Date: Thursday, October 19, 2000

NuMI Low Energy Target Design Review Summary

**Participants:**

### Name: affiliation email phone

### 

### Peter Barnes LLNL <pdbarnes@llnl.gov> (925) 422 3384

### John Haines ORNL <hainesjr@ornl.gov> (865) 574 0966

### Norbert Holtkamp (chair) FNAL <Holtkamp@fnal.gov> (630) 840 6429

### Ralph Pasquinelli FNAL <pasquin@fnal.gov> (630) 840 4724

### Nikolai Mokhov FNAL <mokhov@fnal.gov> (630) 840 4409

### Lincoln Read (organizer) FNAL <alr@fnal.gov> (630) 840 4274

# General Introduction

The Review was held on Tuesday, October 17^th^ at Fermilab. The
presentations were given by James Hylen from Fermilab, Victor
Zarucheisky from Protvino, Deborah Harris from Fermilab and Kris
Anderson from Fermilab. The committee wants to thank the speakers up
front for the interesting, complete and very enjoyable presentations
given through out the review. We all certainly got a very complete
picture of what has been done so far and where this part of the NuMI
program is heading. We also want to thank Lincoln Read for organizing it
and getting us all together for this very interesting topic.

# The Charge to the committee

Perform a technical review of the conceptual and engineering designs of
the low energy target (LET) system for the NuMI Project. The principal
object of the review is to bless the existing designs; or, failing that,
to suggest alternative approach(es) that should be considered.

> Detailed issues to be considered and commented upon by the committee
> include the following:

a)  The LET system design to be reviewed is that described in NuMI Notes
    501, 543, 675.

> b\) Are any further prototypes or other design revisions, additions,
> modifications, or calculations, needed before proceeding to the Title
> II (Blueprint) and Construction phases of the LET system?
>
> c\) Two decisions need to be made very soon by the NuMI project team:
> in each case, the committee is asked to provide its comments,
> findings, and recommendations concerning these two decisions. The two
> issues to be decided are:
>
> 1 - Should targets be coated with silicon nitride or should they be
> anodized. (There needs to be an insulating layer such that budal (sic)
> monitoring (charge monitoring) works satisfactorily? (the target being
> helium-filled.)
>
> 2 - Should the target be run in vacuum or in helium (prototype tests
> were run in both of these two environments)?
>
> d\) The stress calculations that have been made: are they reasonable?
> And, is there any concern about vibrations, or any other worries about
> stress-related effects?
>
> e\) Look at the cooling scheme and instrumentation techniques for any
> obvious defects in same.

In order to fulfill this task as completely as possible and over the
short time scale the NuMI management wanted us to deliver the report,
the target design review was split up into 5 topics with each member of
the committee focusing on one. These five topics are addressed in the
following sections of the report.

# 1. Target efficiency with respect to neutrino flux in the detector 

# 

## Findings

Simulations concerning the target efficiency with respect to material
choice were presented and explained well. It was pointed out that the
flux in the far detector could be optimized and increased by a few
percent if higher Z materials would have been chosen, on the other hand
strong technical reasons exist not to consider them for long lifetime
operation. Because the focal length of the horn is different for low
energy pions, the target has to move directly into the horn. While the
length of the target is approximately 1 m, 60 cm are within the front
end of the horn. This makes the design of the target more complicated
due to limitations in transverse size, it also raises questions about
proper insulation of the target with respect to the horn. Further on,
the alignment of the target with respect to the horn, the method for
mounting the target, and the optimization of the flux once the target is
in place, are all of them issues that need to be considered. It was
pointed out during the discussion (J. Hylen), that in order to
compensate 3 % loss in flux with a larger detector size (to achieve the
same "Luminosity") corresponds to an upgrade of 1 M\$. Two methods for
monitoring and optimizing the neutrino beam flux were presented. One,
the so called Budal monitor, is based on charge readout of the actual
target segments. The results presented were very convincing and obvious.
The device is definitely useful. The second method, presented by Debbie
Harris, using the near muon detectors, was not convincing. The typical
change in rate is of the order of 1-3 % and has to be detected with that
kind of precision to allow any conclusion to be reached on misalignment,
target density change or even target failure.

## Comments

The committee believes that the choice of graphite as the target
material is well justified and that the incremental increase or gain
with different materials is not worth any further evaluation at this
point in time. From the point of view of neutrino flux generation, the
fabrication of this target should certainly go ahead. In order to
optimize the pion flux coming out of the target, use of the Budal
monitor should be sufficient and should allow one to align the target
with respect to the beam, if the monitor works as well as in the example
presented at the review. For monitoring the behavior of the target,
changes in target density and flux changes as a result of that, the near
muon detectors seem not likely to be useful. The required resolution (2
% or so) can be easily degraded by variations in current from the main
injector, thermal changes, electronic noise and so on. The committee
feels that the signal to noise ratio of this measurement will not allow
conclusive results, but it is certainly worth doing.

## Recommendations

The committee recommends that simulations be continued, to investigate
the parameter dependence of the pion/muon/neutrino flux. Realistic
assumptions on main injector current fluctuations etc should be folded
into the evaluation.

# 

# 2. Reliability and verification of computational tools 

# 

## Findings

The low energy target design looks thorough. The choice of a low-Z
material, with graphite as a preferable one, and the choice of target
geometry seem to be close to optimal for the beam parameters and for
maximal neutrino fluxes towards the detector. The tools to perform the
design studies - MARS for energy deposition and HAST/ANSYS for thermal
and stress analyses - are adequate to the problem. At the same time,
several comments and statements in the presentations have been
unconvincing (see Comments section, following).

## Comments

From the geometry choice it looks like the pion reabsorption in the fin
geometry, steel pipes and molybdenum solder should be higher than the
quoted 2%. It would be useful to calculate the numbers in detail or to
show a plot representing this effect. For the neutrino rates an
uncertainty of 20% was quoted coming from the uncertainties in the pion
production models (quite non-uniform in the neutrino energy region
considered). For the studies that were presented, changes of 1-2% in the
neutrino rates were of interest and given the large 20% spectrum
uncertainty, consideration of the various effects looks incomplete. The
statistics should also be better and the effects of proton beam
variation (size, intensity) should be taken into account too.

The margin being given on the energy deposition as extracted from these
calculations, a factor of 1.6 at this design stage, seems unacceptably
small. This is clearly the case, if one takes into account the quoted
30% variation in energy deposition predictions with several versions of
the MARS code being used at IHEP, the simplifications used in the ANSYS
model, and the variation in the proton beam characteristics that are to
be expected. Other programs (Geant/FLUKA) could be used for
benchmarking. The prototype target radiation test that has been
performed at AP0, which we certainly consider to be valuable under any
circumstances, is incomplete regarding its conclusions. So far it does
not allow one to determine what the actual safety factor is.

## Recommendations

1.  Perform further literature search on radiation damage data for
    graphite, especially for proton beam interactions.

2.  Show explicitly the effect of neutrino rate reduction due to pion
    re-absorption in the geometry accepted.

3.  Perform a sensitivity analysis for energy deposition: code
    predictions (go with the official Fermilab MARS version, add if
    possible FLUKA and GEANT predictions), target performance with
    deterioration of graphite properties (computer experiment with
    variation of graphite density, modulus of elasticity and other
    parameters).

4.  On this basis of such sensitivity analysis studies, we suggest that
    one may well choose to use to a more relaxed set of beam parameters
    (most significantly, larger beam size if necessary).

5.  Derive conclusions from the prototype target radiation test both on
    radiation damage (fatigue) and high intensity scan, and tie them to
    the target design.

# 3. Technical optimization with respect to material choice, geometry, alignment, diagnostics etc

## Findings

The design and construction method of the target was presented very well
and it was clear from the presentation that a lot of work and
preparation has been done in Protvino to successfully complete the task.
Detailed considerations about the material choices, the electric
insulation and the mechanical set-up were presented.

## Comments

While the review committee was asked to focus on the design of the
target, it became clear very soon that issues such as the implementation
of the program, the operating procedures for the target, the exchange of
targets, and the handling and installation of targets, are equally
critical as is the issue of the design of the target itself. After all
is said and done, these issues cannot be treated separately. Very
careful consideration must be given to the location and mounting of the
target near the first horn. Preferences for mounting directly to the
horn or in a separate module were presented. While \"alignment ease\"
supports mounting the target directly on the horn module, a target
failure would require replacement of a \$300K horn assembly. A separate
module for the target allows the most flexibility, but at the expense of
extended installation and alignment periods and greater initial costs.
The committee does not have enough information to recommend either
technique, but realizes that the choice is crucial to achieving optimum
performance. Nonetheless, both techniques are adaptable to the existing
target design. It is with this compatibility in mind that we would
recommend procurement of the first target after some additional
mechanical analysis is performed (see Sections 2 and 5 of this review
report). This choice (of mounting methodology) should be made very soon,
although the construction of the target is independent of it. [It is
obvious, though, that additional engineering manpower is required, in
order that the work to determine which alternative is the most feasible
one, can proceed.]{.underline}

The diagnostics for the target performance is an area where not much
(not enough) information was presented, while the methods presented do
not seem sufficient. This is especially the case for target failures. A
clear method of indication for target failure needs to be developed and
implemented.

The topic of interlocks was not discussed at the review. The target will
need to absorb beam powers that are an order of magnitude higher that
that experienced in the Antiproton source at Fermilab. Interlocks on the
water cooling system, vacuum, and radiation should be carefully thought
out. This is a topic worthy of a future design review.

## Recommendations

1.  Extend the stress simulations that were shown. It seemed that the
    ANSYS simulation was done with only one type of grid. The influence
    of zoning, and varying grids for the simulation, should be
    investigated to see whether the results for the stress calculation
    changes. Pre-stress of the material should be determined and
    included in the simulation.

2.  Consider possible DC discharge path for the target. The highly
    ionized environment near the target could easily charge the
    structure to high voltages. The discharge path and the
    characteristics of the low conductivity water-cooling were not
    clearly described.

3.  Analyze horn to target case short, damage threshold,and specify
    impedance to ground.

# 4. Cooling (H20 versus Gas) etc\...

## Findings

We were presented with written material summarizing the results of
thermal models for the target. The models compute the energy depositions
and temperature distributions resulting from a range of design choices,
including gas or water cooling of the target fin and aluminum or steel
case. The modeling was based on a single method for computing the energy
deposition (MARS), and neglected a number of (probably minor) effects
(discussed below). For the purpose of making the major design decisions
affecting the cooling system, we find the model adequate.

In the case of gas cooling, the average temperature in the carbon is
estimated to be 250°C, compared to 37°C for the water cooled system. The
water flow required is 3.4 liter/min, or 3 m/s

flow velocity, and results in a pressure drop at the target can of 15
psig. Based on the much lower temperatures obtainable with water cooling
and the existing LCW water system, the NuMI target will be water cooled.
We concur with this choice.

Because of the lower atomic number, the aluminum case absorbs 209 W,
while the steel case absorbs 640 W. Assuming only convection cooling to
20°C air in the chase, the aluminum case stabilizes at a temperature of
95°C, while the steel case reaches 300°C. For the thickness analyzed the
case material has a negligible impact on the neutrino beam intensity.
Based on the reduced power load and lower temperature of the aluminum
case, the NuMI target case will be made from aluminum. We concur with
this choice.

We also saw pictures of a completed target prototype. The prototype is
full scale except that it contains 20 graphite segments, not the 47
called for in the real target design. It consists of the

target itself, with cooling lines, and the target case with ceramic
transition. It does not include the target box or mounting flange. The
prototype includes the full cooling path with all material

transitions, and all ceramic insulators.

## Comments

We commend the Protvino group for their attention to detail in designing
and constructing the target prototype, particularly demonstrating each
material transition in the cooling system. This process is likely to
uncover any hidden manufacturing issues, and should be carried through
to completion. We heard no discussion of potential negative effects due
to the water flow velocity, such as vibration or erosion of the cooling
elements. Once the prototype is completed, a flow test would verify the
pressure requirement and uncover any dramatic vibration issues.

Discussion of conductivity requirements for the water did not result in
a specification. Based on the discussion, we presume that the
requirement will be set by electrical issues related to grounding; on
the other hand it is well known that LCW water systems are required
because of the aggressive radicals produced in the radioactive
environment. An issue was raised as to the level of conductivity
maintainable in the high level of ionizing radiation that will be
present in the chase. It would be useful to have these specs before
continuing the design.

We note that the target has, in effect, two cooling systems: water in
the pipes on the target fin itself, and air cooling of the target case.
For modeling the case temperature the interior volume was assumed to be
at vacuum, rendering the two systems largely independent. However, the
vacuum will not be perfect, and it has not been decided finally whether
to operate with vacuum or under helium atmosphere in the target. The
thermal model should take into account the achievable operating pressure
inside the case, considering the long pumping lines involved. A separate
model should consider the heat transport effects of helium gas inside
the case.

It was not clear from the presentations if the assumed chase air
temperature (20°C) is what the target will actually be exposed to, since
the air flowing in the chase has already passed the length

of the target pile between the steel and concrete blocks. It was also
not clear whether the high air velocity in the chase (15 m/s) had been
taken into account. Additionally, the low melting temperature of the
solder used in the target potentially imposes a requirement on the
target pile cooling system in the event of failure of the air flow. The
residual heat in the pile should not result in heating of the air, and
hence the target, to 300°C. In addition the assembly could exhibit
vibration that should be investigated. Some baffling may be necessary to
keep the structure from \"whistling\".

The case was assumed to have zero emissivity on the inside, while the
target was assumed to have unit emissivity. The thermal models should
take into account the power input to the target due to radiation from
the inside wall of the case. Additionally, despite the high emissivity
of the graphite, most of the power will likely be reflected by the case;
this effect should be taken into account.

As noted above, the thermal model is certainly adequate to design the
cooling system. The refinements to the model, listed below, should be
incorporated to more accurately understand the resulting temperature and
stress distributions in the target, not because they will fundamentally
impact the cooling design. We mention these issues here because they
primarily arise from details of the cooling design and model. Other
issues affecting the temperatures and stress levels in the graphite
segments are discussed elsewhere in this review report.

## Recommendations

1.  Perform a water flow test to check the flow characteristics and
    detect any detrimental vibration.

2.  Assess potential negative effects of the water flow velocity.

3.  Investigate whether target failure could be detected by performing
    simple calorimetry on the cooling water supplied to the target. The
    data presented indicated that the vast majority of energy from the
    beam is deposited in the graphite. Such a simple measurement could
    indicate when the target has become fractured or partially disabled.

4.  Specify requirements on the conductivity of the water and an analyze
    the impact of ionizing radiation.

5.  Refine thermal models to include the effects listed in the next
    Section (5) of this review report.

6.  Estimate the likely achievable vacuum and pump down time for the
    target.

7.  Analyze the target and case temperatures, both for the likely vacuum
    and for helium gas fill.

8.  Match the assumed exterior air temperature to the target pile
    thermal model at the location of the target, not to the inlet air
    temperature. The ultimate temperature in the chase under failure of
    the air flow should be noted.

9.  Include in the case thermal model the air velocity.

10. Treat the target thermal model, the case and the target system in a
    unified fashion, taking into account the likely emissivities of the
    materials.

# 5. Material Compatibility, Radiation Damage, and Manufacturing

## Findings

Based on information presented at the review and evidence of work
performed by the project team to date, it is clear that significant
attention has been given to material compatibility and manufacturability
issues. The prototype that has already been fabricated has successfully
addressed the most challenging manufacturing issues associated with
joining of dissimilar materials.

Issues related to radiation damage, especially those associated with the
graphite target, need further consideration in the design and analysis
process. The thermal conductivity, thermal expansion coefficient,
modulus of elasticity, and strength properties of the graphite will
undergo significant changes over the lifetime of a target. Although it
will be difficult to precisely predict the changes in these strength
properties, the neutron irradiation database should be used as the basis
for setting the range of parameters to be analyzed in examining the
robustness of the design. This issue is especially worrisome given the
relatively low safety factor between the calculated peak stress and the
fatigue limit.

Although the use of a silicon nitride coating on the graphite showed a
slight improvement in the performance of the Budal monitor, the use of
this coating is not recommended. The risk of exfoliating this coating in
this severe environment is judged to be significant. Localized loss of
coating may make interpretation of the signal from the Budal monitor
difficult. Using an uncoated graphite surface will make the signal
processing slightly more difficult, but will remove exfoliation as a
concern.

## Comments

The issue of compatibility of solder materials with the radiation and
vacuum environment was likely addressed in the design and prototyping
processes but was not explicitly discussed at the review meeting. If
this issue has not already been considered, it should be at this time
and substitutions of appropriate materials made before fabrication of
the final target.

## Recommendation

The committee recommends that the project team perform a series of
thermal and structural analyses to examine the robustness of the design
to changes in graphite properties. The range of property changes that
could be expected can be estimated using the neutron irradiation
database developed over many years of fission and fusion energy
research. The dpa damage of the graphite in the NuMI environment should
be calculated with existing physics models, and this parameter can then
be used as the figure of merit in translating the neutron irradiation
data to the 120 GeV proton damage expected in the NuMI environment.

## Recommended Action Items

1.  Use physics models to calculate the expected dpa damage in the
    graphite target for a 120 GeV proton beam.

2.  Perform literature review/contact experts on graphite irradiation
    data to determine range of expected property variations given the
    expected dpa damage.

3.  Perform thermal and structural analyses to determine that the design
    remains within allowable limits for the expected range of variations
    in target material properties.

4.  Analyze loading on the case ceramic adapter and the case support to
    show that ceramic and case can sustain the load.

5.  Adjust design parameters if needed to satisfy item 3.

6.  Fabricate and test final target.

# Recommendations for Specific questions from the NuMI Management

A set of specific questions were given to the committee and are listed
on page 1 and 2. The questions are repeated here together with the
recommendation of the committee.

b\) *Are any further prototypes or other design revisions, additions,
modifications, or calculations needed before proceeding to the Title II
(Blueprint) and Construction phases of the LET system?*

Apart from the recommendations on extending the analysis of the target,
the committee feels that the construction should go ahead. Everything
else that should be done, can be done in parallel with target
fabrication.

c\) *Two decisions need to be made very soon by the NuMI project team:
in each case, the committee is asked to provide its comments, findings,
and recommendations concerning these two decisions. The two issues to be
decided are:*

1 - Should targets be coated with silicon nitride or should they be
anodized. (There needs to be an insulating layer such that Budal (sic)
monitoring (charge monitoring) works satisfactorily? (the target being
helium-filled.)

2 - Should the target be run in vacuum or in helium (prototype tests
were run in both of these two environments)?

The committee can not identify any good reason to coat the target, or
parts of the target. The resolution of the Budal monitor does not
increase (given the experimental data that were presented) and, from the
practical point of view, the coating could be damaged or partially
disappear which would make the interpretation of the measurements only
the more difficult. For the running under vacuum or helium the committee
has not have a strong opinion. Both approaches would seem to be fine.
The rational for the final decision should however be documented.

d\) *The stress calculations that have been made: are they reasonable?
And, is there any concern about vibrations, or any other worries about
stress-related effects?*

As being pointed out before, the stress calculations are certainly
reasonable, but should be extended as described in the text above. The
choice of a safety margin of only 1.6 seems not reasonable to the
committee.

> *e) Look at the cooling scheme and instrumentation techniques for any
> obvious defects in same.*

Instrumentation was certainly a weak point, throughout the review. Apart
from the Budal monitor no convincing plans for instrumentation were
presented to the committee. The suggestion to use calorimetry
(temperature rise in the LCW) for inspection and interlock is certainly
a good one. More work should be done in this area.

# General Comment

We would strongly encourage the NuMI management to set the \"threshold\"
for holding reviews a little higher. The design of the low energy
target, while an important aspect of the experiment, is only one aspect
of the impact that the target will have on operation of the experiment.
All of the review information provided to the committee before the
review, was based on the target work at Protvino.

The committee tended to focus during the review on the issues associated
with target installation, alignment, instrumentation, interlocks,
replacement, and radiation damage, to name the more salient points. The
target has a value of \$40-\$50K and should be considered as an
expendable part (as has been the case with other targets both here at
Fermilab and in other laboratories around the world). The other issues
mentioned above merit at least as much attention as the target design
itself and we suggest that these various issues should not be reviewed
separately from one another.

**OVERALL REVIEW COMMITTEE RECOMMENDATION**

**Following the completion of the additional design study efforts
outlined in the above five (5) sections of this report, the committee
recommends that final target fabrication proceed as soon as possible.**