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ALSNews           Vol. 18           March 28, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. USERS PROVIDE FEEDBACK ON WEEKLY, LONG-TERM SCHEDULES
    3. HELIUM EXPERIMENTS ACHIEVE ULTRA-HIGH RESOLUTION

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

Beam availability last week was 96.1% overall and 90.8% during user shifts. Outages of the storage ring rf system, linac modulator #1 focus coil, a water flow switch on linac modulator #2, and a storage ring corrector magnet power supply caused beam interruptions during user shifts.

Weekly scheduling meeting date and time change:
    Based on suggestions from ALS users, the weekly scheduling meeting will now
take place on Fridays at 3:30 p.m. in the Building 6 conference room, beginning
this Friday, March 31.

**NOTE** SCHEDULE CHANGE
    Friday, March 31, 08:00-10:00 -- 1-mA, 320-bunch operations for U8/gas
filter characterization at 9.0.2 (previously scheduled for 400-mA operation)

Operations summary for March 28 - April 12
1.5-GeV, 400-mA, 320-bunch operations for users:
    March 30, 08:00-23:15
    March 31, 10:00-23:15
    April 1-2, 08:00-23:15
    April 5, 16:00-23:15
    April 6-9, 08:00-23:15
1.9-GeV, >250-mA, 320-bunch operations for users:
    April 12, 16:00-23:15
1.5-GeV, 1-mA, 320-bunch operations for U8/gas filter characterization at
9.0.2:
    March 29, 16:00-23:15
    March 31, 08:00-10:00
Maintenance: 
    April 3 & 10, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    March  28 and April 4 & 11, 08:00-23:15
    March 29 and April 5 & 12, 08:00-16:00

2. USERS PROVIDE FEEDBACK ON WEEKLY, LONG-TERM SCHEDULES
(contact: fred_schlachter@lbl.gov)

On March 27, ALS users and management met to discuss the new weekly operations schedule and the long-term operations schedule for the ALS. Users expressed their approval of the change from the former weekly schedule (5 days/week, 3 shifts/day) to the present weekly schedule (7 days/week, 2 shifts/day). At the users' request, ALS management is now considering how to make as many Monday-Friday day shifts as possible available to users.

ALS management also previewed two upcoming shutdowns for major equipment installations. One is scheduled for 4.5 weeks plus one startup week, beginning in September 1995. During this shutdown, the 8-cm-period undulator will be moved from sector 9 to sector 12, and a 10-cm-period undulator will be installed in sector 9. The second shutdown will be scheduled in early 1996 for 2.5 weeks plus one startup week. A 16-cm-period wiggler will be installed in sector 5 during this shutdown. Both shutdowns will also include other, smaller-scale installations and improvements.

3. HELIUM EXPERIMENTS ACHIEVE ULTRA-HIGH RESOLUTION
(contact: fred_schlachter@lbl.gov)

The first independent investigator at the ALS, Gunter Kaindl of the Freie Universitaet Berlin, working in cooperation with ALS staff, has obtained a record-breaking resolving power of 60,000 at Beamline 9.0.1. This resolving power (photon energy divided by resolution at that energy) is a factor of 6 higher than the resolving power promised to users (10,000) and a factor of nearly 4 higher than the best resolving power achieved to date in the photon energy range, 60-79 eV, that is of interest in these experiments. For the past five years, the highest resolving power (16,000) in this range has come from the Freie Universitaet's high-resolution SX700/II monochromator at BESSY, operated by Kaindl's group.

Kaindl's experiments at the ALS focused on autoionizing resonances in the doubly excited states of helium. Helium, as the simplest atom for which quantum-mechanical equations cannot be solved exactly, provides an excellent test case for approximate solutions based on various theoretical treatments of electron correlation (interactions among electrons within an atom). In Kaindl's experiments, photons from the Beamline 9.0 undulator interacted with helium atoms in a gas cell. This interaction led to two processes, indistinguishable from each other because of their identical final states. In one process, direct single ionization, one electron could absorb energy from a photon and leave the atom. In the other process, double excitation followed by autoionization, one photon could excite both electrons to higher-energy states, after which one electron would leave the atom while the other returned to a lower-energy state. Quantum-mechanical interference between direct single ionization and the doubly excited states produced several series of resonances. In the He (2s, np+) series (one electron is excited to the 2s orbital, the other to an np orbital, with n=4 or higher), the group measured resonances all the way to n=24, well beyond the resolution limits of previous experiments.

This ultra-high resolution was possible because of the careful design, conservative engineering, and fine tuning of the beamline, along with careful attention to experimental details. The ability to achieve this high performance makes Beamline 9.0.1 a powerful tool for research in atomic and molecular physics. The beamline may also prove useful in areas of surface and materials sciences which demand very high resolution. Kaindl's doctoral student Konrad Schulz worked with him on this experiment, as did Fred Schlachter, John Bozek, and Phil Heimann of the ALS, with assistance from many other ALS staff members.