Ion pump

Ion pump

VACUUM Classified Abstracts II -- Vacuum Apparatus and Auxiliaries -- II Abstract No. and References Contd 15/II Improvements i.o.r.t. Vapo...

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VACUUM Classified Abstracts

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Abstract No. and References

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Improvements i.o.r.t. Vapour Vacuum Pumps United Kingdom. A m e a n s is described of reducing the loss of fluid from a diffusion p u m p w h e n handling a large t h r o u g h p u t of gas. U n d e r these conditions, which m a y arise if a leak develops or if a s y s t e m is p u m p e d from atmospheric pressure w i t h o u t a b y - p a s s line for r o u g h pumping, a considerable q u a n t i t y of the working fluid m a y be carried a w a y as a v e r y fine mist, difficult to remove b y conventional scrubbers. The invention described operates b y breaking u p the mist-laden air s t r e a m into smaller s t r e a m s which are accelerated and directed against an obstacle so as to cause a s h a r p change in direction. The air molecules change direction, b u t the heavier drops of fluid strike the obstacle and so are removed from the air stream. Three forms of the device are described; each is used in the outlet of the p u m p . I n one, the air s t r e a m first passes t h r o u g h a disc pierced with a n u m b e r of holes to give the acceleration. I m m e d i a t e l y behind this plate is another, also perforated, b u t arranged so t h a t the holes in the second are n o t in line with the holes in the first. Two such pairs of discs are shown, followed b y a pair of more finely perforated plates which m a y be made from wire gauze, to remove a n y large droplets which m a y remain. I n the second form, for use where the outlet tube is vertical, pairs of cylindrical cups, perforated in their curved surfaces, replace the discs. W i t h either form, the size of the accelerating apertures has to be calculated knowing the rate of flow of the gas stream, and so different sets of plates or cups would h a v e to be used to suit different backing p u m p s . The third form of the invention shows a plate w i t h a single slit of adjustable width, followed b y an obstacle in the form of a strip overlapping the slit. Tests in which a v o l u m e of 3 1. was p u m p e d from atmospheric pressure t h r o u g h a h o t v a p o u r p u m p showed a 5:1 i m p r o v e m e n t in the rate of loss of p u m p fluid. I n all cases, means are provided to allow the fluid recovered to drain back into the boiler.

A. S. D. Barrett, B. D. Power & W. Edwards & Co. (London) Ltd. Brit. Pat. 748,376

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Diffusion Pumps with Freon-12 See A b s t r a c t No.: 6/I

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Booster and Molecular Air Pumps See A b s t r a c t No.: 5/I

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Theory of Molecular Pumps Operating at Low Pressures France. I n t e g r a l equations for the pressure distribution in molecular p u m p s are developed, based on operational conditions generally b u t taking into account outgassing from the p u m p walls. The values obtained from these expressions for the case of r o t o r and s t a t o r being concentric cylinders are compared with values obtained b y practical experiment and are found to be in general agreement.

Article by C. Mercier J. Phys. Radium Suppl. 17, March 1956 1A-11A 19/II

High-Vacuum Molecular Pump Holland. Molecular high v a c u u m p u m p s are often constructed in t w o main parts: A rotor, and a s t a t o r which closely s u r r o u n d s it. I n either the r o t o r or the s t a t o r a shallow helical groove is n o r m a l l y provided, which decreases in cross-sectional area from the low to the high pressure end of the p u m p , resulting in m a n y machining difficulties. I n the a r r a n g e m e n t described the r o t o r and s t a t o r are made with a frusto-conical shape. The groove can therefore be machined to a c o n s t a n t depth from the axis of rotation, since the inclination of the surface to the axis n o w produces the necessary change in groove area. The rotor is also spring loaded axially so t h a t an excessive pressure on the high v a c u u m side causes the rotor to w i t h d r a w slightly from the s t a t o r until the pressures are equalised. A f u r t h e r feature is the provision of a hollow r o t o r h a v i n g a second s t a t o r inside it and a second groove in parallel with the first, so t h a t the p u m p i n g capacity is doubled for very little increase in size. This principle m a y be extended to a multiple s y s t e m of co-axial rotors and stators.

A. C. van Dorsten A. Verhooff &

Hartford

National Bank & Trust Co. U.S. Pat. 2,730,297

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Experiments at Very Low Pressure See A b s t r a c t No.: 12/I

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Ion Pump United States. The p u m p described consists essentially of a metal envelope comprising three hollow cylindrical sections sealed together lengthwise. The centre section has a side inlet, for connection ¢o the s y s t e m to be evacuated, and is provided internally w i t h a loosely w o u n d helical coil t h r o u g h which a high c u r r e n t can be passed to produce an axial magnetic field. E a c h end section has external magnetic windings which extend the field along the p u m p axis a t a p p r o x i m a t e l y u n i f o r m strength. The cathode assemblies are m o u n t e d at the outer ends of these sections, and are maintained at a b o u t - - 5 0 0 V relative to the anode envelope. I n operation, one of the cathodes is heated so as to discharge electrons, which are collimated into an axial b e a m b y the magnetic field. The electrons, m o v i n g in t i g h t helixes a r o u n d the axis, travel along the p u m p t h r o u g h the centre coil and on to the second cathode which repels t h e m back along their path. The discharge therefore becomes oscillatory, and the probability of ionisation of gas molecules is v e r y high. The positive ions formed strike the cathodes and are either absorbed there or t r a p p e d on n e a r b y surfaces b y s p u t t e r e d cathode material. A con-

October, 1956

Vacuum Vol. VI

239

VACUUM Classified A b s t r a c t s Abstract No. and Roforenees

E. O. l,awrence J. 8. Foster Jr. & U.S. At. Engy. Comm. U.S. Pat.

2,726,805 2211I Article by E. Thomas Bull. Clasee A'ci. 42 , July 1956 778.7S3 "3:I I

D. AIpert & Westinghouse Electric InternntionM Co. Brit. Pat. 755.074 24/II

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s t r i c t e d portion in front of each c a t h o d e ensures t h a t a n y u n c h a r g e d molectfles formed by r e c o m b i n a t i o n , which m a y m i g r a t e t o w a r d s the inlet, m u s t pa s s b a c k t h r o u g h t h e arc a n d so becnIne re-ionised. A s m a l l gas leak in ay be a d m i t t e d n e a r one cathode, so t h a t t h e local pre s s ure here r e m a i n s high e nough for t he a rc to be m a i n t a i n e d . The fore p u m p m a n i f o l d is c o n n e c t e d t o a p o i n t ne a r each c a t hode , b u t is closed off for n o r m a l w o r l d n g unless inert gases are to be p u m p e d . The p u m p is c a p a b l e of a speed of 6,000 1.%. a n d a v a c u u m of 10 -7 ram. tlg.

Study of an Evapor-Ion Pump l l e l g i u m . A v a c u u m p u m p w o r k i n g on t h e p r i n c i p l e of t h e ion p u m p i n g of gases a m l of g e t t e r i n g by t i t a n i u m which e v a p o r a t e s c o n t i n u o u s l y a n d c onde ns e s on t h e i nne r wall of t he p u m p is described. The m e c h a n i s m for the feeding nf t i t a n i u m is also described. The p u m p is now on trial. (. I u t k v l )

Improvements i.o.r.t. High Vacuum Pumping Devices U n i t e d States. The device described is an i o n i s a t i o n p u m p i n g a p p a r a t u s for p r o d u c i n g v e r y low pressures. It consists of a glass bulb c o n t a i n i n g an e l e c t rode a s s e m b l y a n d h a v i n g a side t u b e l e a d i n g to t he s y s t e m to be e v a c u a t e d a n d to the fore p u m p . Two t i l a m e n t s are p r o v i d e d : One, of t u n g s t e n , to a c t as a source of electrons. and t h e other, of nickel or s i m i l a r g e t t e r m a t e r i a l w hi c h can be e v a p o r a t e d readily, to produc e a l a v e r on tin: walls of t h e lmlb. An a c c e l e r a t o r electrode, in t he form of a helical grid s u r r m l n d i n g t he filaments, is m a i n t a i n e d a t a b o u t [ 125 V r e l a t i v e to the e m i t t i n g filament. To s t a r t ope ra t i ons , t he s y s t e m is first e w u : u a t e d to a b o u t I micron Hg w i t h the fore p u m p while a l a y e r of g e t t e r m a t e r i a l is e v a p o r a t e d on to t h e bul b walls. The e m i t t i n g t i l a m e n t is then s w i t c h e d on, a n d the e l e c t r o n s gi ve n off are a c c e l e r a t e d t h r o u g h t he grid a n d louise t he re s i d u al gas molecules w i t h which t h e y collide; t he s e ions are d r i v e n i nt o t h e w a l l s of t he b u l b a nd absorbed. \Vhen s a t u r a t i o n is reached, more g e t t e r m a t e r i a l is e v a p o r a t e d from t he second f i l a m e n t so as to form a fresh a b s o r b i n g surface, if n e c e s s a r y the e v a p o r a t e d l a y e r m a y be m a i n t a i n e d a t a n e g a t i v e p o t e n t i a l w i t h r e s p e c t to t h e e m i t t ing filament. D u r i n g the fine p u m p i n g s t a g e t h e b u l b is i m m e r s e d in l i qui d air.

Vacuum Container Imcorporating a Hand-Operated Vacuum Pump See A b s t r a c t No.: 1 4 9 / I I I

~5/H Article by 51. 13. Rcynold~ J . Chem. Edue. 32, Deo. 1955 621

The Operation of McLeod Gauges and Toepler Pumps U n i t e d States. A s y s t e m is described for t he c ont rol of t he r a t e of flow of m e r c u r y from t he re s e rvoi r of a McLeod g a u g e or T o e p l e r p u m p which a v o i d s t h e d a n g e r of s u d d e n s urge s of m e r c u r y . I t consists of t w o needle valves, one in the line l e a d i n g to the r ough v a c u u m a n d t h e o t h e r in t h e line l e a d i n g to t h e a t m o s p h e r e . Simult a n e o u s a d j u s t m e n t of t h e two v a l v e s p e r m i t s precise r e g u l a t i o n of t h e pre s s ure in t he reservoir. A s t o p c o c k is p r o v i d e d betwee.n the v a l v e s a n d t h e reservoir; t h i s is closed w h e n t h e s y s t e m is not in use since l e a k a g e el the needle v a l v e s would allow the m e r c u r y to rise if t he source of rough v a c u u m failed.

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The Operation of McLeod Gauges and Toepler Pumps See A b s t r a c t No.: 2 5 / I I

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Note 1)y H. A. Wvllic .I. Sci. lnstrum. 33, Aug. 19,56 317 2s.:I I

Ar~i,'h, l,y P. Lott .4 nalyt. Chen~. 28, Feb. 1956 276-277

240

A McLeod Gauge for Measuring a Wide Range of Pressures A u s t r a l i a . In this McLeod g a u g e the closed c a p i l l a r y consists of a l e n g t h of line c a p i l l a r y for t he m e a s u r e l n e n t of low pressures in the usual m a n n e r b u t b e t w e e n t h i s a n d t he m a i n b u l b t h e r e is a s m a l l bul b COlmected t~) th e m a i n b u l b by a larger bore c a p i l l a r y . A d a t u m line on t hi s c a p i l l a r y is used when it is desired to m e a s u r e pressures up to several m i l l i m e t r e s of me rc ury. A t t he s e pressures, however, it is not p e r m i s s i b l e to n e g l e c t th e p r essure of t h e gas over the m e r c u r y in t he open c a p i l l a r y of a McLeod ga uge of n o r m a l design. In t h i s McLeod g a u g e t h e space a b o v e t h e two c a p i l l a r y tubes, one for each r a n g e of t he i n s t r u m e n t , is isolated from t he rest of t h e s y s t e m a n d is k e p t e v a c u a t e d b y an a u x i l i a r y p u m p . Thi s isolation is a c h i e v e d by, h a v i n g t w o c o n c e n t r i c tutyes l e a d i n g down to t h e m e r c u r y reservoir, t h e m e r c u r y ri s i ng up t he i nne r t u b e s e a l i ng off t he bul b a n d closed c a p i l l a r y from the s y s t e m , while t h e ¢~pen c a p i l l a r y t u b e s are s u p p l i e d w i t h m e r c u r y t h r o u g h t h e a n n u l a r space s u r r o u n d i n g t h e i n n e r tube.

Simplified Method for MeLeod Gauge Design and Calibration U n i t e d States. The c o n s t r u c t i o n of th e McLeod g a u g e is s i m p l e b u t i n f e r , n a t i o n on t he e x a c t size of c a p i l l a r y bore a n d b u l b v o l u m e for a desired pressure r a n g e is difficult to find. A n o m o g r a p h has been designed (and is reproduced) which o v e r c o m e s t h i s difficulty a n d can be used for t h e d r a w i n g up of specifications t o t h e glass blower, for instance. It is u s u a b l e for a n y McLeod g a u g e w he re t he m e r c u r y in t he o u t s i d e a r m is t)rought level to the sealed top of t h e c a p i l l a r y t u b e fixed to t h e b u l b of t he g a u g e a n d pressure r e a d i n g s a re m a d e b y th e l e n g t h of the c o l u m n of gas c o m p r e s s e d in t h i s c a p i l l a r y . The h o m o g r a p h covers pre s s ure s be t w e e n 30 a nd 10 .6 mm. t t g a n d b u l b v o l u m e s from 10 to 250 c m L

Vacuum T7~I

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October, 1956