The heavy ion booster of the Munich MP-tandem is a linear RF accelerator of Interdigital H-type structure. It consists of a single cavity with many accelerating gaps. The chosen structure is extremely efficient. Its effective shunt impedance with the new types of drift tubes is as high as 18 M¿/m·ß-0.83 in the velocity range 0.05¿ß¿0.10 (ß=v/c, v mean particle velocity). For ß=0.1, a RF power input of 41 kW is sufficient for a total effective accelerating voltage of 5 MV. To increase the beam intensity a bunched beam is injected into the booster. The booster delivers ultrashort beam bunches, if it is synchronized with the existing low energy bunching system of the tandem. In first test experiments a 32S tandem beam has been accelerating from 100 to 160 MeV and a 58Ni beam from 130 to 230 MeV.