Patent Description:
An ion source is the component of particle accelerators where the gas is ionised, transforming into plasma, and from which the charged particles are then extracted to be accelerated. Ion sources are mainly used as internal sources in cyclotrons to produce lightweight positive ions and negative hydrogen. These types of machines have been traditionally used in the world of research as multipurpose beam machines for use in multiple fields. They have recently been used for radioisotope synthesis in radiopharmaceutical applications, as well as in proton/hadron therapy machines for the treatment of tumours.

Ion sources have traditionally been very present in the world of research in different fields, including their use in particle accelerators, and in the study of materials or the structure of matter. To generate ions, one starts with the material to be ionised (generally a gas) and electrons are removed or added to the atoms by means of one or more of the following processes: electron impact (direct ionisation and/or charge exchange), photoionisation and surface ionisation.

In its simplest scheme, an ion source is made up of a main chamber where the process takes place, material to ionise (introduced previously or continuously), an energy source for ionisation and an extraction system. According to the process followed, a general classification of the different types of ion sources can be made:.

In the case of internal ion sources for cyclotrons, the preferred application field for the present invention, due to the internal configuration of cyclotrons, with very little space available for internally coupling the ion sources and a very high magnetic field in the vertical direction which traps the paths of the electrons and does not let them move freely, the only internal sources that have been used to date for cyclotrons are Penning sources. Penning ion sources have two cathodes placed at the vertical ends and a hollow tube parallel to the magnetic field that surrounds them. Said cathodes can be externally heated or remain initially cool and then heated with ion bombardment from the discharge. Due to the symmetrical configuration of the cathodes and the magnetic field, the electrons are emitted and accelerated, moving in helical paths that increase ionisation, and when reaching the opposite end they are reflected due to the electric field. The collisions of fast electrons with the injected gas results in the creation of a plasma from which both positive ions and negative ions can be extracted. Penning ion sources have the drawback of the sputtering of cathodes which, despite being commonly made of materials with high resistance and high electron emission (such as tantalum), are subjected to excessive wear that makes frequent replacement necessary.

Penning ion sources are very simple and compact, using DC discharge. The use of an external source adds greater complexity to the system although it allows other methods to be used to generate the plasma, such that manufacturers do not usually include them in their commercial cyclotrons. The problem with all sources that use DC discharges is that this type of discharge erodes the cathodes while the plasma is active, meaning that they must be changed periodically and in these machines that are used for medical applications, it is generally desirable to have it running for as long as possible without interruptions. Furthermore, in the case of producing H-, the high-energy electrons from the DC discharge are the particles that contribute the most to the destruction of H-, so that the drawn current is reduced.

Patent document <CIT> discloses an ion source employing a microwave resonant cavity which is excited in one of its modes of resonance and a static magnetic field adjusted to the electron cyclotron resonance.

Therefore, it is necessary to have an internal ion source for cyclotrons that solves these drawbacks.

The present invention relates to a low-erosion radio frequency ion source, especially useful for use as an internal ion source for cyclotrons.

In one embodiment, the ion source comprises a movable part partially introduced radially into the cavity through an opening made in the body to finely adjust the frequency of the resonant cavity. The moving part is preferably made of conductive material or dielectric material.

The radio-frequency energy supply is provided through a capacitive coupling or inductive coupling. Capacitive coupling is performed by means of a coaxial waveguide whose inner conductor is partially introduced into the cavity through the power supply input. Inductive coupling is performed by means of a loop that short-circuits an interior wall of the body with an inner conductor of a coaxial waveguide introduced through the power supply input.

In one embodiment, a first end of the coaxial conductor is in contact with a circular interior wall of the body, the second end of the coaxial conductor being free. In this embodiment, the conductive protuberance is preferably disposed at the second end of the coaxial conductor. The expansion chamber is preferably cylindrical and is arranged so that the longitudinal axis thereof is perpendicular to the longitudinal axis of the cavity. Alternatively, the expansion chamber can be arranged so that the longitudinal axis thereof is parallel to the longitudinal axis of the cavity.

In another embodiment, the two ends of the coaxial conductor are respectively in contact with the two circular interior walls of the body. In this embodiment, the conductive protuberance is preferably disposed in the central portion of the coaxial conductor.

The ion source can have a double cavity, comprising a second body and a second conductor that form a second coaxial resonant cavity. The cavities of both bodies are connected to each other through a common expansion chamber.

The ion source of the present invention enables solving the drawbacks of the Penning internal ion sources used in cyclotrons, in which the plasma is generated causing erosion on the conductive materials. Erosion occurs because the plasma is positively charged, so the electrons are attracted to the plasma, while the positive ions are rejected and accelerated by the potential difference between the plasma and the wall. Thus, if the energy of the ions at the time of collision with the wall is high enough (>> <NUM> eV), atoms are removed from the material when the ion collides with the conductive material. The number of atoms removed depends on the conductive material.

In the proposed ion source, the plasma is generated without producing erosion on the conductive materials (i.e., the electrodes) used in the ion source, such that the maintenance and interruptions produced when the source is operating are much less than in the case of a Penning source. Thus, in an embodiment of the present invention where the radio frequency energy supply is used by means of capacitive discharge, working at a sufficiently high frequency (for example, <NUM>), no erosion occurs on the source materials. Plasma discharge can operate in two different modes: the alpha mode, where the discharge is maintained thanks to the secondary electrons emitted by the cathode (or the portion that functioned as the cathode at that time), and the gamma mode, where the mechanism for heating the plasma is collisionless heating. The alpha mode occurs in DC discharges and in RF at low frequencies, and the transition to the gamma mode occurs starting at a certain frequency that depends on the characteristics of the plasma.

The formation of a resonator or coaxial resonant chamber makes it possible to increase the electric field and facilitate ignition, so that the ion source of the present invention further achieves much lower energy consumption.

In the ion source of the present invention, it is also not necessary to have hot cathodes at temperatures of the order of <NUM>; therefore, instead of using conductive materials with high resistance and high electron emission, such as tantalum, other less expensive materials such as copper can be used. Due to the collision of the ions with the cathodes, the kinetic energy thereof is converted into thermal energy which increases the temperature of the cathodes, which emit electrons by thermionic effect, which are necessary to maintain the DC discharge in Penning sources. As in the present invention, collisions with cathodes are much less energetic, the heating of the cathodes is much lower, and less thermally restrictive conductive materials can be used (i.e., with lower melting temperature and higher conductivity), such as copper.

Furthermore, in the case of producing H-, since the present ion source does not generate high-energy electrons in the plasma, the drawn current is significantly increased. The cross section for producing H- is the highest at low energy (<NUM>-<NUM> eV); at higher energies the cross section for production decreases significantly, while the cross section for producing the destruction of H-increases notably, as explained in detail in H. Tawara, "Cross Sections and Related Data for Electron Collisions with Hydrogen Molecules and Molecular Ions".

What follows is a very brief description of a series of drawings that aid in better understanding the invention, and which are expressly related to an embodiment of said invention that is presented by way of a non-limiting example of the same.

The present invention relates to an ion source designed mainly for use as an internal source in cyclotrons.

Currently, Penning ion sources are used as an internal source for cyclotrons, such as for example the one represented in <FIG> (longitudinal cross-sectional front view) and in <FIG> (longitudinal cross-sectional perspective view), which corresponds to a double-cavity ion source.

The double-cavity Penning ion source comprises two hollow bodies, each one made up of two parts, a conductive part (<NUM>, <NUM>') and an insulating part (<NUM>, <NUM>'), which fit together so that the interior walls thereof delimit a cylindrical cavity (<NUM>, <NUM>'). At least one of the conductive parts <NUM> has a gas supply inlet <NUM> through which a plasma-forming gas is introduced into the respective cavity <NUM> thereof. In each cavity (<NUM>, <NUM>') there is a coaxial conductor (<NUM>, <NUM>') disposed in the cavity (<NUM>, <NUM>') of the body (<NUM>, <NUM>'), arranged parallel to the longitudinal axis of the cylindrical cavity (<NUM>, <NUM>').

Both cavities (<NUM>, <NUM>') are interconnected by means of a common cylindrical expansion chamber (<NUM>) through respective holes (<NUM>, <NUM>') made in the walls of the conductive parts (<NUM>, <NUM>'). An ion-extraction aperture (<NUM>) disposed in the walls which delimit the expansion chamber (<NUM>), in the central portion thereof, makes it possible to extract ions from the plasma generated from the gas introduced into the cavities (<NUM>, <NUM>').

A conductive element (<NUM>, <NUM>') is introduced into each cavity (<NUM>, <NUM>'), penetrating through the insulating part (<NUM>, <NUM>'), and in electrical contact with the coaxial conductor (<NUM>, <NUM>') of the cavity. The conductive element (<NUM>, <NUM>') is excited with DC voltages of around <NUM> V. To start the discharge, it is necessary to open the gas flow and apply a potential difference of several thousand volts between anode and cathode (i.e., the conductive part <NUM>/<NUM>' and the coaxial conductor <NUM>/<NUM>'). After igniting the plasma, the power supply stabilises it by maintaining a potential difference between <NUM>-1000V with a current of several hundred milliamps. The discharge that is established is of the DC type, requiring the emission of secondary electrons from the conductive material (such that they must be at a high temperature and be a material with high electron emissivity) and the ions that are expelled from the plasma are accelerated at high energy, causing erosion of the cathodes.

<FIG> shows a vertical cross section of an embodiment of the device object of the present invention, ion source <NUM>, according to a cut plane perpendicular to the X-axis, wherein the external magnetic field B (normally generated by an electromagnet or a permanent magnet when the ion source is installed and running) is aligned with the vertical Z-axis of the reference system.

The operation of the ion source <NUM> is based on a coaxial resonant cavity. <FIG> shows a cross section of the ion source <NUM> according to the XY horizontal plane passing through the axis of the resonant cavity. The interior walls (11a, 11b, 11c) of a hollow body <NUM> are electrically conductive and define a cylindrical cavity <NUM>. In one embodiment, the entire body <NUM> is conductive, preferably made of copper.

The body <NUM> has three interior walls: a first interior wall 11a, of circular geometry, a second interior wall 11b, also circular and opposite the first interior wall 11a, and a third interior wall 11c, of cylindrical geometry, which connects both circular interior walls (11a, 11b).

A coaxial conductor <NUM> is disposed in the cavity <NUM> of the body <NUM>, arranged parallel to the longitudinal axis of the cylindrical cavity <NUM>. At least one of the ends (15a, 15b) of the coaxial conductor <NUM> is in contact with one of the circular interior walls (11a, 11b) of the body <NUM>, forming a coaxial resonant cavity. In this way, the coaxial conductor <NUM> can short-circuit both interior walls (11a, 11b) to obtain a λ/<NUM> coaxial resonant cavity, obtaining the maximum electric field in the centre, or it short-circuits a single interior wall to obtain a λ/<NUM> coaxial resonant cavity (with the maximum electric field at the opposite end of the conductor). In the example of <FIG> and <FIG>, only one of the ends of the coaxial conductor <NUM>, specifically the first end 15a, short-circuits one of the circular interior walls of the body <NUM> (in particular, the first interior wall 11a), the body <NUM> and the coaxial conductor <NUM> thus forming a λ/<NUM> coaxial resonant cavity, with the maximum electric field at the second end 15b of the coaxial conductor <NUM>.

The body <NUM> has a gas supply port or inlet <NUM> (i.e., a hole or opening made in one of the walls thereof) through which a plasma-forming gas is introduced into the cavity <NUM>. <FIG> shows a tube <NUM>, hermetically coupled to the gas supply inlet <NUM>, through which the gas is introduced into the cavity <NUM>. These types of ion sources generally work with Hydrogen, and to a lesser extent Deuterium and Helium, depending on the ion to be extracted.

The body <NUM> also has a power supply inlet <NUM> through which radio frequency energy is injected into the cavity <NUM>.

An expansion chamber <NUM> is connected to the cavity <NUM> through a plasma outlet hole <NUM> made in one of the walls of the body <NUM>. An ion-extraction aperture <NUM> is disposed in one of the walls of the expansion chamber <NUM>. The ion source <NUM> is introduced under vacuum into the chamber of a cyclotron, and the gas that is injected is partially transformed into plasma and the rest escapes through the ion-extraction aperture <NUM>.

The coaxial conductor <NUM> has a conductive protuberance <NUM> that extends radially into the cavity <NUM> with respect to the axis of the cylindrical cavity (i.e., perpendicular to said axis), said conductive protuberance <NUM> being opposite the plasma outlet hole <NUM> of the body <NUM> that connects the cavity <NUM> to the expansion chamber <NUM> (i.e., the conductive protuberance <NUM> is opposite the expansion chamber <NUM>). The conductive protuberance <NUM> does not come into contact with the interior wall of the body <NUM>, although it remains very close, usually less than <NUM> millimetres; this separation distance will largely depend on the dimensions of the resonant cavity. The ignition voltage, injected power in the case of RF, will depend in turn on this separation distance and the density of the injected gas.

Depending on where the plasma is to be generated, the body <NUM> is short-circuited by the internal coaxial conductor <NUM> at one end 15a or at both ends (15a, 15b). The coaxial conductor <NUM> is an inner conductor that functions like an electrode opposite the outer conductor, the interior walls of the body <NUM>, in such a way that when power is injected, the cavity <NUM> enters into resonance and the electric field that is established in the gap between the two conductors (<NUM>, <NUM>) changes sign.

In the example of <FIG> and <FIG>, a portion of the free end of the coaxial conductor <NUM>, second end 15b, is modified by means of a conductive protuberance or protrusion <NUM> directed towards the expansion chamber <NUM>, in order to produce a concentration and an increase of the electric field in the area where the plasma is to be produced (plasma production area). The plasma produced escapes from the cavity <NUM> through the plasma outlet hole <NUM> towards the expansion chamber <NUM>, forming a plasma column <NUM> aligned with the magnetic field B, from which the ions are extracted using the ion-extraction aperture <NUM>. The expansion chamber <NUM> is a cavity, also preferably of cylindrical geometry, which performs the function of an expansion chamber for the plasma column <NUM>. In the ion sources applied to cyclotrons, the expansion chamber <NUM> is a cylindrical cavity with a small radius so that after extracting the particles through the ion-extraction aperture <NUM> and accelerating them in the first turn, they do not collide with the source and are lost. The expansion chamber <NUM> also acts as a mechanical support, keeping the two symmetrical portions of the ion source separate, when they are a double-cavity ion source (as shown in <FIG> and <FIG>).

As shown in the embodiment of <FIG>, a coaxial waveguide <NUM> which transports radio frequency/microwave energy is coupled through the power supply access, port or inlet <NUM>, the coupling possibly being of the electrical (capacitive) or magnetic (inductive) type. <FIG> shows a typical capacitive coupling, wherein the dielectric <NUM> that surrounds the inner conductor <NUM> of the coaxial waveguide <NUM> enables the hermetic sealing of the power supply inlet <NUM> (so that portion of the injected gas does not escapes through said inlet), and wherein the inner conductor <NUM> of the coaxial waveguide <NUM> protrudes from the dielectric <NUM>, partially entering into the cavity <NUM>. Unlike this capacitive coupling, a typical inductive coupling uses a loop that short-circuits the interior of the coaxial waveguide with the resonant cavity.

The frequency of the resonant cavity can be adjusted by means of an insert or moving part <NUM> that is partially introduced into the cavity <NUM>. The moving part <NUM> can be displaced radially at the moment of the initial configuration of the ion source <NUM> (i.e., perpendicular to the axis of the cylindrical cavity <NUM>), thus allowing the resonance frequency to be finely adjusted based on the volume of the movable part <NUM> that is introduced into the cavity <NUM>. The moving part <NUM> is an optional element, not strictly necessary for the operation of the ion source, although it improves the operation by making it easier to adjust the resonance frequency. The moving part <NUM> can be made of conductive material (preferably copper) or of dielectric material (such as alumina), depending on the behaviour and the variation in frequency to be achieved.

<FIG> and <FIG> show two additional views of the ion source <NUM>, according to one possible embodiment. <FIG> illustrates a front view of the ion source <NUM>, wherein the portion above the axis of the cavity <NUM> is shown in mid-section. <FIG> shows a three-dimensional view of an ion source <NUM>. The gas supply inlet <NUM> cannot be seen in <FIG> as it is disposed at the rear of the body <NUM> in this view. The projection <NUM> shown in <FIG> is an element with the same function as the movable part <NUM> of <FIG>, an element by means of which the frequency of the resonant cavity is finely adjusted. In this case, the projection <NUM> is integrated into the body of the ion source, but it could be designed as a separate body.

<FIG> and <FIG> show, respectively and according to another embodiment, a front cross section and a perspective cross section of a double-cavity ion source <NUM>, with a plane of symmetry <NUM> in the central portion of the ion-extraction aperture <NUM>, both cavities (<NUM>, <NUM>') being connected by a common expansion chamber <NUM>, which allows the expansion of the plasma column <NUM> produced in each cavity (<NUM>, <NUM>'). The elements of the ion source <NUM> for each of the two cavities (<NUM>, <NUM>') are the same as those shown in <FIG> for the ion source <NUM> having a single cavity (first body <NUM> and second body <NUM>', first coaxial conductor <NUM> and second coaxial conductor <NUM>', first conductive protuberance <NUM> and second conductive protuberance <NUM>', first plasma outlet hole <NUM> and second plasma outlet hole <NUM>', etc.), with the particularity in this case that both cavities (<NUM>, <NUM>') are opposite each other and share the expansion chamber <NUM>. Double-cavity ion sources <NUM> are used to obtain plasma more easily and increase the production of particles, such that at both ends there are two plasma jets that converge at the height of the plane of symmetry <NUM>, forming a single plasma column <NUM> in the central portion, wherein the ion-extraction aperture <NUM> is located to remove the desired particles, whether they are positive or negative ions.

The length of the resonant cavity (along the Y-axis) is of the order of or less than λ/<NUM> (where λ is the wavelength associated with the oscillating electromagnetic field given by the ratio λ = f/c, where f is the oscillation frequency and c speed of light) in the case of resonant cavities short-circuited at one end (quarter-wave cavities). In the case of half-wave resonant cavities, short-circuited at both ends and with plasma formation in the central portion of the inner conductor, the length of the resonant cavity will be of the order of or less than λ/<NUM>. The transverse dimensions, as well as those of the conductive protuberance <NUM> for concentrating the electric field, are determined by the specific parameters of the resonant cavity to be obtained, mainly the quality factor Q and characteristic impedance R/Q, and they will also have an effect on the resonant frequency of the cavity.

The interior walls of the body <NUM> are made of a conductive material with low electrical resistivity and high thermal conductivity, generally copper or copper deposited on another metal, since there is a desire for the Q factor to be high and the power deposited on the walls to be rapidly dissipated.

To operate the ion source (<NUM>; <NUM>), one starts from the initial state, where there is no energy in the cavity <NUM> or cavities (<NUM>, <NUM>'). The radio frequency energy that is introduced into the cavity is produced in a generator, which can be solid state, electron tube (magnetron, TWT, gyrotron, klystron, etc.) or a coil and capacitor resonant circuit, depending on the frequency, power and required working mode. Said power travels through a waveguide, generally coaxial or hollow (e.g., rectangular), to the cavity, wherein the power is transferred to the resonant cavity through a coupling (electrical, inductive or through-hole), minimising reflections and power losses. As electromagnetic energy is introduced into the cavity (with a frequency equal to the resonant frequency of the cavity), the value of the electric field increases in magnitude in such a way that it reaches a point when the plasma ignites (Paschen curve for oscillating electromagnetic fields). Once the plasma is formed, which expands through the plasma outlet hole <NUM> spreading along the magnetic field lines generated by an electromagnet or a permanent magnet, the resonant frequency of the cavity shifts, such that if the frequency of the electromagnetic field supplied to the cavity remains constant, power begins to be reflected due to the difference in impedances, reaching a point when all the power except that which is necessary to maintain the discharge and compensate for losses in the walls of the cavity will be reflected, stabilising the system in the steady state.

According to a possible embodiment, a specific design of the present invention uses a λ/<NUM> coaxial resonant cavity, approximately <NUM> long for a frequency of <NUM>, with one end short-circuited and the other open, and made of copper. In the portion of the open end of the inner coaxial conductor <NUM> there is a conductive protuberance <NUM> protruding in the same direction as the magnetic field (in the vertical direction Z) which is opposite the plasma outlet hole <NUM> and which allows increasing the electric field in that area to achieve plasma formation with less power. The plasma leaves through the plasma outlet hole <NUM> and enters the expansion chamber <NUM>, where it spreads mainly in the direction of the magnetic field lines (parallel to the Z-axis) forming a plasma column <NUM>, and passes close to the ion-extraction aperture <NUM>, wherein the ions are extracted by means of an electric field.

In the embodiment shown in the figures, the gas supply inlet <NUM> is implemented by means of a simple hole connected to a tube <NUM>, while the coupling of the radio frequency system is carried out with electrical coupling by means of a protruding cylinder (dielectric <NUM>) connected to the inner conductor <NUM> of a coaxial waveguide <NUM>. Other alternatives for introducing power are a magnetic coupling through a loop or a hole made in a waveguide. The resonant frequency of the cavity is adjusted by the moving part <NUM>.

<FIG> illustrates an ion source <NUM> according to another possible embodiment, wherein the location of the plasma outlet hole <NUM> (in this case it is located in the circular second interior wall 11b) and the orientation of the expansion chamber <NUM> changes with respect to the cavity <NUM>. Furthermore, the conductive protuberance <NUM> of the ion source <NUM> for this embodiment preferably has a circular cross section, to thereby maintain internal symmetry in the cavity <NUM> (the conductive protuberance <NUM> of <FIG> protrudes on each side, top and bottom, of the coaxial conductor <NUM>). However, the conductive protuberance <NUM> of <FIG> can have different types of cross sections, depending on the geometry and dimensions of the cavity, the coaxial conductor and the plasma outlet hole (the cross section can be optimised by means of simulation to obtain a greater concentration of the electric field opposite of the plasma outlet hole <NUM> that favours the formation and stability of the plasma), so that the conductive protuberance <NUM> only protrudes on one side, at the top. The upper circle illustrated in <FIG> represents the resonator <NUM> (i.e., the coaxial resonant cavity) that forms when ion source <NUM> is in operation.

While in the ion source <NUM> of <FIG> the main axis of the expansion chamber <NUM> is disposed perpendicular to the axis of the cylindrical cavity <NUM>, in the ion source <NUM> of <FIG> both axes are parallel (in the example of <FIG> they coincide), which allows the ion sources to be axially coupled in cyclotrons.

Internal ion sources for cyclotrons can be radially or axially introduced into the cyclotron. <FIG> and <FIG> show, respectively, a front and perspective view (partially sectioned) of a cyclotron <NUM> (in the figure of the cyclotron, components such as the magnet coils, the radio frequency-acceleration system, the extraction system and the vacuum and opening system of the iron have been omitted) with axial configuration for introducing an ion source. In the cyclotron <NUM> of <FIG> and <FIG>, the ion source is introduced with the axial configuration of <FIG>, wherein the electromagnetic and mechanical design of the ion sources is simpler. <FIG> and <FIG> show a cyclotron <NUM> with radial configuration for introducing the ion source, wherein the design of the ion sources is more complicated (it corresponds to the ion sources represented in <FIG>). In <FIG>, <FIG>, <FIG> and <FIG>, the following references are used:.

As indicated above in the description of <FIG>, a coaxial waveguide <NUM> which transports radio frequency/microwave energy is coupled through the power supply input <NUM>. The coupling can be electrical/capacitive or magnetic/inductive. <FIG> shows an embodiment like the one shown in <FIG> but replacing the capacitive coupling with a magnetic coupling, wherein a loop <NUM> short-circuits the inner conductor <NUM> of the coaxial waveguide <NUM> with the interior wall of the body <NUM>. <FIG> and <FIG> show another type of coupling, coupling by rectangular waveguide <NUM>, in two different views (top view and perspective view, with a partial cross section). In this case, the coupling is performed by means of a hole <NUM> that joins the cavity <NUM> to the vacuum of the rectangular waveguide <NUM>. It would act as an electric dipole and a magnetic dipole that radiate on both sides, such that if there is higher energy density on one side, energy is transferred to the other side until they reach equilibrium. In this embodiment, the ion source <NUM> has larger dimensions due to the rectangular waveguide <NUM>, which must also be under vacuum.

<FIG>, <FIG> show different views in partial section (in particular, a front view, a top view, a front perspective view and a rear perspective view, respectively) of an embodiment of the ion source <NUM> wherein the two ends (15a, 15b) of the coaxial conductor <NUM> are respectively in contact with the two circular interior walls (11a, 11b) of the body <NUM>, thus obtaining a λ/<NUM> coaxial resonant chamber.

<FIG> shows, by way of example, a complete radio frequency system <NUM> in which the ion source (<NUM>; <NUM>; <NUM>) of the present invention can be used. The radio frequency system comprises a generator <NUM> of sufficient power and adjustable parameters to achieve the ignition of the plasma, a circulator <NUM> with a load <NUM> to absorb the reflected power and a directional coupler <NUM> with a power meter <NUM> to monitor the incident and reflected power.

The ion source (<NUM>; <NUM>; <NUM>) is placed immersed in a magnetic field generated by an electromagnet or by a permanent magnet <NUM>, wherein the direction of the field lines is not important, only their movement. The ion source (<NUM>; <NUM>; <NUM>) is joined through the gas supply inlet <NUM> to a gas injection system <NUM>, which comprises a gas reservoir or tank <NUM> and is dosed by means of a regulation system <NUM>. The ion source (<NUM>; <NUM>; <NUM>) is disposed in a chamber <NUM> with sufficient vacuum so that the ions are not neutralised by the residual gas and can be accelerated for later use.

Claim 1:
A low-erosion internal ion source for cyclotrons, comprising:
a hollow body (<NUM>) whose interior walls (11a, 11b, 11c) define a cylindrical cavity (<NUM>), wherein the body (<NUM>) has a gas supply inlet (<NUM>) through which a plasma-forming gas is introduced into the cavity (<NUM>);
a coaxial conductor (<NUM>) disposed in the cavity (<NUM>) of the body (<NUM>) and arranged parallel to the longitudinal axis of the cavity (<NUM>);
an expansion chamber (<NUM>) connected to the cavity (<NUM>) through a plasma outlet hole (<NUM>) made in the body (<NUM>);
an ion-extraction aperture (<NUM>) in contact with the expansion chamber (<NUM>);
wherein:
the body (<NUM>) has a power supply input (<NUM>) through which radio frequency energy is injected into the cavity (<NUM>);
the interior walls of the body (<NUM>) are conductive;
at least one of the ends (15a; 15b) of the coaxial conductor (<NUM>) is in contact with at least one circular interior wall (11a, 11b) of the body (<NUM>), forming a coaxial resonant cavity;
characterised in that:
the coaxial conductor (<NUM>) has a conductive protuberance (<NUM>) that extends radially into the cavity (<NUM>), said conductive protuberance (<NUM>) being opposite the plasma outlet hole (<NUM>).