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The vacuum enclosure of the electromagnetic isotope separator “MEIRA”
The vacuum enclosure of the electromagnetic isotope separator “MEIRA” received
15 July 1971
I Levy and I Chavet,
Israel
Atomic
Energy
Commission,
Soreq Nuclear
Research
Centre,
Yavne, Israel
A short survey is given of the main requirements to be fulfilled by the different vacuum enclosure components according to their function in the separator. The methods and materials chosen for the construction of this enclosure are explained and details of its major components described.
1. Introduction
following
The effects of residual pressure on the general performance of EM1 (electromagnetic isotope) separators were discussed by Chavetl who also gave the main principles for proper design of the vacuum system for these machines. The purpose of this communication is to describe some important features and technological details about the particular design and fabrication methods of the vacuum enclosure of “MEIRA”, an EM1 separator whose ion optical principles have been described recently by Chavet”. This separator is now in the final stages of assembly at the Soreq Nuclear Research Centre. Figure 1 shows a general view of the vacuum enclosure. Its main components are: (a) Source chamber containing the ion source, the acceleration electrode and its positioning mechanism. To this chamber is also fixed the source magnet. (b) Deflection chamber confined in the gap between the polepieces of the deflection magnet. (c) Collector chamber containing the collector and its positioning mechanism. (d) Intermediate sleeves, chambers, and flanges.
(a) Good access to all internal components and easy dismantling for maintenance purposes. (b) Possibility of modifications and of introduction of additional accessories, probes, gauges, etc. (c) Accurate positioning of the enclosure components relative to each other, and of the whole enclosure relative to the deflection field geometry. (d) Efficient pumping of the enclosure from the viewpoint of total throughput and location of the diffusion pumps. (e) Ultimate vacuum approaching lo-’ torr.
2. General requirements for the vacuum enclosure The enclosure
was designed
to satisfy
as far as possible
the
requirements:
3. Vacuum enclosure assembly Requirements (a) and (b) dictated independent units with individual connections to all facilities (primary vacuum, cooling water, air, electricity, etc). The enclosure components are fitted together by bolts and positioning pins. These pins are adjustable to a small extent (1 mm) in order to correct, during the first assembly of the enclosure, any small error in the relative position of the successive components due to fabrication tolerances. The whole enclosure is positioned relative to the magnetic field, according to requirement (c), by matching four reference lines marked on
Figure 1. General view of the MEIRA EM1 separator. (1) source insulator; (2) source chamber; (3, 8, 10, 15, 17) intermediate flanges; (4) source magnet; (5) source valve; (6) second differential diaphragm; (7, 18) sleeves; (9, 16) experimental working (intermediate) chambers; (11, 14) magnetic shields; (12) deflection magnet (coils not shown); (13) deflection chamber; (19) collector valve; (20) collector chamber; (21) diffusion pump and liquid nitrogen trap ports. Vacuum/volume
2l/number
8.
Pergamon
Press Ltd/Printed
in Great Britain
325
I Levy and I C/wet:
The vacuum
enclosure
of the electromagnetic
isotope
separator
“MEIRA”
four legs extending from the deflection chamber side flanges to four other reference lines on the supporting consoles, accurately positioned relative to the magnetic field geometry (Figure 2). Each unit is supported by a carriage. The points of contact between zontal
chamber movement
and along
carriage
allow
height
adjustment,
the axis of the separator,
hori-
and flexibility
along the vertical axis, (Figure 3). The horizontal movement allows disengagement of the units from each other. The flexibility is achieved by Belleville washers and allows the supports to carry most of the weight of the units while their exact position is determined by the rigidity of the vacuum enclosure fixed to the four consoles of the magnet. Each carriage is able to roll in a direction perpendicular to the axis of the separator. Thus, any unit can be very easily disengaged from the separator without disturbing other units. An additional feature worth mentioning is the use of intermediate flanges between units of different geometrical sections (Figure 3). The two O-ring grooves are machined in the intermediate flange. This system simplifies the design and the machining of the chambers. Figure 4 shows the assembled separator.
4. Pumping equipment and auxiliary devices Figure 2.
Method of positioning the enclosure relative to the magnetic field, shown at the entrance side of the magnet.
(I) intermediate flange between deflection chamber and intermediate chamber; (2) consoles fixed to the magnet and accurately positioned relative to the magnetic field: (3) reference lines.
The secondary having
pumping
a nominal
by eight
speed of 2000
oil diffusion
I
of a large number
.‘-.. 50
100 150 200
250
mm
A-A
Figure 3. Assembled sleeve and intermediate chamber resting on flexible supports. (I) cylindrical sleeve; (2) intermediate chamber (rectangular cross’[section): (3) intermediate flange fitted with O-rings; short axial movement; (5) flexible support; (6) water cooled baffle: (7) oil diffusion pump; (8) sleeve for liquid nitrogen trap. 326
pumps
each. The pumps which reduce the
I./set
are equipped with water-cooled baffles effective pumping speed to 900 I./set. The use
0
SECTION
is achieved
pumping
(4)
rails
fol
/ Levy and I C/wet:
The vacuum enclosure
of the electromagnetic
Figure 4. ( kneral view of the assembled separator. and the liquid nitrogen traps above the enclosure.
isotope
separator
“MEIRA”
Source chamber is on left, collector chamber on right. Note the independent
of pumps was dictated by the very elongated shape of the enclosure and the various diaphragms fitted inside. These pumps had to be carefully distributed along the separator as shown in Figure 1. In the region of the source chamber, where a substantial gas input from the ion source is expected, three pumps are grouped. Liquid nitrogen traps are fitted on the vacuum enclosure above each pumping post. This position was chosen in order to improve the pumping speed. The pumping equipment is completed by a Roots pump of 400 ma/hr backed by two rotary pumps of 25 m3/hr each and a third identical pump for roughing purposes. For reasons of economy, gate valves were installed on the diffusion pumps only in the collector and source chambers. Quick cooling of the pump heaters is provided to minimize backstreaming of oil when the pumps are shut down. The flange after the source chamber (which holds the electrode mechanism) functions as a “differential diaphragm” minimizing gas input from the source region to the separator. A second differential diaphragm can be placed if necessary after the third pump to enhance this effect. A third narrow diaphragm placed at the magnet entrance to define the beam section2 has an additional role when separating gases and volatile elements. It restricts the passage of gases from the source to the collector and so reduces isotope contamination in these cases. 5. Materials Stainless steel type 304 was chosen for all the enclosure components because of its good corrosion resistance and degassing characteristics. The only exception is the deflection chamber which has special requirements discussed in detail in the next paragraph. Viton O-rings were used throughout. Plastics were avoided as far as possible.
cart iages
6. The deflection chamber One of the requirements is that the deflecting field in the gap between the polepieces should have a homogeneity better than 10-4. This is achieved by sufficiently tight tolerances in the planeness and parallelism of the pole faces. The contribution of the deflection chamber walls to the field inhomogeneity has to be substantially lower, say around 3 x 10-5. Now, the best stainless steels from this point of view have a magnetic permeability around 1.003 (Type 305 annealed). This means that the thickness tolerance for the plates to be used for the deflection chamber walls must be within 0.4 mm. Since any machining, cold working (including polishing) and welding, considerably alter the magnetic permeability of stainless steel, this tolerance is rather difficult to satisfy especially for the large plates required (approx. 1000x500~ 12 mm). Moreover, if the chamber is used without liner, the beam causes sputtering of magnetic particles from the internal walls and this interferes significantly with the ion trajectories. All these difficulties were avoided by the use of titanium (commercially pure grade 55) whose permeability is around 1.00005. The titanium was welded using the electron beam welding technique*. The shape of the deflection chamber and the type of weld needed are shown in Figure 5. (Welding conditions: penetration 13 mm; beam 27 mA, 150 kV; feed 1 m/min; vacuum 5 x 10m5torr). Since no significant distortions are encountered in this welding technique, the walls were first machined and polished, then assembled by clamps and welded. The last step was the machining of about 1 mm around the vertical walls to the exact dimensions. 7. The intermediate and collector chambers The two intermediate chambers (before and after the deflection *At the Israel Aircraft
Industries,
Ltd., Lod.
327
I Levy and I C/wet:
The vacuum
enclosure
of the electromagnetic
isotope
separator
“MEIRA”
u
SECTION
A-A
Figure 5.
Deflection
titanium,
grade
chamber
and detail of weld. Commercially
pure
55, welded by electron beam.
chamber), intended mainly for experimental purposes, should have a rectangular cross section and wide rectangular apertures. Here too, the use of electron beam welding considerably simplified the fabrication of these chambers. Surfaces containing such welds are entirely satisfactory for sealing purposes using an elastomer O-ring, after superficial milling (a depth of 1 mm is enough). Therefore each chamber was assembled simply from two plates separated by four columns, all machined and internally polished before welding (Figure 6). The plates were strengthened by bars welded on the outside. The collector chamber (extreme right, Figure 4) is a large parallelepipedic box (internal dimensions: 700 ~400 x 320 mm) built from thick plates (I 5 mm). The plates were machined to final dimensions, polished internally, and then welded by the electron beam technique. This box is provided with numerous round apertures. The covers are held by screws penetrating partly into the wall.
Figure
7. Source chamber
shown
mounted
unit resting on its carriage. The traps are they can be lowered thus protecting the pumps from corrosive gases.
on their sleeves. When required,
inside the chamber,
8. Sleeves and source chamber In contrast
to the previous
of more conventional
chambers,
the cylindrical
design and welded
by argon
The more complex form of the source chamber is intended to satisfy the following requirements:
1
100
_c~
sleeves arc arc.
(Figure
7)
(a) Unimpeded pumping of the source region on both sides by two pumps and two overhead liquid nitrogen traps. These traps may be lowered between the source and the pumps when it is desired to protect the pumps from chlorine and other corrosive gases emitted from the source. (b) Appropriate shape for properly locating the cumbersome source magnet. (c) Sufficient distance between the insulated source flange and the main body of the source chamber in order to avoid electrical breakdown. As can be seen by the above description, some aspects of the design of the vacuumenclosure have been improved by adopting new techniques, methods, and materials, which could simplify the mechanical construction of its components or improve their performance. References
Figure
6. Design of the intermediate chamber at the magnet entrance side. Electron beam welding allowed the adoption of a neat and simplified construction.
328
’ 1 Chavet,
Vuu/rrr,r. IY, 1970, 547. 2 I Chavet, Pm. Itrt Corrf Ektwmug~wtic I.wtope Srpwutot-.s urrd the Twhrtiyrces of their Applimtiom, Mnrhrrrg. (Eds H Wagner, W Walchcr. Physikalisches Institut drr Unlversit%t Marburg), 1970.
15 July 1971
I Levy and I Chavet,
Israel
Atomic
Energy
Commission,
Soreq Nuclear
Research
Centre,
Yavne, Israel
A short survey is given of the main requirements to be fulfilled by the different vacuum enclosure components according to their function in the separator. The methods and materials chosen for the construction of this enclosure are explained and details of its major components described.
1. Introduction
following
The effects of residual pressure on the general performance of EM1 (electromagnetic isotope) separators were discussed by Chavetl who also gave the main principles for proper design of the vacuum system for these machines. The purpose of this communication is to describe some important features and technological details about the particular design and fabrication methods of the vacuum enclosure of “MEIRA”, an EM1 separator whose ion optical principles have been described recently by Chavet”. This separator is now in the final stages of assembly at the Soreq Nuclear Research Centre. Figure 1 shows a general view of the vacuum enclosure. Its main components are: (a) Source chamber containing the ion source, the acceleration electrode and its positioning mechanism. To this chamber is also fixed the source magnet. (b) Deflection chamber confined in the gap between the polepieces of the deflection magnet. (c) Collector chamber containing the collector and its positioning mechanism. (d) Intermediate sleeves, chambers, and flanges.
(a) Good access to all internal components and easy dismantling for maintenance purposes. (b) Possibility of modifications and of introduction of additional accessories, probes, gauges, etc. (c) Accurate positioning of the enclosure components relative to each other, and of the whole enclosure relative to the deflection field geometry. (d) Efficient pumping of the enclosure from the viewpoint of total throughput and location of the diffusion pumps. (e) Ultimate vacuum approaching lo-’ torr.
2. General requirements for the vacuum enclosure The enclosure
was designed
to satisfy
as far as possible
the
requirements:
3. Vacuum enclosure assembly Requirements (a) and (b) dictated independent units with individual connections to all facilities (primary vacuum, cooling water, air, electricity, etc). The enclosure components are fitted together by bolts and positioning pins. These pins are adjustable to a small extent (1 mm) in order to correct, during the first assembly of the enclosure, any small error in the relative position of the successive components due to fabrication tolerances. The whole enclosure is positioned relative to the magnetic field, according to requirement (c), by matching four reference lines marked on
Figure 1. General view of the MEIRA EM1 separator. (1) source insulator; (2) source chamber; (3, 8, 10, 15, 17) intermediate flanges; (4) source magnet; (5) source valve; (6) second differential diaphragm; (7, 18) sleeves; (9, 16) experimental working (intermediate) chambers; (11, 14) magnetic shields; (12) deflection magnet (coils not shown); (13) deflection chamber; (19) collector valve; (20) collector chamber; (21) diffusion pump and liquid nitrogen trap ports. Vacuum/volume
2l/number
8.
Pergamon
Press Ltd/Printed
in Great Britain
325
I Levy and I C/wet:
The vacuum
enclosure
of the electromagnetic
isotope
separator
“MEIRA”
four legs extending from the deflection chamber side flanges to four other reference lines on the supporting consoles, accurately positioned relative to the magnetic field geometry (Figure 2). Each unit is supported by a carriage. The points of contact between zontal
chamber movement
and along
carriage
allow
height
adjustment,
the axis of the separator,
hori-
and flexibility
along the vertical axis, (Figure 3). The horizontal movement allows disengagement of the units from each other. The flexibility is achieved by Belleville washers and allows the supports to carry most of the weight of the units while their exact position is determined by the rigidity of the vacuum enclosure fixed to the four consoles of the magnet. Each carriage is able to roll in a direction perpendicular to the axis of the separator. Thus, any unit can be very easily disengaged from the separator without disturbing other units. An additional feature worth mentioning is the use of intermediate flanges between units of different geometrical sections (Figure 3). The two O-ring grooves are machined in the intermediate flange. This system simplifies the design and the machining of the chambers. Figure 4 shows the assembled separator.
4. Pumping equipment and auxiliary devices Figure 2.
Method of positioning the enclosure relative to the magnetic field, shown at the entrance side of the magnet.
(I) intermediate flange between deflection chamber and intermediate chamber; (2) consoles fixed to the magnet and accurately positioned relative to the magnetic field: (3) reference lines.
The secondary having
pumping
a nominal
by eight
speed of 2000
oil diffusion
I
of a large number
.‘-.. 50
100 150 200
250
mm
A-A
Figure 3. Assembled sleeve and intermediate chamber resting on flexible supports. (I) cylindrical sleeve; (2) intermediate chamber (rectangular cross’[section): (3) intermediate flange fitted with O-rings; short axial movement; (5) flexible support; (6) water cooled baffle: (7) oil diffusion pump; (8) sleeve for liquid nitrogen trap. 326
pumps
each. The pumps which reduce the
I./set
are equipped with water-cooled baffles effective pumping speed to 900 I./set. The use
0
SECTION
is achieved
pumping
(4)
rails
fol
/ Levy and I C/wet:
The vacuum enclosure
of the electromagnetic
Figure 4. ( kneral view of the assembled separator. and the liquid nitrogen traps above the enclosure.
isotope
separator
“MEIRA”
Source chamber is on left, collector chamber on right. Note the independent
of pumps was dictated by the very elongated shape of the enclosure and the various diaphragms fitted inside. These pumps had to be carefully distributed along the separator as shown in Figure 1. In the region of the source chamber, where a substantial gas input from the ion source is expected, three pumps are grouped. Liquid nitrogen traps are fitted on the vacuum enclosure above each pumping post. This position was chosen in order to improve the pumping speed. The pumping equipment is completed by a Roots pump of 400 ma/hr backed by two rotary pumps of 25 m3/hr each and a third identical pump for roughing purposes. For reasons of economy, gate valves were installed on the diffusion pumps only in the collector and source chambers. Quick cooling of the pump heaters is provided to minimize backstreaming of oil when the pumps are shut down. The flange after the source chamber (which holds the electrode mechanism) functions as a “differential diaphragm” minimizing gas input from the source region to the separator. A second differential diaphragm can be placed if necessary after the third pump to enhance this effect. A third narrow diaphragm placed at the magnet entrance to define the beam section2 has an additional role when separating gases and volatile elements. It restricts the passage of gases from the source to the collector and so reduces isotope contamination in these cases. 5. Materials Stainless steel type 304 was chosen for all the enclosure components because of its good corrosion resistance and degassing characteristics. The only exception is the deflection chamber which has special requirements discussed in detail in the next paragraph. Viton O-rings were used throughout. Plastics were avoided as far as possible.
cart iages
6. The deflection chamber One of the requirements is that the deflecting field in the gap between the polepieces should have a homogeneity better than 10-4. This is achieved by sufficiently tight tolerances in the planeness and parallelism of the pole faces. The contribution of the deflection chamber walls to the field inhomogeneity has to be substantially lower, say around 3 x 10-5. Now, the best stainless steels from this point of view have a magnetic permeability around 1.003 (Type 305 annealed). This means that the thickness tolerance for the plates to be used for the deflection chamber walls must be within 0.4 mm. Since any machining, cold working (including polishing) and welding, considerably alter the magnetic permeability of stainless steel, this tolerance is rather difficult to satisfy especially for the large plates required (approx. 1000x500~ 12 mm). Moreover, if the chamber is used without liner, the beam causes sputtering of magnetic particles from the internal walls and this interferes significantly with the ion trajectories. All these difficulties were avoided by the use of titanium (commercially pure grade 55) whose permeability is around 1.00005. The titanium was welded using the electron beam welding technique*. The shape of the deflection chamber and the type of weld needed are shown in Figure 5. (Welding conditions: penetration 13 mm; beam 27 mA, 150 kV; feed 1 m/min; vacuum 5 x 10m5torr). Since no significant distortions are encountered in this welding technique, the walls were first machined and polished, then assembled by clamps and welded. The last step was the machining of about 1 mm around the vertical walls to the exact dimensions. 7. The intermediate and collector chambers The two intermediate chambers (before and after the deflection *At the Israel Aircraft
Industries,
Ltd., Lod.
327
I Levy and I C/wet:
The vacuum
enclosure
of the electromagnetic
isotope
separator
“MEIRA”
u
SECTION
A-A
Figure 5.
Deflection
titanium,
grade
chamber
and detail of weld. Commercially
pure
55, welded by electron beam.
chamber), intended mainly for experimental purposes, should have a rectangular cross section and wide rectangular apertures. Here too, the use of electron beam welding considerably simplified the fabrication of these chambers. Surfaces containing such welds are entirely satisfactory for sealing purposes using an elastomer O-ring, after superficial milling (a depth of 1 mm is enough). Therefore each chamber was assembled simply from two plates separated by four columns, all machined and internally polished before welding (Figure 6). The plates were strengthened by bars welded on the outside. The collector chamber (extreme right, Figure 4) is a large parallelepipedic box (internal dimensions: 700 ~400 x 320 mm) built from thick plates (I 5 mm). The plates were machined to final dimensions, polished internally, and then welded by the electron beam technique. This box is provided with numerous round apertures. The covers are held by screws penetrating partly into the wall.
Figure
7. Source chamber
shown
mounted
unit resting on its carriage. The traps are they can be lowered thus protecting the pumps from corrosive gases.
on their sleeves. When required,
inside the chamber,
8. Sleeves and source chamber In contrast
to the previous
of more conventional
chambers,
the cylindrical
design and welded
by argon
The more complex form of the source chamber is intended to satisfy the following requirements:
1
100
_c~
sleeves arc arc.
(Figure
7)
(a) Unimpeded pumping of the source region on both sides by two pumps and two overhead liquid nitrogen traps. These traps may be lowered between the source and the pumps when it is desired to protect the pumps from chlorine and other corrosive gases emitted from the source. (b) Appropriate shape for properly locating the cumbersome source magnet. (c) Sufficient distance between the insulated source flange and the main body of the source chamber in order to avoid electrical breakdown. As can be seen by the above description, some aspects of the design of the vacuumenclosure have been improved by adopting new techniques, methods, and materials, which could simplify the mechanical construction of its components or improve their performance. References
Figure
6. Design of the intermediate chamber at the magnet entrance side. Electron beam welding allowed the adoption of a neat and simplified construction.
328
’ 1 Chavet,
Vuu/rrr,r. IY, 1970, 547. 2 I Chavet, Pm. Itrt Corrf Ektwmug~wtic I.wtope Srpwutot-.s urrd the Twhrtiyrces of their Applimtiom, Mnrhrrrg. (Eds H Wagner, W Walchcr. Physikalisches Institut drr Unlversit%t Marburg), 1970.