TUA07P004 1 A Short Model Excitation of An Asymmetric Force Free Superconducting Transmission Line Magnet M. Wake, H. Sato, R. Carcagno, W. Foster, S. Hays, V. Kashikhin, A. Oleck, H. Piekarz, and R. Rabehl Abstract—A short model of asymmetric force free magnet with single beam aperture was tested at Fermilab together with the excitation test of VLHC transmission line magnet. The design concept of asymmetric force free superconducting magnet was verified by the test. The testing reached up to 104 kA current and no indication of force imbalance was observed. Since the model magnet length was only 10cm, A 0.75m model was constructed and tested at KEK with low current to ensure the validity of the design. The cool down and the excitation at KEK were also successful finding very small thermal contraction of the conductor and reasonable field homogeneity. Index Terms—VLHC, transmission line magnet, asymmetric, low field superconducting magnet, force free design. I. INTRODUCTION A prototype of a superconducting transmission line magnet system proposed for an injector accelerator in a staged VLHC p-p collider [1] has been built and successfully tested at Fermilab. A 1.5 m long, twin-aperture, combined function gradient dipole magnet of 2 T field is excited by a single-turn 100 kA superconducting transmission line [2]. The force free design of this type of magnet has a great advantage in the cryogenic efficiency and simplicity of the structure making construction of very high-energy accelerator, such as VLHC, realistic. However, the force free feature of this design is based on the symmetry. Therefore, the application of this technology is limited to colliders with twin apertures. The modification of the design to generalize the idea into a general purpose single aperture C type magnet was proposed by the introduction of a dummy gap filled with stainless steal [3], [4]. The test of VLHC magnet was carried out with very small section of such asymmetric magnet. The cross section of such magnet is shown in Fig. 1. The short model shares the same size of transmission line and has the same 2 cm vertical gap. The length of the magnet is 10 cm. Since the iron permeability changes with magnetic field, force free design of asymmetric magnet requires the balancing of iron saturation. The force balance can not be perfect at every excitation level but the electro-magnetic force can be designed to the level of practically no-force. It is obvious that the 10 cm magnet length is too short to see the full feature of the design in the excitation test. A 0.75 m model was also constructed and tested at KEK with low current. II. MAGNETIC DESIGN AND CONDUCTOR The geometry of the asymmetric no-force magnet shares the transmission line of the VLHC magnet with 80 mm diameter. The pole gap is also the same 2 cm. The magnetic design of the iron yoke was made by the iteration of numerical calculation. The non-linear permeability of the iron is essential to be included in the design. The saturation of the iron can be adjusted by the width and height of the dummy gap and holes in the edge of the gap as shown in Fig. 1. Fig. 2 shows the force balancing result calculated by ANSYS. The force balance is almost perfectly achieved up to 1.6 T. However, it very rapidly becomes unbalanced exceeding 1.6 T. The conductor to be used for this type of magnet has to, of course, carry a very large current. The current of 100 kA is not very difficult for multi-strand cable [5]. Important feature of the cable is to reduce the thermal contraction. Unlike usual superconducting magnet with many turns of thin conductors, this type of magnet does not allow the conductor thermal contraction without deforming the entire magnet system. The solution for this problem is to use braided cable hold in between invar pipes as shown in Fig. 3. Expansion of inner pipe and draw down of outer pipe were found to hold the superconducting braid tight. The thermal shrinkage of the cable can be controlled by this structure of the cable. The measured thermal shrinkage of the Fig. 1. Structure of asymmetric transmission line magnet. The shape of the magnet is almost the same as a general purpose C-type magnet. The dummy magnetic gap made by non-magnetic material produces a counter force on the conductor to cancel the electro-magnetic force. The real test magnet was built without gradient to check the field. Manuscript received September 20, 2005. M. Wake and H. Sato are with High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan. (e-mail: wake@post.kek.jp). R. Carcagno, W. Foster, S. Hays, V. Kashikhin, A. Oleck, H. Piekarz, and R. Rabehl are with Fermi National Accelerator Laboratory, Batavia Illinois, 60510 USA