The design and implementation of a broadband digital low-level RF control system for the cyclotron accelerators at iThemba LABS

Abstract

iThemba LABS has successfully designed a new broadband digital low-level RF control system for cyclotrons, that operates over the wide frequency range of 2–100 MHz and can achieve peak–peak amplitude and phase stabilities of 0.01% and 0.01°, respectively.The presented system performs direct digital synthesis (DDS) to directly convert the digital RF signals to analog RF and local-oscillator (LO) signals with 16-bit amplitude accuracy, programmable in steps of 1μHz and 0.0001°. Down-conversion of the RF pick-up signals to an optimal intermediate frequency (IF) of 1 MHz and sampling of the IF channels by 16-bit, single sample-latency 10 MHz ADCs was implemented to allow digital high-speed low-latency in-phase/quadrature (I/Q) demodulation of the IF channels within the FPGA. This in turn allows efficient real-time digital closed-loop control of the amplitude and phase of the RF drive-signal to be achieved.The systems have been successfully integrated at iThemba LABS into the K=8 and K=10 injector cyclotrons (SPC1, and SPC2), the K=200 separated sector cyclotron (SSC), the SSC flat-topping system, the pulse-selector system and the AX,J, and K-line RF bunchers.The systems have led to a substantial improvement in the beam quality of the SSC at iThemba LABS with a reduction in beam losses by more than 90%. The design, implementation and performance is discussed.