CERN -- European Laboratory for Particle Physics Proton Synchrotron Division

Project to prepare the PS Complex to be a Pre-injector for the LHC

Overview  Challenge of space charge  Challenge of emittance  Challenge of bunch spacing 
Results of tests  Task list for protons  Ions

Overview

The drawing shows the various stages of acceleration of both protons and ions on their way to injection into LHC.

For protons, the Linac feeds the PS Booster synchrotron, the PS, the SPS and the LHC successively, whereas for ions there is an intermediate storage after the ion Linac (in the old LEAR machine, now called LEIR), and the Booster is omitted from the accelerator chain.

The preparatory work for LHC is divided into two parts, dealing with the PS complex and the SPS respectively.

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Challenge of space charge

The principal challenge for the PS complex is to obtain sufficient brightness for the beams delivered to LHC. As beams get brighter, space charge becomes a more significant effect, and so it was necessary to invent a new filling scheme for the PS, which requires a new radiofrequency system for the Booster.

Details of both the new scheme and the present one are shown in the diagram. By using h=1 in the Booster, the 4 bunches from the 4 rings can be sent to the PS where they fill half the circumference,thus permitting a second batch to complete the filling at the next Booster cycle. The tune shift in the Booster is halved by this scheme.

It was also decided to raise the energy of the PS Booster from 1.0 GeV to 1.4 GeV, which reduces space charge at PS injection in order to be certain to keep within the emittance budget at the highest intensities (see the results of the beam tests below).




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Challenge of emittance

Another challenge is the rather small emittance blowup which can be tolerated as the beam travels through the PS machines, to keep within the allowed "emittance budget" of LHC. The table below lists the machines of the LHC injector chain, and shows the various energies and intensities expected for the two cases of "nominal intensity" and the ultimate "beam-beam" intensity limit; the red figures are the normalised r.m.s emittances which in an ideal world would stay constant as the particles are accelerated, but in reality increase for a variety of reasons, and must be kept within the allowable "budget".

Machine Energy Nominal Intensity Emittance Budget
(normalised, rms)
in micro-metres
Ultimate
Beam-Beam Intensity
Remarks
RFQ 750 keV 200 mA 0.5-1.0 200 mA
Linac 2 50 MeV 180 mA 1.2 180 mA
  • 20 microsec pulse length
PS Booster 1.4 GeV 1.05 1012 protons/ring 2.5 1.8 1012 protons/ring
  • 4 rings
  • 1 bunch/ring
PS 25 GeV 0.84 1013 protons/pulse 3.0 1.4 1013 protons/pulse
  • filling with 2 PSB batches of 1.2 sec each
  • 4 nsec bunch length
  • 25 nsec bunch spacing
SPS 450 GeV 1011 protons/bunch 3.5 1.7 1011 protons/bunch
  • filling with 3 PS pulses of 3.6 sec each
LHC   7 TeV 1011 protons/bunch 3.75 1.7 1011 protons/bunch
  • filling with 12 pulses of 18.0 sec each
  • filling time just over 3 minutes per ring
  • 25 nsec bunch spacing
LHC
Luminosity 1034 cm-2s-1
Luminosity 2.5 1034 cm-2s-1
  • with 2 experiments

To keep within this budget, several actions are underway:

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Challenge of bunch spacing

The third challenge is to produce the required bunch spacing of 25 nsec in the PS at 26 GeV/c, just before transferring 81 of the 84 circulating bunches to the SPS (3 bunches are lost due to the rise-time of the kickers). This job is accomplished by two new RF systems in the PS. The RF "gymnastics" required to prepare the beam in this way is as follows:
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Results of tests

Measurements were made on the Booster in December 1993 by simulating LHC conditions in one ring. By making use of a prototype h=1 cavity and by pushing the performance of the main power supplies, it was possible to investigate what would happen to the beam emittance as the intensity was increased, under different accelerator conditions.

Measurements were made as a function of beam intensity at the normal Booster energy of 1.0 GeV andat the higher value of 1.4 GeV. The data obtained are shown on the graph (where dashed and solid lines represent data obtained with different instruments), which clearly show the difference between 1.0 and 1.4 GeV. The conclusion was that to be sure of staying within the LHC "emittance budget", the Booster energy would have to be increased, otherwise there was a danger that there would be problems at the higher intensity values.

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Task list for protons

The following is a list of the different jobs involved in converting the PS complex to be a proton pre-injector for LHC. After the project had been defined, it was decided that as a Canadian contribution to LHC, the TRIUMF laboratory in Vancouver would take care of certain parts of the work, as indicated below.


System Status in May 1998 TRIUMF collaboration part
PSB h=1 RF systems (x 4)
  • all four systems operational
  • >8 1012 p accelerated in each ring
  • ferrites
  • high voltage power supplies
PSB h=2 RF systems (x 4)
  • these cavities flatten the bunches to reduce space charge
  • all four h=2 cavities are operational

PSB main power supply
  • transformers and VAR compensators installed and working
  • tests with 1.4 GeV later in 1998
  • 10 transformers
  • VAR compensators
PSB-PS kicker pulsers (x 7)
  • 3 recombination kickers: strength increased by about 30%
  • 4 ejection kickers to be upgraded in 1999
  • study of pulse flattening
PSB-PS septa and pulsed supplies
  • PSB ejection septa (4), one vertical recombination septum, PS injection septum are operational
  • PSB recombination septa to be installed in 1998

PSB-PS transfer line magnets and power supplies
  • all but one magnet installed and working
  • all installed power supplies operational
  • a few supplies to come in 1999
  • all magnets
  • all power supplies
PS 40 MHz RF systems (x 2)
  • one cavity (300 KV) works
  • a second cavity is being tested
  • model studies
  • tuners
  • high order mode (HOM) dampers
PS 80 MHz RF systems (x 3)
  • two cavities (300 kV each) are working
  • a third cavity is under test
  • nominal LHC intensity with 25 ns bunch spacing but >5 ns length produced
  • model studies
  • tuners
  • high order mode (HOM) dampers
Wire scanners for 4 PSB rings
(H and V in each ring)
  • successful prototype tests in PSB
  • design and construction
Fast blade profile monitor in PSB
  • prototype being manufactured
  • design and construction

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Ions

Work on ions for LHC will only start in PS Division in the year 2001, after the proton work has finished. At the present time, final tests are being made using Pb ions in LEAR, prior to this machine being moth-balled for several years until it is needed as an ion storage ring for LHC. The current tests are the final confirmation of earlier results which show that indeed the scheme for collecting ions and then injecting them into the PS should give the intensity and luminosity needed by LHC. In order to convert LEAR into LEIR (low energy ion ring), a number of items are involved:


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Copyright CERN, modified 18/05/98