Patent ID: 12245357

DETAILED DESCRIPTION OF THE INVENTION

The Figures show a linac10that comprises an electron source section, including an electron source12, and a cavity section14. The electron source12may be any type of conventional electron source12, and so will not be described in further detail.

The cavity section14includes a cavity16comprising a series of cavity cells101-106joined by a channel18that passes through the centre of each cavity cell101-106. The channel18extends from one side15of cavity section14, through an entrance aperture20of a first cell101, through cells101-106, through an exit aperture22of the final cell106, and to the other side17of the cavity section14. The longitudinal axis of the linac10extends from the centre of the entrance aperture20to the centre of the exit aperture22. Electrons generated by the electron source12enter the cavity section14at the channel18on side15, and exit the cavity section14from the channel18at side17. The cavity section14also comprises a series of side cells201-205. Each side cell201-205is 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 passage34extends between each cavity cell101-106and each overlapping side cell (201-205) and so couple those cells (101&102,102&103,103&104,104&105,105&106). The side cells201-205alternate from side to side, for example alternating between being positioned above and below the cavity cells101-106as shown inFIG.1.

As is well understood in the art, the side cells201-205are RF injection chambers that are used to couple a RF field with the cavity cells101-106via the passages34. Namely, the first side cell201couples the RF field with the first cavity cell101and the second cavity cell102via passages34201-101and34201-102, the second side cell202couples the RF field with the second cavity cell102and the third cavity cell103via passages34202-102and34202-103, and so on. The side cells201-205are coupled to RF field generators that are conventional and so are not shown in the Figures, and not described further.

The cavity cells101-106are not of a uniform size. The third to sixth cavity cells103-106are of a common size and shape, and form an acceleration section120. The first cavity cell101and the second cavity cell102have different sizes, relative to each other and also relative to the cavity cells103-106of the acceleration section120. The first cavity cell101and the second cell102form a capture section110. The second cavity cell102has smaller width (dimension transverse to the longitudinal axis of the linac10) than the cavity cells103-106of the acceleration section120. The first cavity cell101is narrower than the second cavity cell102, which is narrower than the cavity cells103-106of the acceleration section120.

The bore of the channel18is relatively small in its first part from the side15of the cavity section14to the entrance aperture20. This stops the RF field leaking from the first cell101. The bore of the channel18is then relatively large for each of the parts that link the subsequent cavity cells102-106and for the part extending from the exit aperture22to the side17of the cavity section14. This can be seen most clearly in the detail ofFIG.2.

Each cavity101-106cell is approximately cylindrical in shape, and comprises a front wall30101-30106and a back wall32101-32106. Each of the front walls30101-30106have a central re-entrant part. The back walls32102-32106of the cavity cells102-106of the acceleration section120also have a central re-entrant part. However, the back wall32101of the first cavity cell101does not have a central re-entrant part and is flat instead, as can be seen most clearly from the detail ofFIG.2. This shortens the electron path length between the first cavity cell101and the second cavity cell102.

The side cells201-205also have a generally cylindrical shape with a cylindrical central section flanked by annular sections. The side cells201-205have the same size and shape, and are offset the same distance from the longitudinal axis with the exception of the first side cell201which is positioned closer to the longitudinal axis. Each side cell201-205overlaps with two cavity cells101-106, and is coupled to each adjacent cavity cell101-106by respective passages34. The smaller widths of the first and second cavity cells101-102means that the lengths d201-101, d201-102and d202-102of the passages34201-101and34202-102coupling to the first cavity cell101and the second cavity cell102are longer than the passages34for the other cavity cells103-106(as best seen inFIG.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 d201-101:d201-102and d202-102:d202-103of the passages34201-101/34201-102and34202-102/34202-103is larger than for the successive ratios (i.e. d203-103:d203-104, d204-104:d204-105, and so on). The longer passages34201-101and34202-102relative to passages34201-102and34202-103leads to a weaker coupling of the RF field to the first cavity cell101and the second cavity cell102, thereby reducing the RF field amplitude in the first and second cavity cells101-102.

The geometry of the cavity cells101-106, the side cells201-205and the passages34results in a relatively high field amplitude in the cavity cells103-106of the acceleration section120, and a relatively low field amplitude in the first and second cavity cells101-102of the capture section.

The relatively low field amplitude in the first and second cavity cells101and102ensures that electrons travel relatively slowly through the capture section110. The relatively high field amplitude in the cavity cells103-106of the acceleration section120sees the electrons accelerate rapidly. Consequently, the lengths (dimension in the same direction as the longitudinal axis) of the cavity cells103-106in the acceleration section120is longer than the length of the cavity cell101-102in the capture section110. The length of the second cavity cell102is longer than the first cell101due to the small acceleration of electrons as they pass through the first cell101.

The electron source12is operated to deliver a 25 keV DC electron beam to the entrance aperture20of the first cell101. The cavity16is operated using an S-band radio frequency field to create a π/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.

A high capture efficiency is achieved by the first and second cavity cells101-102having 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 cells101-106would make the linac10undesirably long as many more cavity cells101-106would be required. To avoid this, a step in the RF field amplitude is imposed after the first and second cavity cells101-102of the capture section110. It is not straightforward to have a cavity16in which a low field amplitude is achieved in the capture section110, while having the higher RF field amplitude in the cavity cells103-106of the acceleration section120. As explained above, this is achieved though varying the lengths of the passages34to alter the RF field coupling in the first and second cells101-102.

In order to fine-tune the RF field amplitudes achieved in the first and second cells101-102, the first and second cells101-102are detuned such that the adjacent side cell201has finite field amplitudes. Detuning the first and second cells101-102sees the diameter of the first and second cells101-102adjusted from the values calculated to create a resonant RF field. Achieving a resonant RF field in a cavity cell101-106requires 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 cells101-102out of resonance. The more detuned the first and second cavity cells101-102, the lower the field amplitude in the first and second cavity cells101-102and the higher the field amplitude in the adjacent side cell201. When the first and second cavity cells101-102are detuned as described above, the first side cell201and its passages34control the amplitude of the RF field in the first and second cavity cells101-102, but the ratio of the amplitudes in the two cavity cells101-102stays constant hence the optimal configuration has both the first and second cavity cells101-102detuned. This effect may be used either in combination with varying the lengths of the passages34, as described above, or as an alternative to varying the lengths of the passages34.

The lower RF field amplitude in the first and second cavity cells101-102, and the shorter lengths of the first and second cavity cells101-102are expected because the main function of the capture section110is capturing and bunching the electrons from the electron source12, not accelerating these electrons. The RF field in the first cavity cell101gives 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.

The linac10of the Figures has been designed such that the early electrons reach 20% of the speed of light at the exit of the first cavity cell101, 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 cell102. Hence, bunching continues in the second cavity cell102, as too does acceleration of the electrons. The electrons reach around 90% of the speed of light at the exit of the second cavity cell102. As described above, the lower average speed of the electrons through the entire length of the second cavity cell102means that the length of the second cavity cell102is less than that of the cavity cells103-106of the acceleration section120.

The design of the cavity cells101-106, the side cells201-205, the passages34and the resulting RF field is important for the optimal performance of the linac10. The lower field amplitude in the first and second cavity cells101-102avoids back-streaming electrons and also sees far more electrons caught by the next RF cycle and so re-accelerated along the channel18to the second cavity cell102. Also, the lower field amplitude in the first and second cavity cells101-102accelerates the electrons to travel with sub-relativistic speeds for longer, which allows them to be bunched.

However, if the field amplitude in the first cavity cell101is 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 cell101is 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 cells101-102need to be scanned and optimized, which may be performed as follows.

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 cavity16and 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.

Such 1D tracking codes may be used to optimize the lengths of the cavity cells101-106and the resulting RF field in the cavity cells101-106by 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.

The optimum length and field amplitude of each cavity cell101-106depends on the initial velocity particle and purpose of the cavity cell101-106, i.e. acceleration and/or capture and bunching. The main function of the first and second cavity cells101-102of the capture section110is 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 cells103-106of the acceleration section120are used primarily for acceleration. The electrons are provided by the electron source12with 25 keV kinetic energy, which means they are not relativistic and so space charge can dominate. As the electrons are accelerated through the cavity16, 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.

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/˜mpyflo/). Several rounds of beam dynamics optimization and RF cavity modelling may be required to obtain a cavity design with high capture efficiency.

The applicant has found that, using the above method, it is possible to design an S-band linac10, 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 linac10requires less RF power, and therefore lowers the accelerator acquisition cost and the ongoing running costs.

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.

For example, the linac10has 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.

Also, the number of cavity cells101-106may be varied, as too may the number of side cells201-205. The capture section110may comprise two (or more) cavity cells101-106and/or the intermediate section may comprise two (or more) cavity cells101-106. The number of cavity cells101-106in the acceleration section120may also be varied.