Abstract:
A method for packetizing a beam-charged particle, in which the particles pass through an electric field in a device is provided. The device includes an annular shaped central electrode which, in the direction of the beam, is arranged between a first outer electrode and a second outer electrode. A time-dependent electric voltage signal is applied to the central electrode, the temporal course thereof being selected such that particles inside the device undergo a position-dependent speed modification. The course of the speed modification is approximately sawtooth-shaped in the direction of the beam. An associated device is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2012/060273 having a filing date of May 31, 2012, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The following relates to a method for bunching a beam of charged particles, a device for bunching a beam of charged particles and an instrument for performing particle therapy. 
       BACKGROUND 
       [0003]    Accelerated charged particles, for example electrons and protons, are used for a multitude of technical, scientific and medical purposes. The generation of such particles using particle sources and the acceleration thereof using particle accelerators is known. 
         [0004]    Particle sources often generate continuous beams of charged particles. Some particle accelerators, for example RF linear accelerators, are not suitable for accelerating continuous particle beams. Therefore, it is necessary for the particle beams to be bunched by means of a bunching device (buncher), i.e. to subdivide these beams into discrete particle bunches. 
         [0005]    The prior art, for example of U.S. Pat. No. 5,719,478, has disclosed various bunching devices for bunching continuous particle beams. However, these known devices are disadvantageous in that they result in non-ideal bunching in the case of small beam currents, in which a space charge distribution does not influence the bunching process. 
       SUMMARY 
       [0006]    An aspect relates to an improved method for bunching a beam of charged particles. A further aspect relates to providing an improved device for bunching a beam of charged particles. A further aspect includes providing an instrument for performing particle therapy. 
         [0007]    In a method according to embodiments the invention for bunching a beam of charged particles, the particles pass through an electric field in a device. Here, the device comprises a ring-shaped central electrode which is disposed between a first outer electrode and a second outer electrode in a beam direction. A time-dependent electric voltage signal is applied to the central electrode, the electric profile of which electric voltage signal is selected in such a way that particles situated within the device experience a position-dependent change in velocity, the profile of the change in velocity being approximately sawtooth in the beam direction. Advantageously, a sawtooth change in velocity of the particles in the beam direction leads to very high quality bunching with good bunch properties, both in the case of partial and complete bunching. 
         [0008]    In a preferred embodiment of the method, the electric voltage signal has an approximately triangular time profile. Advantageously, this constitutes a suitable option of obtaining a change in velocity with an approximately sawtooth profile in the beam direction. 
         [0009]    In a development of the method, a first gap is formed between the first outer electrode and the central electrode and a second gap is formed between the central electrode and the second outer electrode. Here, the centers of the first gap and of the second gap have a fixed gap distance from one another. The electric voltage signal has a set excitation frequency. The particles have a set velocity prior to passing through the device. In the process, a bunch distance emerges as a quotient of the speed and the excitation frequency. The excitation frequency is selected in such a way that at least the three lowest Fourier components of the position-dependent change in velocity differ from zero. Advantageously, what then emerges from this is an expedient approximation of the profile of the change in velocity in the beam direction to a sawtooth form. 
         [0010]    In one embodiment of the method, the excitation frequency is selected in such a way that the bunch distance is four times the size of the gap distance. Advantageously, then at least the three lowest Fourier components differ from zero. 
         [0011]    In one embodiment of the method, the particles have a nonrelativistic velocity. 
         [0012]    In one embodiment of the method, the outer electrodes are grounded. Advantageously, what emerges from this is a potential difference between the outer electrodes and the central electrode. 
         [0013]    A device according to embodiments of the invention for bunching a beam of charged particles comprises a ring-shaped central electrode which is disposed between a first outer electrode and a second outer electrode in a beam direction. Here, a first gap is formed between the first outer electrode and the central electrode and a second gap is formed between the central electrode and the second outer electrode. Here, the centers of the first gap and of the second gap have a fixed gap distance from one another. The device is moreover embodied to be operated according to a method of the type mentioned above. Advantageously, the device is then suitable for subdividing a particle beam into bunches with excellent bunch properties. 
         [0014]    An instrument according to embodiments of the invention for performing particle therapy comprises a device of the type set forth above. Advantageously, the particle therapy can then be performed with bunches of charged particles. 
     
    
     
       BRIEF DESCRIPTION 
         [0015]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0016]      FIG. 1  shows a schematic block diagram of an embodiment of a particle therapy instrument; 
           [0017]      FIG. 2  shows a schematic of an embodiment a bunching scheme; 
           [0018]      FIG. 3  shows a schematic illustration of an embodiment of a bunching device; 
           [0019]      FIG. 4  shows a schematic illustration of an axial field distribution within the bunching device; 
           [0020]      FIG. 5  shows a schematic illustration of an ideal field distribution; 
           [0021]      FIG. 6  shows a schematic illustration of a real field distribution; 
           [0022]      FIG. 7  shows a first Fourier decomposition; 
           [0023]      FIG. 8  shows a second Fourier decomposition; and 
           [0024]      FIG. 9  shows an optimized field distribution. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  shows a schematic block diagram of a particle therapy instrument  100 . The particle therapy instrument  100  serves as an example of an instrument in which a bunching device can be used. However, bunching devices according to embodiments of the invention can also be used in a multiplicity of further instruments. 
         [0026]    The particle therapy instrument  100  can be used for performing particle therapy on a patient. During particle therapy, a diseased body location of the patient is irradiated with charged particles. By way of example, the charged particles can be protons. 
         [0027]    The particle therapy instrument  100  comprises an ion source  110 , which emits a particle beam  115  of charged particles in a beam direction  101 . By way of example, the ion source  110  can be a proton source. By way of example, the ion source  110  can generate particles with an energy of between 10 keV and 20 keV. The particles leave the ion source  110  in the beam direction  101  as a continuous particle beam  115 . 
         [0028]    Following the ion source  110  in the beam direction  101 , the particle therapy instrument  100  comprises a bunching device  120 . The bunching device  120  is provided for subdividing the continuous particle beam  115  into a sequence of discrete particle bunches  125 . The bunching device  120  can also be referred to as a buncher. The bunching of subdividing of the particle beam  115  into particle bunches  125  can also be referred to as packetizing. The particle bunches  125  leave the bunching device  120  in the unchanging beam direction  101 . 
         [0029]    Following the bunching device  120  in the beam direction  101 , the particle therapy instrument  100  comprises a deflection device  130 . The deflection device  130  can serve for deflecting individual particle bunches  125  in relation to the beam direction  101 . A stop  140  is disposed following the deflection device  130  in the beam direction  101 . Depending on the strength of the deflection of the particle bunches  125  from the beam direction  101  by the deflection device  130 , the particle bunches  125  may pass the stop  140  completely, only partly or not at all. Therefore, the combination of deflection device  130  and stop  140  may serve for selective filtering and/or thinning of individual particle bunches  125 . 
         [0030]    Following the stop  140  in the beam direction  101 , the particle therapy instrument  100  comprises a particle accelerator  150 . By way of example, the particle accelerator  150  can be a linear accelerator, preferably an RF linear accelerator. The particle accelerator  150  serves to accelerate the particle bunches  125  to a higher kinetic energy of e.g. 80 MeV to 250 MeV. 
         [0031]      FIG. 2  shows a simplified illustration of a bunching scheme  200  in order to explain the bunching performed by the bunching device  120 . 
         [0032]    The continuous particle beam  115  runs into the bunching device  120  in the beam direction  101 . By means of the bunching device  120 , the particle beam  115  is subdivided into particle bunches  125 , the centers of which have a bunch distance  210  in the beam direction  101 . Here, the bunch distance  210  need not correspond to the length of the bunching device  120  in the beam direction  101 . 
         [0033]    The bunching is brought about by means of electric fields active within the bunching device  120 , which electric fields influence the velocity of the particles of the particle beam  115  in the beam direction  101 . Leading particles of each particle bunch  125  are decelerated in such a way that they obtain a reduced relative velocity  230 . Late particles of each particle bunch  125  are accelerated such that they obtain an increased relative velocity  220 . The reduction or increase in the relative velocity  220 ,  230  of a particle increases with distance thereof from the center of the particle bunch  125  thereof. 
         [0034]    During the further movement of the particles in the beam direction  101 , the particles toward the back of each particle bunch  125  in the beam direction  101  increasingly catch up with the leading particles of the respective particle bunch  125  as a result of their increased relative velocity  220 . The leading particles of each particle bunch  125  are caught by the remaining particles of the particle bunch  125  during the further movement of the particles in the beam direction  101  due to their reduced relative velocity  230 . The degree of bunching of the particle bunch  125  therefore increases in the beam direction  101  until maximum bunching is achieved at a point in the beam direction  101 . From then on, the particle bunch  125  runs apart again during the further movement of the particles in the beam direction  101 . In the particle therapy instrument  100 , the point of maximal bunching of the particle bunches  125  can, for example, coincide with the location of the stop  140  or with the inlet of the particle accelerator  150 . 
         [0035]      FIG. 3  shows a schematic illustration of a section through the bunching device  120 . In the beam direction  101 , the bunching device comprises a first outer electrode  310 , a central electrode  330  and a second outer electrode  320  in succession. The electrodes  310 ,  320 ,  330  in each case have a hollow cylindrical or tubular design. In the beam direction  101 , the central electrode  330  is shorter than the outer electrodes  310 ,  320 . Therefore, the central electrode  330  can also be referred to as ring-shaped. The particle beam  115  extends in the interior along the longitudinal axis of the tubular electrodes  310 ,  320 ,  330 . 
         [0036]    A first gap  315  is formed between the first outer electrode  310  and the central electrode  330 . A second gap  325  is formed between the central electrode  330  and the second outer electrode  320 . The gaps  315 ,  325  insulate the electrodes  310 ,  330 ,  320  from another electrically. 
         [0037]    In the beam direction  101 , the centers of the gaps  315 ,  325  have a gap distance  340  from one another. A center of the central electrode  330  in the beam direction  101  forms a center  335  of the bunching device  120 . 
         [0038]    During the operation of the bunching device  120 , a time-dependent electric voltage is applied between the central electrode  330  and the outer electrodes  310 ,  320 . Here, the outer electrodes  310 ,  320  preferably are at a common potential. By way of example, the outer electrodes  310 ,  320  may be grounded. A potential difference between the central electrode  330  and the outer electrodes  310 ,  320  causes the formation of an electric field, the equipotential lines  350  of which are depicted schematically in  FIG. 3 . 
         [0039]    The field distribution in the beam direction  101  can approximately be described by Gaussian functions along the central axis (longitudinal axis) of the electrodes  310 ,  320 ,  330  of the bunching device  120 . This is depicted schematically in the axial field distribution  400  in  FIG. 4 . The beam direction  101  in the region around the center  335  of the bunching device  120  is plotted on the horizontal axis of the graph in  FIG. 4 . The vertical axis of the graph in  FIG. 4  shows the electric field strength  401  in the beam direction  101 . A Gaussian approximation  410  approximates the profile of the electric field strength in the beam direction  101 . The field distribution profile is Gaussian at each gap  315 ,  325 . Therefore, the two Gaussian functions have the gap distance  340  from one another. 
         [0040]    If a time-dependent electric voltage is applied to the central electrode  330  of the bunching device  120 , the field distribution E(z) in the beam direction  101  (z), schematically depicted in  FIG. 4 , is modulated by the time-dependent electric field S(t) caused by the voltage applied to the central electrode  330 . The instantaneous field E z  in the beam direction  101  therefore emerges as a product of the axial field component E(z) and of the time-dependent field S(t): 
         [0000]        E   z ( z,t )= E ( z ) S ( t ). 
         [0041]    A particle of the particle beam  115  entering the bunching device  120  in the beam direction  101  experiences a force in the beam direction  101  that is proportional to the instantaneous field E z , and to its charge q. This results in a change in velocity 
         [0000]    
       
         
           
             
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         [0042]    which is proportional to a convolution of the axial field distribution E(z) and S(t). Here, the z-position in the beam direction  101 , the velocity v of the particles of the particle beam  115  and the time t are linked by the bunch position w=z−vt. Here, m denotes the mass of the particle. 
         [0043]    It would be most expedient if the convolution, and hence the change in velocity of the particles of the particle beam  115 , were sawtooth-shaped in the beam direction  101 . This would then result in a velocity variation which increases the further a particle is distanced from the center of the respective particle bunch  125 .  FIG. 5  shows a schematic graph of a change in velocity of the particles of the particle beam  115  emerging in the case of an appropriate field distribution  500 . The bunch position w along the beam direction  101  is plotted on the horizontal axis. A relative change in velocity of the particles of the particle beam  115  is plotted on the vertical axis  501 . An approximate sawtooth function  510  describes an approximately ideal relative change in velocity, which the particles of the particle beam  115  experience in order to obtain bunching with ideal bunching properties. 
         [0044]    However, in practice the sawtooth function in  FIG. 5  can only be achieved with difficulties.  FIG. 6  shows a schematic graph of conditions prevailing in a real field distribution  600 . The z position of the beam direction  101  and the bunch position w in the beam direction  101  and the path  601  traveled by the particles of the particle beam  115  in the time vt along the beam direction  101  are plotted on a horizontal axis of the graph depicted in  FIG. 6 . What is shown is the Gaussian approximation  410  of the axial field profile E(z). Moreover, the time profile of a voltage signal  610  applied to the central electrode  330  of the bunching device  120  is depicted. The voltage signal  610  has a triangular time profile. Moreover,  FIG. 6  shows the resulting change in velocity  620  of the particles of the particle beam  115 . It is possible to identify that a change in velocity  620  with a sinusoidal profile emerges despite the triangular time profile of the voltage signal  610 . Therefore, the change in velocity  620  does not have an approximately sawtooth-shaped profile. 
         [0045]    This can be explained by considering the Fourier coefficients: 
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         [0046]    Here, t2 is the gap distance  340 , t1 is the width of the Gauss pulses of the Gaussian approximation  410 , n is the order of the Fourier coefficients and λ is the bunch distance  210  emerging as the quotient of the particle speed v and the excitation frequency f of the electric voltage signal S(t). 
         [0047]      FIG. 7  shows the first five Fourier coefficients as a function of the bunch distance  210  in an exemplary fixed gap distance  340  of t2=4.6 in a first Fourier decomposition  700 . Plotted on the horizontal axis of the graph depicted in  FIG. 7  is the bunch distance  210  (λ). The amplitude of the respective Fourier coefficients is depicted on a vertical axis  701 . The shown curves specify the profile of the first Fourier coefficient  710 , of the second Fourier coefficient  720 , of the third Fourier coefficient  730 , of the fourth Fourier coefficient  740  and of the fifth Fourier coefficient  750 . 
         [0048]    In  FIG. 7 , a first bunch distance  760  of λ=9.2=2 t2 is marked. These are the parameters used in the illustration in  FIG. 6 . It can be seen that all even Fourier coefficients  720 ,  740 , i.e. all harmonics, are filtered out in the first bunch distance  760 . This is the reason for the sinusoidal profile of the change in velocity  620  in  FIG. 6 . 
         [0049]      FIG. 8  shows a further Fourier decomposition  800 . This time, the gap distance  340  (t2) is plotted on the horizontal axis. The bunch distance  210  is λ&lt;9.2. A vertical axis  801  shows the amplitudes of the Fourier coefficients. Curves depict the profile of the first Fourier coefficient  810 , of the second Fourier coefficient  820 , of the third Fourier coefficient  830 , of the fourth Fourier coefficient  840  and of the fifth Fourier coefficient  850 . Moreover, a first gap distance  860  of t2=2.3 and the second gap distance  870  of t2=4.6=½λ, as used in  FIG. 7 , are marked. While the second Fourier coefficient  820  and the fourth Fourier coefficient  840  are filtered out in the second gap distance  870 , as was already explained above on the basis of  FIG. 7 , the first three Fourier coefficients  810 ,  820 ,  830  have amplitudes that differ from zero in the case of the reduced first gap distance  860  of t2=2.3=¼λ. Thus, if the bunch distance  210  is selected to be four times greater than the gap distance  340 , at least the first three Fourier coefficients  810 ,  820 ,  830  have amplitudes that differ from zero. 
         [0050]      FIG. 9  shows the emerging relative change in velocity of the particles of the particle beam  115  in a graph of an optimized field distribution  900 . The beam direction  101  is plotted on the horizontal axis. The emerging relative change in velocity of the particles of the particle beam  115  is depicted on the vertical axis  901 . A first approximation  910  of a sawtooth function emerges if the gap distance  340  and the bunch distance  210 , as described above, are selected in such a way that at least the first three Fourier coefficients have amplitudes that differ from zero. If the amplitudes of the individual Fourier coefficients are additionally optimized, this results in a second approximation  920 , which is even more similar to a sawtooth function. 
         [0051]    Although the invention has been described and depicted in greater detail by means of the preferred exemplary embodiment, the invention is not restricted by the disclosed examples. Other variations may be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.