### Target System for a Long Baseline Neutrino Beam

**N. Simos, H. Kirk, J. Gallardo, S. Kahn, BNL**

**N. Mokhov, FNAL**

**OVERVIEW**

The need for multi-MW class primary proton beams generating powerful,
intense secondary neutrino beams for use in future neutrino experiments
has prompted a host of feasibility studies of target concepts that will
be capable in supporting such initiative. This is due to the fact that
the survivability of a target that meets the physics demands at such
power level holds the key to the success of any such experiment. To
ensure that such target is even feasible at these power levels, a
dedicated effort is necessary to explore physical limitations of
materials considered in the target scheme. It is well understood that
solid materials experience limitations that primarily stem from the
following: 1) thermal management of the large target-heating generated
by the interacting/showering proton beam; 2) radiation damage that
alters material properties leading to possible degradation of target
performance; and 3) generation of intense pressure (shock) waves that
can lead to target destruction. While liquid targets are not bound by
these limitations, they face a host of technical challenges such as
implementation into a workable scheme (liquid jet) or effects on the
target infrastructure (contained liquid) where cavitation-induced loads
can limit the lifetime of the target scheme.

Since experience data at these power levels (approximately an order of
magnitude higher than that of operating accelerators) are not available
to-date, the viability of any target system concept (either based on
solid or liquid targets) supporting a multi-MW proton source (2-4 MW)
needs to be validated with a comprehensive R&D. Such effort must focus
on materials and/or target concepts holding promise of both
functionality and longevity under high-power conditions. Uncertainties
that can only be resolved through experimental efforts are, for solid
target options, primarily linked to the radiation damage resistance of
the target material expressed by its ability to withstand the induced
thermal shock in its irradiated state while maintaining the key physical
properties (thermal diffusivity, thermal expansion, etc.) that enable
the entire target scheme to function in place for practical time
durations of weeks to months without replacement. On the other hand,
proof-of-principle experiments of liquid target schemes that may be
applicable to a neutrino initiative are vital in ensuring that the all
elements of the concept work. Of paramount importance in these
experiments should be (a) the validation of delivery of such target
(when in jet form) into the extremely tight configurations required,
including delivery at higher frequencies that are dictated by the higher
power, and (b) the interaction and integration with the pion collector
(solenoid or magnetic horn) that provides the magnetic field.

Discussed in the following sections are: (a) target concepts that have
been implemented in the operating neutrino beam experiments and their
potential or limitations in operating at the multi-MW level, (b) the
technical challenges of proton drivers associated with the multi-MW
class and their direct impact on the production target, (c) the ongoing
R&D efforts to assess the viability of solid targets at the multi-MW
power levels and their integration with collector devices such as horns,
and (d) the ongoing R&D on liquid target schemes that might be of
interest to a long baseline neutrino experiment.

**1.0 Current state of the art in solid targets for a conventional
neutrino beam**

Conventional neutrino beam targets (those in the kW range) have relied
on a variety of solid materials for pion generation. While aluminum and
beryllium targets have been used in the K2K and MiniBooNE experiments
respectively, the material that has been selected to support the CNGS,
the NuMI and the T2K target concepts is graphite. Shown in Figure
1.1-1.3 are graphite-based target schemes implemented in some of the
conventional neutrino beam experiments.

While graphite (in all different grades used in these targets) appears
to satisfy the requirements at the current levels of power demand, the
effects of the higher levels of irradiation on the key properties of the
target material that control its functionality (i.e., thermal
conductivity, fatigue, shock absorption, etc.) is either unknown or not
very favorable for graphite (at least the graphite grades that have been
in use). Shown in Figure 1.4 is the dramatic change in thermal
conductivity of graphite with irradiation (even at low levels of damage
expressed in terms of displacements per atom or dpa). The levels of dpa
shown are expected to be achieved very quickly in a multi-MW operation.
Since the data shown are from neutron exposure, it is apparently
important that dedicated R&D efforts focusing on proton irradiation and
damage assessment are needed. As it will be discussed in the following
section, such studies have been initiated at BNL attempting to assess
proton irradiation damage on graphite as well as other promising
material candidates such as carbon composites. Figure 1.5 depicts
another aspect of the BNL effort. Specifically the response of targets
to an intense proton pulse (in terms of strain) is shown indicating that
carbon composites are better able to absorb shock than graphite and
therefore be in position to operate at higher accelerator power.

![](media/image1.png){width="4.785416666666666in"
height="3.1902777777777778in"}

**Figure 1.1: NuMI target system (low energy)**

![](media/image2.png){width="5.710416666666666in"
height="2.9902777777777776in"}

**Figure 1.2: CNGS graphite target**

![](media/image3.png){width="6.413194444444445in"
height="3.2555555555555555in"}

**Figure 1.3: Graphite-based target scheme implemented in the T2K
experiment**

![](media/image4.png){width="3.504861111111111in"
height="4.254166666666666in"}

**Figure 1.4: Effects of irradiation on thermal conductivity of
graphite**

![](media/image5.png){width="4.675in" height="3.34375in"}

**Figure 1.5: Comparison (experimental) of shock response between
graphite and carbon-carbon composite. Data generated during the BNL E951
target validation experiment**

**2.0 The Challenges of a multi-MW Power Accelerator**

**[Proton Driver Issues]{.underline}**

The challenge of delivering 2+ MW of power on a target is governed by a
number of controlling parameters. One parameter set relates to the
production target and specifically the choice of material as well as its
integrated design that allows it to operate as a functional unit. The
second set is linked with the proton pulse structure delivered to the
target and the parameter choices have a direct impact on the
survivability of the target. Whether liquid or solid the target
feasibility issues stem from the inherent material limits that in turn
are a function of the deposited energy density. The energy density is a
result of the pulse intensity, the proton energy, and pulse spatial
structure as well as material density. Balancing of these interconnected
parameters along with the repetition rate of beam delivery on target can
lead to an optimal operational space within which the power delivered
and safely absorbed by the target can be optimized. While studies
exploring the optimal space for a functional multi-MW proton driver have
been initiated exploring the possibilities in a rather generic sense,
focused efforts based on the parameters of a neutrino beam are needed.

**[Energy Choice]{.underline}**

The choice of energy to deliver 2 MW will have effects on muon
production and collection as well as target survivability.

Figure 2.1 depicts muon collection from a recent BNL neutrino factory
study.

![](media/image6.wmf){width="4.245833333333334in"
height="3.3805555555555555in"}

Figure 2.1: Efficiency of muon collection versus proton driver energy

While energy density distribution in a given solid target will vary
within the target depending on the energy of the incoming protons, an
important parameter in transferring deposited heat from the target, the
maximum energy density, as several studies have shown, increases with
increasing energy. Figure 2.2 shows the dependence of energy density on
a copper target as a function of proton energy. Since the overall effect
of higher power (i.e. 2 MW) is a combination of energy and repetition
rate to achieve the desired power level, the optimal combination needs
further study.

![](media/image7.png){width="4.154861111111111in"
height="4.293055555555555in"}

Figure 2.2: Energy density on target versus proton driver energy

The way proton energy will affect solid and liquid targets two is
distinctively different. On one hand, solid targets are vulnerable to
thermo-mechanical shock induced by high energy densities that can lead
to failure even with a single pulse on target, fatigue due to the cyclic
nature of the problem that will lead to premature failure of the target.
Most importantly, solid targets are susceptible to irradiation damage
manifesting itself in altering the key properties of the material, both
physical and mechanical that is responsible for shock absorbance and
heat diffusion towards the heat sink. The onset of irradiation damage is
always expected to compromise the longevity and functionality of a solid
target. In addition to the issues brought upon by the proton beam, solid
targets, even under the best of circumstances, must enable the removal
of the significant heat load through a feasible and "smart" design. This
is particularly challenging because of the constraints brought onto the
target by physics requirements that limit the size of the target to
avoid re-absorption of secondary particles and thus limiting the
available target surface area for heat transfer to the heat sink.

While experimental efforts are currently being pursued to evaluate the
limits and feasibility of both liquid and solid targets, it is has been
assessed, based on our understanding of the governing processes and
extrapolated simulations, that solid targets are appear to be capable of
reaching powers of 2 MW at best and only under the condition of low Z,
high performance materials such as carbon composites. Liquid targets, on
the other hand, appear to have a much greater chance in supporting a \>
2 MW proton driver provided the technical challenges of its
implementation into a workable scheme are solved.

**3.0 Target R&D Studies**

Dedicated experimental studies have been under way at BNL to address the
issues (a) of irradiation damage on targets, (a) the effects of pulse
intensities and the shock absorption capabilities of different material
choices, and (c) of the feasibility of liquid targets in jet form and
their interaction with proton pulses and strong magnetic fields.

The BNL solid target R&D program has been organized and pursued in
response to the problems recognized and faced by any solid target
material choice. As a result a material matrix of wide range of atomic-Z
numbers has been adopted for study. One should also keep in mind that
different types of secondary beams have different optimal production
characteristics.

In the experimental effort thus far, solid targets have been
investigated with a focus on the survivability of the targets.
Materials, which are resistant to fracturing and break-up due to the
generation of intense pressure waves, are being sought. This
experimental effort entails the irradiation of candidate materials in
order to evaluate their properties after prolonged exposure to
radiation. However, the pace and scope of the current BNL target R&D
needs to be intensified and expanded in order for it to provide answers
for a neutrino experiment at the higher power levels sought in a timely
manner and have the most effect.

**BNL Solid Target R&D Overview**

***Beam-on-Targets (E951)***

The beam on target experiment conducted under the BNL E951 aimed at
evaluating whether carbon composites are better suited to absorb shock
as compared with graphite counterparts. Figure 1.5 shows that indeed
carbon-carbon composites, because of the low thermal expansion
coefficient they exhibit, can absorb shock and therefore can intercept
higher intensity pulses which in turn indicate higher power proton
beams. A second aim of the E951 solid target study was to validate
special simulation processes based on which assessments of target-proton
interaction will be made. This step is considered vital given that
understanding of how solid targets survive the high intensity pulses can
only be done through extrapolation of the available data at much lower
power levels through benchmarked simulation models.

![](media/image8.png){width="4.271527777777778in"
height="2.7604166666666665in"}

![](media/image9.png){width="3.057638888888889in"
height="2.2631944444444443in"}
![](media/image10.png){width="3.1819444444444445in"
height="2.361111111111111in"}

***Target Irradiation Damage***

The aim of the study is to experimentally confirm that new alloys and
composites, attractive for their mechanical and physical properties in
the un-irradiated state and under consideration as baseline target
options, retain these properties after being exposed to high power, high
intensity proton beam. This task has two sub-tasks:

1.  Irradiation of target materials - Achieve radiation damage levels
    that are representative of the anticipated operational levels for
    both the super neutrino beam and the neutrino factory proton
    targets. The study is exploring radiation damage (via intense proton
    exposures in BNL Isotope facility) in the following materials:

    -   3D and 2D carbon-carbon composites

    -   Different grades of graphites

    -   Super alloys (super Invar and gum metal)

    -   Titanium alloys

    -   Beryllium

    -   Albemet

    -   Vascomax

    -   Copper and Glidcop

    -   Tungsten

    -   Tantalum

    -   Nickel-plated aluminum (NuMI horn material)

    -   Graphite-Titanium bonded targets

2.  Post-irradiation materials study - The work focuses on the
    resilience on the changes caused by radiation damage in the thermal
    diffusivity, thermal expansion, resistivity and density reduction.
    All of the mentioned functions/properties hold the key to a feasible
    target and horn system. Since recent experimental evidence suggests
    that some materials may undergo a special annealing that corrects
    irradiation damage, explore how common this behavior is to the
    materials considered.

The study has thus far led to some very interesting observations on the
behavior of these material candidates, especially in the way thermal
annealing plays a role in irradiation damage control. Also observed are
the effects of irradiation, combined with the cooling environment such
as water, on some materials and especially the baseline horn material.
The effects of high proton dose that approaches the dose that might be
expected in a 2 MW scheme on carbon composite (2D type) is also
intriguing and needs further assessment.

![](media/image11.png){width="5.05625in" height="3.6631944444444446in"}

![](media/image12.png){width="5.305555555555555in" height="4.175in"}

![](media/image13.png){width="4.152777777777778in"
height="2.7041666666666666in"}

![](media/image14.png){width="3.9451388888888888in" height="2.75625in"}

![](media/image15.jpeg){width="3.7743055555555554in"
height="2.8361111111111112in"}

![](media/image16.png){width="4.49375in" height="1.6125in"}

![](media/image17.png){width="2.6430555555555557in"
height="2.3027777777777776in"}![](media/image18.png){width="3.061111111111111in"
height="2.2104166666666667in"}

![](media/image19.png){width="2.8020833333333335in"
height="2.3631944444444444in"}
![](media/image20.png){width="3.2256944444444446in" height="2.59375in"}

***[High-Power Horn-Focused Neutrino Beams:]{.underline}***

The feasibility, as well as the functionality, of an integrated
target/horn system supporting the physics requirements of a
multi-megawatt Super Neutrino Beam pose significant challenges that
require a substantial new R&D effort. While the experience of running
similar neutrino beam experiments at power levels an order of magnitude
lower than those proposed can be of value in conceptualizing a target
and horn system with the potential of operating at much higher power
levels, this experience cannot be simply extrapolated to the design of a
feasible and functional system at the megawatt power levels. This
circumstance stems primarily from material limitations related to both
target and horn, especially their ability to retain the key physical and
mechanical properties for which they were chosen in the first place. The
severe radiation damage conditions induced by the high-power proton
beam, along with the cyclic thermal shock encountered with each current
pulse, provide severe challenges to the operational goal of pulsing for
the longest possible time before replacement of the target or horn
components is needed. Attractive candidate materials from the physics
performance perspective are certainly available but most of them lack
crucial information on radiation-induced degradation of their key
materials properties. Some of these candidates (i.e., graphite or
carbon-carbon) have been exposed to high levels irradiation, but the
radiation exposure data comes from reactor experience and is based on
neutron rather than proton irradiation. This experience also derives
from steady-state conditions in the absence of shock and rapid thermal
cycling. Variations in the material response to different types of
radiation are not yet measured experimentally.

[Horn/Target Integrated System Performance Issues:]{.underline}

Performance limitations and/or failures of the integrated target/horn
system of either past or running neutrino beam experiments (i.e., K2K
horn, MiniBoone horn, NuMi target, etc.), operating at much lower power
than what will be required for the super neutrino beam application,
provide clear indications that technical issues remain and that that
these will increase in severity as the required beam power rises.

The inability to reliably extrapolate the performance of low-beam-power
experiments and operating experience into the design of the proposed
multi-megawatt neutrino beam requires that the following R&D issues must
be addressed to resolve the integrated target/horn system technical
design and materials suitability questions:

1\) Target Issues/Optimization:

-   identification, design and proto-typing of an optimized target/horn
    configuration for parent pion capture in the desired wide-band
    neutrino beam, including robust and radiation-hard materials choices
    able to provide optimized pion collection for a multi-megawatt
    target/horn system;

-   analysis of the optimal target configuration and materials for their
    physics application compatibility (including atomic numbers and
    densities) and the relationship of these to the inventory of
    radiation-hard materials or composites identified in the first task;

-   analysis of the behavior of the target configuration and the
    expected cyclic beam-induced mechanical/thermal shock at the
    anticipated radiation-damage levels, and prediction of the longevity
    in service of the derived optimal target/horn system.

2\) Horn Issues/Optimization

> While the horn structure does not intercept the proton beam directly,
> it is still exposed to high levels of radiation from secondary
> particles and thus its ability to maintain the intended focusing
> function as radiation damage progresses is of primary importance. The
> ability of the horn material to maintain its resistivity as low as
> possible in the presence of progressive radiation damage is key to the
> selection and optimization of its design. This criterion is coupled
> with the requirement that serious degradation of the structural
> integrity of the horn (as it experiences radiation damage, thermal
> cycling due to electric current and proton beam heating, mechanical
> vibration and potentially corrosive cooling fluids) is strongly
> mitigated or avoided altogether.
>
> In recent neutrino experiments, the failures of focusing horns that
> occurred earlier than anticipated, relative to their expected lives,
> point toward horn materials inability to resist radiation damage and
> to chemical corrosion degradation. Recent BNL radiation damage studies
> of horn materials showed that the material (even with a protective
> coating for fatigue resistance) experiences integrity degradation with
> modest levels of irradiation in a corrosive environment. At
> anticipated radiation damage levels that are an order of magnitude
> higher than current experience in the super neutrino beam era, the
> technical issue of getting the longest operational lifetime from the
> horn structure is of paramount concern.
>
> ![](media/image21.jpeg){width="2.06875in"
> height="1.5840277777777778in"}
> ![](media/image22.jpeg){width="2.088888888888889in"
> height="1.5729166666666667in"}
>
> ***Figure 3.1:** Effects of modest levels of proton irradiation
> (\~0.25 dpa) on nickel-plated aluminum of the NuMI Horn. Experiment
> was performed*
>
> *at the BNL BLIP facility with 200 MeV protons.*
>
> In order to achieve the horn performance required by the super
> neutrino beam and to extend its operational life as much as possible,
> alternative materials options need to be considered in place of
> aluminum, the traditional horn body material. Significant technical
> progress in materials development has benefited other technical
> applications and now needs to be applied to horn and target
> optimization. Alloys such as AlBeMet, that exhibit favorable
> properties for horn use still lack radiation damage characterization.
> Such non-traditional alloys could provide a way to meet the technical
> challenges of a super neutrino beam horn with its associated
> multi-magawatt operating levels. The recent BNL materials irradiation
> study (while still at modest irradiation levels) produced results that
> favor the use of AlBeMet. However, further studies at higher
> irradiation levels are needed to establish the viability of this
> materials option.

3\) Integral Target/Horn Challenges/Optimization  Planned feasibility
experiments

> While the two primary elements of particle generation and focusing
> (i.e., target and horn) may be optimized to achieve their intended
> functions separately, they still need to be compatible in an
> integrated configuration. This is especially challenging because these
> two components need to be coupled in some respects of the operation
> (i.e., sharing cooling path) and be decoupled in others (i.e., remain
> electrically insulated). For both aspects (coupling or de-coupling)
> the beam power plays a pivotal role. The energy deposited in the
> target, as well as in the horn conductor, is directly proportional to
> the beam power as is the demand for a heat transfer path out of the
> integrated system. While the configuration geometry is similar to that
> of the low power neutrino experiments, the demand for heat removal
> capacity is about an order of magnitude higher under the same boundary
> conditions. Added to that is the expected degradation due to radiation
> damage of the material properties that control the flow of heat such
> as thermal conductivity or diffusivity; this is also expected to occur
> at much higher rates due to the higher beam power.
>
> To achieve the performance required of the integrated system, the
> configuration as well as the materials that are involved, must ensure
> that both the heat transfer and the electrical insulation between the
> target and horn are maintained in satisfactory condition even under
> long-term irradiation. Experience from low power neutrino experiments
> is not directly transferable to the design of the proposed high power
> system and therefore dedicated R&D effort is necessary to address the
> following issues

-   ability of helium gas flowing in the small annular space between the
    target and the horn inner conductor to remove heat deposited on the
    target. Helium gas is being considered to replace water cooling for
    several reasons: (a) the geometrical configuration to reach the
    optimal neutrino spectrum calls for an embedded target in the horn
    inner conductor that, in turn, has a very small diameter (\~ 19mm).
    This geometry precludes the use of concentric annular spaces that
    allow coolant to flow past the surface of the target and never be in
    contact with the current flowing in the horn inner conductor. Helium
    gas represents the solution to the interface problem as long as it
    is capable of removing the target deposited heat at the rate
    required; (b) experience with water and its corrosive nature in
    recent experiments has not been positive. Protecting the horn
    surface from corrosion as well as the target surface from erosion
    will ensure a longer operational life. To assess the feasibility of
    helium cooling under such tight configuration, a dedicated heat
    transfer experiment needs to be performed while simulating the exact
    configuration of the proposed target/horn integration and materials
    as well as with the actual thermal loads anticipated.

-   An assessment of the radiation hardness of special insulating
    materials that perform the role of horn/target coupling to endure
    the radiation damage expected over the maximum operating life of the
    integrated system.

-   An assessment of the life expectancy of the integrated system
    subjected to the combined loading conditions. By utilizing
    radiation-induced material property changes, obtained by parallel
    efforts, simulations based on state-of-the-art software and modeling
    of the entire system are necessary to identify the weak points that
    may limit the life of the system. Since failure in one component, no
    matter how peripheral, would mean failure of the system as a whole,
    assessing the integrity under these high beam powers is vital. The
    NuMi experiment experience, in which a peripheral system failure
    caused significant beam downtime, is a reminder of the seriousness
    of the issue.

![](media/image23.png){width="6.238194444444445in"
height="4.0881944444444445in"}

***Figure 3.2:** Conceptual design of target/horn system for the BNL
neutrino superbeam*

> ![](media/image24.png){width="2.8361111111111112in"
> height="2.5729166666666665in"}
> ![](media/image25.png){width="2.870833333333333in"
> height="2.3465277777777778in"}

![](media/image26.png){width="2.397222222222222in"
height="2.2222222222222223in"}![](media/image27.png){width="2.6180555555555554in"
height="2.0444444444444443in"}

> ***Figure 3.3:** BNL neutrino target/horn integration baseline model*

[Target/Horn Integration R&D:]{.underline}

Collaborations with CERN and T2K to validate the concept of He cooling
under severe conditions.

To address the above issues, the following R&D tasks are proposed to be
performed over a 2-year period:

> *Conceptualize, simulate, design and test the critical section of the
> integrated target/horn system (i.e. target, horn inner conductor,
> insulator, and forced helium integration) for helium cooling
> verification and feasibility of the target/horn conductor coupling:*
> To maximize the life of the two key components (target and horn) the
> option of using helium gas flowing in the annulus between the target
> and the inner conductor to remove the target thermal load will be
> explored. The thermal loads induced by the high power beam are about
> an order of magnitude higher than what target/horn systems have
> experienced in experiments to date. In addition, the low heat capacity
> of the inert gas proposed places further challenges in meeting the
> requirements. However, a proof-of-principle experiment showing that
> helium can be used in the proposed target/horn configuration, will
> eliminate far more serious issues associated with water cooling. The
> task will consist of three (3) subtasks, namely:

-   [*Helium cooling feasibility experiment*:]{.underline} In a
    configuration simulating the horn inner conductor interfacing with
    the target (made of carbon-carbon composite or graphite) the ability
    of forced helium in removing heat deposited on the target
    inductively (similar to beam thermal load) will be tested and the
    maximum heat removal capacity of this scheme will be assessed.

-   *[Integrated target heating, helium flow and horn current pulsing
    experiment:]{.underline}* Key to the long term survival of the
    system is the ability of the various components to withstand fatigue
    and induced vibrations from all three sources. Experimental testing
    of the proposed target/horn configuration will reveal potential weak
    spots thus allowing for the system modifications.

-   *[Testing of alternative material options:]{.underline}* Explore
    other than the baseline materials for use as target, horn and
    insulators. Experimental verification of the erosion resistance of
    materials such as different grades of carbon composites and graphite
    to forced flow is of interest. In addition, assessment of electrical
    resistivity and corrosion resistance of materials other than
    aluminum as will provide potential alternatives for a horn with
    maximized life. Further, an experimental study, including
    irradiation, of different insulating materials that allow coupling
    between the horn and the target will provide much needed information
    on this key component of the system.

> ***[Simulation studies of the super neutrino beam integrated
> target/horn system]{.underline}***

-   Simulate particle interaction with various target systems using
    transport codes and couple the hadronic analyses with
    thermo-mechanical simulations of the entire system.

-   Use both aspects of simulation (hadronic and thermomechanical) as
    well as irradiation data from a concurrent R&D to optimize the
    overall performance and design.

-   Focus on the simulation of the unique characteristics of the CC
    composite such as weaving of reinforcing fibers, anisotropic bulk
    structure and customization of fiber configuration to maximize
    important properties such as thermal diffusivity, erosion resistance
    and thermal expansion.

-   Identify the MW power capacity and life expectancy of the proposed
    target/horn scheme based on these comprehensive simulations. This is
    a very important task given that limitations in the target/horn
    system directly translate to limitations of the actual experiment.
    Understanding the limits of any proposed scheme (for this neutrino
    or any other experiment) in terms of useful life and power capacity
    is vital. Given that no proton driver to-date are capable of
    producing the powers proposed, the only path to assessing the system
    is based on simulations that analyze the exact system that has been
    benchmarked to powers that are achievable as of today.

**4. Funding Support Necessary for Target System R&D**

**5. Comments on very advanced work on liquid targets**

The problems presented by solid targets being exposed to intense proton
beams has also led to a consideration of the merits of utilizing liquid
targets. The Muon Collaboration, a national consortium that has been
pursuing R&D for muon colliders and muon storage rings for a number of
years, has developed a scenario for the production of intense low energy
muon beams based on immersing a high-Z liquid material (either Hg or
perhaps a Pb-Bi eutectic) within a high-magnetic-field solenoid for the
purpose of producing and capturing the soft pions generated when the
proton beam interacts with the target. This effort includes a leadership
role in the approved CERN experiment (nTOF11) to demonstrate the
technical feasibility of a mercury jet target under conditions suitable
for a Neutrino Factory.

Liquid target either in the form of jets or contained volumes, do not
suffer from thermal shock, fatigue or irradiation damage. While these
serious limitations are avoided altogether, liquid targets face
challenges of a different kind. Specifically, interaction of the proton
beams with a high Z liquid jet target will lead to an explosive style
destruction that, while of no consequence to the secondary particle
production, could have serious consequences in the target
infrastructure. The ability to replenish a liquid jet to meet the
repetition requirement of the high power proton driver and the
difficulties of adopting a feasible jet scheme to tight geometrical
constraints pose additional challenges. In the case of a contained
liquid, the generation of high cavitational pressures can induce damage
on the target infrastructure.

**Liquid Targets R&D:**

This effort attempts to experimentally confirm the concept of using
high-Z liquid target materials in an intense proton beam. Prior R&D work
in this area has been proceeding for several years under the Muon
Collider/Storage Ring R&D program and could be important to the neutrino
physics program of a future muon storage ring facility. Emphasis is
placed on the ability to replenish the target after successive beam
pulses while confirming that the dispersal of the liquid jet is
manageable. An approved target experiment, nTOF11 at CERN, designed to
answer key questions concerning the performance of liquid-metal targets
in a high-field magnetic field environment while being exposed to an
intense proton beam pulse is the vehicle to address these important
questions. The technical issues to be resolved include: trajectory and
integrity of the liquid-metal jet in the magnetic field; explore the
feasibility of 50 Hz operations; and the impact of μs proton bunch
spacing on secondary particle production.

![](media/image28.png){width="4.145138888888889in"
height="3.0305555555555554in"}

![](media/image29.png){width="4.402777777777778in"
height="3.3222222222222224in"}

![](media/image30.png){width="6.435416666666667in"
height="3.107638888888889in"}

**Summary**

The envisioned R&D target studies have wide appeal given that the target
feasibility issues are common to all initiatives associated with
multi-MW power accelerator drivers. All neutrino beams and neutrino
factory initiatives fall into that category.

For the solid targets, the limitations arise from the fact that
materials available to-date, including alloys and composites, may not be
able to endure the kinds of demand these beam initiatives require.
Advancements in material science hold part of the answer in solving
material limitations, i.e., target resilience to beam induced shock.
Comprehensive and systematic material irradiation studies are necessary
in assessing whether new materials, alloys and composites can withstand
the levels of radiation damage expected with long exposure to a multi-MW
beam. This R&D is focused on covering a broad range of material Z and
therefore increasing its relevance to a wide field of current and future
initiatives for neutrino physics and beyond.

For the liquid target options where the target destruction is not the
primary issue, issues associated with (a) the current lack of
understanding of the behavior of conductive liquids in strong magnetic
fields and (b) the thermodynamic processes that take place during
beam-target interaction need both experimental and simulation-based
verification. Through this R&D, both aspects are being proposed.
Benchmarking of simulations that include magneto-hydro-dynamic and
thermodynamic relations on the experimental efforts will provide the
basis to assess how feasible a multi-MW liquid target is and what are
its limitations.

The underlying statement regarding the value of high power target R&D is
that all initiatives involving multi-MW proton beams have to address
these target issues. Therefore, results of this R&D will serve any
future neutrino beam and neutrino factory initiatives.

Further, with the ever-increasing demand for high power neutrino beams
the pool of workable and realistic target/horn schemes reduces
dramatically. This is due to the limitations of the materials, alloys
and composites, available to-date, to endure the kinds of demands these
beam initiatives require. This issue is common to all of present or
future neutrino initiatives. Advancements in material science hold the
key to solving individual component limitations, i.e., target resilience
to beam shock or horn resilience to corrosion and electrical
degradation. Solving the individual component issue does not, however,
solve the issue as a whole because it is the integration of the system
based on compatibility between components
(target/horn/insulators/coolant) and functionality that is important.
This R&D will address both the individual component integrity and the
feasibility/functionality of the integrated system. By utilizing
on-going and future planned studies of irradiation effects on materials
as well as state-of-the-art simulations, areas where the proposing team
has extensive expertise, limitations of the proposed neutrino beam
initiatives stemming from the engineering side of the target/horn system
as well as possible path toward these high powers will be identified.
Results of this R&D will serve any future neutrino beam initiative.