US2531050A - Ion trap - Google Patents

Description

Nov. 21, 1950 R. G. E. HUTTER ION TRAP Filed Nov. 50, 1946 INVENToR. Rudolf 6fE.Hul'e/ His Horney Patented Nov. 2l, 1950 l10N TRAP Rudolf G. E. Hutter, Flushing, N. Y., assigner to Sylvania Electric Products, Inc., a corporation of Massachusetts Application November 30, 1946, Serial No. 713,243

(Cl. Z50-158) V9 Claims. 1

` This invention relates to apparatus which in its operation produces a beam of charged particles; and more particularly to the separation of a desired type of particles from undesired types. AThe invention is conceived as having primary application to ion traps in cathode-ray devices and also to mass spectrometers.

Various forms of ion traps have been devised to prevent the formation of an ion spot in the target or fluorescent screen of cathode--rayl tubes of the magnetic deiiection type. These traps separate the ions from. the electrons before the beam enters the deflection zone by subjecting the combined ion and electron beam to a relatively strong magnetic eld. The ions are not appreci- `ably deflected, but the electron beam is bent through a relatively wide angle. The neck of the cathode-ray tube, according to one prior art arrangement, is bent so that the deected' electron beam passes along the bent axis, ofthe tube after being separated from the combined ion and electron beam. This arrangement is disclosed in the U. S. Patents 2,211,613and 2,211,614 issued on August 13, 1940, to R. M..Bowie. Alternatively, the electron. stream can be maintained nearly in its original course despite the magnetic iield by means of an electrostatic deection eldjacting in the zone of `the magnetic eld, which affects both the electrons and the ions substantially to the same extent. The electric eld disposes of the ion stream by delecting it away. from anything approximating the straight axis ofthe `cathode-ray tube, and has the overall veffect of maintaining the electron beam on a course approximately coincidingV with the axis of. the tube. This arangement is also described in the Bowie patents cited above.

In cathode-ray tubes having a known form of ion trap the electron beam which is initially circular in cross-section passes through an ion-sepa.- ration eld that affects one part of the electron beam differently from kother parts because of differences `in the composite lield at various crosssectional parts of the beam, and as a consequence the initially circular electron beam tends to be,

come roughly elliptical as it leaves the ion trap.

An object of this invention is to provide an ion trap for cathode-ray devices wherein this beam distortion is minimized and image deiiection is improved. A further obiect is to provide a method of separation of electrons from ions such that a minimum of distortion of the electron beam is caused. A further object is to provide a method of producing the improved type of ion trap.

As applied to mass spectrometers, the invention j contemplates separation of the ions of .a given charge-to-mass ratio from all other particles emanating from an ion gun together, whether of larger or smaller charge-to-mass ratios. Mass spectrometers have long been known wherein ions of various charge-to-mass ratios have been projected into4 a magnetic eld that caused the various particles to described semi-circular paths of various radii. By adjustment of the magnetic intensity, particles of any vdesired charge-to-mass ratio can be collected at a given point. Another type of mass spectrometer is disclosed by Oliphant, Shire, and Crowtherin Proceedings of the Royal Society for 1934 at pages 922-929. The mass spectrometer there described involves a generally linear path for the desired. type of ion, which is achieved by impelling a .heterogeneous stream of ions from an ion gun through a transverse magnetic eld and a coextensive transverse electrostatic. field, these y lields being mutually perpendicular. All of the ions-A except those of a given charge-to-mass ratio from a-straight path are deected in one direction or another. In that spectrometer it is necessary to-adjust the direction of the gun orfocusing system for the ion beam in relation to the ion-separation iields, so that desired particles would emerge from the system at a given collection point l The linear characterization is only roughl proper since the ions enter, leave, or both enter and leave 01T the axis of symmetry of the electrostatic and magnetic elds. The prior art linear mass spectrometer depends on achieving an overall balance between the tendency to deflection of thebeam components into a curved path by the magnetic eld, and a correction of that curvilinear tendency for ions of a given charge-tomass ratio by means of the electrostatic eld. That overall balance is denitely not a point-topointbalance, with these two difliculties resulting: First, the initially sharp focus of the desired beam is impaired; secondly, the gun must be so directed in relation to the separation-field tube as to introduce an error compensating for the non-linearity of the linear" mass spectrograph.

It is accordingly a further object of the present invention to provide an improved linear massspectrometer. A more general object of the invention is to provide an improved system for separation of particles of a given charge-to-mass ratio from others.

In the illustrated embodiment of mv invention a beam formed in a gun and containing both .iQllS and letrons in a cathode Vray device, or

containing ions of various charge-to-mass ratios in a mass spectrometer, is passed through a composite electrostatic and electromagnetic field. The joint effect of the electric and magnetic iields on particles of a given charge-to-mass ratio when established in accordance with this invention is not to deect them appreciablyat any point along their straight-line path through thel separation fields. The discovery that s-uch a system is possible is an underlying feature of the present in.- vention, and the system itself yields notable advantages in both the mass-spectrometer and`in the cathode-ray device.

The amount of deflection of' a beam ofcharged particles depends on the charge-to-mass ratio of the particles in the case of magnetic deflection but is independent of this ratio in case of 'electro-w static deiiection. By establishing a magnetic and an electric iield in sucha way that the deflection of certain particles at all' points along the axis of symmetryofthe. systemis zero,` there will be a minimum of dispersion or transverse distortion of'the'beam which is'to be isolated; The objectionableY particles are nonetheless d'e'ected.

A- better understanding' of'the invention together with' its further objects and" features will be gained' from the following' detailed disclosure ofv a specic embodimentthereof;.and from the drawings wherein:

Fig. 1 is a cathodeeray tube'to which my invention is applied;-

Fig. 2 is a diagram indicating the'operation of a prior-artform of iorrtrap;

Fig. 3 is a4 graph-of the'transverse magnetic 'eld'strength of an ion-trapmagnet'as it isu distributedalong'the'axis'of aY cathode-ray tube to whiclrthetrap is'applied: w

Fig; 4 is-V a' diagram` similar to Fig. 2' showing theoperation oiv an ion *trapl improved'in accordL ance `with the present invention: and

Fig; 5' isan enlarged" perspective View of' the improved' ion trap which is also Ashown in4 Fig. 1.

The invention` is illustrated as applied to a cathode-raytelevisiontube. The envelope of'the cathode-rayV tube in Fig. 1 includesa cylindrical portionIU an'd'a funnel-shapedportionv I2, the wide end of`vl which constitutes the-image area'. Within `cylindrical portionvl I0 is an'assembly I4 for generating and focusingan electron beam.

This assembly commonly termed al gun is suit ably energized at terminals IS'on the base I8`of the cathode-ray tube. All the particles of the beam are accelerated tothe' extent measuredby the-gun'potential. Ion trap'ZDii'spositioned be-` tween gun I4 and deflectionsystem22 (schemati callyl illustrated)` which' deflection systemY is locatednear the junction'of'portions I0' and I2 of the envelope. The electron beam when suitably deflected'produces an image on screen'24, conventionally ofuorescent material. This invention is applicable to cathode-ray tubes having any other kind of 'target 'likelyy to be'deleteriously af` :tectedbyions The ioncbeam; deiiectedfrom the electronl path, impinges against 'apertured metal disc'25;

Ion trap comprises an iron core 26 and-iafcoil 28 outsidefthe envelope fwhich are-energized from a suitable D.CI supply 36 through rheostat 33, andareshapedf to provide a\ heldv symmetrical about the -e plane. A' pair of plates 32 and 34 aresupported insidef'theenvelopein the gapof core 26. These plates are so shaped'and oriented as-toproduce an' electrostatic field symmetrical about the @1 -z planeof the cathode-ray tube and generally parallel to the a: axis. The energizing potential for the plates is supplied by D.C. source 36 and by adjustable contacts 38 and 40 on potentiometer 42.

It is desirable, though not absolutely essential, that the ion trap be located in a zone of uniform potential. This simplifies design, as will be gathered from the specific discussion which follows. In order to provide this 'uniform-potential field and for other known reasons, the internal surface of the cathode ray tube is coated, except in the zone of gun I4 and at its screen, with a conductive material such as aquadagf The potentialsof plates 32 and 34 advantageously are above and below the potential of this coating, and to this end..terminal 44 of the coating is connected to the center tap 46 of potentiometer 42.

To the extentdescribed, the cathode ray tube is entirely conventional, being much the same as that' in Fig. 5 of the Bowie patents mentioned above. It is the purpose in this prior-art type ofion trap to maintain a substantially-straight path for the desired type of'particles. It is` my conclusion, however',v this desired linearityA has been merely an overall consideration. 'I'hus (see Fig.A 2' herein illustrating ay prior' art ion trap) an electron beam emergingv from gun 50 might ultimately reach a part* ofv target 52 whichis along the axis of the gun', after passing through the composite electric and magneticheld of electrostatic plates 54, 56, and'of poles 58 of 'an elec-` tromagnet. However, due to'edge e'ects of the poles and plates the electronbearn departsl from the true axis of` thek system, and" travels off the axis of symmetry of the composite electric and magnetic field. In Fig; 2 the'heavy dashedline indicates a probable path of electrons, while 'the dot-dash line represents the axis'of symmetryjof thegun andthe iields: The path of the ionsis omitted. to avoid confusion.

Along that part ofthev axis of A'thecathode'ray tube which is opposite rectangular poles 58'ofthe magnet the fieldV may be taken' as nearly uniform and of relatively highintensity. Outside the edges of pole pieces 58 and along' the axis'tliere l" is a magnetic field which tapers in intensity but which nevertheless has an important effect on'the electron path. There is asimilarly slight exten.- sion of the transverse electric 'eld. along the axis of the cathode ray tube beyond the edges of plates 54and 56. As a result,. while there is anoverallv approximate balance between the effects of the magnetic eld'and ofthe electric eld on the electron beam, this beam is deliected. fromr the axis of the tube. somewhat as indicated, and travels through an asymmetric part of theeldl It is a known fact thata` beam. is distorted..when traveling in this manner. A beam which is circular in section ron entering the gun may become elliptical or flat after traveling asymmetrically through an ion trap. Where the beamY travels a long, the axis of symmetry ofthe composite field, the distortion is at a minimum.

The remainder ofl this specification is. devoted to the development of suitable means for producing optimallelectric andmagnetc fields to min'- imize beam distortion by balancing the'eiectsof those fields on the desired'particles, electrons in the case ofthe cathode ray tube, so that there will be zero deflection at. each point along, the axisof the tube.

Inth-is ideal ion trap therconditionsought. to, be achieved, is that the eiect of'l the electric eld on electronsof'charge en havinga mass' Mo' at every point along thezaxis of the cathode-ray .75 tube, and where the electrons are accelerated to a potential qs, shall be the same as the effect of the transverse magnetic field at vthat point but opposite in sense. Expressing this mathematically, we have the following equation:

:v 2611 (1) EU)` M OOPHZ) Thus at each point along the axis (z), the electricV (E) and the magnetic (H) field strengths are to be related to each other by a constant. There remains the problem of devising magnetic poles and electrostatic electrodes which shall have mutually compensating effects on an electron beam of uniform speed entering along their axis of symmetry. With respect to the ions, which have other ratios of charge-to-mass, the deflection required for separation will still be achieved, and the deleterious effect of the ions on the screen is avoided as in the prior art.

In Fig. 3 there is shown the distribution curve of the magnetic field strength of a pair of unsaturated rectangular pole pieces 60 (Fig. 4) forming part of the improved ion trap, from point to point along the e axis of the cathode-ray tube. The separation between the magnetic poles in this example is equal to the extent of the pole facing along'the axis of the cathode-ray tube and is four centimeters. From the foregoing discussion of the electric field in relation to the magnetic eld, it willV be apparent that the distribution of the transverse electric field along the axis should have a 4curve like that in Fig. 3. A field of this desired distribution will result when the electrostatic plates of an ion trap are made the same in spacing, size, and shape as the poles and are charged to the proper potential difference, provided that the related magnetic poles are not saturated. Such a pair of charged electrodes cannot in practice be associated with the magnetic poles of the same shape in an ion trap because the poles would affect the electrostatic field, because prohibitively high voltages would be required, and because the poles are preferably mounted outside the cathode-ray tube whereas the electrostatic plates preferably are mounted internally.

It is possible to use electrodes shaped like pole pieces in order to determine the necessary shape of the electrostatic plates of practical dimen-i sions, that is, narrow enough to fit within the cylindrical portion of the cathode ray tube and between the magnetic poles. By immersing the electrodes shaped like the poles in an electrolyte and establishing an A.C. voltage between them, it is possible to plot numerous unipotential surfaces constituting the field pattern. The electrodes used in making these unipotential plots may be replaced by other electrodes having the configurations of unipotential surfaces, insofar as the field pattern between them is concerned. Equipotential plots on opposite sides of the axis of symmetry can be used as the model for shaping the desired electrostatic plates. Since the plates are close to each other they may be made narrower than the widely separated electrostatic replicas of the poles. Where'the plates diverge, as they do under the practical requirements of the improved ion trap, the equipotential plot is not a perfect model since the plates cannot be as wide as the equipotential surface; they are narrow enough for enclosure within the cathoderay tube and taper as they curve toward the cylindrical envelope portion I0 (Fig. 5). Allowance for these factors may be realized either by com- 6 putation or by cut-and-try shaping and testing, as in an electrolytic plotting tank.

Where the pole-pieces are significantly saturated, they cannot serve as models for the electrolytic plotting procedure described above. The lines ol magnetic force should instead, be plotted directly and from these lines a pattern of equipotentialmagnetic lines can be derived by known techniques. The equipotential lines can be extended laterally of the plot to constitute surfaces which will serve as models for the desired electrostatic surfaces. The shapes of plates so determined are correct for only the one degree of pole-piece saturation used in their design, so that significant saturation is preferably avoided if practical, unless constant field-strength can be maintained during use. l

It was mentioned above that the problem of shaping the plates is simplified where the ion trap is to operate in an electrostatically equipotential space. Just as in the case of the narrow diverging portions of the electrodes it is possibleV to allow for non-uniformity in the ion-trap space by critical shaping of the electrostatic plates, checked by computation or by electrolytic plotting or both. However, unipotential spaces may be found both in the electron gun and between the gun and the deflection system, and therefore the allowance for ambient non-uniformity of electrostatic field is not of great concern.

When electrodes are shaped so as to compensate for the tendency of the magnetic field to deflect the electrons at all points in the zone of the ion trap, both between the poles and in the fringe region, a pair of closely spaced and nearly parallel plates generally result for the region directly between the poles, and these plates diverge in regions outside the poles, whether the poles are saturated or not. A typical assembly utilizing rectangular-faced poles is shown in Fig. 5, and in Fig. l on a much smaller scale.

The desired beam rmay be maintained on ythe initial course by plates and poles properly shaped in relation to each other using another approach: the electrostatic surfaces can first be arbitrarily chosen, and the shape of the magnetic system can then be found which is required to compensate, at each point along the straight-line course, for the tendency of the electric field to defiect the electrons. This is an equivalent procedure to the one discussed in detail above, but is-by far the more difficult from a practical standpoint.

`As a compromise, the pole-pieces may be judiciously shaped to simplify the problem of shaping the plates.

In operation of the cathode-ray tube it is necessary to charge the ion-trap plates to a voltage difference which is properly related to the magnetic field strength. The respective potentials of the electrostatic plates may be determined from the related equipotential plots, with the equation given above.

The foregoing discussion on the shaping of electrodes to achieve true linearity of the electron path in a cathode-ray tube also applies to linear mass speotrometers. By properly shaping the electrostatic pates in relation to the pole pieces, it becomes possible to have a truly linear mass spectrometer. The necessity for providing an adjustable coupling between the gun and the composite electric and magnetic field structure can thus be avoided, and the spectrometer can be adjusted for separation of any desired ion simply by adjusting the charges on the electrostatic plates or the magnetic field strength or both.

- Having Vthus-y described. a. specic embodiment of my invention and what I believe to be the operative/principlesthereof, what I claim is:

l. The method. of separating desired particles of a giVenCharge-to-mass ratio from a composite beam. containing particles of other ratios of charge tol mass which comprises the steps. of subjecting the composite beam. to magnetic field generally transverse of the initial axis of the beam and electrostatically substantially nullifying the deflecting' inuence ofk the transverse componentof said magnetic field on the. desired particles at each point along the axis.

2. The method of separating desired particles of a given charge-to-mass. ratio. from a compos.- ite beam containing particles of. other ratios of charge to mass which comprises the steps of subjecting the composite beam to a magnetic eldtransverse ofv and symmetrical to. the initial axisof the beam andsubstantially nullifying the deiiecting force ofthe transverse. component of said. magnetic eld. on the desired particles at each point along said axis witha transverse electrostatic eld symmetrical about said axisand at right angles to said field.

3. Apparatus for separating particles of a given type from particles of another type comprising common.- means for accelerating both types of particles andforming a beam thereof, and mutually perpendicular pairs of elements for establishingelectric and magnetic fields transverse to theinitial course. of said beam, the elements for establishing the electric iield being shaped substantially in accordance with an equipotential surface of saidl magnetic field.

4. Apparatus according to claim 3 wherein said electric elements lie between said magnetic elements.

5. Apparatus according to claim 3 wherein said electric elements are narrow plates supported edgewise between said magnetic elements, and have extensions diverging outside the gap between said magnetic elements.

6. An ion trap for cathode-ray tubes and the like which comprises an electron gun, a deflection system, and an ion trap all arranged along a common. axis, said ion trap comprising means for producing a magnetic eldtransverse to said axis which varies along said axis, and means for producing a transverse electrostatic field having substantially the same variation along the axis.

.7. An instrument for separation of. charged particles according toV their relativev charge-tomass ratios comprising means for producing a beam of charged particles, means symmetric about the initial course of said beam for establishing a magnetic eld transverse to said beam, and means for maintaining an electrostatic eld transverse of the initial course and varying in strength from point to point along thecourse in such manner as substantially to counterbalance the eieot of said magnetic field on particles of a given charge-to-mass ratio, whereby particles of the given ratio will travel a linear path from said Source through the separation system.

8. Means for establishing mutually perpendicular electric and magnetic elds jointly causing no appreciable deflection of charged particles. of a given ratio of charge to mass at each point along alinear path comprising a pair of poles symmetric about the, path and a. pair of' electrostatic plates symmetric about said path, the central portions of the plates being generally parallel and the end-portions in the direction of the path being divergent.

9.. The method of making a tube for separating undesired charged particles from a beam containing desired particles by means of mutually perpendicular electric andv magnetic fields established symmetrically about an ideally linear path of the desired particles comprising. the steps of providing a magnetk proportionedto embrace said tube symmetrically, determining the iield` pattern corresponding to the shape of said magnet, shaping a pair electrostatic plates essentially according to. equipotential lines on opposite side of the plane of symmetry of said eld, and arranging said plates and said magnet in relation to said tube to yield elds as aforesaid.

RUDOLF G. E. HUTTER.

REFERENCES CITED The followingv references are of. record inthe le of this patent:

UNITED STATESI PATENTS Number Name Date 2,211,613 Bowie Aug. 13, 1946 FOREIGN PATENTS Number Country Date 498,491 Great Britain Jan. 9, 1939

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