Patent Description:
The operating principle of electron cyclotron resonance (ECR) ion sources is represented in <FIG>, where <NUM> indicates a chamber made of metal material configured to contain a plasma P, and <NUM> indicates an inlet for the supply of gas to the chamber <NUM>. Associated with the chamber <NUM> are a multipole <NUM>, in particular a hexapole, and a solenoid <NUM>, configured to generate respectively a radial component and an axial component of a magnetic field inside the chamber <NUM>. Reference <NUM> indicates a waveguide configured to inject microwaves inside the chamber <NUM>.

In an ECR ion source the power provided by the microwaves is combined with heating induced by electron cyclotron resonance in a plasma confined in a magnetic bottle in B-minimum configuration to produce ions.

It is known that electrons moving in a magnetic field revolve around the magnetic field lines due to the Lorentz force. The gyration frequency is known as the cyclotron frequency. If a microwave radiation propagates in such a region, electrons are accelerated or decelerated in a resonant manner (according to the phase of their transverse velocity component with respect to the electric field vector) when the following electron cyclotron resonance condition is satisfied: <MAT> where ωf is the microwave frequency, ωc is the electron cyclotron frequency, e and m are the charge and mass of the electron, and B is the local magnetic field.

The electrons of the plasma are confined in a superposition of an axial component of the magnetic field (produced by the solenoids <NUM> or by permanent magnets) and of a radial component of the magnetic field produced by the multipole <NUM>. This gives rise to a so-called B-minimum structure since the magnetic field has a minimum in the middle of the structure and from there it grows in all directions. Thus, a closed surface is created where the electron cyclotron resonance condition is satisfied. The electrons passing through this surface may be accelerated in a resonant manner. Furthermore, an elevated mirror ratio of the magnetic field results in long confinement times for the electrons of the plasma. These electrons may then pass many times through the resonance region, reach high energies, and ionize the atoms of the plasma, as well as the ions themselves, toward higher charge states, by means of successive single ionizations. The magnetic field of the B-minimum structure is shown in <FIG>. In particular, <FIG> represents-according to different perspectives-a longitudinally sectioned view of a traditional cylindrical chamber, where IS indicates an isomagnetic surface and FL indicates the field lines of the magnetic induction vector. <FIG> represents the magnetic field on the longitudinal axis of the chamber produced by the solenoids <NUM>, and <FIG> represents the modulus of the magnetic field on a longitudinal section of the chamber.

The ions in the plasma are not accelerated due to their high mass and remain thermal. These ions are therefore not confined by the magnetic field but by the spatial charge potential of the electrons.

The magnetic confinement, however, is not perfect and the electrons may leave the plasma, particularly in the axial direction. Since the plasma tends to remain neutral, the ions follow the electrons. By taking advantage of a suitable extraction geometry and applying a high voltage, the ions may be extracted from the ion source, as represented by reference <NUM> in <FIG>.

The experimental results obtained by the X-ray imaging technique [<NUM>,<NUM>] and the selfconsistent simulations provide information on the plasma structure of the ECR sources (ECRIS) [<NUM>]. Both the experimental and the numerical results show that the microwave power deposition is not concentrated in the nucleus (central core) of the plasma as would be desirable to maximize the brilliance of the ion beam; instead a plasma "void" is often created along their central axis. The symmetry of the plasma chamber could be a major cause. In effect, in a cylindrical cavity resonator coupled through a rectangular aperture at the bottom of the chamber, eigenmodes often characterized by an off-axis field distribution are excited.

Furthermore, even the "aperture" coupling by means of a rectangular waveguide facing the cylindrical chamber (as commonly occurs in the ECRIS) suffers from an intrinsic mismatch, of both geometry and impedance.

In light of the above, a reworking of the plasma chamber and microwave launching system is a possible solution. <NPL>) discloses a microwave resonator comprising a plasma chamber whose shape follows a magnetic field structure. <CIT> discloses a microwave launching device. <NPL>) discloses an electron cyclotron resonance ion source.

An object of this invention is to propose solutions for improving the wave-plasma coupling in an ECR ion source and for maximizing the absorption of the radiofrequency power in the plasma core.

According to the invention a microwave resonator according to claim <NUM> is provided.

In particular, wherein at said intermediate plane said chamber has a cross section of multi-lobed shape having a plurality of lobes identical to each other and in a number equal to twice the number of lobes of each end face.

Preferably, said chamber is formed as a single piece, in particular by means of an additive manufacturing technique. In this context, the microwave launching device may also be formed as a single piece with said chamber.

Specifically, the resonator further comprises an assembly of magnets or coils configured to generate the magnetic field with B-minimum geometry within the chamber. In the resonator according to the invention, in essence, the cylindrical symmetry used for the chambers of conventional resonators is abandoned in favor of a geometry that copies the three-dimensional structure of the plasma and, therefore, of the magnetic field. This configuration allows, compared to conventional resonators, a better coupling of the electromagnetic field, an improvement of the maximization of the brilliance of the ion beam, and an optimization of the confinement of the plasma.

The structure according to the invention also allows the use of microwave launching systems consisting of waveguides obtained in the same structure as the chamber, from which diffractive slots obtained in the walls of the plasma chamber are opened and allow a profound optimization also of the cooling systems and spaces in general. The slots (radiating slots) obtained along the waveguides allow high efficiency diffractive irradiation inside the plasma chamber, due to the fact that they allow a distributed injection of microwaves along the length of the chamber, instead of a localized injection at an end face of the chamber as occurs in conventional resonators.

Further features and advantages of the device according to the invention will become clearer from the following detailed description of an embodiment of the invention, made in reference to the accompanying drawings, provided purely for illustrative and non-limiting purposes, wherein:.

With reference to <FIG>, a microwave resonator according to the invention is indicated collectively with <NUM>.

The resonator <NUM> comprises a chamber <NUM> made of metal material configured to contain a plasma. The chamber <NUM> comprises a first and a second end face, indicated respectively with 101a and 101b and shown in particular in <FIG>, and a side wall 101c, which extends along a central axis z of the chamber between the first end face 101a and the second end face 101b.

At the first end face 101a, the chamber <NUM> has a cross section of multi-lobed shape having a symmetry of rotation about the central axis z. The number of lobes must be equal to half the number of poles of the magnet or coil assembly intended to generate the radial component of the magnetic field inside the chamber <NUM>. In the illustrated example, the cross section of the first end face 101a of the chamber <NUM> has three lobes as the chamber <NUM> is intended to be associated with a hexapole such as the one described in reference to <FIG>. Furthermore, the cross section of the first end face 101a tapers toward the second end face 101b.

At the second end face 101b, the chamber <NUM> has a cross section of multi-lobed shape identical to that of the first end face 101a and rotated by <NUM>° relative thereto about the central axis z. Furthermore, the cross section of the second end face 101b tapers toward the first end face 101a.

Along the direction of the central axis z the shapes of the end faces 101a and 101b progressively merge with each other; in other words, these shapes combine to generate a shape resulting from their envelope. As a result of the tapering, at each intermediate plane along the direction z the contribution of a single shape is greater than that of the other depending on whether this intermediate plane is closer to one or the other.

In an intermediate plane, i.e., a plane placed at mid-length (see <FIG>), the contribution of the shape of the cross section of the first end face 101a is equal to the contribution of the shape of the cross section of the second end face 101b. Therefore, in this intermediate plane the chamber <NUM> has a cross section of multi-lobed shape 101d having a plurality of identical lobes and in a number equal to double the number of lobes of each end face.

With the geometry described above, the shape of the cavity resonator <NUM> follows the twisting magnetic structure of the B-minimum geometry and, therefore, the shape of the last iso-density surface of the electrons.

The chamber <NUM> is made in one piece and may be made by additive manufacturing.

The resonator <NUM> further comprises a microwave launching device <NUM> configured for the injection of microwaves into the chamber <NUM>. The microwave launching device <NUM> comprises a waveguide <NUM> arranged on the side wall 101c of the chamber <NUM>. The waveguide <NUM> has a substantially rectangular cross section and extends along the central axis z. For the purposes of this invention, "extending along the central axis" means that the waveguide <NUM> has an extending component parallel to the central axis z. As may be seen in the figures, however, the waveguide <NUM> develops according to a three-dimensional curve.

At one end of the waveguide <NUM> adjacent to the first end face 101a of the resonator <NUM> an input 111a for microwaves is made. The input 111a of the waveguide <NUM> may be arranged level with the first end face 101a of the resonator <NUM>.

The waveguide <NUM> has an array of diffractive apertures or slots <NUM> interposed between the waveguide <NUM> and the chamber <NUM> and facing the interior of the chamber <NUM>. In particular, the diffractive apertures <NUM> are obtained through the side wall 101c of the chamber <NUM>. In other words, the waveguide <NUM> shares a wall, i.e., the one in which the diffractive apertures <NUM> are formed, with the chamber <NUM>.

The microwave launching device <NUM> described above forms a slotted waveguide antenna. The design of a slotted waveguide is generally based on the procedure described by Elliott [<NUM>-<NUM>], according to which:.

Thus, the resonant array of slots is obtained by making narrow apertures (relative to the wavelength in vacuum) on the wide wall of the rectangular waveguide, arranged in the longitudinal direction (parallel to the axis of the guide), spaced from each other by an amount equal to half the wavelength in the guide and placed alternately on either side with respect to the central longitudinal axis of the wide wall of the guide, with offset (distance between the centers of the slots and the longitudinal axis) equal on both sides.

The exact dimensions in each case must be optimized. In particular, the offset of the slots, i.e., their distance from the central axis of the wide wall of the slotted waveguide, determines the power levels of the side lobes irradiated by said slotted antenna.

The inventors have optimized the aforesaid features at the design stage to work in the <NUM>-<NUM> frequency band.

Regarding the number of slots, several factors should be considered. In general, the fewer the number of slots the larger the bandwidth, but the efficiency of antennas with only <NUM> or <NUM> slots is usually too low.

Therefore, the optimal number of slots must be determined for each particular case. For example, the optimal number of slots may be estimated in order to minimize the reflection coefficient or to obtain a certain radiation pattern.

The optimal number of slots may be for example equal to <NUM>, but this number (intended as the maximum) depends on the total length available for the plasma chamber of the resonator and therefore may be rescaled.

The microwave launching device <NUM> may be made in one piece with the chamber <NUM>, for example by means of additive manufacturing.

<FIG> shows an example of an ion source inside of which the resonator <NUM> described above is arranged.

Associated with the chamber <NUM> of the resonator <NUM> is an assembly of magnets or coils configured to generate the magnetic field with B-minimum geometry inside the chamber <NUM>. This assembly of magnets comprises a multipole <NUM>, in particular a hexapole, and a solenoid <NUM> configured to generate respectively the radial component and the axial component of the magnetic field inside the chamber <NUM>. Reference <NUM> indicates a waveguide, in particular a waveguide with a standard rectangular section, configured to feed microwaves to the microwave launching device <NUM>. One end of the power waveguide <NUM> is conventionally coupled to a radio frequency generator <NUM>, via a microwave amplifier <NUM>. At the other end, the power supply waveguide <NUM> is coupled to the input 111a of the waveguide <NUM> of the microwave launching device <NUM>.

Claim 1:
A microwave resonator (<NUM>), comprising
a chamber (<NUM>) made of metal material configured to contain electrons of a plasma confined in a B-minimum magnetic field generated within the chamber (<NUM>), said chamber comprising a first and a second end face (101a, 101b) and extending along a central axis (z) between said first and second end face, and
a microwave launching device (<NUM>) configured for the injection of microwaves into said chamber,
wherein, at the first end face (101a), said chamber has a cross section of multi-lobed shape tapering toward the second end face (101b),
wherein, at the second end face (101b), said chamber has a cross section of multi-lobed shape identical to the shape of the cross section of the first end face (101a) and rotated by <NUM>° relative to the first end face about the central axis (z), the cross section of the second end face (101b) tapering towards the first end face (101a), and
wherein, at an intermediate plane, the shape of the cross section of the first end face (101a) merges with the shape of the cross section of the second end face (101b),
characterized in that the microwave resonator (<NUM>) is configured to have a shape that follows the shape of the last iso-density surface of the electrons,
wherein said microwave launching device comprises a waveguide (<NUM>) arranged on a side wall (101c) of said chamber and extending along said central axis, said waveguide having an array of diffractive apertures (<NUM>) interposed between the waveguide (<NUM>) and the chamber (<NUM>) and facing the inside of said chamber.