The 3MV stage of the Heidelberg Heavy Ion Postaccelerator being the first one to use the independent phasing concept of short linear accelerator cavities in a booster application behind an MP-Tandem accelerator, went into operation in December 1977 and is in the state of full user-availability since May of 1978. The machine uses spiral resonators at room temperature. Ten such resonators with design velocity Ã0 = 0.10 at 108.48 MHz are operated either in CW at 20 kW or in pulsed mode (df = 0.25) at 80 kW. Due to the flexibility of the independent phasing of the 10 resonators presently in operation, acceleration voltages as high as 3.3 MV (CW) could be demonstrated for ions between 12C and 58Ni. 5.5 MV have been achieved in pulsed operation. An energy resolution of Â¿E/E2 4 . 10-4 allowing rebunched pulse widths of Â¿tFWHM < 70 ps could be measured, showing the tandem like beam quality. The machine is exclusively computer controlled, prerequisite for the operation of the many parameters of a tandem-postaccelerator-combination. The overall availability of the postaccelerator together with the MP Tandem was above 80% of the scheduled user beam time. The operation of the 3 MV stage will be interrupted for a 4 months' shutdown late 1979 to allow for the extension of the machine to its full 10 MV (CW) acceleration voltage by the addition of 20 more spiral resonators.