ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol.9           January 10, 1995

Table of Contents


    1. OPERATIONS OVERVIEW
    2. MACHINE PHYSICS
    3. BEAMLINE BREAKTHROUGHS
    4. SCINTILLATING SUBJECTS
    5. TIMELY TOPICS

ALSNews will not be published on January 17 because of the Martin Luther King, Jr., Day holiday and the ALS shutdown. We will return on January 24.

1. OPERATIONS OVERVIEW
(contact: rmmiller@lbl.gov)

The ALS began a shutdown period for equipment installations on January 3 (see #4 below, "Since You Asked," for one example of the work going on). The first user shift after the shutdown is scheduled for February 22, and work is proceeding on schedule.

2. MACHINE PHYSICS
(contact: robin@lbl.gov)

** 200 mA ACCELERATED TO 1.9 GeV **

The accelerator physics group successfully ramped a 200-mA stored electron beam from 1.5 GeV to 1.9 GeV in a recent machine test to see if larger currents than the 50 mA used in previous experiments could be accelerated to 1.9 GeV. It is believed that still larger currents could reach 1.9 GeV if more rf cavity power were applied (99 kW was used in this test), but even at 200 mA, insertion device third and fifth harmonics produce more radiation intensity than at 400-mA current with 1.5-GeV electron energy.

** QUADRUPOLE AXIS MEASUREMENTS WILL REDUCE POWER-SUPPLY-RELATED BEAM MOTION **

In an effort that will reduce beam motion due to quadrupole power supply jitter, accelerator physicists used a beam-based alignment technique to find the spatial relationships between beam position monitor (BPM) centers and quadrupole centers with a precision of better than 50 microns. They measured 72 out of 96 vertical and 48 out of 96 horizontal BPM offsets by varying a quadrupole's magnetic field to center the electron beam in the quad, and recording the beam position in adjacent BPMs. These measurements will allow the electron orbit to be centered exactly in the quadrupoles, so that power supply variations' effects on beam motion will be minimized, as will vertical dispersion and orbit distortion.

3. BEAMLINE BREAKTHROUGHS

** FIRST RESULTS FROM THE RECONSTRUCTED DIAGNOSTIC BEAMLINE **
(contact: trrenner@lbl.gov)

On December 23, members of the experimental systems and accelerator physics groups made the first observations of the electron beam size on Beamline 3.1.1, the diagnostic beamline, following its reconstruction. Beam size was measured by imaging the synchrotron radiation from a bend magnet with the ring filled to 3 mA in the standard multi-bunch pattern, and the resulting measurement agreed with predictions, assuming a vertical-to-horizontal emittance ratio of 1%. Another set of measurements, with the ring tuned to a betatron-tune coupling resonance, showed the expected dramatic increase in vertical beam size. Beamline 3.1.1 should prove a valuable diagnostic tool for observing beam sizes and for decreasing undesired beam motion, thus helping ALS operators improve the quality and consistency of beam delivered to users.

** "SECOND LIGHT" TO BEAMLINE 6.3.2 **
(contact: jhunderwood@lbl.gov)

Beamline 6.3.2 was first featured in Volume 3 of ALSNews last November. At that time, the innovative varied line spacing plane grating monochromator (VLS-PGM) built for BESSY in Berlin had just been tested successfully at the newly-commissioned beamline. In December a similar VLS-PGM optimized for conditions at the ALS was installed, and in characteristically short order the endstation received its first light from the new monochromator on December 16. During the current shutdown, two additional gratings will be installed, bringing the total to three and allowing the monochromator to function over its full 50-1000 eV energy range.

4. SCINTILLATING SUBJECTS

** SINCE YOU ASKED **
(contact: rmmiller@lbl.gov)

The major activity of the current shutdown is to remove the 5-cm-period undulator (U5.0) in straight section 7.0, take out its current vacuum chamber, install a smaller vacuum chamber (minimum magnetic gap 14 mm, down from 23 mm; internal vertical height 10 mm), and re-install the undulator in the storage ring. "Why replace a perfectly good vacuum chamber with a smaller one?" you might ask. Here is the reason.

A smaller vacuum chamber allows the undulator to be used with a smaller gap, and therefore a higher field, between the magnetic poles. Because electrons are bent more strongly and so travel a longer path in each oscillation, the photons emerging from the undulator at high field have lower energies. In effect, the possibility of using a smaller undulator gap lowers the threshold for the energy range the undulator is capable of producing. This makes it possible to perform certain lower-energy experiments without reducing the storage ring electron energy and thus forcing every beamline to cope with reduced photon energies. On Beamline 7.0, for example, the minimum photon energy will decrease from 124 eV to around 50 eV, giving access to the silicon 2p edge at 99 eV. This will allow important studies on the oxidation of silicon and its interaction with adsorbed metal layers.

So why wasn't a smaller chamber installed in the first place? When the storage ring was first designed, no one knew for certain just how tightly focused the ALS electron beam would be. Some model calculations showed that a smaller vacuum chamber might not allow beam to be stored. If the dynamic aperture (the space required for the electron beam, including beam position errors and betatron oscillations) had been too large for the vacuum chamber, we would have been left with a $146M paperweight incapable of storing beam at all. Now that measurements have been made with beam scrapers, which can simulate the effects of a smaller chamber, it is clear that the smaller undulator vacuum chamber will not pose a problem for beam storage.

** NEW FACES **
(contact: bob_batterman@macmail.lbl.gov)

The ALS receives many visitors from other institutions in the USA and abroad. This occasional column will introduce some of our long-term visitors to the ALSNews audience.

Bob Batterman has been visiting the ALS since mid-September on sabbatical leave from his positions as the Director of the Cornell High-Energy Synchrotron Source (CHESS) and as a Professor of Applied Physics at Cornell University. Here until the end of May, he is dividing his time in the Bay Area between the ALS and Stanford Synchrotron Radiation Laboratory (SSRL). Batterman is participating in a joint program with representatives from the ALS, SSRL, and the semiconductor industry, to develop techniques for diagnosing contaminants in silicon wafers. His scientific interests include dynamical diffraction of x rays, solid state physics, and neutron diffraction.

5. TIMELY TOPICS

** REMINDER: HALBACH SYMPOSIUM ON MAGNET TECHNOLOGY **

Ever wondered whom you should thank for all those great insertion device x rays? Start with Klaus Halbach, the primary force behind today's permanent magnet wigglers and undulators. To honor Klaus, the ALS is sponsoring a special Halbach Symposium on Magnet Technology on February 3, 1995--with a day long symposium of scientific and informal talks, the release of a volume of his famous notes, and a festschrift in his honor written by the alumni of the Halbach Institute of Technology. (The festschrift is a tradition in which the students and colleagues of an individual of distinction publish a volume of their creative work and dedicate it to him.)

Registration is required. For more information contact:

Elizabeth Saucier
ALS Administration
Re: Halbach Symposium
Lawrence Berkeley Laboratory, MS 80-101
Berkeley, CA 94720

Phone: (510) 486-6166
Fax: (510) 486-4960
Email: ecsaucier@lbl.gov

Writers: jccross@lbl.gov, deborah_dixon@macmail.lbl.gov