**Report of the Meeting of the 2nd Collider-Accelerator Department**

**Machine Advisory Committee**

Brookhaven National Laboratory, November 8-9 2004

**Committee:** Alexander Chao, SLAC; David McGinnis, FNAL; L. Merminga,
JLAB, Ferdinand Willeke, DESY

**Apologies**: Oliver Boine-Frankenheim, GSI; Jean-Pierre Delahaye, CERN

**Introduction**

The Collider-Accelerator Machine Advisory Committee Meeting was held for
the 2nd time on November 8-9 2004 in Brookhaven National Laboratory. The
committee was charged to review the plans and preparation for a
luminosity upgrade program for gold-gold collisions and polarized
proton-proton collisions. The charge and the agenda of the meeting are
attached at the end of this report.

The committee is quite impressed with the continuous progress in RHIC
operation which has been achieved so far. The accelerator operates well
above the original design. Polarized protons have been made available
already. This is an excellent base for further progress. The committee
would like to congratulate the accelerator team on these successes.

The meeting was very effective with well-prepared presentations and open
exchange of information. The formal presentations were complemented by
breakout session for clarification of some of the technical issues. The
committee acknowledges the effort of the CAD-team.

The committee bases its assessments on the goals as presented during the
meeting which may be summarized as:

-   Increase of the Au-Au integrated luminosity by a factor of 2.5 by
    increasing the number of bunches from presently 45 to 112 in each
    ring.

-   Increase of the proton beam polarization in RHIC from presently 45%
    to 70% and extend it to beam energies of 250GeV per beam.

-   Increase of the proton-proton luminosity by a factor of 16 by
    increasing the number of bunches by a factor of two and by
    increasing the bunch intensity from 0.7 ∙ 10^11^ to 2∙ 10^11^
    protons per bunch.

These goals are quite ambitious and represent a considerable challenge.
They require careful study and well planned measures to be undertaken.

Heavy Ion Luminosity Operation

The committee learned that the luminosity for Gold-Gold collisions is
limited in RHIC by various vacuum effects which do not allow increasing
the total beam current above the levels achieved in the 2004 luminosity
run. The ion intensity per bunch is exceeding already the design value
by 10% and there are no immediate further plans to increase this
parameter. The Committee agrees that the vacuum limitations are the only
serious issues to be solved in order to obtain the desired increase in
Au-Au luminosity. The high vacuum pressure observed at highest beam
currents go along with intolerable background condition and poor
luminosity lifetime.

The vacuum problems have been assessed by analyzing accelerator data and
by performing dedicated experiments.

Evidence has been provided that the vacuum problem at injection energy
can be explained by electron cloud instability. The committee agrees
that the arguments which lead to this conclusion are quite compelling.

At luminosity operation, however where e-cloud appears to be a likely
cause of strong vacuum pressure rise with high intensity, direct
evidence for the presence of electron cloud instability (ECI) can not be
demonstrated. Moreover, a number of other reasons such as compromises in
the preparation of vacuum components are quite likely the cause of
pressure instabilities which are observed occasionally. Nevertheless the
committee believes the assumption that electron cloud is present at high
energy as well is reasonable.

The committee endorses the approach to resolve the intensity limitation
by improving the beam vacuum.

However, the fact that ECI can not be clearly identified as the cause is
unsatisfactory. It appears to be highly desirable to provide direct
evidence for ECI forming at high energy in order to assure that the
optimum way to improve the vacuum is chosen and in order to make sure
that the effort of adding solenoids around the beam pipe is justified.

The committee therefore recommends pursuing the quest of ECI in
luminosity operation and attempting to clarify this issue. The committee
can see possible way of making further progress with this issue by
measuring the coherent tune shift across the bunch train or additional
effort to observe the electrons directly in the beam pipe.

The committee already learned on its last meeting that stochastic
cooling is considered as a means to improve the luminosity lifetime and
to suppress backgrounds due to loss of particle from the RF bucket.
Meanwhile Schottky signals have been observed and transfer function
measurements have been observed which indicate that stochastic bunched
beam cooling in RHIC has a good chance to be successful. The plans for
implementing stochastic cooling systems have been advanced and a layout
of the cooling system is in the process of being implemented.

The committee endorses the plan for the installation of longitudinal
kickers with half the required bandwidth already this year. This will
provide a chance to demonstrate cooling and will provide the opportunity
of gaining expertise and experience from which the further development
of the full system will profit. This appears to be an effective way of
proceeding in this challenging and exciting project.

The upper frequency of 8 GHz has very small design margin for a 2/3rd
turn delay and stability at the desired cooling rates may become an
issue. The committee therefore suggests investigating the possible
benefit of a signal connection across the accelerator ring ("cutting a
cord") which would provide more margins in the achievable cooling rates
and in addition would open the possibility to increase the bandwidth to
16GHz, which might be feasible. This would result in a significant
increase in the cooling rates of the system.

Vacuum and Technical Improvements

Plans to improve the RHIC vacuum in response to the experienced problems
to achieve highest beam intensities have been presented to the committee
which include

-   In situ bake out of warm beam pipe sections

-   NEG Coating of the warm beam pipe sections

-   Better pumping of the cold beam pipe prior to cool down

As the committee believes that improved vacuum conditions will very
likely be beneficial for high intensity operation of RHIC, the committee
endorses the planned measures.

Moreover the committee encourages the vacuum support groups to start as
soon as possible with the planning of the various actions. The detailed
plan to work out should be flexible enough to take into account possible
fall back positions or shift of emphasis in case new information becomes
available during the upcoming RHIC luminosity running 2005.

The committee is somewhat concerned about the vacuum inside the cold
part of the accelerator. The vacuum prior to cool down should be reduced
to a level of at least 10^-6^ mbar or less to avoid saturation of the
cold beam pipe surface and relatively high vacuum gas density during
beam operation.

In order to study the impact of many layers of gas on the cold beam
pipe, one sextant has been equipped with more pumps resulting in a
significantly reduced gas pressure prior to cool down.

The committee endorses in particular plans to install additional turbo
pumps around the accelerator in case that the improved sextant shows a
better vacuum behaviour with respect to the rest of the ring. In case
those additional pumps cannot be provided in time for the next shut down
a staged cool down with concentration of the available pumping speed
should be considered to be included in the shut down.

Pressure instabilities in the RHIC warm vacuum sections have been
observed. They appear to coincide with recent vacuum installations where
compromises between vacuum requirements and schedule constraints have
been made.

The committee would like to encourage rigorous vacuum certifications.

Polarization

The committee is very pleased to see an impressive progress and success
of the RHIC polarized proton program. These include application of an AC
dipole in the AGS and a 5% Siberian snake to overcome the strongest
intrinsic depolarizing resonances, the successful operation of the OPPIS
polarized H^-^ source, the functioning of the RHIC Siberian snake and
the spin rotators and last but not least the pC polarimeter. This
success constitutes a solid base for the future performance
enhancements.

The committee takes note of an ambitious upgrade plan with the goal to
achieve 70% polarization in RHIC at a beam energy of 250GeV with a very
high beam intensity of 2∙10^11^ protons per bunch in 112 bunches. To
achieve this, a perfect polarization transmission efficiency of 100% in
both AGS and RHIC acceleration is required under more demanding
conditions. The simultaneous increase in polarization, beam intensity
and beam energy represents a considerable challenge which requires
careful analysis and planning.

Reviewing the plans of the polarization team to achieve these goals
which are based on the installation of a superconducting 25% Siberian
snake in the AGS and an improvement of the polarized ion source OPPIS,
the committee can not recognize any principal problems with the planned
upgrade. However, the committee would like to see a more clear estimate
and identification of the necessary resources and study time.

As far as the polarization in the AGS is concerned, the committee
expects that the addition of a superconducting Siberian snake, together
with a combined optimal tuning with the existing warm snake, should help
to cancel the depolarization resonances (3 weak intrinsic depolarising
resonances at 24-Q~y~, 24+Q~y~, and 48-Q~y~, as well as the strong
resonance at 36+Q~y~) which limited the performance in the previous run.
The committee also believes that there is good chance to provide the
necessary resonance free space in the working diagram to accommodate the
tune spread of beams of intensities of up to 2∙10^11^. It is also
expected that the AC dipoles will not be necessary anymore.

A challenging issue however is to operate the AGS with the vertical tune
very close to the integer resonances, Q~y~=8.96. The enhanced
sensitivity to orbit error, dispersion error, distortion of the beam
envelope function, as well as the presence of non-linear snake
resonances will require considerable set-up and tuning time.

The committee suggests that the polarization team performs a simulation
of ramping with orbit and beta errors and realistic orbit correction in
order to assess the operational difficulties and as a base for the
planning of the necessary machine study time. Particular attention
should be paid to nonlinear depolarization resonances to avoid
unpleasant surprises.

The committee supports the idea of using the planned luminosity run
period with collisions of Cu ions parasitically to set up the AGS for
high polarization and high intensity running. This however will require
careful planning.

With regard to intensity dependent depolarization effects, the committee
suggests that the polarization team should keep up the effort to provide
more understanding of the issue. There are some obvious checks to
perform such as comparison of the space charge enlarged tune footprint
with the expected available resonance free space in the tune diagram. If
space charge still turns out to be problem, the polarization team should
design another way of operating the Siberian snake with higher strength.

The committee supports the idea of back-up plans for a second cold
Siberian snake in case the problems in the AGS cannot be solved
completely.

The achieved polarization in RHIC amounts to respectable 40% and 45%
with a bunch intensity of 0.7∙10^11^ protons per bunch in 56 bunches and
at a beam energy of 100GeV per beam in the two RHIC rings respectively.
The proper functioning of the Siberian snakes and the spin rotators has
been impressively demonstrated this way. The working point for RHIC has
been optimized for simultaneous high polarization and accommodation of
strong beam-beam forces.

The committee would like to note that this optimization is quite time
consuming and need to be continued consequently. As beam-beam effect has
to be observed in RHIC with the desired level of beam intensity, the
committee is concerned about the depolarizing effect of the beam-beam
interaction which might be of particular importance in the case of
beam-beam emittance growth.

The committee would like to encourage the RHIC team to consider
injection with both the Siberian snake and the rotator on in order to
shorten the injection time, and hopefully increase the polarization
transmission efficiency.

While the committee is quite confident about achieving high polarization
at beam energies of 100GeV, to preserve polarization during acceleration
to 250 GeV will be a new challenge. Confidence level is higher for 100
GeV/c (remove this sentence). Resonances get stronger with beam energy.
The multitude of intrinsic and imperfection resonance to overcome will
make it quite difficult obtaining 100% polarization transmission
efficiency.

The committee believes that a condition of 250GeV polarization will be
stringent orbit control resulting in RMS orbit deviations from the
centre of the quadrupole magnets of less than 0.3 mm. Such an almost
planar orbit will require careful realignment of the accelerator magnets
and will require beam based alignment techniques.

The committee however would like to stress that beam based alignment is
also very tedious and time consuming. The RHIC design team needs to
develop and to test an appropriate algorithm.

The committee further believes that unfortunately harmonic orbit
correction resonance by resonance is impractical. However, the committee
would like to encourage the RHIC polarization team to make use of HERA
experience where a very successful method of preserving high
polarization without empirical retuning the bumps has been applied:
Depolarization due to unavoidable orbit correction can be avoided if the
orbit correction kicks are projected into the space of harmonic bumps
and compensated with bumps of opposite sign.

The committee can see a considerable need for simulation studies and
accelerator study time. The committee believes that good use of the time
between now and 2008, when 250 GeV luminosity running is planned, needs
to be made.

The committee would like to mention further optimization possibilities:\
The betatron tunes need to be fixed quite well during acceleration and
low beta squeezing to stay within the limited window between
depolarizing resonances.

There has been a good effort to search for a single working point which
is optimum for both injection and luminosity operation. The committee
would like to encourage the RHIC team to continue this effort as it
believes there is more to be gained by this optimization. For example,
it might be beneficial to attempt compensation of the strong persistent
current driven 3^rd^ integer resonances at RHIC injection in order to
avoid depolarization during the change of the betatron tune from
injection to luminosity values.

If betatron phase changes between the interaction points are employed to
minimize the beam-beam effect, its effect on polarization must be
studied in parallel.

The committee is pleased to take note of the good performance of the
OPPIS polarized ion source. For the time being a polarization 80% has
been achieved with beam intensities of (3-5)∙10^11^ H^-^ ions per pulse.
The limitations are well understood. An improvement program based on a
new solenoid magnet to be installed in 2005 is underway. This is
expected to increase the polarization of the source to 85% at a beam
intensity of 5∙10^11^ H^-^ ions per pulse with a beam current stability
of ∆I / I \<10^-3^ and a small momentum spread of ∆P/P \< 1%. The
committee is looking forward to hear the results from the source
upgrade.

The committee takes notice that the pC and jet polarimeters both
function well. The committee would like to congratulate the polarimeter
team for the success with these devices which have been the subject of
concern and uncertainty in the past.

The committee agrees that faster electronics is needed to measure
polarization during the ramp. The committee endorses the efforts to
resolve this issue.

The committee urges the polarimeter team to calibrate the polarimeters
at 100 GeV/c. The uncertainty of this calibration has cast more
uncertainties on the RHIC polarization effort more than necessarily, and
should be resolved as soon as possible.

The committee is excited about the future option to perform spin
"turn-by-turn" measurement of spin motion which allows detecting spin
precession with an AC dipole magnet. This would be a fascinating
diagnostics tool which allows further in-depth research of spin
dynamics. The committee encourages the full pursuit of this exciting
direction.

pp Luminosity

An ambitious upgrade of the proton-proton luminosity by a factor of 16
above design value is planned. This large factor is based on an increase
of the bunch intensity from presently 0.7∙10^11^ to 2∙10^11^ protons per
bunch and on an increase of the number of bunches from 56 to 112.

One limitation of the bunch intensity is given by the intensity
dependent depolarization effects in the AGS which have been discussed
above and are expected to be overcome by the planned measures in the
AGS.

The remaining important constraints appear to be

-   Beam-beam effect limiting the bunch intensity and

-   Intensity dependent pressure rise in the cold beam pipe of the arcs.

Beam-beam experiments in RHIC with bunch intensities of 1.7∙10^11^ using
the 2004 betatron tunes between the 5^th^ and 4^th^ integer resonance
have revealed a considerable emittance growth in the order of 50% within
2 hours of operation. This would presumably lead to a drastic reduction
of luminosity lifetime, achievable integrated luminosity and possibly
deteriorate the background conditions.

The committee has been presented with several proposals to overcome
these difficulties:

-   Optimization of the working point

-   Optimizing the betatron phase difference between the interaction
    points (IP)

-   Minimizing the observed tune modulation

A thorough study of the working point by weak-strong beam-beam
simulations including the lattice nonlinearities showed that a
considerable increase of the dynamic aperture in collisions can be
obtained by using the SPS working point between the 2^nd^ and 3^rd^
integer resonance. This tune is also quite favourable with respect to
polarization, while other successful tunes, such as the HERA working
point would not qualify for high polarization operation. The unfortunate
disadvantage of the SPS working point is its closeness to the third
integer resonance which is quite strong at injection due to the presence
of persistent current sextupoles. The use of these tunes thus either
requires to shift working points somewhere between injection and storage
or to compensate for the strong 3^rd^ integer resonances at injection.

The committee suggests pursuing both options in order to maximize the
possibility to be able to operate at the most favourable working point
in collisions.

The attempts to minimize the beam-beam effects by optimizing the
betatron phases between the interaction points (IP) were apparently not
very successful so far. The committee does not expect much benefit from
this effort.

The low beta quadruple magnet triplet has been observed to perform low
frequency oscillations in the order of 10Hz. These will translate into
orbit oscillations and will lead due to the non-linear beam-beam
focusing to a low frequency tune modulation. Such modulation in presence
of strong nonlinear forces is known to be potentially detrimental to the
quality of beams.

The committee endorses plans to employ a closed orbit feedback to
minimize the tune modulation due to orbit oscillation.

The committee acknowledges that the beam-beam effect is addressed quite
thoroughly. However, the committee is nevertheless concerned, that the
planned measures are not sufficient or that the planned measures will
not address the beam-beam problems completely.

The committee therefore suggests to keep up to study the RHIC beam-beam
effects by both simulation and accelerator study in a systematic fashion
to make a serious attempt to clarify the remaining difficulties. A
number of obvious studies could be performed: The effect of modulation
should be clarified by weak strong beam-beam simulations and by
accelerator experiments. The RHIC team should investigate the emittance
growth versus beam intensity over a wider range and investigate the role
of working points experimentally.

The committee believes that there is a good chance that the vacuum
issues in conjunction with large proton beam intensity will be improved
considerably by the improved pumping prior to cool down as discussed in
the previous section. It will be however important to perform the
improvement as soon as possible to verify this assumption.

The committee is somewhat more sceptical whether the beam-beam problems
can be resolved completely by the planned measures. The committee
further suggests considering fall back positions in case the beam
intensity can not be brought to the desired level. A possibility might
be the reduction of beta function at the interaction point. This will
most likely increase luminosity somewhat and might even reduce the
residual orbit induced tune modulation.

Collider -- Accelerator Department Machine Advisory Committee

**Nov 8-9, 2004 Meeting**

**[Charge]{.underline}**

The main goal for RHIC operation over the next four years is to reach
enhanced luminosity and polarization performance as laid out in recent
planning documents (see i.e.
http://www.rhichome.bnl.gov/RHIC/Runs/RhicProjections.pdf). The main
charge to the Committee consists of reviewing the development and
commissioning plans. In particular please review, comment on, and offer
recommendations as appropriate in the following areas:

-   [RHIC luminosity development for heavy ion operation:]{.underline}
    Are the limitations to the RHIC luminosity understood? Are the plans
    to overcome the present luminosity limitations well developed and
    properly focused? Are the commissioning plans realistic and
    developed in sufficient detail?

-   [RHIC luminosity development for polarized proton
    operation:]{.underline} Are the limitations to the

-   RHIC luminosity understood? Are the plans to overcome the present
    luminosity limitations well developed and properly focused? Are the
    commissioning plans realistic and developed in sufficient detail?

-   [Polarization development in the AGS:]{.underline} Are the sources
    of beam depolarization in the AGS understood? Are the plans to reach
    full polarization (75%) at the nominal bunch intensity (2 x 10^11^
    protons) at AGS extraction well developed and properly focused? Are
    the commissioning plans realistic and developed in sufficient
    detail?

-   [Polarized beam acceleration in RHIC to 250 GeV:]{.underline} Is
    there sufficient understanding of polarized proton acceleration in
    RHIC to 250 GeV to plan a successful commissioning effort? Are the
    plans to reach full polarization (70%) at 250 GeV in RHIC well
    developed and properly focused?

In additions, any comments or suggestions the committee might wish to
make on other topics of the C-AD accelerator complex are very welcome.

It is requested that a concise report responsive to this charge be
forwarded to the C-AD Chair Derek Lowenstein by December 15, 2004.

+-----------------------------------------------------------------------+
| **C-AD Machine Advisory Committee Meeting**                           |
|                                                                       |
| November 8 - 9, 2004 Large Conference Room, Bldg. 911B                |
+-----------------------------------------------------------------------+

+-----+-----------------------+-----------------------+--------------+
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| res |                       |                       |              |
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| ati |                       |                       |              |
| ons |                       |                       |              |
| **  |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| #   |                       |                       |              |
| **[ |                       |                       |              |
| Mon |                       |                       |              |
| day |                       |                       |              |
| , N |                       |                       |              |
| ove |                       |                       |              |
| mbe |                       |                       |              |
| r 8 |                       |                       |              |
| th] |                       |                       |              |
| {.u |                       |                       |              |
| nde |                       |                       |              |
| rli |                       |                       |              |
| ne} |                       |                       |              |
| **  |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 08  |                       | Executive Session     |              |
| :30 |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 09  | Welcome               |                       | D.           |
| :00 |                       |                       | Lowenstein   |
+-----+-----------------------+-----------------------+--------------+
| 09  | Response to last MAC  |                       | T. Roser     |
| :15 | Recommendation        |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 09  | Setting the Scope:    |                       | W. Fischer   |
| :45 | RHIC 4-year goals (HI |                       |              |
|     | & Polarized Protons)  |                       |              |
+-----+-----------------------+-----------------------+--------------+
| **  |                       | ***Coffee Break -     |              |
| *10 |                       | Small Conference      |              |
| :00 |                       | Room***               |              |
| *** |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| **  |                       |                       |              |
| *He |                       |                       |              |
| avy |                       |                       |              |
| I   |                       |                       |              |
| ons |                       |                       |              |
| *** |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 10  | Update Stochastic     |                       | M. Brennan / |
| :30 | Cooling               |                       |              |
|     |                       |                       | M.           |
|     |                       |                       | Blaskiewicz  |
+-----+-----------------------+-----------------------+--------------+
| 11  | Vacuum Limitations    |                       | W. Fischer   |
| :00 |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 11  | Vacuum Upgrade Work   |                       | H. C. Hseuh  |
| :20 |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 11  | ***reserve***         |                       |              |
| :40 |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| **  | ***Lunch - Small      |                       |              |
| *12 | Conference Room***    |                       |              |
| :30 |                       |                       |              |
| *** |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+
| 13  | Tour - AGS Cold Snake |                       |              |
| :00 |                       |                       |              |
+-----+-----------------------+-----------------------+--------------+

  -------------- ------------ -------------------------------------------- -----------------
  ***Polarized                                                             
  Protons***                                                               

  14:30                       Polarized Proton Operations in AGS and RHIC  M. Bai

  15:00                       Polarized Source Upgrade                     A. Zelenski

  15:30                       AGS Cold Snake Commissioning                 W. MacKay

  ***16:00***    ***Coffee                                                 
                 Break -                                                   
                 Small                                                     
                 Conference                                                
                 Room***                                                   

  16:30                       RHIC Polarization at 250 GeV Beam Energy     V. Ptitsyn

  17:00                       Polarimetry                                  H. Huang

  17:30                       pp Luminosity Development                    R. Tomas
  -------------- ------------ -------------------------------------------- -----------------

  --------------------- --------------------------------------------------------------
  ***[Tuesday, November 
  9th]{.underline}***   

  08:30                 Executive Session / Discussion

  10:30                 Coffee Break - Small Conference Room

  11:00                 Executive Session / Discussion

  13:00                 Executive Session / Discussion

  15:00                 Closeout

  16:00                 Adjourn
  --------------------- --------------------------------------------------------------