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CERN's Large Hadron Collider (LHC) [1][2] will be supplied with protons from the injector chain Linac2-PS Booster (PSB)-PS-SPS (Fig. 1). The required transverse beam brilliance (intensity/emittance) is almost twice that of current PS beams and the LHC bunch spacing of 25 ns must be impressed on the beam before its transfer to the SPS. The scheme involves new RF harmonics in the PSB and the PS, an increase of the PSB energy, and two-batch filling of the PS. After a successful test of the main ingredients, a project for converting the PS complex was launched in 1994. Major additions are (i) h=1 RF systems in the PSB, (ii) upgrading of the PSB main magnet supply from 1 to 1.4 GeV operation, (iii) new magnets, septa, power supplies, kicker pulsers for the PSB-PS beam transfer, (iv) 40 and 80 MHz systems in the PS, (v) beam profile measurement devices with improved resolution. A significant part of the effort is being provided by TRIUMF under the Canada-CERN co-operation agreement on the LHC. LHC REQUIREMENTSSupplying the LHC with protons from the existing chain of injectors may appear straightforward at first sight. However, the beam has to fit into the tiny LHC dynamic aperture, and must also have the brilliance required for LHC's desired luminosity. While the intensity is well within the capabilities of the PS complex (Linac2, PSB, PS), the beam brilliance requested (here defined as intensity per bunch/normalised rms emittance) needs to be up to twice that of current beams. LHC proton beam parameters at collision (7 TeV per beam) at various stages of operation are summarised in Table 1 The injector chain has to produce beams corresponding to the nominal performance; wherever possible, a higher performance level is aimed at, to potentially fill the LHC up to the beam-beam limit and to provide an operational margin. A SCHEME TO REDUCE SPACE CHARGEWith a beam brilliance corresponding to the LHC beambeam limit, the space-charge tune shifts ('Q) are almost twice the present figures at the PSB (50 MeV) and PS (1 GeV) injection energies, leading to unacceptable beam blow-up and loss. Double-batch filling of the PS to reduce space charge in the PSBUsually the four PSB rings (each 1/4 of PS circumference) are transferred sequentially (3-4-2-1), to fill the PS in one pulse. The LHC beam would generate, at PSB injection, a 'Q = 0.9, far beyond the "hard" limit of 0.55. With two PSB pulses to fill the PS, the intensity per PSB pulse is halved and 'Q reduced to 0.45. RF harmonic 1 in PSB for two-batch filling of the PSWith the RF equal to the PSB revolution frequency, there is just one bunch per ring (5 at present) and two-batch filling of the PS becomes feasible. Appropriate phasing of the bunch in each ring before extraction enables the four

II. PRINCIPLE OF ONE-TURN-DELAY FEEDBACKThe periodic transient beam loading in the CERN PS ferrite cavities was diagnosed in 1989 to be the source of performance limitations for the antiproton production beam [l]. A project was then launched to lower the transient beam induced voltage by a factor of four, with a "one-turn-delay feedback" system reducing the equivalent cavity impedance on the first 3 revolution frequency side-bands around the cavity tune frequency. The design is able to cope with a wide frequency range due to particle acceleration (15 % velocity variation) and choice of harmonic number (h= 10 to 20). Loop gain is above 0 dB in the vicinity of revolution frequency harmonics over an instantaneous bandwidth of 3 to 4 times the 3 dB bandwidth of the original RF system. Fast digital electronics is applied extensively, resulting in a very reliable and compact implementation. The various functions are described, closed loop performance of a cavity is shown and measurements with beam are presented.