Development of sub-100 femtosecond timing and synchronization system

Impact and trends in embedding field programmable gate arrays and microcontrollers in scientific instrumentation

Microcontrollers and field-programmable gate arrays have been largely leveraged in scientific instrumentation since decades. Recent advancements in the performance of these programmable digital devices, with hundreds of I/O pins, up to millions of logic cells, >10 Gb/s connectivity, and hundreds of MHz multiple clocks, have been accelerating this trend, extending the range of functions. The diversification of devices from very low-cost 8-bit microcontrollers up to 32-bit ARM-based ones and a system of chip combining programmable logic with processors make them ubiquitous in modern electronic systems, addressing diverse challenges from ultra-low power operation, with sub-µA quiescent current in sleep mode for portable and Internet of Things applications, to high-performance computing, such as in machine vision. In this Review, the main motivations (compactness, re-configurability, parallelization, low latency for sub-ns timing, and real-time control), the possible approaches of the adoption of embedded devices, and the achievable performances are discussed. Relevant examples of applications in opto-electronics, physics experiments, impedance, vibration, and temperature sensing from the recent literature are also reviewed. From this bird-eye view, key paradigms emerge, such as the blurring of boundaries between digital platforms and the pervasiveness of machine learning algorithms, significantly fostered by the possibility to be run in embedded devices for distributing intelligence in the environment.

Measurements of the Growth and Saturation of Electron Weibel Instability in Optical-Field Ionized Plasmas

The temporal evolution of the magnetic field associated with electron thermal Weibel instability in optical-field ionized plasmas is measured using ultrashort (1.8 ps), relativistic (45 MeV) electron bunches from a linear accelerator. The self-generated magnetic fields are found to self-organize into a quasistatic structure consistent with a helicoid topology within a few picoseconds and such a structure lasts for tens of picoseconds in underdense plasmas. The measured growth rate agrees well with that predicted by the kinetic theory of plasmas taking into account collisions. Magnetic trapping is identified as the dominant saturation mechanism.

Application of a drift compensation low-level radio frequency system based on time-multiplexing pick-up/reference signals

The high accuracy, low drift low-level radio frequency (LLRF) system is essential for the long-term stability of the accelerator RF and the acquirement of low emittance, high intensity electron beams. A time-multiplexing pick-up/reference signal based LLRF system is proposed to deal with the component temperature related phase drift and has been deployed and applied at the Xi'an Gamma-ray Light Source (XGLS) injector. The long term dual-receiver out-of-loop stability experiments with a continuous wave laser based phase reference distribution system (PRDS) show that the LLRF system can achieve ∼40 fs Root-Mean-Square (rms) phase accuracy and 51 fs/52 fs peak-peak drift (in 7 days/17 h with the high power RF system, respectively) while the reference phase varies both ∼30 ps. An ∼4 h beam-based experiment has also been conducted to evaluate the overall performance of the whole XGLS timing and synchronization system, which shows that the PRDS, LLRF system, high power RF system, and laser oscillator laser-RF synchronization system can keep long-term phase stability.

A low level radio frequency system drift compensation technique by time-multiplexing pick-up/reference signals

The Low Level radio frequency system long-term stability is critical for the operation of accelerator facilities. The RF cavity field phase drift observed at the Tsinghua Thomson scattering X-ray source showed the correlations with devices temperature characteristic. We proposed a drift compensation technique by time-multiplexing cavity pick-up and phase reference signals, which guaranteed that they shared the same route with the same change. The preliminary ∼84 h Dual-Receiver out-of-loop stability test showed phase drift of 100 fs peak-peak (∼45 fs rms) when the reference signal phase changed ∼40 ps peak-peak.

Phase Space Dynamics of a Plasma Wakefield Dechirper for Energy Spread Reduction

Plasma-based accelerators have made impressive progress in recent years. However, the beam energy spread obtained in these accelerators is still at the ∼1% level, nearly one order of magnitude larger than what is needed for challenging applications like coherent light sources or colliders. In plasma accelerators, the beam energy spread is mainly dominated by its energy chirp (longitudinally correlated energy spread). Here we demonstrate that when an initially chirped electron beam from a linac with a proper current profile is sent through a low-density plasma structure, the self-wake of the beam can significantly reduce its energy chirp and the overall energy spread. The resolution-limited energy spectrum measurements show at least a threefold reduction of the beam energy spread from 1.28% to 0.41% FWHM with a dechirping strength of ∼1  (MV/m)/(mm pC). Refined time-resolved phase space measurements, combined with high-fidelity three-dimensional particle-in-cell simulations, further indicate the real energy spread after the dechirper is only about 0.13% (FWHM), a factor of 10 reduction of the initial energy spread.

Femtosecond time synchronization of optical clocks off of a flying quadcopter

Future optical clock networks will require free-space optical time-frequency transfer between flying clocks. However, simple one-way or standard two-way time transfer between flying clocks will completely break down because of the time-of-flight variations and Doppler shifts associated with the strongly time-varying link distances. Here, we demonstrate an advanced, frequency comb-based optical two-way time-frequency transfer (O-TWTFT) that can successfully synchronize the optical timescales at two sites connected via a time-varying turbulent air path. The link between the two sites is established using either a quadcopter-mounted retroreflector or a swept delay line at speeds up to 24 ms⁻¹. Despite 50-ps breakdown in time-of-flight reciprocity, the sites’ timescales are synchronized to < 1 fs in time deviation. The corresponding sites’ frequencies agree to ~ 10⁻¹⁸ despite 10⁻⁷ Doppler shifts. This work demonstrates comb-based O-TWTFT can enable free-space optical networks between airborne or satellite-borne optical clocks for precision navigation, timing and probes of fundamental science.

Optical clock networks have many applications from precision time keeping, sensing to fundamental physics. Here the authors demonstrate robust and free-space femtosecond time synchronization of optical clocks via a moving quadcopter.