Patent Application
20130033201
The present invention relates to a charged particle accelerator that accelerates charged particles and a method for accelerating charged particles. More specifically, the present invention relates to a linear trajectory accelerator and a spiral trajectory accelerator that generate accelerating electric fields using a combination of a high-voltage pulse generation device and a controller, and to a method for accelerating charged particles using these charged particle accelerators.
In the cyclotron, a period Tp of revolution of the charged particle 74 satisfies the relationship Tp=2πm/eB, where n denotes the ratio of the circle's circumference to its diameter, m denotes the mass of the charged particle 74, e denotes the electric charge of the charged particle 74, and B denotes the magnetic flux density on a particle trajectory attributed to the magnet 70. Therefore, provided that m/eB is constant, the period of revolution of the charged particle 74 is constant regardless of the radius of revolution. For example, when a period Trf of the accelerating radio frequency of the radio-frequency power supply 73 satisfies the relationship Trf=Tp/2, the charged particle 74 is constantly accelerated in an electrode gap between the accelerating electrodes 71 and 72, and therefore can be accelerated to a high energy.
When the speed of the charged particle 74 approaches the speed of light, the value of the mass m of the charged particle 74 increases due to relativistic effects. As a result, in the cyclotron shown in
The above conventional charged particle accelerator with the spiral trajectory is problematic in that the energy gain cannot be increased due to the loss of the isochronous properties in a relativistic energy range, and it requires a function of changing the accelerating radio-frequency voltage or magnetic field distribution to correct the loss of the isochronous properties, which results in an increase in the number of device components and the cost.
The present invention has been conceived to solve the aforementioned problem with conventional configurations, and its main object is to provide a charged particle accelerator and a method for accelerating charged particles that are less expensive and yield a higher energy gain than the conventional ones.
In order to solve the above problem, one aspect of the present invention is a charged particle accelerator including: a charged particle generation source for emitting a charged particle; an accelerating electrode tube through which the charged particle emitted from the charged particle generation source passes and which is for accelerating the charged particle that passes; a drive circuit for applying voltage for accelerating the charged particle to the accelerating electrode tube; and a control unit for controlling the drive circuit so that application of the voltage to the accelerating electrode tube is started while the charged particle is traveling through the accelerating electrode tube.
With respect to the above aspect, it is preferable that the accelerating electrode tube be provided in plurality, the plurality of accelerating electrode tubes be arranged in a linear fashion, the charged particle emitted from the charged particle generation source pass through the plurality of accelerating electrode tubes in sequence, and the control unit control the drive circuit to start applying the voltage to any accelerating electrode tube through which the charged particle is traveling, thus applying the voltage to the plurality of accelerating electrode tubes in sequence.
Furthermore, with respect to the above aspect, it is preferable that the charged particle accelerator further include a bending magnet for changing a traveling direction of the charged particle that has passed through the accelerating electrode tube.
Furthermore, with respect to the above aspect, it is preferable that the bending magnet change the traveling direction of the charged particle that has passed through the accelerating electrode tube so as to cause the charged particle to pass through the same accelerating electrode tube again, and the control unit control the drive circuit to start applying the voltage to the accelerating electrode tube while the charged particle is traveling through the accelerating electrode tube, thus applying the voltage to the same accelerating electrode tube multiple times.
Furthermore, with respect to the above aspect, it is preferable that the charged particle accelerator further include an adjustment unit for adjusting the traveling direction of the charged particle to a direction that intersects the traveling direction.
Furthermore, with respect to the above aspect, it is preferable that the charged particle accelerator further include an ammeter for measuring an accelerating current that is generated in an accelerating electrode tube when the charged particle passes through the accelerating electrode tube, and the control unit adjust a timing to start applying voltage to an accelerating electrode tube based on a result of measurement of the accelerating current by the ammeter.
Furthermore, with respect to the above aspect, it is preferable that the drive circuit be capable of changing a value of voltage applied to an accelerating electrode tube.
Furthermore, with respect to the above aspect, it is preferable that the charged particle accelerator further include a detection unit for detecting whether or not the charged particle accelerated by an accelerating electrode tube is traveling along a predetermined trajectory, and the control unit stop the drive circuit when the detection unit has detected that the charged particle is not traveling along the predetermined trajectory.
Another aspect of the present invention is a method for accelerating a charged particle, including: a step of emitting the charged particle from a charged particle generation source so as to cause the charged particle to pass through a plurality of accelerating electrode tubes in sequence; and a step of starting to apply voltage for accelerating the charged particle to any accelerating electrode tube through which the charged particle is traveling, thus applying the voltage to the plurality of accelerating electrode tubes in sequence.
A charged particle accelerator and a method for accelerating charged particles pertaining to the present invention are less expensive and yield a higher energy gain than the conventional ones.
A description is now given of embodiments of the present invention with reference to the drawings and tables.
The following describes operations of the linear-trajectory charged particle accelerator configured in the above manner. Note that the following description provides a representative example in which a hexavalent carbon ion is accelerated. The 20-kV direct current power supply 3 constantly applies a voltage of 20 kV to the ion source 1. When the controller 8 outputs “1”, the switching circuits S#1 to S#28 connect the O terminals and the I terminals and output the same voltage as the voltage applied to the I terminals from the O terminals. On the other hand, when the controller 8 outputs “0”, the outputs from the O terminals are at ground potential. In an initial state prior to the acceleration, the controller 8 outputs “1” only to the switching circuit S#1 and outputs “0” to the remaining switching circuits S#1 to S#28. In other words, in the initial state, only the accelerating electrode tube LA#1 has an electric potential of 20 kV, and the remaining accelerating electrode tubes LA#2 to LA#28 are all at ground potential. Therefore, in the initial state, the charged particle 2 is not extracted because the ion source 1 and the accelerating electrode tube LA#1 have the same electric potential.
In order to perform an accelerating operation, the controller 8 first outputs “0” to the switching circuit S#1 for a predetermined time period so as to place the accelerating electrode tube LA#1 at ground potential. When the accelerating electrode tube LA#1 is at ground potential, the charged particle 2 (hexavalent carbon ion) is extracted from the ion source 1. The ion source 1 has been adjusted such that the ion current is 1 mA and the ion beam diameter is 5 mm. For example, if the accelerating electrode tube LA#1 stays at ground potential for 100 nanoseconds, a plused ion beam including about 2.7×108 charged particles 2 (hexavalent carbon ions) will be obtained. In order to produce an ion beam including more charged particles 2 to increase the amount of radiation, it is sufficient to place the accelerating electrode tube LA#1 at ground potential for a time period longer than 100 nanoseconds. Conversely, in order to decrease the amount of radiation per pulsed ion beam, it is sufficient to place the accelerating electrode tube LA#1 at ground potential for a time period shorter than 100 nanoseconds. Therefore, the linear-trajectory charged particle accelerator shown in
The pulsed ion beam is injected into the accelerating electrode tube LA#1 while being accelerated by a difference in electric potential between the ion source 1 and the accelerating electrode tube LA#1. When the leading edge of the pulsed ion beam substantially reaches the center of the accelerating electrode tube LA#1, the controller 8 outputs “1” to the switching circuit S#1, thus switching the electric potential of the accelerating electrode tube LA#1 to 20 kV. When the pulsed ion beam is emitted from the accelerating electrode tube LA#1, it is accelerated for the second time by a difference in electric potential between the accelerating electrode tubes LA#1 and LA#2.
Thereafter, when the leading edge of the pulsed ion beam substantially reaches the center of the accelerating electrode tube LA#2, the controller 8 switches the electric potential of the accelerating electrode tube LA#2 to 20 kV. When the pulsed ion beam is emitted from the accelerating electrode tube LA#2, it is accelerated again, this time by a difference in electric potential between the accelerating electrode tubes LA#2 and LA#3. The controller 8 increases the accelerating energy of the pulsed ion beam, namely the charged particle 2, by repeating the above sequence control for applied voltage with respect to the accelerating electrode tubes LA#2 to LA#28.
The speed of the pulsed ion beam increases each time the pulsed ion beam passes through an accelerating electrode tube. Hence, considering a delay in response of a switching circuit S#n, in order to reliably switch the electric potential when the pulsed ion beam is substantially at the center of an accelerating electrode tube LA#n, it is necessary to increase the lengths of subsequent accelerating electrode tubes. In Embodiment 1 of the present invention, the accelerating electrode tubes have the lengths presented in Table 1. Table 1 also presents reference values of the energy and pulse width of the pulsed ion beam injected into the accelerating electrode tubes. The pulsed ion beam is accelerated by a difference in electric potential between the accelerating electrode tube LA#28 and the dummy electrode tube 7 at the end, thus obtaining an accelerating energy of 2 MeV/u in total. Note that in an application where beam convergence is required, such as the case of acceleration of a large-current pulsed ion beam, quadrupole electrostatic lenses or other beam convergence circuits may be disposed in the accelerating electrode tubes or on an ion beam transport path. Specific optical designs, i.e. the locations and properties of the beam convergence circuits, will be adjusted on a case-by-case basis in accordance with the intensity of the ion beam and a required beam diameter.
When the pulsed ion beam is emitted from one accelerating electrode tube and injected into a subsequent accelerating electrode tube, it is accelerated by a difference in electric potential between the two accelerating electrode tubes. At this time, an accelerating current flows through the 20-kV direct current power supply 3 or the 200-kV direct current power supply 5. The ammeters 4 and 6 measure this accelerating current and notify the controller 8 of the measured accelerating current. Based on the value measured by the ammeters 4 and 6, the controller 8 learns a timing when the pulsed ion beam is accelerated, namely a timing when the pulsed ion beam passes between the two accelerating electrode tubes. The controller 8 calculates the actual accelerating energy of the pulsed ion beam from this timing data, and when there is a large deviation between the calculated value and a scheduled value, it judges that some sort of abnormality has occurred in the device and executes, for example, processing of warning an operator to that effect.
The values of time periods presented in Table 2 have been calculated under the precondition that the direct current power supplies 3 and 5 output a complete rated voltage. If the voltage output from the direct current power supply 3 or 5 is disturbed, e.g. if its voltage value fluctuates due to a sudden change in the primary power supply voltage and the like, then the values of time periods presented in Table 2 need to be corrected depending on the situation. For this reason, the controller 8 executes processing for correcting times to start applying voltage to the accelerating electrode tubes based on values measured by the ammeters 4 and 6.
The following describes processing for correcting a timing to apply voltage to an accelerating electrode tube LA#n (n=2, 3, . . . , 28) in more detail. Assume that an ion beam is in a preceding accelerating electrode tube LA#n−1 and proceeding to the subsequent accelerating electrode tube LA#n at a speed of v_n−1. At this time, the accelerating voltage is applied to LA#n−1. Also assume that when the ion beam passes through a gap between LA#n−1 and LA#n, it is accelerated by a difference in electric potential between the two accelerating electrode tubes, and when it arrives at LA#n, the speed thereof reaches v_n. During the accelerating operation, an accelerating current flows through a direct current power supply. As the gap between the accelerating electrode tubes can be approximated to a uniform electric field, a time period T_ai(n−1) in which the accelerating current flows through LA#n−1 can be obtained by Expression 1.
Here, d denotes the length of the gap between the accelerating electrode tubes, and w_ib denotes the pulse length of the ion beam. As v_n is a known value, the speed v_n of the accelerated ion beam can be obtained from Expression 1 by measuring T_ai(n−1).
In the present embodiment, as a voltage of 20 kV is extracted from the ion source 1, the ion beam is accelerated to 1.39×10˜6 msec when it arrives at LA#1. Furthermore, as a time period for which the ion beam is extracted is 100 ns, the pulse width of the ion beam is 0.139 m. Therefore, v—≈1.39×10˜6 m/sec, w_ib≈v—1×10˜9 ns=0.139 m, and an electrode gap d is 5 cm, that is to say, d=0.05 m. The value of Tai(1) can be obtained by measuring the accelerating current of LA#1, and v—2, namely the speed of the ion beam in LA#2, can be calculated from the relationship of Expression 1. As the value of the length of the accelerating electrode tube LA#2 is known, a timing when the ion beam is at a central portion of LA#2, namely the best timing to output “1” to the switching circuit S#2, can be obtained from the value of v—2.
While the device is performing a rated operation, the ion beam is subjected to 20-kV acceleration in a gap between LA#1 and LA#2, and therefore v—2≈1.96×10˜6 msec. In this case, the best value for t1 shown in
When there is a deviation from a rated value during the accelerating operation due to disturbances, such as fluctuations in the power supply voltage, the value of v—2 calculated from the measured value T_ai(1) deviates from 1.96×10˜6 m/sec. In this case, the controller 8 re-sets t1 based on v—2 calculated from the measured value and continues the timing control using the re-set t1. The controller 8 corrects and optimizes a timing to apply voltage to each accelerating electrode tube using the above recursive procedure.
By measuring an accelerating current flowing through an accelerating electrode tube in the above-described manner, it is possible to control a timing to apply the accelerating voltage to a subsequent accelerating electrode tube more accurately, and to detect occurrence of any device failure when the flow of the accelerating current cannot be confirmed within a predetermined time period. Furthermore, as a timing of travel of an accelerated charged particle can be measured based on an accelerating current flowing through an accelerating electrode tube, it is possible to perform timing control that is resistant to disturbances such as fluctuations in the power supply, and thus to provide a high-quality accelerator.
Although a power supply of a fixed voltage is used as a direct current power supply in
As set forth above, in the present embodiment, when a charged particle extracted from an ion source or an electron source is injected into the first accelerating electrode tube, the controller applies the accelerating voltage to the accelerating electrode tube at a timing when the charged particle has completely entered the accelerating electrode tube. As a subsequent accelerating electrode tube is maintained at ground potential (0 V) at first, the charged particle emitted from the first accelerating electrode tube is accelerated by a difference in electric potential between the first and second accelerating electrode tubes. Thereafter, the controller applies the accelerating voltage to the second accelerating electrode tube at a timing when the charged particle has entered the second accelerating electrode tube. By repeatedly performing such timing control on n accelerating electrode tubes arranged in a linear fashion, the accelerating energy of the charged particle can be increased. Note that the electric potential of any accelerating electrode tube that comes after the first accelerating electrode tube is reset to ground potential after the charged particle has entered a subsequent accelerating electrode tube. With the above configuration, accelerating electric fields can be generated through distributed control of voltage applied to each accelerating electrode tube. In this way, a radio-frequency power generation circuit that has been conventionally required becomes no longer necessary, and an inexpensive and highly reliable accelerator can be provided.
Detailed configurations of the acceleration unit 41, the adjustment unit 42 and the detection unit 43 are shown in
In the present case, the acceleration unit 41 is constituted by 157 accelerating cells. Similarly, the adjustment unit 42 is constituted by 157 adjustment cells, and the detection unit 43 is constituted by 157 detection cells. As shown in
As shown in
The adjustment unit 42 is constituted by 157 adjustment cells TU#1 to TU#157, and the detection unit 43 is constituted by 157 detection cells DT#1 to DT#157.
The following describes operations of the spiral-trajectory charged particle accelerator configured in the above manner. As with Embodiment 1, the following description provides an example in which a hexavalent carbon ion is accelerated. That is to say, the following describes operations in which a hexavalent carbon ion is injected as the charged particle 40 at an energy of 2 MeV/u and is accelerated to about 430 MeV/u. Note that the following description is provided under the assumption that permanent magnets with a magnetic field strength of 1.5 tesla are used as the bending magnets 44 and 45. As shown in
The pulsed ion beam emitted from the dummy electrode passes through the bending magnet 44, an adjustment cell TU#m, a detection cell DT#m, and the bending magnet 45, and is injected into the accelerating cell AC#m again to be further accelerated through the above operation. By repeating this, the pulsed ion beam of the charged particle 40 is accelerated multiple times in the same accelerating cell.
Once the accelerating energy of the pulsed ion beam has reached a predetermined energy through multiple accelerations in one accelerating cell, the controller 46 transfers the pulsed ion beam from an accelerating cell AC#x to an accelerating cell AC#x+1 by operating the sending electrode plates and the receiving electrode plates of the accelerating cells. First, a description is given of an operation for transferring the pulsed ion beam of the charged particle 40 from an odd-numbered accelerating cell to an even-numbered accelerating cell.
Next, a description is given of an operation for transferring the pulsed ion beam from an even-numbered accelerating cell to an odd-numbered accelerating cell.
That is to say, in the spiral-trajectory charged particle accelerator shown in
Injection radius: 0.27 m
Emission radius: 4.99 m
Injection energy: 2 MeV/u
Emission energy: 432 MeV/u
Next, a description is given of the functions of the adjustment cells TU#1 to TU#157 with reference to
The following describes the functions of the detection cells with reference to
As has been described above, in the present embodiment, the accelerating electrode tubes are connected in a loop via the bending magnets, that is to say, there is no need to arrange the accelerating electrode tubes in a linear fashion, and therefore the total length of the accelerator can be reduced. Furthermore, by selecting bending magnets with appropriate shapes and magnetic field strengths, it is possible to design a trajectory along which a charged particle accelerated by an accelerating electrode tubes returns to the same accelerating electrode tube, and therefore the charged particle can be accelerated multiple times by one accelerating electrode tube. Since a charged particle can be thus accelerated multiple times by one accelerating electrode tube with the use of bending magnets, a high energy gain can be yielded. Furthermore, when permanent magnets are used as the bending magnets, an accelerator that consumes low power during operation can be provided.
The following describes operations of the charged particle detection system configured in the above manner. A fixed voltage is applied to the three detection electrode tubes placed in a rear portion of the transport path 56. More specifically, ground potential is applied to the detection electrode tubes #1 and #3, whereas an electric potential of 1 kV is applied to the detection electrode tube #2. The charged particle 40 passes through these detection electrode tubes before being injected into the accelerating cell AC#1 via the transport path 56. At this time, the charged particle 40 is decelerated by a difference in electric potential between the detection electrode tubes #1 and #2, and then accelerated again by a difference in electric potential between the detection electrode tubes #2 and #3. As the values of the decelerating energy and the accelerating energy are substantially the same, the accelerating energy of the charged particle 40 is not substantially changed by the charged particle 40 passing through these detection electrode tubes.
When the charged particle 40 is decelerated in the gap between the detection electrode tubes #1 and #2, a negative accelerating current flows through the 1-kV direct current power supply 54. On the other hand, when the charged particle 40 is accelerated in the gap between the detection electrode tubes #2 and #3, a positive accelerating current flows through the 1-kV direct current power supply 54. The ammeter 55 measures these positive and negative accelerating currents and notifies the controller 46 of the measured accelerating currents. The controller 46 can obtain the location, the speed and the total amount of charge of the charged particle 40 based on the values measured by the ammeter 54. Based on these data, the controller 46 can calculate an appropriate timing to apply the accelerating voltage (200 kV) to the accelerating electrode tube embedded in the first accelerating cell AC#1.
Note that when the linear-trajectory charged particle accelerator shown in
The above Embodiment 2 has described a configuration for changing a direction in which the charged particle travels by using the bending magnets so as to make the charged particle pass through the same accelerating electrode tube multiple times. However, the present invention is not limited in this way. Alternatively, it is possible to have a configuration in which a plurality of accelerating electrode tubes are arranged in a non-linear fashion with bending magnets provided between neighboring accelerating electrode tubes. With this configuration, the direction in which the charged particle travels can be changed by the bending magnets so that the charged particle passes through the accelerating electrode tubes arranged in a non-linear fashion in sequence. This type of charged particle accelerator can be made shorter and smaller than a linear trajectory accelerator. A conventional charged particle accelerator generates the accelerating voltage using a radio-frequency power supply, and therefore cannot be made smaller as the distance of a gap between accelerating electrode tubes always needs have a constant value. The aforementioned small charged particle accelerator is advantageous in that it can be installed in a place with a limited space, such as on a ship.
A charged particle accelerator pertaining to the present invention is useful as a linear trajectory accelerator and a spiral trajectory accelerator, and a method for accelerating charged particles pertaining to the present invention is useful as a method for accelerating charged particles that uses these charged particle accelerators.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-101291 | Apr 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/060044 | 4/25/2011 | WO | 00 | 7/16/2012 |
