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.8           December 20, 1994

Table of Contents


    1. AN EARLY HAPPY NEW YEAR
    2. ADMINISTRATIVIA
    3. OPERATIONS OVERVIEW  
    4. MACHINE PHYSICS
    5. BEAMLINE BREAKTHROUGHS

1. AN EARLY HAPPY NEW YEAR

As 1994 draws to a close, the ALS would like to thank all of you who have shown an interest in our progress and activities this year by attending our user meeting or workshops, sending suggestions and compliments for our publications, taking the time to thank a particularly helpful staff member, or just expressing curiosity about what we are and what opportunities we offer the scientific community. We look forward with anticipation to an exciting 1995, and wish everyone a pleasant holiday and a happy new year.

2. ADMINISTRATIVIA
(contact: alsnews@lbl.gov)

To give everyone a much-deserved rest and to help save energy, Lawrence Berkeley Laboratory (home of the ALS) shuts down from December 24 until January 3. As such, there will be no ALSNews for the next two weeks and the regular Tuesday-delivery schedule will resume on January 10.

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

This week is the last week of operations before the January-February shutdown, which begins January 3, 1995. User operations are scheduled to resume on February 22, 1995.

Operations for Wednesday, Dec. 21, through Friday, Dec. 30:
Standard 320 bunch, 400 mA operation for users:
    Dec. 21, 00:00-16:00
Accelerator physics: 
    Dec. 21, 16:00-24:00
Maintenance:  
    Dec. 22, 23
Holidays: Dec. 24-Jan. 2

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

**POSSIBLE ORBIT CHANGES AFTER JAN./FEB. SHUTDOWN**
Attention users: after the upcoming shutdown, the storage ring lattice-magnets will be operated on their lower hysteresis branches, to facilitate ramping up to 1.9 GeV. Even though orbit corrections at this new working point have been performed we anticipate minor orbit changes (on the order of 100 microns) that may affect beamline source-point locations.

**LONGITUDINAL FEEDBACK TEST WITH 200-W POWER AMPLIFIER**
A borrowed 200-watt traveling wave tube amplifier was used in a test of the new feedback system for suppression of multi-bunch instabilities that cause a small energy spread in the electron beam at high current and a consequent spectral broadening of the undulator third and fifth harmonics. During the test, the ring was filled to 200 mA with the feedback system turned on. The feedback system successfully controlled the longitudinal coupled-bunch motion which is excited by RF-cavity higher-order modes. At currents larger than 200 mA, the feedback system lost control of these longitudinal oscillations. It is anticipated that a more powerful amplifier will be purchased once experiments have indicated the appropriate power requirements for the ALS.

**AIR TEMPERATURE CORRELATED WITH BEAM MOTION**
The ALS accelerator physics and experimental systems group are collaborating on several studies to provide increased beam stability. In a recent test to determine possible causes of minor beam motion observed by some users, a correlation was found between measured horizontal beam motion (measured by an insertion-device beam-position monitor located in straight section 4) and air temperature (which has a cycle of about 1 hour during which it changes by plus or minus 0.75 degrees C). The measurements showed a horizontal beam motion of 40-micron peak-to-peak amplitude with a similar periodicity as the temperature. Whether some or all of this motion can be explained as movement of the beam position monitor itself is still to be determined.

5. BEAMLINE BREAKTHROUGHS

**NEW PHOTOIONIZATION TECHNIQUES IN USE AT ALS**
(contact: dmitri_mossessian@macmail.lbl.gov)

Researchers at Beamline 9.0.1 have applied a new technique using multilayer mirrors to the study of helium photoionization with excitation. A group from the University of Nevada-Reno collaborated in this experiment with Russian scientists from Ioffe Physico-Technical Institute (St. Petersburg) and the Institute for Physics of Microstructures (N. Novgorod).

The physics of photoionization with excitation are as follows: an incoming photon ejects one electron from a helium atom, making it He+, and excites the remaining electron in He+ to an unoccupied orbital. Then the excited electron drops back to the 1s orbital, emitting a photon in the process. These photons were measured to obtain spectra, and the spectra had such high resolution that features inaccessible to previous studies were visible. Four spherical, normal-incidence MoSi multilayer mirrors were manufactured specifically for reflection of the first four radiation lines in the He+ Lyman series. The mirrors were assembled with filters and detectors in a polychromator, to allow the simultaneous detection of the four lines. The throughput of the polychromator channels exceeds that of a conventional grating spectrometer by 2 orders of magnitude. This high throughput, together with the high intensity of the ALS undulator radiation, enabled the researchers to observe features never seen before in the sequence of resonances for the radiation decay of n=2p and n=3p, 3d levels. This experiment demonstrated the feasibility of studying processes with cross sections as low as 10e-21 square centimeters while obtaining a record resolution of 3 meV.

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