Room temperature quarter wave resonator re-buncher development for a high power heavy-ion linear accelerator

In the medium energy beam transport (MEBT) line system of the RAON which consists of several quadrupole magnets, three normal-conducting re-bunchers, and several diagnostic devices, a quarter wave resonator type re-buncher was chosen for minimizing longitudinal emittance growth and manipulating a longitudinal phase ellipse into the longitudinal acceptance of the low energy linac. The re-buncher has a resonant frequency of 81.25 MHz, geometrical beta (βg) of 0.049, and physical length of 24 cm. Based on the result of numerical calculations of electromagnetic field using CST-MWS and mechanical analysis of the heat distribution and deformation, an internal structure of the re-buncher was optimized to increase the effective voltage and to reduce power losses in the wall. The criteria of the multipacting effect was estimated and it was confirmed by the experiment. The position and specification of cooling channels are designed to recover a heat load up to 15 kW with reasonable margin of 25%. The coaxial and loop type RF power coupler are positioned on the high magnetic field region and two slug tuners are installed perpendicularly to the beam axis. The frequency sensitivity as a function of the tuner depth and cooling water temperature is measured and the frequency shift is in all cases within the provided tuner range. The test with a high power of 10 kW and the continuous wave is performed and the reflection power is observed less than 450 W.