Development of superconducting crossbar- H -mode cavities for proton and ion accelerators

The crossbar-$H$-mode (CH) structure is the first superconducting multicell drift tube cavity for the low and medium energy range operated in the ${H}_{21}$ mode. Because of the large energy gain per cavity, which leads to high real estate gradients, it is an excellent candidate for the efficient acceleration in high power proton and ion accelerators with fixed velocity profile. A prototype cavity has been developed and tested successfully with a gradient of $7\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. A few new superconducting CH cavities with improved geometries for different high power applications are under development at present. One cavity ($f=325\text{ }\text{ }\mathrm{MHz}$, $\ensuremath{\beta}=0.16$, seven cells) is currently under construction and studied with respect to a possible upgrade option for the GSI UNILAC. Another cavity ($f=217\text{ }\text{ }\mathrm{MHz}$, $\ensuremath{\beta}=0.059$, 15 cells) is designed for a cw operated energy variable heavy ion linac application. Furthermore, the EUROTRANS project (European research program for the transmutation of high level nuclear waste in an accelerator driven system, 600 MeV protons, 352 MHz) is one of many possible applications for this kind of superconducting rf cavity. In this context a layout of the 17 MeV EUROTRANS injector containing four superconducting CH cavities was proposed by the Institute for Applied Physics (IAP) Frankfurt. The status of the cavity development related to the EUROTRANS injector is presented.