US2954156A

US2954156A - Apparatus for the production of high vacuum, especially in metal evaporation vacuum pmps

Info

Publication number: US2954156A
Application number: US630944A
Authority: US
Inventors: Meyer Enrico
Original assignee: Galileo Societa Per Azioni Off
Current assignee: Officine Galileo SpA
Priority date: 1955-12-29
Filing date: 1956-12-27
Publication date: 1960-09-27
Anticipated expiration: 1977-09-27
Also published as: DE1045041B

Description

Sept. 27, 1960 2,954,156

ION OF HIGH VACUUM, ESPEC IALLY E. MEYER APPARATUS FOR THE PRODUCT IN METAL EVAPORATION VACUUM PUMPS Filed D90. 27. 1956 nite APPARATUS FOR THE PRODUCTION OF HIGH VACUUM, ESPECIALLY IN METAL EVAPORA- TION VACUUM PUMPS Filed Dec. 27, 1956, Ser. No. 630,944

Claims priority, application Italy Dec. 29, 1955 8 Claims. (Cl. 230-69) For some requirements in the production and preservation of high vacua, the conventional diffusion pumps do not always represent the most suitable solution, an example of this is found, in the accelerators for elementary particles, wherein it is required to avoid the deposit of oil vapours on the walls of the acceleration chamber, and this independently of any type of protection.

Apart from this, but also in the case of accelerators, in which the hermetic seal is an element of fundamental importance, the use of the diffusion pumps is not appropriate, because it is more important to preserve the attained vacuum in these cases than it is to realize high extraction capacities at a low level of pressure. A diffusion pump is highly appropriate when a limited degree of vacuum is to be attained in a short time, but not for exceeding this limit. Beyond the predetermined limit the actual requirements of the plants would be exceeded, and this to no purpose, since it is sufficient to continuously compensate the losses due to the failure of the seals of said plants.

The more recent introduction of a new type of pump in which the getter action of a metallic vapour (titanium) is combined with the pumping action of an ionic current has heretofore avoided the aforesaid disadvantage of the presence of oil vapours, and has thereby ob-' viated the necessity of using protective devices, such as traps and the like. 7

These pumps however have other disadvantages. They are extremely expensive, because they require a high frequency heating system for the heating and evaporation of the titanium, together with a mechanism adapted to feed said metal continuously into a melting crucible. For these reasons, even a pump of small capacity is very expensive. This accentuates the cost for plants of the type involved (accelerators) because such a plant generally employs a high number of pumping elements. In addition to the foregoing, these pumps have other drawbacks including, for example, the fact that when the deposit of titanium on the walls has reached a substantial thickness, it tends to scale and then fall down. Impairments and failures of the electrical circuits result.

The present invention relates to an improvement in pumps of the type having the aforesaid combined action. According to the invention, a vacuum pump of the getter metal evaporation type is characterized by the fact that the pumping action is automatically adjusted according to the value of the pressures measured by a vacuum gauge.

More particularly the pressure measuring means or vacuum gauge directly controls the heating device and also controls the supply device of the furnace of the getter through a suitable direct or indirect adjustment system. Thus when the vacuum gauge marks an increase of pressure, the adjustment system produces an appropriate increase in the rate of heating of the crucible in which the molten metal is contained. The metallic vapours are thereby formed with increased rapidity and States Patent Patented Sept. 27, 1960 ICC the rate of pumping is consequently increased. A dey crease of the rate of heating is similarly caused to occur when the vacuum gauge marks a decrease of pressure. The pumping elfect is thus caused to be proportional,

at any moment, to the pressure existing in the vacuum chamber. In accordance with the invention, the capacity of the pump may at the start, attain a high predetermined value getter action with a relatively low evaporation temperature. This obviates t-heproblems and difliculties of high temperature heating and in particular the need for a high frequency heating system, enabling a resistance heating'system to be used instead.

The single figure of the drawing diagrammatically shows in section a practical and advantageous illustrative embodiment of the invention.

The pump includes an ionization chamber I and an evaporation chamber E in communication with each other.

The evaporation chamber E can be hermetically sealed.

off from the ionization chamber I by means of a door valve 1. The ionization system is of the cold cathode type and is located-under the evaporation chamber E. It serves simultaneously as an ionization pump and as an element of the vacuum gauge.

The ionization chamber I includes the conventional combination of an emitting cathode 2 and an anode 3, the latter having an annular shape in the illustrative embodiment. The cathode and anode form a part of a high voltage generator G and produce an electric field. A magnetic field is produced by a coil 4 in a direction parallel to that of the electric field. The combined action ofthe two fields causes the electrons to travel along a long helical trajectory, and this increases the probability of their ionizing the gas molecules which arepresent. The use of such a device is particularly appropriate in the illustrative embodiment. It is known that the use of a cold cathode vacuum gauge, at vlow pressures, is restricted by the fact that the cathode tends first to harden and finally to fail when the pressure falls under about 10- mm. In the case of accelerators, however, such a high vacuum is not required, and the automatic control of the adjustment here employed prevents the attainment of such a high vacuum. The regular and continuous operation of the ionization apparatus, formed by and within the chamber I, is therefore assured. The ionic current, which is generated in the chamber I, is

initially accelerated by means of a grid 5 which collects A a portion of the current.

There is a metallic hood 6 in the upper section of the evaporation chamberE and said hood is raised to a negative potential very much higher than that of the grid 5.

The hood 6 operates as a plate for the remaining portion of the ionic flow, which exerts the pumping action.

The current collected by the grid 5, which is a function of the pressure in the vacuum chamber V, passes through an amplifier A. The amplified current affects an adjusting device R which controls the current supply to a resistance furnace 7. The furnace 7 heats a crucible 8 located in the evaporation chamber E. The adjustment takes place in such a way that the heating of the furnace 7 decreases when the pressure of the vacuum chamber V decreases and increases when said pressure increases. 7

In the generator G there is included an amplifier "and an electronic emission regulator of a conventional type, generally designated G The amplifier A is a differential amplifier of a conventional type. The regulator R includes a magnetic amplifier which is affected by the output circuit of the amplifier A.

The adjustment device R further includes means adapted to intervene-for example to act on an alarm sign-alwhen the pressure in the interior of the vacuum chamber increases beyond a maximum predetermined limit. The single means depends, directly or indirectly, upon the above defined vacuum gauge. In fact, while a decrease in the intensity of the discharge, acting on the regulator through the amplifier, decreases the heating of the furnace, an abnormal increase of the intensity of said discharge, due to an excess of gas, actuates the alarm device.

The excess of gas present in the vacuum chamber may be due to either of two causes: (1) an abrupt failure of the seal of the vacuum chamber; or (2) exhaustion of the getter metal which causes the evaporation pump to cease to operate. In the latter case, for the purpose of actuating the signalling device before arriving at the exhaustion of the getter metal, the crucible 8 for holding the metal to be evaporated-which is of porcelain, metal or other suitable material-4s formed as a frustum of a cone, tapering downward or is of an equivalent shape, such that the evaporating surface of the metal, and hence its pumping efiect, gradually decreases as the surface of the metal approaches the bottom of the crucible. With this arrangement, the intensity of the ionic current of the associated vacuum gauge increases, causing the control current to the regulator R to be increased to such an extent that the alarm device is operated before the getter metal is exhausted.

In an embodiment of the kind above described, barium is advantageously used as a getter metal, because of its relatively low evaporation temperature.

Since, in an arrangement of pumps of the kind above described, the consumption of the metal is minimized (it being known that a furnace having a charge of 50 grams may last many days), it is possible to avoid complication of the assembly of said pump, by omitting to provide means for automatically feeding the getter metal, and by providing instead for replacing the evaporation chamber E containing the nearly exhausted crucible 8 with another similar chamber E containing a fully charged crucible 8 as often as the need arises.

The substitution may be simply and rapidly effected without interrupting the usual operation of the pumping unit. In fact, the valve 1 is merely closed and in this way the space under vacuum of the chamber I is insulated and separated from the evaporation chamber E, which may therefore be removed.

The advantages offered by the invention include the following: (1) automatic adjustment of the capacity of the pump, according to the value of the measure of the pressure, through a measuring instrument (vacuum gauge) forming part of the described assembly; (2) saving and economy in the consumption of the getter metal as soon as the condition of dynamic equilibrium is attained; (3) safety deriving from limitation of quantity of the metal deposited on the walls of the evaporating chamber with a consequent reduced disability of detachment of small metallic scales or strips which may cause impairment or failure of the electrical circuit; (4) a remarkable economy in the cost of plant, including particularly the elimination of the need for expensive high frequency heating and the expensive feed system, which are indispensable for a titanium furnace; and ,(5) an indefinite extension of the useful life of the pumping unit.

What I claim is:

1. A vacuum pump of the getter metal evaporation type including in combination, a first hollow closure defining an evaporation chamber, a second hollow closure operatively connected to said first closure and defining an ionization'chamber, ionization apparatus mounted in said ionization chamber, a container to be evacuated operatively connected to said ionization chamber, a crucible of getter metal mounted in said first closure, an electrical heater mounted in said first closure adjacent said crucible, an electrical pressure responsive device mounted in said ionization chamber, and a regulator for said heater mounted externally of said closures and electrically connected to said pressure responsive device, said regulator being responsive to said pressure. responsive device, whereby the temperature of said heater is varied in accordance with variations in pressure in said ionization chamber.

2. A vacuum pump of the getter metal evaporation type including in combination, a first hollow closure defining an evaporation chamber, a second hollow closure operatively connected to said first closure and defining an ionization chamber, ionization apparatus mounted in said ionization chamber, a container to be evacuated operatively connected to said ionization chamber, a crucible of getter metal mounted in said first closure, an electrical heater mounted in said first closure adjacent said crucible, an electrical pressure responsive device mounted in said ionization chamber, and a regulator for said heater mounted externally of said closures and electrically connected to said pressure responsive device, said regulator being responsive to said pressure responsive device, whereby the temperature of said heater is varied in accordance with variations in pressure in said ionization chamber, and said pressure responsive device comprising a grid.

3. A vacuum! pump of the getter metal evaporation type including in combination, a first hollow closure defining an evaporation chamber, a second hollow closure operatively connected to said first closure and defining an ionization chamber, a crucible of getter metal mounted in said evaporation chamber, an electrical heater mounted in said evaporation chamber adjacent to said crucible, means for producing an ionized current in said ionization chamber, a grid mounted in said ionization chamber in the path of said ionized current, said grid adapted to collect current from said ionization chamber in accordance with the pressure therein, an amplifier mounted externally of said chambers and electrically connected to said grid for amplifying current collected by said grid, a regulator electrically connected to the output of said amplifier and having its output electrically connected to said heater whereby to vary the temperature of the latter in accordance with the pressure in said ionization chamber.

4. A pump as in claim 3 wherein the getter metal is barium.

5. A pump as defined in claim 3 wherein said closures are removably interconnected.

6. A pump as defined in claim 5 wherein said second closure is provided with a valve, said valve being encompassed by said first closure when said closures are operatively connected, said valve being normally open when said closures are interconnected and normally closed when said closures are disconnected.

7. A pump as defined in claim 3 wherein said crucible is of inverted frusto-conical shape.

8. A getter metal evaporation vacuum pump, including in combination, means forming a vacuum chamber; means forming an ionization chamber communicating with said vacuum chamber; means forming an evaporation chamber communicating with said ionization chamber through. an opening; valve means operable to separate said ionization chamber with a perfect seal from said evaporation chamber; a cathode and an anode in said ionization chamber connected to electrical supply means on the outside of said ionization chamber, said anode located between the cathode and said opening; a grid arranged between said anode and said opening; a

' said grid; an amplifier for current collected by said =grid,

said current varying with the pressure in said vacuum chamber, a current regulator for said furnace electrically is connected to the output of said amplifier to control the heat output of said furnace in accordance with the current collected by said grid whereby to increase the evap0.

ration of metal in said crucible with the increase of pres- 5 sure in said vacuum chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,850,225 Herb Sept. 2, 1958