LIBO - A linac-booster for protontherapy: Construction and tests of a prototype

LIBO is a proton accelerator that operates at 3 GHz, the same frequency as the one adopted in the about 7500 electron linacs used for radiotherapy all over the world. Such a high frequency was chosen to obtain a large gradient (on average more than 10 MV/m), and thus a short linac (about 15 m) to boost the energy of the protons, extracted at about 60 MeV from a cyclotron, up to the 200 MeV needed for the treatment of deep-seated tumours. This paper describes the design study of the full 3 GHz Side Coupled Linac (modular structure, nine modules) and the construction and tests of the LIBO prototype (first module), which was built to accelerate protons from 62 to 74 MeV with an RF peak power of 4.4 MW. The items discussed are the beam dynamics parameters of the module (longitudinal and transverse acceptances), the constructional elements and procedures, the accuracies of the various mechanical elements, the cooling system, the RF tuning, the RF measurement and the RF power tests. These tests showed that, after a short conditioning time, the gradient in each of the four tanks of the module could reach 28.5 MV/m, much larger than the nominal project value (15.8 MV/m). The last section of the paper describes the successful acceleration tests performed at the Laboratori Nazionali del Sud of INFN in Catania with a solid-state 3 GHz modulator lent by IBA.