Frequency jump using 704.4 MHz radio-frequency quadrupole and cross-bar H -type drift tube linear accelerators

A large-scale proton or ${\mathrm{H}}^{\ensuremath{-}}$ accelerator often requires a jump in radio frequency, typically in the beam velocity range of $\ensuremath{\beta}=0.4--0.6$. The MYRRHA linac has two frequency jumps: one at $\ensuremath{\beta}\ensuremath{\cong}0.2$ and another one in the typical beam velocity range at $\ensuremath{\beta}=0.56$. Using the MYRRHA linac as an example, this study investigates two new solutions, whereby the frequency jump in the range of $\ensuremath{\beta}=0.4--0.6$ is eliminated. A single frequency jump at $\ensuremath{\beta}\ensuremath{\cong}0.2$ can shorten the whole linac considerably and reduce the construction and operation costs. The proposed frequency jump sections will use 704.4 MHz radio-frequency quadrupole and cross-bar $H$-type drift tube linear accelerators. To ensure a safe and reliable cw operation at such very high frequency for these kinds of accelerating structures, careful design studies with respect to beam dynamics and rf structures including water-cooling channels have been performed. The results demonstrated the feasibility of both solutions.