WO2016185103A1

WO2016185103A1 - Tuning device for a radio-frequency accelerating cavity

Info

Publication number: WO2016185103A1
Application number: PCT/FR2016/050680
Authority: WO
Inventors: Pierre-Alain LEROY
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2015-05-15
Filing date: 2016-03-25
Publication date: 2016-11-24
Also published as: FR3036232B1; EP3295773A1; FR3036232A1

Abstract

The invention concerns a tuning device for an RFQ cavity (13) comprising; an inlet flange (10) comprising: o a nut (50); o a sealing flange (11); o a transverse wall (18) located between the nut (50) and the sealing flange (11); - a piston (21) comprising: o a piston nose (40) intended to be inserted into the cavity; o a piston rod (23) provided with a threaded portion screwed into the nut (50); - a bellows (32) surrounding the piston rod (23), the bellows being surrounded by the sealing flange (11) of the inlet flange, the bellows comprising o a first end (33) sealingly attached to the piston nose (40), and; o a second end (35) sealingly attached to the transverse wall (18) of the inlet flange (10).

Description

ACCORDING DEVICE FOR RADIO FREQUENCY ACCELERATOR CAVITY

TECHNICAL AREA

The field of the invention is that of tuning devices for radio frequency accelerating cavity, also called RFQ cavities for "quadrupole radio frequency". STATE OF THE PRIOR ART

Particle accelerators use radio frequency accelerating cavities, also called RFQ (Radio Frequency Quadrupole) cavities. These cavities are generally placed at the output of the "low energy" lines that conduct the continuous beams from the particle sources to the RFQ cavity. The role of an RFQ cavity is to focus the particle beam and group the particles of the beam into packets, while beginning to accelerate them, so that they can then be injected into a superconducting linear accelerator. The RFQ cavity is generally composed of several sections of copper. These sections are machined and assembled by brazing or mechanically. They are composed of four machined poles with a modulated undulation. The general manufacturing tolerance of these cavities is of the order of 50 μιτι. The radio frequency waves injected into the cavity generate:

an electric field that accelerates and packages the beam and

a magnetic field that focuses the particles.

These phenomena depend on the frequency and the voltage law, which themselves depend on the geometry of the cavity. The particle beam in particular is very sensitive to frequency variations and the voltage law due to mechanical inaccuracies during the manufacture of the cavity. To compensate for these mechanical inaccuracies, several tuning pistons are spaced longitudinally in the cavity. These tuning pistons are also called "tuning devices". These tuning devices are intended to modify the internal volume of the cavity by sinking more or less inside, which has the effect of correcting the mechanical defects of the cavity.

FIG. 1 represents a tuning device 1 of the prior art. This tuning device 1 comprises an inlet flange 2 in which a piston 3 slides. The piston 3 comprises a piston nose 4 and a sealing flange 6. The volume of the cavity is modified by modifying the length L1 the piston nose 4 inserted into the cavity. The length L1 of the piston nose 4 inserted into the cavity is chosen by inserting a wedge 5 of height H1 chosen between the sealing flange 6 of the piston and the inlet flange 2. Such a tuning device must imperatively be watertight. because the operating cavity is placed under a vacuum of about 10 ^"7 mbar. for this, a first seal 7 is placed between the sealing flange of the piston 6 and the spacer 5, while a second seal is placed between the wedge 5 and the inlet flange 2. The method of tuning the cavity using such a tuning device is complex.

Indeed, it comprises firstly a step of determining the height H1 of the wedge 5 to be inserted between the collar of the piston and the inlet flange. For this, it is first necessary to determine the length L1 of the piston nose to be inserted into the cavity. This length L1 is determined by means of an intermediate tuning device 1 'shown in FIG. 2. This intermediate tuning device 1' also comprises an inlet flange 2 'and a piston 3'. The inlet flange 2 'is fixed, while the nut 5' is provided with a thread, which makes it possible to move the piston 3 'with respect to the inlet flange 2' so as to modify the length L1 piston nose inserted into the cavity.

The optimum length L1 'of the piston nose 4 inserted into the cavity is determined experimentally by placing the cavity at atmospheric pressure, at low power and at a low radio frequency level. Once this length L1 'is determined, it dismounts the intermediate tuning device 1', which can not remain in place since it is not sealed due to the presence of the thread and the complementary thread. The length L1 'is then measured and the height of the shim H1 which must be used to have a length L1 equal to the length L1' is deduced therefrom. The wedge is then machined and the final tuning device is put in place in the cavity.

A radio frequency measurement is then carried out with the definitive tuning device to check whether the tuning is correct. If this is not the case, the shims must be re-machined.

The method of tuning a cavity using the tuning devices of the prior art is therefore long and complex. SUMMARY OF THE INVENTION

The invention aims to overcome the disadvantages of the state of the art by proposing a tuning device for simplifying the tuning process of the cavity, while having the same guarantees in terms of sealing as the devices of agreement of the prior art.

To do this, is proposed according to a first aspect of the invention, a tuning device for a radiofrequency accelerating cavity comprising:

an inlet flange comprising:

o a nut;

a sealing flange arranged to be sealingly attached to an outer wall of the cavity;

a transverse wall located between the nut and the sealing flange;

a piston comprising:

o a piston nose to be inserted into the cavity;

a piston rod provided with a threaded portion screwed into the nut of the inlet flange; a bellows surrounding the piston rod, the bellows being surrounded by the sealing flange of the inlet flange, the bellows comprising:

a first end sealingly attached to the piston nose;

a second end sealingly attached to the transverse wall of the inlet flange.

The device according to the first aspect of the invention is particularly advantageous since it has the advantage of having a piston nose whose length inserted in the cavity is adjustable, while being sealed. For this, the threaded portion of the piston rod is screwed into the nut of the inlet flange, which allows to adjust the length of the piston nose inserted into the cavity. The bellows makes it possible to seal the device by isolating the threaded parts of the device from the parts in fluid communication with the cavity. Thus, the threaded portions are in communication with the interior of the bellows, while the parts in fluid communication with the interior of the cavity are located outside the bellows. The zone of the device outside the bellows can therefore be at the same pressure as the inside of the cavity, even if the zone of the device inside the bellows is at atmospheric pressure. In addition, the bellows is deformable longitudinally, so that it tolerates the longitudinal movements of the piston relative to the inlet flange.

The tuning device according to the first aspect of the invention may also have one or more of the following features taken individually or in any technically possible combination.

According to one embodiment, the tuning device has a symmetry of revolution around the piston rod, which allows to distribute the forces exerted on the device. According to one embodiment, the tuning device comprises at least two locking screws arranged to block the rotation of the piston rod with respect to the inlet flange, which makes it possible to immobilize the piston in translation with respect to the entrance flange. According to one embodiment, the piston rod is pierced by a longitudinal opening capable of being traversed by a cooling fluid, which makes it possible to cool the piston so that it is at the same temperature as the inside of the piston. the cavity.

According to one embodiment, the piston nose comprises:

an end skirt surrounding the first end of the bellows;

a transverse wall connecting the end skirt to the piston rod, the first end of the bellows being fastened to the transverse wall of the piston nose.

Thus, the piston nose is hollow, which allows for a more compact tuning device. According to one embodiment, the transverse wall of the piston nose is provided with a ring projecting longitudinally, the first end of the bellows being fixed on the ring. This ring facilitates the attachment of the bellows on the piston nose.

According to one embodiment, the piston nose has an outer wall provided with a groove, an electrically conductive seal being inserted into the groove so as to be in contact with both the piston nose and the inlet flange. . This seal prevents the radio frequency waves present in the cavity from entering the tuning device beyond this seal. Thus, the portion of the piston nose that allows tuning the cavity is delimited by this seal.

Advantageously, the second end of the bellows is attached to the inlet flange by a weld, the first end of the bellows being fixed to the piston nose by a weld. The bellows is thus sealed to the inlet flange on the one hand and the piston on the other hand.

Advantageously, the sealing flange of the inlet flange is pierced by an annular groove in which a seal is inserted, which makes it possible to ensure the seal between the outer wall of the cavity and the flange. Entrance. According to one embodiment, the device comprises:

means for measuring the radio frequency wave in the cavity;

- Control means adapted to control the translation of the piston relative to the inlet flange.

It is thus possible to automatically control the position of the piston nose as a function of the measurement of the radio frequency wave in the cavity.

These means for measuring the radiofrequency wave could for example be measuring loops configured to measure the electromagnetic field present in the cavity.

A second aspect of the invention relates to a radiofrequency accelerating cavity comprising at least one tuning device according to the first aspect of the invention, the sealing flange of the inlet flange of the device being sealingly attached to a wall. external cavity, the piston nose of the device being inserted into an opening of the cavity.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge on reading the detailed description which follows, with reference to the appended figures, which represent:

- Figure 1, a sectional view of a tuning device according to the prior art;

FIG. 2 is a sectional view of an intermediate tuning device of the prior art;

- Figure 3, a sectional view of a tuning device according to one embodiment of the invention;

- Figure 4, a perspective view of the device of Figure 3.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

A tuning device according to one embodiment of the invention will now be described with reference to Figures 3 and 4. The tuning device 1 comprises an inlet flange 10. The inlet flange 10 comprises a tubular wall 15 having a lower end 16 and an upper end 17. The inlet flange 10 also comprises a sealing flange 1 1 radially projecting from the lower end 16 of the tubular wall 15. The sealing flange 1 1 is intended to come into contact with an outer wall 12 of the cavity 13 to be tuned. The sealing flange 1 1 is pierced by orifices 14 for fixing the sealing flange 1 1 on the outer wall 12 of the cavity 13 by means of screws. The sealing flange 1 1 also preferably comprises an annular groove 41 in which can be inserted a seal 42 for sealing the contact between the sealing flange 1 1 and the outer wall 12 of the cavity.

The inlet flange also has a transverse wall 18 which closes the upper end 17 of the tubular wall 15.

The transverse wall 18 of the inlet flange 10 is pierced, preferably at its center, by an orifice 19 surrounded by an annular skirt 20. The annular skirt 20 and the tubular wall 15 are preferably concentric. As shown in FIGS. 3 and 4, the annular skirt 20 can protrude longitudinally from the orifice 19.

The inlet flange 10 also comprises a nut 50 pierced by an orifice 51. The nut 50 is provided with a threaded inner wall 52. The threaded inner wall 52 of the nut 50 surrounds the orifice 51 of the nut. The nut 50 is integral with the annular skirt 20. According to various embodiments, the nut 50 and the annular skirt 20 may be two separate parts secured, for example by screws to allow adjustment if necessary.

The tuning device also comprises a piston 21. The piston 21 extends along a reference axis 22. The tuning device preferably has a symmetry of revolution with respect to the reference axis 22 which allows to distribute the forces.

The piston 21 comprises a piston rod 23 inserted into the orifice 19 of the skirt 20 and in the hole 51 of the nut 50. The piston rod 23 is provided with a threaded portion 26 screwed into the nut 50 of the inlet flange 10. By screwing more or less the threaded portion 26 of the piston in the nut 50, the piston can be translated relative to the inlet flange. The piston rod 23 further preferably has a smooth portion 53 inserted into the annular skirt 20 of the inlet flange. This smooth portion 53 of the piston rod is slidable in the annular skirt 20 of the inlet flange when the threaded portion 26 of the piston rod 23 is screwed into the nut 50.

The tuning device also comprises screws 39, preferably two, for blocking the movement of the piston rod 23 with respect to the inlet flange 10. The piston rod 23 can thus be immobilized with respect to the flange. entrance 10.

The piston rod 23 has a distal end, called "outer end" 27 intended to remain outside the cavity 13 and a proximal end, called "inner end" 28 intended to be inserted into the cavity 13. The end Inner 28 of the piston rod is surrounded by an end skirt 29 connected to the piston rod 23 by a transverse wall 30. The end skirt 29 of the piston is inserted into the tubular wall 15 of the inlet flange 10.

The portion of the inner end 28 surrounded by the end skirt 29, the end skirt 29 and the transverse wall 30 form a piston nose 40. The piston nose 40 is intended to be inserted into the cavity 13 of the piston. how to tune it in frequency.

The end skirt 29 preferably has an outer wall 43 in which is hollowed a groove 44. A seal 45, called "tuning seal" 45 in the following, is inserted into the groove 44. The gasket 45 is made of an electrically conductive material. The tuning joint 45 is arranged to prevent radiofrequency waves from passing between the piston nose 40 and the inlet flange 10. For this, the tuning seal 45 fills the space between the piston nose and the inlet flange 10. The cavity is tuned by moving the position of the gasket 45 longitudinally. The end skirt 29 and the transverse wall 30 of the piston on the one hand, and the tubular wall 15 and the transverse wall 18 of the inlet flange 10 on the other hand define a variable volume 31 depending on the position of the piston. piston 21 with respect to the inlet flange 10.

The tuning device also comprises a bellows 32 connecting the piston 21 to the inlet flange 10. The bellows 32 surrounds the piston rod 23. The bellows 32 is surrounded by the tubular wall 15 of the inlet flange 10.

The bellows 32 comprises:

a first end 33 fixed to the transverse wall 30 of the piston 21. For this, a ring 34 preferably protrudes longitudinally from the transverse wall 30 of the piston. The first end 33 of the bellows is preferably welded to the ring 34. The ring 34 facilitates the attachment of the bellows 32 to the transverse wall 30 of the piston. The ring 34 is preferably brazed to the transverse wall 30 of the piston;

a second end 35 fixed to the transverse wall 18 of the inlet flange. For this, a ring 36 preferably projects longitudinally from the transverse wall 18 of the inlet flange 10. The second end 35 is preferably welded to the ring 36. The ring 36 also facilitates the fastening of the bellows on the inlet flange. The ring 36 is preferably brazed to the transverse wall 18. The second end 35 of the bellows 32 is disposed inside the annular wall 15 of the inlet flange 10. The bellows 32 and the annular wall 15 of the inlet flange are concentric. Furthermore, the second end 35 of the bellows surrounds the annular skirt 20 of the inlet flange. The first end 33 of the bellows is disposed within the end skirt 29 of the piston nose. The bellows 32 and the end skirt 29 are also concentric. The bellows 32 is deformable longitudinally. In other words, the bellows 32 may be deformed along the reference axis 22. The bellows 32 is preferably metallic. The bellows 32 longitudinally divides the volume 31 into two chambers 37, 38 sealed with respect to one another: the first chamber 37 corresponds to the space inside the bellows 32, while the second chamber 38 corresponds to the space in the volume 31, outside the bellows 32. The first and the second chamber are concentric with respect to each other.

The first chamber 37, inside the bellows, is intended to be placed at atmospheric pressure. Indeed, the first chamber 37 is in communication with the outside of the cavity via the threaded portion 26 of the piston rod and the nut 50. The second chamber 38, outside the bellows, is intended to be placed at the same pressure as the interior of the cavity 13. In fact, the second chamber 38 is in fluid communication with the interior of the cavity 13 via the space between the piston nose 40 and the inlet flange 10. thus, when the pressure inside the cavity is 10 ^"7 mbar, the pressure in the second chamber 38 is also ^{10" 7} mbar. The threaded portion 26 of the piston rod and the nut 50 are thus isolated from the inside of the cavity by the bellows 32, so that the tuning device is sealed despite the presence of the thread.

In addition, the tuning device is preferably provided with cooling means. Indeed, in operation, the radio frequency waves can heat the piston nose 40. It is therefore advantageous to provide cooling means 46 for cooling the piston nose 40. To do this, the piston nose 40 is preferably hollowed out. by a longitudinal opening 47 in which can be injected a cooling fluid.

Naturally, the invention is not limited to the embodiments described with reference to the figures and variants could be envisaged without departing from the scope of the invention. Thus, other cooling means could be considered. It would be possible to use, as cooling means, pipes distributed over the circumference of the piston. In addition, the piston, and in particular the piston nose could have another geometry. Indeed, in the embodiment described, the piston nose is recessed between the end skirt 29 and the piston rod 23, essentially for reasons of weight and cost. However, one could also consider using a full piston nose for reasons of simplicity of manufacture. Similarly, in the embodiment described, the inlet flange is recessed between the tubular wall and the annular skirt. However one could also consider making a non-recessed entrance flange.

Moreover, one could also consider automating the process of tuning the cavity using the tuning device. For this, the device could also include means for measuring the radiofrequency wave in the cavity. These measuring means could for example be integrated in the piston nose. The device could further comprise control means for automatically screwing the piston rod into the inlet flange. The screwing of the piston rod could be slaved according to the measurement of the radiofrequency wave.

Claims

1. Tuning device for a radiofrequency accelerating cavity (13) comprising:

an inlet flange (10) comprising:

a nut (50);

o a sealing flange (1 1) arranged to be sealingly attached to an outer wall (12) of the cavity (13);

a transverse wall (18) located between the nut (50) and the sealing flange (1 1);

a piston (21) comprising:

a piston nose (40) for insertion into the cavity; o a piston rod (23) provided with a threaded portion screwed into the nut (50) of the inlet flange (10), the piston rod (23) being pierced by a longitudinal opening (47) adapted to be traversed by a cooling fluid;

- a bellows (32) surrounding the piston rod (23), the bellows being surrounded by the sealing flange (1 1) of the inlet flange, the bellows comprising:

a first end (33) sealingly attached to the piston nose (40);

a second end (35) sealingly attached to the transverse wall (18) of the inlet flange (10).

2. Tuning device according to the preceding claim, having a symmetry of revolution around the piston rod (23).

3. Device according to one of the preceding claims, comprising at least two locking screws (39) arranged to block the rotation of the piston rod relative to the inlet flange.

4. Device according to one of the preceding claims, wherein the nose of piston (40) comprises:

- an end skirt (29) surrounding the first end (33) of the bellows (32);

- a transverse wall (30) connecting the end skirt (29) to the piston rod (23), the first end of the bellows being attached to the transverse wall (30) of the piston nose (40).

5. Device according to one of the preceding claims, wherein the piston nose (40) comprises an outer wall provided with a groove (44), an electrically conductive seal (45) being inserted into the groove (44) so to be in contact with both the piston nose (40) and the inlet flange

(10).

6. Device according to one of the preceding claims, wherein the second end (35) of the bellows (32) is fixed to the inlet flange (10) by a weld, the first end (33) of the bellows (32). being attached to the piston nose (40) by a weld.

7. Device according to one of the preceding claims, wherein the sealing flange (1 1) of the inlet flange (10) is pierced by an annular groove (41) in which is inserted a seal.

8. Device according to one of the preceding claims, further comprising:

means for measuring a radio frequency wave in the cavity;

9. radiofrequency accelerating cavity (13) comprising at least one tuning device according to one of the preceding claims, the sealing flange.

(1 1) of the inlet flange (10) of the device being sealingly attached to an outer wall (12) of the cavity, the piston nose (40) of the device being inserted into an opening of the cavity.