US2935634A - Ion source - Google Patents

Description

May 3, 196() Filed June 13, 1957 A. LERBS ION SOURCE 2 Sheets-Sheet 1 @www ALF/P62) L ERES ATIOFNEK A. LERBS May 3, 1960 10N SOURCE 2 Sheets-Sheet 2 Filed June 13, 195'? ION SOURCE Alfred Lerbs, Frankfurt am Main, Germany, assigner to Compagnie Generale de Telegraphie sans Fil, Paris, France The present invention relates to an improved source of monokinetic ions. More particularly, the present invention relates to an improved source of monokinetic ions in which all the elements of the ion beam have exactly the same speed. The attainment of a beam of ions of which all the elements have strictly the same speed is particularly important in certain applications, such as accelerators, mass spectographs or isotope separators.

Ion sources are already known in the prior art which are designated in the literature under the name of Sources of Dempster-Nier. In these last-mentioned sources, the ionization results from'the impact between the electrons of a beam which is focused by a magnetic iield and the molecules of a gas which till an enclosure forming the ionization chamber. An electric iield perpendicular to the direction of propagation of the electrons enables the evacuation or extraction of the ions in these prior art devices.

ln the ion sources of this type, each electron is suscep tible of successively encountering several gaseous molecules. Since such electron loses a little of the energy thereof with each impact, the ions which result from such impacts have necessarily different speeds.

The present invention makes it possible to avoid these inconveniences of the prior art and enables the attainment in particular of ions having rigorously the same speed.

The ion source according to the present invention comprises in combination au electron optical means with crossed fields which allows the obtainment of a linear electron beam and an ionization space with crossed iields from which the ions are extracted by the intermediary of a slot or hole provided in the negative electrode.

According to one embodiment of the present invention, the ion source comprises two parallel electrodes which may be either planar or curved, and which are carried at diierent potentials in such a manner than an electric field exists therebetween `at all points perpendicular to these electrodes, while the space formed between the two electrodes is additionally subjected to` a magnetic eld perpendicular to the electric lield; a cathode emitting electrons; electron-optical means enabling the realization of a laminar beam of electrons, to introduce the same into the space defined hereinabove with a perfectly determined level, adjacent the electrode having the higher potential, with the desired speed, the value of the electric and magnetic fields being additionally chosen and selected in such a manner that the electron beam is propagated in this space in parallel to the two electrodes and perpendicularly to the electric and magnetic ields with a speed equal to the desired speed, and to maintain essentially the linearity of the beam in the course of such propagation, the electrode having the lower potential being provided with a longitudinal slot to evacuate or extract the ion beam, means being additionally provided to lead the gas to be ionized' into the' space between the two electrodes.

According to a preferred embodimentl of thepresent 2,935,634 n Y, Fatented Maty 3,

- 2 invention, the ionization space is provided with a supple mentary electrode which may or may not be an exten-A sion oi one of the electrodes of the electron-optical system, and which has a form and is carried at such a po-Y tential that it favorably etiectsat the same time focusing of the electrons and those of the ions. y

Accordingly, it is an object of the present inventionto provide an ion source which is greatly superior to those ot the prior art. n Y, l

It is a further object of the presentinvention to provide an ion source in which the ions produced therein have rigorously the same speed. y y

A still further object of the present inventionis to provide an ion source of simple structure which includes as few as possible electrodes and which, nevertheless, provides completely adequate electron-optical means.

A still further object of the present invention is to pro vide an ion source in which essentially the entire beam of electrons is collected, i.e., in which the losses of electrons due to absorption by the electrodes is minimized, thereby permitting the use of a high intensity electric field.

Still another object of the present invention is to provide a system of ion source which operates at higherv efliciency. Y v V- y A further object of the present invention is to provide a strncturefor an ion source in which the ions produced therein are completely or essentially completely extracted in a proper manner. l n y These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing, which shows, for purposes of illustration only, several embodiments in accordance with the present invention and wherein: Y

Figure 1 is a cross-sectional view taken along line 141 of Figure 2 showing a prior art ion source;

Figure 2 is a cross-sectional view taken along line Il II of Figure 1 of the same prior art ion source;

Figure 3 is a cross-sectional View taken along line Ill-Ill of Figure 4 of an ion source in accordance with the present invention; v

Figure 4 is a cross-sectional view taken along line IV-IV of Figure 3; M l

Figure 5 is a cross-sectional vie'w similar to Figure'fl of another embodiment of an ion sourcev in accordance with the present invention; Y

Figure 6 is a cross-sectional view similar to Figures/1 and 5 of still a further embodiment of an ion source in accordance with the present invention;

Figure 7 is a partial plan view showing the electrode/21 of Figure 6; v Y

Figure 8 is a cross-sectional view, on a smaller scale, similar to Figure 3 of still another embodiment of an ion source in accordance with the present invention;

Figure 9 is a cross-sectional view similar to Figure 8 of still a further modification of an ion source in accordance with the present invention; Y l

Figure 10 is a cross-sectional view taken along line X--X of Figure l1 of still a further embodiment of an ion source in accordance with the present invention; and

Figure l1 is a crossl sectional view taken along line Xi--Xi of Figure l0. Y

Referring now to the drawing, wherein like reference numerals are used throughout the various viewsto designate like parts, and more particularly to Figures 1 and 2 thereof which illustrate an ion source of the prior art of the type most commonly used heretofore, namely a source of Dempster-Nien schematically illustrated therein, these two iFigures 1 and 2 illustrate such an ion source of the type which comprises various elements .disposed at the interior of an envelope (not shown), into which is introduced a gas under weak pressure, for example, of the order of 10-2 mm. of mercury.

More specifically, of these elements, reference numeral 1 designates a cathode which emits an electron beam 2. Electrode 3 is a focusing electrode while electrode 4 is an accelerating electrode. A collector elecfrode 5 is disposed opposite electrodes 3 and 4, while an electrode 6 which is placed in front of collector electrode 5 is intended to impede the secondary emission from the collector 5 which might trouble the operation of the ion source. The electrodes 3, 4 and 6 are provided with apertures extending therethrough permitting the passage of an electron beam which is essentially cylindrical and linear.

The planar or Lfiat electrodes 7 and 8 arranged essenttially perpendicularly to the electrodes mentioned hereinabove define the ionization space. Electrodes 7 and 8 permit the establishment in this ionization space of an electric field suitable for the extraction of the ions. The electrode 9 which is disposed essentially parallelly to the electrode 8 permits the establishment of an electric 'field between these two electrodes S and 9 which accelerates the ion beam. The electrode 7 is provided with van aperture 10 for the introduction of the gas to be ionized. The electrodes 8 and 9 are provided with apertures 11 and 12 respectively for the evacuation of extraction of the ion beam. A magnetic field B which has the sense of'the theoretical direction of propagation of the electron beam 2 serves to focus the latter.

` The operation of such an ion source is as follows:

. The electrodes in the course of the passage thereof through the ionization space defined by the electrodes 4, 7, -6 and 8 encounter and impinge on molecules of Vthe gas and thereby produce the formation of positive ions which are displaced in the direction of the electric field existing between the plates 7 and 8. The ions produced thereby are evacuated by the slots 8 and 9 toward the utilization apparatus.

, Such a source of ion as shown in Figures 1 and 2 offers numerous inconveniences and disadvantages, as

follows:

`(l) It utilizes a high number of electrodes at different potentials;

(2) The elements of the electron-optical means are insufficient. Measurements in the laboratory, in the absence of gas to be ionized, have indicated that only approximately fifteen percent of the electrons emitted by the 'cathode 1 may be recovered on the collector 5. This indicates what considerable portion of electrons is `finally lost;

(3) The combined action of the electric and magnetic fields imparts to the electrons a slow helicoidal movement having a certain transverse component. A tendency Vresults therefrom for the entire beam to deviate from its initial direction of propagation which renders problematical the passage of the electrons through the aperture provided in the plate 6. Ordinarily, the importance of the helicoidal movement of the electrons is diminished by reducing the intensity of the electric field which exists between the plates 7 and 8;

(4) The reduction of the intensity of the electric field, for reasons pointed out under 3, increases the harmful action of the magnetic focusing field on the ion trajectories or paths. As a matter of fact, the ion paths are curved or bent thereby in the directions perpendicular to the slots 11 and 12. The ions, instead of passing through the apertures 11 and 12, run the danger of impinging upon the electrodes 8 and 9;

(S) The helicoidal movement of the beam 2 even though of weak `amplitude is responsible for the fact that ,the electrons of the beam are not all at the same potential and, therefor, that the ions produced thereby do not have the same kinetic energy. The ions thus produced, therefore, do not have the same speed. Furthermore, an electron may produce several successive ionizations. Since the energy of the electron diminishes with each impact, it is not possible to have a beam of monokinetic ions:

(6) The presence of the electrode 6 which is negative with respect to the collector 5, indispensible for reasons indicated hereinabove, is a cause of instability inthe operation of the source.

Consequently, the output of tlie ion source Dempster- Nier is particularly disastrous and the ions produced thereby have different speeds.

According to the embodiment of the present invention shown in Figure 3 and Figure 4, three electrodes 20, 2l and 22 are disposed essentially in parallel. The electrode 26 is provided with an aperture on which is fixed a tubulure 23. The electrodes 21 and 22 comprise narrow longitudinal slots 24 and 25 of which the axes are parallel and which are situated or located in the same plane of'symmetry of the system. The electrode 26 forms a unitary or integral piece with the collector 26. The electrode 21 is extended by a small portion 27 forming an essentially right angle with the electrode 21 and is thereupon further extended by a second portion 28 forming again an essentially right angle with the portion 27. A trough-shaped recess is provided in the portion 28 of which the cross section is seen in Figure 3 and which is designated therein by reference numeral 29. Along this trough-shaped recess 29 extends a cathode 30 which emits an electron beam 31 propagating in the ionization space which itself is defined by the electrodes 20, 21 and 26, respectively. An electrode 32 which serves as accelerating and focusing electrode is provided in the extension of electrode 20 but electrically insulated therefrom and disposed to face the electrode 28.

The assembly is disposed in a tight enclosure. of potential 33 enables to carry the different c at the necessary potentials. 30 being chosen as referenc and 26 are carried at a pot hundred volts. The accele 32 is carried at a potential order of magnitude as the electrodes 21, 27 and 28 whi are placed at essentially the 30, while the electrode is strongly negative. A tion, which may be eith or electromagnet type,

A source ectrodes The potential of the cathode e potential, the electrodes 20 ential of the order of several rating and focusing electrode which is less but of the same electrodes 20 and 26. The ch may all be interconnected saine potential as the cathode 22 is carried at a voltage which magnet of any suitable construcer of the permanent magnet type and of which the pole pieces are schematically indicated by reference numerals 34 and 35 in Figure 4, provides a magnetic field B perpendicular at the same time to the electric field which exists in the ionization space by the fact of the difference of potential between electrodes 20 and 21 and to the direction of propagation of the electron beam 31 which is established after the electron beam has described a half arc of a cycloid under the action of the electric and magnetic fields in the space between the electrodes 32 and 28. The electrodes 27, 28 and 32 form the electron-optical means of the ion source.

It is known that when the electrons propagate in the space in which there exist electric and magnetic fields crossed with respect to each other, such electrons possess cycloidal paths or trajectories. The speed of movement of drift thereof is U0=Z B i.e., proportional to the ratio of the intensities of the electric eld to the magnetic field. It is also known that the form of the trajectory or path of the electrons, and therewith, the amplitude of the relative movement, depends on the speed and direction of the electrons vat the moment when they enter the space in which the homogeneous electric and magnetic fields prevail. vIn particular, if the ele'ctrons of the beam arrive at point 36 with a'Jspeed and with a. direction perpendicular at the same time to the magnetic field B and to the electric field E, then the amplitude of relative movement of the laminar beam of wherein B is expressed in gausses, V in volts, and D and d in centimeters. The distances D and d are indicated in Figures 3. D is the distance between the positive electrode 20 and the equipotential zero which coincides with the electrode 2.1 if the latter is atthe potential of the cathode Sii, which is the case indicated herein. V is the potential of the electrode 20 with respect to the cathode, and d is the distance of the beam 31 to the electrode 20. The potential of the equipotential line of the ionization space along which propagates the electron beam is given by the following equation:

V: 1 -DX V (2) According to the invention, the ratio d/D is selected relatively small, for example, at a value less than 0.5. The values of the electric and magnetic yfields and the voltage of the electrode 32 are thereupon calculated in such a manner as to have a laminar beam of electrons traversing the interaction space `in parallel to the electrode 2u precisely at the distance ai therefrom and with the y speed attained in the plane r36. Otherwise stated, the value of the magnetic field is not any random value. It is furnished by the Equation l if the choice of the intensity of the electric field and of the distance d separating the electron beam from the positive electrode has lbeen preselected. Of course, it is also understood that the value of the electromagnetic eld B may be fixed a priori. In this case, the intensity of the electric field Iis controlled to obtain the desired distance Typical values which have been determined "experimentally and which are given for purposes of illustration only are as follows:

V=150 volts B=53 gausses D=0.5 cm.A

V0=90 volts A high value of the electric field is interesting for purposes of accelerating the ions and to make them leave the ionization space as rapidly as possible.

The functioning of the ion source of the present invention as illustrated in Figures 3 and 4 is "as follows:

The gas to be ionized is introduced under weak pressure into the ionization space through the tubulure 23. The molecules ionized by the impact with the electrons are accelerated by the electr-ic field, in particular, by that which exists between the `electrodes 21. and 2,2, and a beam of ions is collected in the workingspace 37 disposed below the electrode 22 after traversal through the slots 24 and 25 of the electrodes 21 and 22, respectively.

The magnetic field is perpendicular -t`o the ,path -or trajectory of the electron beam. The value thereof vis so chosen as explained hereinabove that vthe trajectory or .path of the beam is established at a desired distance d from the electrode 20 which is relatively small. It is quite obvious that such a condition cannot be rigorously realized except to the extent that the laminar beam does not possess too large a thickness. This is the reason why a linear filament is used which extends all along the electrode 23 following the dimension thereof parallel to Figure 4 but having only av slight cross section 30 as shown in Figure 3. If a cathode were used which has a sufficiently large size inthe direction parallel to the electrode 28 according to Figure 3, the conditions indicated hereinabove, in Equation 1 could not be realized except for the electrons emitted by a portion of the cathode, for example, by the central portion thereof. These conditions, however, could not be realized any more for the electrons emitted by the peripheral parts of the cathode. Cycloidal electron paths would result therefrom which would oscillate about a plane or flat path.

The theory which has been confirmed by experiments indicates that the arrangement according to the present invention offers, with respect to anion source Dempster- Nier multiple advantages of which a few only are indicated hereinbelow:

(a) The electron path or trajectory 31 traverses the interaction space parallel to and in direct proximity of the positive electrode 2G; as a result thereof all the ions produced pass through the slot 24 with the same speed; an electron which participates in a phenomenon of ionization by impact with a gas molecule has lost a part of the energy thereof. Its speed, therefore, is reduced. it no longer finds itself on the electrical equipotential line which would correspond to a linear path; the linear reference path thereof is displaced toward the anode 20 by a value proportional to the energy which it has lost. lt commences immediately thereupon a Cycloidal trajectory and since it is in proximity of the positive electrode 20 it has the greatest chances of being also collected by the electrode 20 without having had time to ionize a second gas molecule. The same reasoning is valid, a fortiori, for the electrons liberated during the course of ionization. One may, therefore, operate with a stronger current Without reducing the stability of the phenomenon if discharge; the proximity of the anode 20 from the electron path 21 greatly facilitates the control and adjustment of the electron-optical means. It sufiices to separate for this adjustment the collector 26 from the anode 2t? and to search for the maximum current on the collector in the absence of ionization;

`(b) The number of electrodes at diEerent potentials is reduced. v

(c) The magnetic field still acts on the ion paths by bending the same, however: the magnetic field has a great- 'ly reduced intensity, for example, approximately fifty gauss, to that which it necessarily would have to have in an ion source of Dempster-Nier where intensities of the magnetic Ifields are used ordinarily comprised between and 250 gausses. The influence of the magnetic field is, therefore, less noticeable in the ion source according to the present invention. It is the more so since the electric field has a much more relatively elevated intensity than in an ion source of Dempster-Nier; since the magnetic field is directed perpendicularly to the plane of Figure 3, 'it curves the ion paths or trajectories in the plane of'this figure, i.e., in parallel -to the slots 24- and 25 and not perpendicularly thereto. This is such that the ions may still pass through the slots Without the risk of being collected by the electrodes 21 and 22. The effect is furthermore less important and the discrimination of mass which results therefrom is practically negligible;

(d) The electron beam 31 being of relatively little thickness, all the electrons which compose the beam may have rigorously the same speed;

(e) The electric field has an intensity With respect to the magnetic field which-is considerably superior to what it was in an ion source Dempster-Nien The evacuation of the ions is facilitated thereby, and the influence of the magnetic field on the ion paths or trajectories diminished;

(f) The danger of secondary emission produced by the collector does no longer exist since it is united with the positive electrode of the tube. Consequently, the stability of operation of the device is greatly increased.

In conclusion, the ion source according to the present invention offers the following advantages: important ionization rate, that is, a high ratio of number of ions produced by the source to the number of molecules of gas consumed during unit of time. Consequently, a slight consumption of gas to be ionized is had; high electric output. The electric output is defined as the ratio of intensity of ion beam to the electric power applied to the source. All the electrons emitted by the cathode attain or reach the ionization space; weak dispersion of ion speeds resulting from linear paths of the electrons; simplicity of manufacture; great stability in operation; and increase in the pumping speed as a result of the first point mentioned hereinabove.

Figures 3 and 4 indicate only one embodiment of a source according to the present invention. The other figures described more fully hereinafter show different improvements of the combination defined hereinabove in the form of arrangements and lay-outs susceptible to be added to the form of the electrodes of ythe system and capable to increase still further the remarkable properties mentioned hereinabove.

Figure 5, which is a cross-sectional view similar to Figure 4, shows an improvement with respect to Figure 4 which consists in providing7 the electrode 21 with edge portions 38 and 39. The equipotential lines of the electric field which prevail in the spaces 20-21 and 21-22 have been indicated in this figure in dashed lines, these spaces being defined between the electrodes 20 and 21 and between the electrodes 21 and 22 respectively. It is quite clear from Figure 5 that with the respective polarities of the different electrodes in this embodiment, the existence of the edge portions 38 and 39 favor the focusing of the electrons in the space 20 and 21 and the focusing of the ions in the space 21 and 22.

According to the improvements shown in Figures 6 and 7, the electrode 21, instead of being provided with a single longitudinal slot provided therein which extends in parallel to the direction of the electron beam, is provided with a series of parallel longitudinal slots. The separations 40 between the successive slots 24 are realized by suspending or stretching wires or with the aid of narrow blades, for example, made of graphite. The electric field which exists between the electrodes 20 and 21 in this embodiment is much more homogeneous and much more suitable to appropriate focalization of the electron beam. By utilizing blades 40 having a height which increases from the center of the electrode to the edges thereof, an electrode 21 may be obtained which presents an essentially plane upper face and a lower face which is cylindrical as shown in Figure 6. Thus, the electrode 21 has a concave surface essentially circularly shaped in the plane defined by the crossed electric and magnetic lines of force. The distribution of the equipotential lines of the electric field existing in the ionization space between the electrodes 20 and 21 is such that a larger electron beam may be utilized together with excellent focusing thereof, This permits a more dense ion beam. The blades 40 do not impede the passage of the ions and the form of the surfaces with respect to the electrodes 21 and 22 favors focusing of the ion beam at the level of the aperture 25 provided in the electrode 22.

Figures 8 and 9 show two other improvements of an ion source in accordance with the present invention which distinguish themselves from the embodiment of Figure 3 by the form and construction of the electron-optical means. In connection with the construction of the electron-optical means illustrated in Figure 3, the electric field at the level of the cathode is relatively weak as also l8 the electron current, as a consequence thereof. For certain applications, it might well be desired to have higher intensities of the beams. The constructions and shapes of the electron-optical means illustrated in Figures 8 and 9 permit to satisfy such exigencies.

In Figure 8, the extension 41 of the electrode 21 in which is arranged the trough-shaped recess 29 accommodating the filament cathode 30 is so arranged obliquely with respect to the electrostatic lines of force as to approach the electrode 20 instead of deviating or turning away from the electrode 32 as was the case in Figure 3 in which electrode portion 27 served this purpose. The optical effect is obtained principally by the orientation at the departure of the electron beam.

In Figure 9 the trough-shaped recess 42 receiving the cathode filament 3f) is lodged in the extension 43 of the electrode 22 which is obliquely positioned similarly to extension 41 of electrode 21 and placed at a potential very negative as indicated hereinabove. The extension 41 of the electrode 21 is provided with an aperture 44 of any shape, appropriate for that of the electron beam 3i. The electrode 41 serves as focusing electrode and accelerating electrode of the electron beam 31. This electronoptical means is particularly appropriate for the extraction from the cathode 30 of currents yof high intensity.

Figures 10 and 11 finally illustrate a preferred embodiment of an ion source according to the present invention, curved in the shape of an arc of a circle. Instead of comprising two plane or fiat electrodes 20 and 21 as in Figure 3, the embodiment illustrated in Figures l0 and l1 comprises two electrodes 50 and 51 having sections following planes parallel to that of Figure 10 in the form of arcs of circumferences over the greatest part of length thereof. The electrode 50 is'provided with a tubulure `53: for the admission of gas to be ionized. The electrode 51 is provided with an aperture 54 for the passage of the ions. The electrode 22 of Figure 3 is transformed into an electrode 52 of very particular shape, constituted by a box of which the top is provided with an aperture 55 and the bottom pierced by a circular hole 65. The electrodes 50 and 51 and the top of the electrode 52 are constituted by three portions of coaxial cylinders, having circular bases, the common axis thereof being indicated by point 65. An electrode 58, secured to the electrode 51 in any suitable manner, for example, by brazing, and presenting a transverse trough-shaped recess 59 to accommodate therein the cathode filament 60 constitutes a first electrode of the electron-optical means of the source, of which the second electrode is constituted by the extremity 67 of electrode 50 of which the curvature has been modified thereat. A collector electrode 56 is secured, for example, by brazing, to the electrode 50. The electrode 51 is provided additionally with protuberances 57, 62, 63 and 64. Magnets, which may be permanent magnets and of which only sections have been indicated at 67 and 68 in Figure 11, have a form which corresponds to the configuration of the electrodes 50 and 51 in such a way that a homogeneous magnetic field exists in the space defined by these two electrodes 50 and 51 which is perpendicular to the plane of Figure 10. A battery 66 or other suitable source of voltage is provided to furnish the different voltages necessary for operation of the device. y

The electrode 51 is at a voltage slightly negative with respect to that of the cathode 60; the electrode 50 is at a positive voltage and the electrode 52 at a voltage which is strongly negative.

With the electrodes S0 and 51 parallel, it is quite obvious that if a difference of potential is established therebetween, an electric field is established in the space delimited by the two electrodes 50 and 51 which s perpendicular thereto at all points and of which the equipotential surfaces are parallel to the surfaces of these electrodes. Nothing, therefore, impedes a laminar beam of electrons 61 emitted by the cathode 60 to traverse the ioni-tation space defined by the electrodes lS andy 51`b`y remaining constantly parallel to vtlieae electrodes. -It 'simply suiiices, as indicated in 'detail hereinabove, that the electron beam 61 is injected into the ionization l'space with a suitable speed and direction. lIf these conditions are fulfilled, then the beam may propagate in immediate proximity to the electrode u up to the moment when it encounters the collector 556. The ions resulting from the ionization then follow the lines of the electric field, and since the electrodes Sti, 51 and 52 are portions of coaxial cylinders of which the common 'axis is indicated at 65, the ions accelerated by the difference of potential existing between the velectrodes 51 and 52 will all converge toward the point 65 which may be called point of concentration or cross-over. The protuberances 57, 62, 63 `and 64 further permit to improve the focusing or at least to diminish any adverse iniiuences which might exist.

This embodiment of ion source according to the pres- 'ent invention is, therefore, particularly favorable since, in addition to all the advantages which have been mentioned hereinabove as a result of the Yinvestigation of the functioning of the embodiment illustrated in Figures 3 and 4, it further permits focusing of the ion beam at the point 65 which may be a point located i'n the working space. After traversing the cross-over point 65, the diverging ion beam may, at the will of the person utilizing the same, be transformed into a parallel beam or converging beam by means of appropriate ion-optical means. For example, for the utilizationthereof in a mass spectograph, it is possible to obtain, with the aid of convenient suitable optical means that the image of the cross-over point be formed on the detector. Thus, one obtains simultaneously a great intensity of the ion beam as well as a high power of resolution.

Another advantage of this embodiment is that it permits to reduce considerably the pumping power necessary to maintain the vacuum in the utilization chamber. This follows from the fact that the ionization source communicates therein with the utilization apparatus by a -hole 65 of reduced dimension, whereas with the classical ion sources, this communication may only take place by the intermediary of slots of more considerable and significant dimensions.

It is well understood, of course, that the present invention is not limited to the particular embodiments illustrated and described herein which have only been given for exemplary purposes. In particular, by utilizing structures comparable to those described in Figures and 1l,

but with an electron beam which is very large and by providing the electrode 51 with the improvements of Figures 6 and 7, the ions leave the ionization space and may thereupon be focused all along aline, this latter case being of particular interest if a great intensity of ion current is required.

While I have shown and described one preferred embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of many changes and modifications within the spirit of the present invention, and I intend to cover all such changes and modifications as encompassed by the appended claims.

I claim:

1. An ion source comprising, in an envelope filled with gas under small pressure, two parallel electrodes, means for applying a potential difference to said electrodes to establish therebetween an electrostatic field perpendicular thereto, means for establishing through the space between said electrodes a magnetic field having lines of force perpendicular to said electrostatic field lines of force, means comprising an electron source and electron-optical electrodes for injecting an electron beam for ionizing said gas into the space between said electrodes perpendicular to said electrostatic and magnetic lines of force existing therein and at a level near the one of said electrodes carrying a higher potential, collecting means 310 for said electren beam, `and means comprising at least one aperture in the other 'of said electrodes carrying a lower potential, for extracting ioiisfrom said space.

2. An ion source as claimed in claim l, wherein said extracting means further comprises an apertured electrode having an aperture positioned to be traversed by 'said ions having traversed said other electrode aperture, said apertured electrode being-biased negatively with respect to said other electrode.

3. An ion source as claimed in claim 2, wherein said two electrodes andthe surface of said apertured electrode facing said other electrode are circularly curved in the common plane of propagation direction of said beam and of said electrostatic eld, said apertured electrode having a form of a box having a circular aperture in the wall thereof opposite to said circularly curved surface.

4. An ion source as claimed in 'claim 1, wherein said 'two electrodes are planar.

5. An ion source as claimed in claim 1, wherein said Vtwo electrodes are circularly curved in the common plane vof propagation direction of said beam and of said elec'- trostatic field.

6. An ion source as claimed in claim v1, wherein said other electrode carrying a lower potential has an elongated-plate shape with two edge portions essentially parallel to the direction of propagation of said beam, each 4of said edge portions forming an elongated plate having its plane essentially perpendicular to the direction of said magnetic field.

7. An ion source as claimed in claim 1, wherein said aperture in said other electrode is a slot extending in "the direction of Vsaid beam.

8. An ion source las claimed in claim 1, comprising a plurality of .said apertures in said `other electrode, said apertures bemg in the form of parallel slots extending in 'the direction of said beam, and conducting walls separating said slots.

9; An ion source as claimed in claim l, wherein said other electrode has la concave 'surface essentially circularly shaped in the plane defined by said velectrostatic and lm'a'gr'ietic lines of force, saidsurface being on the side of said electrode opposite to said beam propagation space.

10. An ion source as claimed in claim 1, wherein one of said electron-optical electrodes is a plate parallel to said magnetic field lines of force and obliquely positioned with respect to said electrostatic field lines of force, said plate being connected to said other electrode carrying a lower potential.

l1. An ion source as claimed in claim 1, wherein said extracting means further comprises an apertured electrode provided with an aperture positioned to be traversed by said ions having traversed said other electrode aperture, said apertured electrode being biased negatively with respect to said other electrode, and wherein said electronoptical electrodes include two-plate shaped electrodes parallel to said magnetic field lines of force and obliquely positioned with respect to said electrostatic field lines of force, said plate-shaped electrodes being respectively connected to said other electrode carrying a lower potential and to said apertured negatively biased electrode.

l2. An ion source as claimed in claim 1, wherein said other electrode carrying a lower potential is connected with means for focusing the ion beam traversing said aperture provided therein.

13. An ion source comprising, in an enveloped filled with gas under relatively small pressure, two electrodes defining an ionization space therebetween, means for establishing an electrostatic field between said two electrodes, means for establishing a magnetic field yhaving lines of force extending through said ionization space at an angle to the lines of force of said electrostatic field, means for injecting au electron beam into said ionization space at an angle to both said electrostatic and magnetic lines of force existing therein and with a speed and direction to assure 11 essentially laminar flow of all the electrons in said Vbeam along one of said electrodes, and means providedwith at least one aperture in the other of said electrodes for extracting ions from said envelope.

14. An ion source according to claim 13, wherein said one electrode is positively biased with respect to said other electrode.

15. An ion source comprising, in an envelope filled with gas under relatively slight pressure, two electrodes defining an ionization space therebetween, means for establishing `an electrostatic field between said two electrodes, means for establishing a magnetic field having lines of force extending through said space and crossing the lines of force of said electrostatic field, means including an electron source and electron-optical means for injecting an electron beam into said space at an angle to both the electrostatic and magnetic lines of force existing therein near one of said electrodes to thereby produce ions by the impact of electrons with the molecules of said gas, and means provided with at least one aperture in the other of said electrodes for extracting ions from said envelope.

16. An ion source adapted to produce ions having the same speed comprising, in an envelope filled with gas under relatively slight pressure, two electrodes defining an ionization space therebetween, means for establishing an electrostatic field between said two electrodes, means for establishing a magnetic field having lines of force extending through said space at an angle to the lines of force of said electrostatic field, means including an electron source and electron-optical means for injecting an electron beam into said space essentially perpendicular to the electrostatic and magnetic lines of force existing therein and at a level near one of said electrodes, collecting means for collecting said electron beam, and means provided with at least one aperture in the other of said electrodes for extracting ions from said envelope.

17. An ion source adapted to produce ions having the same speed comprising, in an envelope filled with gas under relatively slight pressure, two electrodes defining an ionization space therebetween, means for establishing an electrostatic field between said two electrodes, means for establishing a magnetic field having lines of force extending through said space and at an angle to the lines of force of said electrostatic field, means including an electron source and electron-optical means for injecting an electron beam into said space essentially perpendicular to the electrostatic and magnetic lines of force existing therein with a speed essentially equal to the ratio of the electric field to the magnetic field, and means provided with at least one aperture in one of said electrodes for extracting ions'from said envelope produced by impact of electrons with gas molecules.

18. An ion source comprising means including two electrodes defining therebetween an ionization space containing a gas, means producing cross electrostatic and magnetic fields in said space, means including an electron source and electron-optical means for injecting simultaneously with the operation of said field producing means an electron beam into said space at a speed predetermined by the ratio of said fields and at a predetermined distance from one of said electrodes, and means-comprising at least one aperture in the other of said electrodes for extracting ions produced by collision of electrons with the molecules of said gas.

19. An ion source as claimed in claim 18, wherein said fields are mutually perpendicular, and wherein said electron beam is injected into said space essentially perpendicularly to both said fields.

Berterottiere et al. Sept. 24, 1957 www.

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