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. 21           April 18, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. OPERATION AT 1.9 GeV -- WHAT IT MEANS TO USERS
    3. NEW X-RAY MICROSCOPES FOR SPECTROMICROSCOPY
    4. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

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

Beam availability last week was 90.5% overall and 90.8% during user shifts. The major outage was caused by a faulty relay in booster-to-storage-ring bend magnet 3. (This relay caused down time during the week of April 3-9 as well. A correct diagnosis of the outage's cause was made and the relay replaced on April 11, just before we went to press with ALSNews. Therefore, by the time ALSNews came out last week, the problem was solved, but April 11 is included in this week's figures for beam availability.)

**STANDARDS RAISED IN CALCULATION OF PERFORMANCE FIGURES**

The process we use to calculate ALS performance percentages has changed as of April 3, to reflect the ALS commitment to maximizing user beamtime. Formerly, a 15-minute period was allowed as the standard time for storage ring fills, and only fill time in excess of 15 minutes was considered down time. Now we consider the entire fill time as down time. This change results in a systematic lowering of our performance percentages from previous levels, but it will allow the percentages to reflect progress in shortening fill times below 15 minutes.

Operations summary for April 18 - May 7
1.9-GeV, 250-mA, 320-bunch operations for users:
    April 19, 16:00-23:15
    April 20-23, 08:00-23:15
1.5-GeV, 400-mA, 320-bunch operations for users:
    April 26, 16:00-23:15
    April 27-30, 08:00-23:15
1.5-GeV, 2-bunch operations for users:
    May 3, 16:00-23:15
    May 4-7, 08:00-23:15
Maintenance: 
    April 24 & May 1, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    April 18 & 25 and May 2, 08:00-23:15
    April 19 & 26 and May 3, 08:00-16:00

2. OPERATION AT 1.9 GeV -- WHAT IT MEANS TO USERS

April 12 marked the first shift of user operation at a storage ring energy of 1.9 GeV (nominal storage ring energy is 1.5 GeV), and the long-term schedule through September 3, 1995, includes six weeks of 1.9-GeV operation. The availability of 1.9-GeV user operation increases the variety of experiments which can be performed efficiently on ALS beamlines by providing more usable photon flux to experiments needing maximum flux at high photon energies (above ~1 keV). This includes investigations using x-ray fluorescence spectroscopy, x-ray lithography, hard x-ray microscopy, protein crystallography, or total-reflection x-ray fluorescence. Whether a particular experiment shows a flux improvement at 1.9 GeV depends on the photon energy the experiment uses, as follows:

-- High-photon-energy experiments (above ~1 keV) receive much higher flux at 1.9 GeV than at 1.5 GeV. This is most pronounced in the hard-x-ray regime, where bend-magnet experiments using ~10 keV photons get about 20 times more flux at 1.9 GeV than at 1.5 GeV.

-- Medium-photon-energy experiments (between the lowest available photon energies in the table below and ~1 keV) receive slightly more flux at 1.9 GeV than at 1.5 GeV.

-- Low-photon-energy experiments may not be able to run at high storage ring energies, because the energy of undulator harmonics varies as the square of the storage ring energy. The table below gives the lowest available photon energies for the ALS undulators at three different storage ring energies. The narrow-gap vacuum chamber at Beamline 7.0 and the longer-period undulator at Beamline 9.0 both give lower available energies, making it possible for lower-photon-energy experiments to stay on-line at higher storage ring energies.

    Lowest Photon Energies Available 
    from ALS Undulators

                     1.0 GeV    1.5 GeV    1.9 GeV
                   -------------------------------
    BL 7.0 (U5)   |  21   eV     63  eV     105 eV
    BL 8.0 (U5)   |  58.5 eV     132 eV     211 eV
    BL 9.0 (U8)*  |  8    eV     18  eV     29  eV
    BL 9.0 (U10)* |  2.2  eV*    5   eV     8   eV

Many beam characteristics, such as lifetime, change very little from 1.5 to 1.9 GeV. The position of the electron beam at 1.9 GeV differs by only 0.2 mm (both horizontally and vertically) from its position at 1.5 GeV, so small steering corrections and/or minor realignment of beamline optics are sufficient to correct any changes in beam position at endstations. One change visible on the beam current monitors around the ALS is that during 1.9-GeV operation, the ring is filled only to 250 mA rather than 400 mA. This is to protect the oldest of the rf cavity windows, which is susceptible to breakage under the high power load that high currents at 1.9 GeV would impose. A new rf cavity window is now being constructed to allow 1.9-GeV operation at currents above 250 mA.

3. NEW X-RAY MICROSCOPES FOR SPECTROMICROSCOPY
(contact: t_warwick@lbl.gov)

One of the driving reasons to construct the ALS was the need for a source with sufficient brilliance to make x-ray microscopy a practical reality. Spectromicroscopy (or spatially resolved spectroscopy) is now a large area of activity for user experiments. To take full advantage of the ALS's capabilities in this area and LBL's expertise in engineering, the ALS Experimental Systems Group is working with the Mechanical Engineering Group and the Beamline 7.0 Participating Research Team to design two zone plate microscopes for use on undulator Beamline 7.0.

The first, a scanning transmission x-ray microscope (STXM), was completed in March and is now in use. In this instrument, focused radiation from the beamline illuminates a pinhole, overfilling it so that the pinhole defines the characteristics of the downstream illumination, and then a Fresnel zone plate images the light from the pinhole to a microfocus. The image is formed by scanning the sample across the fixed focus of the zone plate, and recording either the transmission of x rays or the yield of electrons as a function of position. This process creates an x-ray absorption map of the surface with a resolution of 100 nm, which will improve to 30 nm with the use of better zone plates.

At this resolution, it is critically important to maintain the stability of the entire system and to accurately reproduce the motion (and thus the location) of the scanning stages. Several types of motion in addition to scanning must be produced by the STXM; for example, the zone plate must move longitudinally to stay in focus as the incoming photon energy changes. Some of these motions are small (on the order of microns), must be highly precise (some tolerances are as small as +/- 5 nm), and may involve components separated by only a fraction of a millimeter.

The first results from the STXM show that it performs to specification. Its first experiments are being carried out in air, but an enclosure is being installed which will permit operation in a helium atmosphere or in rough vacuum to reduce the effects of atmospheric absorption. In the first week of STXM operation, a Kevlar fiber was imaged with 1-micron resolution, and much work was done to characterize the resolution, contrast, and overall performance of the microscope. (Kevlar, a registered trademark of the DuPont Company, is a polymer used in over 200 applications including tires, windsurfing sails, and bullet-proof fabrics.) Planned experiments include looking at the chemical structure of co-polymer mixes and examining the structures of metal interconnect lines in integrated circuits.

The second microscope, a scanning photoemission microscope (SPEM), will be a more advanced version of the STXM that will be housed in ultra-high vacuum (UHV) and use a hemispherical electron energy analyzer as a detector. The complexities of UHV operation and the proximity of the electron analyzer to an already-crowded sample area will make the SPEM more challenging to design and build, but working experience with the STXM will provide useful background in this endeavor.

4. 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) 486-7696
Email: fred_schlachter@lbl.gov