RF measurement and tuning for an Alvarez-type drift tube linear accelerator without post couplers

Alvarez-type Drift Tube Linacs (DTLs) are commonly adopted to accelerate the proton or heavy ion beam with the speed range about 0.04 similar to 0.4 times of the light speed in vacuum. Acquisition of appropriate frequency and distribution of the electromagnetic field, and the field stability inside the Alvarez-type DTL cavity are essential to achieve good beam characteristics. The field requirement can be achieved through tuning by tuners and post couplers when the cavity is off-line. The field stability of cavity is usually measured by the tilt sensitivity and the minimum frequency interval between the operation mode and the nearest neighbor mode. The frequency interval will increase with the reduction of the number of the acceleration units (cells) inside the cavity, which is beneficial to fulfill the tilt sensitivity requirement. If the field stability condition is naturally satisfied, the post couplers inside the DTL cavity are not needed, and the cavity frequency and field distribution can be tuned merely by tuners. In this paper, the tuning method based on the pseudo-inverse matrix is applied to the Alvarez-type DTL cavity without post couplers in Xi'an 200 MeV proton application facility (XiPAF). The method is improved by an expanded matrix algorithm and the tuning process is optimized to expedite the iteration during the tuning. The XiPAF DTL cavity has 23 cells and 5 tuners. By adjusting the insertion depths of the tuners, the tuning was achieved in merely one round of iteration. After tuning, the relative error of the peak field distribution and resonant frequency of the cavity reached to within +/- 3% and 325 +/- 0.007 MHz (25 degrees C, vacuum), respectively. It was verified that the field stability meets the requirement from the physical design both before and after tuning.