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ALSNews           Vol. 47           March 6, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. HIGHER CURRENT TO BE AVAILABLE WITH COUPLED-BUNCH FEEDBACK ON
    3. FLUORESCENCE REVEALS STRUCTURE OF MYSTERIOUS MAGNETIC MULTILAYER
    4. WHAT'S A SEMICONDUCTOR, ANYWAY?
    5. LONG-TERM SCHEDULE SET FOR MAY-NOVEMBER 1996
    6. USERS' TOWN MEETING APRIL 12

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

Beam availability for the last two weeks was 92.7% overall and 93.9% during user shifts.

Operations Summary for March 6 - March 25

Mar 06, 00:00-08:00           User Scrubbing & Special Operations
                              (1.9-GeV/260-mA/320-bunch operation 
                              requested by users)
Mar 06, 08:00- Mar 11, 07:15  1.5-GeV/400-mA/320-bunch user operations
Mar 11, 07:30-24:00           Maintenance & Startup
Mar 12, 00:00-24:00           Accelerator Physics
Mar 13, 00:00-08:00           User Scrubbing & Special Operations
Mar 13, 08:00- Mar 18, 07:15  1.5-GeV/400-mA/320-bunch user operations
Mar 18, 07:30-24:00           Maintenance & Startup
Mar 19, 00:00-24:00           Accelerator Physics
Mar 20, 00:00-08:00           User Scrubbing & Special Operations
Mar 20, 08:00- Mar 25, 07:15  1.9-GeV/260-mA/320-bunch user operations

Weekly operations scheduling meetings: Fridays at 3:30 p.m. in the Building 6 conference room.

2. HIGHER CURRENT TO BE AVAILABLE WITH COUPLED-BUNCH FEEDBACK ON
(contact: ajackson@lbl.gov)

Commissioning of the coupled-bunch longitudinal and transverse feedback systems has progressed very well in the past few months. The ALS now offers scheduled operation with these systems on, but with a few restrictions. As before, the current at 1.9 GeV is limited to 260 mA because of concerns about the power-handling capabilities of the disk window in rf cavity number 1. (Cavity number 2 has a cylindrical window that has no such problems, and a similar window is being prepared for installation in cavity number 1, probably during the 1997 shutdown). At 1.5 GeV with feedback on, the current is also restricted to 260 mA but for a different reason -- heating of the ceramic tubes in the injection straight section. A forced-air cooling system is now being tested to address this problem. If it is successful, operation at 1.5 GeV with feedback on at the full current of 400 mA will be made available to users on request.

The overheating of the ceramic tubes is a consequence of the shorter bunches associated with the feedback system. The problem was discovered in the last shutdown, when it was observed that polycarbonate supports at the ends of the four ceramic tubes had all melted, indicating that the local temperature had exceeded 150 #161#C. Before the ALS came back into operation, technicians attached thermocouples to the ceramics, and it was found that the heating was isolated to the ends of the tubes.

It is not yet clear whether the problem results from heating of the adjacent flex bands or from a deterioration of the conductive coating (along the inside surface of each tube) at the tube ends. The amount of power involved is quite small, however, and the problem can be alleviated by forced-air cooling. The injection equipment has now been modified to include nozzles that direct air from the storage ring's compressed air system onto the ends of the tubes, and an air-flow interlock has been added to dump the beam automatically should this air be turned off.

3. FLUORESCENCE REVEALS STRUCTURE OF MYSTERIOUS MAGNETIC MULTILAYER
(contact: jcarlisle@cms1.llnl.gov)

An iron/silicon multilayer under testing at Argonne National Laboratory in 1993 surprised researchers when it exhibited giant magnetoresistance (see below), in seeming defiance of the current understanding of multilayer magnetic materials. A group working at the ALS has since used two forms of soft x-ray spectroscopy to determine the multilayer's composition, resolving the conflict between theory and experiment. The group included researchers from Lawrence Livermore National Laboratory, the University of Tennessee, and Tulane University.

Giant magnetoresistance (GMR) is a property of certain multilayer magnetic materials whereby their electrical resistance changes dramatically in response to changing magnetic fields. GMR materials are thus strong candidates for use in read heads for the next generation of magnetic data-storage systems (for more on GMR and magnetic data storage, see ALSNews Vol. 36, Sept. 19, 1995). Normally, GMR is observed only in structures with alternating layers of a magnetic material (such as cobalt) and a non-magnetic metal (such as copper), each layer a few tens of angstroms thick. The Argonne research group had grown an iron-silicon multilayer which exhibited GMR, unexpectedly because silicon is a semiconductor rather than a metal (see item #4 below for more on semiconductors). Subsequent electron microscopy experiments at Lawrence Livermore National Laboratory suggested that the non-magnetic layer in the sample was actually an iron-silicon compound (iron silicide) formed by intermixing of iron and silicon atoms during the preparation process. However, it was not possible to determine whether the material was a metal or a semiconductor, so the researchers turned to soft x-ray spectroscopy on Beamline 8.0 of the ALS to measure the occupied and unoccupied orbitals of the iron silicide. Semiconductor materials have a gap (called a band gap) between the energies of occupied and unoccupied orbitals, whereas metals do not.

The group was able to demonstrate the absence of a band gap and, hence, the metallic nature of the iron-silicide compound by combining soft x-ray fluorescence spectroscopy (SXF) with fluorescence-yield near-edge x-ray absorption spectroscopy (NEXAFS). In both these methods, photons from the ALS excite an electron in an atom (a silicon atom, for this experiment) from a core state to a higher-energy unoccupied state, whereupon a second electron drops from a valence state to fill the hole at the core level. The energy lost by the second electron is radiated as a fluorescent photon. In SXF, the excitation energy is held constant. The fluorescent photon energies correspond to the differences in energy between various occupied states and the core state, so SXF maps the energy levels of occupied states. NEXAFS, in contrast, maps the energies of unoccupied states. The excitation energy is swept through the range of interest while a detector measures the yield of fluorescent photons. Whenever the excitation energy matches the energy difference between the core state and an unoccupied state, excitation occurs and is followed by fluorescence, leading to a high fluorescence yield. If there were a band gap in the iron silicide, the SXF and NEXAFS spectra would not overlap, but since they did, the researchers concluded that the iron silicide was a metal.

The ALS, with its high-brightness beams, provided a key tool in this investigation. The researchers needed to use fluorescence because fluorescent photons from the iron silicide layer could escape to the surface through intervening layers, whereas electrons (for electron microscopy) could not. Very few fluorescent photons are emitted compared to the number of incoming photons, however, so using fluorescence as an experimental tool requires a highly efficient spectrometer (like the Tennessee-Tulane instrument used in this experiment) coupled with an intense, bright source such as the ALS.

4. WHAT'S A SEMICONDUCTOR, ANYWAY?

Solid materials can be divided into three main types based on their electrical resistivity (the difficulty with which electrical current flows through them): conductors (metals) have very low resistivity, insulators have very high resistivity, and semiconductors have moderate resistivity.

These materials can also be classified by examining their band structures (the distribution in energy and momentum of their electron orbitals; a "band" is a group of orbitals with closely spaced energies). For electric current to flow, the highest-energy band with electrons in it must be only partially filled. In conductors, the valence band is only partially filled, so electric current can flow easily. In insulators and semiconductors with all atoms in their ground states, the valence band is full and the conduction band (containing the lowest-energy unoccupied orbitals) is empty; moreover, there is an energy gap between the valence and conduction bands. It is only when electrons are excited from the valence band to the conduction band (partially filling it) that electric current can flow through the material. In an insulator, this excitation is unlikely without an outside stimulus, such as light or heat, because the energy gap between the bands is large (greater than 2 eV). In a semiconductor, the band gap is smaller than 2 eV, so a few electrons can be excited to the conduction band relatively easily (e.g., by thermal vibrations of the atoms in the semiconductor).

5. LONG-TERM SCHEDULE SET FOR MAY-NOVEMBER 1996
(contact: fred_schlachter@lbl.gov)

The ALS operating schedule for May 22 to November 4 has been reviewed by ALS users and beamline spokespersons. The final operating schedule for May to November is given below by week. A graphic schedule detailing each shift of operation is available on the World Wide Web. Active and recent users will receive a copy by mail; others may request a copy by sending their complete mailing address to alsuser@lbl.gov with a "please send long-term schedule" message. The schedule for the period following November 4 will be determined during the summer.

The first part of each heading is the storage ring energy in GeV; MB and 2B stand for multi-bunch and two-bunch operation respectively. Accelerator physics time, maintenance/startup, and holidays occupy the time not accounted for in the "Dates" column, as well as some shifts on the first and/or last days of most periods listed.

Dates (MM/DD/YY)    1.5/MB   1.9/MB   1.9/2B   1.1/MB   Shutdown

05/01/96-05/21/96                                       Shutdown/Startup
05/22/96-05/27/96            1.9/MB
05/30/96-06/03/96            1.9/MB
06/05/96-06/10/96   1.5/MB
06/12/96-06/17/96   1.5/MB
06/19/96-06/24/96   1.5/MB
06/26/96-07/01/96            1.9/MB
07/03/96-07/08/96            1.9/MB   (3 holiday shifts on July 4 and 5)
07/10/96-07/15/96            1.9/MB
07/17/96-07/22/96            1.9/MB
07/25/96-07/29/96   1.5/MB
07/31/96-08/05/96   1.5/MB
08/07/96-08/12/96                              1.1/MB
08/14/96-08/19/96                     1.9/2B
08/21/96-08/26/96                     1.9/2B
08/28/96-09/01/96   1.5/MB
09/05/96-09/09/96   1.5/MB
09/11/96-09/16/96            1.9/MB
09/18/96-09/23/96            1.9/MB
09/26/96-09/30/96            1.9/MB
10/02/96-10/07/96   1.5/MB
10/09/96-10/14/96   1.5/MB
10/16/96-10/21/96            1.9/MB
10/23/96-10/28/96            1.9/MB
10/30/96-11/04/96            1.9/MB

6. USERS' TOWN MEETING APRIL 12

A meeting of ALS management, users, and the ALS User Executive Committee (UEC) will occur on April 12, 1996, at 9:00 a.m. in the Building 4 conference room. Please send any issues you may wish to raise at the meeting to Jeff Bokor, UEC Chair (jbokor@eecs.berkeley.edu) or to Fred Schlachter (fred_schlachter@lbl.gov) in advance of the meeting.