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ALSNews           Vol. 26           May 23, 1995

Table of Contents


    1. CAUSE OF OUTAGE CONFIRMED AND CORRECTED
    2. OPERATIONS UPDATE
    3. DESIGN UNDERWAY FOR ELLIPTICALLY POLARIZING UNDULATOR BEAMLINE
    4. LONG-TERM ALS SCHEDULING MEETING MAY 26
    5. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

1. CAUSE OF OUTAGE CONFIRMED AND CORRECTED
(contact: rmmiller@lbl.gov)

It has now been confirmed that the cause of the April 25-May 12 outage at the ALS was a broken flex band. The flex band has been replaced, and the machine is now operating at its nominal magnet settings. For more information on flex bands and the diagnostic process, please see ALSNews Vol. 25, May 16.

As we reported last week, beam tests on May 12 indicated that the probable reason that the electron beam could not be stacked in its normal orbit was an obstruction at the upstream end of the straight section in sector 2. On Monday, May 22, technicians working under a dry tent opened the vacuum bellows at this location, exposing the two flex bands there, and found that the upstream flex band of the pair had one broken "finger" which bent down into the center of the vacuum chamber, blocking the normal electron orbit. ALS staff will analyze the broken flex band in the coming weeks to determine the probable cause of its breakage.

Technicians removed the broken flex band and installed a replacement which was pre-assembled, cleaned and baked last week in preparation for this operation. Once the storage ring vacuum was restored (an overnight process thanks to the dry tent procedure, which prevented contamination of the vacuum chamber during repairs), beam stacking and storage proceeded normally. The orbit has changed from pre-outage conditions by +/- 500 microns in both the horizontal and vertical planes, and the horizontal betatron tune is 0.02 units higher than before; neither of these changes is expected to affect user operations. Beam lifetime is currently about 6.5 hours at 400 mA, but this should increase as the storage ring vacuum continues to improve.

In the course of investigating this outage, ALS staff performed systematic checks of all accelerator subsystems, including magnets, power supplies, rf systems, and the injection system. Minor anomalies were found and corrected, including several loose magnet connections. One possible explanation for the loose connections is thermal ratcheting, driven by the large number of on-off cycles the ALS has been through (the ALS, like most synchrotrons, is designed for continuous operation, but our limited funding currently enforces gaps in operations). None of these anomalies caused the outage, but their discovery and correction will likely allow more stable ALS operation in the future.

2. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam availability last week was 89.3% overall and 93.0% during user shifts.

Operations summary for May 23 - June 11
1.5-GeV, 400-mA, 320-bunch operations for users:
    May 24, 08:00-16:00
    May 25-28, 08:00-23:15
    June 1-4, 08:00-23:15
1.9-GeV, 250-mA, 320-bunch operations for users:
    June 7, 08:00-16:00
    June 8-11, 08:00-23:15
Maintenance: 
    May 30 & June 5, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    May 23 & 31 and June 6, 08:00-23:15
    May 24 & June 7, 16:00-23:15
Holiday
    May 29

3. DESIGN UNDERWAY FOR ELLIPTICALLY POLARIZING UNDULATOR BEAMLINE
(contact: hapadmore@lbl.gov)

Scientists in many fields, studying topics from the enzymes controlling photosynthesis to the magnetic patterns on computer storage disks, will benefit from a new insertion device-beamline combination planned for ALS Beamline 4.0. The new beamline will deliver high-flux, high-brightness beams of circularly polarized photons suitable for spectroscopy and microscopy, to probe the properties and interactions of magnetic materials and the structures of oriented materials (materials whose molecules are arranged in non-random, repeating structures).

At the heart of these plans is a newly designed elliptically polarizing undulator (EPU), of a type pioneered by Shigemi Sasaki. The EPU produces a continuously adjustable magnetic field which can cause storage ring electrons to follow paths of arbitrary elliptical helicity, producing light ranging from pure right or left circular polarization to pure horizontal or vertical linear polarization, with changes of polarization taking a few seconds.

A novel design scheme will provide maximum flexibility of photon energies and polarizations: there will be spaces for two EPUs, placed end to end, within the straight section for the beamline. Small bending magnets will direct the electron beam through one EPU, then through the other. In addition, a translating mechanism will allow each EPU to be moved out of the beam path during machine operations and replaced by another EPU with a different spatial period (to produce a different range of photon energies). The output from each operating EPU can be directed to either of two branchlines, or the output from both EPUs can be directed to the same branchline. Thus rapid changes in polarization can be produced by setting the two EPUs for opposite polarization and alternating between the two beams with a mechanical chopper.

To make use of the EPU-generated light, two "application-specific" branchlines are planned, one for spectroscopy and one for microscopy, to give the best possible performance for each type of experiment. For microscopy, this means a medium-resolution, high-throughput, entrance-slitless monochromator and a highly efficient optical system. The microscopy branchline will have two branches of its own: one for full-field photoelectron microscopy, the other for zone-plate-based scanning microscopy. The zone plate technique is flexible, allowing any detection method, and will achieve sub-100-nanometer resolution. The full-field technique images photoemitted electrons, and its resolution is normally limited to 200 nm by chromatic aberration. An aberration correction scheme should improve this resolution by a factor of five or more.

The spectroscopy branchline will provide high spectral resolution over a wide energy range (25 eV - 1800 eV), using a monochromator with multiple gratings and included angles. This energy range covers the important core levels for magnetic materials -- the L[2,3] edges of transition metals and the M[4,5] edges of rare earths -- as well as allowing access to low energies for valence band measurements.

Among the many systems to be studied at the new beamline are enzymes (where chemical reactions are catalyzed at the locations of transition metal atoms), interactions among layers in multilayer magnetic materials, and spatial variation of magnetization on a sub-micron scale. The beamline is scheduled to begin operation at the end of 1996. The EPU project represents a re-direction of effort from the elliptical wiggler beamline previously planned for ALS Beamline 11.0.

4. LONG-TERM ALS SCHEDULING MEETING MAY 26
(contact: fred_schlachter@lbl.gov)

On Friday, May 26, all users are invited to attend a long-term operations scheduling meeting to discuss the period between the end of the September-October 1995 shutdown and the start of the next shutdown (provisionally scheduled for February 1996). The meeting will be held at 3:00 p.m. in Building 4, Room 102B. The ALS Users' Executive Committee, which will be meeting that day, will participate in the scheduling meeting.

5. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

The ALS has two cycles per year for soliciting proposals from scientists who wish to conduct research at the facility as independent investigators: April-September and October-March. The next submission deadline is June 1, 1995, for beamtime between October 1995 and March 1996.

To request a proposal form, contact:
Elizabeth Saucier, ALS User Administrator
Tel: (510) 486-6166
Fax: (510) 486-4960
Email: alsuser@lbl.gov

For beamline and PRT information, contact:
Fred Schlachter, ALS User Liaison
Tel: (510) 486-4892
Fax: (510) 486-4960
Email: fred_schlachter@lbl.gov