Longitudinal and vertical focusing with a field gradient spiral inflector

Abstract

Traditional spiral inflectors of the Belmont-Pabot type are commonly used for axial injection of external ion beams into cyclotrons. These inflectors are designed to control the trajectory of the central path, and do not actively focus the beam in the vertical and longitudinal directions. This can introduce effects such as a large vertical divergence and a debunching longitudinal spread, making it difficult to match the injection line emittance to the cyclotron acceptance. In an attempt to overcome this, some recent inflectors have started incorporating electrodes specially shaped to produce field gradients along the central path, thereby influencing the inflector optics. This method has shown some success, and at iThemba LABS an inflector was built exhibiting good vertical focusing. However, it performed poorly longitudinally, worse than traditional spiral inflectors. In this article a generalized field gradient spiral inflector design is presented, based on a mathematical description of all possible first-order field gradients along the central path. Such a design is numerically optimized to simultaneously focus longitudinally and vertically. Experimental studies of this design show a 60% improvement in overall current extracted from the cyclotron.