DESIGN AND FINITE ELEMENT ANALYSIS OF INSERTION DEVICE
VACUUM CHAMBER FOR INDUS-2 STORAGE RING
D.P.Yadav#, Ram Shiroman and R.Sridhar
Raja Ramanna Centre for Advanced Technology, Indore-452013
Abstract
This paper describes design and finite element analysis
of identical vacuum chamber for U-1 and U-2 undulators
of Indus-2 (2.5 Gev, 300mA) storage ring. For effective
pumping of photon induced desorbed gas load and fast
conditioning of these chambers advanced technique of
distributed pumping called non-evaporable getter (NEG)
film coating is proposed to be applied on the vacuum
exposed surfaces of these undulator chambers. Like other
chambers of machine operating pressure of ~ 2X10-9 mbar
is required in these vacuum chambers to give longer
stored beam lifetime. Finite element pressure profile
simulation predicts achievement of average pressure
~2X10-9 mbar after 50Ah of accumulated beam dose with
this non-evaporable getter coating.
INTRODUCTION
Indus-2 storage ring will be augmented with 05 nos of
insertion device sources for high brilliance radiation
during XII plan period. Two room temperature out of
vacuum permanent magnet type undulators namely U-1
and U-2 will be installed in long straight sections LS-2
and LS-3 respectively in first phase. U-1 undulator will
generate synchrotron radiation having maximum flux
between 6eV to 250eV (in 1st harmonic) for atomic,
molecular and optical science (AMOS) experimental
beamline. U-2 undulator will generate synchrotron
radiation having maximum flux between 30eV to 600eV
(in 1st harmonic) for angle resolved photoelectron
spectroscopy (ARPES) experimental beamline. Since the
undulator parameters are based on the standard available
pure permanent magnet blocks of Nd-Fe-B, the low limit
of the tunable photon energy range depends significantly
on the minimum pole gap of the undulator [1]. The
minimum pole gap of both the undulators is 23 mm. For
maximum flux of usable photons, optimised length of
each device is 2.5m. Existing design layout of LS-2 &
LS-3 was modified in order to make undulator
compatible ultra high vacuum (UHV) system. Modified
common layout of U-1 and U-2 is shown in Fig. 1.
Vacuum chambers for these two devices are identical in
design. Like other chambers of machine required
operating pressure inside these vacuum chambers is ~ 2.0
X 10-9 mbar for longer stored beam life time.
DESIGN OF VACUUM CHAMBER
Geometrty Design
Sectional views of extruded vacuum chamber is shown
in Fig. 2a and 2b.. The undulator is located outside of the
vacuum chamber. Therefore, the external cross-section
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height of the chamber, which determines the minimum
gap of the undulator, becomes a critical parameter to
achieve the desired tunability of the photon energy. 2700
mm long chamber has overall cross-sectional dimension
of 150 mm x 21 mm. Chamber outer surface shall be
machined for increased straightness requirement due to
only 1 mm clearance between magnet pole face and outer
surface of chamber each side. In order to minimize the
beam impedance problem shape of internal aperture of
chamber has been kept race-track profile mating with
surrounding chambers. Dimension of internal aperture is
17 mm (v) x 81 mm (H) for meeting the requirements of
good field region and fabrication tolerances. Two water
cooling holes of diameter 6mm each side of beam channel
are co-extruded for low conductivity (LCW) water flow
for dissipating synchrotron radiation heat during
operation. 3D isometric view of undulator and vacuum
chamber assembly is shown in Fig. 3.
Material of Construction
EN-AW 6060-T6 grade Aluminium alloy has been
chosen as material of construction of this extruded
vacuum chamber due to: non-magnetic, UHV
compatibility (low specific outgassing rate), extrudability,
good electrical and thermal conductivity, weldability and
low residual radioactivity. Relevant properties of EN-AW
6060-T6 are shown in Table-1. Atlas make explosion
bonded bi-metallic (combination of S316L and AA6061T6 materials) con-flat (CF) flanges shall be used for end
flanges. Extrusion chamber will be welded to aluminium
portion of this flange by gas tungsten arc welding
(GTAW) process.
Table 1: Properties of EN-AW 6060-T6
Chemical
Composition (in wt %)
Tensile strength
0.2% proof strength
% elongation
Modulus of Elasticity
Poissoin Ratio
Density (g/cm3)
Thermal Conductivity
W/(m K)
Electrical
Conductivity (MS/m)
Si:0.30-0.6,
Mg:0.35-0.6,
Fe:0.10-0.30,
Cu:0.1max,
Mn:0.1 max, Ti: 0.1 max Al:
remainder
170 190 MPa
140- 150 MPa
6%
69.50 GPa
0.33
2.7
200-220
34-38
Pumping Scheme
For effective pumping of thermal and photon induced
desorbed gas loads in highly conductance limited
undulator vacuum chamber it is proposed to install
lumped pump at both ends and distributed non-evaporable
getter (NEG) coating of Ti-Zr-V film on the internal
surface of chamber. A lumped pump is the combination
of 270 l/s sputter ion pump (SIP) and 1000 l/s titanium
sublimation pump (TSP). Salient feature of NEG coating
are: high pumping speed for reactive gases like CO and
H2, reduced thermal as well photon induced desorption
(PID) rate, low secondary electron yield, reduction of
bremsstrahlung radiation [2], no additional space
requirement for installation, vibration free and no need
electrical feedthrough or isolation during operation.
Thickness of coating is 1 micron. Activation temperature
of this coating is ~ 180C for 24 hour.
Supports
Undulator vacuum chamber will be supported by three
supports. Two end supports (one at each end of chamber)
will be floating type support that will allow axial
movement of chamber ends towards connected RFshielded bellows during baking. Third support will be
mounted on mid span of chamber and will be fixed type.
FINITE ELEMENT ANALYSIS
Structural Analysis
Due to conflicting requirements of minimum pole gap
and beam good filed region very limited space is left for
wall thickness of chamber each side. In proposed design
nominal wall thickness is 2mm each side. Stress analysis
using Ansys software was carried out to check the
adequecy of this thickness against atmospheric pressure
loading. Due to symmetry only quarter portion of
chamber cross section was modelled. Vertical deflection
and Von-Misces stress are shown in Fig. 4a and 4b
respectively. Finite element stress analysis shows that
with a Young's Modulus of 69.50 GPa at room
temperature the maximum deflection due to one
atmospheric pressure at the 2 mm thick wall at the top and
bottom of the aluminium chamber will be 0.13 mm as
shown in Fig. 4. Maximum Von-Misces stress of 41.5
MPa is well within the yield strength of material.
Anealing the chamber to 180°C activation temperature
will reduce Young’s Modulus only 5%. It was ensured
that the maximum deflection will be 0.25 mm per side
including the annealing effect and fabrication tolerance of
wall thickness. This value of estimated deflection ensures
clear 16 mm vertical height internal aperture of chamber
for good field region as required for beam operation.
Pressure Profile Simulation
Low internal aperture size of vacuum chamber leads to
limited gas conductance. Pressure profile simulation using
Ansys finite element software shows the pressure profile
(shown in Fig. 5.) in various conditions. Pressure profile
shows the achievement of pressure ~ 2X10 -10 mbar after
50 Ah of integrated beam dose in case of NEG coated
aluminium chamber. This shows the increased efficiency
of NEG coating in pumping of PID dynamic gas load for
achieving the desired pressure level required for longer
beam life time in the machine.
CONCLUSION
FEM analysis results clearly show that 16 mm clear
good field region height is available for beam channel
considering the deflection due to evacuation, annealing
effect and fabrication tolerance of wall thickness.
Proposed NEG coating as advanced technique of
distributed pumping is very effective in pump down of
PID dynamic gas load for achieving the desired pressure
level in reasonable period of time as required for longer
beam life time operation of machine.
ACKNOWLEDGEMENT
Sincere efforts of Shri Vinod Vishwakarma for
preparing the 2D drawings and 3D model for this
manuscript is duly acknowledged.
References
[1] S. Kim et al, “Vacuum chamber for an undulator
straight section” Proceedings of the 1987 IEEE Particle
Accelerator Conference
[2] R. Kersevan, “NEG coated vacuum chambers at the
ESRF: present status and future plans” EPAC’02, Paris,
France, P 2565-2567
e-
Q1 END
RGA
Q1 END
RGA
e-
NEG
NEG
SIP
FAST CORRECTOR
85
260
RFS GV
IMG
( 145mm BELLOWLENGTH FOR
FAST CORRECTOR)
INSERSION DEVICES
TSP
RIGHT ANGLE 1000 l/s
VALVE
MODIFIED BPI (TYPE-1) WITH
RFS BELLOWAT ONEEND
U1/U2
TAPERCUM PUM MANIFOLD
WITH RF SCREEN
RIGHT ANGLE
VALVE
LOWGAP BPI (TYPE-II)
TSP
LOWGAP BPI (TYPE-II)
TAPERCUM PUM MANIFOLD
WITH RF SCREEN
( 145mm BELLOWLENGTH FOR
FAST CORRECTOR)
RFS GV
MODIFIED BPI (TYPE-1) WITH
RFS BELLOWAT ONEEND
TAPER
IMG
267.5
2500
100
200
FAST CORRECTOR
2700
200
267.5
260
85
4325
Figure 1. Modified Layout of LS-2 & LS-3
0.2 B
0.2 A
0.2 A
± 1.0
2700
-B-
3
4
1
OUTSIDEOF RING
± 0.8
150
17
21
80
-A-
ELECTRON BEAM DIRECTION
4 PLUGS, TIGWELDED
(WATERTIGHT)
NEGCOATED 1MICRON. ± 20%THK
17
81
INSIDEOF RING
SECTION-EE
Figure 2a. Longitudinal Sectional View of Chamber
Figure 3. 3D Isometric View of Undulator
& Chamber Assembly
Figure 2b. Cross- Sectional View of Chamber
Figure 4a. Vertical Deflection
Figure 4b. Von-Misces Stress
Thermal outgasing + PID (uncoated
chamber) after 50Ahr
Thermal outgasing + PID (NEG
Coated Chamber) after 50 Ahr
Thermal
chamber)
Figure 5. Pressure Profile across the Undulator Chamber
outgassing
(Uncoated