Development of Specially Designed Small Doublet for Compact Cyclotron Beam Injection

A number of compact cyclotrons accelerating H^- ions to energies between 10 MeV and 100 MeV have recently been constructed for research and medical diagnostics/therapy in several countries. However limitations in extracted beam intensity are brought up by inadequate injection efficiency from the ion source into the machine. To improve the optics matching from the external source to machine acceptance, a specially designed small doublet has been developed to replace the traditionally used solenoid in the vertical injection line before the spiral inflector. Due to the narrow space inside the compact cyclotron, the doublet is designed based on the following special consideration: 1) To use the central plug of the main magnet as a part of the return yoke of the doublet; 2) To use the capillary water-cooling conductor for the coil to keep the doublet sufficiently small; 3) To adopt an integral design of the two quadrupoles into one assembly so that they could be installed in the channel of the central plug in high vacuum environment. In this paper, the design of the doublet is introduced. The outer diameter of 118 mm is precisely fit to the central plug of the Verification Facility of High Intensity Cyclotron at CIAE. Its inner diameter is 50 mm for beam injection. It can provide a maximal magnetic field gradient of 2.6 T/m from the measurement. Various kinds of detailed techniques for the doublet fabrication, e.g. the enwinding of capillary water-cooling conductor with outer diameter of 3 mm and inner diameter of 2 mm, the poles and yoke assembling, will be presented. The beam test results will be given as well.