CA1071770A - Device for electrical deceleration of flow of charged particles - Google Patents

Abstract

ABSTRACT

A device for electrical deceleration of a flow of accelerated charged particles, comprising three electrodes positioned downstream, the first electrode having a potential equal to the energy of the electron flow, the second having an approximately zero potential and the third electrode, which is intended for reception of particles, having a small potential of the same sign as that of the first electrode. The second electrode is made as a set of sharp members, their edges being directed toward the incoming flow of charged particles. The device can be provided with a fourth electrode positioned after the third electrode, the potential of the fourth electrode being of the same sign as that of the third electrode.
Besides, the device can be provided with a fifth electrode positioned between the third and the fourth electrodes and having a potential of the same sign as that of said third and fourth electrodes but smaller in absolute value.

Description

" ~o7~q70 This invention relates to devices for deceleration of flows of charged particles, and, in particular, to devices for electrical deceleration of a flow of accelerated charged particles. Such devices can be employed in high-voltage elec-tron beam recuperating tubes and in future electron beam energy transmission lines.
There is known a device for electrical deceleration of a flow of accelerated charged particles comprising three electrodes arranged one after another down the particle flow.
The first of these electrGdes located down the particle flow has a potential equal to the flow energy, *he potential of the second one is approximately zero and the third electrode intended for reception of particles has a small potential of the same sign as that of the first electrode.
The basic characteristics of -the deceleration zone are;
the current of the decelerated flow, the energy of partic]es capture (the potential of the third electrode) and the value of the flow of secondary particles moving toward decelerated particles.
The current of the decelerated flow should be as close to the maximum possible and the capture energy and the flow of secondary particles as close to the minimum as possi-ble.
The current of the decelerated flow can be increased either by increasing the current density or by increasing the cross-section of the flow.

107177-~) In the known device the current in the decelera-ted flow should be increased by increasing the cross-section of the flow (flow density cannot be increased over a certain~value because the flow is expanded more than the hole in -the sec-ond electrode).
But the increasing cross-section of the decelerated flow should be also accompanied by increase of the hole in the second electrode. In the known devlce the local current value in the decelerated flow does not exceed 20 - 30 a.
When a flow with a local current of 1,000 a and more is to be decelerated, the flow cross section and the respective diameter of the hole in the second electrode and the cross--section of the third electrode should be considerably inc-reased, which presents certain difficulties in design.
At the same time a larger hole in the second electrode facilitates the passage of secondary particles into the space between the first and second electrodes, where these ~econ-dary particles are accelerated by the electric field existing in this space, since they move in the direction opposite to the decelerated flow, which results in increase of the flow of secondary particles.
The flow of secondary particles in a dele-terious flux and it should be brought down to minimum and in some cases amount to a value less than 10 3 or 10 4 of -the decelerated flow.

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Thus in the k~own device it is impossible to attain a sharp increase of current in the decelerated flow retaining a relatively small flow of secondary particles.
It is an object of this invention to provide a device for electrical deceleration of a flow of accelerated charged particles, wherein deceleration of flows with currents of the order of 1,000 a can be achieved.~
It is another object of this invention to provide a device, wherein the flow of secondary particles is so small as to constitute less than 0.1% of the current in the decelerated flow.
These objects are achieved by a device for electrical deceleration of a flow of accelerated charged particles which comprises: a first electrode positioned across the flow and having a potential equal to the energy of the flow being de-celerated, a second electrode positioned behind said first .
electrode downstream of the flow and having a potential approximately equal to zero, said second electrode being made as a plurality of pointed members with their points directed toward said flow of charged particles, a third electrode for i ! .
reception of said particles positioned behind said second electrode downstream of said flow and having a small potential of the same sign as that of said first electrode.
The sharp members can be made as blades.
- It is advisable to arrange the blades parallel to one another, It is also advisable to arrange the blades in two per-pendicular directions, thus forming a grid structure.

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The sharp members can be also made as needles.
It is advisable that holes be made in the third electro- -de in central areas of portions posi-tioned opposite the gaps between the sharp members of the second electrode and a fourth electrode be placed behind the third electrode down the particle flow, which is made as a set of chamber elec-trodes equal in number to the number of~holes in the third electrode and each being positioned opposite the respective hole and having a potential of the same sign as that the third electrode.
Besides, it is desirable that a fifth electrode be pla-ced between the third and the fourth electrodes, which is of the same geometry as the third electrode, its holes being positioned opposite the holes of the third electrode, and has a potential of the same sign as the potentials of the third and fourth electrodes but less in absolute value.
It is also advisable that a device be provided with a means for producing a magnetic field, which lines of force in the space between the first and second electrodes are directed along the trajectories of charged particles.
A device for electrical deceleration of a flux of char-ged particles made in accordance with this invention permits deceleration of currents of the order of 1,000 a at a low capture energy and small (up to 0.1% of the decelerated flow) flow of secondary particles.

Accordingly, in accordance with a more specific embodi-ment, a device for electrical deceleration of a flow of accelera-ted charged particles comprises: a first electrode positioned . .
across the flow and having a potential equal to the energy of the flow being decelerated, a second electrode positioned behind said first electrode downstream of the flow and having a poten-tial approximately equal to zero, said second electrode being made as a plurality of pointed members with their points directed toward said flow of charged particles, a third electrode for reception of said particles positioned behind said second elec- -trode downstream of said flow and having a small potential of the same sign as that of said first electrode, said third elec-trode being provided with holes positioned in alignment with said pointed members of said second electrode, and a fourth . electrode positioned behind said third electrode downstream of said flow of particles and having a potential of the same sign as that of said third electrode, said fourth electrode being ~ -made as a plurality of chamber electrodes, the number of chamber electrodes being equal to the number of holes in said third elec-trode, each of said chamber electrodes being positioned in alignment with one of the holes in said third e1ectFode.

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- 4a -1~71770 The invention will now be described in greater detail with reference to specific embodiments thereof taken in con-junction with the accompanying drawings, wherein:
Fig. 1 shows schematical a general view of a device for acceleration and electrical deceleration of a flux of accelerated charged particles;
Fig. 2 shows a diagram of distribution of potentials on electrodes of the device;
Fig. 3 shows a longitudinal section view of an enlarged part A of Fig. l;
Fig. 4 shows electrodes 11 and 12 as viewed from the electrode 10 of Fig. l;
Fig. 5 shows a section view taken along line V-V of Fig. 4.
Fig. 6 shows an embodiment of a second electrode made as a set of blades;
Fig. 7 shows a section view taken along line VII-VII of Fig. 6;
Fig. 8 shows an embodiment of a second electrode made as a grid structure;
Fig. 9 shows a longitudinal section view of schematic of an embodiment of a device for deceleration of electrons provided with four electrodes;
Fig. 10 shows a plan view of a section of a third elec-trode provided with round holes;

~7~77-3 Fig. 11 shows a plan view of a section of a third elec-trode provided with holes made as slits;
Fig. 12 shows an enlarged part B of Fig. 9;
Fig. 13 shows a longitudinal section view of an embodi-ment of a device for deceleration of electrons;
Fig. 14 shows a longitudinal sec-tion view of a device for deceleration of electrons provided with a means for pro-ducing a magnetic field;
Fig. 15 shows an enlarged view of a part C of Fig. 14;
Fig. 16 shows a longitudinal section view of a unit of a device for deceleration of electrons provided with five electrodes;
Fig. 17 shows a diagram of distribution of potentials on the electrodes of the device of Fig. 16.
The proposed embodiments of a device are intended for deceleration of accelerated electron flows. But these devi-ces can be employed for deceleration of flows of other charg-ed particles, e.g. ion flows.
Referring to Fig. 1 a device for acceleration and elec-trical deceleration of an accelerated electron flow compri-ses a cathode 1 and an anode 2, a source 3 of accelerating voltage being inserted therebetween. It is assumed that the potential of the cathode 1 of the electron source 3, where the electron beam received its energy, is equal to zero. A
channel 4 for transportation of an electron flow 5 is elec-trically coupled to the anode 2. The anode 2 and the cathode 10717'7V

l form an electron injector 6. A device 7 for deceleration of the accelerated electron flow 5 is connected to voltage sources 8 and 9. The deceleration device 7 comprises three electrodes 10, ll and 12 positioned one after another down the electron flow. In this case the first electrode 10 has a potential equal to the energy of the electron flow S (ap-proximately equal to the potential of the anode 2)o The se-cond electrode ll has an almost zero potential, which is achieved by inserting the voltage source 8 between the elec~
trodes lO and ll. The third electrode 12 is intended -to cap-ture electrons and has a small potential of the same sign as the first electode 10 and is electrically coupled to the se-cond electrode 11 via the voltage source 9. The second elec-trode 11 is made as a set of sharp members 13 directed by their edges toward the electron flow 5.
Referring to Fig. 2, a line 14 shows a diagram of rela-tive distribution of potentials on the electrodes of the de-vice of Fig. 1. Points I, II, X, XI and XII on the horizon-tal axis of the diagram correspond to the electrodes 1, 2, 10, 11 and 12 of Fig. 1.
Fig. 2 does not demonstrate the true distribution of potentials in the space between the electrodes, where the potential is reduced in relation to the potentials of elec-trodes due to the inherent charge of the electron flow.

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Referring to Fig. 3 the part A of Fig. 1 is shown as an enlarged longitudinal section view thereof. Arrows 15 indicate trajectories of electrons in the vicinity of the sharp member 13, an arrow 16 shows trajectories of secondary electrons hitting the butt ends of the sharp members 13.
Fig. 4 shows the electrodes 11 and 12 as viewed from the electrode 10 of Fig. 1, whereas Fig. 5 shows a section view taken along line V-V of Fig. 4. The electrode 11 of Figs 4 and 5 is shown as a plate provided with uniformly distributed over its surface sharp members, which are need-les 17. The electrode 12 is made as a plate provided with square holes 18 distributed with respect to position of the needles17 so that the latter enter the holes 18. The distan-ces between the adjoining needles 17 are chosen so as to provide in spaces therebetween an electric field sufficient for screening (producing a potential trough) the main mass of secondary electrons moving from the third electrode 12.
The characteristic size, which determines the flow of secondary particles, is the size of the needle 17 at the sharp tip. In the proposed embodiment the size of the needle tip can be 2 - 3 um and the size at the base of the needle 17 can be 80 - 100 um.
Figs 6 and 7 show a plan view of a second electrode and a section view taken along line VII-VII of Fig. 6 respective-ly. There is another embodiment of the second electrode which :: .

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is a set of blades 19 stretched parallel to one another. The collector, which is the electrode 12, is in this case made as a flat plate positioned behind the second electrode (the blade 19) down the decelerated electron flow.
Referring to Fig. 8 one more embodiment of a second electrode is a grid structure formed by a set of blades 19 stretched parallel to one another and a set of blades 191 also stretched parallel to one another, but arranged perpen-dicularly to the blades 19 in the plane of the second elec-trode. Distances between the blades 19 and 191 and their characteristic sizes are selected on the basis of the recom-mendations given in the description of Figs 4 and 5. All bla-des 19 and 191 are coupled to ~ne another electrically and mechanically (not shown in Figs 7 and 8).
Fig. 9 shows schematically an embodiment of a device for deceleration of an accelerated electron flow, wherein in contrast to the deceleration device of Fig. 1 the third elec-trode 12 is provided with holes made in central areas of por-tions located in the gaps between the sharp members 13. Besi-des, a fourth electrode 21 is placed behind the third elec-trode 12 down the electron flow 5. This electrode 21 is made as a set of chamber electrodes 22 equal in number to the num-ber of the holes 20 in the electrode 12. Each chamber elec-trode 22 is positioned opposite the respective hole 20. The fourth electrode 21 has a potential of the same sign as that of the third electrode, which is a little higher in absolute 10717'~

value. The fourth electrode 21 can be made both of a plurali-ty of separate members, which are electrically and mechani-cally connected, and of a whole plate, whereon the required planes are done by machin ng~
Fig. 10 shows a plan view of a part of the third elec- -trode 12 for the case, when the second electrode is made as a set of the needles 17 (or as a grid structure formed by two sets of perpendicular blades). The holes 20 are round and the chamber electrodes 22 shown in Fig. lO by a dotted line are cylindrical because they are positioned behind the electrode 12.
Fig. ll shows a plan view of a part of the third elec~
trode 12 for the case when the second electrode is made as a set of parallel blades l9. Holes 23 in this embodiment are made as elongated slits and chamber electrodes 24~ which are also shown by a dotted line, are made as elongated cavities.
Fig. 12 shows an enlarged view of the part B of Fig. 9 which is a unit of the proposed deceleration device. Dotted lines show trajectories of secondary electrons, whereas full lines 25 show equipotentials of the electric field. The equi-potential lines 25 of the electrical field between the se-cond electrode (the members 13) and the third electrode 12 form elec-trostatic lenses which focus the electron flow 5.
Fig. 13 schematically shows one of the embodiments of the proposed device for deceleration designed to recuperate the energy of the intense electron beam. As may be seen from 107~77~

Fig. 13 the geometry is basically similar to the optics of electron guns with a compressed beam, which arefairly wel1 known. The second, third and fourth electrodes in this embo-diment are arranged on a spherical surface. An electrode 26 is provided to eliminate end effects, the shape of the elec-trodes 10 and 26 being selected either by calculation or on an electrolyzer cell.
It is known that the longitudinal magnetic field of electron injectors (in particular the converging magnetic field in compressed beam guns) permits a more distinct boun-dary of the beam. In injectors characterized by a high degree of compression this can actually reduce the cross-section of the beam. When a gun with a magnetic field is used as an in-jector and the beam is further enclosed in a longitudinal magnetic field, the deceleration device is also advisable to be provided with a means for producing a magnetic field which lines of force in the space between the first and second electrodes are directed along the trajectories of the charged particles. An embodiment of a deceleration device featuring a means for producing a magnetic field is schematically shown in Fig. 14. In contrast to the device of Fig. 13 this device is provided with an electric magnet comprising a mag-netic circuit formed by two sections 27 and 28 and two coils 29 and 30. Sharp members 31 are arranged on the spherical surface of the section 27 of the magnetic circuit. They are made of ferromagnetic material and their number is equal to 1~17'70 the number of chamber electrodes 22. The tip of each member 31 is positioned inside a respective chamber electrode 22.
Fig. 14 shows lines 32 of force of the magnetic field of the electric magnet and a reverse magnetic current 33. To close the current 33 a ferromagnetic yoke composed of a plurality of isolated members (not shown) can be used.
Fig. 15 shows an enlarged view of the part C of Fig. 14 which is a unit of the deceleration device. As may be seen from Fig. 14, the lines 32 of force of the magnetic field ar~e concentrated on the ferromagnetic members 31.
Fig. 16 schematically shows a unit of one more embodi-ment of the proposed deceleration device. In this embodiment in contrast to those of Figs 13, 14 and 15 a fifth electrode 34 is placed between the third and the fourth electrodes, which geometry is similar to the geometry of the third elec-trode 12, holes 35 of thefifth electrode 34 being positioned opposite the holes 20 of the third electrode 12.
The fifth electrode 34 has a potential of the same sign as the potentials of the third and fourth electrodes but less in absolute value.
Referring to Fig. 17 a line 36 indicates relative dist-ribution of potentials on the electrodes of the device of Fig. 16. Digits I, II, III, IV and V on the horizontal axis of the diagram designate the points which dëtermine the po-tentia]s on the first, second, third, fourth and fifth elec-trodes respectively.

1(1717'7~) The proposed device for deceleration of a flow of ac-celerated electrons operates as follows.
The flow 5 (Fig. 1) of accelerated electrons comes into the field of action of the decelerating electric field ap-plied between -the electrode 10 and the electrode 11, the electrons lose their energy and move to the electrode 11.
Here some electrons hit butts of the sharp members 13, are reflected and move toward the main beam accumulating energy again. But this is but a minor part of the flow, because the area of the butt ends of the sharp members 13 can be less than 10 S _ 106 of the total flow area.
The main part of electrons comes into the space between the sharp members 13 and goes on moving in the initial direc-tion, that is toward the third electrode 12~ As it has al-ready been said, an accelerating potential difference is ap-plied between the elec-trodes 11 and 12. Thus, when approach-ing the electrode 12 electrons have the energy which is close to the potential of the electrode 12. It is significant that the equipotentials 25 (Fig. 12) of this accelerating field have the shape of a focusing lens and the electron flow is broken by the electrode 11 into spearate currents, each being focused on the electrode 12 (Fig. 3, Fig. 12) in -the central part of the unit of the second electrode 11.
In the embodiment of Fig. 9 holes are provided in the electrode 12 approximately to the size of the focused curren-t.
The holes may be round (Fig. lO)or elongated (Fig. 11) de-1~7177f~ !

pending on the shape of the focused electron flow which is in its turn determined by the structure of the second elec-trode 11.
In this embodiment of the device the streams of the electron flow 5 (~ig. 9) come into the space of the fourth electrode 21. There is a small accelerating voltage for the main flow between the electrodes 12 and 2] and, respectively, a decelerating voltage for the secondary electrons from -the electrode 21. Thus each unit of the electrode 21 is one of the optimal geometries for maximum suppression of secondary electrons. The electrode 12 acts in thus case as a suppressor.
The size "a" (Fig. 12), like other sizes of the electrode 21, should be selected by calculation or experiment. It is pos-sible that sometimes the size "a" is zero. Other measures can be taken to reduce secondary emission: selection of the material for the electrode 21~ of the shape of the inner sur-face of cavities, etc.
Accelerating fields near the electrodes 12 and 21 in-crease the electron capture energy to a certain extent, but these values are sufficiently low. Thus with the initial ene-rgy of the beam equal to 100 - 200 keV the potential of the electrode 21, which determines the capture energy, can amount to mere 500 - 1,000 eV, which corresponds to the electron capture energy of 500 - 1,000 eV.
Thetransmitta~ce of the second electrode 11 (Fig. 9) is very high: it ensures a flow of secondary electrons from 107177~

the electrode 11 less than 10 - 10 of the ma in flow. But the most significant in this case are the secondary particles leaving the electrode 12 with a full energy approximately perpendicular to the capture surface (that is the surface of the electrode 12)o It is well known that 1 - 2% of the secon-dary particles possess energy equal to the energy of the par-ticles of the primary flow, a part of them moves at angles close to normal. It is natural that the depth of the potential trough stopping secondary particles cannot be rated for the full energy of falling particles.
It can only lock the majority of secondary particles which energy is equal to 0.1 - 0.2 of the energy of the beam.
As a result the flow of secondary particles in the absence of the proposed means amounts to no less than 10 3 - 10 4 of the main flow.
At the same time the passage of the flow separated into streams into cavities of the chamber electrodes 22 of the electrode 21 permits reduction of the flow of secondary par-ticles by one or two orders. This is quite normal for collec-tors shaped as a Faraday cylinder.
~^ Thus, the flow of secondary particles can be brought to less than 10 5 - 10 6 of the main flow if the proposed device is employed.
Such high degree of screening of secondary electrons can permit deceleration of superpowerful stationary electron flows.

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10~71770 One of the limitations of the proposed device apart from the Faraday cylinder properties is the part oE the beam which is focused. It is na-tural that some electrons on the boundary of two streams (two focusing electrostatic lenses) may not be focused and thus miss the point of the electrode 12 where the hole 20 is provided for passage of electrons into the electrode 21. From this point of view the system with the blades 19 (Fig 6, 7 8) is better than that with the needles 17 (Figs 4, 5).
In the embodiment of Fig. 13 the electron flow 5 decele-rated in the electric field between the first and second electrodes widens and passes nearly perpendicular to the spherical surface formed by the second, third and fourth electrodes. When the flow approaches each unit in this way to be then decelerated after being forcused by the electric field of the unit, it comes precisely into the hole 20 and into the cavity of the chamber electrode 22. In this case a minimum quantity of particles hit the electrode 12 and the number of secondary particles originating on this electrode becomes close to zero. This is very important because the secondary particles from the electrode 12 are practically not screened.
The geometry of Fig. 13 can be employed for deceleration of monochromatic electron beams with the energy of 100 - 200 keV and more at currents of many hundreds of amperes.

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Radical improvement of optical qualities of the system can be obtained in a device similar to an electron gun with a compressed beam, which uses a guiding megnetic field (Figs 14 and 15). More distinct boundaries of the beam reduce losses of particles during transportation. In some cases they reduce the cross-section of the par-ticle flow (which in its turn permits reduction of the aperture of the system and its overall size~.
In this case the magnetic field contribues to focusing streams of the electron flow 5 into the holes 20 of the elec-trode 12. The part of the electron flow, which is subjected to magnetic focusing, is determined by the portion of the magnetic flow falling on the members 31. A certain part of the flow is naturally closed right on the ferromagnetic sec-tion 27 of the magnetic circuit bypassing the points 31. An optimum height of the members 31 should be found to improve the focusing action of the magnetic flo2. Selection of opti-mum sizes of elements of the proposed deceleration device de-pends on the concrete parameters of the decelerated flow, as well as on the requirements set to the device: current value in the flow 5, permissible capture energy, the magnitude of the secondary particles flow, etc.
Parameters of the deceleration zone can be improved (cap-ture energy and secondary flow reduced) by further elabora-tion of the structure of the receiving collector. The embodi-ment of Fig. 16 of the device for deceleration of electron ' ~''.'' flow comprises an additional el~ctrode 34 as compared to that of Fig. 13. The additional electrode 34 permits the following mode of operation: to improve focusing of the beam (reduction of its diameter in the plane of the plate of the electrode 12) a higher potential is supplied to the electrode 12, e.g. 3 - 5 kV against 500 - 1,000 V in the above descri-bed embodiment (Fig. 13). The electron flow is decelerated ~-in the space between the electrodes 12 and 34, then it is somewhat accelerated before it is captured by the walls of the chamber electrode 22. Such mode of operation permits suf-ficiently low capture potential (sell than 500 - l,OOOV) the electrode 34 acting as a suppressor. Better focusing of the beam in the plane of the electrode 12 and smaller hole 20 in said electrode 12 permits reduction of secondary electrons.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A device for electrical deceleration of a flow of ac-celerated charged particles comprising: a first electrode posi-tioned across the flow and having a potential equal to the energy of the flow being decelerated; a second electrode positioned be-hind said first electrode downstream of the flow and having a potential approximately equal to zero, said second electrode being made as a plurality of pointed members with their points directed toward said flow of charged particles; a third electrode for reception of said particles positioned behind said second electrode downstream of said flow and having a small potential of the same sign as that of said first electrode.

2. A device as claimed in claim 1, wherein said pointed members are made as parallel blades.

3. A device as claimed in claim 2, wherein said blades are set in two perpendicular directions forming a grid structure.

4. A device as claimed in claim 1, wherein said pointed members are made as needles.

5. A device for electrical deceleration of a flow of accelerated charged particles comprising: a first electrode posi-tioned across the flow and having a potential equal to the energy of the flow being decelerated; a second electrode positioned be-hind said first electrode downstream of the flow and having a potential approximately equal to zero, said second electrode being made as a plurality of pointed members with their points directed toward said flow of charged particles; a third electrode for reception of said particles positioned behind said second electrode downstream of said flow and having a small potential of the same sign as that of said first electrode, said third electrode being provided with holes positioned in alignment with said pointed members of said second electrode; and a fourth electrode posi-tioned behind said third electrode downstream of said flow of particles and having a potential of the same sign as that of said third electrode, said fourth electrode being made as a plurality of chamber electrodes, the number of chamber elec-trodes being equal to the number of holes in said third elec-trode, each of said chamber electrodes being positioned in alignment with one of the holes in said third electrode.

6. A device as claimed in claim 5, further comprising a means producing a magnetic field, lines of force in the space between said first and second electrodes being directed along the trajectory of said charged particles.

7. A device as claimed in claim 5, further comprising: a fifth electrode positioned between said third and said fourth electrode and having a potential of the same sign as that of potentials of said third and fourth electrodes but being less than the latter in absolute value; said fifth electrode having a geometry similar to the geometry of said third electrode, and having holes arranged opposite said holes of said third elec-trode.

8. A device as claimed in claim 7 comprising a means for producing a magnetic field, lines of force in the space between said first and second electrodes being directed along trajectories of said charged particles.

9. A device as claimed in claim 1, wherein said third electrode has holes positioned in alignment with said pointed members of said second electrode.