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Differential cross sections for elastic Compton scattering from $^4$He have been measured with high statistical precision at the High Intensity $\gamma$-ray Source at laboratory scattering angles of $55^\circ$, $90^\circ$, and $125^\circ$ using a quasi-monoenergetic photon beam with a weighted mean energy value of $81.3$ MeV. The results are compared to previous measurements and similar fore-aft asymmetry in the angular distribution of the differential cross sections is observed. This experimental work is expected to strongly motivate the development of effective-field-theory calculations of Compton scattering from $^4$He to fully interpret the data.

The short wavelength operation of free-electron laser (FEL) oscillators is limited by the availability of high-reflectivity, thermally stable, and radiation-resistant FEL mirrors in the vacuum UV (VUV) wavelength. We report our recent work to extend the shortest lasing wavelength of the oscillator FEL to 168.6 nm using a storage ring FEL. This progress has been made possible by developing a new FEL configuration with substantially reduced undulator harmonic radiation on the FEL mirror, a thermally stable FEL optical cavity, and a new type of high-reflectivity fluoride-based multilayer coating with a protective capping layer. Using these fluoride-based mirrors, we have demonstrated storage ring FEL lasing from 168.6 to 179.7 nm with excellent beam stability. Employing this VUV FEL in Compton scattering, we have produced the first 120 MeV gamma rays at the High Intensity Gamma-ray Source (HIGS). Operating the HIGS in this new high-energy region will create many new opportunities for photonuclear physics research, in particular, the low-energy quantum chromodynamics research.

Compton scattering, where the electromagnetic field of a real photon induces radiation multipoles by displacing charges and currents inside the nucleon, is an important probe of both the structure of light nuclei and the nucleon polarizabilities. This paper presents the first extensive and precise measurements of elastic Compton scattering on ${}^{4}$He. These results should allow precision comparisons with new calculations expected from both lattice gauge and effective field theories.

Touschek lifetime of an electron beam in a storage ring depends on the beam polarization through the intrabeam scattering effect. Therefore, the electron beam polarization can be determined from the Touschek lifetime measurement. In this paper, we report on a systematic experimental procedure we developed to study the radiative polarization of a stored electron beam. Using this technique, we successfully observed the radiative polarization of an electron beam in the Duke storage ring, and determined the equilibrium degree of polarization and the time constant of the polarization build-up process.

With a wide wavelength tuning range, free-electron lasers (FELs) are well suited for producing simultaneous lasing at multiple wavelengths. We present the first experimental results of a novel two-color storage ring FEL. With three undulators and a pair of dual-band mirrors, the two-color FEL can lase simultaneously in infrared (IR) around 720 nm and in ultraviolet (UV) around 360 nm. We have demonstrated independent wavelength tuning in a wide range (60 nm in IR and 24 nm in UV). We have also realized two-color harmonic operation with the UV lasing tuned to the second harmonic of the IR lasing. Furthermore, we have demonstrated good power stability with two-color lasing, and good control of the power sharing between the two colors.

In this paper, we present an attempt to improve the field quality in the pseudo-helical wigglers used in the OK-5 VUV FEL at Duke. One type of “imperfection” of a wiggler intrinsic for any magnetic system and affecting the dynamic aperture is non-linear focusing. We tried to find an approach to reduce affect of the wiggler on dynamic aperture. Another kind of imperfection of the field in the pseudo-helical wigglers are caused by asymmetry of design. In our case the asymmetry of the coils allows the presence of small average quadrupole and octupole fields. We used small asymmetric cuts in the poles to eliminate these terms. The magnetic field in the regular part of the wiggler was simulated with the use of 3D code “MERMAID”. All four wigglers have been fabricated and measured. Magnetic measurements show good agreement with the simulations.

The OK-5/Duke storage ring free electron laser (FEL) project was started in 1998. Presently, the components of the OK-5 FEL and the new South straight section are in the final stage of manufacturing. This paper describes the design and the main features of the OK-5/Duke storage ring FEL. The basic concepts and main compromises made in the design process are presented. Plans for the OK-5 FEL commissioning are discussed.

In this paper, we present the design and parameters of electromagnetic wigglers with controllable polarization for the VUV OK-5/Duke storage ring FEL. The OK-5 FEL, the first distributed optical klystron, is comprised of four wigglers and three matching sections with individually controlled quadrupoles and bunchers. The geometry and the relative strength of horizontal and vertical fields determine the polarization of the radiation from the OK-5 wigglers. We compare the predicted and measured quality of the wiggler fields.

In this paper, we present a possible lattice for the Duke storage ring with horizontal emittance of 1.4 nm rad at an energy of 1 GeV. The new lattice was constrained to fit the layout of the existing storage ring and to re-use existing magnets. Within these constrains, the improvement of the emittance (from current 18 nm rad) is possible due to the use in the arcs of combined function bending magnets with strong dipole, quadrupole and sextuple fields. We present the results of 2D and 3D simulations of magnetic fields for these magnets. We discuss the choice of the arc's lattice cell and the per-cell tune advance. The lattice is based on the concept of local compensation of the non-linear geometrical aberrations. Preliminary studies of the dynamic aperture for this lattice are very encouraging. We briefly discuss the possibility further emittance reduction using new magnets for the arcs.

The Duke storage ring is a light source ring optimized for driving the Free Electron Lasers (FELs). To take advantage of the high brightness of the electron beam in the storage ring, a next generation FEL source — an elliptically polarized OK-5 FEL system, has been designed to match the electron beam quality of the Duke storage ring. In this paper, we present the storage ring lattice modifications which are necessary to accommodate the 24 m long OK-5 FEL. Because of its length and strong nonlinear focusing, the OK-5 FEL is expected to have a significant impact on the electron beam dynamics. We also present the preliminary results on the dynamic aperture calculation for the OK-5 FEL operated in different modes.

The quadrupoles and sextupoles for the Swiss Light Source, a 2.4 GeV electron storage ring, had severe requirements on the manufacturing tolerances and the alignment of their magnetic axis. The 306 multipoles were manufactured, and magnetically measured at BINP and after delivery also at Paul Scherrer Institute. In this paper, the Rotating Coil Systems for precise magnetic measurements is described, and for the series magnets the main results of the magnetic measurements are also presented.

Summary form only given. For the FEL (Free Electron Laser) complex in Duke University five pulse generators (kickers) were developed and built. As fast switches TPI-type thyratrons (pseudospark switches) are used. In contrast with other pseudospark switches TPI-thyratrons are capable of operating in circuitry with a grounded cathode which allows obtaining service life time up to 20 000 hours. The thyratrons can be considered as an alternative to well-known switches, including hot cathode hydrogen thyratrons and up-to-date power solid state switches, whereas being more compact and cost effective compared to the latter. The absence of a hot cathode provides for turn-on time for the thyratrons of about 3-5 ns in modes with thousands-amperes peak currents and sub-nanosecond jitter. Injection generator is built using compact TPI1-1k/20 thyratron, making in a load with impedance of 50 ohm square bi-polar pulses with duration of 100 ns, pulse leading edge of 5-10 ns and "forepart flattop" instability not greater than plusmn5% at the peak overall voltage on deflecting plates from 5 to 10 kV. Extraction generator is built also on TPI1-1k/20 thyratron. It provides in a load with impedance of 50 ohm pulses with duration at level 0.1 not more than 15 ns at the peak overall voltage on plates from 8 to 40 kV. Generator of injection into a big ring is built on power TPI1-10k/20 thyratrons. With plate impedance of 25 ohm it provides for pulse duration at level 0.1 not more than 100 ns at the peak overall voltage on plates from 10 to 50 kV. Results of pulse generators service within over 1.5 years are presented.

At Duke University, a booster synchrotron was recently commissioned as part of the high intensity gamma-ray source (HIGS) upgrade. For the ramping magnet power supply controls, a scheme was developed to present the high level operator interface in terms of the physics quantities of the accelerator, i.e. the effective focusing strength of the magnets. This scheme allows for the nonlinearities of the magnets - a result of the extremely compact footprint of this booster - to be incorporated into the low level software. This facilitates machine studies and simplifies the use of physics modeling. In addition, it simplifies operation, allowing the booster to ramp to any energy from the 0.24 GeV of the injector linac to the 1.2 GeV maximum of the Duke storage ring. The high level of flexibility of this system is further advanced by incorporating the level of tunability typically found in a storage ring control system. Tuning changes made during steady-state operation are automatically propagated to the waveforms which make up the booster ramp. This approach provides a good match to the wide operation modes of the Duke storage ring and its associated free electron laser (FEL), and may be useful for other compact booster synchrotrons.

A booster synchrotron has been recently commissioned at Duke University FEL Laboratory as a part of the High Intensity Gamma-ray Source (HIGS) facility. The booster provides top-off injection into the storage ring in the energy range of 0.24-1.2 GeV. In order to minimize the cost of the project, the booster is designed with a very compact footprint. As a result, unconventionally high field bending magnets at 1.76 T are required. A main ramping power supply drives all dipoles and quadrupoles. Quadrupole trims are used to compensate for tune changes caused by the change of relative focusing strength during ramping. Sextupoles compensate for chromatic effects caused by dipole magnet pole saturation. All these compensations have to be performed as a function of beam energy. Above 1.1 GeV, where the magnets are heavily saturated, the reduction of dynamic aperture is compensated by redistribution of strength among the sextupole families. With these compensations, effects of the magnet saturation do not cause any considerable beam loss during injection, energy ramping, and extraction.

Low cost power supply system for compact full energy booster synchrotron was designed, developed and successfully commissioned at Duke University. 500 kW second hand thyristor controlled power supply has been completely rebuilt to provide high accuracy ramping of current in the range between 150 A and 700 A in a 1.3 sec repetition cycle. Reproducibility of current at injection and extraction energy of better than 0.2 % was achieved. Conflict of requirements of a fast ramp operation and a magnet protection in the case of emergency shutdown was resolved by means of additional thyristor switches. All trim power supplies involved in ramp have been matched with main power supply for the time response and voltage range. Vertical injection to and extraction from the booster requires a strong Y-bump. Combination of low voltage DC power supply and pulse boosting circuit has eliminated the need of expensive power supply for peak power about 4 kW. Challenges of design, main parameters of the booster power supply system and discussion of operation experience are presented in this paper.

A betatron tune measurement system is one of the most important diagnostic tools for any circular accelerator. For the commissioning of a booster synchrotron newly developed for top-off injection into the Duke storage ring, a versatile tune measurement system employing a network analyzer, a short stripline kicker, a space filter and a photomultiplier tube has been developed to allow tune measurements over a wide range of beam energies from 0.24 to 1.2 GeV, and for low current measurements at a few micro-amperes. The signal from the highly sensitive optical detection system is also used for fast tune measurements with a realtime oscilloscope. This technique is being developed for live tune monitoring during booster energy ramping.

As part of the high intensity gamma-ray source (HIGS) upgrade, the booster synchrotron has been built and commissioned in 2006. It ramps the electron beam between 0.24 and 1.2 GeV for top-off injection into the Duke storage ring. The booster has vertical injection/extraction which uses symmetrical schemes with a bumped orbit. The injection/extraction kickers and corresponding septum magnets are located in the opposite straight sections of the booster ring separated by about 1/4 of the vertical betatron wave. Due to the non-ideal properties of the magnetic material, the magnetic field leaks out into the stored beam chamber, which results in orbit distortion, tune and chromaticity shifts, and change of coupling. The dynamics impacts due to the leaked septum fields have been successfully compensated to achieve good injection efficiency.

A booster synchrotron (Duke booster) has been built and recently commissioned at Duke University Free Electron Laser Laboratory (DFELL) as part of the High Intensity Gamma-ray Source (HIGS) facility upgrade. HIGS is collaboration between the DFELL and Triangle Universities Nuclear Laboratory (TUNL). The booster provides top-off injection into the Duke FEL storage ring in the energy range of 0.24 -1.2 GeV. When operating the Duke storage ring to produce high energy Compton gamma ray beams above 20 MeV, continuous electron beam loss occurs. The lost electrons are replenished by the booster injector operating in the top-off mode. The present operational injection and extraction rate of the machine allows us to routinely replenish up to 5-108 electrons per second. The compactness of the booster posed a challenge for its development and commissioning. The booster has been successfully commissioned in 2006. This paper reports experience of commissioning and initial operation of the booster.

First operational experience has been gained with the linac and booster diagnostic system during the commissioning of the booster synchrotron at Duke University. The booster was designed and developed as an injector for the storage ring as a part of the High Intensity Gamma-ray Source (HIGS) upgrade of Duke FEL storage ring. Booster beam instrumentation includes: beam charge measurements (Faraday cups, Integrated Current Transformers (ICT), Modular Parametric Current Transformer (MPCT)), beam position monitoring (BPM), betatron tune measurements using synchrotron radiation (SR), transverse profile and temporal beam structure monitoring (insertable screens, striplines, dissector). The diagnostics provided good understanding of electron beam behavior and allowed us to adjust important beam parameters within design specifications. An overview of the diagnostic instrumentation of the Duke Booster synchrotron is given along with measurement examples and discussion of operational experience.

This Letter reports the first experimental results from the world's first distributed optical-klystron (DOK) free-electron laser (FEL), the DOK-1 FEL, at Duke University. The DOK-1 FEL is a hybrid system, comprised of four wigglers: two horizontal and two helical. With the DOK-1 FEL, we have obtained the highest FEL gain among all storage ring based FELs at 47.8% ($\ifmmode\pm\else\textpm\fi{}2.7%$) per pass. We have also demonstrated that the FEL gain can be enhanced by increasing electron bunching using wigglers with a different polarization. Furthermore, we have realized controlled polarization switches of the FEL beam by a nonoptical means through the manipulation of a buncher magnet.

A new 178 MHz RF system has been commissioned at Duke Storage Ring. It consists of a 140 kW tetrode transmitter; higher order modes (HOM) damped RF cavity and the necessary frequency and voltage control electronics. The cavity walls are made of copper-on-stainless steel bimetal (8 mm Cu, 7 mm SS). The cavity has a larger beam pipe opening (700 mm in diameter) in the down-stream side, which allows the HOM propagating out of the cavity and being absorbed by the ceramic loads. The design details and the commissioning results are presented in this paper.

This paper presents the methodology and initial results for mechanical alignment of the booster synchrotron for the Duke Free Electron laser Laboratory (FELL) storage ring. The booster is a compact design and requires special considerations for alignment. All magnets and vacuum chambers in the arcs have been designed such that a laser tracker can be best utilized for alignment. A parametric 3D design package has been used to determine target coordinates. These target coordinates evolve from design goals to physically verified dimensions by modifying the parametric model to match mechanical measurement data after fabrication. By utilizing the functionality of a laser tracker system [1] and a parametric 3D modeler, a direct and efficient measurement and alignment technique has been developed for the booster ring’s complex geometry.

A dedicated booster synchrotron is being constructed at the Duke FEL Laboratory to provide fullenergy injection into the main electron storage ring. A new timing system has been developed to coordinate the injection of electron bunches from the linac to the booster, the ramping of energy in the booster, and extraction of bunches into the main ring. The timing system will allow the extraction of any bunch in the booster into any selected bucket in the main ring to provide top-off injection for any of the various operational bunch patterns of the main ring. A new master oscillator has also been developed for the RF system of the booster. The oscillator may be tuned independently or phase-locked to the master oscillator of the main ring. The issues of the soft phase locking process of the new master oscillator are discussed. The timing system and new oscillator have been fabricated and tested and are ready for operation.

The Duke Free Electron Laser Laboratory is developing a booster synchrotron to provide full energy injection into the Duke electron storage ring. In this paper, we describe the development of the control system for the booster. Requirements include the competing needs of simple and reliable turn-key operation for the machine as a booster; and the sophistication and flexibility of operation of the machine as a storage ring for commissioning, machine studies and as a light source. To simplify operations and machine studies, the high level controls will present the system in terms of the physics quantities of the accelerator, allowing a tight integration between the physics model and the low level hardware control, as we have previously implemented for the Duke storage ring.

The full energy booster injector for the Duke FEL storage ring is presently under installation [2]. The booster is designed to provide continuous injection into the Duke FEL storage ring in top-off mode at the energy variable from 0.27 GeV to 1.2 GeV. The magnetic elements for the booster have been fabricated and magnetically measured at Budker Institute of Nuclear Physics, Russia. The paper presents magnetic and mechanical design of the booster dipole and quadrupole magnets and results of their magnetic measurements. Results of simulation of the booster lattice taking into account residual field and non-linearity of the magnets are also presented.

The next generation electromagnetic OK-5/Duke storage ring FEL wigglers [1], [2] require three 3kA/70V power supplies with current stability about 20 ppm and current ripples less than 20 ppm in their full operating range. Duke FEL Laboratory acquired three out-of-service SCR controlled power supplies (Trans-Rex, 5kA/100V), which were built almost 30 years ago. The existing archaic firing circuit, lack of any output voltage filtering and with an outdated DCCT, would not be able to meet the above requirements. To deliver the desirable high performance with very limited funds, all three Trans-Rex power supplies have been completely rebuilt in-house at DFELL. Modern high stability electronic components and a high precision Danfysik DCCT have been used. A new symmetrical firing circuit, efficient passive filter and reliable transformer-coupled active filter are used to reduce output current ripples to an appropriate level. At the present time, all three refurbished power supplies are in operation. One of these power supplies was used since August, 2004 to feed OK-4 wigglers with good overall performance. Others two have been tested and used as power supplies for magnetic measurements.

This paper presents the current status of the booster synchrotron for the Duke Free Electron Laser Laboratory (DFELL) storage ring. The booster will provide full energy injection into the storage ring in a wide energy range from 0.27 to 1.2 GeV. When operating the DFELL storage ring as the High Intensity Gamma Source (HIGS) to produce gamma photons above 20 MeV with Compton scattering, continuous electron loss occurs. The top-off mode operation of the booster injector will enable the continuous operation of the HIGS facility by replenishing the lost electrons. The design requirement for a compact booster with the single bunch extraction capability remains a challenge for the machine development. Presently, the booster project is in the installation phase. The magnetic elements, vacuum chambers, injection and extraction kickers have been developed in collaboration with and fabricated at the Budker Institute of Nuclear Physics (BINP), Russia. The diagnostic and control system is being developed at DFELL. The commissioning of the booster synchrotron will start at the end of 2005.

Photon analyzing power for the three‐body breakup of 3He has been studied at Eγ = 15.0 MeV. The measurements were done at the HIγS facility at Duke University.

Big-bang nucleosynthesis (BBN) is an observational cornerstone of the hot big-bang cosmology. According to [1] the p(n,γ)d reaction is of special interest because the big-bang abundance of deuterium is most sensitive to the baryon density. However, in the most important energy range for BBN, i.e., from 25–200 keV in the c.m. system, experimental data are completely missing. In fact, published data exist only at thermal energies and at n-p c.m. energies above 275 keV. Therefore, theoretical models have been used [1] in the BBN energy range that are normalized to the high-precision thermal neutron capture cross-section data.

The photon analyzing power for the photodisintegration of the deuteron was measured for seven gamma-ray energies between 2.39 and 4.05 MeV using the linearly polarized gamma-ray beam of the High-Intensity Gamma-ray Source at the Duke Free-Electron Laser Laboratory. The data provide a stringent test of theoretical calculations for the inverse reaction, the neutron-proton radiative capture reaction at energies important for Big-Bang Nucleosynthesis. Our data are in excellent agreement with potential model and effective field theory calculations. Therefore, the uncertainty in the baryon density obtained from Big-Bang Nucleosynthesis can be reduced at least by 20%., Comment: 5 pages, 5 figures

The OK-4 is the first storage ring FEL operating in the United States. It was commissioned in November, 1996 and demonstrated lasing in the near UV and visible ranges (345-413 nm) with extracted power of 0.15 W. In addition to lasing, the OK-4/Duke FEL generated a nearly monochromatic (1% FWHM) /spl gamma/-ray beam. In this paper we describe the initial performance of the OK-4/Duke storage ring FEL and /spl gamma/-ray source.

At the Duke FEL Lab, we have developed a new storage ring control system in terms of the physics quantities of the accelerator. Instead of controlling power supply currents in Amperes, this system controls the effective focusing of magnets. By directly controlling the physics quantities, this control system allows tighter integration of the physics model based high level controls with the EPICS based low-level controls. EPICS events have been extensively used to provide time synchronization during the energy and lattice ramping. This new control system also facilitates the implementation of multiple functions on shared control channels. As a result, the physics based control system simplifies many complex control tasks, improves the beam stability during ramping, and facilitates machine studies. With better understanding of the accelerator, it is possible to fine tune this control system to present users with a virtual accelerator whose operation is independent of the ring energy.

Single particle dynamics in the storage ring can be significantly influenced by strong nonlinearities from long and strong-field insertion devices. This paper reports our preliminary results on the dynamics impact of the 24 m long OK5 FEL in the Duke ring. Initial studies are performed using an intermediate lattice with two OK5 wigglers. The dynamic aperture is computed using a recently developed symplectic wiggler integrator and the frequency map technique, NAFF We have observed significant dynamic aperture reduction due to OK5 wigglers at lower beam energies. We also report our preliminary findings on the means to improve beam dynamics with wigglers.

As part of the recent Duke storage ring hardware upgrade (2001-2002), a power supply improvement program was put in place to bring all major DC supplies to their specifications. In carrying out this program, power supplies have been modified, tuned, and thoroughly tested. In its actual operation configuration, each power supply was subject to extensive testing to determine its DC stability, reproducibility and linearity, AC ripple and noise, and ramping performance. As a result, all major DC supplies have been improved to meet most important performance specifications for 1 GeV operation.

In this paper we present current status of the Booster Synchrotron for the Duke FEL storage ring. The Booster which is recently under design, fabrication and construction, will provide full energy injection into the storage ring at energy from 0.3 to 1.2 GeV. The Duke storage ring FEL (SR FEL) operates in lasing mode with 193-700 nm wavelength range. The geometry of the Duke SR FEL provides for interacting head-on collision of e-beam and FEL photons. This mode of operation is used to generate intense beams of /spl gamma/-rays from 2 MeV to about 200 MeV (currently from 2 MeV to 58 MeV). Generation of /spl gamma/-rays with energy exceeding 20 MeV causes the loss of electrons, which will be replaced by injection from the Booster operating in a top-off mode. The paper presents design and status for elements of magnetic system and vacuum system, as well as design and parameters of fast extraction kicker with 11 nS pulse duration. All these element are designed and will be fabricated by Budker Institute of Nuclear Physics, Novosibirsk, Russia.

In this paper we present a project of a Booster-injector for the Duke FEL storage ring. The Booster will provide full energy injection into the storage ring at energy from 0.25 to 1.2 GeV. The Duke storage ring FEL (SR FEL) operates in lasing mode with 193-700 nm wavelength range. This range will be extended into VUV in near future. The geometry of the Duke SR FEL provides for head-on collision of e-beam and FEL photons. This mode of operation is used for High Intensity /spl gamma/-ray Source (HI/spl gamma/S facility) generating intense beams of /spl gamma/-rays from 1 MeV to about 200 MeV (currently from 2 MeV to 58 MeV). Generation of /spl gamma/-rays with energy exceeding 20 MeV causes the loss of electrons, which will be replaced by new electrons from the Booster, operating in a top-up mode. The Booster has a robust FODO lattice. In this paper we present the design of the Booster and its magnetic elements. For 2D and 3D magnet design of the magnetic elements we used MERMAID 2D/3D code.

Two types of electromagnetic undulators are described. The first one has 33.5 periods with a length of 10 cm and a magnetic field amplitude of 5.3 kG. The other, developed for future FEL experiments, has 39.5 periods with 7 cm length and a magnetic field amplitude of 55 kG.