Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation at high currents. Furthermore, it is difficult to probe their density over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave transmitted over a section of the accelerator and used it to measure the average electron cloud density over a 50 m section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center. Low energy background electrons in the beam pipes of high energy accelerators of positively charged beams present a serious challenge to increasing current in these machines. Under the right machine conditions, such as bunch repetition rate, peak current, etc., amplification of the electrons can occur from secondary emission when the electrons strike the beam pipe walls, creating a growth in vacuum pressure along with a number of adverse effects on the circulating beam including severe two-stream instabilities, transverse beam blowup, and heating of cryogenic vacuum chambers. The net result is that the beam intensity is limited and beam quality reduced [1,2]. This effect is important for several future accelerators Electron cloud effects have been primarily observed in a number of high intensity synchrotrons and storage rings [4 -13]. Experimental studies of the electrons have mainly used local detectors (retarding field analyzers) to measure the time dependence, density, and energy spectrum of the electron cloud in a small region near the detector [14 -16]. However, the electron cloud density (ECD) can vary significantly along the beam pipe depending on local beam pipe geometry and surface conditions. Furthermore, the local measurement only detects those electrons that reach the beam pipe walls and can only infer the ECD with the help of computer simulation. Therefore, it is important to develop means of directly measuring the electron clouds over longer sections of the accelerator. Of course, one method of inferring the ring average ECD is from effects on the high energy beam itself, which usually only appear at relatively high beam intensities, however.In this Letter, we present a novel idea [17] and its successful demonstration for measuring the ECD over a much longer section of a storage ring. This idea is based on measuring the time delay (i.e., phase shift) of a microwave signal propagating in the beam pipe due to the change of the index of refraction caused by the electron cloud. In practice, it would be challenging to measure the absolute phase shift of the signal, which is expected to be at most only a few degrees over a hundred meters, in an accelerator environment. Our idea relies instead on measuring the modulation of the phase shift of the microwave signal through the electron plasma by taking advantage of the modulation of the density of the electron cloud from gaps in the fill pattern of the circulating posi...

The two LHC injection kicker magnet systems must produce a kick of 1.3 T.m with a flattop duration variable up to 7860 ns, and rise and fall times of less than 900 ns and 3000 ns, respectively. Each system is composed of two resonant charging power supplies (RCPSs) and four 5 Ω transmission line kicker magnets with matched terminating resistors and pulse forming networks (PFNs). A beam screen is placed in the aperture of the magnets: the screen consists of a ceramic tube with conductors on the inner wall. The conductors provide a path for the image current of the, high intensity, LHC beam and screen the ferrite against Wake fields. The conductors initially used gave adequately low beam coupling impedance however inter-conductor discharges occurred during pulsing of the magnet: an alternative design was discharge free at the nominal operating voltage but the impedance was too high for the ultimate LHC beam. This paper presents the results of a new development undertaken to meet the often conflicting requirements for low beam coupling impedance, fast magnetic field risetime and good high voltage behaviour. High voltage test results and thermal measurements are also presented. Large Hadron Collider Project AbstractThe two LHC injection kicker magnet systems must produce a kick of 1.3 T.m with a flattop duration variable up to 7860 ns, and rise and fall times of less than 900 ns and 3000 ns, respectively. Each system is composed of two resonant charging power supplies (RCPSs) and four 5 Ω transmission line kicker magnets with matched terminating resistors and pulse forming networks (PFNs). A beam screen is placed in the aperture of the magnets: the screen consists of a ceramic tube with conductors on the inner wall. The conductors provide a path for the image current of the, high intensity, LHC beam and screen the ferrite against Wake fields. The conductors initially used gave adequately low beam coupling impedance however inter-conductor discharges occurred during pulsing of the magnet: an alternative design was discharge free at the nominal operating voltage but the impedance was too high for the ultimate LHC beam. This paper presents the results of a new development undertaken to meet the often conflicting requirements for low beam coupling impedance, fast magnetic field risetime and good high voltage behaviour. High voltage test results and thermal measurements are also presented.

In the CERN SPS microwave transmission measurements through beampipe sections with a length of 30 m and 7 m respectively have been carried out in the frequency range 2-4 GHz since spring 2003. Here we report on new results obtained with improved measurement techniques during the 2004 run. Observation techniques include a fast real time scope, spectrum analyser IF and video output signal registration and baseband signal observation using a PC soundcard. The unexpected beam-induced amplitude modulation has been confirmed on all kinds of available beams including single bunches. It was found that there is a correlation between the amount of beam induced signal attenuation and the beam losses registered by external scintillators. Potential theoretical models are discussed. EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN -AB Department

We describe a dedicated electron cloud experiment which was installed in the CERN Proton Synchrotron in 2007. The setup comprises shielded button-type pickups, a fast vacuum logging, a dipole magnet, and a stripline electrode to experimentally verify the beneficial effect of electron cloud clearing. The electron cloud effect was observed within the last milliseconds before ejection of the nominal LHC proton beam consisting of 72 bunches with 25 ns spacing. Measurements of electron flux at the wall and vacuum pressure are presented for a set of magnetic fields and bias voltages on the clearing electrode, showing that efficient electron cloud suppression can be obtained for appropriate clearing voltages but revealing an unexpectedly complex dependence on magnetic field and voltage.

The prediction of the transverse wall beam impedance at the first unstable betatron line (8kHz) of the CERN Large Hadron Collider (LHC) is of paramount importance for understanding and controlling the related coupled-bunch instabilities. Until now only novel analytical formulas were available at this frequency. Recently, laboratory measurements and numerical simulations were performed to cross-check the analytical predictions. The experimental results based on the measurement of the variation of a probe coil inductance in the presence of (i)sample graphite plates, (ii)stand-alone LHC collimator jaws, and (iii)a full LHC collimator assembly are presented in detail. The measurement results are compared to both analytical theories and simulations. In addition, the consequences for the understanding of the LHC impedance are discussed