Hi,

Here are brief responses to questions 2.c. and 2.d. on the list of
questions posed by NSERC.

Glen
-- 
 Glen Marshall                mailto:Glen.Marshall@triumf.ca
 TRIUMF, 4004 Wesbrook Mall   phone: 604-222-7466
 VANCOUVER, BC V6T 2A3        fax:   604-222-1074




Q 2.c. Will the magnet power supply feedback be on the currents or
the magnetic field?

Answer: 

It is anticipated that the power supply regulation on the
current, as determined from very stable and reproducible DC
current transformers which are now being installed, will be
adequate. However, we will install temperature-compensated Hall
effect devices in all magnets (dipole and quadrupole) to
complement the existing NMR devices in the dipoles (field
nonuniformity in quadrupoles is too large to permit the use of
NMR devices). The fields thus determined will be monitored for
stability and reproducibility, to learn whether current control
alone will provide fields within the tolerance range. If it does
not, the communication envisaged between the slow monitoring
system and the muon beam line control system does allow software
feedback to maintain Hall or NMR device readings, and in any case
these will be recorded into the data stream, with alarms to
notify us of any excursions beyond a very restricted range of
values. One problem that may occur is the degradation of the Hall
device characteristics with radiation. While we are designing the
system to avoid this as much as possible, the first quadrupoles
in the M13 beam line are in a high radiation field and their Hall
devices may have a limited lifetime in beam. For this reason, we
hope to learn by recipe how to control the quadrupole field to
the required precision via current control only, characterizing
and understanding any instability, non-reproducibility, or
hysteresis effects, before exposing the Hall devices to beam
radiation.

Q 2.d. Can TRIUMF provide a smaller 1AT1 target and are any other
target modifications needed (you mention tailoring the shape)?

Answer: 

We believe that TRIUMF can provide a smaller 1AT1 target, and
with some development, can provide a shape of target which is
more advantageous for some parts of the TWIST program. No other
modifications to the 1AT1 target have been considered.

In the past, TRIUMF routinely provided graphite targets of up to
10 mm thickness at 1AT1. Graphite is preferred for TWIST because,
unlike the beryllium targets, they are not encased in a water
cooling jacket.  For surface muons, the lack of enclosure leads
to an increased muon rate (typically at least 30% higher,
depending on proton beam characteristics). The graphite targets
are edge-cooled, i.e., they are bonded on one edge to a
water-cooled heat sink. Targets which are thinner in the beam
direction are usually easier because radiation cooling is more
effective, and the demands of thermal transfer for edge cooling
are not as great. On the other hand, development some years ago
of a thicker graphite target for use at 1AT2 was not successful;
due to greater energy deposition, target failure from breakage
due to thermal stresses occurred.

The muon source size must be kept small in order that the beam
emittance from the channel is small. This makes muon beam
injection into the solenoidal field easier, especially where
maintaining polarization across the fringe field is critical, as
in the $P_{mu}\xi$ measurement. Making a smaller target is one
ingredient to reducing the source size, but there is
another. With the existing target shape, surface muons from both
the upstream face (where the proton beam enters) and the side are
visible in the direction of M13, at 135 degrees to the proton
beam direction. By constructing a graphite target of prismatic
shape, where only the side of the target is visible from the
direction of M13, the effective source size is reduced.

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