# 1998 International Union of Crystallography Journal of Synchrotron Radiation Printed in Great Britain ± all rights reserved ISSN 0909-0495 # 1998 448 J. Synchrotron Rad. (1998). 5, 448±450 A superconductive undulator with a period length of 3.8 mm T. Hezel, a * B. Krevet, b H. O. Moser, a J. A. Rossmanith, c R. Rossmanith d and Th. Schneider e a Projektgruppe Errichtung ANKA, Forschungszentrum Karlsruhe, PO Box 3640, D-76021 Karlsruhe, Germany, b Institut fu È r Mikrostrukturtechnik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany, c Christopher Newport University, Newport News, Virginia, USA, d DESY Hasylab, D-22605 Hamburg, Germany, and e Institut fu Èr Technische Physik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany. E-mail: hezel@anka.fzk.de (Received 4 August 1997; accepted 6 November 1997 ) During recent years several attempts have been undertaken to decrease the period length of undulators to the millimetre range. In this paper a novel type of in-vacuum undulator is described which is built using superconductive wires. The period length of this special device is 3.8 mm. In principle, it is possible to decrease this period length even further. A 100-period-long undulator has been built and will be tested with a beam in the near future. Keywords: undulators; superconductivity; in-vacuum. 1. Introduction Following various publications on concepts of micro-undulators (Granatstein et al., 1985; Tatchyn & Csonka, 1987), work on a superconductive micro-undulator started in Karlsruhe in the early 1990s (Moser et al., 1991; Holzapfel, 1991). Independently, a short prototype of a superconductive undu- lator with an 8.8 mm period was built at Brookhaven (Ben-Zvi et al., 1990) following a slightly different concept. Field calculations and measurements were performed recently with a longer prototype (Ingold et al., 1996). Also independently, a group at Spectra Technology Inc. (Gottschalk et al., 1991) pointed out that FELs built with superconductive electromagnetic undulators might have advantages over a design with permanent magnets. Since that time the interest in reducing the period length of undulators has grown steadily (Stefan et al., 1991; van Vaer- enbergh, 1996; Tanabe et al. , 1997). In 1996, the Forschungszen- trum started experimental work on a superconductive undulator with a period length of 3.8 mm. First results were presented by Hezel et al. (1997) There are many reasons for building such undulators. They include (i) producing higher-energy photons with a given particle beam energy, and (ii) obtaining a given spectrum with lower energy machines with favourable consequences for the brilliance. Millimetre-period undulators might also play an important role in the development of X-ray lasers in the future. In principle, millimetre-period-length undulators can be built in various ways: they can be Halbach-type undulators (with permanent magnets), hybrid-type undulators or so-called elec- tromagnetic undulators. Electromagnetic undulators generate the ®eld by the current in a wire (Biot±Savart). Halbach-type undulators and hybrid undulators are dif®cult to build when the period length is in the millimetre region: mechanical problems make the design dif®cult (Tatchyn & Csonka, 1987; Rakowsky et al., 1997). Electromagnetic undulators, on the other hand, have the disadvantage that the required currents as well as the ohmic losses are relatively high. The use of superconductors instead of normal conductors reduces the ohmic losses to a negligible degree. For this reason ANKA is pursuing their development. The principal layout of the undulator is shown in Fig. 1. Undulators with short periods require a small gap. According to the well known formula of the ®eld strength in the gap as a function of the gap height, B gap  B 0 = coshg= u ; where B 0 is the ®eld at the pole, g is the gap height and  u is the undulator period, the period length should not be less than 4g in order to prevent reduction of the maximum ®eld by more than 20%. This poses a problem since the gap has to be in the millimetre range; therefore, the undulator must be integrated into the vacuum system. Otherwise, the thickness of the vacuum chamber will already signi®cantly reduce the strength of the maximum obtainable ®eld. The different superconductive mate- rials are selected according to their suitability for integration into a vacuum system, e.g. NbTi conductors are integrated into a copper matrix. The metallic copper surface is almost ideal for installment into a UHV environment. Nb 3 Sn technology appears less appropriate, and at the moment high-T c wires cannot handle the required current. Nevertheless, we feel that high-T c super- Figure 1 Layout of the superconductive undulator. Figure 2 Measurement of the quench current as a function of an external magnetic ®eld. Files: c:\acta-doc/el3120/el3120.3d, c:\acta-doc/el3120/el3120.sgm Paper number: S971569^EL3120 Paper type: SC N