CERN
EDMS NO.
CH-1211 Geneva 23 Switzerland
0000000
Date : 2012-11-20
Technical note
The ion injection chain in the HL-LHC era.
D.Manglunki / BE-OP
Abstract
To reach a luminosity higher than 5×1027 Hz/cm2 for Pb-Pb collisions during the HL-LHC era,
several upgrades will have to be performed in the injector chain.
1. Executive summary
To reach the performance expected by the ALICE experiment after its upgrade during the 2nd
long LHC shutdown [1], we recommend the following set of measures in the ion injector chain:
●
Study the transmission through the RFQ
●
Increase Linac3 repetition rate to 10Hz
●
Modify Linac 3 for multiple charge acceleration
●
Implement a batch compression scheme for ions in the PS.
●
Implement a 50ns rise time injection scheme for ions into the SPS
●
Upgrade the SPS LLRF to minimize RF noise on the injection flat bottom
●
Modify equipment to allow for more injections in the SPS.
By themselves, and assuming the nominal * of 0.5 m in the ALICE IP, these measures can
bring the peak luminosity with ions to the level of 5×1027 cm-2s-1. Depending upon the success
in increasing the intensity in LEIR (subject of study in the context of the Ion injector upgrade),
the peak luminosity may potentially be higher.
A detailed cost to completion is included at the end of this document.
2. Introduction
Among the potential solutions for upgrading the injectors, the following scheme is derived from
a proposal presented at the 2012 LHC performance workshop in Chamonix (Fig.5 in ref. [2]).
Based on an alternating 50 ns/100 ns bunch spacing in the LHC, it assumes that the number of
injections in the SPS can be increased to 24, which would increase the number of bunches
circulating in LHC up to 912 and bring the peak luminosity up to 5×1027 cm-2s-1. It uses the
following principles, with the evolution of the bunches sketched in Fig.1:
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Production of two bunches in LEIR.
●
Batch compression in the PS, yielding two bunches of 3×108 ions, spaced by
100 ns.
●
24 transfers of such batches to the SPS, with a batch spacing of 50 ns, giving a
train of 48 bunches, alternately spaced by 50 ns and 100 ns.
●
2×19 transfers of the SPS trains to the LHC, giving 912 bunches per LHC ring.
Figure 1: 100/50 ns spacing scheme for HL-LHC, with 24 PS injections into the SPS.
The following paragraphs list the upgrades necessary in the various machines of the ion
injector chain to achieve this result. In addition, all improvements to the intensity and/or
emittance of the upstream parts of the chain translate to increases of the luminosity of the
collider. The modifications required in the LHC itself to cope and benefit from this improved
injected beam [3], and to mitigate for the twice longer injection time, are not addressed in this
paper.
The present limitations in LEIR are not yet understood enough to allow proposing means to
increase the intensity and brightness of the bunches delivered to the PS.
3. LINAC3
As it has been found recently, the transmission through the RFQ is somewhat lower than
expected (less than 50% vs over 80%) as soon as the current from the source exceeds
100 eA. A programme of investigations and simulations has been devised to try and
understand, and if possible cure, this unexpected loss [4].
This possibility aside, increasing the Linac3 repetition rate, i.e from 5 Hz to 10 Hz is the easiest
feasible means to increase the number of ions delivered per second. Most Linac3 components
are already compatible with a 10 Hz repetition rate, but not the transfer lines between Linac3
and LEIR. A study has been launched to estimate their modifications [5, 6]. In total, 11
magnets have to be upgraded, as well as a fraction of their power supplies. As their
replacement is already part of a consolidation programme, a repetition rate of 10 Hz will be
included in their specifications. Note that the behaviour of the stripping foils under 10 Hz
operation will have to be assessed.
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As a complementary measure, the simultaneous acceleration of multiple charge states in the
Linac is a promising option which has already been proposed during the design phase of the ILHC project [7, 8]. Machine developments have taken place several years ago [9], but the
whole study needs to be re-activated, both on paper and with beam.
4. LEIR
LEIR currently experiences a limitation of about 5×10 10 charges of Pb54+ at the beginning of
the accelerating ramp [10]. Offline studies and simulations, as well as time with beam, will be
needed to diagnose and if possible cure or mitigate it. To this effect, the transverse emittance
measurement system has to become operational from the CCC.
As a result of this study, the LEIR electron cooler may need upgrading.
5. PS
The batch compression gymnastics h=16-18-21 has been tested using the existing Low Level
equipment [11]. For the sake of optimization, some Low Level RF development is nevertheless
planned. Machine development time will be necessary to set it up and anticipate problems
6. SPS
The design of an injection system capable of a rise time inferior to 50 ns has started [12, 13].
The transverse blow-up and beam loss currently experienced on the injection flat bottom can
be mitigated by two solutions, currently being implemented:
 Using a lower tune (“Q20”) to fight the space charge detuning as well as the IntraBeam Scattering [14].
 Switching the low level RF to a fixed harmonic system on the flat bottom to fight the
RF noise [15].
Both techniques have already shown encouraging results during machine development periods,
and will be experimented at least partially in operation during the coming p-Pb run.
With a 50 ns rise-time injection system and provided that blow-up and beam loss issues on the
flat bottom are solved, the accumulation of 24 PS ion batches in the SPS as proposed in Fig. 1,
will be feasible. The flat bottom will then be twice longer than presently, which will require
modifying some equipment:
 The current ROCS function generators surveillance system which does not allow to
extract beam more than 48 seconds after the start of the supercycle.
 The number of injections is currently limited to 15. This number came from the
original LHC design report, and made sure the proposed schemes were possible.
These latter two improvements are part of the SPS hardware modifications which will take
place during LS1.
7. Conclusion
Taking for granted a scheme where 24 PS batches are injected into the SPS, factors up to 2.5
can be expected in the number of bunches circulating in the LHC (Fig. 2). Even a partial
implementation of the upgrades which are presented in table 1 below, can significantly
increase the peak luminosity of the collider.
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3.0
2.5
200ns Intermediate beam
(performance of 2011)
2.0
Proposed 100/50 ns
beam
1.5
1.0
Nominal (as in design
report)
0.5
0.0
0
50
100
150
200
250
Fig. 2: Summary of possible luminosity performance increase with 24 injections, and a bunch
spacing of 100 ns within the PS batch. The vertical axis is (nB×IB2) scaled to the performance
obtained in 2011 with 12 injections, the horizontal axis is the SPS injection kicker rise
time (ns). The nominal design values and 2011 performance are plotted for reference.
Topic
Source & Linac
Multiple charge acceleration
RFQ transmission
Operation at 10Hz
LEIR
Transverse emittance
measurements from CCC
Beam studies and simulations
Delivery
Resources
LS2
LS1
LS2
10 man-months (BE/ABP)
8.5 man-months (BE/ABP)
400kCHF (TE/EPC, TE/MSC)
LS1
6 man-months (BE/OP)
2016
Electron cooling upgrade – if
needed
PS
RF gymnastics for batch
compression (h=16-18-21)
SPS
Fixed harmonic on flat bottom
Q20 for ions
LS2
12 man-years [4 staff, 4 fellow,
4 PhD student](BE/ABP)
(BE/BI)
50 ns rise time injection
LS2
Suppress limit on cycle length
Higher number of injections
LS1
LS1
November (BE/RF)
2012
6 man-month (BE/ABP)
LS1
Jan 2013
(BE/RF)
1 man year [staff]
2 man-years [fellow]
2 man-years [PhD] (BE/ABP)
8.1MCHF, 19.3 man-years
(TE/ABT)
(BE/CO, TE/EPC)
(BE/RF, BE/CO)
Table 1: Summary of recommended upgrades, expected delivery times, and needed additional
resources [4, 5, 6, 13].
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References
[1] J. P. Wessels, "Will ALICE run in the HL-LHC era? " , Proc. Chamonix 2012 Workshop
on LHC Performance, Chamonix, France, 6 - 10 Feb 2012
[2] D.Manglunki, "Plans for ions in the injector complex", Proc. Chamonix 2012 Workshop
on LHC Performance, Chamonix, France, 6 - 10 Feb 2012
[3] J.M.Jowett, "Heavy Ions in 2012 and the Programme up to 2022", Proc. Chamonix
2012 Workshop on LHC Performance, Chamonix, France, 6 - 10 Feb 2012
[4] G. Arduini, C. Carli, D. Kuchler, R. Scrivens « Activities and resources required to study
how to deliver the Lead Ion beams required for a Lead Ion Luminosity Upgrade »,
EDMS xxxxxx.
[5] D.Bodart, “Preliminary magnet review of the Linac 3 Ion Beam Line and LEIR for the 10
Hz injection upgrade“, Private communication.
[6] J.M.Cravero, « Linac III transfer line @10Hz », private communication, 6/7/2012
[7] LHC design report, Vol III, part 4, chapter 34, “The LHC Ion Injector Chain, Source and
Linac3 ”
[8] V.Coco et al., “Acceleration of several charge states of lead ion in CERN Linac3”, in
Proc. Linac’2004, Lübeck, Germany, 16-20 August 2004.
[9] M.O’Neil & R.Scrivens, “Linac 3 MD – Multiple Charge Acceleration“, Private
communication, 16 Jan 2001.
[10] D.Manglunki et al, "Performance of the CERN Heavy Ion production complex", 3rd
International Particle Accelerator Conference, IPAC12 May 20-25, 2012, New Orleans,
USA. CERN-ATS-2012-104
[11] S.Hancock, H.Damerau, Private communication.
[12] D.Manglunki, "Required parameters for the SPS ion injection system in the HL-LHC
era", CERN-ATS-Note-2012-077 (TECH)
[13] B.Goddard et al.“New fast ion inflector for SPS”, EDMS xxxxxx
[14] Y. Papaphilippou et al, “LHC ion beam at the SPS with Q20 optics ”, Presented at the
LIU-SPS-BD working Group, 31 May 2012.
[15] T.Bohl, Private communication.