M. Benedikt
DRAFT
06/03/2016
General considerations and parameters for the normal conducting PS2
1. Introduction:
The PS2 is considered as a medium/long term option to replace the existing PS
machine. It is a normal conducting synchrotron with an injection energy of 3.5 GeV
and a maximum extraction energy of around 50 GeV, significantly higher than the
present PS that delivers 25 GeV protons. The new machine has to be integrated into
the existing CERN accelerator complex.
At a first stage, the proton injector chain will consist of Linac4, PS-Booster and the
existing PS, and will provide protons at around 3.5 GeV. (Alternatively, the beam
could also be injected directly from the PS Booster at 1.4 GeV but with lower
performance due to increased space charge tune spread at the lower energy.) Later on,
following a PS-Booster replacement based on either the SPL or an RCS, the injection
energy will be around 3.5 GeV.
The ion injector chain consists of Linac3, LEIR and the existing PS. Alternatively,
ions could be injected into the PS2 directly from LEIR at a rigidity, equivalent to the
one of 1.25 GeV protons (6.6 Tm).
For the present purpose of a first-order cost and performance analysis, a simple
machine model based on a pure FODO lattice structure is used. Important design
questions like e.g. avoiding transition crossing have not been considered. Possible
improvements in beam manipulations (bunch splittings etc.) were also not analyzed.
Beam handling was assumed to be similar to what is presently being done in the PS.
From this assumption the corresponding hardware requirements (e.g. RF systems,
extraction systems, etc) were derived.
2. PS2 performance estimation:
The performance of the PS2 is estimated using the present performance of the
PS complex, the improvements expected from Linac4 and the length of the new
machine as input parameters.
Linac 4 is expected to double the intensity (brightness) of all PSB beams at
unchanged transverse emittances, resulting in a factor 2 performance increase
compared to present. The PS will accelerate the beams and inject into the PS2 at
3.5 GeV1. Assuming that the new machine is twice longer than the PS, double batch
filling from PS to PS2 can be applied.
For CNGS type beams, the resulting total intensity in the PS2 will be four times
higher than presently obtained in the PS (1.2E14 instead of 3E13). The average line
density in the PS2 will be twice the present PS value. Assuming a five-turn extraction
towards the SPS, the PS2 can fill the complete SPS in a single pulse with twice the
present line density.
1
The question whether the PS can handle these beams has still to be studied. Space charge effects at
injection will be twice higher than at present but transition crossing is no longer needed since the
beams are already extracted below transition energy.
M. Benedikt
DRAFT
06/03/2016
The LHC beam could be produced with a new scheme once Linac4 is available [RG].
All four PSB rings will be used and before extraction the beams will be triple split to
h=3 (asymmetric) to fill 12 buckets of the PS that accelerates on harmonics 142. The
12 bunches are then sent to the PS2 that is waiting on h=28 and after a second PS
batch, 24 buckets are filled. The PS2 accelerates on h=28 to top energy. Each bunch is
then split in six (triple and double splitting), resulting in 144 bunches on h=168 before
extraction to the SPS.
Using this scheme, 18 LHC bunches are produced from one Booster ring, compared to
12 LHC bunches at present. Taking into account the factor 2 intensity increase from
Linac4, the LHC beam brightness (or bunch intensity) will increase by a factor 1.5.
Filling of the SPS will take only two PS2 batches instead of four PS batches.
Alternatively, the present scheme with double batch injection from PSB to PS could
be used and would result in a factor 2 brightness increase.
A general performance estimate for the PS2 can be based on space charge
considerations. Compared to present, space charge effects will be a factor 2 higher in
the PS due to the intensity increase, originating from the Linac4 upgrade that is
expected to double the bunch intensity while leaving all other transverse and
longitudinal parameters unchanged. Space charge effects at a given energy for a given
bunch will be twice bigger in the new PS2 than in the PS because of the twice larger
circumference. Summing up, the PS2 will experience typically four times larger space
charge effects than presently observed in the PS at a given energy. This increase will
be counteracted because of the higher injection energy of 3.5 GeV in the PS2. The
ratio in *2 between 1.4 GeV and 3.5 GeV is 0.26, almost fully compensating the
factor 4. Therefore the PS2 with injection at 3.5 GeV, will be able to handle beams
with at least twice the line density and four times more total intensity than presently
available in the PS.
3. Geometry and main magnets
The PS2 will be a separated function machine in contrast to the PS. The dipole field at
maximum energy of 50 GeV is set to 1.8 T resulting in a total bending length of
nearly 600 m. Applying the rule of thumb that half the circumference will be filled
with the main dipoles, the total machine length will be around 1200 m, roughly twice
the length of the present PS.
Main magnets
There will be 200 H-type dipole magnets of 3 m length each. Since the sagitta is only
12 mm, the magnets can be built as straight units with rectangular edge focusing. The
gap height will be 10 cm and the pole width around 25 cm.
The issue of injection energy during the first stage of operation is not only important
for the machine performance (see above) but also most relevant for the magnet
design. Since the beam size decreases by a factor 2 from 1.4 GeV to 3.5 GeV, the
dipole gap height could be reduced to 8 cm when definitely excluding high intensity
proton injection at 1.4 GeV.
2
The reduced bunch spacing will require upgrade of the PS Booster ejection and recombination kickers
to reduce the rise-times.
M. Benedikt
DRAFT
06/03/2016
Assuming a regular FODO type lattice with two dipoles between the quadrupoles in
the arc, the typical cell length will be around 20 m. Therefore the machine will consist
of around 60 FODO cells or 120 quadrupole magnets. The quadrupole magnets will
be 1.75 m long. The gradients at maximum energy should not exceed 16 T/m
translating into pole-tip fields of 1.2 T for 15 cm bore diameter. Again the bore
diameter could be reduced if proton operation at 1.4 GeV is excluded.
Auxiliary magnets
A classical orbit correction scheme, where each FODO cell is equipped with a
horizontal and a vertical correction magnet, would require a total of about 60 h/v
dipole orbit correctors. In case horizontal and vertical correctors are separated,
120 units will be needed. The required integrated field strength is around 0.05 Tm.
Four skew quadrupole families (4 magnets each) will be needed for compensation of
coupling and the nearest sum resonance (PS: 20 magnets).
One quadrupole family will be needed for controlling the tune during the slow
resonant extraction (4 magnets) (PS: 2 magnets).
This gives a total of 20 auxiliary quadrupoles grouped in 5 families.
Two sextupole families are required to control the chromaticities (8 magnets each).
Another two sextupole families will be used for controlling the slow resonant
extraction and the island extraction (2 magnets each) (PS: 2 & 2 air core magnets).
Additional sextupoles may be needed for compensation of resonances.
This gives a total of 20 sextupoles grouped in 4 families.
Two octupole families will be needed for the island extraction (2 magnets each)
(PS: 4 magnets).
Eventually a third octupole family will be installed to counteract instabilities via
transverse Landau damping (4 magnets) (PS: 10 magnets).
This gives a total of 8 octupoles grouped in 3 families.
4. Power converters
Main power converters
All main dipoles (200 magnets) will be powered in series. The typical requirements
are maximum current 7 kA, maximum total voltage 10 kV, rms power dissipation
12.5 MW.
The main quadrupole magnets will be arranged in (estimated) four families, powered
by separate converters. It is assumed that there will be two families with 50 magnets
each, covering the arc cells, and two families with 10 magnets each, covering the long
straight section cells. The typical requirements are maximum current 1.5 kA,
maximum total voltage 7 kV, rms power dissipation 3.0 MW for 50 magnets and
maximum current 1.5 kA, maximum total voltage 1.5 kV, rms power dissipation
0.6 MW for 10 magnets.
M. Benedikt
DRAFT
06/03/2016
Auxiliary magnets power converters
The orbit correction system will be based on 120 magnets, powered by individual
power converters. Typical voltage and current requirements are 50V/20A, with a
maximum dissipated power per magnet of 150 W.
The remaining auxiliary magnets (quadrupoles, sextupoles and octupoles) will be
grouped in 12 families, each consisting of 2 to 8 individual magnet and powered by
single converter.
5. Beam transfer insertions
The PS2 will have four main insertions, two dedicated to injection and two dedicated
to extraction.
Injection:
A classical single-turn type injection system (orbit bump, septum, fast kicker) with
variable kick length is needed from the very beginning, when the PS2 will operate
with the present injectors. The system will have to cover the energy range from
1.25 GeV (proton equivalent for ion injection from LEIR) to 3.5 GeV, i.e. a maximum
magnetic rigidity of B = 14.5 Tm.
For the fast kicker the required pulse length is up to 2.5 s, with rise and fall time
of ≤100 ns. The maximum vertical beam size (3) at injection will be ±25 mm.
At a later stage, an H- charge exchange injection system will be needed if the PSB is
replaced with an SPL type machine and not an RCS. This system will consist of an
injection septum and a short special dipole housing the stripping foil. Fast orbit bumps
(horizontal and vertical) will be needed to allow for so-called phase space painting
during the injection process (≤100 turns i.e. ≤500 s). The vertical orbit bump could
be replaced by fast steering of the vertical injection trajectory. Dumps fort the
partially or unstripped H0 and H- beams are needed. The injection energy will be
3.5 GeV, corresponding to a magnetic rigidity of B = 14.5 Tm.
The classical single-turn injection system will still be needed for ion operation.
Extraction:
A classical fast extraction system (orbit bump, septum, fast kicker) with variable kick
length is needed as principal extraction system. The system should be combined with
an island (or CT) extraction system allowing for four- or five-turn extraction towards
the SPS. This will require an electrostatic septum, special multipole magnets and
additional fast bumpers. The system has to be designed for variable extraction
energies up to 50 GeV, i.e. a maximum magnetic rigidity of B = 170 Tm.
For the fast kicker the required pulse length is up to 4.2 s, at a rise time of ≤150 ns.
For the island extraction fast bumpers, the pulse length requirements are 21 s (first
step, five turns) and 4.2 s (second step, one turn) with rise times of ≤150 ns.
The maximum vertical beam size (3) at extraction (injection) will be ±7 (±25) mm.
M. Benedikt
DRAFT
06/03/2016
A slow extraction system for physics from the PS2 is also needed. This system will
require multipole magnets an orbit bump, electrostatic and magnetic septa. The
system should allow for extraction spills of around one second. The system has to be
designed for variable extraction energies up to 50 GeV, i.e. a maximum magnetic
rigidity of B = 170 Tm.
6. RF systems
The PS2 will be equipped with several RF systems, following a concept similar to the
present PS. For a first estimate, the system parameters of the PS are used as input and
scaled were appropriate. Furthermore it is assumed that the lattice parameters relevant
for the RF are not radically different from those of the existing PS machine. Based on
the performance estimate from Section 2, the total beam power will be 4 times higher
in the PS2 than in the PS and the beam density a factor 2 higher. The revolution
period will be a twice longer.
Main accelerating system:
The main RF system for acceleration will be a classical tunable system covering the
frequency range from 3 MHz to 10 MHz, operating at harmonic numbers between
14 and 42. This choice is motivated by the need for compatibility with the existing
injector chain and it is also well-matched to chopping of the SPL beam (for injection
only inside buckets to reduce capture losses).
To reach an acceleration rate comparable to the PS, about twice more peak voltage
will be needed. Around a factor four more RF power will be needed to cope with the
higher beam intensity.
Fixed frequency systems:
For the fixed frequency systems used for longitudinal gymnastics, the voltage
requirements are to first order identical to those in the PS. The various manipulations
(splitting, bunch rotation, etc.) will take twice longer due to the factor two increase in
revolution time. More RF power will be needed to cope with the more important beam
loading effects.
Several fixed frequency RF systems are needed for production of the different LHC
beam variants. A 20 MHz system to cover h=84 and a 40 MHz system to establish the
25 ns bunch spacing on harmonics 168. Finally an 80 MHz system is required (on top
of the 40 MHz) for bunch shortening. In case 75 ns beams will still be required, an
additional 13.3 MHz system is needed for operation on h=56.
A 200 MHz frequency system will be needed for controlled longitudinal blow-up and
eventual pre-bunching of beams for the SPS 200 MHz system.
M. Benedikt
DRAFT
06/03/2016
7. Beam instrumentation
The beam intensity measurement system will have to work over a large dynamic
range of five orders of magnitude to cover from the single LHC pilot bunch up to
highest intensity CNGS type beams.
The trajectory (orbit) measurement system will be based on 60 BPMs in each plane,
one monitor per quarter betatron wavelength.
Other main diagnostics systems required are a transverse emittance measurement
system and a tune measurement system.
Feedback systems will be required to counteract longitudinal and transverse
instabilities.
An internal beam dump system has to be foreseen.
8. Vacuum system
A vacuum quality of 1E-10 mbar is required for the machine.
The dipole vacuum chamber will be of elliptical or super-elliptical cross-section with
half axes of 70 mm x 45 mm (horizontal x vertical).
The standard chamber outside dipoles will be of circular cross-section with 70 mm
radius. There will be 600 m dipole chamber and 600 m standard chamber.
9. Radiation protection aspects
10. Transfer lines
The length of associated transfer lines will depend on the implementation of the new
machine in the existing accelerator complex. A cost estimate per unit length is
required.
11. Civil engineering and technical infrastructure
A new 1200 m tunnel with proper shielding will be needed to house the machine.
New transfer line tunnels depending on the implementation on the machine will be
needed.
M. Benedikt
DRAFT
06/03/2016
The main requirements on the technical infrastructure will have to be estimated
(electrical power and reactive power compensation, cooling and ventilation
requirements).
An estimate for surface buildings housing the technical equipment is needed.
12. Non-baseline R&D and upgrade scenarios
The normal conducting PS2 is featuring a relatively conservative design approach. It
is therefore worth considering also an upgrade version based on superconducting
magnets. This option should be seen mainly as a trigger for an aggressive R&D on
AC magnets. The aim of the R&D is twofold and the decision branching is in three or
four years from now.
One could aim at developing a 3 T, 1 Hz dipole with 150 mm coil diameter, in view
of designing a fast-cycling accelerator at 75 GeV with a typical beam power of around
1 MW. Alternatively, one could aim at developing a 4 T, 0.3 Hz dipole in view of a
100 GeV machine at lower beam power, comparable to the normal conducting PS2.
The common R&D point is the management of the thermal load in the SC dipole. The
decision on the final dipole design and on the specific parameters can be made in due
time say in three years.
Additional linked items for R&D are the RF systems about 5-6 times larger than in the
PS2 for the 1 Hz variant (and possibly the kickers that need to be about 1.5 to 2 times
more performing than in the PS2).
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