US3379984A

US3379984A - Shortening device for relativistic electron pulses downstream of an accelerator

Info

Publication number: US3379984A
Application number: US438656A
Authority: US
Inventors: Pinel Jacques
Original assignee: CSF Compagnie Generale de Telegraphie sans Fil SA
Current assignee: Thales SA
Priority date: 1964-03-13
Filing date: 1965-03-10
Publication date: 1968-04-23
Anticipated expiration: 1985-04-23
Also published as: FR1396524A; CH423020A

Description

J. PINEL. 3,379,984 SHORTENING DEVICE FOR RELATIVISTIC ELECTRON April 23, 1968 PULSES DOWNSTREAM OF AN ACCELERATOR 5 Sheets-Sheet 1 Filed March 10, 1965 INVENTOR:

J.P/NEL Qmmm 19 ATTORNEY April 23, 1968 J. PINEL 3,379,984

SHORTENING DEVICE FOR RELATIVISTIC ELECTRON PULSES DOWNSTREAM OF AN ACCELERATOR 5 Sheets-Sheet 2 Filed March 10, 1965 INVENTOR Z J. P/IVEL BY m &

,4. ATTORNEY April 23, 1968 J. PINEL 3,379,984

SHORTENING DEVICE FOR RELATIVISTIC ELECTRON PULSES DOWNSTREAM OF AN ACCELERATOR 3 Sheets-Sheet 5 Filed March 10, 1965 INVENTOR ATTORNEY United States Patent 3,379,984 SHORTENHNG DEVICE FUR RELATIVESTTC ELECTRGN PULSES DQWNSTREAM 9F AN ACCELERATOR Jacques Pine], Paris, France, assignor to CSF-(Iornpagnie Generals de Telegraphic Sans Fil, Paris, France Filed Mar. 10, 1965, Ser. No. 438,656 Claims priority, application France, Mar. 13, 1964, 967,266 16 Claims. (Cl. 328-230) ABSTRACT OF THE DISCLQSURE An electron accelerator installation comprising an accelerator for accelerating electrons to relativistic velocities and for delivering pulses containing a spatial sequence of bunched electrons having successively decreasing energies, and a magnet positioned on the path of said electrons downstream of the output of the accelerator for shortening the duration of each of said pulses. The accelerated electrons propagate in the gap between the two pole pieces of the magnet. The induction in the gap is so chosen that after describing their orbit the bunches reach simultaneously the same point in space and are extracted from the gap at said common point.

The present invention relates to a device for shortening the pulses of accelerated particles.

The device of the present invention is intended to be used at the output of a particle accelerator, more particularly of an accelerator of electrons carried to relativistic velocities, i.e., to velocities near the velocity of light, and wherein the accelerated electrons are delivered in the form of pulses. The aim of the device in accordance with the present invention is to shorten the pulses so as to enable proper handling in the course of measurements or experiments in the field of nuclear physics.

It is known that owing to the inertia of circuits and sources of particles, it is hardly feasible to operate an accelerator in pulsed condition with pulses shorter than several nanoseconds (for instance, 3 nanoseconds or 3-10 seconds). During each of these pulses, a burst of electrons is delivered from the output of the accelerator which has a velocity substantially equal to the velocity of light, this burst forming a chaplet of washers separated by a distance equal to the wave-length of the high frequency wave which was used for operating the accelerator, each of the washers containing substantially mono-energetic electrons. However, the electrons in each washer have respectively different energies, the first washer having the highest energy and the following washers having decreasing energies, according to a substantially linear law as a function of their position within the chaplet. This decrease of energy is due to the fact that the wave which has accelerated the first washer has yielded to said washer a fraction of its energy, thereby keeping for accelerating the second washer only relatively lower energy, etc. The length of the chaplet of washers might not be shorter than L=0.9 m. if the pulse duration could not be made shorter than 3 nanoseconds, since, with the velocity of light 0:3 l0 m./sec.,

Now, the phenomena to be measured or utilized in nuclear physics are associated with durations which are of the same order of several nanoseconds. It follows therefrom that the results of measurements are altered when the pulses have a length of the same order, similar to the effect of the internal resistance of a voltmeter or a-mrneter which may cause the measurements of voltage or current to be altered. Therefore, there is a need to shorten the pulses so that their length or duration could be neglected when compared with the measuring time. The device according to the invention provides a solution of this problem.

According to the invention, there is provided at the output of a pulsed accelerator a device called regrouper, which includes substantially a magnet having a gap through which is directed the chaplet of washers formed by the accelerated particles. The washers follow within the gap, about the lines of force of the magnet, circular orbits the radius of which varies with the energy of each washer considered. The induction in the gap is chosen so that different particles come or arrive, at the same instant, substantially at a common point of their orbits; the input chaplet of washers is then re-grouped at the output so as to form a short bunch having a length appreciably smaller than the length of the input chaplet of washers.

According to another feature of the invention, the magnet is so positioned with respect to the input chaplet of washers that the axis of the chaplet is slightly inclined with respect to the plane perpendicular to the lines of force in the gap, the particles following thereby within the gap helicoidal paths having these circular orbits as projections of the paths on this perpendicular plane, with a view to enable the particles to leave the re-groupcr without interfering with the particles entering the same.

According to a further feature of the invention, the faces of the pole pieces of the magnet are shaped to form a spiral or corkscrew staircase winding about an axis passing through a peripheral point of the piece.

According to a further development of the invention, the lateral faces of the stairs of the staircase (the counterstairs) are formed by cylindrical surfaces orthogonal to the family of circles representing the projections of the paths of the particles.

Accordingly, it is an object of the present invention to provide a device which enables shortening of pulses derived from an accelerator to avoid the danger that the results of measurements or experiments of nuclear physics are altered or rendered false owing to excessive pulse lengths.

Another object of the present invention resides in a provision of a magnet having a novel shape and located between the output of a particle accelerator and further utilization devices for the particles, thereby acting on the duration of the pulses of the particles in the sense favorable to physical measurements or experiments.

A further object of the present invention resides in the provision of an installation of an electron accelerator, delivering pulses at relativistic velocity, having a duration more appropriate to physical measurements or experiments.

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 drawings which shows, for purposes of illustration only, a manner of embodying the principle in accordance with the present invention, and wherein:

FIGURE 1 is a basic schematic representation of an electron accelerator installation in accordance with the present invention,

FTGURE 2 is a perspective view of the paths followed by the particles in the re-grouper according to the present invention,

FIGURE 3 is a cross-sectional view of a bunch of particles leaving the re-grouper, taken along line HIIII of FIGURE 1,

FIGURE 4 is a perspective exploded view of an example of a constructive embodiment of the re-grouper in accordance with the present invention,

FIGURE 5 is a perspective assembled view of the same re-grouper,

FIGURE 6 is an end face view of one of the pole pieces of the re-grouper of FIGURE 4,

FIGURE '7 is a schematic diagram showing a further development of the accelerator installation in accordance with the present invention, and

FIGURE 8 is a schematic diagram showing a still further development of the accelerator installation of the present invention.

FIGURE 1 shows an extremely schematic representation or" the device in accordance with the invention, exclusively for the purpose of better understanding of its operation.

The block 1 represents a conventional accelerator of electrons which are carried to a relativistic velocity, this accelerator being in pulsed operation. The electrons coming from the output of the accelerator during a pulse are grouped or re-assemhled within a voiurne of space 2, and form therein a chaplet of washers extending over a total length L, equal to the product of velocity near th velocity of light :3 10 iii/sec. by the duration of the pulse. Assuming, for instance, that the washers are circular, the envelope of the Chaplet would be cylindrical if the paths of electrons were parallel, but in fact there is always a slight divergence of these paths, and therefore the volume 2 is a truncated cone with small opening. This chaplet of length L moves within the space at the output velocity of the electrons, the diameter of the washers increasing slightly and pro ressively during this movement due to the aforementioned divergence so that the chaplet is contained between boundary paths 3 and 4. The energy of the subsequent washers decreases between boundary levels E and E in proportion to the position of each washers under consideration within the chaplet,

According to the invention, the chaplet of washers distributed in the manner specified above, enters into the gap of a magnet having pole pieces shown in FIGURE 1 in projection in dash lines 5, and having magnetic lines of force perpendicular to the plane of the drawing. The induction in the gap is denoted by B (FIGURE 4). Within this gap, the electrons begin to follow in the plane of the drawing circular paths having radii proportional to the momentum of each washer under consideration, and hence proportional to the energies of the subsequent washers, since their velocity is substantially the same, due to the relativistic acceleration of the electrons. Thus, the washer E follows a circle C an intermediate washer E follows a circle C and the last washer E follows a circle C FIGURE 1 shows a position wherein the washer E just arrived at the beginning of its circular path, while the washer IE at the same instant has already progressed by a distance L along the circumference of the circle (3;. 0

Intermediate washers occupy positions nearer E along their respective circles. The radii of the outermost and innermost circles C and C are denoted respectively by R and r.

The result aimed at by the invention will be obtained if the washer E having described the circle C returns to the same point as E at the same instant when the washer E having described the circle C returns to its initial position indicated in FIGURE 1. All intermediate washers would then return substantially to the same point substantially at the same instant, due to proportionality between the energies and the positions of the subsequent washers in the chaplet.

This condition corresponds to the numerical expression:

21rR27rl=L (1) As, on the other hand:

E l r E,, (2)

We have two equations from which R and r are calculated as follows:

Induction B required for a circle of radius R be described by electrons having a given energy E is, accord ing to a known formula:

E B r wherein B is in Tesla (a Tesla being a unit of the international MKS system equal to 10,000 gauss), if E, is eX- pressed in mev. and R in meters.

Thus, using one of the Expressions 3 or 4, it is found in my case:

21r(E E (B in Tesla, when E; and E in rnev. and L in meters).

Since all washers arrive simultaneously at the output point, it is seen that if one causes the electrons to leave the gap at this very instant, all electrons remain grouped or re-assembled in a short bunch 6 (FIGURES 1 and 3), having a length L substantially smaller than L, and corresponding to the depth of a washer, which represents a fraction of the wavelength of the wave which was used for acceleration in the accelerator 1. It may 'be admitted in practice that this reduced length will correspond to a duration of the order of a fraction of one nanosecond, if the length L corresponded to 3 nanoseconds. This short bunch 6 represents thus a shortening of the initial pulse and is therefore more readily utilizable for physical handling.

However, for purposes of a practical embodiment of the principle schematically shown in FIGURE 1, the device should be so constructed that the output point of the short bunch 6 is prevented from coinciding in space with the input point of the chaplet of washers 2. This coincidence could be troubling due to interference of input electrons with output electrons. The output point must be for this reason shifted with respect to the input point, in the direction parallel to lines of force B. This shift can be attained by slightly tilting the magnet with respect to the direction of the input chaplet of washers so that the direction may have a slight slope to the plane perpendicular to the lines of force B. There are then no more circles described by the electrons but turns of helices having circles C etc., as projections onto the plane normal to the lines of force B. FIGURE 2 shows a perspective View of a turn of helix 7 described by electrons having energy E and also of a turn of helix 8 described by electrons having energy E It is assumed here that both turns originate from a common point A, and that the tangent to each of the turns 7 and 8, passing through the common point A and located in a tangent plane parallel to lines of force B, is common to both turns, i.e., has the same slope angle with respect to the plane normal to these lines of force B. It is seen that the output point 8; or S is shifted with respect to the common input point in the direction of the lines of force B, and moreover this shift is different for electrons having respective extremity energies E and E while output points of electrons having intermediate energies are of course distributed between S and S It results therefrom that the cross section of the short bunch 6, taken perpendicular to the plane of FIGURE 1 and parallel to the width d, has an elongated shape in the direction of height h, as shown in FIGURE 3. In this cross section the electrons are grouped following their energies progressively decreasing from the bottom upwards in FIG- URE 3, as symbolized by the cross hatching which is less and less dense in the upward direction.

FIGURES 4 and 5 show in perspective, respectively, an exploded and an assembled view of an example of structural embodiment of the above-explained principles. In FIGURE 4, reference numerals 5 and 5' are used to designate two pole pieces corresponding to the projection 5 in FIGURE 1. The magnet is energized by

coils

9 and 10, and the magnetic circuit is completed by a yoke 11.

In the gap between pole pieces 5 and 5' is located a guide 12 having an input 13 intended to be connected to the evacuated enclosure of an accelerator corresponding to the block 1 of FIGURE 1, and having an output 14 intended to be connected to the enclosure (not shown) of a utilization device of the derived short bunch. This guide 12 is made of a non-magnetic metal and has a helicoidal shape with variable cross section, properly sized for passing all paths comprised between the extreme helices 7 and 8 of FIGURE 2. The radii R and r of these helices can be calculated from Equations 3 and 4; for instance, if the length L of the pulse 2 in FIGURE 1 is 0.9 meter, and if the energies of the first and the last With the same data, the induction B in the gap is calculated from Equation 6:

=O.7 Tesla The input branch of the guide 12 is passed through a magnetic channel provided according to the invention in the body of the pole piece 5; the input of this channel is seen at 15 and the output at 16. In the same manner, the output branch of the guide 12 is passed through a magnetic channel provided in the body of the pole piece 5', with the input at 17 and the output at 18. The fringing areas of the magnet are thereby traversed without disturbing the electron paths inside of the guide 12 before their arrival at the starting point of the helicoidal path, or along their trajectory followed after having described the helix. If to the contrary, these channels were manufactured separately from the pole pieces and thereafter introduced in the gap, their presence would disturb the distribution of induction in the gap.

The size of the guide 12 parallel to lines of force B should be, of course, slightly larger than the height h in FIGURE 3. This height it corresponds to the shift between points S and S in FIGURE 2, added to the diameter of the washers in the chaplet 2, with consideration of the divergence of the chaplet. The distance between points S and S is substantially equal to:

where c: is the slope of the helix described by the guide 12, expressed in radians, and taken with respect to the plane normal to lines of force B. On the other hand, this slope should be such that the output branch of the guide passes below the input branch, the thickness of the Walls of the guide 12 being duly considered. For instance, with oc=0.04 radians, the distance between the output points of electrons is 21r(O.29-0.145) -0.04=0.036 rn.=3.6 crn.

Assuming the diameter of the washers in the chaplet 2 at the input into the magnet to be equal to 0.4 cm. the height of the cross section of the guide 12 would be taken slightly greater than 4 cm. It is readily checked that with a slope of 0.04 and a height of the order of 4 cm., the output branch of the guide passes below the input branch thereof.

The size of the gap of the magnet should then be chosen slightly greater than this height with the addition of safety distances on both sides of the guide with respect to the metallic faces of both pole pieces. For instance, with a guide height of the order of 4 cm., the gap width could be chosen of the order of 6.5 cm.

With the output face 16 of the input channel located in the same plane as the input face 17 of the output channel, and with this plane disposed parallel to lines of force B and containing the normal line common to said lines of force and to the movement direction of electrons, the magnetic field distribution along the helicoidal paths followed by the electrons between output face 16 and input face 17 must be progressively varied or matched in order to avoid defocusing effects. This matching can be attained if the pole faces are shaped in the form of a

staircase having stairs

19, 19", 19", etc., on the

pole

5 and 26', 20", 20", etc., on the pole 5'. These stairs each turn in the fashion of a corkscrew staircase, having for its axis a vertical line passing through the plane of opening 16 or 17, respectively. The faces of the stairs are substantially perpendicular to the desired direction of the lines of force B.

Each stair, when considered together with an opposite facing stair on the opposed pole piece, forms a pair of pole pieces capable of being assimilated to a partial magnet so that the electrons within the gap successively traverse a series of such partial magnets joined by their lateral faces.

These partial magnets, having gaps traversed successively by electrons, define magnetic fields which are uniformly distributed in each partial magnet, and moreover equal in all magnets if the width of their gaps is the same. Thus, the gap chosen, for example equal to 6.5 cm. as indicated above, will be maintained between each pair of facing stairs.

On the other hand, in order to avoid as much as possible a defective focusing, it is desirable that the lateral faces joining the partial magnets be normal to the trajectories of electrons. This is why, according to a further development of the invention, the lateral faces of the stairs (counter-stairs), such as 21 or 22, are cylindrical surfaces orthogonal to the family of circles representing projections of helices described by electrons. In FIG- URE 6, showing a face View of the pole piece 5, and indicating in dash line the circle C of FIGURE 1, it is seen that the lines or circles 22 representing the projection of counter-stairs of the staircase composed of

stairs

20, 20", 20, etc., are orthogonal to the circle C and have a common point A so that circles 22 are orthogonal with respect to the family of circles mutually tangent at the point A.

If it is desired that the short bunch 6 shown in FIG- URES 1 and 3 be energy-focused, i.e., that its height h be reduced to practically pin-point size, it is possible to apply for this purpose a focusing device, known per se, for instance a system of two

magnets

23, 24 with parallel faces (FIGURE 7), properly dimensioned and mutually spaced according to Well known teachings of prior art. The lines of force of these magnets must then be directed along the width [1, i.e., perpendicular to lines of force B of the re-grouping magnet 5. After passing through the

magnets

23 and 24, the height h of the short bunch 6 is reduced to practically pin-point size, and the cross section of the bunch becomes such as shown in 25.

Finally, the shift between points S and S could be reduced if between the accelerator 1 and the re-grouper 9, 1G, 11 (FIGURE 8) there is inserted a deflecting magnet 26 having lines of force perpendicular on the one hand to the initial direction of movement of the electrons, and on the other, to the induction in the re-grouper. By means of this magnet 26, the paths of electrons with different energies are spread in the plane perpendicular to its lines of force, so that the slope of these paths varies inversely with energy. It results therefrom that while in FIGURE 2 the helices 7 and 8 had the same slope but different pitches, the interposition of the magnet 26 modifies the helical paths so that their slopes becomes different, the helix 8 having a smaller slope. The difference between their pitches would then be reduced and could even decrease to zero, both helices '7 and 8 having then the same pitch, and both points S and S coming into coincidence. The cross section of the short bunch 6 in FIGURE 3 would then become circular as the cross section of the truncated cone 2 in FIGURE 1. The gap ally parallel, by means of a second deflecting magnet 27,

acting oppositely to the magnet 26.

While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art. Especially, the pole pieces of the regrouping magnet could be incorporated in the evacuated enclosure. The system of focusing magnets shown in FIGURE 7 could be replaced by a system of quadru-polar lenses or similar focusing means. The induction of the re-grouper could be made adjustable for adapting the device to operate with different maximum and minimum energy levels. The re-grouper could be made orientable in space, to vary the impact slope of the electrons. The described device is of course applicable to particles other than electrons, if the accelerator of said particles is powerful enough for carrying them to relativistic velocities. Therefore, I do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. In a particle accelerator installation, an accelerator operative to accelerate electrons to relativistic velocities and to deliver the electrons in the output thereof in the form of pulses containing a spatial sequence of electrons having successively decreasing energies, and means positioned along the path of said electrons downstream of the output of said accelerator for shortening the duration of each of said pulses by grouping electrons contained in a respective pulse into relatively reduced spatial length.

2. In a particle accelerator installation, an accelerator operative to accelerate electrons to relativistic velocities and to deliver the electrons in the output thereof in the form of pulses containing a spatial sequence of electrons having successively decreasing energies, and means posi tioned along the path of said electrons downstream of the output of said accelerator for shortening the duration of each of said pulses by grouping electrons contained in a respective pulse into a relatively reduced spatial length, said means including re-grouping magnet means producing lines of force and having a pair of pole pieces with a gap therebetween traversed by said accelerated pulses, with said electrons describing orbits about the magnetic lines of force in said gap, the induction in said gap being so chosen that after describing one orbit substantially all electrons are re-grouped at a given instant in a substantially common point of space, and means for extracting said re-grouped electrons from said gap at said common point.

3. The combination according to claim 2, wherein said orbits are substantially closed.

4. The combination as claimed in claim 2, wherein the induction in said gap is chosen substantially according to the expression:

wherein:

B is the induction, in Tesla, E the energy of electrons having the highest energy,

in mev., E,,, the energy of electrons having the lowest energy,

in mev., L, the length of an accelerated pulse, in meters. 5. The combination as claimed in claim 2, wherein said re-grouping magnet means is positioned in space so that the impact direction of an accelerated pulse is lit slightly inclined with respect to the plane perpendicular to the lines of force in said gap.

6. The combination as claimed in claim 5, further comprising helicoidal guide means for said electrons within said gap.

7. The combination as claimed in claim 6, wherein said guide means has a variable width along the path of said electrons, said width first progressively increasing from the input of said electrons and then progressively decreasing toward the output thereof.

8. The combination as claimed in claim 5, wherein the faces of said pole pieces are shaped to form a corkscrew staircase turning about an axis passing through the periphery of both pieces.

9. The combination as claimed in claim 8, wherein the lateral faces of the stairs of said staircase are substantially cylindrically shaped, said cylindrical surfaces being orthogonal to the family of circles representing projections on said pole pieces of the paths of said electrons in said gap.

19. The combination as claimed in claim 5, further comprising magnetic channel means in the body of each of said pole pieces, respectively, for the input and for the output of said electrons in said gap.

'11. In a particle accelerator installation, an accelerator operative to accelerate particles to relativistic velocities and to deliver the particles in the output thereof in the form of pulses containing a spatial sequence of particles having successively decreasing energies, and means positioned along the path of said particles downstream of the output of said accelerator for shortening the duration of each of said pulses by grouping particles contained in a respective pulse into a relatively reduced spatial length, and focusing means positioned along the path of said shortened pulses.

12. In a particle accelerator installation, an accelerator operativeto accelerate particles to relativistic velocities and to deliver the particles in the output thereof in the form of pulses containing a spatial sequence of particles having successively decreasing energies, and means positioned along the path of said particles downstream of the output of said accelerator for shortening the duration of each of said pulses by grouping particles contained in a respective pulse into a relatively reduced spatial length, and deflecting means for the particles between said accelerator and said shortening means.

13. In a particle accelerator installation, an accelerator operative to accelerate particles to relativistic Velocities and to deliver the particles in the output thereof in the form of pulses containing a spatial sequence of particles having successively decreasing energies, and means positioned along the path of said particles downstream of the output of said accelerator for shortening the duration of each of said pulses by grouping particles contained in a respective pulse into a relatively reduced spatial length, and deflecting means for the particles between said accelerator and said shortening means, and second deflecting means for correcting the effects of said first-named deflecting means, said second deflecting means being positioned along the path of said shortened pulses.

14. In a particle accelerator system having an accelerator producing in the output thereof pulses of accelerated electrons moving at relativistic velocities, each pulse consisting of a chaplet of washers separated by predetermined distances, the improvement essentially consisting of a corrective system disposed along the path of the accelerated electrons downstream of the accelerator output including re-grouping means having input means and output means for re-grouping the washers in each chaplet so that the length of a respective chaplet in said output means is considerably shorter than the length thereof corresponding to the pulse length in said input means.

15. In a particle accelerator system having an accelerator producing in the output thereof pulses of acelerated particles moving at relativistic velocities, each pulse consisting of a chaplet of Washers separated by predetermined distances, the improvement essentially consisting of a corrective system disposed along the path of the accelerated particles downstream of the accelerator output including i e-grouping means having magnet means, input means and output means for re-grouping the washers in each chaplet so that the length of a respective chaplet in said output means is considerably shorter than the length thereof corresponding to the pulse length in said input means, said magnet means effectively producing paths of different lengths followed by each washer proportional to the average energy level thereof.

16. In a particle accelerator system having an accelerator producing in the output thereof pulses of accelerated particles moving at relativistic velocities, each pulse consisting of a chaplet of washers separated by predetermined distances, the improvement essentially consisting of a corrective system disposed along the path of the accelerated particles downstream of the accelerator output including re-grouping means having magnet means, input means and output means for re-grouping the washers in each chaplet so that the length of a respective chaplet in said output means is considerably shorter than the length thereof corresponding to the pulse length in said input means, said magnet means effectively producing paths of different lengths followed by each washer proportional to the average energy level thereof in such a manner as to produce coincidence of all washers in a respective chaplet in said output means.

References Cited UNITED STATES PATENTS 2,395,647 2/1946 Strobel 33180 2,872,574 2/1959 McMillan et al 328-234 3,031,596 4/1962 Leboutet et al 3l3 84 3,089,092 5/1963 Plotkin et al 328155 3,268,729 8/1966 Adachi et a1. 250-845 IAMESXV. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.