Undulator Gap Research Articles

Optimization of accelerator system performance at the National Synchrotron Light Source (invited)

There is an active program of accelerator development at the National Synchrotron Light Source (NSLS) aimed at improving reliability, stability, and brightness. Work is primarily focused on providing improved performance for the NSLS user community, however, important elements of our work have a generic character and should be of value to other synchrotron radiation facilities. In particular, we have successfully operated a small gap undulator with a full vertical beam aperture of only 3.8 mm, with no degradation of beam lifetime. This provides strong support for the belief that small gap, short period devices will play an important role in the future.

First observation of undulator radiation from APPLE-1

Various polarized radiation was observed in the visible region generated by the new type undulator APPLE-1 (Advanced Planar Polarized Light Emitted - 1). The undulator was installed in the low energy electron storage ring JSR and we have succeeded in observing linearly polarized radiation in both planes and circularly polarized radiation with the aid of a Wollaston prism. During the process of shifting the arrays and changing the undulator gap, no noticeable change of radiation axis was observed.

High Brilliance Beamline for Time Resolved X-ray Absorption Spectroscopy at ESRF

Beamline 8 at the ESRF equipped with a low K undulator, coupling x-ray optics and an energy dispersive XAFS spectrometer is under construction and shall be in operation by 1995. Undulator gap tuning and tapering the magnetic field by a linear variation of the undulator gap will be provided to choose the spectral characteristics for experiments at defined absorption edges. A flux of 1012 photons per second on the sample within the total spectral range of 5 keV to 25 keV and a time resolution below 100 microseconds shall be reached. Small sample volumes can also be analyzed due to the high brilliance of the generated synchrotron radiation.

Future Prospects of XAFS Researches Using High Brilliance Photon Sources

Future prospects in XAFS researches to be developed by high brilliance photon sources are discussed. Above a dilute limit, sensitivity and time resolution are enhanced with the increase in brilliance, detector effeciency and focusing capability. It is shown that a multipole wiggler and multi-element solid state detector improve both sensitivity and time resolution by an order of magnitude. Applications to structural studies of dilute systems, in particular, surface and interfaces are presented. A “tunable” X-ray undulator inserted in an ultra-low (<10 nrad) emittance storage ring, which covers a hard X-ray (4-30 keV) region using the fundamental and third higher harmonic radiations, further improves the present limitations on sensitivity and resolutions in time, space and energy. Feasibility of a variable gap/bandpath undulator combined with new quick scanning techniques for applications in time-resolved studies is discussed. By optimizing an undulator band width for a scan, without varying undulator gap, XAFS is obtained by simple scanning of monochromator (Q-scan I). A new quick scanning technique for time-resolved studies on dilute systems based on an energy dispersive geometry (Q-scan II) is proposed. Design considerations of a “tunable” X-ray undulator XAFS beamline at SPring-8 are presented, in which either a high flux or a microfocus high energy resolution beam is obtained by a fixed-exit double-axis (2+2) crystal monochromator followed by crossed eliptical mirrors. A concept of “ring” detector for microfucusing in a fluorescence detection mode is also proposed.

Achromatic and isochronous electron beam transport for tunable free electron lasers

Abstract We have continued the study of a suitable electron beam transport line, which is both isochronous and achromatic, for the free electron laser being designed at Lawrence Berkeley Laboratory. A refined version of the beam transport optics is discussed that accomodates two different modes of FEL wavelength tuning. For the fine tuning involving a small change of the electron beam energy, sextupoles are added to cancel the leading nonlinear dispersion. For the main tuning involving the change of the undulator gap, a practical solution of maintaining the beam matching condition is presented. Calculation of the higher order aberrations is facilitated by a newly developed code.

Performance of the Revolver undulator at the Photon Factory; application to spectroscopy

The Revolver undulator beamline, BL-19A at the Photon Factory, constructed to be dedicated to the solid state research and laid out for photon energies from 20 to 250 eV, has been calibrated and the resolution was measured at several energies. To cover a wide spectral range with the maximum intensity of the first harmomics of undulator radiation, the simultaneous scanninf of the undulator gap and the monochromator has been accomplished in the normal user beam time. The degradation of the beamline by the high heat loading was considerably reduced by adopting SiC substrate mirrors and gratings.

Performance and upgrades of the NSLS storage rings and photon sources

The usefulness of synchrotron radiation sources is not only determined by current, energy, and magnetic field but also orbit stability and lifetime. The status and developments at NSLS in storage ring performance are discussed. Efforts at NSLS to look toward the future in source development are presented. In particular, small gap undulator development and studies toward development of an ultraviolet free-electron laser at the Accelerator Test Facility are described.

The undulator beamline at the SRS Daresbury

The design and performance of the undulator beamline 5U.1 at the SRS used for soft x-ray spectroscopy is reported and compared with the theoretical expectation of the intensity, resolution, and polarization. The beamline consists of a ten-period permanent magnet variable gap undulator followed by an entrance slitless plane grating monochromator. The energy region covered is from 0.1 to 1 keV. The design is similar to the SX700 but uses a spherical instead of an ellipsoidal focusing mirror. To eliminate the large horizontal beam width at the exit slit the beamline has been modified to include a post-focusing mirror. The spherical focusing mirror has been changed to one with a larger radius resulting in a decreased demagnification, thereby essentially eliminating primary coma. The monochromator contains an ion-etched and lamellar profile 1200-lines/mm plane grating of Pt-coated CVD SiC on a graphite substrate. The high flux of the undulator makes the beam line suitable for SEXAFS studies and for work involving soft x-ray fluorescence detection. The combination of high flux and high resolution is especially advantageous for atomic and molecular spectroscopy and to resolve the quasiatomic structures which are observed in localized 3d transition metal compounds.

Initial results of operating the rocketdyne undulator in a tapered configuration

The near-infrared Rocketdyne/Stanford free electron laser (FEL) uses a very-high-quality precision undulator whose field strength and field taper are adjustable. The Rocketdyne undulator has been operated in both an amplifier configuration, as in the master-oscillator power amplifier (MOPA) experiments, and an oscillator configuration, as in the photocathode and tapered-undulator experiments. The tapered-undulator experiment was performed at the Stanford Photon Research Laboratory (SPRL) using an electron beam supplied by the Mark III rf-linac. During the experiment we observed sustained oscillations as the undulator magnetic-field taper was continuously tuned from 0% to 10%. We observed ∼1.2% extraction efficiency for a magnetic-field taper of 9.6%. During the same experiment we observed sustained oscillations as the undulator gap was continuously varied over 120 mil. Details of the experiment are presented.

Implementation and effects of undulator scanning at Aladdin

The stability of the Aladdin closed orbit while scanning the U2 undulator gap is reported. Over the range of undulator gap settings the entire optimum closed orbit has been corrected to 20 μm rms vertically and 40 μm rms horizontally. The correction scheme utilizes computer control of the two undulator end correctors for the horizontal orbit and two of the regular ring correctors for the vertical orbit. Data showing the output of several other beam lines while the undulator is scanned over its entire range is presented. Small fluctuations in the intensity observed in some user beamlines has been attributed to the discrete steps used by the orbit correctors. A change in vertical beam size with undulator gap is shown to be related to a sextupole component in the undulator field. Possible remedies for these problems are discussed.

Implementation of an undulator beamline on Aladdin

The first undulator monochromator on the Aladdin storage ring has been installed and recently tested. The design and alignment of an undulator beamline presented several unique problems not normally encountered in the design of bending magnet beamlines. The primary problem is the lack of visible radiation from the undulator source when it is operated under normal conditions. In this case a temporary beamline was installed to measure the intensity of undulator light transmitted through a pinhole which could be horizontally and vertically scanned through the undulator beam. This was used to adjust the steering of the beam to be centered in the port and to do some nondispersive characterization of the undulator. This temporary line was designed to permit precision alignment of the components of the beamline according to the measured position of the full energy beam. The monochromator used on this beamline is a 6 m toroidal grating monochromator (TGM). It allows further dispersive testing of the undulator and provides dispersed undulator flux for user experiments. The beamline is identical in terms of expected reflectivity losses and grating efficiencies to two similar beamlines implemented on bending magnets. This allows direct comparison of the flux available to experimenters from undulator sources to flux from bending magnets. Initial measurements indicate that with the undulator gap set for 50 eV, at the peak of the first harmonic, the undulator beamline produces 40 times more flux than a similar beamline using a bending magnet source.

The undulator at BESSY: First operational experience

Results of the first operation of the undulator at the BESSY storage ring are presented. The influence of this device on machine operation has been investigated. Closed orbit corrections and tune shifts as a function of the undulator gap were determined. Spectra of the emitted light were measured using the new toroidal grating monochromator (TGM-5) undulator beamline. The spectra can be understood from a knowledge of the undulator and monochromator characteristics. The undulator intensity is compared with that from the fringe field of the neighbouring dipole magnets.

The Stanford Mark III infrared free electron laser

We have built and operated a compact infrared free electron laser using a conventional linear accelerator and a broadband optical cavity. The gain was measured to be 28% and saturation was obtained with a 2.5 μs macropulse. The laser was tuned from 3.1 to 2.6 μm by changing the undulator gap. Coherent spontaneous radiation at the fifth and seventh harmonics was seen.

Storage Ring Parameters for High Gain FEL

We use one-dimensional free electron laser (FEL) theory to find criteria for choosing electron beam and undulator parameters for operation of a high gain FEL at 400 A. The criteria are moderate electron beam energy (<1 GeV), high peak current (several hundred amperes), small emittance ( about10/sup -8/ m-rad), small relative momentum spread ( about0.001), and narrow undulator gap ( about3 mm). Results of two-dimensional simulations on FEL performance are also presented.