JP3881854B2 - Charged particle energy control method and charged particle accelerator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In the present invention, charged particles such as electrons and ions are accelerated using a high-frequency linear accelerator (Linac, also called linac), and crosslinking, modification, sterilization, sterilization, insecticide, etc. by electron beam irradiation are performed. In particular, the present invention relates to a charged particle accelerator used for ion implantation and modification by ion irradiation, and a method for controlling charged particle energy. More specifically, by using a superconducting accelerator as a linear accelerator, the charged particle energy is controlled. It relates to means to facilitate.
[0002]
[Prior art]
A charged particle source that generates charged particles such as electrons and ions, a linear accelerator that accelerates charged particles given from the charged particle source by a high frequency electric field in an acceleration cavity, and a high frequency that supplies high frequency power for acceleration to the linear accelerator A charged particle acceleration device including an oscillator is called a high-frequency linac or the like and has already been used in various fields.
[0003]
[Problems to be solved by the invention]
Unlike an accelerator using an electrostatic field such as a cockcroft type, it is very difficult to control the energy of charged particles accelerated by a conventional linear accelerator using a high-frequency electric field as described above.
[0004]
This is because conventional linear accelerators use normal-type acceleration cavities, and most of the input high-frequency power is consumed as Joule heat loss in the accelerating cavities. This is because uniform and large distortion occurs and operation instability easily occurs. It is very difficult to suppress the occurrence of instability, and even if the generation of instability is suppressed at a certain energy, the generation of instability occurs easily at other energy.
[0005]
In addition, it is not easy to know how much energy the charged particles have after acceleration. This is because, as described above, most of the input high-frequency power is consumed as Joule heat loss in the acceleration cavity, and it is not precisely known how much of the rest is converted into charged particle energy. Because. The rate of energy conversion varies depending on the device configuration, the frequency of high-frequency power, power, input adjustment, and the like, and is not constant.
[0006]
Conventionally, in order to obtain the energy of the charged particles after acceleration, (a) the intensity of the high-frequency electric field generated in the acceleration cavity is measured using an antenna or the like, and the acceleration electric field strength is estimated to estimate the energy, (B) Bending charged particles emitted from a linear accelerator with a bending electromagnet and calculating the energy from the bending state, or (c) Necessary parameters using a known energy event such as a nuclear reaction Was calibrated with energy, and during that time, it was determined appropriately by interpolation.
[0007]
However, the means (a) above is a guess and is not reliable. The means (b) requires a deflecting electromagnet, and the configuration becomes large. In addition, bending charged particles requires a large space. In some cases, it is difficult to adopt a device arrangement that bends charged particles using a deflecting electromagnet. The means (c) also takes time since a calibration system such as a calibration graph is used. Moreover, it is necessary to periodically check whether the calibration has changed over time.
[0008]
SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a method and an apparatus that can easily and accurately control the energy of charged particles after acceleration.
[0009]
[Means for Solving the Problems]
The charged particle energy control method according to the present invention uses, as the linear accelerator, a superconducting accelerator having a superconducting acceleration cavity that is maintained in a superconducting state and accelerates the charged particles by a high-frequency electric field. The reflection of the high frequency from the superconducting acceleration cavity is adjusted to substantially zero, and the output P of the high frequency power output from the high frequency oscillator and substantially all the current values i of the charged particles accelerated by the superconducting accelerator are obtained. And the energy E of the charged particles after being accelerated by the superconducting accelerator is controlled based on the relationship E = P / i.
[0010]
The charged particle accelerator according to the present invention includes, as the linear accelerator, a superconducting accelerator having a superconducting acceleration cavity that is maintained in a superconducting state and accelerates the charged particles by a high-frequency electric field. A reflection adjustment mechanism that adjusts the high-frequency reflection from the superconducting acceleration cavity in the accelerator to substantially zero, an output measuring instrument that measures the high-frequency output P output from the high-frequency oscillator, and acceleration with the superconducting accelerator Accelerated by the superconducting accelerator using a current measuring device that measures substantially all current values i of charged particles, an output P measured by the output measuring device, and a current value i measured by the current measuring device. And an energy control device that controls the energy E of the charged particles after that based on the relationship E = P / i.
[0011]
According to the above configuration, the high-frequency power output from the high-frequency oscillator adjusts the high-frequency reflection from the superconducting acceleration cavity to substantially zero, and is thus supplied to the superconducting acceleration cavity.
[0012]
In this superconducting accelerating cavity, because of superconductivity, Joule heat loss is substantially zero, i.e., no loss is caused in the accelerating cavity, and all supplied high-frequency power is used to accelerate charged particles, Converted to the power of charged particles.
[0013]
Therefore, the relationship of the following equation is established between the output P of the high frequency power output from the high frequency oscillator and the power W of the charged particles accelerated by the superconducting accelerator.
[0014]
[Expression 1]
P = W
[0015]
On the other hand, the power W of the charged particle after acceleration is the product of the energy E of the charged particle and the current value i of the charged particle, and is expressed by the following equation.
[0016]
[Expression 2]
W = E ・ i
[0017]
From the above equations 1 and 2, the following relationship is obtained.
[0018]
[Equation 3]
E = P / i
[0019]
The inventors of the present application have found that the relationship of the above formula 3 is established in the charged particle accelerator by using the superconducting acceleration cavity and adjusting the high frequency from it to a non-reflective state.
[0020]
The output P of the high frequency oscillator can be easily and accurately measured by a known technique such as a high frequency power meter.
[0021]
The current value i of the charged particles can also be easily and accurately measured by a known technique such as a current transformer.
[0022]
The present invention controls the energy E of charged particles based on the relationship of the above equation 3 using the output P and the current value i that can be measured easily and accurately in this way. The energy of the charged particles after acceleration can be easily and accurately controlled with high reliability.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic diagram showing an example of a charged particle acceleration apparatus that implements the energy control method according to the present invention. FIG. 2 is a schematic cross-sectional view showing an example around the superconducting accelerator in FIG.
[0024]
This charged particle acceleration device includes a charged particle source 2 that generates (emits) charged particles 4 and a superconducting acceleration that accelerates charged particles 4 that are supplied from the charged particle source 2 while being kept in a superconducting state by a high-frequency electric field. A superconducting accelerator 6 having a cavity 14 (see FIG. 2) and a high-frequency oscillator 16 for supplying high-frequency power for accelerating charged particles to the superconducting accelerator 6 (more specifically, to the superconducting accelerating cavity 14 therein). A reflection adjusting mechanism 30 for adjusting the high-frequency reflection from the superconducting accelerator 6 (more specifically, from the superconducting acceleration cavity 14 inside the superconducting accelerator 6) to zero, and after acceleration based on the above equation (3) And an energy control device 36 for controlling the energy E of the charged particles 4.
[0025]
The charged particles 4 are, for example, electrons or ions.
[0026]
When the charged particle 4 is an electron, the charged particle source 2 is, for example, an electron gun. When the charged particles 4 are ions, the charged particle source 2 is, for example, an ion source.
[0027]
In this example, the high-frequency oscillator 16 is a high-frequency output from an output measuring instrument 18 that measures the output P of high-frequency power that is output therefrom, and the superconducting accelerator 6 (more specifically, from the superconducting acceleration cavity 14 inside it). And a reflection measuring instrument 20 for measuring the reflection R of the light. The output measuring instrument 18 is, for example, a high frequency wattmeter. The reflection measuring instrument 20 is, for example, a reflection wattmeter.
[0028]
The high-frequency power output from the high-frequency oscillator 16 is supplied to the superconducting acceleration cavity 14 in the superconducting accelerator 6 via the coaxial tube 22 in this example. However, a waveguide or the like may be used instead of the coaxial tube 22. In this specification, “high frequency” is used in a broad concept including microwaves.
[0029]
The superconducting accelerator 6 is a kind of high-frequency linear accelerator, and in this example, has a structure as shown in FIG.
[0030]
That is, the superconducting accelerator 6 accommodates a plurality (5 in this example) of superconducting accelerating cavities 14 connected in series in a liquid helium container 10 filled with liquid helium 12, and further this liquid helium container. 10 is housed in a vacuum vessel 8. That is, the superconducting acceleration cavity 14 has a five-cell structure.
[0031]
Although a helium refrigerator for supplying liquid helium 12 to the liquid helium container 10 is provided, the illustration thereof is omitted here. In addition, a nitrogen temperature shield or the like is usually provided between the liquid helium vessel 10 and the vacuum vessel 8, but this is also omitted from the drawing.
[0032]
Each superconducting accelerating cavity 14 is made of niobium (Nb) in this example, and is kept in a superconducting state at a temperature of 4.2 K of liquid helium 12. However, a superconducting material other than niobium may be used for the superconducting accelerating cavity 14, and a cooling temperature and a cooling medium for keeping it in a superconducting state may be determined according to the material.
[0033]
Each superconducting accelerating cavity 14 is supplied with high-frequency power for accelerating charged particles from the high-frequency oscillator 16 via a coaxial tube 22 and an antenna 26 in this example. Thereby, a high-frequency electric field is generated in each superconducting acceleration cavity 14, and the charged particles 4 can be accelerated by the high-frequency electric field.
[0034]
The maximum acceleration energy of the charged particles 4 is generally determined by the number of superconducting acceleration cavities 14 (number of cells), the frequency of the high frequency power, and the like. For example, when accelerating electrons, if 5 cells are used at 500 MHz, energy of about 10 MeV can be obtained at the maximum. Within this range, according to the present invention, the energy E can be easily adjusted according to the relationship of the above formula 3.
[0035]
In this example, as shown in FIG. 2, the reflection adjusting mechanism 30 is connected to the coaxial tube 22 and supplies high-frequency power to the superconducting acceleration cavity 14, and the antenna 26 is indicated by an arrow B. And a drive unit 28 for taking in and out. The drive unit 28 has a motor. In the penetration portion of the antenna 26, an inner vacuum atmosphere and an outer atmosphere are partitioned by a partition plate 24 made of ceramics or the like.
[0036]
The reflection adjustment mechanism 30 moves the antenna 26 in and out to adjust the high-frequency coupling state with the superconducting acceleration cavity 14 so that the reflection R from the superconducting acceleration cavity 14 is substantially zero (that is, zero or zero). Can be adjusted). In this example, the reflection R can be monitored by the reflection measuring instrument 20 described above.
[0037]
In this example, there is further provided a reflection control device 32 that automatically controls the reflection R from the superconducting acceleration cavity 14 to substantially zero by controlling the drive unit 28 according to the reflection R measured by the reflection measuring instrument 20. ing. However, the adjustment of the reflection R as described above may be performed manually while viewing the reflection measuring instrument 20. In that case, the reflection control device 32 is unnecessary, and the drive unit 28 may be a manual mechanism.
[0038]
The charged particle accelerator further includes a current measuring device 34 that measures a current value i of the charged particle 4 accelerated by the superconducting accelerator 6.
[0039]
In this example, the current measuring device 34 is provided at the exit of the superconducting accelerator 6 and is a winding that measures the current that flows when the accelerated charged particles 4 pass through. It can also be called a vessel. The current measuring device 34 may be provided at the entrance of the superconducting accelerator 6 to measure the current value i of the charged particles 4 immediately before entering the superconducting accelerator 6 or immediately after leaving the charged particle source 2. The current value i of the charged particle 4 may be measured. Alternatively, when the charged particle source 2 is an electron gun, a current measuring device that measures the cathode current may be used instead of the current measuring device 34 described above. In this apparatus, it can be considered that all the charged particles 4 output from the charged particle source 2 are accelerated without any loss on the way, so even if the current value i of the charged particles 4 is measured at any of the above points. good.
[0040]
The energy control device 36 performs the calculation of Equation 3 using the output P measured by the output measuring instrument 18 and the current value i measured by the current measuring instrument 34, and outputs charged particles output from the superconducting accelerator 6. 4 is obtained, and the energy E is controlled to a desired (target) value. For example, while maintaining the output P output from the high-frequency oscillator 16, the charged particle source 2 can be controlled to control the current value i of the charged particle 4 output from the target so as to achieve the target energy E. . Alternatively, the output value P of the high-frequency power output from the high-frequency oscillator 16 and the current value i of the charged particles 4 output from the charged particle source 2 can be controlled so that the target energy E and current value i can be obtained.
[0041]
The energy control device 36 and the reflection control device 32 may be incorporated in one control device. That is, an integrated control device having the functions of both 32 and 36 may be provided.
[0042]
As shown in this example, a scanner (scanning coil in the illustrated example) 38 that scans the charged particles 4 output from the superconducting accelerator 6 and a scanning tube 40 that spreads toward the end are provided on the downstream side of the superconducting accelerator 6. Further, it may be configured such that the window foil 42 provided at the tip of the scanning tube 40 is transmitted, the scanned charged particles 4 are taken out in an irradiation atmosphere (for example, in the air) and irradiated on the irradiated object 44. . In this case, if the charged particle 4 is an electron, this device becomes a scanning electron beam irradiation device. As a result, the irradiated object 44 can be subjected to processing such as crosslinking, modification, sterilization, sterilization, and insecticidal treatment.
[0043]
As described above, a conventional high-frequency linear accelerator uses a normal-type acceleration cavity, and most of the input high-frequency power is Joule heat loss, and the ratio is not constant. , The relationship of Equation 1 cannot be obtained.
[0044]
In contrast, the superconducting accelerator 6 uses the superconducting accelerating cavity 14 as described above, and its Joule heat loss is substantially zero (that is, zero or negligibly small). Therefore, all of the input high frequency power is converted into the power of the charged particles 4. In addition, the reflection adjusting mechanism 30 adjusts to a non-reflecting state. As a result, in this charged particle accelerator, the relationship of the above formula 3 is established.
[0045]
Moreover, the output P of the high-frequency oscillator 16 can be easily and accurately measured by the output measuring instrument 18, for example. The current value i of the charged particle 4 can also be easily and accurately measured by the current measuring device 34, for example.
[0046]
In this charged particle acceleration device, the charged particle 4 based on the relationship of the above equation 3 using the output P of the high-frequency oscillator 16 and the current value i of the charged particle 4 that can be measured easily and accurately as described above. Thus, the energy E of the charged particles after being accelerated by the superconducting accelerator 6 can be controlled to a desired value easily and accurately with high reliability. The reproducibility of the energy E is also very good.
[0047]
Unlike the case of using a deflecting electromagnet for conventional energy measurement, the charged particles 4 need not be bent, and the trajectory of the charged particles 4 can be kept linear, and the charged particles 4 can be used immediately after acceleration. Therefore, the apparatus configuration is simplified and the space is saved.
[0048]
Moreover, as can be seen from the above equation 1, the power W of the charged particles 4 output from the superconducting accelerator 6 can be kept constant by keeping the output P of the high-frequency oscillator 16 constant, and this power W can be kept constant. Up to now, it is also a great feature that the energy E of the charged particle 4 can be easily changed by changing the current value i of the charged particle 4 as can be seen from the above equation (3). For example, while maintaining the power W of the charged particle 4 output from the superconducting accelerator 6 at a constant value of 100 kW, the charged particle 4 with an energy E of 10 MeV and a current value i of 10 mA is output, or the current E with an energy E of 5 MeV. Output of charged particles 4 having a value i of 20 mA can also be easily realized.
[0049]
This is particularly convenient when the accelerated charged particles 4 are used for the treatment of an object to be irradiated, such as the electron beam irradiation apparatus described above. This is because the power W of the charged particles 4 is directly related to the processing capability of the irradiated object, and there are cases where it is desired to change only the energy E of the charged particles 4 without changing the processing capability. For example, when irradiating an irradiated object with an electron beam while carrying irradiated objects of different heights on the same conveyor, the energy E of the electron beam is changed without changing the power of the electron beam. It is preferable to change the transmission power of the electron beam (that is, the depth of the sterilization treatment) according to the height of the object to be irradiated, and this can be easily dealt with.
[0050]
In conventional linear accelerators using normal-conducting accelerating cavities, it is very difficult to change the acceleration energy as described above, and the reproducibility of the acceleration energy after energy adjustment is also poor. It is impossible to respond by changing energy.
[0051]
Further, in the conventional linear accelerator using the normal conduction type accelerating cavity, the Joule heat loss in the accelerating cavity is very large and it is difficult to cool the accelerating cavity, so that the duty ratio is extremely small (for example, 1% or less). In contrast, in the superconducting accelerator 6, there is almost no problem of heat generation due to Joule heat loss in the superconducting acceleration cavity 14. Therefore, continuous operation for continuously taking out the accelerated charged particles 4 is performed. Can do. Or, of course, intermittent operation with a much larger duty ratio (for example, 25% or 50%, etc.) is also possible. Therefore, the utilization efficiency of the charged particles 4 is dramatically improved as compared with the conventional case. Therefore, for example, by using such charged particles 4 for processing the irradiated object, the processing efficiency can be dramatically increased.
[0052]
【The invention's effect】
As described above, the present invention uses a superconducting accelerating cavity and adjusts high-frequency reflection from the superconducting accelerating cavity to substantially zero, thereby converting all the supplied high-frequency power into the power of charged particles. In this way, the energy E of the charged particles is controlled based on the relationship E = P / i, and the energy of the charged particles after acceleration can be easily and accurately determined. Can be controlled. The reproducibility of the energy is very good.
In addition, unlike the conventional case of using a deflecting electromagnet for energy measurement, it is not necessary to bend the charged particles, the charged particle trajectory can be kept straight, and the charged particles can be used immediately after acceleration. Since it can be used for processing, the equipment configuration is simplified and the space is saved.
Moreover, the power of the charged particles output from the superconducting accelerator can be kept constant by keeping the output P of the high-frequency oscillator constant, and by changing the current value i of the charged particles while keeping this power constant. As can be seen from the above equation E = P / i, it is also a great feature that the energy E of the charged particles can be easily changed.
[0053]
Moreover, since the superconducting accelerating cavity is used and there is almost no problem of heat generation due to Joule heat loss, continuous operation in which accelerated charged particles are continuously extracted, and intermittent operation with a much higher duty ratio than conventional ones are also possible. It becomes possible.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a charged particle acceleration apparatus that implements an energy control method according to the present invention.
2 is a schematic cross-sectional view showing an example around a superconducting accelerator in FIG. 1. FIG.
[Explanation of symbols]
2 charged particle source 4 charged particle 6 superconducting accelerator 14 superconducting acceleration cavity 16 high frequency oscillator 18 output measuring instrument 20 reflection measuring instrument 30 reflection adjusting mechanism 32 reflection control device 34 current measuring instrument 36 energy control device

Claims (2)

A charged particle source that generates charged particles, a linear accelerator that accelerates and outputs charged particles given from the charged particle source by a high-frequency electric field in the acceleration cavity, and supplies acceleration high-frequency power to the acceleration cavity in the linear accelerator In a charged particle acceleration device comprising a high-frequency oscillator that
As the linear accelerator, a superconducting accelerator having a superconducting accelerating cavity that is kept in a superconducting state and accelerates the charged particles by a high-frequency electric field is used. After being accelerated by the superconducting accelerator using the output P of the high-frequency power output from the high-frequency oscillator and substantially all the current values i of the charged particles accelerated by the superconducting accelerator. The charged particle energy control method is characterized in that the charged particle energy E is controlled based on the relationship E = P / i. A charged particle source that generates charged particles, a linear accelerator that accelerates and outputs charged particles given from the charged particle source by a high-frequency electric field in the acceleration cavity, and supplies acceleration high-frequency power to the acceleration cavity in the linear accelerator In a charged particle acceleration device comprising a high-frequency oscillator that
The linear accelerator includes a superconducting accelerator having a superconducting acceleration cavity that is maintained in a superconducting state and accelerates the charged particles by a high-frequency electric field, and further includes a high-frequency from the superconducting acceleration cavity in the superconducting accelerator. A reflection adjustment mechanism for adjusting the reflection of the light to substantially zero, an output measuring instrument for measuring the high-frequency output P output from the high-frequency oscillator, and substantially all current values of the charged particles accelerated by the superconducting accelerator Using the current measuring instrument for measuring i, the output P measured by the output measuring instrument and the current value i measured by the current measuring instrument, the energy E of the charged particle after being accelerated by the superconducting accelerator is expressed as E A charged particle acceleration device comprising: an energy control device that performs control based on a relationship = P / i.

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