Collimation

The collimator system o f a hadron collider fulfils multiple functions, in particular:

(1) it must minimise beam loss in the cold parts o f the machine during periods o f

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F i g . 2 . 2 2 . F L U K A s im u la t e d p e a k d o s e a lo n g t h e s e p a r a t io n d i p o l e D 1 fo r a t o t a l in t e g r a t e d l u m in o s it y o f 10 a b - 1 a n d a h o r i z o n t a l ( b la c k ) o r v e r t ic a l h a l f c r o s s in g a n g le (r e d ) o f 165

^ r a d , w i t h a n in n e r W s h ie ld in g o f 2 c m .

F i g . 2 . 2 3 . B e t a f u n c t i o n a n d a p e r t u r e s fo r t h e i n j e c t i o n o p t i c s a t 4 5 0 G e V in t h e e x p e r im e n t I R s 1 a n d 5, w i t h ß * = 1 1 m , i n c lu d in g a 2 m m c l o s e d - o r b i t u n c e r t a in t y , w i t h b e a m - b e a m s e p a r a t io n a t t h e I P a n d a h a l f c r o s s in g a n g le o f 1 8 0 ^ r a d ( b o t h s e p a r a t io n a n d a n g le c o r ­ r e s p o n d in g t o 1 6 .8 a ) .

Fig. 2 .2 4 . O p t i c s f o r t h e R F a n d d ia g n o s t ic s in s e r t io n I R 4 in t h e p r e s e n t L H C (le f t ) a n d w i t h lo n g e r R F s e c t i o n a n d l o n g e r s e p a r a t io n d i p o l e s f o r t h e H E - L H C (r ig h t ).

minimum beam lifetime (assumed to be 12 min) to prevent quenches o f superconduct­

ing magnets; (2) it must protect the machine from failures (e.g. in case o f injection errors or an asynchronous beam abort) by intercepting bunches which would oth­

erwise destroy machine com ponents; (3) it must keep experiment backgrounds at an acceptable level. To achieve the second task, the collimators need to be made from highly robust materials. Furthermore, the collimators are the closest elements to the beam and significant sources o f impedance. Development o f novel, robust, low- impedance collimators is ongoing as part o f the HL-LHC effort [54,55]. It is likely that such new low-impedance collimators will be required for the HE-LHC.

W ith the reduced physical aperture at injection and smaller beams at higher energy, collimating the HE-LHC beams is significantly more challenging than for the HL-LHC. The baseline solution is to use the HL-LHC collimation layout as a starting point [56,57], building on the well-tested LHC collimation system [58,59], with the necessary modifications. As is already the case for the LHC and HL-LHC, the collimator system will be multi-staged, consisting o f primary, secondary and tertiary collimators, plus others, such as those used for protection o f the extraction system or capturing collision debris.

The betatron collimation straight in IR7 and the momentum collimation straight in IR3 are challenging from an optics/m agnet point o f view. Due to the intrinsically high beam losses in these regions, the LHC collimation straights can only accom m o­

date warm magnets. The HE-LHC design strategy has been to maintain or approxi­

mate the LHC optics with its carefully optimised collimator locations [60] and phase advances between collimators. Keeping exactly the same optics would require a dou­

bling o f the integrated bending and focussing fields. Minimising longitudinal gaps, eliminating any weakly excited quadrupoles and spare collimator slots and increasing the length o f all magnets to the maximum extent possible, all help accomplish this goal. For IR3 the remaining lack o f integrated magnet strength in the region hosting the primary and secondary collimators was com pensated by length scaling, leading to beta function values that are increased by the same scaling factor. For IR7 such a length scaling was not necessary.

The introduction o f dispersion-suppressor (DS) collimators requires additional space in the DS. The solution adopted for HL-LHC, which relies on replacing one standard dipole by two shorter and stronger dipoles with space for the collimator in between [61- 63], can probably not be applied to the HE-LHC, since the standard dipoles already have a field o f 16 T. Therefore an alternative solution which implies

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F i g . 2 . 2 5 . O p t i c s o f t h e H E - L H C m o m e n t u m c o l l i m a t i o n s e c t i o n in I R 3 .

moving several dipoles to create space, has been adopted. This layout uses ideas previously considered for HL-LHC [64].

Based on these considerations, a layout and optics design were established for the two cleaning insertions o f the HE-LHC. The optics for IR3 and IR7 are shown in Figures 2.25 and 2.26, respectively.

Possible future improvements include the following three points:

1. Em pty areas inside the cross section o f the warm twin quadrupoles M Q W could be filled with shielding material, if this reduces the radiation levels downstream.

2. To make the most efficient use o f the space available in the IR7 and IR3 straight sections, the outer dipoles o f the separation doglegs, which are subject to fairly low radiation levels, could be replaced by shorter superconducting dipoles.

3. In addition, since for HE-LHC the inter-beam separation in the arcs is increased to 250 mm, com pared with 194 mm at the LHC, the necessity and optimum size o f the IR 7 /3 dogleg (and, hence, the integrated strength o f the corresponding dipoles) need to be re-examined.

The hierarchy must be preserved in the presence o f errors, which requires a min­

imum transverse distance o f 1-2a between the different levels o f collimators, taking into account machine imperfections, optics and orbit stability, injection oscillations and possible failure modes. Table 2.6 compares the settings planned for the HL-LHC [48,65] with settings for the HE-LHC at two different injection energies, for the 23 x 90 optics. For injection, LHC-like settings in units o f a are chosen to protect the (reduced) aperture, noting that the HE-LHC emittance is about 30% smaller than the LHC design emittance. Injecting at 900 G eV would allow the same settings and margins to be used as for the LHC or HL-LHC. In view o f the larger triplet aperture in units o f a com pared with the HL-LHC, the HE-LHC top-energy settings in Table 2.7 are based on the HL-LHC collimation settings, but result in a slightly smaller physical half gap o f 0.82 mm. In addition, since the HL-LHC DS collimator

5[m] x1°4

F i g . 2 . 2 6 . O p t i c s o f t h e H E - L H C b e t a t r o n c o l l i m a t i o n s e c t i o n in I R 7 .

T a b l e 2 . 6 . L H C / H L - L H C a n d p r e lim in a r y H E - L H C c o l l i m a t o r s e t t in g s a t t w o d iffe r e n t i n j e c t i o n e n e r g ie s f o r a r e fe r e n c e e m i t t a n c e o f 2 .5 p m , a n d t h e 2 3 x 9 0 o p t ic s .

HL-LHC HE-LHC

Beam energy 450 GeV 450 GeV 1.3 TeV

Aperture (half gap)

(

)

(

)

(

)

Primary collimator TCP 6.7 4.3 5.7 3.81 9.7 3.81

Secondary collimator TCS 7.9 4.75 6.7 4.21 11.4 4.21

Active absorber TCLA 11.8 5.9 9.0 4.45 15.3

Dump protection TCDQ 9.5 15.0 8.0 11.96 13.6 11.96

Tertiary collimator TCT 15.4 7.5 13.0 22.1

DS collimators TCLD in IR7 >20 n/a 10 and 12 3.55 and 5.33 17 and 20.4 3.55 and 5.33

Machine aperture 12.6 - 9.5 - 16.2

-N o t e s . T h e c o l l i m a t o r g a p s in m m r e fe r t o t h e m in im u m g a p p e r fa m ily .

gaps would result in an extremely tight physical gap o f less 0.8 mm, for the HE-LHC the two T C L D ’s were opened to 18.1/22.2a, respectively, without any loss in clean­

ing efficiency. Some further optimisation o f these settings could be done to reach an optimum balance between machine protection and minimum impedance.

Injection at 450 G eV will be significantly more challenging, due to a physical aperture below 10a, and the feasibility o f high-energy operation with the 5.7a primary cut in Table 2.6 remains to be demonstrated, as well as the tight margin between the primary collimators and the machine aperture.

Table 2.7 presents a similar comparison for the collision optics at top energy, where the aperture and settings are determined by the experiment insertions and not by the arcs. At top energy, the triplet aperture remains large enough that the collimation settings can be chosen similar to the HL-LHC, either in units o f beam size or, alternatively, even in physical dimensions. Indeed, for the same number o f a the

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T able 2 .7 . L H C / H L - L H C a n d p r e lim in a r y H E - L H C c o l l i m a t o r s e t t in g s a t t o p e n e r g y in c o l l i s i o n f o r a r e fe r e n c e e m i t t a n c e o f 2 .5 p m .

H L -L H C H E -L H C

B e a m energy 7 T e V 1 3 .5 T e V

A p e r t u r e ( h a l f g a p ) ( m m ) ( m m )

P r i m a r y c o l l i m a t o r T C P 6 .7 1.1 6 .7 0 .8 2

S e c o n d a r y c o l l i m a t o r T C S 9 .1 1 .4 9.1 1 .3 2

A c t i v e a b s o r b e r T C L A 11.8 1.6 1 1 .5 1 .0 4

D u m p p r o t e c t i o n T C D Q 10.1 4 .0 10.1 2 .7 5

T e r t ia r y c o l l i m a t o r T C T 1 0 .4 3 .6 1 0 .5 0 .9 4

D S c o l l i m a t o r s T C L D in I R 7 1 8 .1 1 .7 8 1 8 .1 a n d 2 2 .2 1 .1 7 a n d 1 .5 4

M a c h in e a p e r t u r e 1 1 .9 *

-

-N o te s. T h e c o l l i m a t o r g a p s i n m m r e fe r t o t h e m in im u m g a p p e r fa m ily . ł A f t e r ß * -le v e llin g ,

**B o t t l e n e c k a t t r ip le t .

F i g . 2 . 2 7 . S im u la t e d c le a n in g e ff ic ie n c y a r o u n d I R 7 a t c o l l i s i o n e n e r g y w i t h p r im a r y c o l ­ lim a t o r s , T C P , s e t a t 6 .7 a a n d s e c o n d a r y c o l l i m a t o r s T C S G a t 9 .1 a , f o r t h e 2 3 x 9 0 a rc o p t ic s .

gap size shrinks by roughly v/2, so for example, a gap o f 140 pm at HL-LHC becomes 100pm at the HE-LHC. The retraction between the dump protection (T C D Q ) and the tertiary collimators (T C T ) imposes constraints on the phase advance from the extraction kickers to the TC T s, as it does also for the LHC and HL-LHC [66].

Figures 2.27 and 2.28 present simulated cleaning inefficiencies at collision energy and at an injection energy o f 450 G eV for the 23 x 90 optics based on the collimation

F i g . 2 . 2 8 . S im u la t e d c le a n in g e ff ic ie n c y a r o u n d I R 7 a t a n i n j e c t i o n e n e r g y o f 4 5 0 G e V , w it h t h e p r im a r y c o l l i m a t o r s , T C P , s e t a t 5 .7 a a n d s e c o n d a r y c o l l i m a t o r s , T C S G , a t 6 .7 a , fo r t h e 2 3 x 9 0 a r c o p t i c s . T h e T D I a n d T C L I i n j e c t i o n p r o t e c t i o n c o l l i m a t o r s a r e s e t a t 6 .8 a n d 8 .0 a , r e s p e c t i v e l y ( L H C s e t t in g w o u ld b e 6 .8 a , H L - L H C s e t t in g 8 a ) .

settings o f Tables 2.6 and 2.7. The cleaning efficiency o f the collimation system with this layout and optics was studied using S IX T R A C K [38,67,68], using the setup and assumptions described in [59]. The local cleaning inefficiency is excellent, significantly less than 10- 5 , for all cold sections (shown in blue, while losses in warm areas are dis­

played in red). The cleaning simulations also include two IR7 dispersion-suppressor (DS) collimators set at 10 and 14a at injection, respectively, and to 18.1 and 22.2a at collision. The second DS collimator has a larger opening in order not to affect the momentum cleaning at IR3. As can be seen in Figure 2.28, the DS collimators inter­

cept practically all protons that otherwise would have been lost on the cold magnets in the DS. This is a highly promising result. Nevertheless, FL U K A simulations o f the full shower development are still needed to fully validate this design, i.e. to judge the risk o f quenches or o f any damage to the collimators themselves. In addition, further studies are needed to finalise the collimator settings outside IR7, in particular for the injection protection.