Multiperiodic accelerator structures for linear particle accelerators

Abstract

High efficiency linear accelerator structures comprising a succession of cylindrical resonant cavities which are accelerating cavities, and coupling annular cavities which are located at the periphery thereof, each of these annular cavities being coupled to two adjacent cylindrical cavities.

Description

Multiperiodic structures in linear accelerators are generally constituted by groups of two or three resonant cavities which are accelerating cavities; these accelerating cavities are coupled with one another by a coupling cavity, each of such groups(two accelerating cavities and one coupling cavity) corresponding to one period of the electromagnetic wave which is created within this accelerating structure.

It is important that, in such structures, the energy stored in the accelerating cavities should be maximum and that the energy stored in the coupling cavities should be as low as possible.

It is an object of this invention to achieve this aim in a particularly effective manner, while, in the same time, providing a structure which is simple to manufacture, is readily adjustable and has an operating frequency which is not sensitive to minor machining or adjustment inaccuracies.

In accordance with the invention, there is provided a linear structure for a linear particle accelerator comprising an input cavity and a plurality of successive accelerating cylindrical resonant cavities, means for coupling said cylindrical resonant cavities to each other and means for feeding, said electromagnetic energy into said input cavity, said coupling means comprising a plurality of annular cavities coaxial with said cylindrical resonant cavities and located at the peripheries thereof, each of said annular cavities being coupled to two adjacent cylindrical resonant cavities.

For the better understanding of the invention and to show how the same way be carried into effect, reference will be made to the drawings accompanying the ensuing description in which:

FIG. 1 schematically illustrates a linear structure in accordance with the invention,

FIG. 2 illustrates a vector diagram indicating the phase shift in the incident wave and reflected wave in one set of three associated cavities (two accelerating cavities and one coupling cavity),

FIGS. 3 and 4 schematically illustrate an example of a coupling cavity in accordance with the invention, and the equivalent circuit diagram of said cavity,

FIG. 5 illustrates in detail the elements of a structure according to the invention,

FIGS. 6 and 7 respectively illustrate a triperiodic structure according to the invention and in a somewhat simplified manner, the arrangement of the coupling apertures between the different cavities of this structure,

FIGS. 8, 9 and 10 respectively illustrate another embodiment of an accelerator structure according to the invention, and two tuning systems therefor,

FIGS. 11 to 14 illustrate still other embodiments of a structure according to the invention and,

FIGS. 15 and 16 respectively show resonance frequency curves of two biperiodic structures in accordance with the invention.

The accelerator structure in accordance with the invention, schematically illustrated in FIG. 1, comprises a succession of cylindrical cavities a.sub.1, b.sub.1, a.sub.2, b.sub.2, a.sub.3, b.sub.3 having a common axis X.sub.1 X.sub.2 substantially coincidental with the mean trajectory of the accelerated particle beam.

The pairs of adjacent cavities a.sub.1 and b.sub.1, b.sub.1 and a.sub.2, a.sub.2 and b.sub.2, b.sub.2 and a.sub.3, a.sub.3 and b.sub.3, are preceded by an input cavity a.sub.o.

Two adjacent cavities have a common walls m.sub.1, n.sub.1, m.sub.2, n.sub.2, m.sub.3, n.sub.3, perpendicular to the axis X.sub.1 X.sub.2. In the embodiment shown by way of example, every wall m.sub.1 is provided with two holes u and v for coupling purposes, whilst the other walls n.sub.1 have no coupling apertures at all. The cylindrical cavities a.sub.1, b.sub.1, a.sub.2, b.sub.2, a.sub.3, b.sub.3, are respectively coupled, in pairs to annular cavities c.sub.1, c.sub.2, c.sub.3, disposed coaxially at the periphery of the cylindrical cavities a.sub. 1, b.sub.1, a.sub.2, b.sub.2 a.sub.3, b.sub.3, with which they are associated by means of coupling holes s.sub.1, t.sub.1, s.sub.2, t.sub.2, s.sub.3, t.sub.3 formed in the walls p.sub.1, p.sub.2, p.sub.3, respectively common to an annular cavity and to the pair of associated cylindrical cavities. In FIG. 1, the path followed by the electromagnetic which has been fed into the first cavity, that is to say into the input cavity a.sub.o, for example, by means of a coupling loop B, is illustrated by a full line and the reflected electromagnetic wave by a broken line.

The shape and dimensions given to the first and last cylindrical cavities a.sub.o and b.sub.n of the structure, and the shape and dimensions given to the annular cavities of coupling cavities c.sub.i are so selected that, as known, in the standing wave operation the components of incident and reflected waves add to each other in the cylindrical cavities and cancel one another out in the annular cavities. The vectoral diagram of FIG. 2 illustrates the phase shift 2.pi./3 which exists between the cavities a.sub.1, c.sub.1, b.sub.1, of a "2.pi.mode" triperiodic structure.

FIG. 3 schematically illustrates an example of an annular cavity and FIG. 4 shows its equivalent circuit diagram.

Designating by r and r + .DELTA.r the outer radius of the annular cavity Ci, by .DELTA.z the length of the terminal inductive portions S.sub.1 and S.sub.2 of the annular cavities C.sub.i, by 2 .DELTA.z of central inductive portions S.sub.3, by 1 the length of the re-entrant portions R.sub.1 and R.sub.2, these portions S.sub.1, S.sub.2, S.sub.3, R.sub.1, R.sub.2 being respectively illustrated in the equivalent circuit diagram of FIG. 4 as inducators L.sub.1, L.sub.2 and L.sub.3 and the capacitors C.sub.1 and C.sub.2, the value of the inductions L.sub.1 and L.sub.2 is given by: ##EQU1## whilst that of the capacitances C.sub.1 and C.sub.2 is given by: ##SPC1##

where d is the width of the capactivie space formed by the re-entrant portions.

If the inductance of the intermediate inductive portion L.sub.3 is equal to 2 L.sub.1, the resonance frequency of the annular cavity is given by: ##EQU2##

FIG. 6 illustrates, in longitudinal section, an example of a triperiodic structure in accordance with the invention. In this example, the accelerator structure is produced by stacking, into a cylindrical sleeve 13, elements e.sub.1 and e.sub.2 which are solids of revolution, such as shown in FIG. 5. Each element e.sub.1 has substantially the shape of a circular plate m exhibiting a central portion 2 having an increase thickness through which a hole 3 extends. Each element e.sub.2 is in the form of a cylindar the lateral wall of which is constituted with an embattle ring 6. This cylinder is provided with a circular wall n comprising a central portion 4 having an increased thickness and through which a hole 5 extends. The elements e.sub.1 are furthermore provided with apertures u and v formed in their circular plate m and the rings 6 constituting the lateral walls of the elements e.sub.2 contain two apertures s and t disposed symmetrically at either side of the circular plate n. The rings 6 of the elements e.sub.2 exhibit two shoulders 11 and 12 between which accomodate the plates m of the elements e.sub.1.

The elements e.sub.1 and e.sub.2 are assembled together in the manner shown in FIG. 6, within a cylindrical sleeve 13 thus forming cylindrical cavities a.sub.1, b.sub.1, a.sub.2 b.sub.2, a.sub.3, b.sub.3, and annular cavities c.sub.1, c.sub.2, c.sub.3 . . . . The coupling between the cavities b.sub.1 and a.sub.2 ; b.sub.2 and a.sub.3 . . . . is effected through apertures u.sub.1, v.sub.1 ; u.sub.2, v.sub.2, . . . . formed in the circular plates m. To avoid direct coupling between the cavities c.sub.1 and a.sub.2, c.sub.2 and a.sub.3 . . . . , the coupling apertures u.sub.1, v.sub.1, u.sub.2, v.sub.2, . . . . are arranged in such a fashion that they are staggered in relation to the apertures s.sub.1, t.sub.1, s.sub.2, t.sub.2, as shown in the cut-away perspective view of FIG. 7.

The cavities a.sub.o and b.sub.n at the ends of the structure are identical to one another but differ slightly from the cavities a.sub.1, a.sub.2, a.sub.3 . . . b.sub.1, b.sub.2 . . . Their dimensions are such that at the resonance frequency of the accelerating cavities a.sub.1, a.sub.2, . . . b.sub.1, b.sub.2, when the latter are operating in the 2.pi. mode for example, they subject the reflected wave to a phase shift of .pi./2 in relation to the input wave so that within the accelerating structure a standing wave situation is created in which the electromagnetic field is cancelled in all the annular coupling cavities so that optimum efficiency on the part of the structure is ensured.

FIG. 8 illustrates embodiment of a triperiodic structure in accordance with the invention. The elements 20 and 21 are assembled by means of rods which are, for example, four and are 90.degree. angularly spaced. These rods extend through tubular passages 22 longitudinally disposed at the periphery of the elements 20 and 21. At least one adjustable tuning plunger 23 is associated with each annular coupling cavity c.sub.i (FIG. 9), and at least two adjustable tuning plungers 24 and 25 are respectively associated with each two adjacent cavities a.sub.i and b.sub.i to tune these cavities c.sub.i, a.sub.i and b.sub.i.

The structures described hereinbefore are of the triperiodic type but biperiodic structures can be produced in a similar way. FIG. 11 illustrates in longitudinal section a biperiodic structure operating in the .pi. mode. The length of the cylindrical accelerator cavity is in this case equal to .beta..lambda./2, being the reduced velocity of the particles propagating through the cylindrical cavities and .lambda. the operating wavelength of the structure. Such a structure is particularly suitable because it is constituted by identical elements 30, which build up cylindrical cavities d.sub.1, d.sub.2, d.sub.3 . . . The coupling between two adjacent cylindrical cavities d.sub.1, d.sub.2 ; d.sub.2, d.sub.3 ; . . . is effected solely through the medium of the annular cavities f.sub.1, f.sub.2 . . . by means of coupling apertures s.sub.1, t.sub.2 ; s.sub.2, t.sub.3 ; . . .

FIG. 12 illustrates another embodiment of a biperiodic structure in which the phase shift between two successive cylindrical accelerator cavities is 2, the electrical length of each of these cavities being .beta..lambda.. The structure comprises cylindrical cavities g.sub.1, g.sub.2, g.sub.3 . . . coupled in pairs through annular cavities h.sub.1, h.sub.2 . . . having a profile with three re-entrant portions r.sub.1, r.sub.2, r.sub.3, the components of the electric field H of the electromagnetic wave being distributed within the structure in the manner indicated in FIG. 12.

This "2.pi.mode" biperiodic structure is better suited to accelerators of relatively low energy whereas ".pi.mode" biperiodic structures and "2.pi.mode" triperiodic structures are better suited to high energy accelerators. In other words, the efficiency of a structure is better if the length of the accelerator cavity is substantially equal to their radius; however this radius depends essentially upon the operating wavelength .lambda.. Thus, the fact that the length of a cell of a ".pi. mode" biperiodic structure is .beta..lambda. (instead of .beta..lambda./2 as in the case of ".pi. mode" biperiodic structures or "2.pi. mode" triperiodic structures), favours the acceleration of particles of relatively low velocity.

The structures in accordance with the invention have been described by way of non-limitative examples and their characteristics generally depend on the selected dimensions of the coupling apertures, on their location and number. By a suitable selection of the parameters it is possible to obtain the desired predetermined operating pass band.

It is also possible to improve the operation of a ".pi./2 mode" biperiodic structure having annular coupling cavities c.sub.1, c.sub.2 . . . c.sub.n with two re-entrant portions such as illustrated in FIG. 6, by arranging the coupling apertures s and t not at the ends of the annular cavity c.sub.1, c.sub.2 . . . c.sub.n as in FIG. 6 but in the central part of said annular cavity as shown in FIG. 13.

As a matter of fact, the resonance frequency of the system formed by the accelerating cavities a.sub.i, b.sub.i and coupling cavities; c.sub.i highly depends upon the coupling factor, and therefore upon the dimension of the coupling apertures s, t when said apertures s, t are arranged at the ends of the re-entrant section of the annular cavity c.sub.i. It is possible to remedy this drawback by arranging the coupling apertures s, t at the centre of the annular cavity c.sub.i (FIG. 13), as mentioned before.

Apertures s and t are therefore formed obliquely in the central zone of the wall p (FIG. 13) common to the accelerating cavities a.sub.i, b.sub.i and to the coupling cavity c.sub.i associated therewith, these apertures s and t, which respectively couple the annular cavity c.sub.i with the accelerating cavities a.sub.i and b.sub.i, being arranged on different radii making an angle with one another in order to avoid direct coupling between the two apertures s, t. The dimensions of these apertures s and t arranged in the central zone of the annular cavity c.sub.i are not critical in so far as the resonance frequency of the accelerating structure is concerned. This makes it possible to adjust separately the frequency of the structure and the coupling between the cavities.

FIGS. 15 and 16 respectively illustrate, in the case of a biperiodic structure operating in the .pi./2 mode, the variation of the resonance frequency of the structure as a function of the dimensions d.sub.1, d.sub.2, d.sub.3, of the coupling apertures s, t when these apertures are arranged at the ends of the annular cavity c.sub.i (FIG. 15) and in the case where the apertures are arranged at the centre of said annular cavity c.sub.i (FIG. 16).

FIG. 14 illustrates a biperiodic "2.pi. mode" structure, the annular cavity c.sub.i of which has three re-entrant portions. The coupling apertures s and t are in this case arranged at either side of the central re-entrant portion and in different planes, in order to avoid direct coupling between the apertures s and t.

The particle accelerator structures in accordance with the invention can advantageously be utilised in linear electron or proton accelerators.

Claims

What we claim is:

1. A linear structure for a linear accelerator comprising an input cavity and a plurality of successive pairs of accelerating cylindrical resonant cavities, means including coupling apertures for coupling said pairs of cavities, means for coupling said cavities forming each said pair to each other comprising a plurality of annular cavities each coupled through apertures to one of said pairs of cavities, said annular coupling cavities arranged coaxially with said cylindrical resonant cavities and located at the periphery thereof; and means for feeding electromagnetic energy into said input cavity.

2. A linear structure as claimed in claim 1, wherein said annular cavities have a section of re-entrant profile type.

3. A linear structure as claimed in claim 2, wherein said structure is biperiodic.

4. A linear structure as claimed in claim 3, wherein every two adjacent cylindrical cavities are coupled to each other by means one of said annular cavities, said annular cavities having a re-entrant section, and being provided with coupling apertures (s, t).

5. A linear structure as claimed in claim 4, wherein said apertures (s, t) are arranged at the ends of said re-entrant section, said section having two re-entrant portions.

6. A linear structure as claimed in claim 4, wherein said apertures (s, t) are located in the central portion of said re-entrant section, said section having two re-entrant portions.

7. A structure as claimed in claim 2, wherein said structure is triperiodic.

8. A linear structure as claimed in claim 7, wherein adjacent pairs of cylindrical cavities are coupled through apertures formed in a transverse common wall of said adjacent pairs of cylindrical cavities.