Adaptive Tuner Control in Triumf Isac 2 Superconducting Linac Using Kalman Filter

The TRIUMF ISAC 2 RF control system uses phase locking self-excited control. Amplitude, phase and frequency control is achieved with I/Q voltage injection, and forward RF power is minimized with a tuner feedback loop. The phase difference between the input coupler and the output pickup drives a velocity servo system to provide tuning control. However, the presence of microphonics in the cryomodule, although under control by the quadrature loop, still presents significant interference to the phase error signal for the tuner. The tuner will follow this noise and generate more microphonics as a result. A first-order Kalman filter is used for an estimation of the phase difference and reduces the movement of the tuner. INTRODUCTION The ISAC 2 superconducting RF cavities have unloaded Q’s of the order of 10. Even when operating under overcoupling condition the loaded Q’s are still of the order of 10. At this high operating Q any slight detuning of the cavity will cause a large increase in power in order to keep the cavity at the operating voltage and frequency. This detuning arises from the fluctuation in helium pressure and microphonics. In-phase and quadrature-phase feedback is used to control the voltage and phase to a stable reference. The function of the tuner is to reduce the RF power requirement by keeping the detuning to within a prescribed limit. This frees all the reserve power to be used for phase locking in the presence of microphonics. To achieve this goal two tuning conditions are monitored: the phase difference between the input and output of the cavity, and the quadrature drive of the modulator required to maintain phase lock. However, both of these signals contain high frequency information whose sources are microphonics and power supply noise. The tuner controller, however, operates at a relatively low frequency of 10 samples/s and will not be capable of cancelling out the microphonics. Instead, a digital filter is used to remove the microphonics components, and Kalman filters are used to perform this task. We have chosen a Single Input Single Output Kalman filter as a test of principle. RESONATOR TUNING CONTROL The TRIUMF ISAC 2 superconducting RF cavities operate under phase-locked self-excited operation[1]. The basic phase-locked self-excited loop is shown in Fig. 1. i V and q V are the in-phase and quadrature phase components of the drive signal, respectively. Let l θ be the phase delay around the main loop, f θ be the additional phase delay around the quadrature correction branch and c ω be the resonant frequency of the cavity. The system will oscillate at a frequency t se α ω = and the voltage at the cavity Vo is [2] ( ) [ ] ( ) q i i i c se V ie V e i V q l θ θ γ ω ω τ + ≈ − + 1 0 (1) where τ is the loaded time constant of the cavity and γ is the voltage multiplication ratio. The argument part of Equation 1 gives