# MUON ACCELERATORS

**19.d. [Background]{.underline}**

The Muon Collaboration (MC), formed by Brookhaven National Laboratory,
Lawrence Berkeley National Laboratory and Fermilab, and supported by
many universities and research institutes, is studying the design and
the key components needed for an intense muon source with
high-brightness and high-intensity suitable for a Neutrino Factory based
on a Muon Storage Ring, and ultimately for a Muon Collider. Following a
recommendation in 1999 from MUTAC (Muon Collaboration Technical Advisory
Committee) that was supported by the Fermilab management the first
neutrino factory design study was conducted in FY00. This initial study
(Neutrino Factory Feasibility Study I) was used to refine the intense
muon source R&D program and to refocus the muon cooling R&D towards a
cooling channel for a Neutrino Factory. Study I showed that a neutrino
factory is technically feasible, but the baseline solution requires a
well-supported R&D program in order to develop all the necessary
components.

Complementing the first neutrino factory design study, there was a
parallel six-month study of the physics potential of a neutrino factory.
This study produced a report in April 2000. The study group continued to
look at physics and detector issues at both neutrino factories and at
future conventional neutrino beams driven by an upgraded proton source.
An addendum to the original report was produced in FY01. The physics
studies showed that there is a unique and exciting physics program that
could be pursued at a Neutrino Factory. However, a high performance
neutrino factory, capable of measuring CP violation in the lepton
sector, requires a higher beam intensity than that achieved in the
Feasibility Study I design. Ideas for achieving this factor led to a
second-generation design study in FY01, initiated by the Brookhaven
management, and supported by the full Muon collaboration, including the
Fermilab group. The FY01 study (Neutrino Factory Feasibility Study II)
resulted in an improved design that met the intensity goals without
increasing the estimated cost of a neutrino factory.

During the course of the Neutrino Factory design studies, it become
apparent that the design and development of a cooling channel to reduce
the muon beam emittance by about a factor of 100 for a Neutrino Factory
(10^6^ for a Muon Collider) is not only a challenging task, but is also
the critical task that needs to be accomplished before a Neutrino
Factory could be proposed. Following Feasibility Study I, the Fermilab
management encouraged the Fermilab group working on the neutrino
factory/muon collider R&D to focus their efforts on developing a muon
cooling channel for an intense muon source suitable for a neutrino
factory. The MUCOOL collaboration was formed to pursue this muon cooling
R&D. The center of gravity of the MUCOOL effort is at Fermilab, but with
strong participation from LBNL, MSU, ICAR and many other university
groups. There are about 70 accelerator and particle physicists and
engineers in the MUCOOL Collaboration. In FY00, ICAR, a consortium of
Illinois universities was formed, and it successfully obtained state
funds to participate in accelerator R&D, with MUCOOL as its initial
flagship project.

A muon ionization-cooling channel consists of liquid hydrogen absorbers
with very thin windows interspersed with high gradient 201 MHz cavities.
The absorbers and cavities are integrated within a lattice of high-field
solenoids that confine the muon beam in the transverse direction. The
MUCOOL R&D program consists of (i) developing and testing the absorbers,
(ii) understanding the performance of high gradient cavities for use in
a high field solenoid, initially at 805 MHz, (iii) prototyping and
testing 201 MHz cavities in a high field solenoid, (iv) integrating an
absorber-cavity-solenoid system for an engineering bench test, and
ultimately (v) testing this system in a 400 MeV proton beam from the
Fermilab Linac: a high intensity engineering test. Beyond the MUCOOL
program, it is proposed to conduct a cooling experiment (called MICE) in
a muon beam at the Rutherford Lab. The absorbers and RF cavities
developed by MUCOOL institutions would be used in that cooling
experiment. The Fermilab group is a vital component of the (MICE)
cooling experiment, which is an international endeavor.

Prior to FY04 the MUCOOL activity had already some significant
accomplishments:

1.  An 805 MHz high power RF test facility has been built at Lab G,
    equipped with a 5T solenoid to enable cavity studies in high
    magnetic fields, required for a cooling channel. \`

2.  An open-cell 805 MHz test cavity has been built and tested at Lab G,
    and operated within the 5T solenoid. The magnetic field is observed
    to facilitate breakdown and large dark currents. This work enabled a
    thesis student (Vincent Wu) to obtain his Ph.D. A second 805 MHz
    cavity with beryllium windows, constructed at LBNL and the
    University of Mississippi, has been completed, installed and tested
    in Lab G. These cavities have been operated within magnetic fields
    and conditions for breakdown have been studied.

3.  Initial tests of a high-gradient 805 MHz cavity filled with gaseous
    H~2~ were performed in Lab G, in collaboration with Muons, Inc.

4.  Various types of detectors have been tested at Lab G to see which
    instrumentation technologies can be used within a cooling channel.
    Measurements of the X-ray flux near the high gradient cavity suggest
    that the instrumentation for the single-particle experiment concept
    being pursued by MICE must be carefully designed. In preparation for
    the MICE experiment, studies of detector performance within the
    harsh environment near an operating RF cavity were pursued at Lab G.

5.  A 201 MHz cavity has been designed at LBNL, and plans have been
    developed to build and then test this cavity at Fermilab.

6.  A liquid hydrogen absorber has been designed and the safety process
    for filling absorbers has advanced. Thin windows for the liquid
    hydrogen absorber have been manufactured and tested, including
    destructive tests at liquid nitrogen temperatures.

7.  A test area for testing absorbers and 201 MHz cavities has been
    designed and civil construction has been completed. This is the Muon
    Test Area (MTA) facility at the end of the 400 MeV Linac.

In addition to the hardware activities at Fermilab, there are also
design activities focused on improving the Study II Neutrino Factory
design with an emphasis on cost reduction. These design activities
include work on ring coolers, a new RF bunching and phase rotation
scheme, and a new acceleration scheme. The ring cooler studies have led
to designs of ionization cooling systems which can cool longitudinally
as well as transversely. The cooling, RF capture and acceleration ideas
that have been developed are novel and may result in significant new
inventions that have accelerator applications beyond their use for
intense muon sources. Based on the rate of current progress, we believe
we will be ready to initiate a Neutrino Factory Design Study within the
next two years. The goal of this study will be to arrive at a design
that is more cost effective than the previous designs, with sufficient
engineering to produce a reliable cost estimate. This design study
should be a solid basis for construction of a future facility

**19.f. [Accomplishments in FY04]{.underline}**

The civil construction for the MUCOOL Test Area (MTA) has been
completed. Beneficial occupancy has been established and an initial test
of operation of a Liquid Hydrogen absorber within the MTA area is being
completed. A beam line from the 400MeV Linac into the MTA area will be
designed and reviewed, with radiation assessment. This work would be in
collaboration with the Accelerator Division external beams department.

We anticipate equipping the MTA for (i) 201 MHz high power testing, and
if funding permits, (ii) liquid hydrogen absorber filling. Fermilab
would contribute the effort for this, as specified in the MOU with the
Muon Collaboration.

Facilities for both 805 and 201 MHz RF will be installed in the MTA. The
RF and diagnostic equipment from the Lab G area is being transferred
into the MTA area, as well as the magnets and test cavities. RF tests
will be reestablished. The RF program will continue and extend studies
of the Be-Cu 805 MHz closed cell cavities and initiate 201MHz studies.

A hollow tube grid structure for an 805 MHz cavity, has been constructed
by an IIT Ph.D. student (M. Al'Sharoa), and will also be tested in the
MTA. In collaboration with LBNL, design studies for a grid of hollow
tubes within high gradient 201 MHz cavities have been developed. The
805MHz studies will give us useful experience which will determine
whether to extend these designs into a neutrino factory cooling RF
cavity (201MHz).

Further tests in Lab G of a H~2~ gas RF cavity were performed, obtaining
peak fields \> 70 MV/m with Mo-coated copper surfaces. These tests will
be continued in the MTA area, with studies of breakdown within magnetic
fields.

The MICE collaboration has consolidated into a coherent effort and has
submitted a proposal to the Rutherford Lab, which has been favorably
reviewed and accepted. The development and testing of 201 MHz cavities
has been identified as the critical path item for the MICE experiment.
The prototype cavities are to be built by LBNL and tested in the MTA.
The new MUCOOL Test Area is the critical resource required to test the
MICE cavities.

A Research & Development plan to develop promising instrumentation types
will be formulated once the first detector measurements have been
completed.

Fermilab is participating in the APS neutrino study, developing plans
for future neutrino factory feasibility. We are presenting a new
approach for the capture, phase rotation and cooling of muons, which
uses only high-frequency RF cavities. This approach should be as
effective and much more affordable than the previous induction-linac
based version.

We have also developed the concept of "non-scaling" FFAG acceleration
for muons, which could make the acceleration much more practical. The
present studies will develop this concept, assess its potential and
develop a prototype design (low-energy electron ring), which could test
its feasibility.

These design activities are expected to result in a significantly
improved cost effective Neutrino Factory design.

**19.g.1. [Accomplishments Expected in FY05]{.underline}**

Following design and assessment, the beam line components will be
identified, and assembled in the MTA transfer line. Design and
construction of new components will be initiated.

The RF test program, which should be established in the MTA before the
end of FY04, will continue with testing of cavity structures within
magnetic fields, and tests of gridded and foil cavities, as well as
gas-filled cavities. Analyses of these tests should lead to improved
"second-generation" designs, which should be initiated by the end of
FY05.

If funding permits, a first prototype absorber will be filled with
liquid hydrogen, and preparations made for an eventual test in which the
absorber is operated next to an RF cavity operating under high power.

The acceleration concepts discussed above should be further developed
and a complete FFAG-based scenario should be developed, and compared
with previous RLA approaches. A low energy e-based prototype design
could be developed to the point that a specific R&D device could be
proposed for construction.

Simulation studies for ring coolers, muon acceleration, and bunching and
phase rotation will advance, in preparation for an eventual Neutrino
Factory Design Study that will be focused on a cost effective Neutrino
Factory design.

###### **19.g.2. Future Accomplishments Expected in FY06 and Beyond**

We envision completion and commissioning of the 400 MeV beam line from
the linac into the MTA, with the assistance of the External Beams and
Proton Source departments. With completion of installation of
high-voltage RF facilities, cryogenic capability, and liquid-hydrogen
absorbers, we would then have the ability for complete testing of
cooling systems. The facility would be available for a wide variety of
accelerator component research and development projects.

Thus, in the MUCOOL Test Area, we anticipate testing an assembly of the
components needed for effective ionization cooling, which would include
a liquid hydrogen absorber within a 5T solenoid (which is moved from Lab
G), and adjacent to an operating 201 MHz cavity, which is itself within
a solenoid (to be built at LBNL). The 400 MeV beamline would enable the
MUCOOL test setup to be exposed to beam, for a full engineering test of
the cooling channel components needed in a short cooling cell.

As the acceleration R&D progresses, for example, if the non-scaling FFAG
retains its promise, a prototype accelerator (20MeV electron ring?)
could be constructed and operated. (The device may also have non-muon
applications.)

In parallel with the 400 MeV beam test, the Fermilab group would
participate in production and testing of the cooling components for the
MICE experiment at the Rutherford Lab. This would be complemented by
participation in the design and implementation of the MICE experiment.

On the Neutrino Factory design and simulation side, the group would
participate in the Neutrino factory design study, which might be
initiated in FY05 and completed by the end of FY06. This research and
design work will result in significant accelerator technology
improvements and innovations of general applicability, in addition to
preparing the way for Neutrino Factories and possibly Muon Colliders.