  Based on the design requirements proposed by the Beijing On-Line Isotope
Separation project (BISOL), four Sn$^{22+}$-based,81.25MHz continuous wave (CW)
drift tube linac (DTL) cavities have been designed. These DTLs are capable of
accelerating Sn$^{22+}$ of 0.1 pmA from 0.5 MeV/u to 1.8 MeV/u over a length of
7 m, with an output longitudinal normalized RMS emittance of 0.35$\pi \cdot$
mm$\cdot$mrad, and transmission efficiency higher than 95%. The dynamics design
adopted the KONUS (Kombinierte Null Grad Struktur Combined $0^\circ$ Structure)
scheme. Comprehensive error study implies that these DTLs can accommodate a
wide range of non-ideal beams and cavity alignment errors while maintaining
high transmission efficiency. The electromagnetic design employed a Cross-bar
H-mode (CH) structure for superior water-cooling characteristics, and a
detailed tuning analysis was conducted to derive an optimal tuning scheme. The
results of the multiple-physics analysis indicate that the frequency shift of
each cavity is within an acceptable range. Comparing the dynamics requirements
with the RF design results, similar particle output phase distribution,
equivalent energy gain and consistent emittance growth are observed. Detailed
designs will be presented in this manuscript.
