7/29/02

#### Baffle and target/baffle carrier design

Jim Hylen, Valeri Garkusha, Victor Zarucheisky, Ernie Villegas

TABLE OF CONTENTS

[Carrier and Baffle Definition 3](#carrier-and-baffle-definition)

[Target/Baffle Carrier Functions and Requirements
3](#targetbaffle-carrier-functions-and-requirements)

[Baffle Functions and Requirements
4](#baffle-functions-and-requirements)

[Target Motion Capability 5](#target-motion-capability)

[Low Energy Target Insertion 5](#low-energy-target-insertion)

[Other Neutrino Energy Tunes 5](#other-neutrino-energy-tunes)

[Extended target motion by relocating target module
6](#extended-target-motion-by-relocating-target-module)

[Restriction of motion by carriage, hotcell
6](#restriction-of-motion-by-carriage-hotcell)

[Target/baffle carrier hardware design
7](#targetbaffle-carrier-hardware-design)

[Carrier mechanical design 7](#carrier-mechanical-design)

[Carrier thermal considerations 8](#carrier-thermal-considerations)

[Carrier Frame Stiffness 9](#carrier-frame-stiffness)

[Flexible utility lines 9](#flexible-utility-lines)

[Target Module Endwall Penetrations
9](#target-module-endwall-penetrations)

[Baffle hardware design 10](#baffle-hardware-design)

[Baffle hole size, baffle location -- beam accident geometry
10](#baffle-hole-size-baffle-location-beam-accident-geometry)

[Baffle material 11](#baffle-material)

[Baffle outer radius 12](#baffle-outer-radius)

[Baffle length 12](#baffle-length)

[Baffle atmosphere and windows 12](#baffle-atmosphere-and-windows)

[Baffle thermal warp 12](#baffle-thermal-warp)

[Baffle cooling and monitoring 13](#baffle-cooling-and-monitoring)

[Accident scenario 14](#accident-scenario)

[Continuous scraping 14](#continuous-scraping)

[Calibration 15](#calibration)

[Commissioning alignment scan 15](#commissioning-alignment-scan)

[Effect of scraping on the neutrino spectrum
15](#effect-of-scraping-on-the-neutrino-spectrum)

[Figures 17](#figures)

[References 31](#references)

TABLE OF FIGURES

[Table 1 Length of target/baffle/carrier unit in various configurations.
6](#__RefHeading___Toc15715017)

[Table 2 Approximate mass of carrier components, and loads.
7](#__RefHeading___Toc15715018)

[Table 3 Allocation of target module endwall utility penetrations.
10](#__RefHeading___Toc15715019)

[Table 4 Target carrier electrical pin allocation.
10](#__RefHeading___Toc15715020)

[Table 5 Maximum baffle hole radius allowed for different layouts of
baffle, horn, and target. 11](#__RefHeading___Toc15715021)

[Table 6 Thermal parameters for two different air heat transfer
coefficients. Temperatures in parenthesis ignore radiative cooling, are
prevented from occurring by a 120 deg C thermocouple limit input to beam
interlock, but are included for illustration.
14](#__RefHeading___Toc15715022)

[Figure 1 Modules in target hall shielding. Target/baffle carrier
mounted below target/baffle module, with target and baffle extended into
the low energy position. Also visible are carriage cross beams, steel
shielding, horn 1 mounted below horn 1 module.
17](#__RefHeading___Toc15715023)

[Figure 2 Target/baffle carrier with target and baffle mounted. Target
shown extended into the low energy neutrino beam position.
18](#__RefHeading___Toc15715024)

[Figure 3 Baffle. 150 cm long, 11 mm diameter hole, 57 mm diameter
graphite core, 61 mm diameter aluminum casing. Cooling fins not shown.
19](#__RefHeading___Toc15715025)

[Figure 4 (Rotated) target carrier side view. Target and baffle are
shown in both Low Energy Beam extended position, and in fully retracted
highest energy beam position. 20](#__RefHeading___Toc15715026)

[Figure 5 Target carrier end view. 21](#__RefHeading___Toc15715027)

[Figure 6 Summary from study of beam energy deposition in carrier using
MARS. 22](#__RefHeading___Toc15715028)

[Figure 7 Top view of Horn 1 module upstream endwall, which will be
copied for the target module. Utility penetrations include those labeled
clamp, outer, inner, and the two large holes to the right of those
labeled inner. 23](#__RefHeading___Toc15715029)

[Figure 8 Plan view of baffle, target, horn geometry. Since target is
rectangular rather than cylindrical, limits of its horizontal and
vertical extent are shown. Note large factor between horizontal and
vertical scales. 24](#__RefHeading___Toc15715030)

[Figure 9 Beam-eye view of baffle inner radius and comparison to beam
spot, target fin, horn neck, and target cooling and support structure.
25](#__RefHeading___Toc15715031)

[Figure 10 Temperature rise of protected components for an accident
spill as a function of the length of the baffle. The distance Lbt
between the downstream end of the baffle and the target vertical fin is
176 cm (circles), 86 cm (boxes), and 41 cm (triangles).
26](#__RefHeading___Toc15715032)

[Figure 11 Temperature rise of protected components for an accident
spill as a function of the position of a 1.5 m long baffle. Lbt is the
distance from the end of the baffle to the start of the target vertical
fin. The chosen design value of 68 cm is marked.
27](#__RefHeading___Toc15715033)

[Figure 12 Energy deposited in baffle (J/cm3/pulse) as a function of
position along baffle for 4e13 protons hitting baffle. Integral is 42
kJ/pulse, corresponding to 22 kW at 1.87 second repetition rate, or 5.5%
of the 400 kW total beam power. 28](#__RefHeading___Toc15715034)

[Figure 13 Section of pin radiator. 29](#__RefHeading___Toc15715035)

[Figure 14 Effect of various amounts of beam-baffle scraping on neutrino
spectrum. Top plots show spectra separately for 100% of beam on target
or baffle. (Mark Messier, 6/12/02) 30](#__RefHeading___Toc15715036)

# Carrier and Baffle Definition

"Module" refers to the heavy shielding module that supports beam
components below it, and has positioning motors mounted on top of it.
The target/baffle carrier is a frame mounted on shafts through the
target/baffle module. The module provides motion control of the target
transverse to the beam by moving the shafts vertically and horizontally.
The carrier supplies precision motion control of the target along the
beam. When maintenance is required that cannot be done in situ, the
entire module with carrier and components still mounted on it is moved
to the hot work cell. The module, carrier, target and baffle are shown
in Figure 1, Figure 2, and Figure 3.

*Please refer to the engineering drawing set for up-to-date definitive
dimensions and layout.* The integration drawing for the modules,
carrier, and horn 1 is **8874.000-ME-363028**. The final carrier and
baffle drawings have not been made yet.

# Target/Baffle Carrier Functions and Requirements

Target/Baffle Carrier must:

-   Remotely drive target into horn along beam axis after module is set
    in place: target casing extends 66.2 cm into horn, target module has
    7.1 cm clearance to horn, thus minimum of 73.3 cm forward travel.

-   Provide target motion capability to 250 cm retracted from the
    standard low energy running position, to enable changing the energy
    tune of the neutrino beam.

-   Set position of target and baffle to 0.5 mm transversely, and 1 mm
    longitudinally along beam line.

-   Maintain position of target and baffle to 0.5 mm transversely and 1
    mm longitudinally under beam heating from 4x10^13^ protons / 1.87
    sec striking target; this includes other beam energy tunes in
    addition to the standard low energy beam position.

-   Provide the motion capability of the target and baffle utilities:
    target water supply line, target water return line, target vacuum
    line, two Budal target monitor electrical lines, 8 thermocouple
    electrical lines.

-   Maintain 1 inch clearance to target hall chase walls, in addition to
    the allowance for +/- 8 mm horizontal and +8 mm / -9 inch vertical
    motion of support shafts.

-   Provide a 2 cm radius "stay-clear" through the support structure
    that allows beam scans of downstream components when the
    target/baffle carrier is remotely lowered off beam axis (by \~ 9
    inch).

-   Survive radiation dose of up to 10^11^ rad/year for up to 10 years
    (except for the drive shaft end which is above the module).

-   Be short enough that it can be extracted through the target module
    carriage (199 inches) and inserted in the hot work cell (216 inches)
    with the target frozen in the extended position, or frozen with the
    baffle sticking out the back end in the target fully retracted
    position.

In addition, carrier will provide:

-   Four tooling balls for survey, located at beamline elevation, one on
    each side of the beamline at each module endwall, visible through
    holes in the endwall.

-   A pair of limit switches at each end of the travel of the
    target/baffle.

-   Rotary encoder on drive shaft at the top of the module to give
    remote readback of location of target along beamline.

-   Include baffling to direct part of the chase air stream to the
    target that is otherwise blocked by the target base and support
    tube. Air flow must also be maintained over the length of the baffle
    and the target.

-   Rust prevention coating of drive shaft and any other (non-stainless)
    steel parts.

Note secondary containment of radioactive water leaks is built into the
target pile design, and is not a carrier design consideration.

# Baffle Functions and Requirements

The baffle must:

-   Prevent mis-steered primary proton beam from causing damage to the
    horn neck and target cooling/support components.

-   Be capable of operating continuously with 3% beam scraping at design
    luminosity.

-   Withstand full intensity of beam for a number of pulses it will take
    to detect mis-steering of beam and shut beam off.

-   Incorporate monitoring that will shut beam off in case of continued
    large loss of beam on baffle. It may be useful for efficient running
    that loss of one pulse on baffle not turn off the beam.

-   Construction and alignment tolerance of hole through baffle must be
    0.5 mm or better.

In addition:

-   Incorporate monitoring that allows detection and monitoring of
    beam-baffle scraping at order of 1% accuracy. (E.g. if 2% of the
    beam is being scraped, measure (2+/-1)%). Since actual full power
    beam running is expected be around 2x10^13^ during the first couple
    years running, this means an accuracy of +/-2x10^11^ or better.

-   Incorporate monitoring that allows a cross-check of alignment via
    e.g. low intensity beam-scan to 0.5 mm.

-   Have the baffle move along the beamline with the target, as this
    increases the available target travel (the overall length for
    extraction of a broken target/baffle module is limited, and by
    having the target stick out one end of the carrier at one extreme of
    motion and the baffle stick out of the carrier at the other extreme
    one makes maximum usage of this length).

# Target Motion Capability

## Low Energy Target Insertion

For the Low Energy Beam, the target will extend 66 cm into the horn with
only 3 mm transverse clearance at the tip. The choice was made not to
mount the target on the horn itself for fear of vibration damage to the
target from horn pulsing. The target module will be set in place with
the target in a retracted position. After the target module is surveyed
into position, and the axis of the carrier aligned with the beamline
(and horn) axis using the module motor drives, the target will be
extended along the beam line into the horn using the carrier motor
drive.

## Other Neutrino Energy Tunes

In NuMI-NOTE-BEAM-0783 \[ref. [^1]\], the case was made that by
extending the range of motion of the low energy target upstream one
obtains the capability to quickly and easily change the energy of the
neutrino beam. Such alternate energy tunes are labeled "semi-medium" and
"semi-high" energy beams, and produce decent rates and spectra, although
not quite the performance that can be obtained with optimized targets
and horn 2 positions for the alternate tunes.

When moving the target, one is taking the chance that it could become
stuck in some location. The target is in a high-radiation environment
(e.g. humidity can turn to nitric acid), and repairs to the carrier
mechanisms after being irradiated is mostly limited to replacement.
Proper choice of materials can help mitigate this. However, this is a
risk explicitly taken, and reinforces the need to have a complete spare
target/baffle/carrier unit at beam-turn-on.

## Extended target motion by relocating target module

The target carrier allows for 2.5 m of target motion without moving the
target module. The target can be moved even further upstream by moving
the target module, although this is not a trivial exercise. The current
baseline design allows space for one to move the target module 2 m
upstream by unstacking some shielding, using the crane to move the 20
ton module, restacking shielding, and then doing a survey to check the
final position. This would result in a total target range of 4.5 meters,
but the last 0.5 m or so may be precluded by interference between the
baffle and target hall upstream wall and beam pipe. (The standard high
energy target position was 3.65 m upstream of the low energy position,
and is reachable in this scheme.) The target/baffle carriage cross beams
which limit the upstream movement of the module can be seen in Figure 1.
Three pieces of steel shielding required for this new position are not
in the baseline project, and would have to be acquired before the move.

## Restriction of motion by carriage, hotcell

  ------------------------ ------------------------- ---------------------
  **Carrier frame**        **Fixed frame**           **160 in.**

                                                     

  **Moving components**    **Total moving            **315.97 cm = 124.40
                           components**              in.**

                           Baffle length             150 cm

                           Baffle to vertical Target 68 cm
                           fin                       

                           Target fin length         95.38 cm

                           Target casing beyond fin  2.59 cm

                                                     

  **Length normal          **Components inside       **160 in.**
  extraction**             frame**                   

                                                     

  **Total length L.E.      **Frame with target       **188.86 in.**
  position**               extended**                

                           Target extension          28.86 in.

                           Frame length              160 in

                                                     

  **Total length fully     **2.5 m upstream of       **193.97 in**
  retracted**              L.E.**                    

                           Baffle extension          33.97 in.

                           Frame length              160 in
  ------------------------ ------------------------- ---------------------

[]{#__RefHeading___Toc15715017 .anchor}Table 1 Length of
target/baffle/carrier unit in various configurations.

The opening in the target carriage shielding is 199.389 inches. The hot
work cell is 216 inches long. These two openings set the maximum length
of the target/baffle carrier and range of motion. The module with
target/baffle possibly frozen in the fully extended or fully retracted
position must be able to navigate through these openings, with operators
controlling the crane by camera and remote control due to the high
residual radiation.

In the target extended downstream position, the length of carrier plus
target is 189 inches. With 250 cm of motion capability, with the baffle
extended out the upstream end, the baffle plus carrier is 194 inches
long. See also Figure 4.

# Target/baffle carrier hardware design

## Carrier mechanical design

The target and baffle frames have rollers which ride on aluminum rails,
as shown in Figure 5. The rails are at beam height, and are supported by
an aluminum tube space frame. Two hangers (sometimes miscalled clamps)
connect the space frame to the shafts (not shown in the figure), which
come through the module.

  ------------------ ---------------------------------- ---------- -------
  Components                                                       215 lb

                     Target Assy.                       150 lb     

                     Baffle Assy.                       65 lb      

  Coils                                                            47 lb

                     One steel loop                     0.581 lb   

                     Water in loop                      0.194 lb   

                     3x22 steel loops                   38.3 lb    

                     2x22 water loops                   8.5 lb     

  Carriage Assy.                                                   598 lb

                     Frame Assy.                        231 lb     

                     Target baffle left support         120 lb     

                     Target baffle right support        110 lb     

                     Shaft support                      44 lb      

                     Coil support                       23 lb      

                     Drive system                       70 lb      

  Drive shaft loads                                                82 lb

                     Components on 0.058 slope          12 lb      

                     Rolling friction (0.05 coef.)      11 lb      

                     Coil contraction, max.             30 lb      

                     Friction coil on supports (0.61    29 lb      
                     coef.)                                        
  ------------------ ---------------------------------- ---------- -------

[]{#__RefHeading___Toc15715018 .anchor}Table 2 Approximate mass of
carrier components, and loads.

The aluminum rails are coated with tungsten-disulfide to provide a tough
surface for the rollers to ride on.

The rollers are commercially available cam-followers, black oxide coated
steel in an aluminum housing. (These may be replaced with stainless
steel if we can locate a vendor). They will be run dry, i.e. without
grease. It is expected that the target location will only be changed a
few hundred times, so the number of cycles is relatively small.

The drive shaft is 1 inch diameter 17-4 PH heat treated stainless steel.

## Carrier thermal considerations

When running in the ME or HE locations, some of the target support
shafts and rails will be downstream of the target, and thus receive
substantial energy deposition from the secondary beam. MARS was used to
calculate the energy depositions \[ref. [^2], see Figure 6 for summary
of study\]. In order to maintain thermal position stability, several
measures are taken.

Water cooling is supplied to the downstream module-to-carrier hanger
(sometimes known as the clamp, in analogy to its use for horn 1), and
the lower part of the support shaft is made of low-expansion steel. The
water-cooled clamp design is copied from the horn 1 module design \[ref.
[^3]\]. The bearing between this clamp and the carrier is on
beam-centerline vertically. The clamp water cooling is not required for
low energy beam running. Thus its water loop is separate on the module
from the target water loop, so that failure in this system would not
require replacing the target for low energy running.

The carrier frame and rails are made of minimal volume to surface ratio
aluminum and are kept at larger radius to the beam centerline. For
initial design studies, a frame constructed of ¼ inch thick wall ¾ inch
square aluminum tubing was assumed. The beam heating of the tubing
plateaus about 1 m downstream of the start of the target (Figure 6). At
a radius of 25 cm from the beam, the energy deposition in the aluminum
is around 0.032 watts/gram. Assuming a heat transfer coefficient of 10
w/m^2^K to the 18 deg C air flowing in the chase, the asymptotic
temperature calculated was 55 deg C, which is acceptable for the frame.
The final design uses thinner 1/8 inch wall 2 inch square tubing, and
thus should experience less than half the temperature rise. The beam
heating upstream of the target is negligible.

The drive shaft is placed at a radius of 40 cm from the beam-centerline.
At that radius and downstream of the target, it is estimated that its
temperature will be between 100degC and 150degC. The part that sets the
target location is upstream of the target, and will remain cool. The
downstream end of the shaft can be constrained transversely, but
allowance must be made for a 2 mm longitudinal expansion of the
downstream part when running higher energy tunes.

## Carrier Frame Stiffness

Analysis of carrier support frame distortion has been carried out for
three positions of the target; fully extended, fully retracted, and
halfway in between. The vertical mis-alignment of points on the target
and baffle due to changes in the loading of the carrier frame was at
most about 0.2 mm.

During the assembly of the target on the carrier frame, the target will
be aligned such that the upstream and downstream ends of the target fin
are vertically on axis; because of the sag of the target tube due to
gravity, the middle of the target will be a fraction of a mm above the
beam axis.

The sag of the drive shaft is 5/16 inch when the target is in the fully
extended position.

## Flexible utility lines

The 2.5 m travel of the target is accommodated by compressing a coil of
22 loops of pipe, 3/8" O.D. 0.028" wall stainless, total length 38 m per
line. Three lines are used: water supply, water return, vacuum. In the
LE position, the coil is unloaded. Fully retracted by the 2.5 m, the
coil force is of order 30 lbs. The drive shaft is kept under tension,
eliminating worry about buckling. (Putting the drive shaft in
compression might have forced use of a larger diameter shaft,
exacerbating cooling problems due to the larger volume to surface
ratio). The maximum stress on the water line is calculated to be 20 ksi,
which is below the yield stress of 35 ksi.

Radiation hard thermocouple cable is produced commercially, consisting
of a stainless steel sheath encapsulating a pair of wires in magnesium
oxide insulation. We will use this cable for all electrical routing on
the carrier. The flex necessary for the target motion will be obtained
in similar fashion to that for the utility lines, i.e. a coil that
expands or contracts, resting on an immobile support shaft. The voltage
standoff required is reasonably modest. The target tube may see a
voltage of order 50 Volts from the horn, since it follows the horn
voltage closely due to beam ionization of the air between the horn and
target tube. The Budal monitor may be run with a bias voltage of up to
75 volts, although a smaller bias (down to zero) can be used if
necessary.

## Target Module Endwall Penetrations

The Horn 1 module upstream endwall has six penetrations for utilities,
as shown in Figure 7. Target module upstream and downstream endwalls
will copy this, with the assignments for usage shown in Table 3.

Each electrical feed through contains five two-pin plugs. Connections
are as show in Table 4, where the third baffle thermocouple could be
re-allocated if some other electrical connection is required.

  ------------------------ ----------------------------------------------
  Upstream endwall         

  1                        Upstream Left Survey Port

  2                        Upstream Right Survey Port

  3                        Drive Shaft for 2.5 m motion

  4                        Target water supply

  5                        Target water return

  6                        Target vacuum line

  Downstream endwall       

  1                        Downstream Left Survey Port

  2                        Downstream Right Survey Port

  3                        Clamp water supply

  4                        Clamp water return

  5                        Electrical feed through #1

  6                        Electrical feed through #2
  ------------------------ ----------------------------------------------

[]{#__RefHeading___Toc15715019 .anchor}Table 3 Allocation of target
module endwall utility penetrations.

  ------ -------------------------------- --------------------------------
  Plug   Pin 1                            Pin 2

  1      Target Budal horizontal fin      Shield for horizontal fin signal

  2      Target Budal vertical fin        Shield for vertical fin signal

  3      Carrier thermocouple 1           Carrier thermocouple 1

  4      Carrier thermocouple 2           Carrier thermocouple 2

  5      Baffle thermocouple 1            Baffle thermocouple 1

  6      Baffle thermocouple 2            Baffle thermocouple 2

  7      Baffle thermocouple 3 (or spare) Baffle thermocouple 3 (or spare)

  8      Wire from target vacuum jacket   Ground for limit switches

  9      U.S. limit switch 1              U.S. limit switch 2

  10     D.S. limit switch 1              D.S. limit switch 2
  ------ -------------------------------- --------------------------------

[]{#__RefHeading___Toc15715020 .anchor}Table 4 Target carrier electrical
pin allocation.

# Baffle hardware design

The baffle is mounted with the target to eliminate the expense of
another module to support and align it. The baffle and target can be
pre-aligned to each other before attachment to the module, and move as a
unit. The baffle construction tolerance is 0.2 mm RMS; the goal for
overall baffle position accuracy is 0.5 mm, including thermal effects,
survey tolerance, and carrier wobble.

## Baffle hole size, baffle location -- beam accident geometry

The baffle hole is made as large as possible while still providing
protection to the horn and target. Figure 8 and Figure 9 show the layout
of the baffle relative to the target and horn.

The horn protection system actually consists of two baffle locations, a
relatively large opening after the last bend string (but unfortunately
before the last quad), and a second baffle in the target hall. Peter
Lucas produced a spreadsheet which takes into account the magnetic
field, and gives the maximum radius that the beam could reach while
still passing through the two baffles. These limits are set entirely by
the two baffles and the intervening magnet, and do not try to identify
magnet failures that could actually produce these deviations. Examining
some variations run by Adam Para, the pattern is roughly that the fully
protected region is the baffle radius plus (0.5 mr horizontal, 0.7 mr
vertical) times the distance downstream of the baffle.

The following table lists three items to be protected by the baffle, and
derives maximum baffle radii using the above rule of thumb for two
cases: (1) having the baffle close to the target, and moving with the
target, or (2) at a fixed upstream location which would still allow
target movement to the HE location without moving the baffle. Baffle
radius is reduced by 0.5 mm to allow for alignment tolerance. Numbers
are in coordinate system where horn 1 starts at Z=0cm (i.e. the GNUMI
Monte Carlo coordinate system). The horn neck is at (Z,R) = (0.8m,9mm);
the top edge is shadowed by the target, so only the horizontal edge is
included in the calculation. The target cooling line solder starts at
(Z,R) = (-0.35m,7.5mm) at top and bottom of graphite fin; after some
distance down target, the graphite contributes to shielding the tube.
The cooling loop tube between supply and return is at the end of the
target (Z,R) = (0.6m,7.7mm), and is exposed in the horizontal direction.

  ---------------------------------------- -------------- ---------------
  ***Baffle hole R(max)***                 Baffle Z=-2m   Baffle Z=-5m

  Horn 1 neck, X direction                 7.1 mm         5.6 mm

  Target cooling tube, Y direction, LE     5.8 mm         3.7 mm
  position                                                

  Target cooling loop, X direction, LE     5.9 mm         4.4 mm
  position                                                
  ---------------------------------------- -------------- ---------------

[]{#__RefHeading___Toc15715021 .anchor}Table 5 Maximum baffle hole
radius allowed for different layouts of baffle, horn, and target.

From the above table, it is seen that a 5.5 mm radius baffle hole
protects the horn for all baffle locations, for all different energy
beams. If the target and baffle move together, then a 5.5 mm radius
baffle will also protect the target for all beam configurations; however
a fixed location baffle would require a smaller hole to protect the
target. It is also seen that the X and Y requirements are very similar,
and a circular hole thus makes sense. From the above arguments, the
baffle is mounted near the target, moves with the target, and the radius
of the hole is selected to be 5.5 mm.

In the current layout, the baffle ends at -103 cm in the Monte Carlo
coordinate system, or 68 cm before the leading edge of the target
vertical fin.

## Baffle material

Calculations have been done of energy deposition, heat rise, and stress
for the standard NuMI beam (4e13 protons in a 0.7 mm sigma-x 1.4 mm
sigma-y beam spot) hitting Nickel, Copper, Aluminum, and Graphite.
Nickel and Copper are clearly unsuitable. Aluminum is marginal; it would
exceed its yield point and creep modestly with accident pulses \[ref.
[^4]\]. Graphite is satisfactory \[ref. [^5]\]. Beryllium would also
work, but is rejected because it would generate mixed waste
unnecessarily. We have selected Graphite; it is reasonable to use the
same grade selected for the target, ZXF-5Q, since we already have
experience with it in the target beam test.

## Baffle outer radius

IHEP has proposed shrink-wrapping the graphite in a long aluminum tube
\[ref. Error: Reference source not found\]. This provides good thermal
contact with the graphite that will be maintained under beam heating in
spite of the larger thermal expansion coefficient of aluminum. Also it
can be done with high accuracy. The technology they describe would limit
baffle diameter to 60 mm; 60 mm would appear to be a reasonable match to
NuMI requirements. The graphite diameter is 57.1 mm, which is a standard
stock graphite rod size.

## Baffle length

Calculations have been done to find out how long the graphite baffle
must be to protect the horn inner conductor, the target cooling tube and
solder joint to the graphite, and the target ceramic. The position of
the baffle was also varied. The position/length selected is that which
has the upstream end of the baffle as close to the target as will still
provide a good safety margin. This allows the greatest possible target
motion. The result chosen is150 cm long baffle with 68 cm spacing
between baffle and target fin. Curiously, extending the baffle length in
the space between target and baffle further actually makes the energy
deposition in the target fixturing worse, although it reduces the energy
deposition in the horn neck. Please see \[ref. Error: Reference source
not found\] for derivations of acceptable temperature rises; Figure 10
and Figure 11 include temperature rises for baffle lengths and positions
calculated after \[ref. Error: Reference source not found\] was written.

## Baffle atmosphere and windows

\[ref. Error: Reference source not found\] documents that the maximum
temperature in the core of an accident shower along the inner edge of
the baffle is 344 deg C, at which temperature no detectable oxidation is
expected. The oxidation threshold (temperature resulting in 1% weight
loss in 24 hours) for ZXF-5Q is 450 deg C. Hence an inert atmosphere in
the baffle is not required.

Beryllium windows are provided to prevent erosion of the graphite by the
strong wind in the target pile. A ventilation hole is provided to
prevent air pressure buildup.

## Baffle thermal warp

A thermal gradient laterally across the baffle can be caused by
azimuthally asymmetric beam loss, air flow, or cooling fin contact. This
would cause the baffle to warp. The following arguments indicate such
effects are not a problem for us.

A zeroth order model with a linear temperature gradient across the
cylinder gives a warp of the center relative to the ends of (αTL^2^/8w)
where α is the coefficient of thermal expansion, T is the maximum
temperature difference, L is the length of the baffle, and w is the
width of the baffle. For a warp of 0.5 mm, this implies a temperature
difference of 13.3 K for graphite across the baffle. Numerically
modeling non-realistic extreme cases where the entire fin cooling was on
one half of the baffle or the entire heat deposition was on one half of
the baffle for 3% continuous scraping gave only a few degree temperature
differences even at the higher temperature downstream end of the baffle.
Hence this effect can be safely ignored.

# Baffle cooling and monitoring

Calorimetric methods are a simple and direct way of monitoring the beam
loss on the baffle. For this to work, the cooling of the baffle should
be adjusted so that temperature rises are easily measured with standard
thermocouples but the baffle will not overheat. The simplest is to rely
on heat transfer to the existing air flow, with the addition of fins to
increase the heat transfer. However, heat transfer efficiency is hard to
precisely calculate a priori, so a large safety factor should be built
in for the operating parameters.

Figure 12 shows the energy distribution when beam hits the front of the
baffle \[ref. [^6]\]. This corresponds to an average temperature rise of
11 K per pulse at the downstream end of the baffle for full power beam.
Since the largest temperature rise is downstream, the monitoring
thermocouples should be mounted near the downstream end.

To first order, the time constant for heat transfer to the cooling air
is (Vdc/Ah), and the asymptotic temperature rise is (PV/Ah), where V is
the volume of graphite, d is the density of graphite 1.8 g/cm3, c is the
specific heat capacity of graphite 0.7 J/gK, A is the surface area of
the graphite segment, h is the heat transfer coefficient from baffle to
air, and P is the power deposited per unit volume. For a 3 cm radius
cylinder, V/A = 1.5 cm. The cooling air flow along the surface of the
baffle is about 4 m/s, which would give h\~12W/m2K for flow parallel to
a fin-less cylindrical baffle \[ref. Error: Reference source not
found\]. The actual flow pattern is hard to predict, and with transverse
flow the heat transfer coefficient could be double this. For use in
selecting a strategy, a hypothetical fin with five times the heat
transfer of the cylinder is chosen as a reasonable operating point, and
shown in comparison in the table. The cooling air temperature will be
about 16 deg C. Note that \[ref. Error: Reference source not found\]
includes full three-dimensional modeling of the heat transfers, with
results that agree well at the downstream end of the baffle with the
simple approximation given here.

The baffle fin design has not been finalized, but Figure 13 shows a
concept. It consists

of two half rings 1.5 mm thick with pins 2 mm in diameter. Pins of 15 mm
height are located with period 7 mm along the circumference and 9 mm
along the baffle. The internal radius of a half ring is the same as the
external radius of the baffle casing. These half rings are pressed to
the baffle casing via a stainless steel clamp, providing pressure about
10-20 MPa. Such a pressure should provide a good thermal contact between

radiator and casing. Another method of providing a good thermal contact
is use of a 50 micron thick Indium film. The estimate of effective heat
transfer coefficient of the pin radiator is 300 W/m^2^K at 4 m/s air
velocity, which implies that only about 20% of the surface area of the
baffle would have to be covered with the fins to achieve the desired
heat transfer shown in Table 6.

  ---------------------------------- -------------------- ----------------
                                     No fin, air flow     Fin on baffle
                                     parallel             

  Heat transfer coeff. (h)           12 W/m^2^K           60 W/m^2^K

  Time constant toward asymptotic    1575 sec             315 sec
  temperature rise                                        

  Asymptotic temperature rise,                            
  downstream end                                          

  Full power accident                (9291 K)             (1858 K)

  3% continuous scraping             (278 K)              56 K

  1% desired monitoring accuracy     93 K                 19 K

  Temperature rise per pulse,        11 K                 
  downstream end, full beam power                         
  ---------------------------------- -------------------- ----------------

[]{#__RefHeading___Toc15715022 .anchor}Table 6 Thermal parameters for
two different air heat transfer coefficients. Temperatures in
parenthesis ignore radiative cooling, are prevented from occurring by a
120 deg C thermocouple limit input to beam interlock, but are included
for illustration.

## Accident scenario

To prevent damage to the aluminum baffle tube, an interlock will be set
to prevent further beam pulses at a thermocouple readback of about 120
deg C. The interlock could be set lower if found to be operationally
desirable. This would prevent more than at most a dozen continuous
accident pulses; however the interlock will probably not be set tight
enough to trip the beam from a single accident pulse. With the fin, the
baffle will cool off in a few minutes, allowing one to turn the beam
back on quickly.

## Continuous scraping

We are optimistic that beam scraping will be low. The fin design
nevertheless would allow 3% continuous beam scraping with a safety
factor of 2 in the heat transfer coefficient h before reaching the 120
deg C trip point.

The 19 K temperature rise per 1% beam loss is an order of magnitude
larger than needed for monitoring the beam loss at the required level,
providing a large safety factor on the value of h being larger than the
design specification.

Backgrounds to the calorimetric monitoring of the baffle scraping have
also been calculated. The energy deposit in the baffle is 0.10
J/cm3/pulse for 1% scraping, compared to 0.04 J/cm3/pulse from
interactions in the target pile air and 0.0002 J/cm3/pulse backscatter
from the target \[ref. Error: Reference source not found\].

Note that the thermal monitoring does not give pulse-by-pulse monitoring
of the scraping, but an average with a few minute time constant. We have
not yet found an argument showing a necessity for pulse-by-pulse
monitoring of scraping.

## Calibration

The actual temperature response of the baffle will be calibrated during
commissioning by steering 100% of the beam at low intensity on to the
baffle. Note that a constant beam intensity should be maintained for a
period of several time constants for this measurement.

## Commissioning alignment scan

The fin parameter set also allows an alignment check of the baffle via
beam scan during commissioning, or whenever one wants to recheck
alignment. It is conservative to turn down beam power during scans; beam
intensity as low as 8x10^11^ protons per pulse is easy to achieve. Such
a single pulse (2% of maximum beam) into the baffle would produce only a
0.2 K temperature rise in the baffle, so one would need to use a large
number of pulses per point (say 30 over a one minute period), or else a
higher intensity beam (in which case one could limit the repetition
rate) to produce a robust thermocouple response. With the standard 1 mm
RMS beam spot, an alignment check accuracy of a fraction of 1 mm would
be expected, based on experience with similar temperature scans done
during the NuMI target test.

Usable signals from the alignment check baffle scan will most likely
also be detected in the horn 1 alignment ionization chamber and the
hadron monitor in front of the absorber, providing redundancy.

Since the baffle and target are pre-aligned on a common carrier, the
combination of baffle scan and Budal-monitor target scan can demonstrate
that the combined baffle/target system is at the correct angle as well
as in the correct position.

# Effect of scraping on the neutrino spectrum

The effect on the neutrino spectrum of beam tails hitting the baffle has
been modeled in GNUMI, and is illustrated in Figure 14. The effect of
scraping is mostly significant between 5 GeV and 8 GeV. In a 10 kty
exposure, there will be of order 500 events in this range collected in
the MINOS far detector, giving a statistical error of 4%. Thus knowing a
scraping correction to 1% accuracy would keep the systematic error
negligible compared to the statistical error. As shown in the figure, 1%
beam scraping produces about a 1% change in the double ratio (beam
scraping correction), hence measuring the scraping to +/- 1% of the
total beam power is a reasonable specification.

It would be nice to keep the scraping to less than 1%, so that no
correction at all would need to be applied. For a gaussian beam with 1
mm RMS, 99% of the beam is within 3 mm radius. In our case, the baffle
hole is 5.5 mm radius, and even after allowing for 0.5 mm alignment
tolerances the loss from a gaussian beam would be negligible. However,
only experience with the real beam will tell us if there are significant
non-gaussian tails, or jitter in the beam position that cause losses on
the baffle.

Note this simulation had baffle to target distance of 37 cm instead of
our current choice of 68 cm, but that should have only a minor effect.
In a similar study with the baffle a couple meters further back, Brajesh
Choudhary found the scraping sensitivity to be twice as large, which may
be another motivation for keeping the baffle close to the target.

# Figures

![](media/image1.jpeg){width="5.993055555555555in"
height="2.932638888888889in"}

[]{#__RefHeading___Toc15715023 .anchor}Figure 1 Modules in target hall
shielding. Target/baffle carrier mounted below target/baffle module,
with target and baffle extended into the low energy position. Also
visible are carriage cross beams, steel shielding, horn 1 mounted below
horn 1 module.

![](media/image2.jpeg){width="5.995138888888889in"
height="6.292361111111111in"}

[]{#__RefHeading___Toc15715024 .anchor}Figure 2 Target/baffle carrier
with target and baffle mounted. Target shown extended into the low
energy neutrino beam position.

![](media/image3.wmf){width="5.999305555555556in"
height="4.461111111111111in"}

[]{#__RefHeading___Toc15715025 .anchor}Figure 3 Baffle. 150 cm long, 11
mm diameter hole, 57 mm diameter graphite core, 61 mm diameter aluminum
casing. Cooling fins not shown.

![](media/image4.jpeg){width="3.522222222222222in"
height="8.503472222222221in"}

[]{#__RefHeading___Toc15715026 .anchor}Figure 4 (Rotated) target carrier
side view. Target and baffle are shown in both Low Energy Beam extended
position, and in fully retracted highest energy beam position.

![](media/image5.jpeg){width="5.996527777777778in"
height="5.509722222222222in"}

[]{#__RefHeading___Toc15715027 .anchor}Figure 5 Target carrier end view.

![](media/image6.jpeg){width="5.9944444444444445in"
height="8.64861111111111in"}

[]{#__RefHeading___Toc15715028 .anchor}Figure 6 Summary from study of
beam energy deposition in carrier using MARS.

![](media/image7.jpeg){width="5.991666666666666in"
height="1.5298611111111111in"}

[]{#__RefHeading___Toc15715029 .anchor}Figure 7 Top view of Horn 1
module upstream endwall, which will be copied for the target module.
Utility penetrations include those labeled clamp, outer, inner, and the
two large holes to the right of those labeled inner.

![](media/image8.wmf){width="5.865277777777778in"
height="8.377083333333333in"}

[]{#__RefHeading___Toc15715030 .anchor}Figure 8 Plan view of baffle,
target, horn geometry. Since target is rectangular rather than
cylindrical, limits of its horizontal and vertical extent are shown.
Note large factor between horizontal and vertical scales.

![](media/image9.wmf){width="5.992361111111111in"
height="7.754861111111111in"}

[]{#__RefHeading___Toc15715031 .anchor}Figure 9 Beam-eye view of baffle
inner radius and comparison to beam spot, target fin, horn neck, and
target cooling and support structure.

![](media/image10.wmf){width="5.958333333333333in" height="5.375in"}

[]{#__RefHeading___Toc15715032 .anchor}Figure 10 Temperature rise of
protected components for an accident spill as a function of the length
of the baffle. The distance Lbt between the downstream end of the baffle
and the target vertical fin is 176 cm (circles), 86 cm (boxes), and 41
cm (triangles).

![](media/image11.wmf){width="5.997916666666667in"
height="5.490972222222222in"}

[]{#__RefHeading___Toc15715033 .anchor}Figure 11 Temperature rise of
protected components for an accident spill as a function of the position
of a 1.5 m long baffle. Lbt is the distance from the end of the baffle
to the start of the target vertical fin. The chosen design value of 68
cm is marked.

![](media/image12.wmf){width="5.995833333333334in"
height="5.409027777777778in"}

[]{#__RefHeading___Toc15715034 .anchor}Figure 12 Energy deposited in
baffle (J/cm3/pulse) as a function of position along baffle for 4e13
protons hitting baffle. Integral is 42 kJ/pulse, corresponding to 22 kW
at 1.87 second repetition rate, or 5.5% of the 400 kW total beam power.

![](media/image13.wmf){width="5.993055555555555in"
height="7.538888888888889in"}

[]{#__RefHeading___Toc15715035 .anchor}Figure 13 Section of pin
radiator.

![](media/image14.wmf){width="2.826388888888889in"
height="2.9298611111111112in"}![](media/image15.wmf){width="2.8333333333333335in"
height="3.0027777777777778in"}

![](media/image16.wmf){width="4.725in" height="4.878472222222222in"}

[]{#__RefHeading___Toc15715036 .anchor}Figure 14 Effect of various
amounts of beam-baffle scraping on neutrino spectrum. Top plots show
spectra separately for 100% of beam on target or baffle. (Mark Messier,
6/12/02)

# References

[^1]: Proposal for Continuously-Variable Beam Energy, M. Kostin, S.
    Kopp, M. Messier, D. Harris, J. Hylen, A. Para, NuMI-NOTE-BEAM-0783,
    10/17/01.

[^2]: Energy deposition in target support structure, R. Rameika, B.
    Lundberg, 3/02.

[^3]: Thermal Analysis of Clamp Using MARS Analysis Heat Generations,
    Bob Wands, 9/12/01.

[^4]: Thermal Stress Analysis of Numi Baffle -- II, MSG-EAR-0282, Bob
    Wands, 4/2/01 and Thermal Stress Analysis of Numj Baffle,
    MSG-EAR-00279, Bob Wands, 11/8/00.

[^5]: Technical Design of the Target Pile Protection Baffle-draft,
    S.Filippov, V. Garkusha, R. Gibadullin, F. Novoskoltsev, I.
    Ponimash, T. Ryabova, V. Zarucheisky, (IHEP) 4/30/02.

[^6]: MARS Simulation: Baffle Heating, B. Lundberg, 6/9/02.