Design optimization of the protection heaters for the LARP high-field Nb 3 Sn quadrupoles M. Marchevsky, D. W. Cheng, H. Felice, G. Sabbi, Lawrence Berkeley National Laboratory, T. Salmi, Tampere Institute of Technology, G. Chlachidze and G. Ambrosio, Fermi National Accelerator Laboratory, V. Marinozzi, University of Milan, E. Todesco, European Organization for Nuclear Research (CERN) Peak power density W/cm 2 HFU voltage up to 450 V HFU current: up to 220 A HFU capacitance: mF Distance between heating stations: up to 120 mm (Could be related to the transposition pitch of 109 mm) Trace parameters: Kapton Insulation thickness: 50 µm Stainless Steel thickness: 25 µm Copper thickness: 10 µm Glue thickness: up to 25 µm Coil surface coverage by trace: < 50 % IL Distance from heater to coil or voltage taps: 4 mm or more Large spacing L between the heating stations -> higher surface power density -> shorter  1, but longer  2 of the quench propagation between the heated areas Small spacing L between the heating stations > smaller heater power -> longer  1, but shorter  2 of the quench propagation between the heated areas Power density > 50 W/cm 2 is desirable Reducing insulation thickness is critical for achieving a shorter heater delay Superconducting accelerator magnet normally require active quench protection: upon detecting the quench, the goal is to create the largest normal zone in the shortest possible time. One well-known way to use a protection heaters that spreads quench propagation within the coil winding As heaters are placed on the coil surface, there is an insulation barrier between heater and the supercondutor, and hence, an associated heat diffusion time. When coils are short, a continuous heating strip of higher-resistivity material (usually stainless steel) can be used. Example shown: protection heater of the “HQ” LARP high-field quadrupole For a long magnet, a continuous heater strip cannot be used due to large resistance, and therefore a concept of “heating stations” is used to induce quench zones with some periodicity along the coil. Example shown: heater of the LARP “LQ: long quadrupole. Heater delay time (interval between firing the heater and the voltage development across the conductor) measured experimentally for the LQ magnet as function of the peak heater power Uniform strip (  1,  2 =0) Strip with heating stations (  1,  2 ) Numerical simulations of heater delay A model has been developed calculate the heater delay; details can be found in: Making heater periodicity commensurate with the twist pitch of the cable may be expected to further improves heater performance. If p = 2nw and l = (2n+/-1) w, then the supercurrent in all strands This approach can potentially improve heater efficiency, as all cable strands will get resistive and start dissipating heat at once. of the cable segment of length L=nl can be “interrupted” simultaneously by the normal zones created with n heating stations. Experimental heater patterns for the existing LARP magnets 1. Heater patter for the HQ03 – new LARP quadrupole. Heating station periodicity is 2x the cable twist pitch. Heater for the QXF - new LARP magnet for LHC luminosity upgrade 2. Heater pattern for the outer layer of the LHQ – a 3m LARP coil. The top portion design is based on the optimization algorithm [1-3] to provide a simultaneous quenching of high and low-field zones. The bottom portion is the old-style “LQ” pattern. The test is to be completed in September LPo2K-03 Introduction SQXF Coil 1 - a modification of the periodic heating station concept  Reduced width of the copper-plated bridges (more space available for holes)  Increased width of the heating station  Cooper-plated terminals of the heating stations to improve current flow uniformity Optimization is done by minimizing the sum of (PH delay + quench propagation time between the HS). Other designs with selective copper plating are currently in development for the QXF coils: The following list of heater parameters have been by the LARP-CERN collaboration for QXF magnet development: Inner layer heater layout: Four heater strips per outer layer and two per inner layer are planned for redundancy Power (W/cm 2 ) LHS (mm) Period (mm) PH delay (ms) Tot. Delay (ms) Power (W/cm 2 ) LHS (mm) Period (mm) PH delay (ms) Tot. Delay (ms) Outer layer heater layout: Alternative inner layer heater layout: *by T. Salmi *by E. Todesco 5 segments (303.5 mm length) will provide simultaneous quenching of all strands. ½ twist pitch base (60.7 mm period) : OL: 18 segments (R heater = 1.6  P/A (straight) = 161 W/cm 2 at 150 V) IL: 16 segments (R heater = 1.73  P/A (straight) = 137 W/cm 2 at 150 V) Selective copper plating is the way to scale up the heater length to long QXF. (Still an option for the upcoming QXF coils) T. Salmi, et al., IEEE, Trans. Appl. Supercond., 24 (3), 2014 T. Salmi, et al., IEEE, Trans. Appl. Supercond., 24 (4), 2014 T. Salmi, et al., Proc. WAMSDO, 2013 More simulations and experimental data: HQ01, 25 µm polyimide HQ02, 75 µm polyimide Experimental results versus simulations for the HQ magnet Simulation result (adjusted for the pitch length of 109 mm): τ RC = 36 ms, R = 5.6 Ω, I = 80 A  HS length = mm  period = 91 mm  P(0) = 130 W/cm 2  PH delay = 13 ms  propagation between HS = 4 ms