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. 17           March 21, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. ACCELERATOR PHYSICS -- NEW VERTICAL BEAM INSTABILITY UNDER INVESTIGATION
    3. NEUTRON APERTURE CHARACTERIZATION AT THE ALS

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

Beam availability last week was 94.4% overall and 93.4% during user shifts. The major outage was 3.5 hours caused by air in the low-conductivity water (LCW) lines as a result of maintenance work on the LCW system that serves another area of the Laboratory (LBL).

Weekly Scheduling Meeting:
    March 27 at 11:00 in Building 2-100B followed by discussion of short- and
long-term operations schedule. Don't forget -- this is an opportunity for users
to give feedback on the 7-day, 2-shift schedule and on how 1.5 GeV and 1.9 GeV
time should be scheduled during the weeks of June 6, June 14, August 9, and
August 16.

**NOTE** SCHEDULE CHANGE:
    Saturday March 25, 16:00-23:15, 1.5-GeV, 400-mA, 320-bunch operations
(previously scheduled for 1-mA operation)
    Wednesday March 29, 16:00-23:15, 1.5-GeV, 1-mA, 320-bunch operations for
U8/gas filter characterization at 9.0.2 (previously scheduled for 400-mA
operation)

Operations summary for March 21 - April 5
1.5-GeV, 400-mA, 320-bunch operations for users:
    March 22, 16:00-23:15
    March 23-25, 08:00-23:15
    March 26, 08:00-16:00
    March 30-April 2, 08:00-23:15
    April 5, 16:00-23:15
1.5-GeV, 1-mA, 320-bunch operations for U8/gas filter characterization 
at 9.0.2:
    March 26, 16:00-23:15
    March 29, 16:00-23:15
Maintenance: 
    March 27 and April 3, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    March 21 & 28 and April 4, 08:00-23:15
    March 22 & 29 and April 5, 08:00-16:00

2. ACCELERATOR PHYSICS -- NEW VERTICAL BEAM INSTABILITY UNDER INVESTIGATION
(contact: robin@lbl.gov)

During the winter shutdown a new, narrow-gap chamber (internal gap of just 10 mm) was installed in the Beamline 7.0 undulator allowing that undulator to be closed to 14 mm, or a peak field of 1.0 T. After the shutdown, observations showed that the electron beam became vertically unstable when users closed the gap to less than 25 mm at high beam current (>300 mA). The instability, observed in the diagnostic beamline 3.1, caused vertical oscillations in the beam position as well as growth in the vertical beam size. This effect severely degraded the quality of the beam and made operation at minimum gap impossible.

It is not yet entirely clear what causes this instability. Certainly, the new narrow-gap chamber has increased the vertical impedance of the storage ring, and the higher undulator fields cause larger beta-function distortion and provide stronger higher-order fields which can drive resonances. The instability is current-dependent, and it is relatively insensitive to betatron-tune variation, but it can be suppressed by increasing the vertical chromaticity. This last observation provides a way to assure unrestricted use of Beamline 7.0 while the accelerator group investigates the instability's root cause. Diagnostic beamline 3.1, commissioned in its present form last December, is a valuable resource for this investigation because of its ability to offer real-time, detailed information on electron beam characteristics.

3. NEUTRON APERTURE CHARACTERIZATION AT THE ALS
(contact: ress@llnl.gov)

David Ress, who works in inertial-confinement fusion (ICF) research at Lawrence Livermore National Laboratory, recently used Beamline 10.3 to evaluate apertures for use in his ICF experiments. ICF, one of the major lines of research into fusion energy, uses laser energy (currently about 30 kJ per event, producing temperatures of about 2 million K) to implode target capsules containing the heavy hydrogen isotopes deuterium and/or tritium. In ICF research the goal is to ignite a small fuel capsule in a thermonuclear burn. To initiate ignition, uniform radiation illuminates the capsule surface causing it to implode, and creating a region of hot, low-density material in the center of the capsule, surrounded by cold, high-density material. The hot spot in the center begins fusion, sending out energetic alpha particles that cause a thermonuclear burn wave to propagate through the rest of the material. The current phase in ICF research replaces the outer shell of frozen deuterium or tritium with a glass or plastic shell, so that only the hot spot reaction occurs; ignition and propagating-burn research is the goal of the planned National Ignition Facility, with about 60 times more laser energy per implosion.

To achieve ignition, it is necessary to make the radiation on the target capsule as symmetric as possible, thereby minimizing hydrodynamic instabilities that cause less efficient implosions. One way to measure the radiation symmetry involves taking a neutron image of the hot spot fusion region. Neutron imaging requires a specially tapered gold aperture with a diameter varying from 500 to 900 microns. The aperture taper must be known to much greater spatial resolution than the desired resolution for the neutron image in order for the neutron image to be analyzed accurately. Various research groups have fabricated neutron apertures by several different methods, and Ress wished to evaluate the success of these methods and to confirm the accuracy of an aperture shape used for neutron- imaging experiments in 1988.

To test each aperture, Ress used the collimated x rays at Beamline 10.3, first making them incident on the entrance (small end) of the aperture with the aperture axis parallel to the x-ray beam, so that only the entrance was imaged. He then inclined the aperture relative to the x-ray beam, taking additional exposures in which the beam contacted interior aperture surfaces, until the aperture was tilted so far that no x rays passed through it. He repeated this process for eight different azimuthal (rotational) angles and then used the complete data set to "unfold" the shape of the aperture and evaluate whether it matched its specifications.

The results of these tests showed that the aperture used in the 1988 neutron-imaging experiments was by far the most accurate; this validates the results of those experiments and will help guide future efforts in aperture fabrication. The manufacture of this aperture began with grinding a copper mandrel (negative) for the aperture, then straightening the mandrel by hand, electroplating it with a few millimeters of gold, and finally etching away the copper mandrel with acid.