EP0222786B1 - Cyclotron - Google Patents

Abstract

Non supraconductor cyclotrons. According to the invention such a cyclotron (1) comprises a magnetic circuit comprising at least three sectors (13, 13') called ''hills'', wherein the air-gap (19) is reduced to a size similar to that of the accelerated beam and wherein the magnetic flux is substantially concentrated, said hills being separated by spacings (15) configured like sectors called ''valleys'' wherein the air-gap is of a very large size so that the magnetic flux is essentially null and the magnetic circuit is further comprised of a single pair of coils (2), which are substantially circular and surrounding the hills (13, 13') and the valleys (15, 15'). This form of execution enables to achieve important energy savings, the power consumed being reduced for example from 100kW for a normal non supraconductor cyclotron to 7 kW in the form of execution according to the invention.

Description

The present invention relates to a conventional cyclotron of a new design which makes it possible to significantly reduce the energy requirements.

There are two types of known cyclotrons: cyclotrons using superconductive coils (superconductive cyclotrons) and cyclotrons using non-superconductive coils (conventional cyclotrons).

Superconducting cyclotrons do not use electrical power to maintain the magnetic field necessary to accelerate particles. However, the technology of superconducting coils and associated cryogenics remains complex and expensive. In addition, these coils required liquid helium as the coolant. These considerations strongly restrict the use of superconductive cyclotrons.

On the other hand, in the case of conventional cyclotrons, a large part of the power is used to produce and profile the magnetic field necessary for the acceleration of the particles.

There are currently classic so-called "compact" cyclotrons which have only one pole. In this case, the acceleration electrodes, generally called "dice" are arranged in the air gap. Consequently, the power supplied to the cyclotron must be relatively high to establish the magnetic field in an air gap of increased size. On the other hand, the vacuum box is very simple and inexpensive.

Also known are so-called "separate sector" cyclotrons, in which the magnetic structure is divided into separate units, entirely autonomous, in the form of sectors. In the free spaces left between these "separate sectors" the acceleration devices have been installed. Consequently, the air gap of the magnetic sectors can be reduced and, consequently, the number of amps / turn required to produce the magnetic field is less important.

However, these cyclotrons have a series of drawbacks. First of all, each separate sector is equipped with a pair of coils. These coils are of complex shape (sector-shaped) and, to free the free space between the sectors, they must be of minimum section.

This means that the current density must be high in these coils and, consequently, the electric power required to produce the magnetic field remains high although the number of amps / revolution is lower.

Finally, the sectors being mechanically independent, the mechanical design of the cyclotron, in particular of the vacuum box, is complex and expensive.

A conventional cyclotron of the compact type corresponding to the preamble of claim 1 is known from French Patent Application FR-A-2 176 485.

The object of the present invention is to provide a new type of non-superconductive cyclotron where the electric power required to produce the magnetic field is much lower than in the above-mentioned conventional cyclotrons, namely the "compact" cyclotron and the separate sector cyclotron. ".

This object can be achieved by a cyclotron according to claim 1, comprising a new magnetic structure where there is a small air gap, which reduces the number of amps / revolution required, but also a pair of essentially circular coils and advantageously of large section, which reduces the current density and therefore the electrical power required to produce the number of amps / turn required.

Another object of the invention is to avoid in the new structure the mechanical complexity inherent in so-called "separate sector" cyclotrons.

This new structure specific to the conventional cyclotron according to the invention is characterized in that it comprises at least three sectors called "hills" where the air gap is reduced to a dimension close to that of the accelerated beam and where the magnetic flux is essentially concentrated. , separated by sector-shaped spacings called "valleys", where the air gap is very large (for example, but not limited to, where the air gap is of the order of 30 times that of the hills ), so that the magnetic flux is essentially zero and by a single pair of essentially circular coils essentially surrounding the "hills" and the "valleys", flux returns being advantageously arranged outside the coil opposite the "hills ", with a view to closing the magnetic circuit.

Another advantageous characteristic of the cyclotron according to the invention is that the sectors called "hills" are rigidly assembled on two plates called "yoke" forming covers for the vacuum box and channeling the magnetic flux towards the aforementioned flux returns.

According to the invention, the cyclotron preferably comprises four sectors made of a conventional magnetic material.

A great advantage of the device according to the invention lies in the fact that the acceleration electrodes can be placed in the "valleys" and that, consequently, the air gap can be reduced to a minimum, that is to say at the location necessary for the circulation of the particles to be accelerated. This results in a notable saving in the energy consumed.

Another advantage of the cyclotron according to the design principle of the invention lies in the simplicity of the coils which provide the magnetic induction field.

Geometries with similarities have already been described for superconductive cyclotrons by documents US-A-3,925,676; FR-A-2 234 733; IEEE Transactions on Nuclear Science Vol. NS-30 (1983) Aug., No. 4, Part 1, New York, USA p 2126-2128 E. Acerbi; and Nuclear Instruments & Methods in Physics Research, vol. 220 (1984) Febr., No. 1, Amsterdam, Netherlands, p 186-193 U. Trinks.

However, the similarity between the superconductive cyclotrons mentioned above and the non-superconductive cyclotron according to the invention is limited to the geometry. Magnetic functioning is fundamentally different.

To obtain a low number of amps / revolution in the cyclotron according to the invention, the magnetic flux is concentrated in the "hills" where the air gap is minimum and essentially zero in the "valleys" where the air gap is large.

In superconducting cyclotrons of similar geometry, on the contrary the steel is completely saturated and the magnetic flux is very high in the "valleys" as in the "hills" (see ref. Nuclear Instruments & Methods in Physics Research, vol. 220 ( 1984) Febr., No. 1, page 187, table 1) and the desired effect of reducing the number of ampere turns is not achieved.

Furthermore, unlike existing conventional cyclotrons, the structure has a symmetry of revolution, with flow returns advantageously arranged in alignment with each of the sectors, which completely eliminates the harmful asymmetries of the magnetic field associated with conventional designs.

In addition, the design of the cyclotron according to the invention makes it possible to house the vertical beam accelerator electrodes as well as the final stage of the power amplifier directly in the "valleys". Advantageously, the electrode plate is inductively coupled to the cyclotron cavity. The stability of the system is only improved.

Although such an inductive coupling has already been used in conventional cyclotrons, it has never been used to solve the problems of variable charge in high intensity cyclotrons.

Conventional cyclotrons also use assemblies of the acceleration electrodes on a vertical beam resonating at half wavelength. These cavities are generally excited from a high frequency power generator, located at a certain distance.

Furthermore, in the case of conventional cyclotrons, if the intensity of the beam accelerated by the cyclotron is such that the acceleration power becomes comparable to the power dissipated by the Joule effect in the cavities, the apparent shunt impedance of the cavity is decreased, and the coupling system is out of tune, causing the appearance of reflected power on the transmission line. This effect can cause instabilities in the interactive beam-accelerating system.

• la figure 1 représente une coupe schématique selon le plan médian d'un cyclotron selon l'invention; et
• la figure 2 représente une coupe selon la ligne II-II de la figure 1.

Other details and advantages will appear more clearly in the description which follows, accompanied in figures in which:

• Figure 1 shows a schematic section along the median plane of a cyclotron according to the invention; and
• FIG. 2 represents a section on line II-II of FIG. 1.

It is obvious that the present description is given by way of example only and that it is not intended to limit the scope of the present invention.

Accessory devices such as the outlet conduits, the cyclotron support, the vacuum pumps, are mentioned by way of illustration but are not specific to the cyclotron according to the invention. In the figures, identical references represent identical or analogous elements.

The magnetic structure of the cyclotron has a symmetry with respect to the plane in which the particles are accelerated, called "median plane" 17, for example placed horizontally and with respect to an axis 26 perpendicular to this plane.

This magnetic structure consists of a number of elements made of ferromagnetic material (3, 5, 11, 13, 13 ') and a pair of coils made of a conductive material (21, 23).

• 1) Deux plaques de base 3 et 5, appelées culasses, par exemple en forme de disques situées essentiellement de façon coaxiale par rapport à l'axe 26, parallèle et symétrique par rapport au plan médian 17, l'une étant au-dessus du plan médian, l'autre étant en-dessous de celui-ci.
• 2) D'au moins trois secteurs supérieurs 13 et d'un nombre égal de secteurs inférieurs 13' situés l'un en face de l'autre symétriquement par rapport au plan médian 17, séparés par un entrefer 19 minimum, c'est-à-dire réduit à une dimension voisine de la dimension axiale du faisceau accéléré et juste suffisant pour le passage du faisceau de particules, le flux magnétique étant de cette manière essentiellement concentré à cet endroit. Les secteurs 13 et 13' sont fixés rigidement à la culasse supérieure 3 et inférieure 5 et sont appelés collines.
• 3) D'au moins trois retours de flux 11 réunissent de façon rigide la culasse inférieure 3 et supérieure 5, situés à l'extérieur, en face des secteurs 13 et 13' et séparés de ceux-ci par un espace de forme annulaire dans lequel est située la paire de bobines 21, 23. Outre la fonction mécanique précitée, ces "retours de flux" 11 assurent le retour du flux magnétique tout en laissant accessibles les espaces angulaires 15 et 15' situés entre les collines.

The ferromagnetic structure consists of:

• 1) Two base plates 3 and 5, called cylinder heads, for example in the form of discs situated essentially coaxially with respect to the axis 26, parallel and symmetrical with respect to the median plane 17, one being above the median plane, the other being below it.
• 2) At least three upper sectors 13 and an equal number of lower sectors 13 'located one opposite the other symmetrically with respect to the median plane 17, separated by a minimum air gap 19, that is ie reduced to a dimension close to the axial dimension of the accelerated beam and just sufficient for the passage of the particle beam, the magnetic flux being in this way essentially concentrated at this location. The sectors 13 and 13 'are rigidly fixed to the upper 3 and lower 5 yoke and are called hills.
• 3) At least three flow returns 11 rigidly join the lower 3 and upper 5 yoke, located outside, opposite the sectors 13 and 13 'and separated from these by an annular space in which is located the pair of coils 21, 23. In addition to the aforementioned mechanical function, these "flux returns" 11 ensure the return of the magnetic flux while leaving accessible the angular spaces 15 and 15 'located between the hills.

The coils 21 and 23 are essentially circular in shape and are located in the annular space left between the sectors 13 and 13 'and the flow returns 11. Advantageously, these coils have a large section, which results in a low current density. and therefore a low electrical power dissipated to produce the magnetic field.

The angular spaces 15 and 15 ', located respectively between the sectors 13 and 13', are called "valleys". The air gap is important, that is to say of very large dimension compared to that of the air gap of the hills because it extends from the upper cylinder head 3 to the cylinder head lower 5. This air gap is, for example, on the order of 30 times greater than the air gap 19. The magnetic flux in the valleys is essentially zero.

The various constituent elements are assembled by means known per se as bolts.

The central duct 25 is intended to receive, at least in part, the source of particles to be accelerated which are injected into the center of the device by means known per se.

In the case shown of a cyclotron with four sectors or four "hills", the angle of a sector is advantageously of the order of 54 °.

A cyclotron according to the invention advantageously comprises the final stages of two high frequency power amplifiers 27 inductively coupled by a loop to the acceleration electrodes 28 with vertical beam 29, which are housed in the "valleys" between the sectors 13, 13 '.

In the cyclotron according to the invention, the vacuum chamber (31) can advantageously be very simple. It consists of a ring of non-magnetic material, extending from the upper yoke 3 to the lower yoke 5 in the space left between the sectors 13, 13 'and the coils 21, 23.

Note the advantage of the simplicity of a pair of large coils and the air gap reduced to a minimum, which allows significant energy savings.

As an example, we can mention that, in the case of a cyclotron with an energy of about 30 MeV, the air gap in the hills is 3 cm and the magnetic field 1.8 T (18 kGs) , while in the valleys the air gap is 106 cm and the magnetic field 0.04 T (0.4 kGs). In this case the number of ampere turns required is 33,000 At per coil, which, with a current density of 50 A / cm ² in the coils gives a consumed power of 7 kW for the cyclotron according to the invention against 100 kW for a normal cyclotron.

Note, for example, that for a superconductive cyclotron according to US-A-3 925 676, the number of ampere turns required is 1.8 10 ⁶ At per coil (col. 4, line 33 to 43).

Claims (8)

1. Non-superconductive cyclotron of the compact type intended for the acceleration of a beam of particles, comprising a magnetic structure which exhibits a symmetry with respect to a medium plane (17) in which the particles are accelerated by means of accelerating electrodes (28) and with respect to an axis (26) perpendicular to this plane; said magnetic structure comprising:

two base plates (3, 5) situated substantially in a configuration which is coaxial with respect to the axis (26) parallel and symmetrical with respect to the median plane (17), one (3) being above the median plane and the other one (5) being below the latter;

at least three upper sectors (13) and an equal number of lower sectors (13'), rigidly fixed to the respective plates situated one opposite the other, symmetrically with respect to the median plane (17), and axially separated, the opposite sectors being disposed symmetrically with respect to the axis (26) to form alternated regions called "hills" (13, 13') and "valleys" (15, 15'), the axial separation of the sectors defining an air gap (19) of the "hills" and the axial separation of the base plate (3, 5) defining an air gap of the valleys;

flux returns connecting together rigidly the base plates (3, 5);

the base plates (3, 5), the sectors (13, 13') and the flux returns (11) being carried out in a ferromagnetic material;

and a substantially circular pair of windings (21, 23) substantially surrounding the hills (13, 13') and the valleys (15, 15');

the said cyclotron being characterized in that:

the air gap (19) of the hills is reduced to a dimension close to the axial dimension of the accelerated beam and in that the magnetic flux is substantially concentrated therein, while the air gap of the valleys is of a very large dimension with respect to that of the air gap of the hills, in order that the magnetic flux in the valleys should be substantially zero.

2. Cyclotron according to Claim 1, characterized in that the sectors called "hills" are rigidly fixed to a single component constructed of ferromagnetic material.

3. Cyclotron according to Claim 1, characterized in that the air gap of the valleys (15, 15') is of the order of 30 times greater than the air gap (19) of the hills (13, 13').

4. Cyclotron according to Claim 1, characterized in that it includes flux returns fitted (11) outside the pair of annular windings (21, 23), opposite the hills (13, 13'), to form the magnetic circuit.

5. Cyclotron according to Claim 1, characterized in that the sectors (13, 13') designated as hills exhibit an angle of the order of 54°.

6. Cyclotron according to Claim 1, characterized in that the accelerating electrodes (28) are accommodated to the valleys (15, 15').

7. Cyclotron according to Claim 1, characterized in that the final stage of a power amplifier (27) is mounted in the valleys (15, 15').

8. Cyclotron according to Claim 1, characterized in that the final stage of a power amplifier (27) is inductively coupled with the accelerating electrodes (28).

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