Tsinghua Thomson Scattering X-ray Source Research Articles

Experimental demonstration of polarization control in Thomson scattering X-ray sources

Thomson scattering sources are known for generating quasi-monochromatic, high-brightness, and polarization-tunable X-rays, which are widely applied in fields such as fluorescence imaging, magnetic circular dichroism, and nuclear physics. In this paper, we present an experimental demonstration of polarization control at the Tsinghua Thomson scattering X-ray source (TTX) by adjusting the laser polarization. A specially designed Compton polarimeter is employed to effectively analyze the Degree of Linear Polarization (DoLP) of the X-rays produced at the TTX. Our results indicate that the DoLP of X-rays within a collecting angle of 1/(3γ), where γ is the Lorentz factor of the relativistic electron, closely matches the DoLP of the laser. Furthermore, we demonstrate that the X-ray polarization can be continuously modulated from linear polarization to circular polarization by adjusting the laser polarization, which can significantly enhance the functionality and applicability of Thomson Scattering sources for scientific research.

Multivariable virtual diagnostics and tuning of beam positioning using machine learning

In many experiments, such as Thomson scattering experiments, beam position sensitivity is encountered when using the Tsinghua Thomson Scattering X-ray Source (TTX). In each experiment, the position of the beam is manually tuned, a process that requires several minutes to complete. Moreover, the beam position drifts over time. In addition, in some situations, such as free electron laser operation, the beam position will be affected; thus, a downstream beam position monitor (BPM) cannot offer a reliable beam position. In this study, machine learning is used to perform virtual diagnostics and tuning of the beam position of TTX. For universality, 20 independent parameters that may affect the beam position are imported and tuned over a large range. While universal virtual diagnostics can be achieved using this process, the parameters mutually influence each other, and it is difficult to determine their valid combined range, leading to low data validity; consequently, in some situations, the beam will be lost.A bagged tree classification model is trained to determine the combined range of the parameters. This greatly improves the experimental efficiency from 38% to 64%. The final accuracy achieved using this classification model is 95%. When the beam perturbations have a root mean square error (RMSE) of approximately 0.096 mm, the virtual diagnostic accuracy is 0.13 mm in the range of 20 mm; thus, the normalized root mean square error (NRMSE) is 0.65%. This can satisfy most demands on TTX. In addition, it is experimentally demonstrated that the beam can be tuned to the desired position with an RMSE of 0.03 mm using this model.

Development of a seven-cell S-band standing-wave RF-deflecting cavity for Tsinghua Thomson scattering X-ray source

A 2856-MHz, π-mode, seven-cell standing-wave deflecting cavity was designed and fabricated for bunch length measurement in Tsinghua Thomson scattering X-ray source (TTX) facility. This cavity was installed in the TTX and provided a deflecting voltage of 4.2 MV with an input power of 2.5 MW. Bunch length diagnoses of electron beams with energies up to 39 MeV have been performed. In this article, the RF design of the cavity using HFSS, fabrication, and RF test processes are reviewed. High-power operation with accelerated beams and calibration of the deflecting voltage are also presented.

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.

Lattice design and beam dynamics of a storage ring for a Thomson scattering x-ray source

TTX-II (Tsinghua Thomson Scattering X-Ray Source Phase II) is a Thomson scattering based x-ray source currently under development at the accelerator laboratory in Tsinghua University. The circumference of the storage ring is 5.668 m. The compact nature of the ring gives rise to several design challenges such as chromaticity correction and collective instability, which can be suppressed by carefully optimizing ring parameters. The lattice design and the beam dynamics of TTX-II are presented in this work.

Experimental feasibility of dual-energy computed tomography based on the Thomson scattering X-ray source.

Unlike large-scale and expensive synchrotron radiation facilities, the Thomson scattering X-ray source can provide quasi-monochromatic, energy-tunable and high-brightness X-ray pulses with a small footprint and moderate cost, making it an excellent candidate for dual-energy and multi-energy imaging at laboratories and hospitals. Here, the first feasibility study on dual-energy computed tomography (CT) based on this type of light source is reported, and the effective atomic number and electron-density distribution of a standard phantom consisting of polytetrafluoroethylene, water and aluminium is derived. The experiment was carried out at the Tsinghua Thomson scattering X-ray source with peak energies of 29 keV and 68 keV. Both the reconstructed effective atomic numbers and the retrieved electron densities of the three materials were compared with their theoretical values. It was found that these values were in agreement by 0.68% and 2.60% on average for effective atomic number and electron density, respectively. These results have verified the feasibility of dual-energy CT based on the Thomson scattering X-ray source and will further expand the scope of X-ray imaging using this type of light source.

Design of a L-band cavity optimized for Robinson instability in the TTX-II storage ring

Tsinghua Thomson Scattering X-ray Source Phase II is a Thomson scattering-based X-ray source currently under development at Accelerator Laboratory in Tsinghua University. The purpose of this paper is to design a L-band cavity for the storage ring and to study the coupled bunch instability induced by the HOMs of the cavity. The growth rates and tune shifts of the coupled bunch instability are calculated with approximation formulas and simulated with particle tracking code. The frequency of HOMs is optimized that the Landau damping can suppress the coupled bunch instability. A prototype cavity is manufactured and cold-tested. The HOM frequencies can be tuned away from beam spectrum, and the CBI instability can be easily suppressed with Landau damping. Without HOM dampers or feedback system, the HOMs can be tuned during design phase such that the CBI instability can be suppressed.

Focal spot characteristics of Thomson scattering x-ray sources

With the recent rapid development of x-ray imaging using Thomson scattering x-ray sources, there is an increasing demand for knowledge of the focal spot characteristics of this type of light source. This is because imaging quality is closely related to focal spot characteristics. In this paper, an analytical expression of the focal spot size and beam parameters at arbitrary interaction angles is derived. Based on this, the focal spot can be optimized in a straightforward manner by adjusting the laser and electron focusing at a fixed interaction geometry. As the beam focusing increases, a smaller focal spot and a higher photon yield are obtained, while the bandwidth of scattered x rays deteriorates, leading to a compromise of the above three physical parameters. Considering the beam position and time fluctuations, the focal spot will be broadened and the photon yield will be reduced. Using typical beam parameters at the Tsinghua Thomson scattering x-ray source, it is shown that a micrometer-sized focal spot can be easily achieved. The results will help one to develop the x-ray imaging field, especially in micro-computed tomography applications based on Thomson light sources.

Development of sub-100 femtosecond timing and synchronization system.

The precise timing and synchronization system is an essential part for the ultra-fast electron and X-ray sources based on the photocathode injector where strict synchronization among RF, laser, and beams are required. In this paper, we present an integrated sub-100 femtosecond timing and synchronization system developed and demonstrated recently in Tsinghua University based on the collaboration with Lawrence Berkeley National Lab. The timing and synchronization system includes the fiber-based CW carrier phase reference distribution system for delivering stabilized RF phase reference to multiple receiver clients, the Low Level RF (LLRF) control system to monitor and generate the phase and amplitude controllable pulse RF signal, and the laser-RF synchronization system for high precision synchronization between optical and RF signals. Each subsystem is characterized by its blocking structure and is also expansible. A novel asymmetric calibration sideband signal method was proposed for eliminating the non-linear distortion in the optical synchronization process. According to offline and online tests, the system can deliver a stable signal to each client and suppress the drift and jitter of the RF signal for the accelerator and the laser oscillator to less than 100 fs RMS (∼0.1° in 2856 MHz frequency). Moreover, a demo system with a LLRF client and a laser-RF synchronization client is deployed and operated successfully at the Tsinghua Thomson scattering X-ray source. The beam-based jitter measurement experiments have been conducted to evaluate the overall performance of the system, and the jitter sources are discussed.

Temporal profile monitor based on electro-optic spatial decoding for low-energy bunches

The measurement of electron bunch temporal profile is one of the key diagnostics in accelerators, especially for ultrashort bunches. The electro-optic (EO) technique enables the precise longitudinal characterization of bunch electric field in a single-shot and nondestructive way, which can simultaneously obtain and analyze the time jitter between the electron bunch and the synchronized laser. An EO monitor based on spatial decoding for temporal profile measurement and timing jitter recoding has recently been demonstrated and analyzed in depth for low-energy bunches at the Tsinghua Thomson scattering X-ray source. A detailed description of the experimental setup and measurement results are presented in this paper. An EO signal as short as 82 fs (rms) is observed with $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ gallium phosphide for a 40 MeV electron bunch, and the corresponding length is 106 fs (rms) with $300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ zinc telluride. Owing to the field-opening angle, we propose a method to eliminate the influence of energy factor for bunches with low energy, resulting in a bunch length of $\ensuremath{\sim}60\text{ }\text{ }\mathrm{fs}$ (rms). The monitor is also successfully applied to measure time jitter with approximately 10 fs accuracy. The experiment environment is proved to be the main source of the slow drift, which is removed using feedback control. Consequently, the rms time jitter decreases from 430 fs to 320 fs.

A pulse-to-pulse timing jitter measurement between two synchronized amplified laser beams for TTX.

In China, Tsinghua Thomson Scattering X-ray Source (TTX) is the dedicated hard X-ray source based on the Thomson scattering between a terawatt ultrashort laser and a relativistic electron beam. In the TTX, two synchronized Ti: sapphire laser systems generate the terawatt ultrashort infrared scattering laser and the ultraviolet driving laser for the photocathode RF gun to produce the electron beam; measuring the timing jitter between the electron beam and the laser beam is an essential task for the X-ray source. In the present study, we report on a single shot, non-collinear cross correlator with fs resolution and measured the timing jitter between the two synchronized laser systems with a pulse-to-pulse method, which is beneficial to estimate the jitter of the X-ray yield in the TTX system. Although it is more important to synchronize the scattering laser to the electron beam and not of the driving laser, the laser-laser jitter measurement would be a good first step towards that goal, and the result generated can be considered as the error signal for the potential feedback stabilization.

Diffraction based method to reconstruct the spectrum of the Thomson scattering x-ray source.

As Thomson scattering x-ray sources based on the collision of intense laser and relativistic electrons have drawn much attention in various scientific fields, there is an increasing demand for the effective methods to reconstruct the spectrum information of the ultra-short and high-intensity x-ray pulses. In this paper, a precise spectrum measurement method for the Thomson scattering x-ray sources was proposed with the diffraction of a Highly Oriented Pyrolytic Graphite (HOPG) crystal and was demonstrated at the Tsinghua Thomson scattering X-ray source. The x-ray pulse is diffracted by a 15 mm (L) ×15 mm (H)× 1 mm (D) HOPG crystal with 1° mosaic spread. By analyzing the diffraction pattern, both x-ray peak energies and energy spectral bandwidths at different polar angles can be reconstructed, which agree well with the theoretical value and simulation. The higher integral reflectivity of the HOPG crystal makes this method possible for single-shot measurement.

Recent progress of phase-contrast imaging at Tsinghua Thomson-scattering X-ray source

Due to its small spot size, a Thomson-scattering X-ray source can produce high spatial coherent X-ray pulse, which is the prerequisite for phase-contrast imaging. In this paper, we will introduce the recent progress of phase-contrast imaging at Tsinghua Thomson-scattering X-ray source (TTX). Since the generation of first hard X-ray pulse at TTX in 2012, we have demonstrated the capacity of in-line phase contrast imaging using a refill of gel ink pen. And then, a Monte Carlo simulation tool for in-line phase-contrast imaging based on Thomson-scattering X-ray source has been developed. Taking advantage of this code, we calculate the typical requirement of photon numbers for in-line phase-contrast imaging based on this type of X-ray source. After the upgrade of infrared laser system and control program, the total photon yield of TTX has been increased to ∼2×107photons/pulse at X-ray central energy of 25keV and 50keV with RMS jitter less than 6% and 4% respectively. A new run of experiments about in-line phase-contrast imaging and phase-contrast computed tomography (CT) have been carried out at TTX.

Laser–RF synchronization based on digital phase detector

Laser oscillator synchronization with RF reference signal is ultra-important for a modern light source based on the accelerator. For Tsinghua Thomson scattering X-ray source, we have synchronized the mode-locked laser oscillator to RF reference signal with 48.2 fs RMS relative jitter. Both fundamental and harmonic signals derived from photo diode detection are used for laser–RF synchronization in our scheme. The fundamental signal is for coarse laser–RF synchronization and multiple laser oscillator synchronization. The harmonic signal is for high precise phase locking. The digital phase detector is implemented in the synchronization scheme for less noise, replacing the mixing to DC phase detection scheme. The digital processing algorithm for synchronization is commonly used in low-level RF control field. In order to test the phase locking loop logic without damaging the real laser oscillator, a laser oscillator emulator was developed for phase locking. This paper will report the laser–RF synchronization scheme and its performance. The laser oscillator emulator system will also be introduced here.

Development of S-band photocathode RF guns at Tsinghua University

This study aims to explain the development of S-band photocathode RF guns at Tsinghua University, with especial attention on the progress of guns developed after 2011. Two types of RF guns were developed based on the BNL/SLAC/UCLA gun prior 2011. These guns have been operated as electron sources for Tsinghua Thomson scattering X-ray source (TTX), MeV ultrafast electron diffraction, and Shanghai deep ultraviolet FEL test facility. Based on our operation experiences and other gun modifications worldwide, numerous improvements have been proposed, and seven third-type guns have been designed and fabricated since 2011. Cold tests of all third-type guns have been completed, confirming a significantly improved quality factor and mode separation. The first third-type gun has been installed on TTX and operated for four years, demonstrating high gradient, low dark current, stable quantum efficiency, and small emittances. The optimal transverse emittance was εx=0.56mmmrad, εy=0.66mmmrad for 200pC with a peak current of 25A, and εx=0.78mmmrad, εy=0.92mmmrad for 500pC with a peak current of 62.5A under a 110MV/m gun gradient.

Intrinsic nonlinear effects of dipole magnets in small rings

We find that dynamic aperture depends significantly on the bending radii of dipole magnets when designing a small storage ring for Tsinghua Thomson scattering X-ray source (TTX) mainly because of the nonlinearity of the dipole field. In this paper, we present systematic studies on the intrinsic-geometric nonlinearity of dipole magnets. The Hamiltonian approach is used to determine the expressions of the geometric nonlinear potential and the corresponding third-order resonance strengths. Simulations are conducted to study these resonances. Our analysis results agree well with the tracking results at the third-order resonances $3{\ensuremath{\nu}}_{x}=\ensuremath{\ell}$ and ${\ensuremath{\nu}}_{x}\ifmmode\pm\else\textpm\fi{}2{\ensuremath{\nu}}_{z}=\ensuremath{\ell}$, where $\ensuremath{\ell}$'s are the integer multiple of the number of superperiods.

Generating 10–40 MeV high quality monoenergetic electron beams using a 5 TW 60 fs laser at Tsinghua University

A unique facility for laser plasma physics and advanced accelerator research has recently been built at Tsinghua University. This system is based on a Tsinghua Thomson scattering X-ray source (TTX), which combines an ultrafast TW laser with a synchronized 45 MeV high brightness linac. In our recent laser wakefield acceleration experiments, we have obtained 10–40 MeV high quality monoenergetic electron beams by running the laser at 5 TW peak power. Under certain conditions a very low relative energy spread of a few percent can be achieved. Absolute charge calibration for three different scintillating screens has also been performed using the linac system.

In-line phase-contrast imaging based on Tsinghua Thomson scattering x-ray source.

Thomson scattering x-ray sources can produce ultrashort, energy tunable x-ray pulses characterized by high brightness, quasi-monochromatic, and high spatial coherence, which make it an ideal source for in-line phase-contrast imaging. We demonstrate the capacity of in-line phase-contrast imaging based on Tsinghua Thomson scattering X-ray source. Clear edge enhancement effect has been observed in the experiment.

Design of a 4.8-m ring for inverse Compton scattering x-ray source

In this paper we present the design of a 50 MeV compact electron storage ring with 4.8-meter circumference for the Tsinghua Thomson scattering x-ray source. The ring consists of four dipole magnets with properly adjusted bending radii and edge angles for both horizontal and vertical focusing, and a pair of quadrupole magnets used to adjust the horizontal damping partition number. We find that the dynamic aperture of compact storage rings depends essentially on the intrinsic nonlinearity of the dipole magnets with small bending radius. Hamiltonian dynamics is found to agree well with results from numerical particle tracking. We develop a self-consistent method to estimate the equilibrium beam parameters in the presence of the intrabeam scattering, synchrotron radiation damping, quantum excitation, and residual gas scattering. We also optimize the rf parameters for achieving a maximum x-ray flux.

High time resolution beam-based measurement of the rf-to-laser jitter in a photocathode rf gun

Characterizing the rf-to-laser jitter in the photocathode rf gun and its possible origins is important for improving the synchronization and beam quality of the linac based on the photocathode rf gun. A new method based on the rf compression effect in the photocathode rf gun is proposed to measure the rf-to-laser jitter in the gun. By taking advantage of the correlation between the rf compression and the laser injection phase, the error caused by the jitter of the accelerating field in the gun is minimized and thus $10 \mathrm{fs}$ time resolution is expected. Experimental demonstration at the Tsinghua Thomson scattering x-ray source with a time resolution better than 35 fs is reported in this paper. The experimental results are successfully used to obtain information on the possible cause of the jitter and the accompanying drifts.