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.

Previous Issues are available.


ALSNews           Vol.6           December 6, 1994

Table of Contents


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

1. ADMINISTRIVIA
(contact: alsnews@lbl.gov)

Many thanks to all of you who have expressed an interest in ALSNews and provided suggestions for topics. New science at the ALS seems to be an area everyone is keen on learning about, along with research techniques and their applications. Some of the requests such as for beamline contact information and availability to users are beyond the scope of a weekly bulletin, but we intend to expand the ALS information on the World Wide Web to incorporate this material. We appreciate feedback on all aspects of ALS publications, so please don't hesitate to send your comments to alsnews@lbl.gov.

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

**COMPARISON OF 1.9 GeV AND 1.5 GeV OPERATION**
The intensity of the 5th harmonic peak was measured in Beamline 7.0 (5-cm-period undulator beamline) at electron energies of 1.5 GeV and 1.9 GeV for a circulating current of 50 mA. The intensity of the peak was 3 times larger at 1.9 GeV than at 1.5 GeV, consistent with theoretical predictions. Intensity comparisons for light emitted from dipole magnets at 1.5 GeV and 1.9 GeV are planned in the near future.

**REDUCTION OF NONLINEAR RESONANCE INTRODUCED BY UNDULATORS**
Betatron-tune scans were made while monitoring beam losses in order to observe the onset of nonlinear structural resonances in the beam. With this technique, excited resonances, both allowed and unallowed by the ring's natural 12-fold symmetry, have been identified. (Unallowed resonances are driven when the symmetry of the lattice is broken.) When the gap of the 8-cm-period undulator was decreased from fully open to 24 mm, a strong enhancement of the unallowed resonances was observed with no noticeable increase in the allowed resonances. This was an indication of increased lattice-symmetry breaking due to the increased linear focusing effect of the undulator. With the undulator gap decreased to 24 mm, the beam was unstable at certain tunes, whereas it had been stable with the gap fully open. By tuning two quadrupoles in the ring, we were able to reduce the amplitude of some of the unallowed resonances to the same level they had with the undulator gap fully open. This is the first indication that it is possible to compensate for the symmetry-breaking effects of insertion devices. Being able to restore the lattice symmetry may be important when stronger insertion devices with smaller gaps are installed in the ring.

**LCW TEMPERATURE CORRELATED WITH BEAM MOTION**
Correlations were found between beam motion and low-conductivity water (LCW) temperature. The beam motion was measured by one insertion-device beam-position monitor located in straight section 4. The LCW temperature has a cycle of about 12 minutes in which it changes by +/-0.75 degrees Celsius. A corresponding vertical orbit motion with similar periodicity and 10-micron peak-to-peak amplitude could be detected. It is yet to be determined whether the beam orbit is actually moving or whether it is the BPMs which are moving. Further, a correlation between the LCW temperature and longitudinal flex-band motion of several microns was found.

**SINGLE-BUNCH LENGTHENING MEASUREMENTS**
Preliminary studies of the bunch frequency-spectrum measured from a beam position monitor button indicate the onset of bunch lengthening as the bunch current increases. This bunch lengthening is probably due to potential-well distortion of the rf bucket. Bunch length measurements with a streak camera are planned.

**FAST TUNE MEASUREMENT**
A fast tune-measurement system has been implemented which can measure the betatron tunes at a rate of 5 Hz. This measurement system gives us greater than a 10 times improvement in speed over the previous tune measurement system.

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

Beam availability overall for last week was 95.7%, including 96.8% during user shifts.

Maintenance time will be two days per week during December, to minimize the time required for the January shutdown by completing some tasks early. Additional user operation has been scheduled on weekends to compensate for the additional maintenance days.

Operations for Wednesday, Dec. 7, through Friday, Dec. 30:
Standard 320 bunch, 400 mA operation for users:
    Dec. 8-10, 00:00-24:00
    Dec. 15-18, 00:00-24:00
    Dec. 19, 00:00-08:00 and 12:00-24:00
    Dec. 20, 00:00-24:00 
    Dec. 21, 00:00-16:00
Maintenance and startup:  
    Dec. 12, 13, 19 (08:00-12:00), 22, 23
Accelerator physics (Wednesdays): 
    Dec. 7 and 14, 00:00-24:00
    Dec. 21 16:00-24:00
Holidays: Dec. 24-30

Shutdown for new equipment installations begins January 2, 1995. User operations are scheduled to resume on February 22, 1995.

4. BEAMLINE BREAKTHROUGHS

**BEAMLINE 7.0: FAST, HIGH-RESOLUTION RESONANT PHOTOEMISSION STUDIES**
(contact: jkikuma@csg.lbl.gov)

Researchers at Beamline 7.0, led by Brian Tonner of U. of Wisconsin-Milwaukee, are making significant improvements in the quality of data available on the electronic structures of polymers. They have taken high-density maps of resonant valence band photoemission across the carbon K-edge in polystyrene and related polymers. Their beamline's high flux and small beam spot allow a large amount of data to be collected in a short time; for example, a recent data set of photoelectron emission intensity versus photon energy (in 0.2-eV steps) and photoelectron binding energy (in 0.1-eV steps) comprises 101 spectra taken in 30 seconds each. The entire data set was taken in a period of less than 1.5 hours.

Resonant photoemission occurs when a core-level electron is excited to an unoccupied molecular orbital, initiating an Auger-like decay process that leads to the emission of a valence-level electron. Each unoccupied orbital/valence orbital pair produces a resonant enhancement in the continuous valence band photoemission spectrum; the largest such enhancement corresponds to the lowest unoccupied orbital and highest occupied valence orbital. Thus, a map of valence band emission intensity versus incident photon energy and electron binding energy can be interpreted as a correlation plot of the energy transfer between occupied and unoccupied states in the polymer during the Auger decay process. The rules of this energy transfer correlation are governed by symmetries, localization, and occupancies of the electronic orbitals. The researchers performing these polymer experiments include Jun Kikuma and Jonathan Denlinger of U. of Wisconsin-Milwaukee and Eli Rotenberg of U. of Oregon.

5. TIMELY TOPICS

**BEAMLINE INFORMATION** Several subscribers to ALSNews requested information on the current and future beamlines and areas of research planned for the ALS. This information is available on the World Wide Web on the ALS Beamline page. From NCSA's Mosaic browser, choose "Open URL" from the File menu and type: http://www-als.lbl.gov/als/als_users_bl/als_bl_chart.html

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