Synchrotron radiation source

Abstract: An industrial, compact synchrotron radiation source comprises SR-light absorbers (31, 33) made of a material having a low gas desorption. The absorbers are positioned inside a bending section/vacuum chamber (1). At least the positions marked A and the surface of the electrically conductive beam stabilizers (61) are to be provided with the absorber.

Description: BACKGROUND OF THE INVENTION This invention relates to a synchrotron radiation SR source and more particularly to a SR source having beam stabilisers suitable for realizing a prolonged lifetime of the synchrotron radiation. As discussed in "Proceeding of the 5th Symposium on Accelerator Science and Technology" in the high energy laboratory reports, 1984, pp. 234-236, conventional accelerators and large-scale SR sources are known wherein bending sections, each of which deflects the orbit of a charged particle beam for causing the synchrotron radiation to be taken out of the source, are not collectively disposed in a relatively short range of the beam duct or bending duct, but disposed with spaces between them where straight sections are disposed so that the bending sections are uniformly distributed as a whole in the beam duct or bending duct. Accordingly, sources of gases discharged from the interior wall surface of the vacuum chamber under the irradiation of synchrotron radiation are substantially uniformly distributed along the orbit of the charged particle beam and besides gases discharged from the bending sections under the irradiation of the synchrotron radiation can be evacuated by not only built-in pumps installed inside the charged particle bending section but also vacuum pumps installed in an adjacent straight section, thereby ensuring that the vacuum chamber can be maintained at high vacuum and a long lifetime of the charged particle beam can be maintained. Conventionally, portions irradiated directly with the synchrotron radiation are made of a stainless steel material or an aluminum alloy material. When irradiated with the synchrotron radiation, the above material discharges a large amount of gases under the influence of the photo-excitation reaction. Since the amount of discharged gases is very large amounting to 10 to 100 times the outgassing amount due to mere thermal discharge, a great number of vacuum pumps must be installed in order to maintain the interior of the vacuum chamber of high vacuum. Further, when the bending angle of charged particle beam obtained by one bending section is designed to be large for the sake of realizing compactness of the SR source, the amount of gases discharged from one bending section is increased and a great number of vacuum pumps must be installed. However, because of a limited installation space, the number of pumps to be installed is limited, raising a problem that the interior of the vacuum chamber can not be maintained at high vacuum and the lifetime of the charged particle beam is shortened. Moreover, in compact SR sources for industrial purposes, because of desirable cost reduction, the bending section for delivery of synchrotron radiation has to be laid concentratedly. Taking a compact SR source comprised of two straight sections and two bending sections, for instance, it is necessary for one bending section to 180° deflect the charged particle beam orbit and as a result, the amount of gases generated by each bending section under the irradiation of synchrotron radiation upon the interior wall surface of a portion of the vacuum chamber corresponding to one bending section is increased extremely, reaching about 10 times the amount of discharged gases generated by each bending section in the case of the large-scale SR source. Accordingly, if the configuration of the vacuum chamber and the layout of vacuum pumps in the large-scale SR source are directly applied to the compact SR source without alternation, then there will arise a problem that pressure in the vacuum chamber rises and the lifetime of the charged particle beam is shortened. A countermeasure for solving the above problems has been proposed wherein the shape of the bending section/vacuum chamber is made different from the conventional duct form of the bending section/vacuum chamber of the large-scale SR source so as to take the form of a sector or a semi-circle and in addition, vacuum pumps are installed near the outer circumferential wall of the bending section/vacuum chamber and SR guide ducts extend from the outer circumferential wall. With this proposal, the vacuum evacuation performance can be comparable or superior to that of the conventional large-scale SR source but disadvantageously the orbit of the charged particle beam tends to be unstable. More particularly, the sector or semi-circular form of the bending section/vacuum chamber tends to adversely interfere with the orbit of the charged particle beam guided to the bending section, thereby inducing a high-frequency electric field (called a wake field) which makes unstable the orbit of the charged particle beam. SUMMARY OF THE INVENTION The present invention contemplates elimination of the above problems and has for its object to provide a SR source being capable of making stable the orbit of the charged particle beam so as to prolong lifetime of the synchrotron radiation. According to the invention, the above object can be accomplished by disposing electrically conductive beam stabilizers at positions inside the bending section/vacuum chamber which have a distance from the charged particle beam orbit toward the outer circumferential wall of the bending section/vacuum chamber, which is substantially equal to the distance between the charged particle beam orbit and the inner circumferential wall. By disposing electrically conductive beam stabilizers at positions inside the bending section/vacuum chamber which have a distance from the charged particle beam orbit toward the outer circumferential wall of the vacuum chamber which is substantially equal to the distance between the charged particle beam orbit and the inner circumferential wall of the vacuum chamber, the bending section/vacuum chamber having a cross-sectional form which expands two-dimensionally can electrically be treated as a straight section beam duct having a nearly circular or elliptical cross-sectional form, so that the charged particle beam orbit can be made stable which would otherwise be disturbed by the induced wake field. Thanks to the stable charged particle beam orbit, the charged particle beam will not be attenuated by deviating from the orbit to the interior wall surface of the vacuum chamber and its lifetime can be prolonged. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view illustrating a SR source having beam stabilizers according to an embodiment of the invention. Figure 2 is a sectional view taken on the line X - X' of Fig. 1. Figure 3 is an enlarged fragmentary view of Fig. 1. Figure 4 is a sectional views taken on the line XII - XII' of Fig. 3. Figure 5 is a view as seen in the direction of arrows T in Fig. 3. Figure 6 is a sectional view taken on the line XIV - XIV' of Fig. 5. Figures 7 and 8 are sectional views illustrating other embodiments of the beam stabilizer. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described by way of example with reference to the accompanying drawings. A bending section/vacuum chamber according to an embodiment of the invention incorporates beam stabilizers as will be described below with reference to the drawings. Fig. 1 illustrates, in plan view form, a bending section/vacuum chamber of industrial compact SR source. In Fig. 1, the bending section/vacuum chamber, simply referred to as vacuum chamber 51 hereinafter, has the form of a substantially C-shaped semi-circle and has one end at which a charged particle beam enters the vacuum chamber and the other end at which the charged particle beam leaves the vacuum chamber. The outer circumferential wall of the vacuum chamber 51 protrudes beyond the outer circumferential edge of a core of a bending electromagnet (not shown) to provide an extension from which five SR guide ducts 53 for delivery of the synchrotron radiation extend and at which eight vacuum pump sets 52 are installed. Inside the vacuum chamber 51, elongated supports 55 bridge the upper and lower walls of the vacuum chamber and protrude through these walls to support the bending electromagnet. The supports 55 longitudinally extend, at positions remote from the outer circumferential wall of the vacuum chamber 51, in a direction which is parallel to the SR beam. Each support 55 is provided at a position intermediate to adjacent two of the SR guide ducts 53. Also installed inside the vacuum chamber 51 are six beam stabilizers 61 made of copper and insert plates 62 made of stainless steel serving as supports for the beam stabilizer 61 and also as supports for the side walls of the vacuum chamber. Each insert plate 62 is connected with a water cooling pipes 65 adapted to cool each beam stabilizer 61. The beam stabilizer 61, insert plate 62 and water cooling pipes 65 are put together to form an assembly which can be inserted into the vacuum chamber 51 through an insertion port 64 formed in the outer circumferential wall of the vacuum chamber. The beam stabilizers 61 are disposed at positions which are distant by a distance ℓ (equal to the width of the straight section beam duct) from the inner circumferential wall of the vacuum chamber 51, and the orbit of a charged particle beam is so controlled as to be centered between each beam stabilizer 61 and the inner circumferential wall. For better understanding of the overall construction of the vacuum chamber 51, reference should be made to Fig. 2. In Fig. 2, the vacuum chamber of Fig. 1 is sectioned along the line X - X and the bending electromagnet is designated by reference numeral 58 and associated with the core as designated by 57 to form a magnetic circuit. The vacuum chamber 51 is inserted between upper and lower halves of the core 57 and bending electromagnet 58 and the bending electromagnet 58 is supported by the supports 55 which vertically protrude through the vacuum chamber 51. Ion pump 52a and titanium getter pump 52b of each vacuum pump set 52 are respectively mounted to the upper and lower end surfaces contiguous to the outer circumferential wall of the vacuum chamber 51. Since the vacuum pump sets 52 are mounted to the end portion in this way, their interior can obviously escape the direct irradiation of synchrotron radiation 54. As described previously, the beam stabilizers 61 and insert plates 62 are also installed within the vacuum chamber 51. The configuration of the beam stabilizer 61 will now be detailed with reference to Figs. 3 to 6. The beam stabilizer 61 shown in Fig. 1 is positionally related to an orbit 56 of the charged particle beam and the synchrotron radiation 54, as diagrammatically shown in Fig. 3. SR beams 54a and 54b respectively stemming from points A₁ and B₁ on the charged partial beam orbit 56 reach end points A₂ and B₂, close to the insert plate 62, of the beam stabilizer 61. Line segments A₂A₁ and B₂B₁ are representative of tangents at the points A₁ and B₁ on the orbit 56, respectively, and coincide with the trace of the SR beams. Since the insert plate 62 supporting the beam stabilizer 61 lies within a region between extensions of the line segments A₂A₁ and B₂B₁, opposite side surfaces and the outer end surface of the insert plate 62 can escape the direct irradiation of the synchrotron radiation. The insert plate 62 has a height equal to an inner height of the vacuum cahmber 51, and the beam stabilizer 61 is internally hollowed to form a cross-sectionally rectangular cavity and is suspended within the vacuum chamber 51. The water cooling pipe 65 is fixedly attached by welding to the insert plate 62 and beam stabilizer 61. The above construction will be described more specifically with reference to Figs. 4 to 6. Fig. 4 is a sectional view taken on the line XII-XII' of Fig. 3, demonstrating the positional relation of the insert plate 62 to the vacuum chamber. The upper and lower ends of the insert plate 62 are in contact with the interior surface of the vacuum chamber 51 but they are not fixed thereto by, for example, welding so that the insert plate 62 can be inserted into the vacuum chamber 51 through the insertion port 64 in the outer circumferential wall in airtight fashion. Two sections of the water cooling pipe 65 are fixed by welding to the upper and lower end sides of the insert plate 62. Fig. 5 is a view as seen in a direction of arrows T in Fig. 3, demonstrating the positional relation of the beam stabilizer 61 to the vacuum chamber. As described previously, the beam stabilizer 61 has the rectangular cavity and it is suspended within the vacuum chamber 51, leaving behind upper and lower spaces as illustrated in Fig. 5. The water cooling pipe 65 is also fixed by welding to the outer (back) surface of the beam stabilizer 61, as best seen from a XIV-XIV' section of Fig. 5 illustrated in Fig. 6. Returning to Fig. 3, upper and lower hooks 66 are provided on the interior surface of the vacuum chamber 51 and act to effect positioning of end corners of the beam stabilizer 61 and the insert plate 62. Although not shown, the height of each hook 66 is not so large as to bridge the vacuum chamber 51 but is designed to take a value which is sufficient for positioning of the beam stabilizer 61 and insert plate 62, measuring 3 to 5 mm, for example. Accordingly, the SR beam does not irradiate the hook 66 directly. The operation and effect of this embodiment will now be described. As shown in Fig. 1, a charged particle beam entering the bending section/vacuum chamber 51 traces the nearly circular orbit 56 under the influence of a magnetic field generated from the bending electromagnet and leaves the exit of the vacuum chamber 51. The synchrotron radiation 54 is radiated tangentially of the charged particle beam orbit 56. The radiation 54 is partly guided to the outside through the SR guide duct 53 and partly irradiated directly on the inner end of the support 55, the inner surface of the beam stabilizer 61 and the interior surface of the outer circumferential wall of the vacuum chamber 51 to cause outgassing of a large amount of gaseous molecules on the basis of the photo-excited separation phenomenon. The area of the interior surface of outer circumferential wall of vacuum chamber 51 is much larger than the area of the other portion. Therefore, most of gases prevailing in the vacuum chamber 51 are discharged from a gas discharge source on the interior surface of the outer circumferential wall. Since a number of vacuum pump sets 52 disposed close to the outer circumferential wall of vacuum chamber 51 then lie in the vicinity of the gas discharge source, discharged gaseous molecules can immediately be evacuated to the outside of the SR source. The vacuum pump sets 52 disposed near the gas discharge source can have a larger effective evacuation rate than is disposed at other site and advantageously the SR source can be maintained under high vacuum condition and lifetime of the charged particle beam can be prolonged. Most of gas discharge sources are remote from the charged particle beam orbit 56 and gases discharged from these sources can hardly affect the charged particle beam adversely. The stability of the charged particle beam orbit will particularly be described. The width ℓ and height of the straight section beam duct are designed through analysis of a wake field in the straight section so as to take values by which the charged particle beam orbit can be stabilized. But due to the fact that the configuration of the bending section/vacuum chamber 51 is expanded two-dimensionally in contrast to that of the straight section, the wake field in the vacuum chamber can not be analysed accurately and the charged particle beam orbit tends to be unstable. In the present embodiment, however, the charged particle beam orbit 56 in the bending section is established in a space which is defined by an inner contour corresponding to the inner circumferential wall of the vacuum chamber 51 and an outer contour corresponding to the beam stabilizers 61 and therefore, the bending section/vacuum chamber can electrically be treated as the straight section beam duct. Accordingly, the charged particle beam orbit 56 can be stable in the vacuum chamber as in the straight section. Further, the beam stabilizer 61 is hollowed to form a cross-sectionally rectangular cavity which has a small area irradiated with the synchrotron radiation and besides it is made of a copper material from which a small amount of gases is discharged under the irradiation of the synchrotron radiation. Accordingly, even with the beam stabilizers 61 disposed inside the vacuum chamber 51, the interior of the vacuum chamber can be maintained at high vacuum and lifetime of the charged particle beam can be prolonged. At the commencement of the rise of the charged particle beam, the charged particle beam orbit goes through the vacuum chamber 51 at a height which is about 1/2 of the inner height of the vacuum chamber. On the other hand, the cross-sectionally rectangular cavity is centered with the beam stabilizer 61 per se as will be seen from Fig. 5 and therefore the synchrotron radiation will irradiate only a part of either end of the beam stabilizer 61, with the result that the amount of gases discharged from the beam stabilizer 61 can be minimized and the rise time of charged particle beam can be minimized. The beam stabilizer 61 heated by the irradiation can be cooled through the cooling water pipe 65 and its temperature rise can be suppressed to below a permissible value. As shown in Fig. 3, one end of the insert plate 62 is encompassed with the beam stabilizer 61 to escape the direct irradiation of the synchrotron radiation 54, thereby minimizing the gas generation amount. Advantageously, by manipulating the water cooling pipe 65, the assembly of beam stabilizer 61, insert plate 62 and water cooling pipe 65 can readily be mounted to or dismounted from the vacuum chamber through the insertion port 64 in the outer circumferential wall without resort to disassembling of the being electromagnet and the like parts. In the present embodiment, the beam stabilizer of copper is used to minimize the outgassing amount under the irradiation of the synchrotron radiation but a beam stabilizer made of aluminum may be used to attain substantially the same effect. Fig. 8 illustrates another embodiment of the beam stabilizer wherein the cross-sectionally rectangular cavity shown in Fig. 5 is partitioned into two smaller cavities. With this embodiment, the gas generation amount under the irradiation is slightly increased but the stability of the charged particle beam can further be improved. The Fig. 8 embodiment and also still another embodiment of Fig. 7 of beam stabilizer having one cavity as in the case of the Fig. 5 embodiment are also featured in that the opposite ends of the beam stabilizer 61 are in contact with the upper and lower walls of the vacuum chamber 51 to establish an electrically closed cycle. By this configuration of Figs. 7 and 8, the stability of the charged particle beam can further be improved. It should also be appreciated that because of the absence of the beam stabilizers at portions for delivery of the synchrotron radiation 54, disturbance of synchrotron radiation 54 due to beam stabilizer can be prevented when the charged particle beam is brought into the operation mode in which the charged particle beam is moved vertically. As has been described, in the SR source of the present invention, due to the electrically conductive beam stabilizers disposed at positions inside the bending section/vacuum chamber which are distant by a predetermined distance from the charged particle beam orbit toward the outer circumferential wall of the vacuum chamber, the bending section/vacuum chamber having a cross-sectional form which expands two-dimensionally can electrically be treated as a straight section beam duct having nearly a circular or elliptical cross-sectional form, so that the charged particle beam orbit can be made stable which would otherwise be disturbed by the induced wake field and consequently, the charged particle beam will not be attenuated by deviating from the orbit to the interior wall surface of the vacuum chamber and its lifetime can be prolonged.

Claims: A synchrotron radiation source, comprising a beam duct which includes straight sections as well as a bending section/vacuum chamber (51), the bending section/vacuum chamber (51) having one end which a charged particle beam enters and the other end which the charged particle beam leaves and having a larger cross-section than the straight sections, and a bending electromagnet (58) disposed so as to encompass said bending section/vacuum chamber characterised in that at least one electrically conductive beam stabilizer (61) is provided outside of an orbit (56) of said charged particle beam within said bending section/vacuum chamber (51) so as to be spaced from an inner circumferential wall of said bending section/vaccum chamber (51) by a distance substantially equal to the width (ℓ) of a straight section of the synchrotron radiation source such that said orbit (56) of said charged particle beam is centered between said beam stabilizer (61) and said inner circumferential wall. A synchrotron radiation source according to claim 1, characterised in that said beam stabilizer (61) is made of copper or aluminum. A synchrotron radiation source according to claim 1, characterised in that said beam stabilizer (61) can be inserted into said bending section/vacuum chamber (51) through an insertion port (64) formed in the outer circumferential wall of said bending section/vacuum chamber (51). A synchrotron radiation source according to claim 1, characterised in that said beam stabilizer (61) is internally hollowed to form a cross-sectionally rectangular cavity. A synchrotron radiation source according to claim 4, characterised in that said cavity is partitioned into two smaller cavities. A synchrotron radiation source according to claim 1, characterised in that said beam stabilizer (61) is cooled with water. A synchrotron radiation source according to claim 1, characterised in that said beam stabilizer (61) is supported by an insert plate (62) also serving as an airtight support for said bending section/vacuum chamber (51) and is cooled through a cooling pipe (65) fixed to said insert plate (62) and that said beam stabilizer (61), insert plate (62) and cooling pipe (65) are put together to form an assembly.

Inventors: IDO SATOSHI, IKEGUCHI TAKASHI, KUROISHI KAZUO, MATSUMOTO MANABU, MURASHITA TORU, SHIBAYAMA AKINORI, SONOBE TADASI, UEDA SHINJIROO, IDO, SATOSHI, IKEGUCHI, TAKASHI, KUROISHI, KAZUO, MATSUMOTO, MANABU, MURASHITA, TORU, SHIBAYAMA, AKINORI, SONOBE, TADASI, UEDA, SHINJIROO
Applicants: HITACHI ENG SERVICE, HITACHI LTD, NIPPON TELEGRAPH & TELEPHONE, HITACHI ENGINEERING AND SERVICES CO., LTD., HITACHI, LTD., NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1988
Main classification: H05H13
IPC codes: H05H7