EP0115720B1 - Electron gun for linear accelerator and accelerating structure comprising such a gun - Google Patents

Description

The invention relates to an electron gun for a linear accelerator, allowing modulation of the electronic current as soon as this current is formed; it also relates to an accelerating structure provided with such an electron gun.

Electron cannons of the triode type, for example, are used to generate electrons injected into an accelerating structure of a Jinerary accelerator; an electromagnetic wave of frequency appropriate to the accelerating structure, makes it possible to accelerate these electrons and to produce a beam whose characteristics vary according to its application.

These applications are varied and there are route accelerators used for scientific purposes, or to constitute irradiators intended for therapeutic treatments, and even more and more frequently to constitute irradiation machines used in industry.

These industrial irradiation machines allow for example the sterilization of various products, and can generally produce electron beams having energies between 1 and 10 MeV.

The electrical power consumed by these machines is considerable, it can reach for example 130 kW of which only 20 kW are found in the beam of accelerated electrons; also the overall performance of such a machine has a direct and significant impact on the cost of processing products. Given the industrial nature of these operations and the importance of the electrical power consumed, an improvement in this yield has very favorable economic consequences.

It is known to group electrons in a packet, using one or more so-called pre-grouping cavities located on the path of the electrons; these pre-grouping cavities being commonly arranged between the electron gun and the accelerating structure into which these packets are injected. Generally the electronic current supplied by the gun is established continuously, during a given and repetitive operating time; the electrons constituting this current are grouped by these cavities, in packets having a duration and a period determined by the operating characteristics of these pre-grouping cavities.

Such grouping in packets of electrons can make it possible to improve the efficiency of a linear accelerator, insofar as it avoids consuming the energy conveyed by the electromagnetic wave, for electrons situated outside the domain of acceptance. in the accelerator phase; but the implementation of these pre-grouping cavities is delicate and complex.

In the present invention, an improvement in the overall efficiency of a machine using an accelerating structure is obtained by means of a modulation of the electronic current, produced from the generation of the latter by the electron gun; this modulation making it possible, for example, to obtain packets of electrons injected into the accelerating structure, without requiring pre-grouping cavities for this purpose.

Electron guns for linear accelerator, are frequently constituted by a thermoemissive cathode and an anode, between which a grid controls the electronic current according to a potential difference established between the grid and the cathode; this potential difference can be continuous to determine an operating time, as it was previously explained, or include within this operating time variations at high frequencies or brief transients. In the latter case, it constitutes a signal, generally produced by a complex injection modulator, transported by a transmission line having appropriate characteristics; but the establishment of this signal between the grid and the cathode, in accordance with the signal produced by the injection modulator, presents difficulties due in particular to stray capacitances.

A French patent application published under No. 2,389,992 describes a short pulse electron gun for a linear accelerator. The structure of this gun is such that it makes it possible to extend the transmission line, driving a control signal, up to a very short distance from the cathode which generates the electrons. But the problem which then arises is to close the transmission line on its ca impedance. characteristic, in the vicinity of the cathode.

The present invention relates to an electron gun for a linear accelerator, capable of producing a modulated electronic current; its arrangement avoids the use of a complex injection modulator, and facilitates the establishment of an alternative potential difference between the grid and the cathode. It also relates to an accelerating structure of a linear electron accelerator, provided with a barrel according to the invention, making it possible to improve the overall efficiency of a machine using such an accelerating structure.

According to the invention, an electron gun for linear accelerator, comprising a transmitting cathode, a grid capable of controlling the intensity of the electron beam emitted by the cathode, an anode pierced with a hole centered around an axis along which are emitted electrons and a transmission line conducting the beam intensity control signal, is characterized in that it comprises a resonant cavity closed on the space between the grid and the cathode, and containing a coupling member electromagnetic linked to the transmission line, which organ makes it possible to inject an electromagnetic wave of frequency F into the resonant cavity, in order to bring the latter into resonance and to excite the grid-cathode space so as to produce between the grid and the cathode a difference in alternating potential of frequency F by which the beam current is modulated.

The electromagnetic wave used to modulate the electronic current generated by such an electron gun, can be taken and injected into the resonant cavity of the gun either: from a generator specific to this function; either from a generator intended to supply an electromagnetic wave injected into an accelerating structure, or by taking the latter electromagnetic wave in an accelerating cavity of this structure, and in particular in an accelerating cavity closest to the gun.

• - La figure 1 montre par une vue schématique en coupe un canon à électrons selon l'invention;
• - La figure 2 montre un canon à électrons conforme à l'invention associé à une structue accélératrice et couplé à une générateur d'ondes électromagnétiques;
• - La figure 3 représente des corrélations de phases entre le courant électronique modulé et l'onde électromagnétique injectée dans la structure accélératrice.

The invention will be better understood thanks to the description which follows and to the three appended figures among which:

• - Figure 1 shows a schematic sectional view of an electron gun according to the invention;
• - Figure 2 shows an electron gun according to the invention associated with an accelerating structure and coupled to an electromagnetic wave generator;
• - Figure 3 shows phase correlations between the modulated electronic current and the electromagnetic wave injected into the accelerator structure.

Figure 1 shows an electron gun 1 according to the invention. This gun 1 has an anode 2; in the nonlimiting example described, this is fixed by screws 4 to an element 50 of an accelerating structure 3, partially shown. In this anode 2, a hole 5 intended for the passage of the electrons, is centered on an axis 6 according to which the electrons, emitted by a cathode 8, are attracted by the anode 2. The cathode 8 comprises a filament 9 and the electrons attracted by the anode 2 pass through the plane of a grid 10 intended to control the passage of the electrons; the grid 10 is held by a metal support tube 11, centered around the emission axis 6. This support tube 11 constitutes an extension of a cavity 13 of the coaxial type, itself centered around the axis of show 6; the support tube 11 is fixed to a first transverse wall 12 of this cavity 13, for example by welding. An internal metal tube 14, also centered around the emission axis 6, is fixed to a second internal transverse wall 26 of the cavity 13.

The cavity 13 thus communicates with an additional space 18 delimited by the grid 10, the support tube 11, the internal metal tube 14, with which this cavity 13 constitutes a resonant cavity 13,18 coaxial.

The resonant cavity 13, 18, arranged along a longitudinal axis coincident with the emission axis 6, is thus limited at one end 7 by the grid 10; the cathode 8 being at a distance d from the grid which constitutes a cathode-grid space 8, 10 on which, in operation, the resonant cavity 13,18 is closed.

An electrical insulating tube 20, made of ceramic for example, surrounds the support tube 11, in order to maintain a vacuum necessary for the operation of the barrel 1, by means of sealing means 22; means for fixing the insulating tube 20 and sealing means 22, being conventional, are not shown.

The cathode 8 is secured in a conventional manner to a plate 21, which comprises a first and a second pad P _l , P ₂ , to which the ends F _l , F ₂ of the filament 9 are respectively connected; the cathode 8 being for example also connected to the second pad P ₂ . The plate 21 of circular shape is embedded in the inner metal tube 14 and, it is supported by a central insulating pin 25; the latter is fixed to the internal transverse metal wall 26 of the cavity 13, in a conventional manner, by means (not shown) allowing, thanks to the sealing means 22, to maintain the vacuum. A first electrical connection 28 connects the first pad P _I , to a sealed crossing 30, and a second electrical connection 29 located axially inside the central pin 25, connects the second pad P ₂ to a second sealed crossing 31. These bushings watertight constitute connection means allowing the supply of the filament 9 and the electrical access to the cathode 8; in order to allow electrical insulation between the cathode 8 and the grid 10, the internal metal tube 14 comprises an insulating ring 60. The electrical supply both at high voltage and at low voltage of the electron gun 1, being carried out so conventional, the circuits intended for this operation as well as possible means for cooling the barrel 1 are not shown.

In the nonlimiting example described, there is also installed in the cavity 13 a second insulating tube 33, placed in the extension of the first tube 20, making it possible to ensure a vacuum tightness thanks to the sealing means 22: this being achieved in order to avoid establishing a vacuum in the whole of the resonant cavity 13, 18, which thus comprises a first part B subjected to the vacuum and a second part C at atmospheric pressure for example. This arrangement facilitates the installation of an adjustment piston 42 and an electromagnetic coupling means 50, which will be explained more fully in the following description.

The cavity 13 is formed by a first and a second metal tube 33, 36 coaxial with the emission axis 6, and constituting respectively an outer wall and an inner wall of the cavity 13; the outer wall 35 being secured to the first transverse wall 12, and the inner wall 36 being secured to the second inner transverse wall 26. A rear end 37 of the cavity 13 has a bottom plate 38 serving in particular to hold the outer walls and interior 35, 36.

The cavity 13 also includes a metal ring 42 contained between the external and internal walls 35, 36, this ring being able to be moved parallel to the emission axis 6, by the action of a screw 43 secured to the plate. background 38; this metal ring constitutes the adjustment means 42 already mentioned, making it possible to modify the dimensions of the resonant cavity 13, 18, and to adjust in operation the frequency of re sound of the latter. In operation, the rear end 37 of the resonant cavity 13, 18 is thus closed by a short circuit; it should be noted that this piston 42 being in the part C not subjected to vacuum, it does not pose any problem with regard to sealing.

The outer wall 35 has a passage 45 to which a transmission line 46 is applied; in the nonlimiting example described, this transmission line is a coaxial line, one end of which plunges into the cavity 13, and provided with an electromagnetic coupling means 50. In the nonlimiting example described, this coupling means 50 is located in part C not subjected to the vacuum of the resonant cavity 13, 18, which can facilitate the problems of connection of transmission line 46.

• - application d'une tension de chauffage du filament 9;
• - application de la haute tension entre la cathode 8 et l'anode 2; l'anode pouvant par exemple comme il est souvent pratiqué être à la masse et la cathode au moins haute tension;
• - application éventuellement d'une tension de polarisation entre la cathode 8 et la grille 10;
• - enfin injection dans la cavité résonnante 13, 18, par l'organe de couplage électromagnétique 50, d'une onde électromagnétique amenée par la ligne de transmission 46; cette onde électromagnétique, générée par un générateur non représenté sur la figure 1, ayant une fréquence F égale à la fréquence de résonnance de la cavité résonnante 13, 18 ou multiple de cette fréquence de résonnance.

Assuming that operating conditions are fulfilled, such as for example:

• - application of a heating voltage of the filament 9;
• - application of the high voltage between the cathode 8 and the anode 2; the anode being able for example as it is often practiced to be with the ground and the cathode at least high voltage;
• - optionally applying a bias voltage between the cathode 8 and the grid 10;
• - Finally injection into the resonant cavity 13, 18, by the electromagnetic coupling member 50, of an electromagnetic wave supplied by the transmission line 46; this electromagnetic wave, generated by a generator not shown in FIG. 1, having a frequency F equal to the resonant frequency of the resonant cavity 13, 18 or multiple of this resonant frequency.

Magnetic and electric fields (not shown) at frequency F then develop in a conventional manner in the resonant cavity 13, 18, and excite the grid-cathode space 10.8; this determines a difference in alternating potential of frequency F between the gate 10 and the cathode 8.

This difference in alternating potential or alternating voltage is superimposed on the bias voltage, thus allowing modulation of the electronic current (not shown). The electronic current delivered by the cannon 1 can thus be modulated more or less deeply, depending on the goal sought; the modulation depth or the type of modulation being obtained in particular, as a function of the level of polarization between the grid 10 and the cathode 8, of the level of the electromagnetic wave injected into the resonant cavity 13, 18, and also as a function of the distance d between the grid 10 and the cathode 8.

This constitutes a nonlimiting example of an electron gun 1 according to the invention, the cavity 13 possibly having a different shape, as well as the coupling member 50; the latter can also be constituted by a coupling loop (not shown) rather than by a capacitive coupling as shown in FIG. 1. Similarly, a person skilled in the art can imagine a different structure for supporting the cathode 8, the wafer 21, and feed the filament 9.

As has already been said in the preamble, an electromagnetic wave of frequency F suitable for an accelerating structure into which it is injected, makes it possible to accelerate electrons; having passed through this structure, the electrons constitute a beam of accelerated electrons of given energy.

An accelerating structure generally consists of a succession of n accelerating cavities; in order to use the example of an irradiator with industrial application cited in the preamble, these accelerating cavities can be as described in a patent application by the applicant, published under No. 2,477,827.

FIG. 2 schematically shows by way of nonlimiting example, an accelerating structure 3, comprising such accelerating cavities CI, C ₂ , C ₃ ; a single accelerating cavity CI can also suffice in certain cases. Electrons supplied by the barrel 1 according to the invention are injected into the accelerating structure 3 in a conventional manner, by means of a sliding and focusing element 50.

The electrons are accelerated in the accelerating structure 3 in a medium direction A, merged with the axis 6 of emission of the electrons generated by the gun 1, thanks to an electromagnetic wave OE of frequency F provided by a generator 51. This wave is injected into the accelerating structure 3, in the accelerating cavity C ₂ for example, by means of conventional coupling means, not shown, as well as coupling means between the resonant cavities CI, C ₂ , C ₃ .

In the nonlimiting example described, the electromagnetic wave OE at frequency F is also injected into the barrel 1, by means of a transformer device 52 and an adjustable phase shifter device 53. This makes it possible to inject into the barrel 1, the electromagnetic wave OE of frequency F generated by the generator 51, with an adjustable phase relative to that of the wave injected into the cavity C ₂ ; this arrangement making it possible to modulate the electronic current delivered by the barrel 1 at the same frequency F, as that injected into the accelerating structure 3 for the acceleration of the electrons.

It may also be advantageous for practical reasons to take the electromagnetic wave in the accelerating structure 3, in the first accelerating cavity C _l for example, given its proximity to the barrel 1; this variant is shown in dotted lines in Figure 2: the electromagnetic wave marked OE 'is injected into the barrel 1 via a transformer 52' and a phase shifter 53 '.

In general, electrons injected into the first accelerating cavity CI acquire therein, if the phase of the electromagnetic wave OE is favorable to them, a speed close to that of the light. The time during which the energy conveyed by this wave is given up to the electrons, is at most equal to a half-period of this wave. Also, in order to avoid a decrease in energy of the electrons, a length L to be traversed by these in each cavity C ₁ , C ₂ , C ₃ is less than half a wavelength; a sliding space 61 located between each cavity allowing the electrons to recover a correct phase of the wave, when they enter a next cavity.

Assuming that the electrons arrive in a cavity C ₁ , from an instant, as shown in Figure 3; the instant t ₁ corresponding to the start of a half accelerating period P / 2 of the electromagnetic wave OE: the energy given up to the electrons at this instant t ₁ is zero and increases until the corresponding instant t ₂ at the maximum of the peak c of the wave; it then decreases until time t ₃ which corresponds to the end of the half period P / 2.

This can give electrons different energies according to their arrival in this cavity during the time interval between instant t ₁ and instant t ₃ ; an accelerated electron beam thus obtained could be homogeneous in energy.

It should also be noted that weakly accelerated electrons tend to diverge from direction A and are then lost to the electron beam; these electrons having consumed, although lost for the beam, a non-negligible part of the energy conveyed by the electromagnetic wave O.E.

It is therefore particularly advantageous to control the electron gun 1 in accordance with the invention, so as to obtain pulses 60 of the electronic current which it supplies; these pulses 60 constituting packets of electrons, delivered at the same frequency F as that of the electromagnetic wave.

By thus modulating the electron gun 1, so as to obtain pulses or packets 60, of width 1 less than or equal to half a period P / 2, it is also possible, thanks to the phase-shifter device 53, to coincide in each cavity C ₁ , C ₂ , C ₃ with the peak of the half accelerating period P / 2; the electromagnetic wave OE thus transferring to the electrons the energy which it conveys, during only a fraction f of the half accelerating period P / 2, where this energy is maximum. This also avoids consumption of this energy, to accelerate electrons located outside the range of acceptance in the accelerator phase.

Such an arrangement is applicable to any linear accelerating structure of electrons. It reduces the energy consumed in the electromagnetic wave, while improving the energy homogeneity of an accelerated electron beam thus obtained.

Claims (11)

1. An electron gun for a linear accelerator, incorporating an emitter cathode (8), a grid (10) able to control the intensity of the electron beam supplied by said cathode, an anode (2) perforated by a hole (5) centered around an axis (6), along which the electrons are emitted, and a transmission line (46) conducting the signal for controlling the beam intensity, characterized in that it comprises a resonance cavity (13, 18) closed on the cathode-grid space, said cavity containing an electromagnetic coupling member (50) connected to the transmission line (46) and making it possible to inject an electromagnetic wave of frequency F into said resonance cavity (13,18), in order to make the latter resonate and excite the grid-cathode space (10, 8), so as to determine between the grid (10) and the cathode (8) an alternating potential difference of frequency F by which is modulated the beam current.

2. An electron gun according to claim 1, characterized in that the resonance cavity (13, 18) is coaxial and has a longitudinal axis coinciding with the emission axis (6).

3. An electron gun according to one of the preceding claims, characterized in that the resonance cavity (13, 18) is constituted by a cavity (13), communicating with a supplementary space (18) defined by the grid (10), a tube (11) for supporting said grid and an innertube (14).

4. An electron gun according to the preceding claim, characterized in that the cavity (13) has an adjusting means (42) making it possible to adjust the resonance frequency of the resonance cavity (13,18).

5. An electron gun according to claim 1, characterized in that the resonance cavity (13, 18) has an insulating tube (33) making it possible to ensure vacuum sealing and to establish in the resonance cavity (13, 18) a first part (B) which is subjected to vacuum and a second part (C) which is subjected to atmospheric pressure.

6. An electron gun according to claims 4 and 5, characterized in that the adjusting means (42) is located in the second part (C) which is subjected to atmospheric pressure.

7. An accelerating strucure for a linear accelerator, comprising at least one accelerating cavity (C1) associated to a generator (51) of an electromagnetic wave (O.E) of frequency F injected into the accelerating structure (3), characterized in that it comprises an electron gun (1) according to one of the claims 1 to 6, the electromagnetic wave (O.E) being injected into the gun, so as to modulate the electronic current supplied by said gun, at frequency F.

8. An accelerating structure according to claim 7, characterized in that the electromagnetic wave (O.E) for modulating the electronic current of the gun is taken from the generator (51).

9. An accelerating structure according to claim 7, characterized in that the electromagnetic wave (O.E) for modulating the electronic current of the gun (1) is taken from the accelerating structure (3).

10. An accelerating structure according to one of claims 7 to 9, characterized in that it accelerates clusters (60) of electrons supplied by the gun (1) at frequency (F), having a width (1) which is equal to or less than a half-period (P/2) of the electromagnetic wave (O.E).

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