**Comments on NuMI Beam Optics Design Review**

**Dec 11, 2001**

**from Elvin Harms**

1\. Overall Remarks

The current design for the primary beam line, though improved from the
previous version, still needs work. I suggest taking a fresh look at the
entire design. The results from the MI extraction studies to date put
some urgency on achieving multi-batch MI operation and measuring beam
parameters from an extracted beam in that mode. Powered elements in the
carrier pipe should be implemented only as a last resort.

2\. Itemized suggestions, questions and concerns

\(1\) With all due respect to the effort already expended and improved
beam line design since last time, this line is of sufficient importance
that I believe it deserves a fresh look from a second design team. Such
a new look should be constrained only by \'hard\' limits such as
extraction and target points, bend centers, and constrained physical
apertures. I could see a new design dividing the line into three
sections - matching from the MI into an achromatic section through the
carrier pipe and then another match to the target. Such a design may
require moving bend elements and adding focusing, but could provide a
line with greater room for error.

Beam Physics was asked to take a fresh look at the NuMI Primary Beam
design shortly after this review. They came up with the same basic
design as the NuMI group -- putting devices in the downstream end of the
carrier tunnel. For a more complete discussion see:

<http://www-numi.fnal.gov/numwork/reviews/apr_02.html>. NLG

\(2\) Another plus to looking at a new design for the line could be to
make beam movement less sensitive to quad movement, gradient errors,
skew dipole roll errors, and beam mis-steering. One can almost certainly
expect magnetic elements to shift after installation is complete,
particularly in the upstream part of the beam line. If one can avoid
downtime by relaxing alignment criteria through a good design, so much
the better.

NLG: This has been done, see:

<http://www-numi.fnal.gov/numwork/reviews/nov_01.html>

and:

<http://www-numi.fnal.gov/numwork/reviews/apr_02.html>, especially the
document:

[A FODO-Based NuMI beamline
design](http://www-numi.fnal.gov/numwork/reviews/apr_02/FODO_design.pdf) -
J. Johnstone

\(3\) The results of the MI extraction studies to date suggest putting
some priority on the multi-batch extraction studies. In terms of beam
line design, I\'d suggest using the current values and scale by a factor
of 4 for initial conditions until hard numbers can be obtained.

NLG: This has been clarified somewhat, see:

[Main Injector parameters expected during NuMI
operation](http://www-numi.fnal.gov/numwork/reviews/apr_02/bock302letter.pdf) -
S. Mishra

1.  Avoid putting magnetic elements in the carrier pipe except as a last
    resort. Environmental conditions sound too harsh. To install magnets
    and expect them to eventually fail and then run without them is
    wasteful. If there is no recourse but to add quads, they should have
    BPM\'s included.

Quads and BPMs will be installed in the downstream section of the
carrier tunnel. Environmental conditions in this region have been
upgraded to match those for the Pre Target tunnel. SC

\(5\) The presented design of the beam pipe/LCW lines support needs
refinement. The beam pipe should have stiffer support than what was
shown and be mechanically de-coupled from the LCW lines. Keeping the LCW
below the beam pipe is certainly a good idea should a leak develop, but
will it matter if the carrier pipe is cold and damp to begin with? If
there are no powered elements, does the carrier pipe carry only the beam
pipe?

DP: The recommendation to isolate the beam pipe from the LCW has been
done.

\(6\) Power supply regulation requirements should not be loosened unless
there is a significant financial impact. More often requirements are
later made more stringent. It wasn\'t clear whether the power supplies
were to run DC or ramped (I think ramped? - see Nancy\'s presentation
pp. 2 and 4). Operational experience has shown that this makes a big
difference in beam reproducibility and stability - both in terms of
power supply regulation and magnet hysteresis. Has this been closely
looked at?

We presently target beam for P-Bar production with a regulation system
that is an order of magnitude lower than being used for NuMI. The cost
of changing to DC on the line will impact the operation cost in both
power bill and cooling system. The DC system can have better pulses to
pulse regulation because the bandwidth limit changes from 20 Hz to 720Hz
but is still limited by the ability to measure both the current and
reference. SLH

\(7\) An unrelated but I think pertinent question: is the carrier pipe
to be under vacuum? If so, to what level; are there scattering issues
which could lead to non-trivial beam loss; how is pumping to be
accomplished on such a large volume?

DP: By carrier pipe, I assume you refer to the beam pipe and not the
concrete tunnel. If this is correct, the beam pipe is to be under high
vacuum to a level that is better (lower pressure) than traditionally
used on external beam pipes, but worse (high pressure) than an
accelerator vacuum. Jim Klen is to produce an engineering note on the
vacuum level in each region of the carrier pipe which details the
locations of the 38 or so ion pumps distributed (at great expense) along
the beamline.