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  • The Harmonic Cavity -- A New Addition To Aladdin


    During the last week of January, a harmonic cavity will be added to Aladdin at the end of long straight four, just past the focusing magnets. The harmonic cavity will lengthen the stored electron bunches and therefore reduce the rate of current loss with negligible effects on brightness. This addition to the ring is expected to at least double the current-lifetime product, It, providing a slower decay of the intensity of the synchrotron radiation.

    The cavity is being added to Aladdin to increase the useful beam lifetime by reducing the loss of current through interaction of the electrons within each bunch. The cavity will limit intrabeam interaction by spatially lengthening the electron distribution from a gaussian to a more rounded trapezoidal shape, therefore lowering the electron density. The modified electron density will reduce the Touschek effect and the coupled bunch interaction both of which are dependent on the electron bunch profile. This result is achieved through the electron beam interaction with a higher harmonic cavity, in this case a fourth harmonic.

    Kevin Kleman led the design effort for the new cavity. His work involved running computer simulations and calculating the necessary dimensions of the harmonic cavity to improve the lifetime product. An important goal was to keep the tuning range wide so the cavity could be completely tuned out for users interested in time resolved measurements. According to Kevin the work was, “pretty straight forward design”, with the more difficult aspects lurking in maintaining a high vacuum with a mechanical tuning mechanism.

    Although all of the design work was done at SRC, some of the machining had to be done elsewhere because the main cavity was cut from a solid block of aluminum that was too large to machine here. After machining in early October, the cavity was shipped to Seattle to line the inside with a thick layer of copper for better conduction properties. The company, Industrial Plating, used a chemical process to provide a layer of high purity, ultra-high conductivity copper. When the plating is done, the cavity will be assembled and vacuum conditioned outside the vault and then installed. The actual physical installation is expected to take only a day, however, finding the proper de-tuning and bunch lengthening settings will require trial runs expected to take several additional days.

    The harmonic cavity is not powered directly, rather it is a passive system that will get its power from the main RF drive by interacting with the stored electron beam. The power requirement of around 150W is much less than the power boost needed to replace the 17 keV lost to synchrotron radiation from an electron in each trip around the ring at 800 MeV. Therefore, although the harmonic cavity will cause a dip in the RF waveform, it is negligible.

    There are several ways to reduce the current loss of intermediate energy, high brightness synchrotron radiation sources. Both a harmonic cavity and an electron motion coupler were used on the Tantalus ring. The harmonic cavity was chosen for Aladdin because it reduces the current loss without a significant loss in brightness while retaining the ability to be tuned out for time resolved measurements.

    Challenges resulting from installing the harmonic cavity remain. Although not expected, the cavity may cause injection to become more difficult. This problem would occur if the cavity cannot be completely tuned out. From the analysis that has been done, the benefits of a larger current-lifetime product outweigh the risks and it is expected that the cavity will be integrated into the ring and tested by the operators on schedule.


    Aladdin -- Editor Brian Tonner tonner@src.wisc.edu