AP2 and Debuncher Acceptance

# Keith Gollwitzer

Motivation

The beam of secondaries from the antiproton production target fills a
large phase space. Simulations show that the angular distribution of
antiprotons has an rms angle of 45mrad and projected to be within a few
mm of the primary beam axis at the center of the target (momentum
acceptance of ±2.25%). The divergent beam is transformed by the lithium
lens to a phase space bounded by the radius of the lithium. The AP2 beam
line transports the large phase to the Debuncher ring where the cooling
systems reduce the phase space before transfer to the Accumulator. Both
the AP2 beam line and Debuncher ring need to have as large as possible
antiproton acceptance.

The AP2 beam line and Debuncher ring magnet elements and lattices were
originally designed to have acceptances larger than 40π mm-mrad
(un-normalized). Historically, the Debuncher acceptance has been
measured to be 20-25π mm-mrad. The current antiproton yield is
15-18x10^-6^ per proton on target. The result of simulations, which
optimize the AP2 optics and the lithium lens gradient, plotted in the
High Gradient Lithium Lens section show that the yield should increase
to 33x10^-6^ (lens gradient constant at 75kG/cm). The goal is to realize
an overall AP2 beam line and Debuncher ring acceptance of 40π mm-mrad.

Issues

The main issue is to identify the location of all apertures that are
less than 40π mm-mrad and take corrective action. Limiting apertures can
exist due to misalignment of accelerator components, beam orbit, beam
size due to the beam optics and/or the physical design of the beam pipe
or accelerator element. The corrective actions are alignment, beam orbit
control, change the lattice and/or move/modify/redesign elements.

Since the initial commissioning of the Antiproton Source, there have
been changes and upgrades to the AP2 beam line and Debuncher ring. The
drawings and documentation of the AP2 beam line and the Debuncher ring
need to be collated and verified. The current beam optics needs to be
measured. The combined information of the apertures and lattice will
identify components that will need to be moved or redesign. It may be
possible to change the lattice to decrease the beam size at such
physical apertures. The combined information will also be used to
determine all components' misalignment tolerance. Where the misalignment
tolerance is small, both alignment and beam orbit control will be
corrective actions taken.

Due to the beam of secondaries in the AP2 beam line and the resulting
low intensity antiproton beam that eventually circulates in the
Debuncher ring, traditional orbit measurement methods are not
considered. Orbit and optics measurements are mostly done with dedicated
reverse 8GeV proton beam (delivered from the Main Injector to the
Accumulator and then transferred to the Debuncher). In any case, the
Beam Position Monitoring systems of the AP2 beam line and Debuncher ring
need to be upgraded to make the needed orbit measurements.

Status

A team of Technical Division personnel has begun the research of
documentation and drawings to determine the aperture anywhere the beam
pipe changes and at the entrance, center and exit points of all
accelerator components of the AP2 beam line and Debuncher ring. During
accesses to the Antiproton Source tunnels, they will be verifying the
research.

In the last three months there has been effort to establish the
procedures to reliably transport reverse 8GeV proton beam to the
Debuncher as well as manipulating the beam in the Debuncher (RF to
change beam momentum and to make BPM measurements with the existing
partially working system) and sending beam through AP2 (verified with
SEM grids and attempts to make beam line BPM measurements). The
collimator scraper sets have been exercised and used to measure the
current admittance during reverse protons studies in the Debuncher and
during stacking to determine the current overall acceptance of the
combined AP2 and Debuncher system. The measured admittances are in the
table below. In addition the total momentum acceptance of the AP2 and
Debuncher system has been measured to be 4.6%.

+-------------------+----------------------+-------------+------------+
|                   | Beam Type            | Hor. Admit. | Ver. Admit |
|                   |                      |             |            |
|                   |                      | (mm-mrad)   | (mm-mrad)  |
+-------------------+----------------------+-------------+------------+
| Debuncher         | Reverse Protons      | 26π         | 20π        |
+-------------------+----------------------+-------------+------------+
| AP2 & Debuncher   | Stacking Antiprotons | 20π         | 12π        |
+-------------------+----------------------+-------------+------------+

Previously, areas where it is believed that beam orbit control would be
most useful were identified. One dipole trim per plane has been inserted
in the upstream portion of the AP2 beam line during the past year.
During the January 2003 shutdown, two vertical dipole trims have been
added to the down stream portion of the AP2 beam line.

Due to the crowded Debuncher ring, orbit corrections will be done with
remotely controlled movable quadrupole stands. The stands allow the
quadrupole to be offset in the transverse planes producing a dipole kick
to the intentionally off-centered beam. Ten stands have been
manufactured and await installation.

A new Debuncher BPM electronics prototype has been installed and tested
under beam conditions. During the January 2003 shutdown, a sector has
had the new electronics installed followed by beam tests of a sector's
data acquisition.

The AP2 beam line BPMs located near the Debuncher kicker during reverse
proton studies show pickup of the kicker signal. About half of the BPMs
may be affected. Investigations into this problem have just begun. In
addition, it is not known if the current procedures for transferring
reverse proton into AP2 will provide enough beam intensity for the
current beam line BPM system to work sufficiently.

Plan

The one sector of new Debuncher BPM electronics and data acquisition
system will be thoroughly tested in the near future. The balance of the
system will be completed and bench tested in the next few months with
installation during opportune accesses.

The beam line BPM pickup of the kicker signal will be addressed in the
near future. A possible solution is to replace the existing cables and
move the data acquisition electronics to another building. It may be
possible to do the AP2 beam line optics measurements with the currently
unaffected BPMs. Further study of the reverse proton beam intensity will
be done. This could lead to a change of beam manipulation procedures
and/or changes to the beam line BPM electronics and data acquisition.

The Debuncher movable quadrupole stands will await opportune accesses
and availability of manpower for installation. The order of priority for
the installation of the stands is four in the injection region, four in
the extraction and cooling pickup region and then two in the kicker tank
region. If these stands along with existing Debuncher correction
elements are successful in providing local orbit control, then up to
twenty-five more stands could be manufactured and installed for local
orbit control throughout the Debuncher ring.

Investigations into the feasibility of using a portable quadrupole
alignment fixture will be done. The idea is to make an orbit
measurement, tunnel access to attach the fixture and move a quadrupole a
small desired amount, verify dipole kick was achieved with orbit
measurements, and if needed iterate or return quadrupole to original
position. The goal would be to do this within two shifts by studiers
without surveyors. A prototype fixture exists and needs to be tested on
a spare magnet and stand setup.

By the summer, the documentation research should result in table of the
apertures throughout the AP2 beam line and Debuncher ring. In addition,
the lattice model files will be completed and verified. The combined
information will help identify possible limiting apertures.

The plans for moving/modifying/redesigning suspected and known limiting
apertures have not changed in the past year. The elements previously
identified are the first Debuncher SQC quadrupole after injection, RF
cavities (DRF3 and DRF2), and the band 4 cooling tanks. The proposed
corrective actions are respectively to replace with a larger opening LQD
quadrupole, move the RF cavities, and redesign the arrays and/or change
the lattice through the cooling tanks.

Surveys of portions of AP2 beam line and the Debuncher ring will be done
whenever manpower is available and tunnel enclosure access is possible.
Surveys will not only concentrate on the small alignment tolerance areas
but also on areas where the tunnel enclosure may have moved: AP2 section
that was removed and replaced for the construction of the MI8 beam line
and where in the past tunnel movement has been measured. In addition,
the overall alignment of the AP1 and AP2 beam lines will be checked.

Studies

A majority of the studies are to be preformed using the 8GeV reverse
protons: beam delivered to the Debuncher from the Main Injector via the
P1, P2, AP1 and AP3 beam lines to the accumulator and then transferred
to the Debuncher through the connecting D2A beam line. Reverse proton
can be kicked out of the Debuncher into AP2 for studies. There are some
semi-parasitic studies that can be performed while the Fermilab complex
is stacking. With the reversal of polarity of the magnet elements of the
AP2 beam line and Debuncher, 8GeV forward protons can be used for
studies when the production target and lithium lens are removed from the
beam path (Main Injector, P1, P2, AP1, AP2 and then to the Debuncher).

## Reverse Proton Studies 

The accumulator shutter system can be closed to isolate any antiprotons
circulating on the core orbit from kicker effects of the injection and
extraction of reverse protons passing through to the Debuncher. These
studies occur during dedicated studies periods.

1.  Debuncher Studies

    a.  BPM commissioning

        i.  Single Sector: intensity versus result stability, local
            bump, front-end algorithm, data acquisition

        ii. All Sectors: console application, polarity, scaling,
            archiving

    b.  1-Bump lattice measurements (after BPM system commissioned)

        i.  Nominal Orbit

        ii. Off momentum orbit

    c.  Determine quadrupoles' centers (after BPM system commissioned)

        i.  Vary shunt currents and see closed orbit changes

        ii. Determine if bumps can result in centered orbit

    d.  After installation of each set of remotely controlled motorized
        quadrupole stands:

        i.  Verify quadrupole movement

        ii. Determine dipole kick per unit of movement

        iii. Develop local bumps

    e.  Center orbit in aperture

        i.  Heat beam in transverse plane and watch Beam Loss Monitor
            (BLM) system signals

        ii. Determine which BLM counter shows first beam loss signal

        iii. See if local orbit bump removes loss signal on BLM counter

    f.  Portable quadrupole alignment fixture (after prototype is
        finished)

        i.  Test ability to produce desired dipole kick

        ii. Test ability to put quadrupole back

    g.  After every orbit change that is implemented:

        i.  Center appropriate motorized (non-steering) devices

    h.  At the end of most studies periods:

        i.  Measure closed orbit

        ii. Measure transverse admittances

        iii. Measure momentum aperture

2.  AP2 Studies

    a.  BPM commissioning

        i.  Develop method for measuring orbit position reliably for
            upstream part of AP2

        ii. Commission downstream AP2 BPMs for solution that removes the
            kicker signal

    b.  1-Bump lattice measurements (after BPM system commissioned; may
        be possible to do after upstream BPMs are commissioned)

        i.  Nominal orbit

        ii. Off momentum orbit

    c.  Commission newly installed dipole trims

    d.  Determine quadrupoles' centers (after BPM system commissioned)

        i.  Vary shunt currents and see orbit changes

        ii. Determine if bumps can result in centered orbit.

    e.  At the end of most studies periods:

        i.  Record nominal BPM orbit

        ii. Record nominal SEM orbit

3.  Injection Region Studies

    a.  Center orbit in aperture

        i.  Blow-up/scrape beam to known emittance

        ii. Kick beam out of Debuncher into AP2

        iii. Measure surviving beam in AP2 varying closed orbit, kick
             strength, and septum voltage and position

## Stacking Studies 

These studies are semi-parasitic since antiprotons can still be stacked
at a lower overall rate. The reduction of the stacking rate will be
caused by increasing the stacking cycle time, scraping of the beam due
to mis-steering of the beam, and/or particles are intercepted as part of
the measurement.

1.  AP2 Studies

    a.  Determine quadrupoles' centers (using SEMs)

    b.  Correct secondaries orbit to the reverse proton SEM orbit

2.  AP2 & Debuncher Studies

    a.  After most studies periods:

        i.  Measure combined AP2+Debuncher transverse acceptance

        ii. Measure combined AP2+Debuncher momentum acceptance

        iii. Record beam orbit using SEMs

## Forward Proton Studies 

These studies are disruptive to the collider program since antiprotons
are not available from the Antiproton Source. Each reversal of polarity
of the magnetic elements requires a few shifts. These studies require
both the antiproton production target and lithium collection lens out of
the beam path.

1.  AP1 & AP2 studies

    a.  Make BPM, SEM and intensity monitors operate with the associated
        timing events to deliver forward proton beam

    b.  1-Bump lattice measurements

    c.  Determine quadrupoles' centers

    d.  Record AP1 and AP2 BPM orbit

    e.  Record AP1 and AP2 SEM orbit

2.  Injection Region Studies

    a.  Center orbit in aperture by measuring surviving beam after
        Debuncher injection kicker while varying different AP2
        correction elements and injection elements (septum and kicker)