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Degassing of glass tubes
Letters to the Editor
Degassing
of glass tubes
Electron bombardment processes are widely used to degas metal parts and glass surfaces. In this Laboratory, tubes used for the envelope of experimental discharge lamps are normally baked for many hours at 500°C and degassed by electrical gas discharge in the tubes using, for example, a Tesla coil or direct cold cathode discharge. These methods are quite conventional and tubes are often evacuated to 1O-9 torr before filling. It has been found that tubes processed in the above manner to 1O-s torr vacuum, give up gas immediately when placed in a 100 MHz field and a whitish discharge starts due to the gas. The starting potentials needed between external clip electrodes are 1000-2000 volts. No such effects are obtained if direct voltage, 60 kHz, or 2400 MHz fields are used at quite high voltages. The pressure in tubes of volume 2 cm3 rises from 10-Otorr to several torr. Spectrographic analysis of the gas liberated showed it to be mainly CO. The effect has been obtained consistently with Vycor, Pyrex, soda, and tungsten-sealing glass. The action of the 100 MHz field produces a distinctive fluorescence on the tube walls different from the usual blue-green fluorescence associated with the action of a Tesla coil on a discharge tube. At tube pressures of 1O-3to le4 torr, the fluorescence is a strong pink to red; at 1O-.6to 1O-Btorr it is very intense, and at 10-’ to l@s torr it fades away. The fluorescence is -a fair indication of the pressure in the range 10-a to lO-9 torr. The pressure changes due to gas desorption arising from 100 MHz excitation are permanent if the tubes are left inert after the action. If, however, the power to the tube was reduced after having obtained gas discharge, the reverse process of adsorption of the gas took place and pressures would fall from several torr to less than 0.1 torr immediately. On raising the power, desorption started again. This process could be repeated several times and then the adsorption process decreased markedly. This rapid adsorption immediately after the first desorption is probably an accelerated form of the wellknown clean-up action’. An accurate investigation of how the gas release and discharge are dependent on field frequency, tube dimensions, and tube pressure could only be done with well-defined tube geometry and electrode configuration. A large and continuous range of freqiencies was not available to check the breadth of the freauencv band which will eive the effects. However, it was found that degas&g of all tubes raniing from 3 to 16 mm internal diameter can be done rapidly and efficiently with a 100 MHz field, eliminating the need for baking and pumping for prolonged periods. As there is relatively little heating of the tube walls in this process, the liberation of gas is a mechanical action, and it is believed to be due to the emission of secondary electrons from the glass during bombardment. The mechanism can possibly be explained in general terms by the theory of Gill and von Engelz, on the electrodeless discharge in inert gases at pressures of lea torr. At pressures of 10-Otorr for example, the mean free path is of the order of lo6 m, so that electrons will easily get to the walls of the tube without collision with gas molecules. On arrival at the walls, they will have enough energy under the influence of the electric field to liberate secondary electrons from the walls. These electrons will accumulate at a rapid rate if the transit time for the electrons is half the period of the field. Thus, given the right phase condition between electron velocity and the field, secondary electrons will be emitted in large quantities and gas will be liberated in the process. At the high frequency of 2400 MHz the half period of the field is about 2 x 10-10 set and this is considerably shorter than the transit time for electrons to pass from wall to wall Vacuum/volume
16/number
8.
Pergamon Press Lfd/Prinfed
between the electrodes (lO-’ to IO-* set). It is also much shorter than the transit time between collisions of electrons with gas molecules. Under these conditions gas release and discharge are very unlikely, either through the action of secondary emission from the walls, or from direct ionization from the interaction with gas molecules. Further work will be done on this resonant-type process of gas desorption, with particular reference to its dependence on frequency, when excitation equipment is available. Acknowledgement is made to Mr E Sharkey of the Defence Standards Laboratories, Sydney, for a confirmatory spectrographic analysis of the gas discharge, and to Mr R M Duffy of this Division for the electrical field measurements. References 1 E H Siegler Jr and G H Dieke, Tech Report No 8, John Hopkins Univ, 1952. 2 E W B Gill and A von Engel, Proc Roy Sot, A192,1948, 446. C F Bruce and E Phair Division of Applied Physics National Standards Laboratory CSIRO Sydney, Australia
Pressure fluctuations
in vacuum
systems
Messrs P R Bell and G G Kelly in their article “Pressure Fluctuations in Vacuum Systems”. Vacuum. 16 (5) Mav 1966. 241. anvear to be in some doubt regarding the m&ha&m 0; press&e p&s-or pips in vacuum systems evacuated by diffusion pumps. As the authors of a paper referred to in the above article’, we are convinced that the origin of such pulses is due to make and break of oil films on the oil-coated flange seals, particularly the top flange seal of the diffusion pump. By replacing the mating flange of this top flange seal with a glass plate and with pressure gauges in both fine side and backing sides of the diffusion pump, it was shown that: (4 With each fine side pressure kick, followed by a sudden small pressure rise in the backing line, there was a corresponding visible make and break of the oil film. (b) Heating the underneath of the top flange increased the frequency of the makes and breaks of the oil film, these again coinciding with measured pressure kicks. When the flange was cooled to a low temperature (at which the oil became very viscous), the pulses stopped altogether, but reappeared as expected when the flange warmed. (4 Frequency of pulses differed, for the same flange temperature, with oils having different densities. (4 Greasing the “0” ring with Apiezon grease stopped the pulses for long periods, but heating the underside of the flange reintroduced the pulses. Examination through the glass plate showed that pulses were only prevented as long as the grease remained, as the grease warmed and melted the fluctuations reappeared. (e) Use of a non-melting silicone grease prevented pulses for long periods in a similar manner. (0 Cleaning the “0” ring by washing in a high vapour pressure solvent such as acetone (even though the ring was subsequently surface dried) produced more pulses (due to increased outgassing) than with an untreated ring, again the increased frequency of visible make and break of the oil film could be directly compared with pressure gauge readings.
in Greaf Britain
433
Degassing
of glass tubes
Electron bombardment processes are widely used to degas metal parts and glass surfaces. In this Laboratory, tubes used for the envelope of experimental discharge lamps are normally baked for many hours at 500°C and degassed by electrical gas discharge in the tubes using, for example, a Tesla coil or direct cold cathode discharge. These methods are quite conventional and tubes are often evacuated to 1O-9 torr before filling. It has been found that tubes processed in the above manner to 1O-s torr vacuum, give up gas immediately when placed in a 100 MHz field and a whitish discharge starts due to the gas. The starting potentials needed between external clip electrodes are 1000-2000 volts. No such effects are obtained if direct voltage, 60 kHz, or 2400 MHz fields are used at quite high voltages. The pressure in tubes of volume 2 cm3 rises from 10-Otorr to several torr. Spectrographic analysis of the gas liberated showed it to be mainly CO. The effect has been obtained consistently with Vycor, Pyrex, soda, and tungsten-sealing glass. The action of the 100 MHz field produces a distinctive fluorescence on the tube walls different from the usual blue-green fluorescence associated with the action of a Tesla coil on a discharge tube. At tube pressures of 1O-3to le4 torr, the fluorescence is a strong pink to red; at 1O-.6to 1O-Btorr it is very intense, and at 10-’ to l@s torr it fades away. The fluorescence is -a fair indication of the pressure in the range 10-a to lO-9 torr. The pressure changes due to gas desorption arising from 100 MHz excitation are permanent if the tubes are left inert after the action. If, however, the power to the tube was reduced after having obtained gas discharge, the reverse process of adsorption of the gas took place and pressures would fall from several torr to less than 0.1 torr immediately. On raising the power, desorption started again. This process could be repeated several times and then the adsorption process decreased markedly. This rapid adsorption immediately after the first desorption is probably an accelerated form of the wellknown clean-up action’. An accurate investigation of how the gas release and discharge are dependent on field frequency, tube dimensions, and tube pressure could only be done with well-defined tube geometry and electrode configuration. A large and continuous range of freqiencies was not available to check the breadth of the freauencv band which will eive the effects. However, it was found that degas&g of all tubes raniing from 3 to 16 mm internal diameter can be done rapidly and efficiently with a 100 MHz field, eliminating the need for baking and pumping for prolonged periods. As there is relatively little heating of the tube walls in this process, the liberation of gas is a mechanical action, and it is believed to be due to the emission of secondary electrons from the glass during bombardment. The mechanism can possibly be explained in general terms by the theory of Gill and von Engelz, on the electrodeless discharge in inert gases at pressures of lea torr. At pressures of 10-Otorr for example, the mean free path is of the order of lo6 m, so that electrons will easily get to the walls of the tube without collision with gas molecules. On arrival at the walls, they will have enough energy under the influence of the electric field to liberate secondary electrons from the walls. These electrons will accumulate at a rapid rate if the transit time for the electrons is half the period of the field. Thus, given the right phase condition between electron velocity and the field, secondary electrons will be emitted in large quantities and gas will be liberated in the process. At the high frequency of 2400 MHz the half period of the field is about 2 x 10-10 set and this is considerably shorter than the transit time for electrons to pass from wall to wall Vacuum/volume
16/number
8.
Pergamon Press Lfd/Prinfed
between the electrodes (lO-’ to IO-* set). It is also much shorter than the transit time between collisions of electrons with gas molecules. Under these conditions gas release and discharge are very unlikely, either through the action of secondary emission from the walls, or from direct ionization from the interaction with gas molecules. Further work will be done on this resonant-type process of gas desorption, with particular reference to its dependence on frequency, when excitation equipment is available. Acknowledgement is made to Mr E Sharkey of the Defence Standards Laboratories, Sydney, for a confirmatory spectrographic analysis of the gas discharge, and to Mr R M Duffy of this Division for the electrical field measurements. References 1 E H Siegler Jr and G H Dieke, Tech Report No 8, John Hopkins Univ, 1952. 2 E W B Gill and A von Engel, Proc Roy Sot, A192,1948, 446. C F Bruce and E Phair Division of Applied Physics National Standards Laboratory CSIRO Sydney, Australia
Pressure fluctuations
in vacuum
systems
Messrs P R Bell and G G Kelly in their article “Pressure Fluctuations in Vacuum Systems”. Vacuum. 16 (5) Mav 1966. 241. anvear to be in some doubt regarding the m&ha&m 0; press&e p&s-or pips in vacuum systems evacuated by diffusion pumps. As the authors of a paper referred to in the above article’, we are convinced that the origin of such pulses is due to make and break of oil films on the oil-coated flange seals, particularly the top flange seal of the diffusion pump. By replacing the mating flange of this top flange seal with a glass plate and with pressure gauges in both fine side and backing sides of the diffusion pump, it was shown that: (4 With each fine side pressure kick, followed by a sudden small pressure rise in the backing line, there was a corresponding visible make and break of the oil film. (b) Heating the underneath of the top flange increased the frequency of the makes and breaks of the oil film, these again coinciding with measured pressure kicks. When the flange was cooled to a low temperature (at which the oil became very viscous), the pulses stopped altogether, but reappeared as expected when the flange warmed. (4 Frequency of pulses differed, for the same flange temperature, with oils having different densities. (4 Greasing the “0” ring with Apiezon grease stopped the pulses for long periods, but heating the underside of the flange reintroduced the pulses. Examination through the glass plate showed that pulses were only prevented as long as the grease remained, as the grease warmed and melted the fluctuations reappeared. (e) Use of a non-melting silicone grease prevented pulses for long periods in a similar manner. (0 Cleaning the “0” ring by washing in a high vapour pressure solvent such as acetone (even though the ring was subsequently surface dried) produced more pulses (due to increased outgassing) than with an untreated ring, again the increased frequency of visible make and break of the oil film could be directly compared with pressure gauge readings.
in Greaf Britain
433