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The McLeod gauge vapour stream effect
Vacuum Congress
A study of the cold-cathode magnetron gauge calibration curves G A Nichiporovich add I F Hanina, U S S R It is k n o w n that the c a l i b r a t i o n curves of the c o l d - c a t h o d e m a g n e t r o n gauges are linear at pressures a b o v e 2 to 5 10 TM torr (the tangent o f the angle of slope of the curve o n a "loglog'" scale is unity). At lower pressures deviation f r o m linearity is observed. A c c o r d i n g to some a u t h o r s the tangent o f the angle of slope o f the curve at lower pressures exceeds I and reaches 1.35-1. T h e discontinuity in the c a l i b r a t i o n curve at low pressures leads to serious errors in measurements. Besides, a characteristic feature of the m a g n e t r o n gauge is the difficulty of starting the discharge in u l t r a h i g h vacuum. This report describes the c o n s t r u c t i o n o f a d e m o u n t a b l e M G ( m a g n e t r o n gauge), which provides for discharge ignition a n d electrode heating by electron b o m b a r d m e n t . This is achieved by m a k i n g the c a t h o d e cylindrical part in the form of thin wire helix, which m a k e s it possible to heat it by direct current d u r i n g the electron b o m b a r d m e n t heating a n d at the m o m e n t of switching the gauge on. T h e M G c a l i b r a t i o n curves a n d the factors affecting their linearity at low pressures were investigated in the r a n g e from 10 ':' to 10 :' torr. It is observed t h a t the m a g n e t i c field strength H a n d the a n o d e voltage V A substantially affect the position of the p o i n t of discontinuity o n the c a l i b r a t i o n curve a n d the angle of slope of the curve in its n o n - l i n e a r part. It is s h o w n t h a t on increase of the m a g n e t i c field s t r e n g t h a n d decrease of the a n o d e voltage the onset of the linear range shifts to lower pressures, a n d the t a n g e n t of the angle of slope in the n o n - l i n e a r p a r t decreases a n d a p p r o a c h e s unity. O p e r a t i n g conditions (V A 4 . 5 to 5.5 kV, HI> l I20G) at which the M G c a l i b r a t i o n curve is linear in the pressure range 10 1:, to 1 '~' t o r r were found.
The M c L e o d gauge vapour stream effect
R S Dadson, K W T Elliott and Daphne M Woodman, D/vision o f Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, England In the course o f an investigation at the N P L a i m e d at assessing the p e r f o r m a n c e of the M c L e o d gauge as a p r i m a r y s t a n d a r d for the c a l i b r a t i o n of v a c u u m gauges, a n extensive study has been m a d e o f the mercury v a p o u r stream effect. T h e results of a g r o u p of these m e a s u r e m e n t s have been briefly discussed in a recent p a p e r by the present a u t h o r s ( V a c u u m , 1967, 17, 439) which s u m m a r i s e d the progress of a n u m b e r o f activities at the N P L in the v a c u u m metrology field. T h e results then available suggested that the usual p h e n o m e n o l o g i c a l equation, s t e m m i n g from the work of G a e d e a n d subsequently used by Ishii and N a k a y a m a , M e i n k e a n d Reich, de Vries a n d Rol, a n d others, gave a satisfactory a c c o u n t of the effect. T h e present p a p e r gives the complete results of the N P L investigation, covering a period o f some two years' work with the M c L e o d gauge, using the seven gases helium, n e o n , nitrogen, argon, c a r b o n dioxide, k r y p t o n a n d xenon. This m u c h larger g r o u p of d a t a d e m o n s t r a t e s even m o r e conclusively that this equation, with a consistent basis of calculation for the diffusion coefficients, provides a r e m a r k a b l y accurate r e p r e s e n t a t i o n of the experimental results. A l t h o u g h , as has been indicated, for example, by Takaishi, the e q u a t i o n in q u e s t i o n is not derived from a strict a p p l i c a t i o n 158
of kinetic theory, there seems at prcsem to be n o m a n a g e a b l e alternative, as T a k a i s h i ' s calculation left u n d e t e r m i n e d one of the crucial numerical factors. The stream effect is most simply described m terms of the ratio p~/p~ of the true pressure at the ionisation gauge, p~, to the u n d e r e s t i m a t e d pressure at the M c L e o d gauge, p,. Vor the case of a I cm diameter tube at 23 C, the new experiment:-I values of p~/p~ for the individual gases nowhere differ by more t h a n 1 per cent from the theoretical expression, a n d the mean discrepancy is only of the o r d e r 0.1 per cent. T h e p a p e r reviews the available evidence from the theoretical a n d experimental s t a n d p o i n t s , a n d sets out the basis of derivation of the corrections in a form suitable for practical application.
SESSION III.C Partial Pressure Measurements, Gas Analysis and Mass Spectrometers Quadrupole ionization gauge
He/rout Schwarz, Rensselaer Polytechnic htslitale, Hart/md Graduate Center, East Windsor Hill, Omnecticut 06028. US,4 The new n o n - m a g n e t i c ionization gauge consists of two q u a d r u p o l e configurations. The device can be c o m p a r e d with a Penning type ionization gauge where the electrons oscillate a l o n g the axis of the tube. The difference between the Penning gauge a n d the new gauge lies mainly in the fact that the confinement of the electrons is achieved by tx~o electrostatic q u a d r u p o l e lenses instead of a magnetic field. Electrons are emitted from a hot c a t h o d e at one end o f the tube and accelerated a l o n g the tube by an a n o d e ring at the half-way point. They will then e n c o u n t e r an inverted field, since on the o t h e r end of the tube a flat disc is positioned which has a potential slightly lower than the c a t h o d e potential. This will cause the electrons to return a n d c o n t i n u e to oscillate o b t a i n i n g m a x i n m m velocity each time they pass the a n o d e ring. The produced ions are collected by a cylindrical screen s u r r o u n d i n g the whole electrode structure. This screen is on a potential o f a b o u t 5 volts negative against the cathode. N o fast electrons can hit any solids so that the X-ray p r o d u c t i o n is kept very low. T h e tube can be operated in two modes: ( l ) d c potentials or (2) ultrahigh frequency (about 200 M H z ) are applied to the q u a d r u p o l e rods. In the latter case the two pairs of the rods were just used as a transmission line for the high frequency: a suitable i m p e d a n c e inside the tube provided a peak-to-peak potential of a b o u ! 200-300 volts between the two pairs of the q u a d r u p o l e rods. In the case of the dc operation, the sensitivity factor was by a factor of 3.5 a n d in the high frequency o p e r a t i o n by a factor of several orders of m a g n i t u d e higher than that of a regular Bayart-Alpert gauge. T h e X-ray limit is so low that it plays a lesser role t h a n a d s o r p t i o n a n d desorption p h e n o m e n a at the ion collector. A d s o r p t i o n a n d desorption processes come to an e q u i l i b r i u m in front of the ion collector a n d create a kind of "gas c l o u d " as this had earlier been postulated as being the main reason of the lower limit o f ionization gauges ~ a n d a p p a r e n t l y later observed by P A R e d h e a d < References I H Schwarz, Arch Tech MC'SlgPIIAI'M 1-1341-2 (~,'ept 1~.~51); R Haefer, Act:t Physica Austriaca o 200 (1955). 2 p A Redhead, Trans 13th Vac Syrup p 67 (1966).
A study of the cold-cathode magnetron gauge calibration curves G A Nichiporovich add I F Hanina, U S S R It is k n o w n that the c a l i b r a t i o n curves of the c o l d - c a t h o d e m a g n e t r o n gauges are linear at pressures a b o v e 2 to 5 10 TM torr (the tangent o f the angle of slope of the curve o n a "loglog'" scale is unity). At lower pressures deviation f r o m linearity is observed. A c c o r d i n g to some a u t h o r s the tangent o f the angle of slope o f the curve at lower pressures exceeds I and reaches 1.35-1. T h e discontinuity in the c a l i b r a t i o n curve at low pressures leads to serious errors in measurements. Besides, a characteristic feature of the m a g n e t r o n gauge is the difficulty of starting the discharge in u l t r a h i g h vacuum. This report describes the c o n s t r u c t i o n o f a d e m o u n t a b l e M G ( m a g n e t r o n gauge), which provides for discharge ignition a n d electrode heating by electron b o m b a r d m e n t . This is achieved by m a k i n g the c a t h o d e cylindrical part in the form of thin wire helix, which m a k e s it possible to heat it by direct current d u r i n g the electron b o m b a r d m e n t heating a n d at the m o m e n t of switching the gauge on. T h e M G c a l i b r a t i o n curves a n d the factors affecting their linearity at low pressures were investigated in the r a n g e from 10 ':' to 10 :' torr. It is observed t h a t the m a g n e t i c field strength H a n d the a n o d e voltage V A substantially affect the position of the p o i n t of discontinuity o n the c a l i b r a t i o n curve a n d the angle of slope of the curve in its n o n - l i n e a r part. It is s h o w n t h a t on increase of the m a g n e t i c field s t r e n g t h a n d decrease of the a n o d e voltage the onset of the linear range shifts to lower pressures, a n d the t a n g e n t of the angle of slope in the n o n - l i n e a r p a r t decreases a n d a p p r o a c h e s unity. O p e r a t i n g conditions (V A 4 . 5 to 5.5 kV, HI> l I20G) at which the M G c a l i b r a t i o n curve is linear in the pressure range 10 1:, to 1 '~' t o r r were found.
The M c L e o d gauge vapour stream effect
R S Dadson, K W T Elliott and Daphne M Woodman, D/vision o f Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, England In the course o f an investigation at the N P L a i m e d at assessing the p e r f o r m a n c e of the M c L e o d gauge as a p r i m a r y s t a n d a r d for the c a l i b r a t i o n of v a c u u m gauges, a n extensive study has been m a d e o f the mercury v a p o u r stream effect. T h e results of a g r o u p of these m e a s u r e m e n t s have been briefly discussed in a recent p a p e r by the present a u t h o r s ( V a c u u m , 1967, 17, 439) which s u m m a r i s e d the progress of a n u m b e r o f activities at the N P L in the v a c u u m metrology field. T h e results then available suggested that the usual p h e n o m e n o l o g i c a l equation, s t e m m i n g from the work of G a e d e a n d subsequently used by Ishii and N a k a y a m a , M e i n k e a n d Reich, de Vries a n d Rol, a n d others, gave a satisfactory a c c o u n t of the effect. T h e present p a p e r gives the complete results of the N P L investigation, covering a period o f some two years' work with the M c L e o d gauge, using the seven gases helium, n e o n , nitrogen, argon, c a r b o n dioxide, k r y p t o n a n d xenon. This m u c h larger g r o u p of d a t a d e m o n s t r a t e s even m o r e conclusively that this equation, with a consistent basis of calculation for the diffusion coefficients, provides a r e m a r k a b l y accurate r e p r e s e n t a t i o n of the experimental results. A l t h o u g h , as has been indicated, for example, by Takaishi, the e q u a t i o n in q u e s t i o n is not derived from a strict a p p l i c a t i o n 158
of kinetic theory, there seems at prcsem to be n o m a n a g e a b l e alternative, as T a k a i s h i ' s calculation left u n d e t e r m i n e d one of the crucial numerical factors. The stream effect is most simply described m terms of the ratio p~/p~ of the true pressure at the ionisation gauge, p~, to the u n d e r e s t i m a t e d pressure at the M c L e o d gauge, p,. Vor the case of a I cm diameter tube at 23 C, the new experiment:-I values of p~/p~ for the individual gases nowhere differ by more t h a n 1 per cent from the theoretical expression, a n d the mean discrepancy is only of the o r d e r 0.1 per cent. T h e p a p e r reviews the available evidence from the theoretical a n d experimental s t a n d p o i n t s , a n d sets out the basis of derivation of the corrections in a form suitable for practical application.
SESSION III.C Partial Pressure Measurements, Gas Analysis and Mass Spectrometers Quadrupole ionization gauge
He/rout Schwarz, Rensselaer Polytechnic htslitale, Hart/md Graduate Center, East Windsor Hill, Omnecticut 06028. US,4 The new n o n - m a g n e t i c ionization gauge consists of two q u a d r u p o l e configurations. The device can be c o m p a r e d with a Penning type ionization gauge where the electrons oscillate a l o n g the axis of the tube. The difference between the Penning gauge a n d the new gauge lies mainly in the fact that the confinement of the electrons is achieved by tx~o electrostatic q u a d r u p o l e lenses instead of a magnetic field. Electrons are emitted from a hot c a t h o d e at one end o f the tube and accelerated a l o n g the tube by an a n o d e ring at the half-way point. They will then e n c o u n t e r an inverted field, since on the o t h e r end of the tube a flat disc is positioned which has a potential slightly lower than the c a t h o d e potential. This will cause the electrons to return a n d c o n t i n u e to oscillate o b t a i n i n g m a x i n m m velocity each time they pass the a n o d e ring. The produced ions are collected by a cylindrical screen s u r r o u n d i n g the whole electrode structure. This screen is on a potential o f a b o u t 5 volts negative against the cathode. N o fast electrons can hit any solids so that the X-ray p r o d u c t i o n is kept very low. T h e tube can be operated in two modes: ( l ) d c potentials or (2) ultrahigh frequency (about 200 M H z ) are applied to the q u a d r u p o l e rods. In the latter case the two pairs of the rods were just used as a transmission line for the high frequency: a suitable i m p e d a n c e inside the tube provided a peak-to-peak potential of a b o u ! 200-300 volts between the two pairs of the q u a d r u p o l e rods. In the case of the dc operation, the sensitivity factor was by a factor of 3.5 a n d in the high frequency o p e r a t i o n by a factor of several orders of m a g n i t u d e higher than that of a regular Bayart-Alpert gauge. T h e X-ray limit is so low that it plays a lesser role t h a n a d s o r p t i o n a n d desorption p h e n o m e n a at the ion collector. A d s o r p t i o n a n d desorption processes come to an e q u i l i b r i u m in front of the ion collector a n d create a kind of "gas c l o u d " as this had earlier been postulated as being the main reason of the lower limit o f ionization gauges ~ a n d a p p a r e n t l y later observed by P A R e d h e a d < References I H Schwarz, Arch Tech MC'SlgPIIAI'M 1-1341-2 (~,'ept 1~.~51); R Haefer, Act:t Physica Austriaca o 200 (1955). 2 p A Redhead, Trans 13th Vac Syrup p 67 (1966).