**Comments on a Review of Beam Optics Design**

**December 11, 2001**

## Characterization of Main Injector Beam Parameters

## Presenter: A. Marchionni

1.  ([Reviewer: A. Drozhdin]{.underline}) The main parameters of the
    beam are under investigation. Transverse emittance growth with
    intensity of the bunch was shown. This function seems is natural and
    depends on many parameters of accelerator. Unfortunately this
    parameter improvement takes a years of accelerator work.

Longitudinal emittance behavior during bunch rotation is under
investigation. The momentum spread growth with increasing of bunch
intensity was found but not explained. Unusual oscillation of phase of
different bunches, shown in the presentation, can be a methodological
error of measurements. But most probably it is related to RF frequency
shift by a small amount at the moment of phase rotation. Some of the
bunches get exactly into the synchronous phase, but others have a small
displacement which cause phase oscillations. Any case a reasonable
explanation of this effect should be found. It seems that this can be
understood and fixed shortly.

2.  ([Reviewer: A. Drozhdin]{.underline}) If 10^-4^ of the extracted
    beam loss is a limit for NUMI, this requires to think about the beam
    halo collimation in the MI. These losses correspond to number of
    particles in the Gaussian distributed beam at the level of 3.5-4
    sigma. Even if the real transverse emittance is about 25 pi mm.mrad
    (95%), the 4 sigma corresponds to 70 pi mm.mrad. And the beam size
    is a factor of 1.3 larger compared to a reference value of 40 pi
    mm.mrad. This may cause losses at extraction. The best solution to
    control losses in the MI (not only at the beam extraction, but at
    many other cases) is to design and implement a beam halo collimation
    system. This system should not be too complicated and expensive.

3.  ([Reviewer: E. Harms]{.underline}) The results of the MI extraction
    studies to date suggest putting some priority on the multi-batch
    extraction studies. In terms of beam line design, I\'d suggest using
    the current values and scale by a factor of 4 for initial conditions
    until hard numbers can be obtained.

NLG: This has been clarified somewhat, see:

[Main Injector parameters expected during NuMI
operation](http://www-numi.fnal.gov/numwork/reviews/apr_02/bock302letter.pdf) -
S. Mishra

4.  ([Reviewer: C. Moore]{.underline}) A point to keep in mind about the
    loss on the extraction septa vis a vis an assumed emittance is that
    the scraping will take place on one side. Hence if a certain
    emittance beam scrapes on the Lambertsons it does not necessarily
    imply that the beam will fit through an equivalent aperture
    downstream and we also can not assume that the halo will all be
    scraped away at the Lambertsons.

5.  ([Reviewer: C. Moore]{.underline}) In terms of the detailed
    longitudinal loss pattern at the MI-52 Lambertsons it is probably
    difficult to determine the energy of the loss without a detailed
    Monte Carlo simulation. If I scale using Table 8.1 of Nikolai\'s
    book , \"Passage of High Energy Particles through Matter\", I find a
    difference in the location of the cascade maximum of a factor of 2.7
    between 8 GeV and 120 GeV. Unfortunately the absolute distances are
    small (a few feet at 120 GeV) making measurements to tell the
    energies apart difficult. I have asked the Rad Techs to make a
    detailed survey the next time they survey the area and we will see
    if we can make any sense out of the measurements.

## Beam Optics Design

## Presenter: P. Lucas

1.  ([Reviewer: A. Drozhdin]{.underline}) A momentum dispersion
    generated by the vertical bends was the main problem. The spot size
    increase at the maximum dispersion due to momentum spread of
    4×10^-4^ was comparable to the beam size from betatron emittance.
    The improved optics of the NUMI beam line permits to decrease
    maximum beam size by a factor of 2. This reduces beam size and
    divergence at the target and possible losses at aperture
    restrictions in the beam line and/or reduces requirements to the
    stability of magnet power supplies.

2.  ([Reviewer: A. Drozhdin]{.underline}) A sensitivity analyses of
    magnets installation and field stability to the beam parameters at
    the target and at the main detector in Soudan (Minnesota), as well
    as to the beam loss along the beam line are performed. An additional
    investigations maybe are needed with respect to ground motion to
    predict periodicity of the equipment realignment.

3.  ([Reviewer: A. Drozhdin]{.underline}) Two additional quads in the
    carrier tunnel improve the beam parameters along the beam line
    especially in a vertical plane, mainly reducing vertical beam size
    by about 30% in a few points. But these points are not critical from
    the point of view of aperture restriction. At the same time
    additional quads introduce a lot of complications for the beam line
    construction, operation and cost. An additional magnets, power
    supplies and cables, beam diagnostics, including radiation monitors
    and maybe BPMs, are required. Maybe it is good to have a remote
    control of transverse position of these quads to limit an access to
    the carrier tunnel for realignment. Because of these complications I
    suggest to take a look at the possibility to give up of these quads
    installation in the carrier tunnel.

4.  ([Reviewer: N. Grossman]{.underline}) Peter states that the power
    supply regulation for the large bends is specified at 100 ppm. This
    is not strictly true. Simply refer to the Power Supply Specification
    review presentation and subsequent power supply review presentations
    and you can see what level of PS regulation is budgeted for based on
    the criteria in the document "Stability Limits on Bend Fields in the
    NuMI Primary Proton Beamline, Peter Lucas, June 7, 2001" and then
    updated to December 5, 2001 and presented to me. Some strings (3)
    are \~50 ppm and some (4) are around 200 ppm, worst case. It is very
    likely that the ones at 200 ppm will be close to 100 ppm, but not
    guaranteed. If we end up using the "JITTER" system, then all strings
    will be better than 100 ppm.

5.  ([Reviewer: N. Grossman]{.underline}) Similarly, the quad power
    supply regulation is mentioned at being better than 0.1% - an order
    of magnitude better than 1%, where there is concern. This is not
    technically correct. The quad PS regulate at +/- 0.05% of the
    maximum current of the PS. Some run at the 200 amp tap, others at
    100 amps. If one translates this into +/-% of the operating current,
    the worst case (based on the previous beamline optics design) was
    +/- 0.44%. This is not that far from 1%. Obviously the higher the
    operating current, the better this number, but we are trying to
    minimize quad currents and thus this may be a problem. These numbers
    have been quoted previously at the PS Specification Review and the
    Magnet PS System review. Along these same lines, the trims PS are
    regulated at +/-0.1% of the maximum current (15 amps). These numbers
    have been discussed with Peter and Sam and were said to be
    sufficient, but perhaps we need to revisit this.

6.  ([Reviewer: N. Grossman]{.underline}) As for the quads in the
    carrier tunnel, I think this is a complicated decision. From a
    purely operational viewpoint, we need to fully understand what it
    would gain us. I think the only way we can tell that is with MARS
    runs of the beam with and without the quads to see how much is
    gained with respect to losses in the carrier tunnel. From a cost
    standpoint, it seems like the best way to go would to put them in
    some sort of box or some kind of a partially shielded structure to
    keep water off them. Then, install them with minimal infrastructure
    and run with them. When they die, turn them off and leave them there
    and run without them. This, of course, assumes that the quads gain
    us something, making it worth doing at this level, but do not gain
    us enough such that a full-blown infrastructure is warranted and
    that they are necessary to run.

7.  ([Reviewer: E. Harms]{.underline}) With all due respect to the
    effort already expended and improved beam line design since last
    time, this line is of sufficient importance that I believe it
    deserves a fresh look from a second design team. Such a new look
    should be constrained only by \'hard\' limits such as extraction and
    target points, bend centers, and constrained physical apertures. I
    could see a new design dividing the line into three sections -
    matching from the MI into an achromatic section through the carrier
    pipe and then another match to the target. Such a design may require
    moving bend elements and adding focusing, but could provide a line
    with greater room for error.

Beam Physics was asked to take a fresh look at the NuMI Primary Beam
design shortly after this review. They came up with the same basic
design as the NuMI group -- putting devices in the downstream end of the
carrier tunnel. For a more complete discussion see:

<http://www-numi.fnal.gov/numwork/reviews/apr_02.html>. NLG

8.  ([Reviewer: E. Harms]{.underline}) Another plus to looking at a new
    design for the line could be to make beam movement less sensitive to
    quad movement, gradient errors, skew dipole roll errors, and beam
    mis-steering. One can almost certainly expect magnetic elements to
    shift after installation is complete, particularly in the upstream
    part of the beam line. If one can avoid downtime by relaxing
    alignment criteria through a good design, so much the better.

NLG: This has been done, see:

<http://www-numi.fnal.gov/numwork/reviews/nov_01.html>

and:

<http://www-numi.fnal.gov/numwork/reviews/apr_02.html>, especially the
document:

[A FODO-Based NuMI beamline
design](http://www-numi.fnal.gov/numwork/reviews/apr_02/FODO_design.pdf) -
J. Johnstone

9.  ([Reviewer: E. Harms]{.underline}) Avoid putting magnetic elements
    in the carrier pipe except as a last resort. Environmental
    conditions sound too harsh. To install magnets and expect them to
    eventually fail and then run without them is wasteful. If there is
    no recourse but to add quads, they should have BPM\'s included.

Quads and BPMs will be installed in the downstream section of the
carrier tunnel. Environmental conditions in this region have been
upgraded to match those for the Pre Target tunnel. SC

10. ([Reviewer: E. Harms]{.underline}) The presented design of the beam
    pipe/LCW lines support needs refinement. The beam pipe should have
    stiffer support than what was shown and be mechanically de-coupled
    from the LCW lines. Keeping the LCW below the beam pipe is certainly
    a good idea should a leak develop, but will it matter if the carrier
    pipe is cold and damp to begin with? If there are no powered
    elements, does the carrier pipe carry only the beam pipe?

DP: The recommendation to isolate the beam pipe from the LCW has been
done.

11. ([Reviewer: E. Harms]{.underline}) An unrelated but I think
    pertinent question: is the carrier pipe to be under vacuum? If so,
    to what level; are there scattering issues which could lead to
    non-trivial beam loss; how is pumping to be accomplished on such a
    large volume?

DP: By carrier pipe, I assume you refer to the beam pipe and not the
concrete tunnel. If this is correct, the beam pipe is to be under high
vacuum to a level that is better (lower pressure) than traditionally
used on external beam pipes, but worse (high pressure) than an
accelerator vacuum. Jim Klen is to produce an engineering note on the
vacuum level in each region of the carrier pipe which details the
locations of the 38 or so ion pumps distributed (at great expense) along
the beamline.

12. ([Reviewer: J. Johnstone]{.underline}) The optics design presented
    showed how, with the addition of 2 quads in the carrier tunnel, the
    beam size can be significantly reduced through a few restrictive
    apertures. That\'s good. However, should these magnets fail, it was
    indicated that operations would continue anyway with the remaining
    quads re-tuned. This plan is apparently motivated by the sundry
    difficulties presented by accessing the new quads. I find this
    reasoning puzzling. The desirability of reducing beam size would
    seem to be self-evident, but, if it has been determined that
    operation without the 2 quads is also acceptable, one has to
    question what would be the point in installing them in the first
    place. Supposing that these 2 quads are to be installed, however,
    then it was also pointed out that they would not be accompanied by
    any diagnostics - no BPM\'s, multiwires, or dipole correctors. I
    strongly believe this would be a serious mistake \-- it would be
    much better to never install the magnets at all. It is a given that
    these magnets will be mis-aligned by some unknown amount, and that
    their gradients will only approximate the design values to within a
    few percent at best. The largest operational problems would be:

    a.  shooting blind down \>200\' of tunnel from Q107 to these quads;

    b.  it will not be possible to steer through the magnetic centers of
        both magnets simultaneously;

    c.  the mis-aligned quads will kick the beam by some amount, and
        with only 2 correctors in each plane between here & the target,
        it is not possible to correct the kick & steer through the
        magnetic centers of the remaining quadrupoles, and;

    d.  without at least BPM\'s & dipole correctors there is no way to
        determine what the mis-alignments are, or what the true quad
        gradients are.

13. ([Reviewer: J. Johnstone]{.underline}) A good amount of work still
    needs to be done to adequately characterize the sensitivity of the
    line to errors. Two analyses that come to mind are:

    a.  It was mentioned that arising from a variety of sources,
        including variation from power supply to power supply, read-back
        error from the supplies to the control room, and transfer
        coefficient mismeasurements, the actual quad gradients will only
        approximate the desired values to within a few percent. It is
        planned to fine tune the quads by observing beam profiles. I\'m
        skeptical that this approach will work \-- it is a very poorly
        constrained, multi-variable problem. If this remains the
        preferred technique, however, it should be demonstrated to be
        feasible with realistic tracking simulations of thousands of
        particles and randomly distributed quad field errors. (More
        reliable operational techniques exist for fixing the quad
        gradients in any case).

    b.  Realistic simulations of orbit correction need to be performed.
        The beamline description should include random distributions of
        the full complement of anticipated error sources from quad &
        dipole fields & magnet misalignments. Only then can it be
        determined definitely that correctors & BPM\'s exist in the
        correct numbers & optimum locations.

14. ([Reviewer: G. Krafczyk]{.underline}) If Quads are to be installed
    in the drift tunnel a serious look at the costs should be done first
    since this will impact the final decision that Numi will make as to
    weather the cost of adding quads is worth the added expense.

15. ([Reviewer: G. Krafczyk]{.underline}) A serious look at tuning the
    beam line with the quads installed should be undertaken. The idea
    that you can steer through both quads in a 200' drift space seems
    problematic but then I don't have to tune the beam line.

16. ([Reviewer: G. Krafczyk]{.underline}) I would think that at the
    least you would like to have Horizontal and vertical BPM\'s and both
    horizontal and vertical correction elements with loss monitors at
    the quads. Also increasing the cost.

17. ([Reviewer: P. Martin]{.underline}) The new optics designs have
    accomplished the goal of reducing the dispersion and beta functions
    in the critical regions. While both solutions presented - one with
    two quadrupoles in the drill-and-blast area, and one without -
    achieve this goal, the question remains as to which of these to
    choose. My own opinion is that the one with the additional quads is
    better, because it reduces the beam size at the end of the NuMI
    stub, where losses cannot be tolerated. However, the NuMI project
    must weigh the various benefits and costs to arrive at a decision.
    If the solution with the quadrupoles is selected, I would recommend
    enclosing the quadrupoles in boxes to provide protection from the
    environment. I would also recommend adding Beam Position Monitors at
    these quadrupoles.

18. ([Reviewer: M. Syphers]{.underline}) Can design be modularized at
    all, to more easily facilitate operation? That is, can the major
    bends be turned into achromatic sections \-- either individually or
    as a unit \-- enabling beta-matching at the ends without being
    sensitive to beam momentum?

19. ([Reviewer: M. Syphers]{.underline}) Need to perform Monte Carlo
    study(ies) of parameter sensitivities \-- quad/bend motion,
    settings; beam quality variations; diagnostics and beam tuning
    algorithms, etc.

20. ([Reviewer: M. Syphers]{.underline}) The beam line design needs to
    take into account the layout of diagnostic equipment. The placement
    of devices such as BPM\'s, SEM\'s or Multiwires, loss monitors,
    etc., needs to be carefully and meaningfully coordinated with the
    optical design. It was not clear where these devices would go or how
    they would be used to monitor: a) Main Injector performance; b) beam
    line performance; c) targeting; \...

## Power Supply System Impacts

## Presenter: N. Grossman

1.  ([Reviewer: E. Harms]{.underline}) Power supply regulation
    requirements should not be loosened unless there is a significant
    financial impact. More often requirements are later made more
    stringent. It wasn\'t clear whether the power supplies were to run
    DC or ramped (I think ramped? - see Nancy\'s presentation pp. 2 and
    4). Operational experience has shown that this makes a big
    difference in beam reproducibility and stability - both in terms of
    power supply regulation and magnet hysteresis. Has this been closely
    looked at?

We presently target beam for P-Bar production with a regulation system
that is an order of magnitude lower than being used for NuMI. The cost
of changing to DC on the line will impact the operation cost in both
power bill and cooling system. The DC system can have better pulses to
pulse regulation because the bandwidth limit changes from 20 Hz to 720Hz
but is still limited by the ability to measure both the current and
reference. SLH