US2908837A

US2908837A - Anode structure

Info

Publication number: US2908837A
Application number: US605579A
Authority: US
Inventors: Daniel B Harrington
Original assignee: Bendix Aviation Corp
Current assignee: Bendix Aviation Corp
Priority date: 1956-08-22
Filing date: 1956-08-22
Publication date: 1959-10-13
Anticipated expiration: 1976-10-13

Description

1959 D'. B. HARRINGTON 2,908,837

ANODE STRUCTURE Filed Aug. 22, 1956 POWER SUPPLY PULSE SOURCE OSCILLOSCOPE POW ER SUPPLY INVENTOR. DANIEL B. HARRINGTON BY AGENT e (mils 2398,83? Patented Oct. 13, 1959 AN ODE STRUCTURE Daniel B. Harrington,.Detro it, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich., a corporation of Delaware Application August 22, 1956, Serial No. 605,57 9

2' Claims. (Cl. 313-79) This invention relates to vacuum devices which utilize electron beams and more particularly to an improved anode structure for such devices which prevents any reduction in the devices efficiency which might be caused by anode pollution resulting from the electron beam impacting the anode.

Incertain vacuum devices, such as radio tubes, it is possible to create an atmosphere that is virtually free of undesirableelements which might pollute the 'tubes elements and otherwise reduce the efliciency of the device. Such vacuums are produced under carefully controlled conditions and are maintained through the use of getters which absorb any harmful gases which may come within the vacuum.

However, in other types of vacuum devices, such as massspectrometers, it is often necessary to either provide a vacuum' which is continuously maintained by pumping apparatus or to provide means for creating and removing the vacuum periodically. .In such devices it is extremely difiicult to remove all possible contaminants from the tubes atmosphere *When such apparatus utilizes an electron beam which is continuously or intermittently directed to a single spot on a collecting anode the beam often interacts with the impurities to create a small insulated spot on the anode surface. 7

While the spotis physically small, it may become very large in relation to the size of the electrons which are attracted to the spot by the fields present in the tube. If the spot is ofan insulating nature, its external surface will tend to assume a potential that is less than that of the contiguous anode and, as the spot continues to build up, its outer surface will eventually reach a potential which repels the electrons. V

Asthe electrons which are attracted to the anode get very close to 'the spot they see a repelling potential surface. They may then reversethemselves, and return to other parts of the tube region which have more positive potential surfaces. 'This action is not serious in devices in which the electrons are accelerated by an electric field alone since the distortion of the electric field by the anode spot causes most of the electrons to be diverted to another anode spot... But where the electron stream is prevented from moving to the side because of a collimating magnetic field, the field reversing action may seriously decrease the efficiency of the device.

In order to prevent this anode pollution and the resultant,undesirable electron action, certain devices have included apparatus for maintaining the anode. at a high temperature so that the polluting agents in theatmosphere will be less likely to settle on the anode. This apparatus, compared to the present invention, is expensive and cumbersome and of a delicate nature.

The present invention provides an anode structure that creates fields in the anode region of such a nature that as an insulated spot forms on a particular point on the anode the electron beam is automatically directed to an other uncontaminated point.

These fields consist of a magnetic field imposed at right angles to an electric'field. They act in combination with thefields that accelerate the electron stream toward. the node to give the electron stream an essentially trochoidallike path in a plane perpendicular to the normal path of the electron stream, This motion, coupled with the normal anode directed motion of the electron stream,

causes the stream to intersect the anode at an oblique angle. As the anode offers less attraction to the stream as a result of an insulating layer being built up at this point of intersection, the path of the stream is changed and the point of intersection, accordingly, moves along the anode, instead of being repelled from the low potential point. Since the contaminated spots may be very small in relation to the entire anode surface, anodes utilizing this novel structure have a life which is much greater than the anodes known tothe prior art.

It is, therefore, an'object of the present invention to rovide an anode structure for vacuum devices which employ electron streams that will obviate the inetficient conditions created by anode pollution.

A further object is to provide an anode structure which will itself cause an impacting electron stream to change its course upon the pollution of a particular area of the anode.

Another object is to provide an anode structure which will create such fields that an impacting electron stream will change its course when a particular target area of the anode becomes polluted.

Another object is to provide a vacuum device which has crossed magnetic and electric fields in the anode region whereby the incoming electron stream assumes a trochoidal' path in a direction perpendicular to the main flow of electrons, and intersects the anode at an oblique angle.

Other objects, advantages and applications of the present invention will be made apparent by the following detailed description of an embodiment of the invention. The description makes reference to the accompanying drawings in which: 1

Figure 1 is a view, partly in perspective, partly in block form, illustrating the application of the novel anode structure to a time-of-flight mass spectrometer.

Figure 2 is a schematic diagram illustrating the electric any vacuum device in which electron stream impacts an anode, it will be illustrated as it may be employed in a time-of-flight mass spectrometer.

In such a device the electron source may consist of a wedge shaped cathode 10 made from tungsten or other suitable material. An accelerating grid 12 is disposed at a relatively close distance to the cathode 10 and is provided with a vertical slot 14, the median position of which is disposed at substantially the same level as the cathode 10.

A collector, anode, generally indicated at 16, is disposed in substantial alignment with the cathode 10 and the slot .14 at a relatively large distance from the grid 12. The grid 12 is connected to the. power supply 18 so as to be provided with a potential of approximately 70 volts. The

The electrons are also acted upon by a magnetic field which is set up by pole pieces '19 which are disposed behind the anode 16 and the filament 10. This field is parallel to the normal path of the electrons. Therefore, those electrons which follow the direct path to the anode 16 are not affected by the magnetic field. However, any component of electron motion perpendicular to the motion toward the anode 16 is converted into a spiral motion by the magnetic field. The electron beam is, therefore, retained within a narrow path.

An ion forming region, generally indicated at 22, is disposed between the anode 16 and the grid 12. It comprises a backing plate 24 which is parallel to and slightly separated from the path of the electrons on their way to the anode 16, and an ion accelerating grid 26 disposed parallel to and on the opposite side of the electron stream from the backing plate 24.

A gas which is to be analyzed in the spectrometer is introduced to the region between the plate 22 and the grid 26 from a source 28. As the electron stream impacts the gas it ionizes some of the gas molecules. Those ions which are positively charged are retained in the electron stream because of their afiinity for the negatively charged electrons.

The backing plate 24 is connected to a pulsing source 30 which applies a positive voltage when it is desired to obtain analysis of the gas. This potential causes the positive ions which are retained in the electron stream to be repelled toward the grid 26 which is grounded. Those ions which arrive in the region of the slot 32 in the grid 26 pass through that slot and continue to the collector plate 34 which is also grounded. Since all of the ions have been accelerated by the same potential they will travel to the collector 34 at rates which are dependent upon their masses. Therefore, their time of arrival at the collector 34 is an indication of their masses. Suitable detecting means such as a cathode ray oscilloscope 36 may be provided for indicating the time of arrival of ion groups which represent various components of the gas being analyzed.

The foregoing structure is well known to those skilled in the art and the present invention particularly concerns the structure of the anode 16 and its associated elements.

The anode 16 comprises two major portions which are disposed at right angles to one another. A first portion 38 is disposed at right angles to the path of the electron beam generated by the cathode 10. The second section 40 of the anode 16 is disposed at right angles to the section 38 and parallel to the electron stream. A third and separate plate 42 which is disposed parallel to the plate 40 on the opposite side of the electron stream is also a part of the anode structure.

. In the absence of the

sections

40 and 42 the electron stream would impinge on a particular area of the anode section 38 and after a period of time would so react with impurities in the vacuum so as to form an insulated layer on the area of impaction. If this insulated coating were allowed to develop it would reach a high resistance and its external area would assume a low potential. When the electrons reached a point very near to the insulated spot they would no longer be faced with a high potential but would rather see the voltage of the coating. Because of the collimating magnetic field the electron stream would be unable to move to the sides adjacent to the anode spot under the influence of the distorted electric field. When a certain point was reached, because of the electrons extremely low inertia, they would reverse their direction of travel and head toward another charged region, such as the grid 26.

In order to overcome this condition, the plate 42 is grounded so as to assume a Zero potential. As shown in Figure 2 this causes an electric field 44 to be created between the anode section 40 and the plate 42.

The resultant action of the electric field 44 created between the

anode segments

40 and 42 and the magnetic field 46 created by the poles 19 at essentially right angles to this electric field is to impose a trochoidal-like motion in the vertical plane on the electrons passing through this area to the anode segment 38.

This trochoidal-like motion, coupled with the initial motion of the electrons toward the anode segment 38, causes them to arrive at the anode segment at an oblique angle. The path of the stream is also dependent upon the force imposed upon the electrons by the anode section 38. Depending on the values chosen for the dimensions and fields involved, the electron stream will initially hit the anode at some spot on either

section

38 or 40. As this spot becomes insulating, the resultant decrease in potential of the outer surface of the spot causes the force imposed upon the electron stream to vary and, therefore, changes the path of the electron stream. This causes the stream to impact the

anode segment

38 or 40 at some other clean spot.

As the new spot begins to collect an insulating layer, the path again varies and the point of impaction again changes. Thus, the beam does not hit the same anode spot for a long enough time to build up an insulated layer which is dense enough to impair the operation of the device.

The present invention is thus seen to be applicable to any vacuum device which utilizes a charged particle stream.

Having thus described my invention, I claim:

1. 'In a vacuum tube device having an electron emitter, an electron collector and a magnetic field disposed in parallel relationship to a line extending through the emitter and collector to collimate the electron beam, means for providing an electric field in a direction perpendicular to the magnetic field, the electric field being constantly applied to deflect the electron beam, upon the development of an insulated spot at the point of impact of the electrons on the collector, so that the beam will strike the collector at a clean spot.

2. -In a vacuum tube device having an electron emitter, an electron collector and a magnetic field disposed in parallel relationship to a line extending through the emitter and collector to collimate the electron beam, means for providing in the region contiguous to the collector an electric field disposed in a direction perpendicular to the magnetic field, the electric field being constantly applied to deflect the electron beam, upon the development of an insulated spot at the point of impact of the electrons on the collector, so that the beam will strike the collector at a clean spot.

References Cited in the file of this patent UNITED STATES PATENTS