COMPACT LINAC

Abstract

A linear accelerator comprises side-coupled cavity cells configured to accelerate electrons with a radio frequency field. The field amplitude in the initial cells is lower than in the later cells, and the initial cells are shorter than the later cells. This creates a capture section where electrons are captured and bunched while experiencing low acceleration, followed by an acceleration section where the bunched electrons experience stronger acceleration.

Claims

1. A linear accelerator comprising: an electron source region in which is located an electron source; a cavity comprising a plurality of cells with each cell linked to an adjacent cell by a side chamber, wherein the cavity is configured to accelerate electrons received from the electron source region through the series of cells with a radio frequency field having an accelerating field amplitude in each of the cells; and a radio frequency source configured to generate and inject the radio frequency field into the cells, wherein the side chambers are configured to couple the radio frequency field between the connected pairs of linked cells; wherein the plurality of cells comprises a succession of cells extending from a first cell of the plurality of cells that receives electrons generated by the electron source to a final cell of the plurality of cells; the plurality of cells comprises a capture section, comprising the first cell and a second cell, and an acceleration section, comprising the final cell and a plurality of cells immediately preceding the final cell, and the length of each cell in the capture section is shorter than the length of each cell in the acceleration section; the cavity is configured such that, when the radio frequency source injects the radio frequency field into the cells, the field amplitude in the capture section is a lower field amplitude and such that the field amplitude in the acceleration section is a higher field amplitude relative to the lower field amplitude in the capture section.

2. The linear accelerator of claim 1, wherein the cavity is configured such that electrons decelerated in the capture section in one radio frequency cycle are accelerated by the radio frequency field in the next radio frequency cycle and travel to the acceleration section.

3. The linear accelerator of claim 2, wherein the cavity is configured such that some electrons arriving in a first radio frequency cycle are accelerated and some electrons are decelerated, and such that some electrons arriving in the next radio frequency cycle are accelerated along with the electrons decelerated in the first radio frequency cycle and travel to the acceleration section together.

4. The linear accelerator of claim 1, wherein the length of the cells in the capture section gets progressively longer and the length of all the cells of the acceleration section is the same.

5. (canceled)

6. The linear accelerator of claim 1, wherein: a pair of passages couple each side chamber to its connected cells with one passage coupling the side chamber to one of the connected cells and the other passage coupling the side chamber to the other of the connected cells; a first side chamber couples the first cell to the second cell; and the length of the passage between the first side chamber and the first cell is longer than the length of each passage between the side chambers and the cells of the acceleration section.

7. (canceled)

8. The linear accelerator of claim 6, wherein the ratio of the length of the passage between the first side chamber and the first cell divided by the length of the passage between the first side chamber and the second cell is greater than the ratio of the length of the passages between each side chamber and its connected cells.

9. The linear accelerator of claim 6, wherein a second side chamber couples the second cell to the third cell and the length of the passage between the second side chamber, and the second cell is longer than the length of each passage between the side chambers and the cells of the acceleration section.

10. (canceled)

11. The linear accelerator of claim 9, wherein the ratio of the length of the passage between the first side chamber and the first cell divided by the length of the passage between the first side chamber and the second cell is greater than the length of the passage between the second side chamber and the second cell divided by the length of the passage between the second side chamber and the third cell.

12. (canceled)

13. The linear accelerator of claim 1, wherein the diameter of the first cell relative to the length of the first cell is set to detune the first cell out of resonance and the diameter of the second cell relative to the length of the second cell is set to detune the second cell out of resonance.

14. The linear accelerator of claim 1, wherein: the first cell comprises an entrance aperture and an exit aperture through which electrons pass when accelerated by the linear accelerator, and wherein the entrance aperture is narrower than the exit aperture; the entrance and exit apertures of the cells of the acceleration section are the same size; and the entrance and exit apertures of the cells of the acceleration section are the same size as the exit aperture of the second cell in the capture section.

15. (canceled)

16. (canceled)

17. The linear accelerator of claim 1, wherein the entrance aperture of the first cell is located in a re-entrant section of the first cell and the exit aperture of the first cell is located on a flat or substantially flat section of the first cell.

18. The linear accelerator of claim 17, wherein the entrance and exit apertures of the cells of the acceleration section are located in re-entrant sections of the respective cells.

19. The linear accelerator of claim 1, wherein the cavity and radio frequency field is configured such that the lower field amplitude in the capture section accelerates the electrons to non-relativistic kinetic energies and the higher field amplitude in the acceleration section accelerates the electrons to relativistic kinetic energies.

20. The linear accelerator of claim 19, wherein: the electron source is operable to provide electrons to the entrance aperture of the first cell with kinetic energies between 10 and 50 keV; and the linear accelerator is operable such that electrons exit the cavity with kinetic energies between 3 and 10 MeV.

21. (canceled)

22. The linear accelerator of claim 1, wherein: the cavity further comprises an intermediate section located between the capture section and the acceleration section; the intermediate section comprises one or more cells with each pair of cells coupled by a side chamber; and the cavity is configured such that, when the radio frequency source injects the radio frequency field into the cells, the field amplitude in the intermediate section is an intermediate field amplitude relative to the lower field amplitude in the capture section and the higher field amplitude in the acceleration section.

23. (canceled)

24. (canceled)

25. A method of accelerating electrons using the linear accelerator of claim 1, the method comprising: injecting a radio frequency field into the cavity such that a radio frequency field with a field amplitude is created in the cells; in the electron source region, directing electrons produced by the electron source to the entrance aperture of the first cell such that the electrons enter the first cell; coupling the electrons to the radio frequency field in the capture section such that the coupled electrons are directed from the capture section to the acceleration section, wherein the field amplitude in the capture section is a lower field amplitude; and accelerating the coupled electrons in the acceleration section with the radio frequency field, wherein the field amplitude in the acceleration section is a higher field amplitude relative to the lower field amplitude in the capture section.

26. The method of claim 25, comprising decelerating some electrons in the capture section in one radio frequency cycle and accelerating the decelerated electrons in the next radio frequency cycle so that the electrons travel to the acceleration section.

27. The method of claim 26, comprising decelerating some electrons in the capture section in one radio frequency cycle and accelerating the decelerated electrons in the next radio frequency cycle with other electrons arriving in the capture section during the next radio frequency cycle so that the electrons accelerated in the second radio frequency cycle travel to the acceleration section together.

28. (canceled)

29. The method of claim 25, comprising injecting the radio frequency field into the cavity and directing electrons produced by the electron source to the entrance aperture of the first cell such that that the lower field amplitude produces a field strength less than the energy at which the electrons enter the first cell.

30. A linear accelerator comprising side-coupled cavity cells configured to accelerate electrons with a radio frequency field, wherein the field amplitude in the initial cells is lower than in the later cells and the initial cells are shorter than the later cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order that the invention can be more readily understood, reference will now be made by way of example only, to the accompanying drawings in which:

[0052] FIG. 1 is a side cross-sectional view of a linac; and

[0053] FIG. 2 is a detail of the linac of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0054] The Figures show a linac 10 that comprises an electron source section, including an electron source 12, and a cavity section 14. The electron source 12 may be any type of conventional electron source 12, and so will not be described in further detail.

[0055] The cavity section 14 includes a cavity 16 comprising a series of cavity cells 101-106 joined by a channel 18 that passes through the centre of each cavity cell 101-106. The channel 18 extends from one side 15 of cavity section 14, through an entrance aperture 20 of a first cell 101, through cells 101-106, through an exit aperture 22 of the final cell 106, and to the other side 17 of the cavity section 14. The longitudinal axis of the linac 10 extends from the centre of the entrance aperture 20 to the centre of the exit aperture 22. Electrons generated by the electron source 12 enter the cavity section 14 at the channel 18 on side 15, and exit the cavity section 14 from the channel 18 at side 17. The cavity section 14 also comprises a series of side cells 201-205. Each side cell 201-205 is positioned adjacent a pair of the cavity cells (101&102, 102&103, 103&104, 104&105, 105&106) so as to overlap with the pair of cavity cells (101&102, 102&103, 103&104, 104&105, 105&106). A passage 34 extends between each cavity cell 101-106 and each overlapping side cell (201-205) and so couple those cells (101&102, 102&103, 103&104, 104&105, 105&106). The side cells 201-205 alternate from side to side, for example alternating between being positioned above and below the cavity cells 101-106 as shown in FIG. 1.

[0056] The side cells 201-205 are used to couple a RF field with the cavity cells 101-106 via the passages 34. Namely, the first side cell 201 couples the RF field with the first cavity cell 101 and the second cavity cell 102 via passages 34.sub.201-101 and 34.sub.201-102, the second side cell 202 couples the RF field with the second cavity cell 102 and the third cavity cell 103 via passages 34.sub.202-102 and 34.sub.202-103, and so on. The side cells 201-205 are coupled to RF field generators that are conventional and so are not shown in the Figures, and not described further.

[0057] The cavity cells 101-106 are not of a uniform size. The third to sixth cavity cells 103-106 are of a common size and shape, and form an acceleration section 120. The first cavity cell 101 and the second cavity cell 102 have different sizes, relative to each other and also relative to the cavity cells 103-106 of the acceleration section 120. The first cavity cell 101 and the second cell 102 form a capture section 110. The second cavity cell 102 has smaller width (dimension transverse to the longitudinal axis of the linac 10) than the cavity cells 103-106 of the acceleration section 120. The first cavity cell 101 is narrower than the second cavity cell 102, which is narrower than the cavity cells 103-106 of the acceleration section 120.

[0058] The bore of the channel 18 is relatively small in its first part from the side 15 of the cavity section 14 to the entrance aperture 20. This stops the RF field leaking from the first cell 101. The bore of the channel 18 is then relatively large for each of the parts that link the subsequent cavity cells 102-106 and for the part extending from the exit aperture 22 to the side 17 of the cavity section 14. This can be seen most clearly in the detail of FIG. 2.

[0059] Each cavity 101-106 cell is approximately cylindrical in shape, and comprises a front wall 30.sub.101-30.sub.106 and a back wall 32.sub.101-32.sub.106. Each of the front walls 30.sub.101-30.sub.106 have a central re-entrant part. The back walls 32.sub.102-32.sub.106 of the cavity cells 102-106 of the acceleration section 120 also have a central re-entrant part. However, the back wall 32.sub.101 of the first cavity cell 101 does not have a central re-entrant part and is flat instead, as can be seen most clearly from the detail of FIG. 2. This shortens the electron path length between the first cavity cell 101 and the second cavity cell 102.

[0060] The side cells 201-205 also have a generally cylindrical shape with a cylindrical central section flanked by annular sections. The side cells 201-205 have the same size and shape, and are offset the same distance from the longitudinal axis with the exception of the first side cell 201 which is positioned closer to the longitudinal axis. Each side cell 201-205 overlaps with two cavity cells 101-106, and is coupled to each adjacent cavity cell 101-106 by respective passages 34. The smaller widths of the first and second cavity cells 101-102 means that the lengths d.sub.201-101, d.sub.201-102 and d.sub.202-102 of the passages 34.sub.201-101 and 34.sub.202-102 coupling to the first cavity cell 101 and the second cavity cell 102 are longer than the passages 34 for the other cavity cells 103-106 (as best seen in FIG. 2; the length of a passage is the length running from the side cell to the cavity cell and hence transverse to the longitudinal axis). Moreover, the ratio of the lengths d.sub.201:101:d.sub.201:102 and d.sub.202-102:d.sub.202-103 of the passages 34.sub.201-101/34.sub.201-102 and 34.sub.202-102/34.sub.202-103 is larger than for the successive ratios (i.e. d.sub.203-103:d.sub.203-104, d.sub.204-104:d.sub.204-105, and so on). The longer passages 34.sub.201-101 and 34.sub.202-102 relative to passages 34.sub.201-102 and 34.sub.202-103 leads to a weaker coupling of the RF field to the first cavity cell 101 and the second cavity cell 102, thereby reducing the RF field amplitude in the first and second cavity cells 101-102.

[0061] The geometry of the cavity cells 101-106, the side cells 201-205 and the passages 34 results in a relatively high field amplitude in the cavity cells 103-106 of the acceleration section 120, and a relatively low field amplitude in the first and second cavity cells 101-102 of the capture section.

[0062] The relatively low field amplitude in the first and second cavity cells 101 and 102 ensures that electrons travel relatively slowly through the capture section 110. The relatively high field amplitude in the cavity cells 103-106 of the acceleration section 120 sees the electrons accelerate rapidly. Consequently, the lengths (dimension in the same direction as the longitudinal axis) of the cavity cells 103-106 in the acceleration section 120 is longer than the length of the cavity cell 101-102 in the capture section 110. The length of the second cavity cell 102 is longer than the first cell 101 due to the small acceleration of electrons as they pass through the first cell 101.

[0063] The electron source 12 is operated to deliver a 25 keV DC electron beam to the entrance aperture 20 of the first cell 101. The cavity 16 is operated using an S-band radio frequency field to create a /2-mode standing wave bi-periodic side-coupled accelerator that is only 30 cm long, but that can accelerate the electrons to 6-8 MeV.

[0064] A high capture efficiency is achieved by the first and second cavity cells 101-102 having a lower field amplitude which allows most of the electrons to be captured and formed into bunches. However, using the lower RF field amplitude in all cavity cells 101-106 would make the linac 10 undesirably long as many more cavity cells 101-106 would be required. To avoid this, a step in the RF field amplitude is imposed after the first and second cavity cells 101-102 of the capture section 110. It is not straightforward to have a cavity 16 in which a low field amplitude is achieved in the capture section 110, while having the higher RF field amplitude in the cavity cells 103-106 of the acceleration section 120. As explained above, this is achieved though varying the lengths of the passages 34 to alter the RF field coupling in the first and second cells 101-102.

[0065] In order to fine-tune the RF field amplitudes achieved in the first and second cells 101-102, the first and second cells 101-102 are detuned such that the adjacent side cell 201 has finite field amplitudes. Detuning the first and second cells 101-102 sees the diameter of the first and second cells 101-102 adjusted from the values calculated to create a resonant RF field. Achieving a resonant RF field in a cavity cell 101-106 requires calculating a number of parameters that include the length and diameter of the cell. The calculated diameter may then be reduced to detune the first and second cells 101-102 out of resonance. The more detuned the first and second cavity cells 101-102, the lower the field amplitude in the first and second cavity cells 101-102 and the higher the field amplitude in the adjacent side cell 201. When the first and second cavity cells 101-102 are detuned as described above, the first side cell 201 and its passages 34 control the amplitude of the RF field in the first and second cavity cells 101-102, but the ratio of the amplitudes in the two cavity cells 101-102 stays constant hence the optimal configuration has both the first and second cavity cells 101-102 detuned. This effect may be used either in combination with varying the lengths of the passages 34, as described above, or as an alternative to varying the lengths of the passages 34.

[0066] The lower RF field amplitude in the first and second cavity cells 101-102, and the shorter lengths of the first and second cavity cells 101-102 are expected because the main function of the capture section 110 is capturing and bunching the electrons from the electron source 12, not accelerating these electrons. The RF field in the first cavity cell 101 gives little acceleration to early electrons and more acceleration to later electrons (relative to cycles of the RF field), thereby producing bunching of the electrons. However, the difference in acceleration across all the electrons cannot be too big or too small, else the bunching will be too large or too small.

[0067] The linac 10 of the Figures has been designed such that the early electrons reach 20% of the speed of light at the exit of the first cavity cell 101, while the later electrons reach 40% of the speed of light. This ensures that the later electrons catch up with the early electrons at the centre of the second cavity cell 102. Hence, bunching continues in the second cavity cell 102, as too does acceleration of the electrons. The electrons reach around 90% of the speed of light at the exit of the second cavity cell 102. As described above, the lower average speed of the electrons through the entire length of the second cavity cell 102 means that the length of the second cavity cell 102 is less than that of the cavity cells 103-106 of the acceleration section 120.

[0068] The design of the cavity cells 101-106, the side cells 201-205, the passages 99 and the resulting RF field is important for the optimal performance of the linac 10. The lower field amplitude in the first and second cavity cells 101-102 avoids back-streaming electrons and also sees far more electrons caught by the next RF cycle and so re-accelerated along the channel 18 to the second cavity cell 102. Also, the lower field amplitude in the first and second cavity cells 101-102 accelerates the electrons to travel with sub-relativistic speeds for longer, which allows them to be bunched.

[0069] However, if the field amplitude in the first cavity cell 101 is too low, two effects will reduce the capture efficiency. The first effect is space charge blow-up of the electron beam, and the second effect is under-bunching of the electrons due to insufficient velocity difference between the early and late electrons. Conversely, if the first cavity cell 101 is too long, capture efficiency will be reduced by over-bunching of the electrons. Thus, the RF field amplitudes and the lengths of first and second cavity cells 101-102 need to be scanned and optimized, which may be performed as follows.

[0070] For example, a 1D longitudinal tracking code may be used to determine a required RF field profile. Such code can simulate launching electrons at different phases, track them through the cavity 16 and record the electrons' arrival phase and kinetic energies to evaluate capture efficiency. The cavity geometry is then determined considering passage lengths, and re-entrant sections, using a separate electromagnetic code to achieve the required RF field profile.

[0071] Such 1D tracking codes may be used to optimize the lengths of the cavity cells 101-106 and the resulting RF field in the cavity cells 101-106 by assessing the arrival phases and kinetic energies of electrons as a function of the launch (emission) phase of the electrons. Inputs to the code include profile of the RF electric field, the RF field frequency, and the electron's charge, mass and initial kinetic energy. The code tracks electrons from the beginning of the RF field profile until they reach either end of the field profile. The code outputs electron phase and energy at certain positions as a function of the launch phase or each electron. This allows identification to which electrons are lost, which electrons are successfully captured and how well the electrons are bunched. Any particles that are found to travel backward and pass beyond the initial start point are marked as lost, and indicate that the design is not optimal. The code may neglect space charge effects and may only be used for initial parameter scans, but the speed and advanced methods of cell length optimization provide approximate global optimum values that may be further optimised.

[0072] The optimum length and field amplitude of each cavity cell 101-106 depends on the initial velocity particle and purpose of the cavity cell 101-106, i.e. acceleration and/or capture and bunching. The main function of the first and second cavity cells 101-102 of the capture section 110 is the capture and bunching of electrons, while giving the electrons sufficient acceleration to prevent beam blow-up due to space charge. The third and subsequent cavity cells 103-106 of the acceleration section 120 are used primarily for acceleration. The electrons are provided by the electron source 12 with 25 keV kinetic energy, which means they are not relativistic and so space charge can dominate. As the electrons are accelerated through the cavity 16, the acceleration to relativistic energies means that the cavity cell lengths need to be increased accordingly. All these may be taken into account by the 1D tracking code by varying the cavity cell lengths.

[0073] The 1D tracking optimized parameters may be used to perform further optimizations by using more precise simulation algorithms to include space charge and transverse dimensions. An example is the ASTRA algorithm (see http://www.desy.de/.sup.mpyflo/). Several rounds of beam dynamics optimization and RF cavity modelling may be required to obtain a cavity design with high capture efficiency.

[0074] The applicant has found that, using the above method, it is possible to design an S-band linac 10, for example or use as a medical linac, with a high capture efficiency of over 90%, of which 88% particles are provided in the 6.1-8.7 MeV range. Compared to traditional medical linacs, the number of back-streaming electrons is reduced from 50% to 6.5%, which improves the electron source lifetime and the electron beam quality. The linac 10 requires less RF power, and therefore lowers the accelerator acquisition cost and the ongoing running costs.

[0075] A person skilled in the art will appreciate that the above embodiments may be varied in many different respects without departing from the scope of the present invention that is defined by the appended claims.

[0076] For example, the linac 10 has wide applicability beyond medical linacs. Any linacs that utilize a thermionic gun as an electron source can benefit from the improved capture efficiency and beam power of the present invention.

[0077] Also, the number of cavity cells 101-106 may be varied, as too may the number of side cells 201-205. The capture section 110 may comprise two (or more) cavity cells 101-106 and/or the intermediate section may comprise two (or more) cavity cells 101-106. The number of cavity cells 101-106 in the acceleration section 120 may also be varied.