Operational experience with the LEP2 SC cavity system

The LEP energy upgrade programme (LEP2) consists of increasing the e+e- colliding beams¹ energies far beyond the W pair production threshold, up to 96 GeV. The large increase in accelerating voltage required, from 250 MV for LEP1 at 45 GeV to 2700 MV at 96 GeV, will be provided by 272 superconducting (sc) cavities. Almost all are of the Nb/Cu type, with a nominal accelerating field of 6 MV/m at 352 MHz. A first set of 56 sc cavities was installed during 1995 and made possible a short physics run at energies of 65, 68 and for a short time 70 GeV. The experience gained during this run, as well as that obtained previously on the machine, will be presented. The cavities and their upgraded ancillary equipment worked satisfactorily at their nominal field and with the LEP beam currents (~ 7 mA). Apart from the usual problems of debugging many new pieces of equipment, the difficulties encountered were microphonic oscillations, together with effects due to the much larger impedance at the RF frequency. An RF feedback working on the vector sum of the signals from the eight cavities driven by a common klystron has been implemented to address these problems. The next steps towards the completion of the LEP2 programme will also be presented

The LHC Superconducting RF System

The European Laboratory for Particle Physics (CERN), the largest high energy physics laboratory worldwide, is constructing the Large Hadron Collider (LHC) in the existing 27 km circumference LEP (Large Electron Positron) collider tunnel. For the LHC, superconducting cavities, operating at 4.5 K, will provide the required acceleration field for ramping the beam energy up to 7 TeV and for keeping the colliding proton beams tightly bunched. Superconducting cavities were chosen, not only because of their high acceleration field leading to a small contribution to the machine impedance, but also because of their high stored energy which minimises the effects of periodic transient beam loading associated with the high beam intensity (0.5 A). There will be eight single-cell cavities per beam, each delivering 2 MV (5.3 MV/m) at 400 MHz. The cavities themselves are now being manufactured by industrial firms, using niobium on copper technology which gives full satisfaction at LEP. A complete cavity prototype assembly including cryostat, tuner and couplers is now being tested at CERN. In addition to a description of the LHC RF superconducting system, results on the prototype cavity assembly will be reported

A Superconducting RF Cavity for Bunch Compression of the High Intensity SPS Proton Beam at Transfer to LHC

The bunch length of the high-intensity proton beam ejected from the CERN SPS into LHC must be reduced to fit into the 400MHz LHC buckets. This will be done using a new 400MHz superconducting RF system in the SPS. Above transition bunch compression is obtained with a cavity tuned slightly below the bunch frequency, thus giving a very high capacitive impedeance. Such a system would however be extremely critical and very unstable without strong RF feedback. To keep the required RF power at an acceptable level during the ramp, the beam, which occupies only a third of the SPS, is accelerated in a variable harmonic mode to place the beam spectrum substantially above the cavity bandwidth. On the flat top, when the beam spectrum is moved towards the cavity resonance, the phase and amplitude of the reference voltage, including its modulation at the revolution frequency are programmed to keep the required RF power to a minimum. This scheme is described in detail together with the prototype 400MHz superconducting cavity already installed in the SPS. Initial tests with beam will also be reported

Design Considerations for the LHC 200 MHz RF System

The longitudinal beam transfer from the SPS into the LHC 400 MHz buckets will not be free of losses without a lower frequency capture system and a fast longitudinal damping system in LHC. We present a complete study of a combined system using four identical copper cavities at 200 MHz delivering 3 MV total CW voltage and having still enough bandwidth to achieve fast longitudinal damping. The shape of a cavity was designed according to the accelerating mode performance, its tuning and the higher order mode spectrum with respect to the LHC beam lines and their possible attenuation. The possibility to park the cavities during coast was included. The local heat load and the corresponding cooling water distribution as well as deformations were studied and techniques to build the cavity with all ports at low cost are proposed. The parameters of the RF generators, couplers and detuning are determined. Simulations of the total LHC RF system incorporating real delays, generator bandwidth and the control loops confirm that this system is capable of capturing and damping the beam longitudinally without losses