Patent Application
20210274632
The present invention falls within the field of ion sources for particle accelerators.
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.
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.
The ion source comprises:
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 (>>1 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, 2.45 GHz), 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 2000 K; 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 (1-10 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
The double-cavity Penning ion source comprises two hollow bodies, each one made up of two parts, a conductive part (1, 1′) and an insulating part (2, 2′), which fit together so that the interior walls thereof delimit a cylindrical cavity (3, 3′). At least one of the conductive parts 1 has a gas supply inlet 4 through which a plasma-forming gas is introduced into the respective cavity 3 thereof. In each cavity (3, 3′) there is a coaxial conductor (5, 5′) disposed in the cavity (3, 3′) of the body (1, 1′), arranged parallel to the longitudinal axis of the cylindrical cavity (3, 3′).
Both cavities (3, 3′) are interconnected by means of a common cylindrical expansion chamber (6) through respective holes (7, 7′) made in the walls of the conductive parts (1, 1′). An ion-extraction aperture (8) disposed in the walls which delimit the expansion chamber (6), in the central portion thereof, makes it possible to extract ions from the plasma generated from the gas introduced into the cavities (3, 3′).
A conductive element (9, 9′) is introduced into each cavity (3, 3′), penetrating through the insulating part (2, 2′), and in electrical contact with the coaxial conductor (5, 5′) of the cavity. The conductive element (9, 9′) is excited with DC voltages of around 3000 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 1/1′ and the coaxial conductor 5/5′). After igniting the plasma, the power supply stabilises it by maintaining a potential difference between 500-1000V 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.
The operation of the ion source 10 is based on a coaxial resonant cavity.
The body 11 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 15 is disposed in the cavity 13 of the body 11, arranged parallel to the longitudinal axis of the cylindrical cavity 13. At least one of the ends (15a, 15b) of the coaxial conductor 15 is in contact with one of the circular interior walls (11a, 11b) of the body 11, forming a coaxial resonant cavity. In this way, the coaxial conductor 15 can short-circuit both interior walls (11a, 11b) to obtain a λ/2 coaxial resonant cavity, obtaining the maximum electric field in the centre, or it short-circuits a single interior wall to obtain a λ/4 coaxial resonant cavity (with the maximum electric field at the opposite end of the conductor). In the example of
The body 11 has a gas supply port or inlet 14 (i.e., a hole or opening made in one of the walls thereof) through which a plasma-forming gas is introduced into the cavity 13.
The body 11 also has a power supply inlet 21 through which radio frequency energy is injected into the cavity 13.
An expansion chamber 16 is connected to the cavity 13 through a plasma outlet hole 17 made in one of the walls of the body 11. An ion-extraction aperture 18 is disposed in one of the walls of the expansion chamber 16. The ion source 10 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 18.
The coaxial conductor 15 has a conductive protuberance 22 that extends radially into the cavity 13 with respect to the axis of the cylindrical cavity (i.e., perpendicular to said axis), said conductive protuberance 22 being opposite the plasma outlet hole 17 of the body 11 that connects the cavity 13 to the expansion chamber 16 (i.e., the conductive protuberance 22 is opposite the expansion chamber 16). The conductive protuberance 22 does not come into contact with the interior wall of the body 11, although it remains very close, usually less than 5 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 11 is short-circuited by the internal coaxial conductor 15 at one end 15a or at both ends (15a, 15b). The coaxial conductor 15 is an inner conductor that functions like an electrode opposite the outer conductor, the interior walls of the body 11, in such a way that when power is injected, the cavity 13 enters into resonance and the electric field that is established in the gap between the two conductors (11, 15) changes sign.
In the example of
As shown in the embodiment of
The frequency of the resonant cavity can be adjusted by means of an insert or moving part 27 that is partially introduced into the cavity 13. The moving part 27 can be displaced radially at the moment of the initial configuration of the ion source 10 (i.e., perpendicular to the axis of the cylindrical cavity 13), thus allowing the resonance frequency to be finely adjusted based on the volume of the movable part 27 that is introduced into the cavity 13. The moving part 27 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 27 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.
The length of the resonant cavity (along the Y-axis) is of the order of or less than λ/4 (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/2. The transverse dimensions, as well as those of the conductive protuberance 22 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 11 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 (10; 30), one starts from the initial state, where there is no energy in the cavity 13 or cavities (13, 13′). 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 17 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/4 coaxial resonant cavity, approximately 3 cm long for a frequency of 2.45 GHz, 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 15 there is a conductive protuberance 22 protruding in the same direction as the magnetic field (in the vertical direction Z) which is opposite the plasma outlet hole 17 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 17 and enters the expansion chamber 16, where it spreads mainly in the direction of the magnetic field lines (parallel to the Z-axis) forming a plasma column 23, and passes close to the ion-extraction aperture 18, wherein the ions are extracted by means of an electric field.
In the embodiment shown in the figures, the gas supply inlet 14 is implemented by means of a simple hole connected to a tube 20, while the coupling of the radio frequency system is carried out with electrical coupling by means of a protruding cylinder (dielectric 25) connected to the inner conductor 26 of a coaxial waveguide 24. 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 27.
While in the ion source 10 of
Internal ion sources for cyclotrons can be radially or axially introduced into the cyclotron.
As indicated above in the description of
The ion source (10; 30; 40) is placed immersed in a magnetic field generated by an electromagnet or by a permanent magnet 56, wherein the direction of the field lines is not important, only their movement. The ion source (10; 30; 40) is joined through the gas supply inlet 14 to a gas injection system 57, which comprises a gas reservoir or tank 58 and is dosed by means of a regulation system 59. The ion source (10; 30; 40) is disposed in a chamber 60 with sufficient vacuum so that the ions are not neutralised by the residual gas and can be accelerated for later use.
The necessary radio frequency power is provided by the generator 51, and the transmitted power is measured with the power meter 55 connected to the directional coupler 54. The generator 51 is protected with the circulator 52 which diverts the power reflected by the ion source (10; 30; 40) to the load 53.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201830684 | Jul 2018 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2019/070461 | 7/1/2019 | WO | 00 |
