Linac Zero Degree Beam Dump Window Design

G.R.Murdoch SNS-NOTE-ENGR-40

28 January 2002

**Introduction**

The SNS linac zero degree beam dump is to be passively cooled
dissipating 7.5kW of beam power. The beam flight tube immediately
upstream of the dump will be either evacuated or back filled with
helium, to separate this medium from the accelerator machine vacuum a
beam window is proposed.

# 

# Design Parameters

# (i) Physics

The window must be able to withstand continuous pulses of 2.1e14 H-
particles/pulse with a pulse duration of 1ms at a frequency of 0.2Hz;
continuous running is simulated as the worst case scenario. The 2-D
gaussian proton beam power density profile for Inconel with the above
conditions is given in Figure 1 assuming a window thickness of 2mm
\[1\].

# (ii) Mechanical

Several criteria must be considered during the mechanical design
process. From an operations/maintenance view point a passive window
design satisfying the physics parameters above would be the optimum
solution i.e. no active cooling apart from convective cooling to air.
Also, the window design must be integrated into the overall linac dump
beam line design with consideration for subsequent handling issues such
as shielding and remote removal and installation. These issues are not
discussed in detail but have been addressed.

# Mechanical Design

A circular domed window 80mm in diameter (beam diameter is 40mm) is
adopted as the base design for thermal analysis. Initial runs indicated
that the maximum thermal stress can be reduced by \~40% if the beam
impinges on the outer surface of the dome instead of the inner surface,
it is likely this geometry creates less restriction to thermal expansion
of the high temperature surface. Adopting a domed window is also
advantageous from a structural integrity point of view. Three radial
fins are added to promote convection.

Three materials are considered, Aluminum 6061-T6, GlidCop Al-15 &
Inconel 718. It is intended to integrate the window design with two EVAC
type vacuum flanges, this allows the window and flanges to be designed
as an integral component. Also, to aid handling a remote vacuum clamp
will be designed based upon a standard EVAC chain clamp.

# Thermal Analysis

The window is modeled as a 2-D axisymmetric running a transient thermal
analysis to mimic the pulsed beam parameters. The beam loading is input
to the ANSYS solution solver via a text program written to enable the
specific beam conditions to be applied. Typical Inconel 718 values for
the proton heat generation across a window thickness of 2mm and electron
heat generation (across the first 0.5mm for Aluminum and 0.2mm for
GlidCop & Inconel \[2\]) are given in Figure 2, these are derived from
Figure 1 using the appropriate material deposition ratios \[3\]. The
incremental values are applied to the model nodes as a function of
radius from the center of the window outwards. A nominal natural
convection coefficient is applied to the model body and outer surface of
the fins. The model is run until equilibrium conditions exist. Thermal
stresses are obtained by loading the transient result nodal temperatures
from a chosen time step into a structural model. The model is then run
as a steady state structural analysis to calculate the thermal stresses;
vacuum loads can be added at this point.

![](media/image1.wmf){width="3.1979166666666665in" height="2.25in"}

Figure 1 Figure 2

**Results**

Considering the results table of Figure 3, the maximum stress in
Aluminum after 45.01s is 158 N/mm^2^, which is 80% of the yield strength
of the material at 200 C. More importantly it is \~3 times the fatigue
endurance limit. Running the window consistently above the fatigue limit
would pose questions on potential crack propagation and consequently
lifetime issues.

![](media/image3.wmf){width="5.875in" height="1.8875in"}

Figure 3

The maximum stress seen in the GlidCop after 45.01s is 229 N/mm^2^,
which is above the yield strength of the material at 200 C. Comparison
with fatigue values is difficult as data at elevated temperatures is
scarce. Due to the poor diffusivity of Inconel the analysis is run for a
longer period of time to achieve near equilibrium conditions, Figures 4
shows the transient analysis plot for Inconel-718.

![](media/image4.png){width="5.000694444444444in"
height="3.5618055555555554in"}

Figure 4

The maximum stress seen in the Inconel window is 596 N/mm^2^, (after the
first pulse) but tails off to a maximum value of 486 N/mm^2^, at
195.04s, shown in Figure 5. This is probably due to the window absorbing
heat, slowly warming and consequently causing a reduction in the
temperature differential across the window. These stress values compare
favorably with a yield strength of 862 N/mm^2^, at 650 C and a fatigue
endurance limit of 655 N/mm^2^, for 1e7 cycles at 540 C.

![](media/image5.wmf)

Figure 5

The stress and temperature fringe profiles for Inconel-718 are shown in
Figure 6&7 respectively. Consideration of the principal stresses shows a
maximum tensile stress value of 213 N/mm^2^ and the maximum compressive
stress value of 474 N/mm^2^, this compressive stress is likely to
inhibit crack propagation in the high temperature area.

![](media/image6.png){width="5.877083333333333in"
height="3.973611111111111in"}

Figure 6

![](media/image7.png){width="5.747222222222222in"
height="4.322222222222222in"}

Figure 7

**Conclusion**

For a passively cooled window Inconel 718 is clearly the most robust
choice of the three materials analyzed. One concern is continuous
running at temperatures in the order of 635 C, although this is
localized at beam center. Radiation from the window surface has not been
considered but basic calculations show that the radiated heat is
negligible, being in the order of 1 -- 5 watts depending on the
emissivity value chosen. The stress values are well within the yield and
fatigue limits quoted in the extensive literature available for the
material.

Inconel-718 is a recognized quantity from a proton irradiation lifetime
point of view, however, it does contain longer half-life isotopes than
either GlidCop or Aluminum and consequently will remain more activated.
This will have to be taken into consideration in all work planning as an
ALARA issue. The choice of a high integrity material such as Inconel-718
should lead to less down time and consequently improved machine
availability.

**References**

\[1\] Deepak Raparia, BNL, Private Communication.

\[2\] Stuart Henderson, SNS, Private Communication,

Studies in Penetration of Charged Particles in Matter,

Nuclear Science Series, Report No. 39.

\[3\] Franz Gallmeier, SNS, Private Communication.

**Acknowledgments**

Rudy Damm, SNS -- Technical Advice.

George Dodson, SNS -- Technical Advice.

Arnold DeCarlo, SNS - Design Drafting.

Sangho Kim, SNS - FEA Verification.

Jim Rank, BNL - Initial Design Work / Analysis.

Ken Reece, SNS -- Technical Advice.