Abstract
The low-level radio frequency (LLRF) control system is one of the fundamental parts of a particle accelerator, ensuring the stability of the electro-magnetic (EM) field inside the resonant cavities. It leverages on the precise measurement of the field by in-phase/quadrature (IQ) detection of an RF probe signal from the cavities, usually performed using analogue downconversion. This approach requires a local oscillator (LO) and is subject to hardware non-idealities like mixer nonlinearity and long-term temperature drifts. In this work, we experimentally evaluate IQ detection by direct sampling for the LLRF system of the Polish free electron laser (PolFEL) now under development at the National Centre for Nuclear Research (NCBJ) in Poland. We study the impact of the sampling scheme and of the clock phase noise for a 1.3-GHz input sub-sampled by a 400-MSa/s analogue-to-digital converter (ADC), estimating amplitude and phase stability below 0.01% and nearly 0.01°, respectively. The results are in line with state-of-the-art implementations, and demonstrate the feasibility of direct sampling for GHz-range LLRF systems.
Highlights
Free electron lasers, like the Polish Free Electron Laser (PolFEL) [1] under development at the National Centre for Nuclear Research (NCBJ) in Poland, are an advanced tool for a range of scientific, industrial, and medical applications [2], allowing researchers to examine materials, characterize biological samples, etc. Such lasers are based on highly concentrated electron beams accelerated in resonant cavities, used to generate high-energy coherent light at wavelengths and peak powers that are difficult to achieve with conventional methods, e.g., the ones based on optical resonators [3,4]
The requirements for the amplitude and phase stability of the field can range from around 1% to 0.1% in amplitude and from 1◦ to 0.1◦ in phase [6,7,8], or even up to 0.01% and 0.01◦, respectively, in the most demanding cases [9]. These requirements are derived from the desired beam parameters of the accelerator, e.g., bunch-to-bunch energy spread [10], and must be satisfied for an observation time corresponding to the RF pulse width in which electron bunches are accelerated, typically in the order of tens of milliseconds
IQ demodulation was carried out by means of an analogue IQ demodulator [25,26], where the IQ components are obtained by mixing the input signal with a local oscillator (LO) signal and with the same LO signal shifted by 90◦, respectively
Summary
Like the Polish Free Electron Laser (PolFEL) [1] under development at the National Centre for Nuclear Research (NCBJ) in Poland, are an advanced tool for a range of scientific, industrial, and medical applications [2], allowing researchers to examine materials, characterize biological samples, etc Such lasers are based on highly concentrated electron beams accelerated in resonant cavities, used to generate high-energy coherent light at wavelengths and peak powers that are difficult to achieve with conventional methods, e.g., the ones based on optical resonators [3,4]. The goal of this work is to experimentally evaluate direct sampling approaches utilizing a high-bandwidth, fast-sampling ADC for the purpose of the LLRF system of the PolFEL, operating at the resonant frequency of 1.3 GHz. In particular, we aim at comparing different sampling frequencies as well as at examining the impact of clock signal phase noise on the acquisition in order to estimate the best achievable amplitude and phase stability for the system under development.
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