RF Design of the Re-buncher Cavities for the LIPAC ... - CERN

Proceedings of IPAC2011, San Sebastián, SpainMOPC047Table 2: Comparison results of the RFre-buncher cavities studied for LIPAC (all in copper).Configuration Pillbox HWR QWR 3-GAP QWR 4-GAP QWR 4-GAP IH5-GAP IH UnitsVacuum length (beam axis) 200 240 240 280 280 280Vacuum height 860 876 507 495 495 504Total power 75 41.7 25.8 12.2 9.4 8.5Central stem power - 30.5 18 4.0 1.7 2.2Side stem power - - - - 1990 1447Q 24279 11347 10773 11413 10148 11570E peak 24.3 20 20.6 21.0 14.8 16.0Power density peak 30.3 42 97.4 17.0 13.1 12.0Shunt impedance 0.82 1.46 2.36 4.99 6.51 7.19The HWR power consumption is higher than QWRone, but the cooling system design would be easierbecause its stem geometry allows a direct cooling channelgoing across the re-buncher. Nevertheless, the HWR stemcomplicates the location of a vacuum port. In addition,HWR height is double than QWR one, and RF losses are50% higher.The QWR design decreases power consumptioncompared with the pillbox and HWR cavities, but someimportant issues should be taken into account: Steering effects: there is an asymmetry in thefield maps due to the drift tube position. Multipactoring inherent to coaxial resonators.In order to relax these issues, it can be concluded thatthe study of coaxial resonators with increasing number ofgaps is a very interesting strategy. One expects that bothpower consumption and power density can be furtherdecreased, while the aforementioned drawbacks do notincrease.Multi-gap resonatorsThree to five gap resonators are studied inthis section:3 and 4-gap ones are depicted in Fig. 2, while the five gapone is shown in Fig. 3. A significant decrease of RFlosses and peak electric field is achieved with eachadditional gap. The available length is too small toallocate a 5-gap cavity, but due to the significant benefits,the MEBT layout has been changed, ncreasing thatlength up to 350 mm. The stem cooling is much easier inthe 5-gap design, which clearly compensates the higherfabrication cost.Figure 2: 3 and 4 gap resonators: 3-gap QWR(left), 4-gapQWR (middle), and 4-gap IH (right).07 Accelerator TechnologyOn the other side, RF performancee (shunt impedance)of IH type is better than QWR one. The deformationunder atmospheric pressure will behigher in the IHmodel, but acceptable with steel endplates.Multipactoring will happen mainly in regular orsymmetric geometries, therefore QWR will be moreprone to problems. Finally, an additional advantage isenvisaged, as due to the lower required power, bothcooling system and RF source requirements are relaxed.In short, it is reasonable to choose the 5-gap IH cavityas the best candidate for LIPAC rebunchers.DETAILED DESIGThe next step is the detailed design of the 5-gap IHcavity. A full model, including the fillet radii necessaryfor the fabrication (milling tools) andthe recesses for thestem bases, has been studied. Two CF100 pumping portsare drilled on the side wall.High order modesFigure 3: 5-gaps IH cavity.320 mm460 mm6.6 kW1.2 kW678 W11746 --13.9 MV/m8.9 W/cm29.29 MΩPerturbations due to high orderr modes in a lowfrequency linac are usually moderate. However, due tothe high beam power, the allowed beam losses are verysmall, and high order modes have been analysed. Table 3T06 Room Temperature RF 185Copyright c○ 2011 by IPAC’11/EPS-AG — cc Creative Commons Attribution 3.0 (CC BY 3.0)