Introduction to the I-LHC and LEIR Projects
In addition to proton operation, the LHC machine will run a few
weeks per year with ions to provide collisions for heavy ion
experiments. Operation with Pb ions is part of the approved LHC
program. Collisions between Pb ions and protons and collisions of
lighter ions not yet
approved, but are likely to be included in future LHC upgrades.
The aim of the I-LHC project is to investigate all aspects to
be taken into account and all hardware additions necessary, in
order to allow Pb ion collisions in addition to proton operation.
Wherever possible, design choices should be taken with a view to
later upgrades which allow LHC operation with lighter ions.
With the ion acceleration scheme avaible at present for SPS fixed
target experiments, no Pb ion beam useful for LHC can be prepared.
Amongst the proposals for an LHC ion injector chain were (i) the
implementation of a Laser Ion Source (LIS), and (ii) an accumulator
ring with strong electron cooling. The latter has been approved and
is now part of the LHC scheme. An overview of the approved ion injector
scheme for LHC, with accumulation in LEIR, is shown in Fig. 3.
The design performance and beam parameters have been defined
taking into account the main limitations for LHC operation with Pb ions :
The LHC ion operation scheme is adjusted to above limitations, and takes
limitations along the injector chain into account as well.
The parameters for nominal ion operation are given in Table 1.
For the first ion runs, an special early operation scheme, with
significant simplifications along the whole LHC ion chain, is proposed and the
relevant parameter are given in Table 1 as well. The users
of LHC ion collisions are interested in an early ion run with lower luminosity,
but expect an increase in luminosity for later operation. Thus, early
LHC ion operation using the early opertaion scheme with later upgrades to
achieve design luminosity is a valuable approach.
- Electron Capture by Pair Production (ECPP) :
One of the possible interactions during the encounter
of two ions is the creation of an electron-positron pair.
The electron may be captured by one of the ions, which in
turn will be lost in the dispersion suppressor. The flux of of ions lost
is proportional to the luminosity in the given experiment
must remain below the threshold leading to quenches.
Thus, ECCP sets directly an upper limit to the luminosity
of about 0.5 1027 cm-2s-1
to 1.0 1027 cm-2s-1.
- Sensitivity of Position Pick-Ups :
The limited sensitivity of the Position Pick-ups requires a minimum of
4 107 Pb82+ ions per bunch for reliable
measurements. Note that
this intensity is relatively close to the design intensity per bunch.
Table 1: Design performance and parameter for LHC operation with Pb ions.
| Parameter || unit || nominal operation || early operation |
| Initial Luminosity || cm-2s-1 || 1027 || 5 x 1025 |
| Energy/nucleon || TeV/u || 2.76 || 2.76 |
| Number of bunches || || 592 || 60 |
| Bunch spacing || ns || 100 || |
| b* || m || 0.5 || 1.0 |
| Transverse normalized rms emittances || mm || 1.5 || 1.5 |
| Transverse rms beam size || mm || 16 || 16 |
| Luminosity half-life with 2/3 experiments || hrs || 4.7/3.1 || 9.4/6.2 |
The situation is summarized as well in Fig. 1, showing various options in a diagram
combining intensities per bunch and luminosities. The nominal scheme and the
early stage scheme for Pb ion operation of the LHC are compatible with LHC constraints.
In addition, points corresponding to the performance achievable with the actual ion
acceleration chain for SPS fixed target operation is plotted. It is evident that this
existing hardware cannot satisfy the needs for LHC Pb ion operation. The intensity
of a Pb ion beam delivered with the existing ion chain would be by far too
low for the LHC beam instrumentation and yield luminosities of no interest
for the users.
An Oxygen ion beam compatible with LHC constraints could be delivered and would
yield a high luminosity. However, such a scheme is of minor interest for the
users even for an early LHC ion run and, thus, has been ruled out.
Fig. 1 : LHC limitations and luminosities
The Pb ion intensities achievable with the existing ion accelerator
chain are far below the needs for LHC. Even with various improvements
along the chain (e.g. upgrade of the existing ECR source, new PSB injection
with stacking in the vertical phase space as well, recombination of
bunches in the PS), there is no hope to satisfy the needs for LHC.
Two schemes to provide the Pb ion beam for LHC have been tested
in extensive experimental investigations, namely a Laser Ion Source
(LIS), and accumulation with electron cooling in a low energy LEAR-like
synchrotron. Despite significant effort and investments, it has not been
possible to proof that a reliable injector chain for LHC ion operation
based on LIS is feasible. On the other hand, various measurements and a
proof of principle experiment performed at the LEAR machine
allow to extrapolate that, with accumulation, the needs for LHC Pb
ion operation can be satisfied. Thus, it has been decided to convert
the previous LEAR machine into LEIR, a dedicated ion accumulator ring
for LHC. In addition, various upgrades and modifications are necessary
along the whole accelerator chain for LHC ion operation.
Extensive experiments in view of ion accumulation have been performed
in the LEAR machine from 1994 to 1996. Those tests gave valuable
information and have influenced the final design of the LEIR machine.
For a proof of the principle, accumulation tests have been performed with a
modified LEAR machine in 1997. The electron cooler has been moved to another straight
section, allowing to better adjust the lattice parameters at the injection and
at the electron cooler. The time evolution of the intensity, accumulated with
an electron cooler current limited to only 105 mA due to technical problems
(instabilities) of the electron gun, is shown in Fig. 2.
After about 1.6 s (four Linac pulses), the time available for
accumulation in the future
LEIR machine, an intensity of a about 3.4 108 ions, i.e. about
one third of what is required for LHC, is accumulated. Thus, a factor 3 must be
gained in accumulation rate. Furthermore, one notices that the accumulated intensity
saturates quickly, because the beam life-time is reduced (from observing
the intensity decay between injections). This is caused by beam loss induced
outgassing mentioned above.
- Life-times in presence of electron cooling and choice of
the charge state :
During the first experiments, an unexpected short life-time
of Pb53+ (charge state initially envisaged for
ion accumulation) has been observed in presence of the cooling
electron beam. This is caused by a large cross section for capture
of an electron from the cooling electron beam. Thus, life-times
for neigbouring charge states, which can be delivered from the
Linac in similar quantities, have been measured as well and turned
out to be longer. Finally, it has been decided to accumulate
Pb54+ ions rather than Pb53+
ions as envisaged initially.
- Role of the dispersion and the betatron function :
In cooling down time versus electron cooler current measurements
with different lattices, it was found that (i) a finite dispersion
enhances cooling rates and that (ii) intermediate betatron
functions (around 5 m) yield fastest cooling. Both these observations
are contrary to initial expectations. The lattice
parameters at the electron cooler of the LEIR machine have been
defined taking these observations into account. The betatron functions
will be bH = bV = 5 m.
Since, due to the
injection process (with stacking in momentum as well), the momentum
spread of the injected beam will be large, the nominal dispersion at
the cooler is D = 0 m. However, with the LEIR geometry and hardware,
a small (negative) dispersion is possible.
- Beam loss induced vacuum degradation :
A dramatic reduction of the ion life-time has been observed during
accumulation tests. This could be traced back to a degradation of
the vacuum caused by desorption of molecules from the vacuum chamber
surface due to lost ions. Following these observations in LEAR,
systematic measurements of ion impact induced outgassing have been
done with a dedicated set-up installed at the end of the ion linac 3.
In addition, during these tests a "scrubbing effect", i.e. a
reduction in ion impact desorption rate after continuous bombardment
has been observed. For LEIR, one will rely on this "scrubbing effect" in
addition to a careful design of the vacuum system, aiming at dynamic
pressure of just a few 10-12 Torr.
Fig. 2 : Intensity versus time measured during Pb ion accumulation tests in LEAR.
The Linac 3 repetition time was 400 ms.
Measures to gain the factor 3 for the intensity accumulated
after 1.6 s needed for the LEIR project are :
The whole accelerator chain for Pb ion operation of the LHC is depicted in
Fig. 3 and Fig. 4.
- Increase of the current delivered by the Linac :
An upgrade of the ECR ion source is planned and an increase in current
by a factor 1.5 to 2 is expected. To be fully effective, the beam
emittances must not increase in order not to reduce the injection
efficiency into LEIR.
- Multiturn injection with stacking in vertical phase space as well :
During the accumulation tests, a multiturn injection with simultaneous
stacking in momentum in horizontal phase space has been applied. For
the LEIR project, stacking in vertical phase space as well is planned.
An increase of the intensity injected per multiturn injection by
an additional factor 2 to 3 is expected. However, the resulting larger
vertical emittance (compared to stacking in momentum and horizontal
phase only) may result in somewhat longer cooling down times.
- State-of-the-art electron cooler :
A new electron cooler is constructed for the LEIR project. Amongst
other improvements ("cooler" electron beam by expansion, a
better quality of the magnetic field in the solenoid...), the gun will
deliver higher stable electron currents than those used during the tests.
This decreases the cooling down times and, thus, more injections are
possible within the time available for accumulation.
Fig. 3 : Sketch of the accelerator chain for LHC Pb io operation.
Fig. 4. : Longitudinal structure of the Pb beam along the accelerator chain for
LHC ion operation.
The main stages, necessary to provide the nominal LHC Pb ion beam, along the chain are :
- Linac 3 :
Production of the beam in an upgraded ECR ion source. After a first
spectrometer, the Pb27+ beam with an intensity of
200 mA Pb27+ is
bunched and accelerated in a RFQ. The beam is further accelerated in 3
RF tanks to 4.2 MeV/u. At the end of the Linac, the beam is stripped
and the desired charge state Pb54+ is selected in a filter line.
For the special LEIR multiturn injection, energy ramping, i.e. change
of the energy during the Linac pulse is necessary. This will be done
with a new cavity to be installed and the already existing debuncher
(Note that, for the accumulation tests in LEIR, energy ramping has
been provided with the debuncher only leading to a (slightly) larger momentum
- LEIR :
In LEIR, 4 to 5 Linac pulses will be accumulated in order to build up
the necessary intensity. Ingredients for short accumulation
times are multiturn injection with stacking in all three (momentum, horizontal
and vertical) phase spaces and strong electron cooling.
After the accumulation at 4.2 MeV/u
the beam will be bunched on harmonic number 2, and accelerated to 72 MeV/nucleon.
Finally, the two bunches, each one containing 4.5 108 ions, will
be transferred to the PS. The beam injected into and extracted from
LEIR passes through a common transfer line, leading to some constraints.
- PS :
Amongst acceleration, complicated RF gymnastics (numerous changes of the
harmonic number and bunch splittings) are necessary to provide
the bunch pattern needed for transfer to the SPS. The bunch spacing necessary
for LHC must be generated already in the PS. In addition every LHC bunch
is split into a bunchlet pair distant by 5 ns.
At every LEIR/PS cycle 4 bunchlet pairs, each one corresponding to one
LHC bunch, are transferred towards the SPS. In the transfer line, the beam
passes through an upgraded (minimizing emittance blow-up due to multiple
Coulomb scattering) stripping insertion in order to fully ionize
the Pb ions.
On a long injection plateau, up to 13 LEIR/PS batches are accumulated.
The space charge tune shift at SPS injection, which is expected to be
a limitation, is reduced by transferring every LHC bunch split into a
bunchlet pair. This, in turn, implies that bunchlet pairs must be
recombined before extraction to build LHC bunches.
- LHC :
Finally 12 SPS batches, each one containing up to 48 bunches, are injected in
one LHC ring. After about 20 minutes to fill both LHC rings, the
beams will be accelerated and brought into collision.