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

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:

• decrease space charge effects in the synchrotrons, mentioned above
• use automatic beam steering and matching for the slow corrections
• use transverse injection dampers for fast corrections
• improve the resolution of beam profile measurement devices
• install low-ripple power supplies
• use fast rise-times for the kicker magnets

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Challenge of bunch spacing

• 16 bunches are accelerated to the flat-top momentum of 26 GeV/c in the PS
• the RF voltage is then slowly reduced to allow the circulating beam to de-bunch
• the beam is re-captured by means of a new 40 MHz RF cavity, to generate 84 bunches with 25 nsec spacing
• finally a sophisticated procedure using the new 80 MHz RF cavities (at 600 KV), allows the bunches to be shortened to a length of approximately 4 nsec, as needed by SPS and LHC

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

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:

• the electron cooling system
• modifications to the LEAR ring
• Linac 3 pulsing rate and ramping
• modifications to PS injection
• modifications to the transfer lines between Linac 3 and LEIR , and LEIR and the PS
• new control system for LEIR

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