Superbrilliant xray undulator for the Tristan Super

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pole is adjusted on the basis of a precise magnetic measure- ment using a Hall generator and a precise three-axes transla- tion stage. As a coarse adjustment, ...
Superbrilliant xray undulator for the Tristan Super Light Facility Shigeru Yamamoto, Tatsuro Shioya, Hideo Kitamura, and Kimichika Tsuchiya Citation: Rev. Sci. Instrum. 66, 1996 (1995); doi: 10.1063/1.1145780 View online: http://dx.doi.org/10.1063/1.1145780 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v66/i2 Published by the AIP Publishing LLC.

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Superbrilliant

x-ray undulator

Shigeru Yamamoto,

for the Tristan Super Light Facility

Tatsuro Shioya, Hideo Kitamura, and Kimichika Tsuchiya

Photon Factory, National Laboratory for High Energy Physics, KEK, Tsukuba, Ibaraki 305, Japan

{Presented on 19 July 1994) We have developed a construction method for very long undulators which are potentially capable of having various magnetic specifications and fit long straight sections in third- or fourth-generation light sources. A prototype device was built by constructing three rigid and precise standardized-unit undulators and connecting them to form a 5.4-mm-long undulator. This prototype will also be used as a superbrilliant light source for a feasibility study, which is planned in 1995, using the Tristan Main F&g. 0 199~Americmz Institute of Physics. -

I. INTRODUCTION Since the so-called third-generation light sources have been realized at several sites in the world at present, the design and construction of superbrilliant and coherent light sources should be a next target. Along this line, we have planned to convert the Tristan Main Ring (MR) from a highenergy collider at the National Laboratory for High Energy Physics, KEK, to a superbrilliant light source, which is to be called the “Tristan Super Light Facility (TSLF).r’1Y2 In the original plan, TSLF was characterized by a very low electron-beam emittance of the order of 1 nm rad, by 70-m-long undulators for hard x rays, and by free-electron lasers for soft x rays (Fig. 1).lv3 Figure 2 shows the distinguishing features of the TSLF compared with other synchrotron radiation (SR) facilities. Since the total length of the undulators for the TSLF amounts to more than 400 m, it is not practical to design and construct such undulators using a device-by-device policy from viewpoints of both the design and the construction costs as well as from that of manpower for commissioning and operation. In order to avoid this situation we have been carrying out studies by constructing a prototype undulator,1’5 in which: (1) three unit undulators are used to form a 5.4 m prototype undulator; (2) for long undulators with about 70 m length, the mechanical frames used for this prototype will be reproduced and connected longitudinally. The purpose of this paper is to describe our scheme concerning the design and construction of this 5.4 m prototype undulator (named XU#MRO). Although our original plan for converting the MR was not finally approved, we will be able to carry out a feasibility study using the MR in the autumn of 1995. The XU#MRO will be used for a feasibility study as a superbrilliant x-ray source. Although the MR will not be completely converted, compared to the original TSLF, we will be able to obtain a natural-beam emittance of 5 nm rad at the MR’s operation energy of 10 GeV, owing to an adequate and minimum modification of the MR lattice.’ In this modification, a free space will be created for installing the XU#MRO by replacing two normal bending magnets with two short bending magnets; low emittance will be achieved by increasing the phase advance of the betatron oscillation to 90” from the present value (60”) and by using emittancedamping wigglers. The spectrum obtained in this case is also shown in Fig. 2. Further, our experience obtained by con1996

Rev. Sci. Instrum. 66 (2), February 1995

structing the XU#MRO will be useful in any future plan requiring very long undulators,7 and will also be useful for constructing third-generation light sources, in some of which a segmented-undulator scheme is to be employed.’ II. STRUCTURE OF THE XU#MRO The parameters of the XU#MRO are given in Table I. Three standardized-unit undulators (1.8 m long) are placed very precisely on a rigid common frame to form a 5.4 m undulator, as shown in Fig. 3. We adopted a pure-Halbach type configuration of NdFeB magnets (remanent field, B,=12.8 kG, coercivity, iH,=17 kOe). The selection of the period (=4.5 cm) was made so that the Miissbauer energy of S7Fe(=14.4 keV) could be obtained at the first harmonic at 10 GeV operation of the MR. This requires a deflection parameter of K= 0.97 at a minimum magnet gap of 3 cm. The magnets are arranged in an out-of-vacuum scheme. The support structure of the X&MRO is shaped like the letter “C” so that it can be installed without breaking the vacuum of the MR. In order to standardize the mechanical structure of the XU#MRO and to make it applicable to various magnetic specifications, the following items were considered. A. Adoption

of a spring system

In the present support structure, the magnetic attractive force tends to violate parallelism between the upper and

FIG. 1. Schematic illustration of the TSLF.

0034.6746/95/66(2)/1996/3/$6.00

Q 1995 American institute of Physics

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10

12

14

16

18

20

Photon energy (keV) FIG. 3. Photograph of the XU#MRO. FIG. 2. Comparison of the brilliance of the TSLF and other SR facilities. MR-L and -5, respectively, denote those obtained from a 70 m undulator (N=1600 and X,=4.5 cmj and a 5.4 m one (iv’=120 and X,=4.5 cm) planned in the original TSLF [lo GeV operation with a natural emittance of 1.5 MI rad (Refs. 1 and 3)]. SP8 (Harima, Japan; 8 GeV, N=lOO, and X,=5 cm) and Electron Synchrotron Radiation Facility (ESRF) [Grenoble, France; 6 GeV, N=112 and X,=4 cm] denote those from third-generation SR facilities. The source parameters for the SP8 and the ESRF were picked up from Ref. 4. MRT stands for that obtained from the XU#MRO in the MR [lo GeV with 5 nm rad (Ref. 6)] for the feasibility study. It should be noted that the brilliance for our feasibility study (MRT) is obtained at 10 mA, whereas those from other facilities, including the original TSLF, are obtained at 100 rm4. The undulators are assumed to be in the out-of-vacuum configuration.

longer h, gives a higher field and a larger distortion, its increase can be limited. If the undulator radiation is characterized by K values less than 5, the maximum field is limited to below 6.5 kG at h,=8.3 cm, as shown in Fig. 5. This value of the magnetic field gives a maximum attractive force of 1400 kgf.; the distortion is suppressed below 1 pm regardless of A,, by adopting the present design of the girders, whose height is 35 cm (Fig. 3). C. Flexibility

lower girders on which magnets are attached. In order to eliminate this, both girders are equipped with special spring systems from the open side of the letter C to share the magnetic force. Each system has a set of springs; the stiffer springs work at progressively~ closer gaps to support the 1oad.r” The load-gap relation in the case of the present magnetic specifications is indicated by the squares in Fig. 4. Although the load shows an exponential dependence on the gap, it can be approximated by the force-up relation of the present spring system, which is indicated by the bold line in Fig. 4. B. Optimization

of magnet-mounting

for selection

of X,

The selection of X, is constrained by the reducibility of the girder length with respect to X,/4 for the pure configuration and A,/2 for the hybrid configuration. A girder length of 1.8 m was selected due to this viewpoint: in the pure configuration, the allowable values of h, from 4 to 1.2 cm are 4.0, 4.5, 4.8, 5.0, 6.0, 7.2, 7.5, 3.0, 9.0, 9.6, 10.0, and 12.0 cm. For an east selection of any of these period lengths we can use a set of parahel plates (Fig. 3). Each set has holes through which the magnets are bolted on the plates. The pitch of these holes fits only a particular X,. In order to eliminate problems which are caused by any difference in the thermal expansion between the girder (made of steel) and the

girders

In order to optimize the stiffness of the girders and to suppress the distortion within an allowable range, we need to know the maximum force applied to them. Figure 5 shows the maximum magnetic field as a minimum gap of 3 cm as a function of the magnetic period length (X,). Although the

250 200 150

100 JO

TABLE I. Parameters of XU#MRO Magnetic structure Magnetic material Period length Number of periods Magnet length Maximum peak field Maximum I\: Range of magnet gap Aperture

0 Pure configuration NdFeB (B,=12.8 kG, iH,=17 kOej

h,=4.5

cm

120 [=3X(40/unit undulator)] 5.4 m [=3X(1.8 m/unit undulator)] B =2.64 kG K=l.ll 3-50 cm 2.4 cm

Rev. Sci. Instrum., Vol. 66, No. 2, February 1995

-50

30

40

50

60

Gap (mm) FIG. 4. Load-gap (or stroke) relation of the spring system. The squares indicate the required load to be shared by a single spring system. The load can be supported by this system if it has an effective force-gap relation, indicated by the bold line with effective spring constants (values are given in the figure).

Synchrotron radiation

1997

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8 25 3 t A? 2 P,

(-L-L]

8 3

6

fl ~~~~~~~ [~~&miled

I?‘d/.

2 0

5

10

15

] “:*

8

20 after exchanging ,

Period length (cm) FIG. 5. Maximum magnetic field giverras a function of A, (a bold line). The relations of the magnetic field and X, with a constant K value are also shown (thin lines). The maximum field is limited to below 6.5 kG at X,=8.3 c m under the condition of K values less than 5.

parallel plates (of stainless steel), we made thin openings between the unit undulators when we mounted the magnets. Besides the above items, relaxation of the thermal expansion should be one of the most significant problems when we construct very long undulators, especially in making the vacuum chamber for the out-of-vacuum configuration. This problem seems to be resolved by adopting an m-vacuum contiguration’0 of the magnets, which is also allowed by the present design of the structure of the XU#MUO.’ III. MAGNETIC FIELD OF THE XU#MRO The magnetic field of the XU#MRO is optimized so that the electron orbit is close to an ideal sinusoidal curve. For this purpose, the kick angle of the electron at each magnetic pole is adjusted on the basis of a precise magnetic measurement using a Hall generator and a precise three-axes translation stage. As a coarse adjustment, the magnets at a pole having a larger kick angle (usually a higher field strength) were exchanged with those at another pole having a smaller one (a lower strength). For a fine adjustment, disk-shaped magnet chips were used in order to decrease the scatter of the kick angles. Each holder which mounts the magnet block has holes in which these chips are embedded. Figure 6 shows the result of a field optimization in terms of the electron orbit. As a criterion for the field optimization, we used the following conditions concerning the deflection angle (L&s) and the effective amplitude (s) of the envelope of the electron orbit in order to obtain good transverse coherence of the radiation: @+&a~ =(X/NXJri2, and 6~ CT,.=(XNX,) “‘/47r. Here, ai is the divergence angle of the undulator radiation having a wavelength of X from a single electron; u, is the corresponding size of the radiation. We adopted a value of C: =4 prad as the critical deflection angle, and one of u,=1.8 pm as the critical amplitude, since we designed the XU#MRO so that its first harmonic could be

1998

10”m

4

0

~

Rev. Sci. Instrum., Vol. 66, No. 2, February 1995

1

-2000

-1000

0 z (mm)

...-D.._.” e’ ’ IT -....................... mmonp’ 1 1000

2000

FIG. 6. Electron orbit in the XU#MRO when K= 1.1. It was adjusted by exchanging magnets from its received state, and finally optimized by using magnet chips (see the text).

tuned at 14.4 keV (0.86 A in terms of h). The optimization was performed fairly successfully (Fig. 6), and the deflections of the envelope became less than 0.2 prad and the amplitude less than 1 m. The variation in the ambient magnetic field, which mainly comes from changes in the accelerator operation and from geomagnetism, is crucial for the fine magnetic adjustment mentioned here. We devised a correction system for such a variation, and installed it in the XU#MRO. This system comprises a detector using Hall generators and correction coils wound round the upper and lower magnet arrays of the IXU#MRO. The details have been given elsewhere.” ACKNOWLEDGMENT The authors express their sincere thanks to Professor M. Ando for his help and critical discussion concerning the present study. ‘The Tristan Super Light Facility; Conceptual Design Report 1992, K E K Progress Report No. 92-1, 1993. ’ M. Ando, Rev. Sci. Instrum. (these proceedings). “S. Kamada, 1992 Photon Factory Activity Report, S-2, 1993. ‘R. Haensel, Rev. Sci. Instrum. 63, 1571 (1992); H. Kamitsubo, ibid. 63, 158671992). 5S. Yamamoto, 1992 Photon Factory Activity Report, S-7, 1993. 6S. Kamada, Rev. Sci. Instrum. (these proceedings). ‘Proceedings of the workshop on scientific application of coherent x rays, SLAC Report No. 437, 1994. ‘For example: G. K. Shenoy, P. J. Viccaro, and E. E. Alp, Rev. Sci. Instrum. 60, 1820 (1989); J. Chavanne, E. Chinchio, M. Diot, P. Elleaume, D. Frachon, X. Marechal, C Mariaggi, and F. Revol, ibid. 63,317 (1992); C. Piloni, ibid. 6.3,380 (1992). “S. Yamamoto, 1990 Photon Factory Activity Report, R-11. ‘OS. Yamamoto, T. Shioya, M. Hara, H. Kitamura, X. Zhang, T. Mochizuki, H. Sugiyama, and M. Ando, Rev. Sci. Instrum. 63,400 (1992). “S Yamamoto, Proceedings of the International Workshop on Magnetic Measurements of Insertion Devices, ANL/APS/TM-13, 186 (1993).

Synchrotron radiation

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