Browsing by Author "Wu, Ying K"

The High Intensity Gamma-ray Source (HIGS) at Duke University is a world class Compton light source facility. At the HIGS, a Free-Electron Laser (FEL) beam is Compton scattered with an electron beam in the Duke storage ring to produce an intense, highly polarized, and nearly monoenergetic gamma-ray beam with a tunable energy from about 1 MeV to 100 MeV. This unique gamma-ray beam has been used in a wide range of basic and application research fields from nuclear physics to astrophysics, from medical research to homeland security and industrial applications.

The capability of accurately predicting the spatial, spectral and temporal characteristics of a Compton gamma-ray beam is crucial for the optimization of the operation of a Compton light source as well as for the applications utilizing the Compton beam. In this dissertation, we have successfully developed two approaches, an analytical calculation method and a Monte Carlo simulation technique, to study the Compton scattering process. Using these two approaches, we have characterized the HIGS beams with varying electron beam parameters as well as different collimation conditions. Based upon the Monte Carlo simulation, an end-to-end spectrum reconstruction method has been developed to analyze the measured energy spectrum of a HIGS beam. With this end-to-end method, the underlying energy distribution of the HIGS beam can be uncovered with a high degree of accuracy using its measured spectrum. To measure the transverse profile of the HIGS beam, we have developed a CCD based gamma-ray beam imaging system with a sub-mm spatial resolution and a high contrast sensitivity. This imaging system has been routinely used to align experimental apparatus with the HIGS beam for nuclear physics research.

To determine the energy distribution of the HIGS beam, it is important to know the energy distribution of the electron beam used in the collision. The electron beam energy and energy spread can be measured using the Compton scattering technique. In order to use this technique, we have developed a new fitting model directly based upon the Compton scattering cross section while taking into account the electron-beam emittance and gamma-beam collimation effects. With this model, we have successfully carried out a precise energy measurement of the electron beam in the Duke storage ring.

Alternatively, the electron beam energy can be measured using the Resonant Spin Depolarization technique, which requires a polarized electron beam. The radiative polarization of an electron beam in the Duke storage ring has been studied as part of this dissertation program. From electron-beam lifetime measurements, the equilibrium degree of polarization of the electron beam has been successfully determined. With the polarized electron beam, we will be able to apply the Resonant Spin Depolarization technique to accurately determine the electron beam energy. This on-going research is of great importance to our continued development of the HIGS facility.

The Duke Free-electron laser (FEL) system, driven by the Duke electron storage ring, has been at the forefront of developing new light source capabilities over the past two decades. In 1999, the Duke FEL demonstrated the first lasing of a storage ring FEL in the vacuum ultraviolet (VUV) region at $194$ nm using two planar OK-4 undulators. With two helical undulators added to the outboard sides of the planar undulators, in 2005 the highest FEL gain ($47.8\%$) of a storage ring FEL was achieved using the Duke FEL system with a four-undulator configuration. In addition, the Duke FEL has been used as the photon source to drive the High Intensity $\gamma$-ray Source (HIGS) via Compton scattering of the FEL beam and electron beam inside the FEL cavity. Taking advantage of FEL's wavelength tunability as well as the adjustability of the energy of the electron beam in the storage ring, the nearly monochromatic $\gamma$-ray beam has been produced in a wide energy range from $1$ to $100$ MeV at the HIGS. To further push the FEL short wavelength limit and enhance the FEL gain in the VUV regime for high energy $\gamma$-ray production, two additional helical undulators were installed in 2012 using an undulator switchyard system to allow switching between the two planar and two helical undulators in the middle section of the FEL system. Using different undulator configurations made possible by the switchyard, a number of novel capabilities of the storage ring FEL have been developed and exploited for a wide FEL wavelength range from infrared (IR) to VUV. These new capabilities will eventually be made available to the $\gamma$-ray operation, which will greatly enhance the $\gamma$-ray user research program, creating new opportunities for certain types of nuclear physics research.

With the wide wavelength tuning range, the FEL is an intrinsically well-suited device to produce lasing with multiple colors. Taking advantage of the availability of an undulator system with multiple undulators, we have demonstrated the first two-color lasing of a storage ring FEL. Using either a three- or four-undulator configuration with a pair of dual-band high reflectivity mirrors, we have achieved simultaneous lasing in the IR and UV spectral regions. With the low-gain feature of the storage ring FEL, the power generated at the two wavelengths can be equally built up and precisely balanced to reach FEL saturation. A systematic experimental program to characterize this two-color FEL has been carried out, including precise power control, a study of the power stability of two-color lasing, wavelength tuning, and the impact of the FEL mirror degradation. Using this two-color laser, we have started to develop a new two-color $\gamma$-ray beam for scientific research at the HIGS.

Using the undulator switchyard, four helical undulators installed in the beamline can be configured to not only enhance the FEL gain in the VUV regime, but also allow for the full polarization control of the FEL beams. For the accelerator operation, the use of helical undulators is essential to extend the FEL mirror lifetime by reducing radiation damage from harmonic undulator radiation. Using a pair of helical undulators with opposite helicities, we have realized (1) fast helicity switching between left- and right-circular polarizations, and (2) the generation of fully controllable linear polarization. In order to extend these new capabilities of polarization control to the $\gamma$-ray operation in a wide energy range at the HIGS, a set of FEL polarization diagnostic systems need to be developed to cover the entire FEL wavelength range. The preliminary development of the polarization diagnostics for the wavelength range from IR to UV has been carried out.

Over the past three decades, laser beams with complex amplitude and phase structures, especially orbital angular momentum (OAM) beams, have been extensively investigated. Researchers have found a wide range of applications for OAM beams spanning a vast range of distance scales, from fundamental physics at the atomic level with modified selection rules, to macroscopic use such as optical tweezers, to probing of the universe such as detection of rotating black holes.

While structured light beams in the visible and longer wavelength regimes can be generated using many techniques, at shorter wavelengths, from vacuum ultraviolet to x-rays to gamma rays, it is much more challenging to produce such light beams. In recent years, to generate structured light in the shorter wavelengths, particle accelerator-based light sources, such as magnetic undulators and free-electron lasers (FELs), have been explored as a promising candidate. While the FEL work was mostly limited to single-pass FELs, we recognized that the oscillator FEL is very attractive for producing high-quality OAM beams with high intracavity power. In this work, we report the first experimental generation of a particular kind of structured light, a coherently mixed (CM) OAM beam, using the Duke storage ring FEL. The coherently mixed OAM beams have been generated up to the fourth order. This was made possible by modifying the FEL cavity to obtain cylindrical symmetry, while suppressing the low-order transverse modes. The cavity modification was implemented using a set of specially developed masks, including an annulus mask and a disk mask.

On the other hand, a reliable and rapid assessment of the structured light has a wide range of applications in the laser development, including high-quality OAM beam generation, optical characterization of beam quality and mode contents, and manipulation and correction of distorted OAM beams. While the diagnostic methods for structured light have been widely investigated in long wavelengths during recent years, they are not available for the short-wavelength regimes due to wavelength limitations of optics used. We report here two general diagnostic techniques for structured light: a phase retrieval method for wavefront reconstruction; and a modal analysis method for assessing the mode contents and beam quality of a structured laser beam. These newly developed methods involve very few optics, and in principle, can be used in a wide range of wavelengths, from infrared to visible to UV and x-ray.

The produced coherently mixed OAM FEL beams are found to possess good beam quality, excellent stability and reproducibility, and substantial intracavity power. Using the aforementioned diagnostic techniques, we have analyzed the measured FEL beam images to retrieve the complex wavefront and mode content. These beams have been found to have good mode quality, dominated by two degenerate OAM modes of the same order but opposite helicities. A pulsed mode operation of the OAM FEL beam has also been developed using an external drive, in which the OAM beams exhibit a highly reproducible temporal structure when the pulsing frequency is varied from 1 Hz to 30 Hz.

The development of OAM FEL beams using the storage ring FEL has paved the way for short-wavelength OAM laser beam generation using future FEL oscillators operating in the extreme ultraviolet and x-ray regimes. The operation of the storage ring FEL also paves the way for the generation of OAM gamma-ray beams via Compton scattering.

The Duke storage has been developed as a dedicated driver for the storage ring based free-electron lasers (FEL) and a high flux Compton gamma-ray source, the High Intensity Gamma-ray Source. The storage ring can be operated from about 250 MeV to 1.2 GeV, which can produces FEL lasers over a wide range of wavelengths and gamma-rays with a tunable energy from 1 MeV to 100 MeV. The Duke light source facility conducts world-class researches across a wide range of scientific disciplines and technological applications.

In a storage ring, beam instabilities can cause a signifcant degradation in machine performance. In the Duke storage ring, coupled-bunch instabilities (CBIs) are the main source which limit ultimately achievable beam current in multi-bunch operations. In order to to suppress CBIs in the Duke storage, we developed a bunch-bybunch longitudinal feedback (LFB) system which is based on a field programmable gate array (FPGA) embedded system. During the design and implementation of the LFB system, several novel methods and techniques are developed in numerical analysis of feedback control and kicker cavity design/fabrication. High current are realized at low energies by using the LFB system. In addition, after the successful commissioning of the LFB system, a analog transverse feedback (TFB) system has been upgraded to a digital one using the same technique as the LFB system.

The LFB system has been routinely operated for HIGS. Additional,the LFB and TFB feedback systems become an useful diagnostic tools in researches of electron beam dynamics, FEL lasing process, and background of HIGS. The control of CIBs in different operation modes are studied using the feedback system. Furthermore, based on the TFB system, a novel bunch cleaning method has been developed to reduce the background of gamma-ray.

The Duke electron storage ring, first commissioned in November of 1994, has been developed as a dedicated driver for storage ring free-electron lasers (SRFELs) operating in a wide wavelength range from infrared, to visible, to ultraviolet (UV) and vacuum ultraviolet (VUV). The storage ring has a long straight section for various insertion devices and can be operated in a wide energy range (0.25 GeV to 1.15 GeV).

Commissioned in 1995, the first free-electron laser (FEL) on the Duke storage ring was the OK-4 FEL, an optical klystron with two planar undulators sandwiching a buncher magnet. In 2005, the OK-5 FEL with two helical undulators was commissioned. Operating four undulators -- two OK-4 and two OK-5 undulators, the world's first distributed optical klystron FEL was brought to operation in 2005. Via Compton scattering of FEL photons and electrons in the storage ring, the Duke FEL drives the world's most powerful, nearly monochromatic, and polarized Compton

gamma-ray source, the High Intensity Gamma-ray Source (HIγS). Today, a variety of configurations of the storage ring FELs at Duke have been used in a wide range of research areas from nuclear physics to biophysics, from chemical and medical research to industrial applications.

The capability of accurately measuring the storage ring electron beam energy spread is crucial for understanding the longitudinal beam dynamics and the dynamics of the storage ring FEL. In this dissertation, we have successfully developed a noninvasive, versatile, and accurate method to measure the energy spread using optical klystron radiation. Novel numerical methods based upon the Gauss-Hermite expansion have been developed to treat both spectral broadening and modulation on an equal footing. Through properly configuring the optical klystron, this energy spread measurement method has a large dynamic range. In addition, a model-based scheme has been developed for correcting the electron beam emittance related inhomogeneous spectral broadening effect, to further enhance the accuracy of measuring the electron beam energy spread.

Taking advantage of the direct measurement method of the electron beam energy spread, we have developed another novel technique to simultaneously measure the FEL power, electron beam energy spread, and other beam parameters. This allowed us to study the FEL power in a systematic manner for the first time. Based on the experimental findings and results of the theoretical predictions, we have proposed a compact formula to predict the FEL power using only the knowledge of electron

beam current, beam energy, and bunch length.

As part of the dissertation work, we have developed a self-consistent numerical model to study the storage ring FEL. The simulation program models the electron beam propagation along the storage ring, multi-turn FEL interaction in the undulators, gradual intra-cavity optical power buildup, etc. This simulation code captures the main features of a storage ring FEL at different time and space scales. The simulated FEL gain has been benchmarked against measured gain and calculated

gain with good agreement. The simulation package can provide comprehensive information about the FEL gain, optical pulse growth, electron beam properties, etc. In the near future, we plan to further improve the simulation model, by including additional physics effects such as microwave instability, to make it a more useful tool for FEL research.