摘要:

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 105low 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<5000. The number of aggressive PDC pulses required was approximately unchanged.

ISSN:0734-2101    

DOI:10.1116/1.577230

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Using x‐ray photoelectron spectroscopy (XPS), we have investigated the effects of oxidation on RbxBa1−xBiO3, the degradation of its surface on exposure to air, and the feasibility of cleaning techniques to reverse this degradation. The sample was a superconducting thin film grown on MgO by molecular‐beam epitaxy. XPS spectra of the initial, air‐exposed surface show two O 1speaks. The main O 1speak increases in intensity upon oxidation at 300 °C for 10 min in 100 Torr O2, and decreases upon air exposure. Also, this peak shifts, together with the Ba and Bi peaks, to lower binding energy upon oxidation. These shifts to lower binding energy upon gain of oxygen (and to higher binding energy upon oxygen loss) can be explained by a rise in the conductivity of the film with rising oxygen content. Carbon and chlorine are observed on the initial air‐exposed surface. Higher binding energy contributions in the Rb XPS peaks indicate that impurity compounds containing Rb are also present. Air‐exposed surfaces do not show any XPS features at the Fermi level. However, removal of C and Cl by Ar+sputtering at 1 kV, followed by oxidation, results in the appearance of intensity at the Fermi level. Hence sputter/oxidation treatments may prove useful in restoring the surfaces of films exposed to air (during patterning, for example) prior to overlayer deposition.

ISSN:0734-2101    

DOI:10.1116/1.577231

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Interfacial reactions in a contact system comprising of thin films of two refractory metals, Ti and Ta, codeposited onto a single crystalline substrate of Si have been studied and compared with the behavior of each of these elements separately on Si. The work analyzed systematically the behavior of this alloy system, both in its ‘‘as deposited’’ state as well as in its interaction with Si, as a function of the alloy composition and the applied heat treatments. The analytical tools employed were Auger electron spectroscopy, transmission electron microscopy and x‐ray diffraction. It has been found that only binary compounds, silicides of each of the metals, and their mutual solid solutions, and not any new ternary compounds, are formed. The reaction products at the early stages of the reaction are TaSi2and the metastable (C49 structure) TiSi2. Extra diffraction lines were obtained, they possibly belong to Ta5Si3. The final stage of the reaction consists of a uniform intermixing of the disilicides, TiSi2and TaSi2. Alloying causes a shift in the temperature of silicide formation to intermediate values between those for pure Ti and Ta (500 and 700 °C, respectively), and this value increases with Ta content in the alloy film.

ISSN:0734-2101    

DOI:10.1116/1.577189

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The morphological changes which take place in silver foils placed in microwave generated argon plasmas can be fully explained on the basis of earlier developed models of ionic sputtering. In contrast, morphological changes in oxygen plasmas were found to be complex and not readily explained by earlier models. Morphological changes were found to be a function of the foil position within the plasma, and thus were clearly influenced by plasma species concentrations and temperature. In environments containing oxygen atoms and no charged species the only change which occurred was a very slow rate of silver oxide growth. In environments containing modest concentrations of charged particles and oxygen atoms extremely rapid oxidation took place. In environments containing high concentrations of radical and charged particle species several unprecedented phenomena were observed. First, silver oxide films were found to be stable at temperatures far above the conventionally calculated decomposition temperature. Second, extremely high etch rates were sometimes observed. Third, the nature of the morphological changes could not be fully predicted, as results varied from sample to sample.

ISSN:0734-2101    

DOI:10.1116/1.577190

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

While the deceleration‐versus‐pressure characteristic of the spinning rotor gauge (SRG) shows excellent proportionality under molecular flow conditions, strong deviations built up at the viscous flow regime. By linearization of the SRG characteristic and on‐line thermal correction of the gas parameters entering the SRG pressure evaluation procedure an accuracy of 5% has been achieved for a variety of common gases under viscous flow conditions. For the thermal correction the new system comprises an ohmic temperature sensor attached to the rotor chamber. The linear pressure range of the SRG now extends from 1 μPa up to 1 atm (11 decades). Furthermore, the system is well suited for determination of the gas viscosity and its thermal increment.

ISSN:0734-2101    

DOI:10.1116/1.577191

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A new x‐ray limit measurement method for extractor gauges has been developed using modulation of the hemispherical ion reflector. The measurement is possible from pressures as high as the 10−8mbar range. X‐ray limits for commercial extractor gauges have been examined for various emission currents and different gauge port geometries. The scatter in the x‐ray limits measured is about 30% and the effect of gauge port geometry is more than 70%.

ISSN:0734-2101    

DOI:10.1116/1.577192

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A model for the mechanism of hydrogen wall pumping in fusion devices has been proposed. In this model the following process is assumed as a trigger for wall pumping: carbon evaporation occurs first under high heat loading of a graphite first wall, and pumping for hydrogen is generated simultaneously due to trapping of energetic hydrogen particles onto the fresh carbon film formed as a result of the carbon evaporation on the wall. The demonstration experiment for the hypothesis has been made in a vacuum coater having physical vapor deposition facilities. It is observed that the wall pumping for hydrogen or deuterium gas occurs in the coater, when carbon vapor is generated with an electron beam evaporator in a hydrogen or deuterium plasma produced with a radio‐frequency discharge, and is deposited on surrounding wall surfaces. As a result of the experiment, the typical pumping speed for the hydrogen or deuterium gas is estimated to be as large as 5×10−2l/s cm2(2×1015molecules/s cm2) at a carbon deposition rate of about 2×1016C/s cm2onto a wall having total surface area of 1000 cm2and in the presence of hydrogen or deuterium plasma having electron density of about 2×109cm−3.

ISSN:0734-2101    

DOI:10.1116/1.577193

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

It is commonly assumed that the distribution of molecular velocities follows a Maxwellian distribution also in the range of high and ultrahigh vacuum, i.e., at large Knudsen numbers. This distribution is theoretically derived from statistical analysis of an ensemble of molecules with frequent molecule–molecule collisions. Such a situation prevails at higher pressures, i.e., at small Knudsen numbers. However, at smaller pressures the molecule–wall collisions prevail, and due to the different nature of these collisions, the statistical arguments fail. As a consequence, the molecular velocity distribution cannot be predictedapriori at such pressures. In the present paper the applicability of the Maxwellian distribution in vacuum metrology at large Knudsen numbers, i.e., in the molecular regime is investigated. For this purpose, the pressures generated in primary standards of the static and dynamic expansion type are calculated for an arbitrary velocity distribution after expansion. The pressure generated by static expansion is proportional to the expectation value 〈v2z〉, that generated by dynamic expansion proportional to the ratio of expectation values 〈v2z〉/〈‖vz‖〉 (vzdenotes the one‐dimensional velocity component). Available experimental calibration data obtained by static and dynamic expansion of common gases are compared to each other and with direct pressure measurements with a liquid column manometer. These data provide a decisive test for the expectation values 〈v2z〉 and 〈‖vz‖〉 in the molecular range. Excellent agreement within the uncertainty of less than 1% between experimental values and values calculated assuming a Maxwellian distribution is found. Thus one may conclude that in common vacuum metrology the Maxwellian distribution can be applied for deriving pressures also in the molecular regime with high accuracy.

ISSN:0734-2101    

DOI:10.1116/1.577194

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Dependence of sensitivity on pressure in He, N2, and H2has been investigated for a group of 16 hot‐cathode ionization gauges, representing both extractor and Bayard–Alpert types, for total pressure as low as 5×10−8Pa. Absolute sensitivities were determined using a primary high vacuum standard. An independent method, that of measuring the sensitivity ratio of one gauge to another, was also employed. Within a scatter of about ±3%, ±4%, and ±4% respectively, the N2, He, and H2sensitivity ratio data showed no clearly discernible pressure dependence down to total pressures as small as 5×10−8Pa. The absolute sensitivity measurements in He and N2over the range 10−7to 10−3Pa in total pressure, also were constant within a scatter of about ±4% and ±3%, respectively. As a consequence of drift in the background component of the total pressure, all the absolute sensitivity measurements at total pressures below 10−7Pa exhibited an apparent pressure dependence not evident in the sensitivity ratio results. In the case of H2, drift in the pressure persisted at total pressures orders of magnitude larger than the background pressure, and for all the gauges led to an apparent difference of approximately 10% between the H2sensitivities at 10−8and 10−5Pa. Results of further investigation suggest that the apparent pressure dependence in the sensitivities is an artifact produced by the well‐known phenomenon of thermal dissociation of H2at hot filaments and associated processes of H2pumping and production of other species such as CO and C2H4.

ISSN:0734-2101    

DOI:10.1116/1.577195

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

For many vacuum processes the rate of recovery to a specific base pressure after gas injection is used as a criterion for determining the cleanliness of the vacuum system. When a cryopump is used, a long recovery time does not necessarily mean that undesirable gas species are present, because the recovery time of the cryopump itself increases as it becomes loaded with gases. This paper reports on experimental measurements of recovery time for APD‐8 Cryopumps after flowing Ar and H2at different rates for extended periods of time. Tests were run to measure the effect of using a new supercarbon versus standard charcoal and to see the effect of the cryotrapping of H2by Ar without the use of charcoal. The recovery time after injecting Ar correlates with the time for the surface temperature of the solid Ar to return to base temperature. The recovery time for H2is a function of the amount of H2that has been pumped, cryopanel temperature, type of charcoal, gas flow rate, H2concentration, and sequence of gas injection.

ISSN:0734-2101    

DOI:10.1116/1.577196

出版商:American Vacuum Society    

年代:1991

数据来源: AIP