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  • The Permanent Magnet Undulator (PMU) to be installed on Long Straight Section - 3 (LSS-3) has exceeded its performance goals. The Brobeck Division of Maxwell Laboratories designed and fabricated the undulator to achieve at least 70% of the ideal flux output for the first and third harmonics, but in fact, magnetic field measurements show that the undulator will exceed this value for the 5th harmonic as well.


    The undulator before instalation on the ring

    Following acceptance of the device at the end of March, the Engineering group re-leveled the undulator and realigned the field measuring system. About the middle of June, the Accelerator group started measuring fields - verifying performance and characterizing the undulator over a wide range of parameters. Priority has been given to further trimming of the magnetic field and to the boosting of the calculated 5th harmonic. Recent work has determined excitations for several trim coils that reduce long- wave field/phase errors, resulting in enhancement of the 5th harmonic by 10-20% to a minimum of 88% over the operating range.

    Maxwell-Brobeck’s representatives, Francis Younger and Bob VanDusen, were here in January and February to test and trim the magnitude and direction of the field produced by the array of about 3,000 permanent magnets. The undulator was tested at the production site in Richmond, California, but had to be re-tested to assure no thermal or mechanical degradation had occurred during shipping.

    This is the second undulator built for Aladdin by Maxwell-Brobeck; the first one was constructed with electromagnets for the Iowa State PRT beamline on LSS-4. The new undulator was built with both a higher field strength and a shorter period. As Maxwell-Brobeck representatives were finishing their work in February, they seemed pleased with the job they had done. When asked how the project was going Mr. Younger said, “The magnet quality was even better than expected ... the magnitude and direction of the magnetic field were off by less than 1 percent.”

    The undulator is part of a group project which will result in the PGM beamline. The PGM beamline will combine projects that involve nearly all of the personnel in the Engineering, Optics, and Accelerator groups. The Permanent Magnet Undulator (PMU), Plane Grating Monochromator (PGM), and the Quadruple Reflection Polarizer (QRP) will provide a beamline with improved brightness that overlaps and extends beyond the energy range of the popular 6M TGM beamlines. Moreover, it will provide highly polarized light that can be used effectively by the QRP.

    Although the quality of the fifth harmonic was not totally unexpected, the Accelerator Group effort will significantly improve the potential usefulness of the beamline to SRC users. The Accelerator Group has been characterizing the undulator since April. Once the undulator is installed and the PGM beamline commissioned late this fall, users will have a brighter beam at both lower and higher energies.

    Undulator flux vs Gap size in mm

    Design specifications dictated a minimum flux of 70 percent of ideal flux for the first and third harmonics.

    The high brightness of the undulator source will permit use of smaller apeture beamline optics that deliver correspondingly better resolution. This will give users a beamline that may be able to improve the speed and quality of their measurments. All of this comes with very minimal risk to the quality of the Aladdin electron beam. Mike Green commented that, “Undulators are relatively nonperturbative regarding adverse impact on ring performance compared to other options such as wigglers.” To allow electron injection without degradation of stacked and accelerated current, the gap in the magnet array will be opened to about 22 cm. Once the desired current and energy have been achieved in Aladdin, the undulator gap can be closed.

    The beamline, which will have two user stations, should be available for users this winter. For more information about the beamline, contact the beamline manager, Mary Severson.

    PGM Undulator Exceeds Performance Goals

    Aladdin -- Editor Brian Tonner