The CYCIAE-230 cyclotron operates cw(continuous wave). The extracted beam is characterized by a fixed energy greater than 230MeV. However modern therapy methodology often requires fast modulation of the beam both from intensity and energy. The energy modulation is typically implemented with a passive device namely energy degrader. Unfortunately, when this device changes the energy, it also changes the intensity of the beam greatly. This cross talk between the beam energy and intensity modulation introducing by the degrader is nonlinear and varies with devices. To address this issue, a vertical deflector is designed and installed in the central region, using electrical field to deflect the beam. The vertical deflector together with a collimator several turns downstream can control the beam intensity. However, the voltage applied to the vertical deflector is also nonlinear to the beam transmission ratio due to the vertical betatron oscillation. These two non-linearities require a sophisticated design of the controller for the vertical deflector. The vertical deflector controller is a 6U VME custom designed control card.The controlling unit is based on a Digital Signal Processor which interpolates the lookup table data to implement feed-forward control. In the meantime, an ionized chamber is providing feedback readings of the intensity just before the nozzle. The DSP unit is using PID algorithm and take this feedback as input to correct the vertical deflector voltage. This type of advanced PID controller is able to adapt to the timing requirements for the beam intensity modulation for proton therapy.
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