1. Field of the Invention
This invention relates to a circular accelerator into which a low energy beam is entered, and from which a high energy beam accelerated on an equilibrium orbit is emitted.
2. Description of the Background Art
Heretofore, a circular accelerator such as a synchrotron has been used in a physical experiment in which a charged particle beam is revolved and accelerated, and a beam extracted from the equilibrium orbit of the circular accelerator is transported by a beam transport system, so as to irradiate a desired object with the extracted beam, or in the remedy of a cancer or the diagnosis of a diseased part for particle beam medicine.
In such a circular accelerator, the resonance of the betatron oscillations of the beam has been employed in order to continuously emit accelerated charged particles. The “resonance of the betatron oscillations” is a phenomenon as stated below. The charged particles revolve while oscillating rightwards and leftwards (in a horizontal direction) or upwards and downwards (in a vertical direction) around the equilibrium orbit of the circular accelerator. This is termed the “betatron oscillations”. The oscillation number of the betatron oscillations per a revolution of the revolving orbit is generally called a “tune (a betatron oscillation number)”. The tune can be controlled by a bending electromagnet, a four-pole electromagnet or the like which is disposed on the revolving orbit. When the fractional part of the tune is brought near to a/b (where a and b denote integers), and simultaneously, a multi-pole magnet for generating the resonance (for example, a six-pole electromagnet) disposed on the equilibrium orbit is excited, the amplitude of the betatron oscillations of the charged particles which have betatron oscillation amplitudes of or larger than a certain fixed amplitude, among the large number of charged particles revolving, increases suddenly. This phenomenon is called the “resonance of the betatron oscillations”, and the boundary part between a stable region and an unstable region is termed a “stable limit (separatrix)”. The magnitude of the betatron oscillation amplitude of the stable limit of the resonance depends upon a deviation from the fractional part of the tune, and it becomes smaller as the deviation is smaller. The beam outside the separatrix becomes unstable, and it is gradually extracted out of the circular accelerator. In this manner, the delicate adjustment of the tune is required in the resonance emission, and a long time is expended on the adjustments of emission parameters.
As methods for performing such resonance emissions, the following four methods have heretofore been known extensively and generally:
[Method 1] The magnitude of a separatrix is gradually made small from an initial large state. A resonance is first generated for charged particles of large betatron oscillation amplitude among charged particles revolving, and resonances are thereafter generated for the charged particles of smaller oscillation amplitudes in succession. Thus, charged particle beams are gradually emitted from an emission unit into an irradiation chamber.
[Method 2] A stable limit is made constant by holding a tune constant, and the amplitude of the betatron oscillations of a beam is increased by high frequencies, thereby to generate a resonance.
[Method 3] A stable limit is made substantially constant by holding a tune substantially constant, and the amplitude of the betatron oscillations of a beam is increased by high frequencies, so as to enlarge the beam to the boundary of the stable limit. Thereafter, a four-pole electromagnet is excited to make a separatrix somewhat smaller. Thus, a charged particle beam is gradually extracted.
[Method 4] A stable limit is made substantially constant by holding a tune substantially constant, and a beam is gradually accelerated by a high-frequency acceleration electric field. Thus, the beam having come outside the separatrix is gradually extracted.
With any of the above methods, the charged particles do not revolve round a center orbit only, but they pass through various parts outside the center orbit and inside the center orbit. In that case, in a prior-art example, the change of the tune is corrected by temporally controlling a six-pole electromagnet or the like. As a concrete example, there has been disclosed a technique wherein, in order to prevent the change of the betatron oscillation number (the tune), attributed to the fact that the equilibrium orbit is shifted by the change etc. of the exciting current of a bending electromagnet, a four-pole electromagnet, a function coupling type electromagnet or the like, and to stably emit the charged particle beam, a six-pole electromagnet which cancels the change of the tune attributed to the exciting current of the bending electromagnet or the four-pole electromagnet is disposed in addition to a six-pole electromagnet for the resonance emission, and the additional six-pole electromagnet is fed with an exciting current which gives the revolving beam a diverging force or a converging force that cancels the change of the tune attributed to the exciting current of the bending electromagnet or the four-pole electromagnet (refer to, for example, Patent Document 1 being JP-A-11-074100).
However, a revolving type accelerator indicated in Patent Document 1 has had the following problems:
(1) The six-pole electromagnet or the like needs to be subjected to a complicated control in order to prevent the change of the tune attributed to the discrepancy of the equilibrium orbit as is ascribable to the change of the exciting current of the bending electromagnet or the other electromagnet, and a long time is expended on beam adjustments.(2) Even in the emission of identical energy, in the case of the resonance emission, the charged particle beam passes on different beam orbits in the course of making the separatrix smaller. Therefore, a complicated control is required for preventing the change of the tune attributed to the change of the orbit, and a long beam adjustment time is expended.