Yuanzineng kexue jishu (Jul 2023)
Research on Resonant Amplification of Capacitive Schottky Pickup
Abstract
Schottky noise diagnostic is a very important method to measure beam revolution frequency, momentum spread, tune, and chromaticity, etc., in the synchrotron. Capacitive pickups are widely used to measure Schottky noise. However, the application of the pickups is limited in low-intensity rings due to their low sensitivity. The capacitive pickup can be modeled as an RC parallel circuit, where R is the input impedance of the front-end FET low-noise amplifier and C is the ground capacitance of the pickup. Usually, the impedance of the RC circuit is determined by the ground capacitance C and is much smaller than R. To increase the impedance, a parallel inductance is added to form an RLC parallel resonant circuit, whose impedance is just R at the resonance frequency ideally. Actually, the amplification factor is the quality factor of the resonant circuit, which is mainly affected by the parasitic resistance of the added inductor. Noises may also be amplified by the resonant circuit, which is unfavorable for improving the pickup sensitivity. Noise analysis was performed with an RLC equivalent model, the results show that the input noise current of the FET amplifier is the main contributor at the neighbor of the resonance frequency, and the input noise voltage of the amplifier contributes more at off-resonance frequencies. Take the FEMTO HVA-200M-40-F amplifier, which has equivalent input noise voltage density and current density of 4.5 nV/Hz and 2 pA/Hz, respectively, for example, total noise at the resonance frequency is about 10 times larger than that at off-resonance frequencies owing to the high impedance at the resonance frequency. Due to the small size and high Q-factor of the helical resonator, a helical resonator with 2.57 μH was designed and constructed, and connected to the capacitive pickup in parallel. The performance of the RLC parallel resonant circuit was tested with a wire setup. Signal amplification of 44.5 dB at the resonance frequency is achieved by the resonant circuit with a 19.8 dB increase of the noise at the same time. Overall, adding the helical resonator increases the signal-to-noise ratio (SNR) by 24.8 dB, which means 2 orders of magnitude improvement of the pickup sensitivity at least. Beam experiments were also conducted to verify the effectiveness of the resonance method at the Xi’an Proton Application Facility (XiPAF). The results show that the SNR of the measured Schottky noise signal shall increase by 22.9 dB by adding the helical resonator, which is in accordance with the offline test results. Momentum spread and transverse tune at the storage mode of XiPAF were measured for the first time with the high-sensitivity resonant capacitive pickup. The input noise current of the front-end amplifier and parasitic resistance of the added inductor are the main limiting factors of the low noise and high quality factor of the resonant capacitive pickup, respectively. To further improve the pickup sensitivity, a high input impedance amplifier with a low input noise current and an inductor with a high quality factor are preferred.
