Asian Program

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Dissertation Abstracts

Korea

  The VacuumCenter at KRISS has been establishing vacuum measurement standards from atmosphere to 10^-8 Pa since 1985. The three major standard systems are the ultrasonic interferometer manometer for low vacuum standard, the static expansion system by Boyle's law for medium vacuum and the orifice-type dynamic expansion system with a porous plug for high and ultra-high vacuum. Uncertainties of the standard systems are evaluated according to "Guide to the Expression of Uncertainty in Measurement" of International Organization for Standardization.
  The relative expanded uncertainties of the ultrasonic interferometer manometer operated at pressures from 100 kPa to 10 Pa are between 7.84×10^-5 to 3.2×10^-3 with confidence level 95% and coverage factor of k=2. The relative expanded uncertainties of static expansion system at pressures from 3 Pa to 1 kPa are between 1.8×10^-3 and 4×10^-3. Principle of dynamic expansion system is dynamic gas flow through orifice conductance. KRISS calibration system for ultra-high vacuum is basically two stage flow divider system. It consists of two almost same dynamic calibration systems, one for high vacuum 10^-2-10^-5Pa range and the other for ultra-high vacuum 10^-5-10^-8 Pa range connected to high vacuum system by a porous plug conductance. The relative expanded uncertainties of dynamic expansion system at pressures from 3.01×10^-6 Pa to 9.02×10^-4 Pa are between 1.05×10^-2 and 9.1×10^-3.
  In order to evaluate vacuum standards KRISS has been performed international comparisons with the highly respectable vacuum standards institutes such as NMIJ (Japan), NPL (U.K), NIST (USA), PTB (Germany) and NPLI (India). All the result of the bilateral and key comparison was in good agreement within the uncertainty limit to other national measurement institutes.
  Number of researches are carried out to extend measurement and standard range to extreme high vacuum. Firstly, gas species investigation in the stainless steel ultra-high vacuum chamber with hot cathode ionization gauges is investigated. The ultra-high vacuum made by stainless steel 304 chamber with two different types of hot cathode ionization gauges and a residual gas analyzer for surveying gas species was installed. H2 and H2O were dominant gas species when turning on and degassing of two hot cathode ionization gauges. The relative partial pressure of H2, C, O, H2O, CO and CO2 in percent were 63.6 %, 2.4 %, 5.0 %, 7.4 %, 15.6 %, and 6.0 % respectively at room temperature. As a result we could verify that H2 and CO was dominant partial pressure, and should be decreased their pressure for obtaining ultra-high and extreme high vacuum. Secondly, generation of extreme high vacuum by ordinary procedures with SUS304 vacuum chamber and total and partial pressure measurements by hot cathode ionization gauges and a quadrupole mass spectrometer is performed. Extremely high vacuum of 10^-10 Pa with attached three ion gauges and a mass spectrometer in order to test their characteristics was generated. Temperature effect showed that the uncertainty suggested by ion gauge manufacturer is only valid when room temperature is maintained within ±0.5°C. The total pressures difference of three ion gauges was appeared about 100 times. This suggests that gauges should be appropriately selected and calibrated for the accurate extreme high vacuum measurement. According to the partial pressure measurement with a quadrupole mass spectrometer, most of residual gas was H2 but unexpected fluorine detected and its source was not identified yet. Thirdly, achievement of extreme high vacuum using a cryopump and conflat aluminium gaskets was studied. The extreme high vacuum of 2×10^-10 Pa was achieved with a closed loop helium refrigerator type cryopump. In order to achieve the extreme high vacuum the whole body of the cryopump itself has to be baked up to the temperature of 150°C. Aluminium 2024 gaskets can stand a repeated thermal cycle if the temperature does not go too high. They functioned well even after baking at 240°C for 20 h, and proved to be compatible with the extreme high vacuum systems.
  Various types of ionization gauges, such as AxTran gauge and Extractor gauge, and residual gas analyzers are capable of pressure measurements in ultra-high vacuum and extreme high vacuum region, but there are some limitations. Especially, hydrogen residual and degassing of the electrodes and envelopes generated by radiation of the hot cathode can cause problems in extreme high vacuum measurements. Extreme high vacuum generation and measurement are also seriously affected by the electron stimulated desorption ions. The electron stimulated desorption effect reduced at the extractor gauge, in which the ion collector is surrounded by a hemispherical reflector electrode at grid potential
  An experimental system was designed and fabricated to investigate the electron stimulated desorption ions from ionization gauges as well as hydrogen adsorbed on cold surfaces. An ultra-high vacuum system used in the electron stimulated desorption study was made of SUS304 stainless steel. The electron stimulated desorption behaviors of Kr, Xe, H2/Xe, H2, Kr/H2, and H2/Kr/H2 adsorbed to the cold surface of Cu were investigated. Residual signals from H⁺, H2⁺, CO⁺, CO2⁺ and H3O⁺ along with ion signal from physically adsorbed gases were detected.The intensity of the H⁺ ion signal of H2/Xe was much higher than that of H2/Kr and H2/Cu. Much of H⁺ and H2⁺ ions yielded from hydrogen on a rare gas solid are assumed to be caused by difference in the desorption energy of H2/Xe, Kr/H2 and H2/Cu. Vacuum standards developed in this study and various experimental results related to low pressure measurement and electron stimulated desorption for hydrogen are found to be applicable to extend the range of standard to extreme high vacuum.