1. Field of the Invention
The present invention relates to a circular acceleration apparatus that can accelerate large-current electron beams, an electromagnetic wave generator that generate electromagnetic waves such as X-rays by making electrons accelerated by the circular acceleration apparatus collide with a target, and an electromagnetic-wave imaging system that produce radioscopic images of human bodies, semiconductors and the like, using X-rays and such generated by the electromagnetic wave generators.
2. Description of the Related Art
In the description below, it is assumed that a circular acceleration apparatus is configured with a circular accelerator, an electron injection unit and a power supply necessary for operating them. The circular accelerator handles electrons generated by an electron generator as injection electrons, and accelerates the injection electrons until they have a predetermined energy, while making them move in their orbits; it may not necessarily be circular. For convenience, the term “circular” is given because electrons circulate in orbits.
In electromagnetic wave generators that use the circular accelerators to generate X-rays and such, betatron accelerators and synchrotron accelerators such as electron storage rings have been used as the circular accelerators. However, when the betatron accelerators are used, increase of current is difficult due to effects of Coulomb repulsion between electrons, resulting in a low electromagnetic wave strength of X-rays and such generated by making electrons collide with a target, so that it has been difficult to apply to industry and medical fields the electromagnetic wave generators using the accelerators. When the synchrotron accelerators are used as radiation sources, the electromagnetic waves generated thereby are intense, but have low energy; it has been difficult to apply those to the industry and medical fields. Also, when synchrotron accelerators are used in order to generate highly-energized electromagnetic waves, a method of not using synchrotron radiation, but making electrons collide with a target is to be adopted; however, the method brings the same difficulties as those when the betatron accelerators are used, in that it is difficult to increase current, so that the strength of the generated electromagnetic waves such as X-rays becomes low; therefore, it has been difficult to apply to the industry and medical fields the electromagnetic wave generators using the synchrotron accelerators as highly energized ones.
In order to improve the situation described above, an electromagnetic wave generator using a so-called hybrid accelerator has been proposed in Japanese Patent Laid-Open No. 2004-296164 (Patent Document 1). The hybrid accelerator is the one that employs such an acceleration method as follows: while an acceleration means is accelerating electrons from an instant when their injection into the accelerator begins, a deflecting magnetic field generated by deflection electromagnets included in the accelerator is kept constant during an injection period, and is controlled to change after finishing the injection. In this accelerator, stable electron orbits exist spreading out over a broad radial range; therefore, when electrons are injected in the way described above, the electrons move stably in orbits each having a different diameter depending on their injected instances during the injection period. Therefore, the accelerator makes the electrons move in orbits spreading over a wide orbital range. Thereby, the spatial density of electrons can be lowered, resulting in less Coulomb repulsion among the electrons, which will enable large-current acceleration.
The hybrid accelerator adopts as an accelerating means, so-called induction acceleration by an electric field that an accelerating magnetic field induces. FIG. 10 shows variations with time of deflection-magnetic-field strength and acceleration-magnetic-field strength according to the invention of Patent document 1. In FIG. 10, ‘41’ is the acceleration-magnetic-field strength variation with respect to time, ‘42’, that of the deflection-magnetic-field strength; it is assumed that injecting operations are performed in a pulsational manner. Here, “injecting operations are performed in a pulsational manner” means that each injection is performed during a predetermined period after a pulse of a rectangular waveform reaches its peak wave-height value. More specifically, because the pulse rises to its peak value immediately after it has been generated, the injection period is set as follows: injection starts after a specific time—from an instant at which the pulse has been applied to an instant at which the pulse reaches its peak—has passed, and continues until a specific time has elapsed while the pulse peak value is held. Because the acceleration-magnetic-field strength 41 begins to increase from an instant when the injection of electrons starts, the electrons have been accelerated from the instant when they have been injected. Meanwhile, the deflection-magnetic-field strength 42 is controlled to stay at a constant value from the instant of the injection start to an instant of the injection end; as soon as the injection ends, the strength 42 is controlled to begin to increase, similarly to the acceleration-magnetic-field strength 41. While the deflection-magnetic-field strength 42 stays at the constant value, the injected electrons that have the same energy as each other are accelerated immediately after their injection, and their deflection curvatures gradually become larger. Therefore, at the instant of injection end, each of the electrons has been accelerated differently depending on its injected instant during the injection period; the injection electrons move in orbits spreading out radially. Because the electrons continue to be thereafter accelerated so as to have a predetermined energy, the radially spread orbits are further expanded radially. After the end of the injection, the deflection-magnetic-field strength 42 increases; the degree of the orbit radial expansion usually becomes less than that during the injection. Once the electrons have been accelerated to have the predetermined energy, the radii of the electron orbits can be expanded by, for example, controlling the deflection-magnetic-field strength 42 at a constant value and the like.
When a target is placed in the way of an orbit having a predetermined radius out of the ones in which the electrons can move around stably, the radii of the electron orbits being changed, the electron beams can collide with the target in a controlled manner, so that electromagnetic waves such as X-rays are generated by the collision. Here, because the target has a certain area, the electrons moving in orbits within a specific radial range are ready to be capable of colliding with the target. Such orbits will be referred as collision orbits, hereinafter.
Even while the radii of the electron orbits are being changed, electrons are diverged radially and are moving in orbits; by gradually changing the radii of the orbits, the electron beams can continue to collide with the target, so that X-rays can be generated continuously. Here, all the electrons that have collided with the target do not always disappear, but electrons that have reduced energy remain there after their collision. Because in general, the residual electrons also have energies in the range of enabling stable movement in orbits, some of the electrons can be recharged with sufficient energy from the acceleration magnetic field every turn, so as to return to a collision orbit. Therefore, using the accelerator, electromagnetic waves can be generated efficiently utilizing electrons moving in orbits (refer to Patent document 1).
As has been described, in the accelerators, because the electrons have stable radially-spread-out orbits, causing less Coulomb repulsion between electrons, it becomes easy to accelerate large currents; because the position of the orbits can be changed, while maintaining conditions for electrons to move stably in orbits by controlling the acceleration-magnetic-field strength and the deflection-magnetic-field strength, it becomes possible for electrons moving in orbits to collide efficiently with the target. Thereby, it becomes possible to increase the strength of the electromagnetic wave such as X-rays generated by the accelerator. So far, a hybrid accelerator has been described as a typical example, it is not limited to the hybrid accelerator that current can be increased by increasing the radii of electron orbits. Any type of circular accelerator has more or less a certain radial range of orbits in which electrons move stably; therefore, it is also possible in a similar fashion to increase current by increasing the radii of the electron orbits. Meanwhile, some accelerators adopt an electric-field acceleration method instead of induction one using a magnetic field. In that case, the above description holds true if the term “acceleration-magnetic-field strength” is interchanged with the term “electric-field acceleration” in FIG. 10. However, during an electron injection period, a hybrid accelerator needs to control changes with time both the acceleration-magnetic-field strength and the deflection-magnetic-field strength so as to have predetermined relationships therebetween; it results in complex electromagnet-power-supply controlling in which the electron acceleration unit and the electron deflection unit generate magnetic fields, causing a problem in that the accelerator has been manufactured at high costs. The above problem has also existed when an electric field acceleration method as well as induction one is used as an electron acceleration means. In that case, it results in complexity of controlling power supply for the electric field acceleration and electromagnet-power-supply that lets the electron deflection unit generate magnetic fields, causing a problem in that the accelerator has been manufactured at high costs. Therefore, when it is intended to increase current in the circular accelerator according to the above-described method, it has been a common problem, not limited to the hybrid accelerator, that control of a power supply applying a high voltage to an electron acceleration unit and a power supply supplying currents to an electron deflection unit for generating a deflecting magnetic field becomes complex, causing high costs.