Dispersion suppressors and geometry

The transition between arcs and dispersion-free straight sections is made by means o f a so-called dispersion suppressor (DS). Each LHC dispersion suppressor comprises four individually powered quadrupole magnets which are separated by two (instead o f three, as in the arcs) dipole magnets. This arrangement o f 4 quadrupole and 8 dipole magnets is referred to as two missing dipole cells in the following text.

Reducing the dispersion at the IPs to zero requires special powering o f two more quadrupole magnets on each side o f the arc. In terms o f the machine optics, the dispersion suppressor, therefore, refers to the two missing “irregular” dipole cells plus one additional “regular” arc cell. The regular part o f the DS is identical to the first FO D O cell o f its respective arc. The irregular portion o f the DS is made o f two cells with a total number o f eight dipoles which are presently chosen to be identical to the arc dipoles. The irregular part is located closest to the IR and is connected to the regular part by a drift space varying in length between 10 m and 20 m. All o f the six quadrupoles in the DS are powered individually.

Three different DS layouts, shown in Figure 2.10 for the 23 x 90 lattice, are inte­

grated depending on the IR. In the DS next to IR3 and IR7, presented in the middle o f Figure 2.10, every quadrupole (M Q ) is followed by one or two trim quadrupoles (M Q T LI). Both quadrupole types are used for matching. An orbit corrector (M CB) is located next to every quadrupole. The trim quadrupoles as well as the orbit correc­

tors used in the irregular part o f the DS are longer than their respective counterparts in the arc. Next to IR1 and IR5 6 m long quadrupoles are needed to ensure sufficient matching flexibility [45]. This configuration is shown in the top plot o f Figure 2.10.

Finally, the b ottom picture o f Figure 2.10 illustrates the DS integrated next to IR2, IR4, IR6 and IR8. For the 18 x 90 lattice the irregular part o f the DS is schemat­

ically identical to the 23 x 90 lattice, while the regular part equals the respective regular arc cell o f the 18 x 90 lattice. A modified layout o f the DS at all odd-number IRs [46] enables the installation o f two DS collimators (T C L D ) for improved cleaning efficiency (see Sect. 2.4.6) .

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F i g . 2 . 1 0 . D is p e r s io n s u p p r e s s o r la y o u t f o r t h e H E - L H C 2 3 x 9 0 a r c o p t i c s , in I R 1 a n d 5 ( t o p ) , I R 3 a n d 7 ( c e n t r e ) , a n d I R 2 , I R 4 , I R 6 a n d I R 8 ( b o t t o m ) [3 3 ] .

The LHC design optics had been matched to the footprint o f LEP, with a maxi­

mum offset o f less than 6 cm, as is illustrated in Figure 2.11. The overall geometrical offset o f the HE-LHC lattice with respect to either LEP or LHC may be divided in two parts: arcs, and dispersion suppressors. The offset o f the arcs may generally be reduced to about 2 cm peak-to-peak by adjusting the arc cell length to adapt the global bending radius to the curvature o f the tunnel, and by centring the bends with respect to the IPs in the same way as in the present LHC. On the other hand, reducing the offset o f the DS needs to be done “by hand” and requires more effort.

The geometrical offset o f the HE-LHC or LHC dispersion suppressors with respect to LEP depends strongly on the total deflection o f the 8 bends in each DS (8A -11B ; see Fig. 2.14) , which is equal to 16n /N bend if all N bend bends in the ring are chosen to be identical. This offset has a minimum amplitude if the DS deflections o f LEP (or LHC) and HE-LHC are approximately equal, i.e. in the range 1230 < N bend < 1237.

A bend scheme o f 1232 (a multiple o f 16) identical bends was chosen for the present LHC (23 cells per arc), resulting in the footprint difference o f Figure 2.12 (right picture). Previous design versions (25 cells, 24 cells) used very different bend schemes, and in those lattices one or more bends in each DS needed to be different from the ones in the regular arc cells in order to follow the LEP geometry.

F i g . 2 . 1 1 . H o r iz o n t a l d iffe r e n c e o f t h e L H C f r o m t h e L E P f o o t p r i n t a ll a r o u n d t h e r in g (le f t ) a n d in t h e r e g io n o f t h e d is p e r s io n s u p p r e s s o r ( r ig h t ).

Fig. 2 .1 2 . H o r i z o n t a l d iffe r e n c e f r o m t h e L H C ( l e f t ) a n d L E P f o o t p r i n t ( r ig h t ) a ll a r o u n d t h e r in g f o r b o t h t h e 2 3 x 9 0 w i t h c o l l i m a t o r s ( T C L D s ) in t h e d is p e r s io n s u p p r e s s o r s o f I R s 1, 3, 5 a n d 7, a n d f o r a p r e lim in a r y 18 x 9 0 H E - L H C o p t i c s w i t h o u t s p a c e f o r T C L D s .

In the proposed HE-LHC 23 x 90 lattice N bend is also equal to 1232, and a rea­

sonable offset value o f 78 mm peak-to-peak w.r.t. LEP (compared to 69 mm for the peak-to-peak offset o f the present LHC w.r.t. LEP) could easily be obtained. How­

ever, going away from the number o f 1232 bends (e.g. with a cell scheme like 18 x 90) it becomes more difficult to limit the DS offset. Changing the number o f bends in each DS is excluded with such moderate (~ 50 ) changes o f N bend, as this would pro­

duce a too large an imbalance between the deflections o f the DS and the arcs. If the main bends are to remain all identical the only solution left is to move some bends longitudinally. An offset o f only 64 mm could recently be achieved in an 18 x 90 test lattice (N bend = 1280), but at the expense o f fine tuning the geometry with many bend displacements.

The horizontal offset corresponding to both HE-LHC optics from the LHC and LEP machines is illustrated in Figure 2.12. In the arcs the maximum offset allowed from the LHC or LEP line is about 3 cm. A better fit to the LEP footprint implies a larger difference from the LHC, especially in the DS region. A zoom ed view o f the offset in this region is shown in Figure 2.13. It is thought that the tunnel wall in the dispersion suppressors follows the LEP footprint. Either optics can be matched to com ply with the geometrical constraints.

T w o special collimators (TC LD s) are needed in each DS o f the collimation straights 3 and 7 and o f the high-luminosity straights 1 and 5. It is proposed to displace the dipole pairs ⁸ and ^{1 1}, to make room for these collimators, as is

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Fig. 2 .1 3 . Z o o m e d v ie w o f t h e h o r iz o n t a l d iffe r e n c e f r o m t h e L H C (le f t ) a n d L E P f o o t ­ p r in t ( r ig h t ) in t h e r e g io n o f t h e d i s p e r s i o n s u p p r e s s o r , f o r t h e 2 3 x 9 0 o p t i c s w i t h o u t ( t o p ) a n d w i t h s p a c e f o r T C L D s ( c e n t r e ) , a n d f o r a p r e lim in a r y 1 8 x 9 0 H E - L H C o p t i c s w i t h o u t T C L D s ( b o t t o m ) . T h e 2 3 x 9 0 o p t i c s w i t h o u t T L C D s fe a t u r e s t h e c o r r e c t m in im u m d is ­ t a n c e s b e t w e e n m a g n e t s , a n d it h a s b e e n m a t c h e d b o t h a t i n j e c t i o n a n d f o r c o llis io n . T h e o t h e r t w o o p t i c s a r e b e i n g fin a lis e d .

illustrated in Figure 2.14. Figure 2.15 sketches how such a dipole displacement by, e.g. 3 m pushes the reference orbit towards the centre o f the ring by 30 mm locally around Q9, increasing the peak-to-peak survey offset by the same amount [46]. The resulting larger deviation from the reference footprints in the straight sections with TC LD s can be seen by com paring the top and centre pictures o f Figure 2.13.