LOW ENERGY HIGH POWER SIDE COUPLED LINAC OPTIMIZATION Vittorio Giorgio Vaccaro, Francesca Galluccio, Naples University Federico II and INFN, Napoli Dario Giove, Istituto Nazionale di Fisica Nucleare, Milano Amedeo Renzi, Naples University Federico II, Napoli ABSTRACT - The use of BBAC ® (Back-to-Back Accelerating Cavity) tiles in proton Side Coupled Linacs can be extended down to energies of the order of 20 MeV, keeping more than suitable shunt impedances and energy gradients. However, the considerable energy dissipation from the cavity noses may induce a remarkable increase in their temperature. This may cause both a strong duty-cycle-dependent detuning of the modules, and dangerous thermo-mechanical stress due to the non-uniform temperature distribution. An innovative shape of the BBAC tile is proposed, which allows to limit the temperature rise within a safe range, without introducing detrimental effects on the quality indices (shunt impedance, dissipated power). The design goal to operate at a 30 MeV injection energy or even lower suggested to deeply investigate a new accelerating SCL structure to face the problem which arises in classical SCL structures: indeed the shunt impedance drastically decreases with decreasing energies. The new design of the tiles (covered by a patent), named Back to Back Accelerating Cavity (BBAC) overcomes this problem and gives some remarkable side advantages (Patent Nr 2008 A25) BBAC Tiles Standard Tiles the septum between two adjacent cavities is no longer obtained by setting two tiles back to back, the manufacturing is simplified and the septum width can be reduced without impacting the quality of the brazing process; the increase of the volume surface ratio in the cavity produces an increase of the shunt impedance; the larger width of the accelerating cavity and its asymmetric cut allow the frequency rod tuners to be easily inserted; the reduced number of tiles required in the assembly provides a more relaxed mechanical tolerances and a general in machining cost reduction. AN INNOVATIVE BACK-TO-BACK ACCELERATING CAVITY (BBAC ® ) MANY ADVANTAGES THERMAL ANALYSIS OF FLAT WALL BBAC OPTIMIZED BBAC WITH TILTED WALLS The high field (up to 80 MV/m) induces a local temperature rise much larger (40 %) on the BBAC nose with respect to the standard cavity ( ΔT = 19.7 K ). The increase of the temperature on the nose of the cavity may prejudice the mechanical properties of the structure. The bulk part of the module warms up very little, while the largest temperature increase happens on the nose and in its adjacent parts. These parts have little possibility to expand because of the bulk portion, which behaves as a cage for the heated part. Therefore high internal stresses are produced in this part. At the beginning of this process, the alternate stresses are absorbed by elastic strains. After a certain number of warm-up/cool-down cycles a slow degradation takes place in the crystal structure of copper, changing its elastic properties into the plastic phase. In this conditions, micro-cracks, similar to those that lead to mechanical fatigue break-down, may be produced. They may trigger electric discharge. ΔT = 27.6 K ΔT = 23.7 K The heat flux is negligible on the nose and maximum in the peripheral part of the cavity. WHY? The high flux in the peripheral part behaves likewise a bottleneck for the heat produced on the nose of the cavity. The even small amount of heat produced in this part cannot easily flow and there is a remarkable increase of temperature around the centre of the cavity. HEAT FLUX ONE DISADVANTAGE: THERMO-MECHANICAL STRESS RISK In order to decrease the thermal resistance the septum has to become thicker and thicker from the centre to the periphery of the cavity. The cavity was reshaped: Minimum septum thickness s=1.4 mm, Wall tilt angle α= rad Retuning by cavity diameter variation of 2%. FLAT BBAC CAVITY ACLIP OPTIMIZED BBAC CAVITY WITH TILTED WALLS Shunt Imped. Temp. Rise [MΩ/m] [K] Standard Tile BBAC flat BBAC tilted With an impedance loss of only 5%, the temperature rise is almost halved with respect to the standard tile design. This optimization procedure is quite general. We intend to apply it to tiles designed for 18 MeV and investigate the possibility to further increase the tilt angle. FUTURE DEVELOPEMENTS