Machine Setup and Operation

Weekly Operations Review

Reported by W. Byrne

Providing high quality light to users in a consistent and reliable
manner is one of the main goals of the accelerator physics group. To
meet this goal a large fraction of our time is spent monitoring the
performance of the machine. At the Group's weekly meeting the
performance of the accelerator over the previous week's run is reviewed.
Graphs of various parameters which characterize the performance of the
storage ring are presented and discussed.

Figures 1-3 show plots of the accelerator's performance for a typical
week (December 5 -- 11, 2000). For this particular week the users began
at 8:00 AM on the 5^th^ (1/3 into day 1) and ended at midnight on the
10^th^. The beginning of day 1 and all of day 7 were used for
accelerator physics studies and accelerator maintenance.

Reliability

The plot of beam current over time (Figure 1A) is a measure of how
reliably beam has been delivered. Beam drop-outs are noted (Figure 1B)
and the causes discussed by the group. Action is taken to correct
recurring problems.

Statistics are compiled each week to show how reliably fills were
completed. For typical 1.9 GeV operation, fills are scheduled every 6
hours. Each fill is categorized according to duration. At the end of the
week the data is charted to show the percentage of fills that lasted the
full 6 hours and the fraction that ended earlier than planned (Figure
1C). Our goal is to meet the scheduled 6 hour fill length 95% of the
time. Presently, more than 90% of the fills meet this 6 hour goal.

Brightness, lifetime, current, and beam size

The lifetime, , in the ALS is predominantly determined by intrabeam
(Touschek) scattering. The lifetime is proportional to the transverse
beam size, ~x~ and ~y~, and inversely proportional to current, *I*:

![](media/image1.wmf)

A figure of merit for brightness which we call the Touschek factor,
*Tau*, (Figure 2D) is define as

![](media/image2.wmf).

The brightness of the beam increases with *Tau*.

During a run, *Tau* should remain constant. Changes in *Tau* mean
changes in the beam quality and are usually indicative of a problem with
the machine. The parameters which determine the Touschek factor are
plotted in Figure 2: beam intensity (Figure 2A), beam lifetime (Figure
2B), and beam size (Figure 2C). The smaller beam size during most of the
first day (Figure 2C) is not real and was caused by a filter in the
optical system that was too dense.

The transverse beam size is a function of the betatron tune. The
betatron tune changes when the insertion device gap position changes.
Changes in the insertion device gap position are plotted in Figure 2F
and the resulting vertical tune changes are plotted in Figure 2E. The
vertical tune is shifted higher when the gaps are closed which moves the
tune closer to the coupling resonance. This coupling of the horizontal
and vertical betatron tunes causes the beam size to increase vertically.
These vertical tune changes are partly responsible for changes in the
vertical beam size. A tune feedforward scheme was recently implemented
to correct most of the tune shift caused by insertion device gap
changes. Since implementing the tune correction, the change in vertical
beam size associated with gap changes has been reduced from
approximately 25% to 1-2%. An exception is the elliptically polarized
undulator. At maximum polarization the vertical beam size can increase
up to 15% at minimum gap.

The transverse beam sizes are plotted in Figure 2C. Almost all of the
size variation is due to the change in sensitivity of the measurement
system with beam intensity. The real transverse variation is 1-2 m.

**Positional Stability**

In order to stabilize the beam position, the orbit of the beam in the
storage ring is corrected once every second. This holds the beam
position constant at the upstream and downstream ends of most of the
straight sections.

The performance of the correction algorithm is monitored routinely and
adjusted if required to optimize performance. Figure 3 shows the
vertical position of the beam at beam position monitors at the ends of
the straight sections and in the curved section of Sector 9 over a
week's run. The BPM's with large deviations in sectors 4, 6 and 9 were
not in the correction loop. The position of the beam at the other BPM's,
which were in the correction loop, remained constant within a few
microns for the duration of the run.

![](media/image3.wmf){width="2.7569444444444446in" height="3.74375in"}
![](media/image4.wmf){width="2.7041666666666666in" height="3.75625in"}

*Figure 1. Beam parameters for a typical week (January 25 -- 30, 2000).
Beam current, lifetime, beamsize, Touschek factor, tune changes and
insertion device gap positions are plotted on the left. Vertical beam
motion the insertion device straight sections are plotted on the ring.*