Linear particle accelerator using magnetic mirrors

The present invention relates to a linear accelerator of charged particles being able to be used both in industrial and in medical apparatus when a particle beam of high energy is necessary, this improved linear accelerator making it possible, whilst achieving a reduction in size, to produce a high performance beam of accelerated particles.

An object of the present invention is a linear accelerator for accelerating a charged particle beam comprising a particle source, a linear accelerating structure constituted by a succession of resonant cavities, means for injecting electromagnetic energy into said structure; magnetic deflection means for deflecting said particle beam, said magnetic deflection means comprising at least a first achromatic and stigmatic magnetic mirror capable of reflecting said beam of particles in a direction which is at 180° to the incident direction of the beam, allowing said particle beam to pass at least twice through said accelerating structure; said particle source being arranged on the axis of said accelerating structure; and said particle source having a form such that it can be traversed along its axis by said accelerated beam having effects at least two passes through said accelerating structure.

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

FIGS. 1 and 3 illustrate two embodiments of a linear particle accelerator in accordance with the invention;

FIG. 2 illustrates an example of a gun having an annular cathode, as used in the accelerator in accordance with the invention;

FIG. 4 illustrates an embodiment of an irradiation device operating at two energy levels, utilising a particle accelerator in accordance with the invention.

FIG. 1 illustrates an embodiment of a particle accelerator in accordance with the invention. This accelerator comprises:

-- A PARTICULE SOURCE 1 GENERATING A BEAM F of charged particles;

-- A STANDING WAVE ACCELERATING STRUCTURE S_A constituted by a series of accelerating cavities Ca;

-- A COUPLING SYSTEM 5 WHICH MAKES IT POSSIBLE TO INJECT, INTO THE STRUCTURE S_A, electromagnetic energy furnished by a microwave generator 6 (a magnetron for example);

-- COILS 7 AND 8 FOR MAGNETIC FOCUSING PURPOSES, WHICH FOCUS THE BEAM F along the accelerating structure S_A ;

-- an achromatic and stigmatic magnetic mirror M₁ which enables the incident beam F_i to be deflected through such an angle in order to reflect it into the accelerating structure S_A where it is re-accelerated. The magnetic mirror M₁, as shown in FIG. 1, is an achromatic and stigmatic mirror constituted by two deflectors D₁ and D₂ each imparting a deflection of 270° to the beam F_i issuing from the accelerating structure S_A, so that the reflected beam F_r is substantially coincidental with the incident beam F_i ;

-- a magnetic deflector D_o making it possible to deflect the reflected beam F_r through 270° for example towards a target C₁ after its second pass through the structure S_A and passage through the particle source 1.

The stigmatic and achromatic magnetic deflectors D_o, D₁ and D₂ have been described in U.S. Pat. No. 3,691,374.

FIG. 2, shows a particle source which, in this case, is an electron-gun K. The electron-gun K comprises a cathode 1 of annular form, the opening 11 at the centre of which is circular and has a diameter d_k greater than the diameter of the reflected beam F_r. The cathode 1 can be indirectly heated by a toroidal filament 2, the central hole being substantially of the same size as the opening 11 in the cathode 1.

Electrodes 3 and 4 for controlling the beam (modulating electrode, anode) are provided, at their centre, with circular openings 12 and 13 to pass the incident beam F_i and the reflected beam F_r. The diameter of the opening 13 in the anode 4 is slightly smaller than the diameter d_k of the opening 11 in the centre of the cathode 1, thus forming a screen in order to protect said cathode 1.

FIG. 3 schematically illustrates another embodiment of a linear accelerator in accordance with the invention. At either side of the accelerating structure S_A which is associated with a particle source K as described earlier, there are respectively arranged magnetic mirrors M₂ and M₃ each respectively constituted by two deflectors D₃, D₄ and D₅, D₆.

The magnetic deflectors D₃ and D₄, which are achromatic and stigmatic deflectors, each deflect the particle beam F_i through 270°. They are respectively constituted by electromagnets equipped with pairs of polepieces P_o, P₁, P₂ and P₃, P₄, P_o. The polepieces P₁, P₂, P₃, P₄ have the form of sectors whose angle is substantially equal to 90° and they are disposed symmetrically in pairs in relation to the axis of the accelerating structure S_A as FIG. 3 shows. These polepieces P₁, P₂, P₃, P₄ respectively comprise entry faces E₁, E₂, E₃, E₄ and exit faces S₁, S₂, S₃, S₄. The electromagnet comprising the polepieces P_o is common to the deflectors D₃ and D₄. These polepieces P_o have a rectangular shape and have two entry faces E_o, E_o1 and two exit faces S_o and S_o1. The faces S_o, E₁ ; S₁, E₂ ; S₃, E₄ ; and S₄, E_o1, are arranged in pairs, parallel to each other and are separated by an interval L equal to the radius of curvature R of the mean trajectory of the particle beam deflected by the magnetic field formed respectively between the pairs of polepieces P_o, P₁, P₂, P₃, and P₄.

The magnetic deflectors D₅ and D₆, which are achromatic in nature, are respectively constituted by three electromagnets equipped with pairs of polepieces P₁₀, P₁₁, P₁₂ (deflector D₅) and P₁₃, P₁₄, P₁₀ (deflector D₆), the electromagnet equipped with the polepieces P₁₀ being common to the deflectors D₅ and D₆.

This choice of the shape of the polepieces and parameters defining these deflectors D₅, D₆, leads to the formation of what are known as "diagonal matrix" deflectors having a single term corresponding to the drift space. These deflectors introduce very small focusing aberrations. Moreover, their size is reduced and their saturation magnetic field remains high.

In operation, the particles issuing from the source 1, and having passed once through the accelerating structure S_A, are deflected through 270° by each of the deflectors D₃ and D₄ and are then returned to the accelerating structure structure S_A. After having passed through said structure S_A for a second time, the beam passes through the particle source 1 and is then reflected by the mirror M₃ towards the accelerating structure S_A where the particles are accelerated a third time. The energy of the particles is then such that the beam is no longer reflected by the mirror M₂ but enters the exit deflection system D_S. This magnetic deflection system D_S is achromatic and stigmatic nature. It is constituted by three electromagnets respectively equipped with pairs of polepieces P₅, P₆ and P₇ whose entry faces E₅, E₆ and E₇ and exit faces S₅, S₆ and S₇ are respectively perpendicular to the mean trajectories of the incident and emergent particle beams. The shape of the polepieces P₅ depends upon the energy of the particles passing through them and upon the magnetic field used. In the example shown in FIG. 3, the polepieces P₆ are sectors having an angle a <π/2 whilst the polepieces P₇ are sectors having an angle b >π/2 and the entry face E₅ of the polepieces P₅ is coincidental with the exit face S₀₁ of the polepieces P_o.

Moreover, the exit faces S₅ and S₆ are flat and respectively parallel to the entry faces E₆ and E₇ which are also flat.

Said exit magnetic deflector D_S makes it possible to suitably focus of non-monokinetic particles on a target C₂ arranged on the axis XY of the accelerating structure S_A, or off said axis XY (as shown in FIG. 3).

An accelerator of this kind, in accordance with the invention, thus makes it possible to furnish energies W₁, W₂, W₃ . . . which can be utilised for simultaneously supplying several radiotherapy treatment rooms using irradiating beams having different energies, in the manner shown in FIG. 4.

For example, in order in a treatment room A located at one of the ends of the accelerator in accordance with the invention, to obtain energy particles W₃ corresponding to three passes of the particle beam through the accelerating structure S_A, it is merely necessary to on the one hand, adjust the magnetic field H₄ of the first mirror M₄ to a value h sufficiently high for it to be able to reflect the particles of energy W₁ (corresponding to a single pass of the particles through the structure S_A) and for it to be able to pass the particles of energy W₃ (corresponding to three passes of these particles through the accelerating structure S_A, these particles thus being totally reflected by the second mirror M₅).

Particles of energy W₃ enter the room A along a trajectory T_A and can then be deflected towards the target C_A.

If another treatment room B is arranged at the other end (at the end where the electron-gun K is located) of the structure S_A, then, in this room B, particles of energy W₂ (corresponding to two passes of these particles through the accelerating structure S_A) can be used. In this case, the magnetic field H₂ of the second mirror M₅ can alternately adopt values h₂₁ and h₂₂ (h₂₁ being less than h₂₂) so that the particle beam of energy W₂ is totally reflected by the mirror M₃ when H₂ = h₂₂ and totally transmitted across the mirror M₃ when H₂ = h₂₁. The particles then enter the room B along the trajectory T_B which will be deflected towards the target C_B by the electromagnets E_B1 . . . E_B3.

The linear accelerator in accordance with the invention has the advantage of allowing easy adjustment of the desired energy.

Taking the case of a beam passing through the accelerating section S_A just once, the energy which is required for the particles is achieved in a conventional manner (variation of the amplitude or phase of the HF energy injected into the accelerating structure S_A). For a beam passing several times through the accelerating section S_A, in order to regulate the energy to a desired value, it is possible to act upon the magnetic flux B of the deflector devices, a slight variation in the magnetic field producing a phase-shift between the bunches of particles within the beam.

In other words, if r is the radius of curvature of the trajectory in a magnetic mirror M₄ (or M₅), π the wavelength of operation of the accelerator, then the phase variation φ is given by: ##EQU1## where: π = 10 cm

r = 5 cm

a variation of 1 % in the factor dB/B results in a phaseshift of:

D φ = 8° , 6

it is also possible to vary the phase of the particle beam passing through the accelerating structure S_A, by modifying the interval separating the mirror (or mirrors) from said structure S_A.

The operating parameters of a linear accelerator in accordance with the invention, must be chosen in order to achieve optimum operation, taking account for the phenomenon of automatic compensation of the electrical and magnetic space-charge effects does not exist in the situation where two beams are intersecting. Each of the beams experiences in respect of the other a defocusing magnetic force which is added to the electrical defocusing force due to the space-charge. The electrodes of the particle source will therefore be designed to take account of this phenomenon (the current can be n times the initial current I_o where n is a whole number equal to or greater than 2 and depends upon the number of passes which the beam makes through the accelerating structure S_A).

In the accelerating structure, the current will be substantially equal to (n-1) I_o.

For an accelerated beam having an energy W₃ three times the energy W₁ corresponding to the energy of the particles after they have effected just one pass through the accelerating structure S_A, the high frequency source 6 will in fact experience a load equal to three times that of the current of the accelerated particles. It is therefore necessary to limit said initial current to a third of its value if an accelerator is to be obtained which yields characteristics corresponding to a beam of particles of energy W₃ # 3W₁.

It should be pointed out, finally, that a linear accelerator in accordance with the invention, equipped with two mirrors of the kind M₂ constituted by two deflectors D₃, D₄ as shown in FIG. 3, has certain advantages over an accelerator equipped with mirrors of type M₁ constituted by deflectors D₁, D₂ (FIG. 1). As a matter of fact, these deflectors D₁, D₂ should have entry and exit faces of curvilinear form to enable aberrations to be compensated, whilst the deflectors D₃, D₄ have straight entry and exit faces and negligible aberration.