Chih-Hao Pai

Temporal Characterization of Electron Bunches From Self-Injection and Ionization Injection in a Laser Wakefield Accelerator

Plasma based accelerators (PBAs) have a proven capability to generate high energy electron beams ... more Plasma based accelerators (PBAs) have a proven capability to generate high energy electron beams with ultrashort duration (~ 10 fs) and high peak current (~ 10 kA), which opens the opportunity for compact free electron lasers. To meet the requirements of such challenging applications, controllable injection is highly needed to produce high-quality and highly stable electron beams. As we know, the beam parameters,including the current profile, strongly depend on the injection process. To explore the underlying physics and optimize beam parameters in PBAs, a temporal characterization is highly required for different injection schemes. Based on coherent transition radiation(CTR) method, the preliminary experiment to measure beam temporal profiles from both self-injection and ionization injection schemes in a single-shot mode has been performed at Tsinghua University. And the simulations using the similar experimental parameters have been performed to interpret the different injection p...

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Experimental Demonstration of Energy-Chirp Reduction by a Plasma Dechirper

The preliminary experimental study is presented using a low density plasma dechirper to reduce a ... more The preliminary experimental study is presented using a low density plasma dechirper to reduce a correlated energy chirp from the 41.5-MeV beam at the linac in Tsinghua University. The plasma dechirper operates through the interaction of the electron bunch with its self-wake to dechirp itself, leading to a reduction in energy spread. The experimental results demonstrate that the projected FWHM energy spread of the beam can be reduced from 0.5 MeV to 0.4 MeV with a 12 mm long plasma dechirper, which are in good agreement with full three-dimensional particle-in-cell simulations.

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High-throughput injection–acceleration of electron bunches from a linear accelerator to a laser wakefield accelerator

Nature Physics, 2021

Plasma-based accelerators (PBAs) driven by either intense lasers (laser wakefield accelerators, L... more Plasma-based accelerators (PBAs) driven by either intense lasers (laser wakefield accelerators, LWFAs) 1 or particle beams (plasma wakefield accelerators, PWFAs) 2 , can accelerate charged particles at extremely high gradients compared to conventional radio-frequency (RF) accelerators. In the past two decades, great strides have been made in this field 3-10 , making PBA a candidate for next-generation light sources and colliders 11 . However, these challenging applications necessarily require beams with good stability, high quality, controllable polarization and excellent reproducibility 12,13 . To date, such beams are generated only by conventional RF accelerators. Therefore, it is important to demonstrate the injection and acceleration of beams first produced *

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Region-of-interest micro-focus computed tomography based on an all-optical inverse Compton scattering source

Matter and Radiation at Extremes, 2020

Micro-focus computed tomography (CT), which allows the hyperfine structure within objects to be r... more Micro-focus computed tomography (CT), which allows the hyperfine structure within objects to be reconstructed, is a powerful nondestructive testing tool in many fields. However, current x-ray sources for micro-focus CT are typically limited by their relatively low photon energy and low flux. An all-optical inverse Compton scattering source (AOCS) based on laser wakefield acceleration can generate intense quasi-monoenergetic x/gamma-ray pulses in the kilo- to megaelectronvolt range with micrometer-level source size, and its potential application for micro-focus CT has become very attractive in recent years because of the rapid progress made in laser wakefield acceleration. Reported here is a successful experimental demonstration of high-fidelity micro-focus CT using an AOCS (∼70 keV) by imaging and reconstructing a test object with complex inner structures. A region-of-interest CT method is adopted to utilize the relatively small field of view of the AOCS to ensure high spatial resol...

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Ion acceleration with an ultra-intense two-frequency laser tweezer

New Journal of Physics, 2020

Ultra-intense lasers produce and manipulate plasmas, allowing to locally generate extremely high ... more Ultra-intense lasers produce and manipulate plasmas, allowing to locally generate extremely high static and electromagnetic fields. This study presents a concept of an ultra-intense optical tweezer, where two counter-propagating circularly polarized intense lasers of different frequencies collide on a nano-foil. Interfering inside the foil, lasers produce a beat wave, which traps and moves plasma electrons as a thin sheet with an optically controlled velocity. The electron displacement creates a plasma micro-capacitor with an extremely strong electrostatic field, that efficiently generates narrow-energy-spread ion beams from the multi-species targets, e.g. protons from the hydrocarbon foils. The proposed ion accelerator concept is explored theoretically and demonstrated numerically with the multi-dimensional particle-in-cell simulations.

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High-resolution phase-contrast imaging of biological specimens using a stable betatron X-ray source in the multiple-exposure mode

Scientific Reports, 2019

Phase-contrast imaging using X-ray sources with high spatial coherence is an emerging tool in bio... more Phase-contrast imaging using X-ray sources with high spatial coherence is an emerging tool in biology and material science. Much of this research is being done using large synchrotron facilities or relatively low-flux microfocus X-ray tubes. An alternative high-flux, ultra-short and high-spatial-coherence table-top X-ray source based on betatron motions of electrons in laser wakefield accelerators has the promise to produce high quality images. In previous phase-contrast imaging studies with betatron sources, single-exposure images with a spatial resolution of 6–70 μm were reported by using large-scale laser systems (60–200 TW). Furthermore, images obtained with multiple exposures tended to have a reduced contrast and resolution due to the shot-to-shot fluctuations. In this article, we demonstrate that a highly stable multiple-exposure betatron source, with an effective average source size of 5 μm, photon number and pointing jitters of <5% and spectral fluctuation of <10%, can...

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A rapid auto-indexing technology for designing readable e-learning content

The International Review of Research in Open and Distributed Learning, 2012

A rapid scene indexing method is proposed to improve retrieval performance for students accessing... more A rapid scene indexing method is proposed to improve retrieval performance for students accessing instructional videos. This indexing method is applied to anchor suitable indices to the instructional video so that students can obtain several small lesson units to gain learning mastery. The method also regulates online course progress. These anchored points not only provide students with fast access to specific material but also can link to certain quizzes or problems to show the interactive e-learning content that course developers deposited in the learning management system, which enhances the learning process. This allows students to click on the anchored point to repeat their lesson, or work through the quizzes or problems until they reach formative assessment. Hence, their learning can be guided by the formative assessment results.

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Colliding ionization injection in a plasma wakefield accelerator

Plasma Physics and Controlled Fusion, 2016

The proposal of generating high quality electron bunches via ionization injection triggered by an... more The proposal of generating high quality electron bunches via ionization injection triggered by an counter propagating laser pulse inside a beam driven plasma wake is examined via two-dimensional particle-in-cell simulations. It is shown that electron bunches obtained using this technique can have extremely small slice energy spread, because each slice is mainly composed of electrons ionized at the same time. Another remarkable advantage is that the injection distance is changeable. A bunch with normalized emittance of 3.3 nm, slice energy spread of 15 keV and brightness of 7.2 × 10 18 A m -2 rad -2 is obtained with an optimal injection length which is achieved by adjusting the launch time of the drive beam or by changing the laser focal position. This makes the scheme a promising approach to generate high quality electron bunches for the fifth generation light source.

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Temporal characterization of ultrashort linearly chirped electron bunches generated from a laser wakefield accelerator

Physical Review Accelerators and Beams, 2016

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On the Feasibility of sub-100 nmrad Emittance Measurement in Plasma Accelerators using Permanent Magnetic Quadrupoles

Plasma Physics and Controlled Fusion, 2017

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

Chinese Physics C, 2015

A unique facility for laser plasma physics and advanced accelerator research has been built recen... more A unique facility for laser plasma physics and advanced accelerator research has been built recently at Tsinghua Universtiy. This system is based on Tsinghua Thomson scattering X-ray source (TTX) , which combining an ultrafast TW laser with a synchronized 45MeV high brightness linac. In our recent laser wakefield acceleration experiments, we have obtained 10∼40MeV high quality monoenergetic electron beams by running the laser at 5TW peak power. Under certain conditions, 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.An article usually includes an abstract, a concise summary of the work covered at length in the main body of the article.

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Experimental observation of relativistic cross phase modulation

International Quantum Electronics Conference, 2005.

An intense pump pulse propagating in a plasma can produce relativistic cross phase modulation on ... more An intense pump pulse propagating in a plasma can produce relativistic cross phase modulation on an overlapping copropagating probe pulse to broaden its spectrum. The first experimental observation is presented. When the laser intensity approaches 10 W/cm2, the electron quivering velocity in the laser field approaches the speed of light, leading to relativistic increase of electron mass. This in turn results in change of plasma refractive index, which can be expressed as 2 2

Enhancement of X-ray production in a plasma-waveguide-based laser wakefield accelerator by insertion of a drift space

Research in Optical Sciences, 2014

ABSTRACT Enhancement of X-ray production probability in a plasma-waveguide-based laser wakefield ... more

Coherent phase space matching for staging plasma and traditional accelerator using longitudinally tailored plasma structure

ABSTRACT For the further development of plasma based accelerators, phase space matching between p... more ABSTRACT For the further development of plasma based accelerators, phase space matching between plasma acceleration stages and between plasma stages and traditional accelerator components becomes a very critical issue for high quality high energy acceleration and its applications in light sources and colliders. Without proper matching, catastrophic emittance growth in the presence of finite energy spread may occur when the beam propagating through different stages and components due to the drastic differences of transverse focusing strength. In this paper we propose to use longitudinally tailored plasma structures as phase space matching components to properly guide the beam through stages. Theoretical analysis and full 3-dimensional particle-in-cell simulations are utilized to show clearly how these structures may work in four different scenarios. Very good agreements between theory and simulations are obtained.

Density-Grating-Based Plasma Nonlinear Optics

Nonlinear Optics: Materials, Fundamentals and Applications, 2007

... [11] M.-C. Chou et al., Phys. Rev. A 74, 023804 (2006). [12] E. Esarey et al., IEEE Trans. Pl... more

GeV Electrons and High brightness Betatron X-rays from Petawatt-Laser-Driven Plasma Accelerators

CLEO: 2014, 2014

We identify three regimes of correlated GeV-electron/keV-betatron-x-ray generation by a laser-pla... more

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Enhancement of relativistic harmonic generation by an optically-preformed periodic plasma waveguide

2007 Quantum Electronics and Laser Science Conference, 2007

Enhancement of relativistic third-harmonic generation by using an optically preformed periodic pl... more Enhancement of relativistic third-harmonic generation by using an optically preformed periodic plasma waveguide was achieved. Resonant dependence of harmonic intensity on plasma density and density modulation parameters was observed, which is a distinct characteristic of quasi-phase-matching. The results demonstrate the potential of a modulated plasma waveguide in high-field applications.

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Single-shot visualization of evolving plasma bubble accelerators by frequency-domain streak camera

CLEO: 2014, 2014

ABSTRACT We visualize formation, propagation and collapse of laser-driven plasma bubbles using a ... more

Generation of Dark-current-free Quasi-monoenergetic 1.25 GeV Electrons by Laser Wakefield Acceleration

Conference on Lasers and Electro-Optics 2012, 2012

ABSTRACT We report electron acceleration to 1.25 GeV by laser-driven wakefield acceleration at pl... more

Generation of Quasi-monoenergetic 2 GeV Electrons by Laser Wakefield Acceleration

CLEO: 2013, 2013

ABSTRACT We report self-injected quasi-monoenergetic (5% spread FWHM) acceleration of electrons t... more

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