A muon beam for cooling experiments Page: 3 of 4
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other magnetic element do not distinguish between
different particles with the same momentum. Therefore,
the unwanted particle types must be removed by some
other means. The low energy of the muons makes it
undesirable to put any significant amount of material in
the beam to range out the hadrons, since it would cause
muons to lose energy and therefore reduce the yield of
interesting muons.
An alternative way to remove contaminants is by using
an electrostatic separator or Wien filter. At a given
momentum, a transverse electric field gives a different
kick to different particles, depending on their mass. Since
the pion and muon mass are very close, this is only
possible if the beam divergence is relatively small.
Electronstatic separators are available from
decommissioned beamlines at Brookhaven. Fig. 4 shows
the separation power of a BNL 4.5m separator with a field
strength of 7.5MV/m.
Betatron amplitude functions [m] versus distance [m]
20.0000
' i
" /,
0.000 42.387
Dispersion functions [m] versus distance [m]
0.000 42.387
0.2500 -Horizontal
Vertical ........... .
Figure 5. Preliminary optics for the muon beamline. The
decay channel is matched into a section of FOFDOD
lattice where beam manipulations (momentum selection,
pion and proton separation) take place. Finally, the beam
is matched into the device under test (e.g., the Helical
Cooling Channel).
Optics
A preliminary optics design is shown in Fig. 5. The
FODO lattice of the decay channel is matched by a
quadrupole doublet into a section of FOFDOD lattice.
This provides a series of long straight sections of
alternating high- and low-beta functions. The first straight
section is occupied by a momentum selection chicane.
This generates vertical dispersion at a location of small
vertical beta function, which provides a possibility to
reduce the momentum spread using vertical collimators
without significantly affecting the vertical emittance.In the second straight section, a vertical electrostatic
separator would be used to separate muons from
remaining protons and pions. This is an area of high
vertical beta function, which minimizes the angular
spread of the beam, and therefore the size of the
differential kick that needs to be given to the different
species. The second straight section is used to penetrate
the shield wall between the target and experimental area.
The phase advance of the FOFDOD lattice is roughly 90
degrees, so the differential kick from the separator
translates into a maximum differential position in the
fourth straight section, where again a collimator can be
used to capture the unwanted particles.
After the beam has been separated and momentum
collimated, it is matched into the device under test (e.g.,
Helical Cooling Channel). The total length of the channel
roughly corresponds to the distance available from the
Linac to the beginning of the MTA experimental hall.
Other optics variant, e.g., where the beamline goes all the
way to the end of the hall and the turns back, doing the
cooling experiment in the reverse direction, has also been
discussed.
Instrumentation
In order to commission the beam line and perform any
cooling experiment, the beam position and size need to be
measured in multiple locations.
Scintillating fiber detectors appear to be the tool of
choice, since they are very sensitive yet require little
material in the beam path. A detector prototype has been
developed by Fermilab PPD for the MTEST beam line
[4]. This detector, which achieved single MIP sensitivity
on the bench, could be adapted for use in a muon line.
When used in conjunction with bending magnets, these
detectors could also be used to measure the beam
momentum.
SUMMARY AND OUTLOOK
The possibility of building a 250MeV/c muon beamline
at the MuCool Test Area has been investigated. A muon
yield of 2 10-6 in the momentum range 200-250MeV/c
was achieved at the end of the decay channel.. By
collimating the muons, it is estimated that 104 muons per
linac pulse could be obtained in a small pencil beam of 75
mm mrad transverse emittance and a few percent
momentum spread.
This is expected to be sufficient for a pencil beam
cooling experiment. A full-blown simulation of the entire
beamline, including collimators and electrostatic
separators will be done to validate these numbers and
estimate the achievable beam purity. A complete
evaluation of the energy deposition and activation issues
is also planned.
REFERENCES
[1] M. Popovic et al, report on protons to MTA.
[2] N.V. Mokhov, "The Mars Code System User's Guide",
Fermilab-FN-628(1995),
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Jansson, Andreas; Balbekov, V. I.; Broemmelsiek, Daniel Robert; Hu, M.; Mokhov, Nikolai V. & Yonehara, K. A muon beam for cooling experiments, article, June 1, 2007; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc888000/m1/3/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.