PERFORMANCE OF THE LHC PRE-INJECTORS
M. Benedikt, R. Cappi, M. Chanel, R. Garoby, M. Giovannozzi, S. Hancock, M. Martini, E. Metral,
G. Metral, K. Schindl, J.-L. Vallet, CERN, Geneva, Switzerland.

Abstract
The LHC pre-injector complex, comprising Linac 2, thePS Booster (PSB) and the PS, has undergone a major
upgrade in order to meet the very stringent requirementsof the LHC. Whereas bunches with the nominal spacing
and transverse beam brightness were already available
from the PS in 1999 [1], their length proved to be outside
tolerance due to a debunching procedure plagued by
microwave instabilities. An alternative scenario was thenproposed, based on a series of bunch-splitting steps in the
PS. The entire process has recently been implemented
successfully, and beams whose longitudinal characteristics
are safely inside LHC specifications are now routinely
available. Variants of the method also enable bunchtrains with gaps of different lengths to be generated.
These are of interest for the study and possible cure ofelectron cloud effects in both the SPS and LHC. The
paper summarizes the beam dynamics issues that had tobe addressed to produce beams with all the requisite
qualities for the LHC.

1 THE LHC PROTON INJECTOR CHAIN
1.1 Parameters of the LHC proton beams
For LHC collider operation, three different proton
beams are required: (i) the “initial” or “commissioning”
beam, permitting LHC physics during the first two years
at a luminosity of 1033 cm-2s-1; (ii) the “nominal” beam for
operating the LHC at 1034 cm-2s-1; (iii) the “ultimate”
beam, which is the foreseeable LHC performance limit at
2.5x1034 cm-2s-1.
Table 1: LHC proton injector chain, nominal parameters
PSB(4 rings)

PS

SPS

LHC
7000

kinet. energy [GeV]

1.4

25

450

repetition time [s]

1.2

3.6

21.6

nb. of pulses to filldownstr. machine2(3 rings)

3 or 4bunches/ring

1

6/18/36/72

216/2882808

RF harmonic nb.

1, 2

7/21/42/84

4620

35640

protons/pulse

4x10128x10123.2x1013

3.1x1014

12

11

11

1.1x1011

protons/ bunch
ε*rms [µm]1.3x101.1x102.5

3.0

εL (2σ)/bunch [eVs]1.5

4σ bunch length ns~170

Luminosity [cm-2s-1]

12 ( for one
LHC ring)

1.1x10
3.5

3.75

0.35

1

2.5

4

1.7

1
1x1034This paper deals with the nominal beam whose main
parameters [2] along the LHC injector chain are compiledin Table 1. At PS extraction, the nominal beam features(i) a small transverse emittance of ε*rms =3.0 µm(normalized); (ii) an intensity of Nb = 1.1 1011 p/bunch;(iii) 72 bunches with a spacing of 25 ns. The ratio
Nb/ε*rms, the beam brightness, is about 1.5 times higher
than obtained previously and implies heavy space charge
at PSB and PS injection.

1.2 Space charge in PSB and PS
CERN’s 50 MeV proton Linac 2 has been upgradedand can now provide pulses of 180 mA during >100 µs.
This beam is injected into each of 3 PSB rings bybetatron stacking, yielding a beam with an incoherent
space-charge tune-spread ∆Qy ~ 0.6 after RF capture on
h=1, when using an h=2 cavity for bunch flattening.
Under these conditions, the emittances cannot be kept
within the tight specifications; the way out is to fill the PS
with two consecutive batches, thus halving the beam
intensity per pulse (by stacking only 3 turns per PSB ring)and reducing ∆Q to a more manageable ~0.3.
The double-batch filling scheme exacerbates the impactof space charge in the PS as the first injected batch,
occupying about half the PS circumference, dwells at low
energy for 1.2 s until the arrival of the second PSB batch.
In this time, space charge effects lead to emittance blowup, which was successfully eliminated by raising the PSBPS transfer energy from 1 to 1.4 GeV. This reduces ∆Q in
the PS and prevents the beam from straddling low-ordernon-linear resonances.

1.3 Providing the LHC bunch spacing of 25 ns
The 25 ns bunch spacing is generated in the PS justbefore the beam is sent to the SPS. The bunches must be~4 ns long to fit the SPS 200 MHz RF system. Originally,
this was achieved by debunching, then rebunching thebeam on h=84, followed by bunch rotation (that is, thebunches are ejected when they are shortest). For this, thePS was equipped with 40 MHz (h=84) and 80 MHz RF
systems. However, when trying the scheme, longitudinalmicrowave instabilities, generated at nominal intensity by
the longitudinal impedance of the PS, blew up the
momentum spread during the delicate debunchingprocess: there was no way to make the bunches shorter
than 5 ns.

Rather than embarking on a costly impedance reduction
programme, the scheme to produce the LHC beam wasradically changed. It is now based on the recently
invented longitudinal splitting of one bunch into three(triple splitting [3]) and also on the more familiar splittingof one bunch into two (double splitting). In this scheme, 6
PSB bunches (2 batches of 3 bunches, i.e. only using 3 ofthe 4 PSB rings) are injected into 6 out of 7 PS buckets,
thus leaving a void for the PS extraction kicker. The 6
bunches in the 7 buckets are split into 18 by the new
method, then accelerated to 25 GeV, split into 36 andfinally into 72, so that 12 of the final 84 buckets are
empty (Fig. 1).
PS ejection:
72 bunchesin 1 turn320 ns beam gap

72 bunches
on h=84

3 RESULTS OF RECENT BEAM TESTS
Quadruple splitting
at 25 GeV

3.1 PS Booster [5],[6]Acceleration
to 25 GeV

18 bunches
on h=21

Empty
bucketTriple splitting
at 1.4 GeV

PS injection:
3+3 bunchesin 2 batches

thus refurbishing the main magnet supply and replacing
most of the PSB-PS transfer line magnets and power
converters. At 1.4 GeV (field level ~0.86 T), the PSB top
and bottom rings have a weaker magnetic field (by ~1%)than the inner ones due to unequal saturation. A trim
supply now equalizes the dipole field in each gap. The 40
MHz and 80 MHz RF systems in the PS are required togenerate the bunch spacing and shorten the bunches.
Improved diagnostic devices, such as fast wire-scanners,
enable transverse beam characteristics of the smallemittance LHC beam to be measured. Finally, a prototype
20 MHz cavity was recently installed in the PS to provide
the intermediate bunch harmonic 42 and to allow the newscheme for producing the LHC beam in the pre-injector to
be tested.

6 buncheson h=7The space-charge detuning is largest (~0.3) at 50 MeV
just after RF capture on harmonic 1. Although all secondand third-order stopbands are narrowed by correctionlenses, they still increase the emittances. The time during
which these resonances are straddled is minimized byappropriate programming of the betatron tunes during thecycle (Fig. 3).

Fig. 1: Multiple splitting scheme for the LHC beam.
“Quadruple”splitting means two steps of double splitting.

2 HARDWARE UPGRADES
Major hardware upgrades in the PS Complex [4] areshown in Fig. 2. One quarter of the equipment, indicatedin italics, is a contribution of Canada (via TRIUMF).

Fig. 2: Major hardware upgrades in the PS complex.
With one bunch per PSB ring (h=1 RF systems),
double-batch injection into the PS becomes feasible byappropriate phasing of the bunches in the 3 rings. The
PSB energy increase implied 26% higher field levels andFig. 3: PS Booster tune diagram with stopbands, timevarying tunes Q(t), and space-charge tune-spread at
injection and ejection energies. N=1.4x1012 protons/ring,bunching factor 0.55, eps-x,y normalised rms emittances.
Owing to the horizontal 3-turn betatron stacking, εx > εyafter injection, but are equalized by deliberate beam“shaving” during acceleration. Before extraction towards

the PS, the bunches undergo controlled longitudinal blowup so as to lengthen them and thus further decrease thespace charge detuning in the PS. However, this process is
not yet operationally reliable. Beam parameters measured
at PSB ejection are compiled in Table 2.

critical and even larger-emittance and thus longer bunches(190 ns) from the PSB are preferable. The first and mostdelicate step in the process, bunch triple-splitting at 1.4
GeV, is shown in Fig. 5.

Table 2: Nominal LHC beam, parameters at PSB
extraction with one bunch per ring, 3 rings used.
Achieved
Goal
protons/ring
1.4x1012
1.32x1012
εx* (=σx2 βγ/βTWISS) [µm]2.22.5εy* (=σy2 βγ/βTWISS) [µm]1.82.52-σ long. emittance εl [eVs]0.91
1.5
4-σ bunch length τb [ns]1501
190
2-σ momentum spread ∆p/p
2x10-3 1)2.45x10-3
1
without controlled blow-up

Fig. 5: Bunch triple-splitting of 6 PSB bunches into 18 in
the PS at 1.4 GeV. The process takes about 40 ms.
The longitudinal beam structure after shortening of the 72
bunches at the PS ejection flat top is shown in Fig. 6.

Fig. 4: Intensity of the PS ring (in units of 1010 protons)
vs. time (in ms).

3.2 PS [5],[6]Three PSB bunches are injected into the PS, with the
RF at h=7, by bunch-to-bucket transfer. They dwell 1.2
seconds on the PS injection plateau until the arrival of thesecond batch of 3 bunches and are then subjected toseveral bunch splitting steps, as depicted in Fig. 1. With aworking point of (Qx, Qy) = (6.22, 6.25), which justaccommodates the space-charge detuning of ~0.20, thebunches do not suffer much transverse emittance increase.
However, the long bunches are prone to a horizontal highorder (m=6) head-tail mode generated by the resistive
wall impedance. This instability is tackled by linear
coupling between the horizontal (unstable) and (stable)
vertical motion [7]. The chromaticities are set to change
sign at transition energy so the beam is kept stablethroughout the cycle. Fig. 4 depicts the beam intensity
versus time in the PS machine, from injection of the firstbatch to extraction.
The RF gymnastics in the PS to transform 6 bunches ofsome 150 ns length into 72 bunches of less than 4 ns
length follow the procedure outlined in Fig. 1 [8]. Despite
the many steps which do not strictly conserve thelongitudinal emittance, the “budget” in the PS is not

Fig. 6: Twelve out of 72 bunches on the last turn of the PS(top, 30 ns/div); zoom on one bunch (1 ns/div).
Transverse emittance measurements reveal that thenormalized emittance does not suffer significant blow-up
between PSB and PS ejection, and that it stays
comfortably within the allocated emittance budget (Fig.
7). This is what had already been obtained by end 1999.
The characteristic parameters of the LHC beam, asdelivered by the PS using the new method, are compiledin Table 3. A comparison with the design figures showsthat, indeed, the PS complex can now deliver the nominal
beam.
There are indications that an electron cloud builds up
during the last turns in the PS (when the bunches, spacedby 25 ns, get compressed to 4 ns) and also in the transfer

line TT2 towards the SPS (single-pass). The effect,
predicted by numerical simulations [6], appears at thetrailing end of the bunch train as a baseline distortion in
the TT2 electrostatic pick-up. It can be eliminated by aweak solenoidal field (Fig. 8). Apparently, the beam is notaffected by the electron cloud.
Table 3: LHC proton beam parameters at PS extraction
Achieved
Nominalprotons/bunch
1.1x1011
1.1x1011*2
εx (=σx βγ/βTWISS) [µm]2.53εy* (=σy2 βγ/βTWISS) [µm]2.532-σ long. emittance εl [eVs]
0.35
0.35
4-σ bunch length τb [ns]3.6
42-σ momentum spread ∆p/p
2.4x10-3
2.2x10-3
3.5

BAD

3
2.5

3.4 Alternative types of bunch trains
Ever since the PS first sent trains of short bunches with
25 ns spacing to the SPS, the latter has suffered fromstrong transverse instabilities, increasing towards the endof the bunch train, due to electron clouds whose
appearance seems to be favoured by the particularfeatures of the bunch train. Electron clouds are expected
to be a major issue in the LHC as well. The new way togenerate the LHC beam in the PS has a welcome spin-off:the possibility to provide bunch trains different from thenominal ones to study the dynamics of the electron cloud
build-up as a function of train parameters. Alternative
bunch trains featuring• holes of 12 (24, 36,..) bunches by omitting 1 (2,3,…)
PSB rings, or• 50 ns spacing, or• 75 ns spacing
and others are feasible, under study, or envisaged [8].

2
1.5

H54
H64
V75
V85

GOOD

1
0.5
0
0

500

1000

1500

2000

2500

Time (ms)1st batch injection

PSB output(SEM-grids inmeas. line)

2nd batch
injection

1.4-25GeV
acceleration PS output(SEM-wires
in TT2)

Fig. 7: Evolution of the normalised rms emittances from
PSB exit to PS extraction (H=horizontal, V=vertical).
Measurements in TT2 were done without bunch
compression.

3.3 PS-SPS matching
A well-matched transfer between the PS and SPS (TT2and TT10) is imperative to comply with the extremely
tight emittance budget. After systematic and extensive
studies, the optics model now fits the measurements whichwere performed with Secondary Emission Grids in TT2
and Optical Transition Radiation (OTR) monitors(allowing visualisation of an x-y image of the beam) inTT10. As a result, there is now (i) negligible horizontal
and vertical betatron mismatch; (ii) no dispersion
mismatch; but (iii) horizontal-vertical coupling (visible on
OTR screens as a tilt of the beam ellipse), very likely
generated in the PS machine. There is concern about nonlinear effects due to the fringe field at PS ejection together
with the large momentum spread of the proton beam(±2.4x10-3). This is a potential source of non-linear
emittance blow-up at SPS injection and needs further
studies.

Fig. 8: Baseline drift of the signal from an electrostatic
pick-up in the PS-SPS transfer line (TT2) due to electron
cloud build-up: (top) solenoid off, (bottom) solenoid (~50
G) on. On each oscillogram, from top to bottom: Sumsignal, ∆x, ∆y. The beam features 72 bunches, spacing 25
ns, length 4 ns.

4 FUTURE WORKThe successful production of the LHC nominal beam
does not mean that the preparation of the PS complex as
LHC pre-injector is complete. The following major issues
have still to be dealt with:• Two 20 MHz, 15 kV cavities are required in the PS
to provide harmonic 42 in the bunch splitting
procedure. It is proposed that they also be tuneable to13.3 MHz, which would enable a bunch spacing of
75 ns.

••••New transverse dampers in the PS, with a bandwidthof 20 MHz, to correct injection oscillations and tametransverse instabilities.
The excessive variation of bunch population (up to ±20%) will have to be reduced to ± 10% which at firstsight appears tolerable for LHC.
The study of ways to produce the “initial” beam, at~1/6 of the nominal intensity in ~1/4 of the transverse
emittance. While its transverse density is smaller than
nominal, the very tight emittance budget is a big
challenge.
There are doubts whether the “ultimate” beam (1.6times the nominal intensity in the nominal emittance,
defined as the LHC performance limit) is feasible
with the new scheme, which, compared to the oldscheme, requires 15% more intensity per bunch in the
PSB and at PS injection. Fortunately, there is nourgency.

CONCLUSIONS
The main progress with respect to earlier results with
LHC-type beams has been the implementation of the new
production scheme based on the recent invention of triple
bunch-splitting. The process finally produces bunches of~ 4 ns length which had not proved feasible with theoriginal scheme based on a debunching-rebunchingprocess, due to microwave instabilities in the PS. In thisway, a nominal proton beam comfortably satisfying LHC
requirements was achieved for the first time and isavailable for studies in the SPS. As a fringe benefit, thenew pre-injector scheme enables the generation of avariety of alternative bunch trains and spacings which
may prove invaluable to investigate electron cloud effects
in the SPS. While progress was also very satisfactory on
beam dynamics issues, such as matching between themachines, some hardware has still to be installed andfuture machine studies will deal with the other types of
LHC proton beams.

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