71 research outputs found
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A comparison of an elliptical multipole wiggler and crystal optics for the production of circularly polarized x-rays
Recently, there has been a great deal of interest in polarization modulated x-ray diffraction and spectroscopy techniques. In particular, the importance of photon helicity in spin-dependent magnetic interactions has expanded the need for high quality circularly polarized x-ray sources with fast switching capabilities. Because circularly polarized photons couple differently with the magnetic moment of an atom than do neutrons, they are able to provide unique magnetic information not accessible by neutron techniques. The development of experiments utilizing circularly polarized x-rays, however, has been hampered by the lack of efficient sources. Two different approaches for the production of circularly polarized x-rays have attracted the most attention; (i) employing specialized insertion devices, and (ii) utilizing x-ray phase retarders based on perfect crystal optics. For soft x-rays (0.1--3.0 keV), source development has centered primarily on insertion devices because there are currently no crystal or multilayer polarizing optics available that cover that full energy range. For harder x-rays (>3.0 keV), however, phase retarding optics have been demonstrated, but whether these optics or insertion devices provide the most efficient circularly polarized x-ray source in this energy regime has remained a matter of contention. Advocates of each method have made qualitative statements about their advantages, i.e., insertion devices provide a larger flux and phase retarders provide a higher degree of circular polarization, yet a detailed quantitative comparison has been lacking. In this paper, we attempt to provide such a comparison by examining the efficiencies of an elliptical multipole wiggler (EMW) and a standard undulator followed by phase retarding crystal optics
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An integral equation based computer code for high-gain free-electron lasers
A computer code for gain optimization of high-gain free-electron lasers (FELs) is described. The electron motion is along precalculated period-averaged trajectories, and the finite-emittance electron beam is represented by a set of thin partial beams. The radiation field amplitudes are calculated at these thin beams only. The system of linear integral equations for these field amplitudes and the Fourier harmonics of the current of each thin beam is solved numerically. The code is aimed for design optimization of high-gain short-wavelength FELs with nonideal magnetic systems (breaks between undulators with quadrupoles and magnetic bunchers; field and steering errors). Both self-amplified spontaneous emission (SASE) and external input signal options can be treated. A typical run for a UV FEL, several gain lengths long, takes only one minute on a Pentium II personal computer (333 MHz) which makes it possible to run the code in optimization loops. Results for the Advanced Photon Source FEL project are presented
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Some practical aspects of undulator radiation properties
It is important to be able to accurately predict the spectral and angular distribution of undulator radiation properties when designing beamlines; at new synchrotron radiation facilities or when performing radiation experiments at already existing beamlines. In practice, the particle beam emittance and beam energy spread must be taken into account in modeling these properties. The undulators fabricated today are made with small RMS phase errors, making them perform almost as, ideal devices. Calculation tools for numerical modeling of undulator radiation sources (ideal and nonideal) will be discussed, and the excellent agreement with experimentally obtained absolute spectral flux measurements of undulator A at the Advanced Photon Source verifies the high accuracy of the computer codes and the high quality of the undulators being built today. Our focus here is on flux properties useful in practical beamline designs, and the chosen examples demonstrate the versatility of computer programs available to beamline designers and experimentalists
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On-Axis Brilliance and Power of In-Vacuum Undulators for the Advanced Photon Source.
A request for studying the spectral performance of in-vacuum undulators (IVUs) for the APS storage ring was recently put forward. In-vacuum undulators are prevalent at other synchrotron radiation facilities such as the ESRF and the Spring-8. However, they never made it into the arsenal of undulators at the APS because the brilliance tuning curves were sufficiently wide due to the fact that the undulator minimum gap could be set as low as 10.5 mm. For sector 3, which in the past used a narrow-gap vacuum chamber, the minimum undulator pole gap was allowed to be set as low as 8.5 mm, providing contiguous tuning curves between the first and third harmonic radiation for a 2.7-cm-period device. (Subsequently, the narrow-gap vacuum chamber was removed and replaced with a standard vacuum chamber, which allows a minimum gap of 10.5 mm.) For sector 4, which currently holds the only narrow-gap vacuum chamber at the APS, the minimum gap is 9.5 mm. In this sector, a permanent magnet hybrid undulator with SmCo magnets is used instead of NdFeB magnets because of their higher radiation resistance and their better protection against radiation-induced demagnetization of the magnets. In the realm of looking to the future, new concepts and technologies are being revisited. Most notable is the superconducting undulator (SCU) technology, which provides the ultimate highest magnetic field of any technology and design. The SCU program has been ongoing at the APS for several years and substantial progress has been made.3,4 However, the in-vacuum undulators may bridge some of the user demands, and it is therefore worthwhile revisiting their potential at the APS. In this work, the following were assumed or required: (1) the smallest in-vacuum beam-stay-clear gap is 5.0 mm, (2) a beam-liner of 2 x 0.060 mm, which increases the pole gap by the same amount, (3) both NdFeB and SmCo magnets shall be studied, even though SmCo magnets are the preferred choice for very small gaps, (4) compare the in-vacuum undulators with superconducting NbTi undulators with a wall thickness/space of 2 x 1.0 mm, and (5) all undulators will have an effective magnetic length of 2.4 m. Three short undulator period lengths were chosen somewhat arbitrary and studied. We will compare the performance of undulators with period lengths of 2.5 cm and 2.0 cm to one with a 1.6-cm period, which is the chosen period length of the first designed and tested short-length SCU for the APS. Additionally, we will make comparisons with the undulator A, which has a period length of 3.3 cm
Design considerations for the magnetic system of a prototype x-ray free-electron laser
A number of difficult technical challenges need to be solved in the fields of accelerator and free-electron laser (FEL) technologies in order to build an X-ray FEL. One of the tasks well suited to the Advanced Photon Source Low Energy Undulator Test Line (LEUTL) is to take the intermediate step of solving some of the problems of single-pass FEL operation in the ultraviolet range. The existing Advanced Photon Source (APS) linac, in addition to its role of supply positrons for the APS storage ring, will also be used to generate the particle beam for the LEUTL. Here, the design of the magnetic system for the high gain soft x-ray free electron laser is described
Multi-dimensional free-electron laser simulation codes : a comparison study.
A self-amplified spontaneous emission (SASE) free-electron laser (FEL) is under construction at the Advanced Photon Source (APS). Five FEL simulation codes were used in the design phase: GENESIS, GINGER, MEDUSA, RON, and TDA3D. Initial comparisons between each of these independent formulations show good agreement for the parameters of the APS SASE FEL
MULTI-DIMENSIONAL FREE-ELECTRON LASER SIMULATION CODES: A COMPARISON STUDY*
Abstract A self-amplified spontaneous emission (SASE) free-electron laser (FEL) is under construction at the Advanced Photon Source (APS). Five FEL simulation codes were used in the design phase: GENESIS, GINGER, MEDUSA, RON, and TDA3D. Initial comparisons between each of these independent formulations show good agreement for the parameters of the APS SASE FEL
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Precision Measurement of the Undulator K Parameter using Spontaneous Radiation
Obtaining precise values of the undulator parameter, K, is critical for producing high-gain FEL radiation. At the LCLS [1], where the FEL wavelength reaches down to 1.5 {angstrom}, the relative precision of K must satisfy ({Delta}K/K){sub rms} {approx}< 0.015% over the full length of the undulator. Transverse misalignments, construction errors, radiation damage, and temperature variations all contribute to errors in the mean K values among the undulator segments. It is therefore important to develop some means to measure relative K values, after installation and alignment. We propose a method using the angle-integrated spontaneous radiation spectrum of two nearby undulator segments, and the natural shot-to-shot energy jitter of the electron beam. Simulation of this scheme is presented using both ideal and measured undulator fields. By ''leap-frogging'' to different pairs of segments with extended separations we hope to confirm or correct the values of K, including proper tapering, over the entire 130-m long LCLS undulator
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The insertion device magnetic measurement facility: Prototype and operational procedures
This report is a description of the current status of the magnetic measurement facility and is a basic instructional manual for the operation of the facility and its components. Please refer to the appendices for more detailed information about specific components and procedures. The purpose of the magnetic measurement facility is to take accurate measurements of the magnetic field in the gay of the IDs in order to determine the effect of the ID on the stored particle beam and the emitted radiation. The facility will also play an important role when evaluating new ideas, novel devices, and inhouse prototypes as part of the ongoing research and development program at the APS. The measurements will be performed with both moving search coils and moving Hall probes. The IDs will be evaluated by computer modeling of the emitted radiation for any given (measured) magnetic field map. The quality of the magnetic field will be described in terms of integrated multipoles for the effect on Storage Ring performance and in terms of the derived trajectories for the emitted radiation. Before being installed on the Storage Ring, every device will be measured and characterized to assure that it is compatible with Storage Ring requirements and radiation specifications. The accuracy that the APS needs to achieve for magnetic measurements will be based on these specifications
Self-amplified spontaneous emission saturation at the Advanced Photon Source free-electron laser (abstract) (invited)
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