Determination of Effective Acceleration Voltages in Cyclotrons Using the Phase Compression-Phase Expansion Effect

A radial voltage distribution of the RF acceleration system in cyclotrons produces a time varying magnetic field. This field compresses the bunch size of the circulating beam for a radially increasing voltage or expands it for a radially decreasing voltage. This phase compression-phase expansion effect was first verified experimentally at the SIN 590 MeV ring cyclotron. The effect can in turn be used to determine the radial voltage distribution very accurately. This method works best for cyclotrons with external ion sources. Measuring the phase history of the beam at the SIN cyclotron with internal phase probes to an accuracy of 10 ps or 0.2° in RF phase allowed a determination of the voltage distribution to an accuracy of about 1 %. Details of the SIN phase measurement system are described.