A 3-λ/4 posi coupler for drift-tube linacs

New permanent magnets for quadrupole focusing make possible smaller drift tubes in drift-tube linacs (DTLs), resulting in higher shunt impedance. However, ordinary post couplers cannot stabilize a DTL whose drift-tube-to-wall spacing exceeds one-quarter wavelength (λ/4) by more than a few per cent for the accelerating mode frequency. We have built and tested post couplers that operate in the 3λ/4 mode. These 3λX/4 post couplers, when substituted for λ/4 couplers, had similar stabilization properties. In addition, the coupling between post and drift tube, and the post's resonant frequency can be independently adjusted.