Measured strain in Nb3Sn coils during excitation and quence

Measured Strain in Nb 3 Sn Coils During Excitation and Quench S. Caspi, S.E. Bartlett, D.R. Dietderich, P. Ferracin, S.A. Gourlay, C.R. Hannaford, A.R. Hafalia, A.F. Lietzke, S. Mattafirri, M. Nyman and G. Sabbi Abstract— The strain in a high field Nb 3 Sn coil was measured during magnet assembly, cool-down, excitation and spot heater quenches. Strain was measured with a full bridge strain gauge mounted directly over the turns and impregnated with the coil. Two such coils were placed in a “common coil” fashion capable of reaching 11T at 4.2K. The measured steady state strain in the coil is compared with results obtained using the FEM code ANSYS. During quenches, the transient strain (due to temperature rise) was also measured and compared with the calculated mechanical time response to a quench. Index Terms—Strain gage, Superconducting Magnet, Quench. We describe the technique of mounting strain gauges (Section II), discuss measured strain data during assembly, cool down, and excitation (Section III) and report on transient strain during spot-heater induced quenches (Section IV). II. S TRAIN G AUGES AND T RACES A. Superconducting Coils Winding small Nb 3 Sn racetrack coils (~2kg each) is a good way of testing new ideas before their use in large magnets. The small coils, approximately 300mm long and 90mm wide, are wound as a double pancake and reacted at 650 C. Two such coils, SC13 and SC14, each with a surrounding stainless protection horseshoe, were placed facing each other in a “common coil” fashion and assembled into a structure (Fig. 1). The assembly used keys and bladders, taking full advantage of the difference in thermal expansion between the inner coils, iron yoke and the external aluminum shell. Testing coils this way was sufficient to withstand pre-stress requirements up to 11T (at approximately 10kA). I. I NTRODUCTION re-stress in superconducting magnets has always been considered a necessary part in minimizing conductor motion and reduce “training”. Measuring pre-stress however required special consideration due to the complex and none- linear nature of the assembly. Measuring sensors such as strain gauges or capacitor gauges are usually placed on the coil peripheral structure or collars in an attempt to determine the coil stress [1], [2]. In the past measured strain during assembly, cool down, excitation and warm up, suggested that coil stress may vary in non-linear fashion that is also history and time dependent. A phenomenon called “ratcheting”, measured in different magnet structures, suggested a possible connection between the behavior of coils under Lorentz forces, pre-stress, and training. We proceeded to impregnate strain gauges within the coil for the following reasons: 1) a simpler more reliable way to determine the coil stress 2) make strain measurements anywhere within coils 3) measure thermal and quench effects 4) shed light on training and better understand what can be done to eliminate it 5) compare with ANSYS strain calculations during a quench [3]-[5]. The method of placing strain gauges directly onto coils takes full advantage of the impregnation process typical in Nb 3 Sn coils. An alternative method of attaching strain gauges to coils after impregnation has been tried elsewhere [6], [7]. Manuscript received October 4, 2004. This work was supported under contract DE-AD03-76SF00098 by the Director, Office of Energy Research, Office of High Energy Physics, U.S. Department of Energy. All authors are with Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (phone: 001 510 486 7244; fax: 001 510 486 5310; e-mail: s_caspi@lbl.gov). P Fig.1. Exposed racetr ack coil surrounded by iron pads, iron yoke laminations, an d aluminum shell. Attaching strain gauges to coils and securing their position was done with a coil trace overlay. The trace, a ProE (CAD) drawing, included two full-bridge strain gauges, a spot-heater, and several voltage-taps (Fig.2). Drawn on vellum paper the trace was subjected to a photo etch process that resulted in a 25m thick stainless image over a 50m thick Kapton sheet.