EP3945753A1

EP3945753A1 - Linear particle accelerator with high accelerating gradient made by tig welding

Info

Publication number: EP3945753A1
Application number: EP21187990.3A
Authority: EP
Inventors: Bruno SPATARO; Luigi FAILLACE; Roberto Bonifazi
Original assignee: CoMeB Srl; Instituto Nazionale di Fisica Nucleare INFN
Current assignee: CoMeB Srl; Instituto Nazionale di Fisica Nucleare INFN
Priority date: 2020-07-28
Filing date: 2021-07-27
Publication date: 2022-02-02
Also published as: IT202000018268A1

Abstract

A linear particle accelerator with high accelerating gradient, provided with a body comprising a compact primary vacuum chamber (3, 3') made of high-conductivity material, wherein said chamber (3, 3') comprises at least three adjacent, coaxial accelerator cells, having a cylindrical symmetry shape, configured to operate in the standing wave configuration in the π mode; wherein said accelerator cells are joined together and locked by tightening means and externally welded by means of TIG (Tungsten Inert Gas) welding to avoid high temperature treatments; wherein each cell has a ring beam forming an RF tooth with a contact angle between 10° and 30°, arranged on at least one contact surface between the assemblable parts to form said cells, which tooth is configured to provide, upon the actuation of said tightening means, the RF contact and the vacuum seal.

Description

The present invention relates to the field of linear particle accelerators, and in particular, to a new generation accelerator with high accelerating gradient, the acceleration chamber of which is made of an unconventional "hard" material, i.e., it is not welded by means of traditional high temperature processes, thus allowing higher accelerating gradients to be obtained.
The invention substantially relates to particle accelerators which not only are capable of obtaining higher gradients, but are also characterized by great compactness and robustness, particularly adapted to carry out high-energy experiments and usable in research fields, industrial applications, radiographic systems, or radiotherapy applications in the medical field.
Generally, the accelerating cavities consist of a plurality of single cells joined to one another.
The most common fixing techniques, used all over the world, are high temperature brazing and diffusion welding, which are necessarily performed inside a high temperature furnace.
The first process, i.e., brazing, involves melting copper alloys which join together the metal surfaces of the adjacent cells.
In diffusion welding, the melting occurs directly on the contact surfaces.
These processes, both at high temperatures, require high-level skills, and as it is known in the community of those skilled in the art of the accelerators described above, are often unsuccessful in the production of cavities adapted for high-power radio frequency operations.
In fact, the risks of brazing the bond by diffusion are many, and they may lead to a failure of the process itself, or to damaging the accelerator structure, or to both situations.
For example, if the brazing alloy is not positioned correctly, small pockets of air may remain around the filling areas; these pockets are sources of trapped gas, and may cause leaks during ultra-high vacuum operations or, if the furnace has a vacuum issue, they may result in the oxidation of the internal structure.
Another reason for avoiding the use of high temperature joining processes is the possibility of obtaining higher accelerating gradients with non-annealed copper alloy structures.
Recent experiments have shown that structures made of hard copper alloys may achieve higher accelerating gradients with lower breakdown rates (BDR) with respect to those treated at high temperatures.
Furthermore, the costs of brazing or diffusion welding are also a non-secondary issue, since it is necessary to take into account dedicated devices, filling materials and furnace times, just to name a few of the difficulties.
Particle accelerators operating in the X-band are there, provided with a vacuum chamber generally consisting of three cells in which the central one is the one characterized by the highest gradient, while the first and the third ones are the end cells. Generally, in this configuration the peak of the electric field on the axis of the central cell is two times higher than in the end cells.
It is the main object of the invention to provide a compact particle accelerator with a high accelerating gradient, made without resorting to high temperature welding processes which tend to deteriorate the accelerating structure.
This has been achieved, according to the invention, by providing a linear particle accelerator consisting of at least three accelerator cells, each of which is provided with an RF tooth on the contact surfaces, held in place by tightening means, such as locking screws, for example, and the primary vacuum structure of which is enclosed by a secondary vacuum chamber.
A better understanding of the invention will be achieved by means of the following detailed description and with reference to the accompanying drawings, which show some preferred embodiments merely given by way of a non-limiting example.

In the drawings:

Figure 1A shows a 3D view of a first embodiment of the invention, consisting of three cylindrical-symmetry cells which are adjacent and coaxial to one another, for example circular in shape, joined and held in place by dedicated longitudinal screws, in which the main vacuum chamber (3), the inlet pipe (1) and the vacuum connection flange (6) may be seen.
Figure 1B , similar to the preceding one, shows the invention from another point of view.
Figure 2 shows a side view of the device in Figure 1 .
Figure 3A shows a section along an axial longitudinal plane, marked as A-A in Figure 2 , consisting of three circular cells which are joined and held in place by dedicated screws.
Figure 3B shows an enlarged detail indicated by letter "D" in Figure 3A , as well as a further detail of the shape of the RF tooth of the first embodiment in Figure 1 .
Figure 4 shows a top view of the device in Figure 1 .
Figure 5A shows a section along an axial longitudinal plane, marked as A-A in Figure 4 , consisting of three circular cells which are joined and held in place by dedicated screws.
Figures 5B and 5C show enlarged details indicated respectively with the letter "B" and "C" in Figure 5A .
Figure 6A shows a 3D view of the section in Figure 3A .
Figure 6B shows an enlarged detail indicated by letter "E" in Figure 6A .
Figure 7A shows a 3D view of the section in Figure 5A .
Figure 7B shows an enlarged detail indicated by letter "F" in Figure 7A .
Figure 8 shows a 3D view of a second embodiment of the invention, consisting of two halves (half-shells) joined by locking screws which are perpendicular to the axis of the circular cells, adjacent and coaxial to each other, in which the main vacuum chamber (3'), the inlet pipe (1) and the vacuum connection flange (6) may be seen.
Figure 9 shows a side view of the device in Figure 8 .
Figure 10A shows a section of the vacuum chamber (3') along an axial longitudinal plane, marked as E-E in Figure 9 .
Figure 10B shows an enlarged detail indicated by letter "M" in Figure 10A .
Figure 11A shows a section along a transverse plane, marked as A-A in Figure 9 .
Figure 11B shows an enlarged detail indicated by letter "D" in Figure 11A .
Figure 12 shows a section along a transverse plane, marked as B-B in Figure 9 .
Figure 13A shows a section along a transverse plane, marked as C-C in Figure 9 .
Figure 13B shows an enlarged detail indicated by letter "F" in Figure 13A .
Figure 13C shows an enlarged detail indicated by letter "L" in Figure 13A .
Figure 14A shows a 3D view of a half-shell corresponding to the second embodiment in Figure 8.
Figures 14B and 14C relate to an enlarged detail, indicated by letter "A" in Figure 14A .
Figure 15 shows a front view of the internal side of the half-shell in Figure 14A .
Figure 16A shows a transverse side view in Figure 15 .
Figure 16B shows an enlarged detail indicated by letter "B" in Figure 16A .
Figure 17 shows a cross section along a plane marked as A-A in Figure 15 .
Figure 18 shows a longitudinal side view in Figure 15 .
Figure 19A , similar to Figure 14A , shows a different point of view of a half-shell corresponding to the second embodiment in Figure 8.
Figure 19B shows an enlarged detail indicated by letter "C" in Figure 19A .
Figure 20 shows a 3D view of a longitudinal tightening hollow screw, relating to the first embodiment in Figures 1A and 1B .
Figure 21 shows a longitudinal side view of the hollow screw in Figure 20 .
Figure 22 shows an axial longitudinal section along a plane marked as A-A in Figure 21 .
Figure 23 shows a 3D view of a tightening wrench for the hollow screw in Figure 20 .
Figure 24 shows a longitudinal side view of the tightening wrench in Figure 23 .
Figure 25 shows a transverse side view of the tightening wrench in Figure 23 .

With reference to the above Figures, the present invention is a linear particle accelerator, substantially consisting of a compact vacuum chamber made of high-conductivity material, such as copper or alloys thereof, for example, corresponding to the primary vacuum chamber, operating in the X-band and consisting of at least 3 adjacent and coaxial circular cells (or with another cylindrical symmetry shape), firmly joined to one another avoiding the use of conventional high temperature joining processes, such as conventional brazing or the like, which generally reach about 800°C and which cause the copper morphology to change, which occurs at about 450-500°C, thus resulting in a reduction in the performance of the accelerator itself in terms of accelerating gradient.
According to a first embodiment of the invention, the at least three adjacent and coaxial cells forming the primary vacuum chamber are locked and tightened in place by means of longitudinal screws; thereby, it is possible to proceed with TIG (Tungsten Inert Gas) welding on the outside of the coupled surfaces.
Each of such cells of the accelerating structure has a preferably circular cylindrical symmetry shape, and is designed to operate in the standing wave configuration in the π mode.
Said first embodiment of the invention provides that the primary vacuum chamber (3) is formed by three circular cells in the shape of a lowered cylinder, the height of which is less than the diameter, stacked on one another to form an internal duct and locked in place by means of special longitudinal screws V arranged outside the perimeter thereof (Figures from 1A to 7B and from 20 to 25).
A peculiar feature of the first embodiment described up to now consists of the tightening screws V, shown in Figures 20-22 , which have the dual purpose of being screwable onto the body of the structure and of receiving the next screw. To obtain this double function, they have been specially conceived and designed by calculating the required sections, and it was also necessary to make a special wrench to tighten them. In particular, each screw has a "male" threaded part which is screwable into the element preceding it (body of the device or screw) and a "female" threaded part which receives the next screw.
Such screws V consist of a front male threaded cylindrical part (for example, with M4 thread) which ends at an annular abutment surface defined by a larger rear cylindrical part (head of the screw) - coaxial to the preceding one - with an internal female threaded hole and having a thread corresponding and coaxial to said male part. According to the present invention, the front male threaded part of the next screw is housable in the female-threaded hole present in the head of each screw, and so on.
It should be noted that the screws V lack polygonal parts, since the "heads" of the screws are completely housed in a hole of the accelerator cell ( Figures 20-22 ).
Advantageously, by virtue of the peculiar constructional choice of the screws V according to the present invention, two important results are obtained: the first concerns the overall size, since, with this configuration, the space required is reduced by half; the second concerns the structural resistance, which, in this conformation, is higher and above all durable over time, since by tightening the screws in copper this would not have been possible.
The screws V are all "tubular", to evacuate the air inside the chambers where they are housed.
Another peculiarity of the invention, linked to the special screws V just described, concerns the tightening "wrench" CS, which was suitably designed to engage with the screw V by means of the female threaded part of the latter. To this end, one end of the wrench CS has a male thread M, with a lock nut CD which acts as a bottom stop during the screwing and allows an easy disassembly of the wrench itself upon tightening the screw V. At the opposite end of the wrench CS, a polygonal head TP is provided, to allow a tightening with a calibrated torque by means of a torque wrench.
A second embodiment of the invention ( Figures 8-19B ) provides that the vacuum chamber (3') is divided into two halves along an axial plane, consisting of two half-shells of substantially parallelepiped shape, in which inside each half-shell, semi-circular half-cells are present, so that, once the two halves (i.e., the two half-shells) are joined, the passage duct consisting of the cells themselves and resulting from the joining of the respective "half-cells" is formed. In this second embodiment, the two halves are kept in an assembled position by screws specially arranged outside the cells, perpendicular to the separation plane of the two halves. Once the two half-shells have been assembled, the accelerator consists of a body comprising a vacuum chamber having at least three cells with a cylindrical symmetry shape, adjacent and coaxial to one another.
The tightening of the structure is preferably carried out by means of transverse screws made of stainless steel; said screws are used to tighten the cells together so as to obtain the RF contact and a vacuum seal.
In both examples which are described, the structure of the particle accelerator consists of the main RF vacuum chamber (3 or 3'), which comprises the volume inside the three standing wave accelerator cells with high accelerating gradient, the inlet irradiation tube, which is connected to the circular RF inlet flange (1), and the downstream irradiation tube, which is connected to the downstream vacuum flange (6), which has a higher cutoff frequency than the operating frequency and therefore all the RF fields are located inside the three cells. The primary RF chamber is connected to two vacuum pumps, one through the circular RF inlet flange and the other through the downstream circular vacuum flange.
In both examples described herein, the primary vacuum chamber is enclosed by a second vacuum chamber; in the first embodiment this second chamber is welded upon the assembly and the TIG welding of the cells, while in the second embodiment the secondary chamber is integral with the structure also containing the primary chamber and therefore it does not require further processing to be installed.
A peculiar feature of the invention consists in that each cell has, on a surface in contact with the adjacent one, a ring beam shaped as an RF tooth, characterized in that it provides a contact angle between 10° and 30°, preferably equal to 20°.
In the first embodiment described so far, said RF tooth preferably has a size of 0.26 mm in length and 0.08 mm in height with a round top ( Figure 3B ).
In the second embodiment described herein, said RF tooth consists of a continuous ring beam arranged on the contact surface between the two half-shells and configured to surround the cells. In such a case, in which the cells 3' are preferably in two halves, the RF tooth preferably has a size of 0.05 mm in height, with the top having a plane surface of 0.08 mm ( Figure 14C ).
The sizing of said "RF tooth" between the cells was identified by means of a set of experimental tests to find a compromise between mechanical construction stability and vacuum seal while maintaining a good electrical contact.
As mentioned, the range of variability of the contact angle is between 10° and 30°.
Such values are not a mere constructional choice: if such an angle were less than 10° the contact between the surfaces would not be ensured and this might cause a deformation of the geometries of the RF surfaces and a greater pressure (i.e., tightening force) of the tightening screws would be required.
If the angle were greater than 30°, the force required by the tightening screws would be less, it would not cause deformation of the geometries of the RF surfaces but it would reduce the electrical contact with possible vacuum losses.
Said RF tooth has the dual purpose of ensuring a vacuum seal and of obtaining the necessary radio frequency electrical contact between the cells; to this end, the geometry of the contact surfaces between the cells has been suitably optimized by providing such a tooth.
Each cell is preferably made of oxygen-free high-conductivity (OFHC) copper, and is processed with a computer numerical control (CNC) milling machine. In this operation, excess material was left on all surfaces. After this first milling step, the cells underwent an annealing process in a vacuum furnace at 450°C for one hour. Such a procedure allows the crystalline structure of the metal to regenerate and remove internal tensions. After this low-temperature heat treatment, the cells are processed on a 5-axis milling machine to achieve the desired finish on all non-RF surfaces. The RF surfaces are left about 0.05 mm thicker than the final size. The final finish of the internal RF surfaces is obtained by means of a processing in an environment at a controlled temperature of 21°C+/-1°C. The final tolerance achieved is of +/-2 µm on each geometric dimension. This last processing step was performed with a very high precision lathe and using monocrystalline diamond tools (MCD).
The use of a very high-precision machine, the MCD tool and the controlled temperature environment, made it possible to create single cells with dimensional tolerances <2 µm and surface roughness <30 nm.
The above RF tooth is provided both in the aligned cell configuration and in the configuration made in two halves.
In order to ensure the vacuum seal and maintain the robustness of the entire assembly of the accelerator device with high accelerating gradient during high-power operation, which causes inevitable thermal and mechanical expansion, the accelerator structure was welded by using, as mentioned, the Tungsten Inert Gas (TIG) process.
With this approach, during the TIG welding step, the maximum temperature to which the accelerator structure is subjected is between 280-300 degrees Celsius, respectively, internally and externally.
Thereby, the change in copper morphology, which generally occurs at higher temperatures, is advantageously avoided.
Another peculiarity of the invention consists in providing a secondary vacuum chamber (4), around the primary RF chamber, the presence of which allows:

1) reducing and possibly eliminating any virtual air pockets, created between cells tightened together, which initially lose gas in the primary vacuum chamber;
2) reducing the risk of contamination of the primary chamber caused by the welding process performed on the external surfaces of the cavity which would inevitably cause breakdowns.

According to the present invention, the combined presence of the tightening screws, the TIG welding, the RF tooth and the secondary vacuum chamber allows obtaining better performances in terms of accelerating gradient, further contributing to obtaining a more compact overall structure.
The structure made of copper was experimentally tested at high power, confirming the expected results for the unconventional structures of the 'hard' type which provide accelerating gradients higher than 100 MV/m in the X-band, despite not being built by using conventional high temperature processes.
With a radio frequency pulse length of 150 ns, the resulting accelerating gradient is 145 MV/m with a 'breakdown' value of 10^-3MV/m/pulse. At 10^-7MV/m/pulse, the corresponding accelerating gradient is 130 MV/m.
The variation of the slope of the 'breakdown' behavior as a function of the accelerating gradient, the surface electric field and the 'pulse heating' is clearly obtained as the radio frequency pulse length increases up to 600 ns.
Advantageously, it is also possible to realize the linear particle accelerator described up to now using the copper/silver alloy, to further improve the behavior in terms of accelerating gradient.
The linear particle accelerator described so far operates in the X-band, preferably between 7 and 12.5 GHz.
Finally, it should be noted that, although the embodiments of the invention - described and shown in the present description merely by way of example - provide cells with a circular shape, according to the present invention it is provided that such cavities may be of any cylindrical symmetry shape.

KEY:

1: Flanged cell outlet
2: Accelerator cell with cylindrical primary vacuum chamber
3: Accelerator cell with cylindrical primary vacuum chamber
4: Accelerator cell with cylindrical primary vacuum chamber
5: Flanged cell inlet
6: Secondary chamber pumping flange CF16
7: Secondary chamber pumping pipe
V: Tubular tightening screws
3': Shell-shaped primary vacuum chamber
CS: Tightening wrench
TP: Polygonal head of the wrench CS
CD: Bottom stop lock nut
M: Male thread
RF: Vacuum seal tooth for radio frequency electrical contact between the cells.

Claims

A linear particle accelerator with high accelerating gradient, provided with a body comprising a compact primary vacuum chamber (3, 3') made of high-conductivity material, characterized in that said chamber (3, 3') comprises at least three adjacent, coaxial accelerating cells, having a cylindrical symmetry shape, configured to operate in the standing wave configuration in the π mode ; wherein said accelerating cells are joined together and locked by tightening means and externally welded by means of TIG (Tungsten Inert Gas) welding to avoid high temperature treatments; wherein each cell has a ring beam forming an RF tooth with a contact angle between 10° and 30°, arranged on at least one contact surface between the assemblable parts to form said cells, which tooth is configured to provide, upon the actuation of said tightening means, the RF contact and the vacuum seal.
An accelerator according to claim 1, characterized in that said primary vacuum chamber (3) includes at least three adjacent, coaxial cylindrical-symmetry cells in the shape of a lowered cylinder, the height of which is less than the diameter, stacked on one another to form an internal duct and locked in place by tightening means substantially consisting of longitudinal screws (V) arranged outside the perimeter thereof, which screws (V) are tubular with a front part and a rear part of larger diameter, coaxial to each other and separated by an annular abutment surface, wherein the front part is male-threaded and the rear part has a female-threaded coaxial hole with the same thread as the male part.
An accelerator according to claim 2, characterized in that each of said screws (V) has a "male" threaded part which is screwable into the element preceding it, be it the body of the accelerator or another screw, and a "female" threaded part which receives the next screw; to this end, said screws (V) consisting of a front male threaded cylindrical part ending at an annular abutment surface defined by a larger rear cylindrical part forming the head of the screw (V) - coaxial to the preceding one - having an internal female threaded hole with a corresponding thread coaxial to said male part; wherein the front male threaded part of the next screw is housable in the female threaded hole present in the head of each screw (V), and so on.
An accelerator according to claim 2 or 3, characterized in that said screws (V) lack polygonal parts, since the heads of the screws (V) are completely housed in a hole of the accelerator cell.
An accelerator according to claim 1, characterized in that said primary vacuum chamber (3') is divided into two halves along an axial plane, consisting of two half-shells of substantially parallelepiped shape, wherein inside each half-shell, semi-circular half-cells are present, configured to form, once the two halves are joined, a passage duct consisting of the same cylindrical symmetry cells and resulting from the joining of the respective half-cells; said two halves being kept in an assembled position by tightening means substantially consisting of screws arranged outside the cells, perpendicular to said axial separation plane of the two halves.
An accelerator according to at least one of the preceding claims, characterized in that it has a structure consisting of the primary vacuum chamber (3, 3'), which comprises:
- the volume inside the three standing wave accelerator cells with high accelerating gradient;

- an inlet irradiation tube, which is connected to a circular RF inlet flange (1); and

- a downstream irradiation tube, which is connected to a downstream vacuum flange (6), which has a higher cutoff frequency than the operating frequency;
thus obtaining that all RF fields are located within the three cells: wherein the primary RF chamber (3, 3') is connected to two vacuum pumps, one through the circular RF inlet flange and the other through the downstream circular vacuum flange.
An accelerator according to at least one of the preceding claims, characterized in that said primary vacuum chamber (3, 3') is enclosed in a secondary vacuum chamber, which is welded once the cells have been assembled and TIG welded, or is included in the structure containing the primary chamber.
An accelerator according to at least one of the preceding claims, characterized in that said ring beam forms an RF tooth which has a contact angle of 20°.
An accelerator according to claim 7, characterized in that said secondary vacuum chamber is configured to:
• reduce and possibly eliminate any virtual air pockets, created between cells tightened together, which initially lose gas in the primary vacuum chamber;

• reduce the risk of contamination of the primary chamber caused by the welding process performed on the external surfaces of the cavity which would inevitably cause breakdowns.
An accelerator according to at least one of the preceding claims, characterized in that it is made of high-conductivity material such as copper or copper/silver alloy.
An accelerator according to the preceding claim, characterized in that it consists of assembled parts which are weldable by means of a TIG (Tungsten Inert Gas) welding at temperatures between 280-300 °C such as to avoid the change of copper morphology, thus allowing to enhance the properties resulting from the use of hard copper which allow obtaining higher acceleration gradients with lower RF breakdown rates (BDR) than those treated at high temperature.
An accelerator according to at least one of claims 2 to 4, characterized in that said screws (V) are screwable/unscrewable by means of a special tightening wrench (CS), which is configured to cooperate with the screw (V) through the female threaded part of the latter; to this end, one end of the wrench (CS) includes a male thread (M), with a lock nut (CD) which acts as a bottom stop during screwing and allows easy disassembly of the wrench itself once the screw (V) has been tightened; wherein, at the opposite end of the wrench (CS), a polygonal head (TP) is provided, which is couplable to a torque wrench to allow a tightening with a calibrated torque.