wall and permeation to coolant in reactor life

Abstract In view of safety, it is very important to control the tritium inventory in first walls and permeation to the coolant. A time-dependent diffusion and temperature calculation code, TPERM, was developed. Using this code, a numerical study on the long term effects of the bakeout temperature on tritium inventory and tritium permeation to the coolant was made. In this study, an FER type first wall (stainless steel) was considered and a cyclic operation (one cycle includes a plasma burn phase and a bakeout phase) was assumed. The results are as follows: (i) There is almost no difference in the tritium inventory in the first wall between the operation with 150 °C-bakeout and the continuous burning operation (without bakeout). In both cases there is not tritium permeation to the coolant at 5 years' integrated burn time. The 150 °C-bakeout is effective to release tritium in the surface (to 0.1 mm depth) region on the plasma side, but it is not effective to decrease the tritium inventory over the reactor life. (ii) To decrease the tritium inventory, a bakeout at a temperature higher than 150 °C is necessary. But a high temperature bakeout causes earlier tritium permeation to the coolant. (iii) From these results it is suggested that the decrease the tritium inventory over the reactor life by bakeout, some form of protection against tritium permeation or a decontamination device in the cooling (or bakeout) system becomes necessary.

Dynamic in situ bakeout for the HERA proton ring vacuum system

Due to the restricted space for the heaters and insulation in the normal conducting magnets of the HERA proton ring, the bakeout process must be done very carefully in order to prevent damage of the magnets. A special high temperature thermal insulating material allows bakeout of the stainless steel vacuum chambers by a heating jacket as thin as 3.5 mm. When built into a normal conducting accelerator magnet, a bakeout temperature of 300°C is achieved without overheating the surrounding magnet. After more than 24 h the maximum temperature of 68°C reached at the sensitive coil of the magnet is well below the 80°C critical temperature of the epoxy. With the described bakeout system no heat sink is necessary to protect the magnet's components from the high temperatures. Control and regulation of the temperature of the vacuum chambers are done by means of thermocouple probes, attached to the outer surface of the chambers near the coaxial heating elements.