Qili Tian

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.

Observation of coherent Smith-Purcell and transition radiation driven by single bunch and micro-bunched electron beams

Generation of coherent Smith-Purcell (cSPr) and transition/diffraction radiation using a single bunch or a pre-modulated relativistic electron beam is one of the growing research areas aiming at the development of radiation sources and beam diagnostics for accelerators. We report the results of comparative experimental studies of terahertz radiation generation by an electron bunch and micro-bunched electron beams and the spectral properties of the coherent transition and SP radiation. The properties of cSPr spectra are investigated and discussed, and excitations of the fundamental and second harmonics of cSPr and their dependence on the beam-grating separation are shown. The experimental and theoretical results are compared, and good agreement is demonstrated.

Phase control with two-beam interferometry method in a terahertz dielectric wakefield accelerator

High-gradient, beam-driven wakefield acceleration in THz structures is a promising technology for future free electron lasers and colliders. In this scheme, the main beam is accelerated by the wakefield of the high current drive beam. The time separation between the main and drive beams has to be chosen carefully to ensure that the main beam is in an accelerating phase of the drives wakefield. THz accelerating structures provide high gradient acceleration due to their small apertures, but their phase control is difficult due to the picosecond-scale period. Here, we report on a wakefield acceleration experiment in a 460 GHz dielectric wakefield accelerator (DWA). The optimum phase of the main beam during the experiment is determined with a two-beam wakefield interferometry (TBI) measurement. This is performed without the measurement of the main and drive beam bunch lengths or their separation. In TBI, the interference of the wakefields produced by the drive and main beams is measured with an integrating THz detector. The TBI signal, as a function of separation between the drive and main beams, exhibits a minimum due to destructive interference of these wakefields, which corresponds to maximum acceleration of the main beam as is confirmed by the energy spectrometer measurement. The maximum energy gain of 0.8 MeV and maximum energy loss of 1.2 MeV for the main beam have been measured, which agrees well with theoretical predictions.High-gradient, beam-driven wakefield acceleration in THz structures is a promising technology for future free electron lasers and colliders. In this scheme, the main beam is accelerated by the wakefield of the high current drive beam. The time separation between the main and drive beams has to be chosen carefully to ensure that the main beam is in an accelerating phase of the drives wakefield. THz accelerating structures provide high gradient acceleration due to their small apertures, but their phase control is difficult due to the picosecond-scale period. Here, we report on a wakefield acceleration experiment in a 460 GHz dielectric wakefield accelerator (DWA). The optimum phase of the main beam during the experiment is determined with a two-beam wakefield interferometry (TBI) measurement. This is performed without the measurement of the main and drive beam bunch lengths or their separation. In TBI, the interference of the wakefields produced by the drive and main beams is measured with an integrating T...

Non-perturbing THz generation at the Tsinghua University Accelerator Laboratory 31 MeV electron beamline

In recent experiments at Tsinghua University Accelerator Laboratory, the 31 MeV electron beam, which has been compressed to subpicosecond pulse durations, has been used to generate high peak power, narrow band Terahertz (THz) radiation by transit through different slow wave structures, specifically quartz capillaries metallized on the outside. Despite the high peak powers that have been produced, the THz pulse energy is negligible compared to the energy of the electron beam. Therefore, the THz generation process can be complementary to other beamline applications like plasma wakefield acceleration studies and Compton x-ray free electron lasers. This approach can be used at x-ray free electron laser beamlines, where THz radiation can be generated without disturbing the x-ray generation process. In the experiment reported here, a high peak current electron beam generated strong narrow band (∼1% bandwidth) THz signals in the form of a mixture of TM01 and TM02 modes. Each slow wave structure is completed with a mode converter at the end of the structure that allows for efficient (>90%) power extraction into free space. In the experiment, both modes in these two dielectric-loaded waveguides TM01 (0.3 THz/0.5 THz) and TM02 (0.9 THz/1.3 THz) were explicitly measured with an interferometer. The THz pulse energy was measured with a calibrated Golay cell at a few μJ.

Selective excitation and control of coherent terahertz Smith-Purcell radiation by high-intensity period-tunable train of electron micro-bunches

We report the observation and studies of selective excitation and control of terahertz (THz) coherent Smith-Purcell radiation (cSPr) from a train of sub-picosecond-period electron micro-bunches. The coherence of the radiation from such a train has been demonstrated. The spectrum of cSPr was measured and the selective excitation of the first and the second harmonics was observed, respectively. We also demonstrate experimentally that radiation pulses generated by each electron micro-bunch interfere coherently with the maximum intensity of cSPr observed at the frequency equal to the frequency of the micro-bunch spacing. The experimental results greatly contribute to the understanding of coherent Smith-Purcell radiation from the train of electron micro-bunches, as well as the development of THz diagnostics for accelerators.We report the observation and studies of selective excitation and control of terahertz (THz) coherent Smith-Purcell radiation (cSPr) from a train of sub-picosecond-period electron micro-bunches. The coherence of the radiation from such a train has been demonstrated. The spectrum of cSPr was measured and the selective excitation of the first and the second harmonics was observed, respectively. We also demonstrate experimentally that radiation pulses generated by each electron micro-bunch interfere coherently with the maximum intensity of cSPr observed at the frequency equal to the frequency of the micro-bunch spacing. The experimental results greatly contribute to the understanding of coherent Smith-Purcell radiation from the train of electron micro-bunches, as well as the development of THz diagnostics for accelerators.

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.

Thomson scattering x-ray source: a novel tool for monochromatic computed tomography

Based on the collision of intense laser and relativistic electrons, a Thomson scattering x-ray source can produce quasi-monochromatic x-ray pulses with high brightness in the tens keV or even higher energy regime, which can eliminate the beam hardening effect encountered in computed tomography (CT) by using polychromatic x-rays generated through Bremsstrahlung and make it possible to relate the reconstructed linear attenuation coefficients to the composition of a material. In this paper, we demonstrate the capacity of quantitative CT measurement based on Tsinghua Thomson scattering X-ray source (TTX) and the potential of anatomical segmentation using quantitative linear attenuation coefficient analysis. A peanut sample (Arachis hypogaea L.) was chosen for this study. According to the reconstructed CT image, all anatomical structures except for the testa (i.e. the seed coat) of peanut were identified clearly in terms of the shape and size, and there were high similarities between reconstructed linear attenuation coefficients of cotyledon and its theoretical values. After quantitative analysis of the reconstructed linear attenuation coefficients, the hull can be peeled off the core at the threshold of 0.31 cm-1. Our results pave the way towards fundamental researches and practical applications based on quantitative CT at TTX.