A CHARGED PARTICLE APPARATUS

Abstract: A charged particle apparatus for accelerating charged particles such as an electron beam (1) includes a circular equilibrium orbit (3) for circulating the charged particles and a plurality of inflectors (2a to 2g) so disposed that their centres of curvature are located progressively inwardly towards the centre of the equilibrium orbit (3) so as to enable the structure of the apparatus to be more compact. On order to remove positive ions produced through collisions between the electron beam (1) and gas contained in the equilibrium orbit (3), negative and positive electrodes are disposed vertically in pairs so that the equilibrium orbit (3) is in between them.

Description: This invention relates to a charged particle apparatus in general and more particularly to the one including synchrotrons and storage rings for the use of accelerators of charged particle beams such as electron beams. The charged particle apparatus according to the present invention can be applied for both synchrotrons and storage rings. The following description will be given taking a storage ring into consideration and electron beams are chosen as an example for the representative charged particle beams. In a conventional storage ring plural pairs of quadrupole electromagnets for focusing electrons, plural bending electromagnets for deflecting electrons, bump electromagnets for generating fast-pulse magnetic field and radio frequency cavities for generating radio frequency electric field are disposed along an equilibrium orbit which passes through inflectors located at the place where the injection of electrons takes place. The storage ring thus constructed causes an electron beam of high energy to run along the equilibrium orbit provided and enables the high-energy electron beam to maintain its kinetic energy for several hours to several tens of hours. One of the applications of the storage ring is a light source for manufacturing very large scale integrated circuits (VLSIs) in which synchrotron radiation is utilized. It is injected when the high-energy electron beam is running along the equilibrium orbit. Conventionally, a linear accelerator or a synchrotron of the known kind is provided on the upstream side of the storage ring. The electron beam is fed from the accelerator through inflectors disposed in part of the straight sections of the storage ring. These straight sections are free from any magnetic fields or electric fields and in which the inflectors, bump electromagnets and radio frequency cavities having the following functions are disposed. The electron beam of high energy circulating in the storage ring runs along the equilibrium orbit having a weak focusing magnetic field distribution. The electron beam injected from said accelerator into said storage ring has a fixed angle with respect to the straight section of the orbit. The electron beam passing through the inflector having its center of curvature located directly opposite to that of the bending electromagnets goes parallel with the orbit and then is injected from the inflector into the storage ring. As is well known, the deflection of the electron beam is performed by an electric field produced between the negative and positive electrodes of the inflector whose center of curvature is located opposite to that of the storage ring. Although the electron beam injected from the inflector runs parallel with the equilibrium orbit, its center has a certain amount of deviation from the center of the orbit thus the sectional center of the electron beam oscillates around the equilibrium orbit to result in a collision with a vacuum tank containing the electron beam to cause partial loss of the beam. The amplitude of the oscillation mentioned above is equal to said deviation of the electron beam. Because of this, the electron beam after passing through the inflector, tends to reduce in its electron beam central amplitude and intersects with the orbit. If the angle between the electron beam and the orbit at this first intersection made after the injection can be made zero, the loss in the electron beam can be minimized. Said bump electromagnet is disposed at this intersection thereby achieving above mentioned purpose. The time requirement of its high speed pulse magnetic field is determined by the speed at which the magnetic field of the bump electromagnet becomes zero before the electron beam completes one whole circle after passing the intersection. The electron beam fed into the equilibrium orbit loses its kinetic energy with a braking action through the six bending electromagnets when it emits synchrotron orbital radiation. The radio frequency cavities are provided to compensate for this kinetic energy loss. That is, the electron beam maintains its position on the orbit by obtaining kinetic energy from an accelerating electric field produced within the radio frequency cavities. The path for the electron beam is made up of said vacuum tank which is kept at a vacuum. The inflector as well as the bump electromagnet are usually installed within the vacuum tank. In the conventional charged particle apparatus this construction requires straight sections for installing inflectors, quadrupole electromagnets, radio frequency cavities and the like. This makes it difficult to provide an apparatus compact in size. German Patent Specification 3148100 relates to an apparatus for accelerating charged particles comprising means defining a circular equilibrium orbit for circulating charged particles and means having a weak focusing electromagnetic field. A single inflector is provided for directing the charged particles from a particle source into the circular orbit. Resonators divided into upper and lower halves relative to the plane of the equilibrium orbit, with a potential difference of several kV between them are provided for ion sweeping. The resonators, which are driven by high frequency power, are also provided for compensating for energy loss of the electrons. SUMMARY OF THE INVENTION In view of the foregoing, it is the main object of the present invention to provide an improved charged particle apparatus in which the use of straight sections in the conventional apparatus is omitted and thus the structure of the same is made compact. According to the present invention, there is provided an apparatus for accelerating charged particles comprising means defining a circular equilibrium orbit for circulating charged particles having a weak focusing electromagnetic field, and inflector means for directing said charged particles from a particle source into said orbit, characterised in that the inflector means comprises a plurality of inflectors which are disposed along the introducing area of the charged particles with their radii of curvature directed towards the interior of the equilibrium orbit, so as to form an approximately circular arc, in such a way that their centres of curvature are located progressively inwardly towards the centre of the said equilibrium orbit. Embodiments of the invention, capable of circulating an electron beam for a long period of time, may be improved by removing positive ions produced through a collision between the electron beam and gas contained inside the equilibrium orbit which is kept at a vacuum. This improvement is accomplished by providing a charged particle apparatus comprising plural pairs of positive and negative electrodes disposed to have the equilibrium orbit in between. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic plan view of a charged particle apparatus embodying features of the present invention; Fig. 2 is a diagram illustrating arrangement and magnetic field intensity distribution of air-core coils in the embodiment of Fig. 1; Fig. 3 is a schematic side view in section of another embodiment derived from the modification of the charged particle apparatus shown in Fig. 1; and Fig. 4 is a schematic plan view of the portion shown in Fig. 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, there is shown a storage ring 10 which is used as a light source for exposing work in the manufacture of very large scale integrated circuits (VLSIs), embodying the present invention. In the figure a train of inflectors starting from a first inflector 2a through a seventh inflector 2g are arranged in sequence to form an approximate circular arc in the introducing area of electron beam 1 so that a round equilibrium orbit 3 for the electron beam 1 is shaped. Because of this arrangement the centres of curvature of these inflectors are located progressively inwardly toward the centre of the equilibrium orbit 3. The introducing area is also provided with rectangular bump electromagnet 6a, and a first radio frequencey cavity 7a and a second radio frequency cavity 7b are disposed along the equilibrium orbit 3 together with a fine adjustment bump electromagnet 6b. At a point 9 on a magnetic field boundary 4 of the storage ring 10 touches a tangent line 5, with respect to which the electron beam 1 runs at a fixed angle of ϑ and passed through the point 9 in the direction of a radius vector 8. Synchrotron orbital radiation emitted from part of the circular equilibrium orbit 3 passes through the gap between the fourth inflector 2d and fifth inflector 2e for example to be utilized as intended. In the apparatus having the aforesaid structure, the electron beam 1 which is fed from an accelerator located upstream the storage ring 10 will be introduced at a fixed angle ϑ, 30 degrees for example, with respect to the tangent line 5 at the intersection point 9 and fed through the first to the seventh inflectors 2a - 2g. It is further passed through the rectangular bump electromagnet 6a to be placed parallel with the equilibrium orbit 3. The sectional center of the electron beam 1 then slowly oscillates around the center of the equilibrium orbit 3. This oscillation is eliminated by the bump electromagnet 6b which makes the angle between the electron beam 1 and the equilibrium orbit 3 zero. This occurs at the point where the center of the electron beam 1 initially crosses the equilibrium orbit 3 having a weak focusing electromagnetic field distribution. The radiation loss of a synchrotron depends on the kinetic energy of the electron beam 1. Where it is about 800 MeV with the circular equilibrium orbit 3 having a diameter of about 1.6 m, the loss will be approximately 45 KeV. To lengthen a quantum life sufficiently with such a high radiation loss it is necessary to produce a higher accelerating voltage in radio frequency cavities. This is sometimes difficult to achieve with a single radio frequency cavity. Taking this into consideration, the present embodiment utilizes the two radio frequency cavities, 7a and 7b. Of course, there are cases in which a single cavity will perform the intended purposes. Fig. 2 shows the magnetic field distribution 17 of the storage ring 10 thus constructed and an example of the coil arrangement to form said field distribution. The axis of abscissa 11 starting from the origin 13 coincides with the radius vector 8 and represents the lateral position of the equilibrium orbit 3. The axis of the ordinate represents the relative positional dimensions of the coils and the relative magnetic field intensity 17. By determining the dimension of the equilibrium orbit 3, dimensions for large diameter upper and lower coils 14a - 14b, middle diameter upper and lower coils 15a - 15b, and small diameter upper and lower coils 16a - 16b for forming the required magnetic field can be relatively obtained. As shown in Fig. 2, the magnetic field of the storage ring 10 is built up of air-core coils, which can be replaced with either superconducting coils or normal conducting coils according to the required magnetic field strength and the diameter of the orbit 3. A positive magnetic field is formed inside the magnetic field boundary 4 shown in Fig. 1. It provides a weak focusing magnetic field of the known kind near the equilibrium orbit 3 to prevent the horizontal and vertical dissipation of the electron beam, which serves an equivalent function to the quadrupole electromagnets in the conventional apparatus. For the portion of the electron beam 1 outside the magnetic boundary 4 where the magnetic field becomes negative, the magnetic intensity is reduced to be negligible by an appropriate magnetic shelter. The first inflector 2a through the seventh inflector 2g are, different from the conventional ones, disposed to form an approximate circular arc in such a way that their centers of curvature are located inwardly toward the center of the circular equilibrium orbit 3. The inflectors 2a through 2g shown in Fig. 1 are located within the magnetic field encompassing the center thereof. The electric field applied by these inflectors formed by applying voltage between negative and positive electrodes, however, is orientated in a direction to decrease the radius of curvature of the electron beam as compared with the radius of curvature if the inflectors are absent. In this way, the inflectors according to the present invention have the same function as that of the conventional inflectors. Because of these inflectors 2a - 2g, the beam 1 introduced at the intersection 9 can be fed to the equilibrium orbit 3 without an excessive bending by the magnetic field. The number of the inflectors will be selected depending upon the magnetic intensity of the storage ring 10 and the kinetic energy of the electron beams 1. Electron flux steadily circulating along the equilibrium orbit 3 will run for several hours to several tens of hours with emitting synchrotron orbital radiation (not shown) and with having lost energy consumed by the synchrotron radiation supplied through the first and second radio frequency cavities 7a and 7b. The electron beam thus circulating the orbit is under a vacuum of 1.33 x 10⁻⁷ - 1.33 x 10⁻⁹ Pa (10⁻⁹ - 10⁻¹¹ torrs). When the circulating current increases more than hundreds of milliamperes, positive ions in the vacuum are accumulated in the electron beam and a collision occuring between the positive ions and the electron beam can become a problem. This will limit the amount of the electron beam running along the equilibrium orbit 3 and thus the current flow in the storage ring 10, which may lay down a large restriction on increasing the circulating current considerably when a high intensity light source using the synchrotron orbital radiation is required. That is, without providing means to eliminate positive ions produced by the collision between the electron beam circulating the orbit and gas in the vacuum, as accumulated current increases its volume, the circulating period of the current shall be inevitably shortened because of the positive irons being trapped in the electron beam. Figs. 3 and 4 show another embodiment in the concerned portion of the present invention which solves this problem. In the figures a first pair of negative and positive electrodes 18a and 18b respectively are disposed vertically which have the equilibrium orbit 3 in between, to remove the positive ions from the electron beam running along the equilibrium orbit 3. Likewise, a second pair through a fifth pair of negative and positive electrodes 19a and 19b to 22a and 22b respectively are diposed in sequence. The construction of this embodiment is identical to the embodiment shown in Fig. 1 other than the provision of electrodes in pairs. The operation of the modified embodiment will now be explained with reference to the first pair electrodes 18a and 18b. Between the first negative electrode 18a and first positive electrode 18b a voltage of several kV for example is applied to form an electric field between them. In the electric field charged particles with a positive charge will be accelerated toward the negative electrode 18a and charged particles with a negative charge will be accelerated toward the positive electrode 18b to obtain respective fixed energy. Since positive ions existing between the electrodes 18a and 18b along the electron flux have low kinetic energy, they are accelerated toward the first negative electrode 18a with their speed and direction depending on the distance between the electrodes 18a and 18b as well as their length. They then collide with the first negative electrode 18a and are neutralized. Kinetic energy with each electron in the electron beam running between the electrodes 18a and 18b is very large compared with the energy the electrons obtain from the electric field formed by the electrodes 18a and 18b. Therefore, the pair of negative and positive electrodes 18a and 18b installed along the orbit 3 will not adversely affect on the stable movement of the electron beam but will be effective in removing the positive ions. In the case when plural pairs of negative and positive electrodes are required as shown in the figures for removing a considerable amount of ions accumulated in tens of thousands of the electron beam circulation along the equilibrium orbit 3, however, an accumulated effect of the plural pairs of electrodes cannot be ignored. To minimize the abovementioned effect, adjacent pairs of negative and positive electrodes are disposed to be reversed in polarity to each other in this embodiment. For example, the second positive electrode 19b is disposed next to the first negative electrode 18a and the second negative electrode 19a is disposed next to the first positive electrode 18b, thus a stable electron beam circulation along the equilibrium orbit 3 is effected for a considerable number of times. The negative and positive electrodes in the abovementioned embodiment may be composed of flat conductors, curved conductors or plates in which conductors are installed in insulation. When eddy current induced on the electrodes by the interaction between the electron beam and electrodes should be avoided, electrodes having a mesh structure can be considered. Although the inflectors 2a through 2g for introducing the electron beam 1 gradually into the equilibrium orbit 3 using an electric field have been described for the aforesaid embodiments, a system using a magnetic field such as the bump electromagnet 6a can also be utilized. Likewise, the number of the inflectors will not be limited to seven and an inflector using an electric field can be used in place of the bump electromagnet 6a. The magnetic field for a storage ring formed by aircore coils as described can be replaced with iron-core electromagnets in the area near the equilibrium orbit 3 as is well known. Therefore, the present invention will not be limited to a system using air-core coils. Furthermore, the storage ring can be equipped with both synchrotron function and storage ring function, in which case the intended purposes can be achieved with the lesser number of inflectors since the kinetic energy of an injected electron beam can be made much lower than the energy stored in the ring.

Claims: Apparatus for accelerating charged particles comprising means defining a circular equilibrium orbit (3) for circulating charged particles having a weak focusing electromagnetic field, and inflector means for directing said charged particles from a particle source into said orbit, characterised in that the inflector means comprises a plurality of inflectors (2a to 2g) which are disposed along the introducing area of the charged particles with their radii of curvature directed towards the interior of the equilibrium orbit, so as to form an approximately circular arc, in such a way that their centres of curvature are located progressively inwardly towards the centre of the said equilibrium orbit (3). Apparatus as claimed in claim 1 further comprising a pair of negative and positive electrodes (18a, 18b) disposed vertically with the said equilibrium orbit between them. Apparatus as claimed in claim 2 in which a plurality of the said pairs of electrodes (18a, 18b - 22a, 22b) are disposed along the equilibrium orbit. Apparatus as claimed in claim 3 in which adjacent pairs of the said electrodes are reversed in polarity with one another. Apparatus as claimed in any of the preceding claims further comprising a plurality of radio frequency cavities (7a, 7b) along the said equilibrium orbit.

Inventors: FUKUMOTO, SHINTARO MITSUBISHI DENKI K.K., MIZOTA, MANABU MITSUBISHI DENKI KABUSHIKI KAISHA, NAKAMURA, SHIRO MITSUBISHI DENKI K.K., NARIKAWA, TAKEBUMI MITSUBISHI DENKI K.K., UEDA, KOJU MITSUBISHI DENKI KABUSHIKI KAISHA
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1986
Main classification: H05H7
IPC codes: H05H7