Measurements of the SPS transverse impedance in 2000

Abstract

We report on measurements of coherent tune shifts, head-tail growth rates, and current-dependent betatron phase advances at the CERN SPS in the year 2000. Comparing results obtained at two different energies shows that there is no notable contribution from space charge. Within the measurement resolution the impedance is the same as in 1999, consistent with the expected small effect from changes to only a small number of pumping ports. In 2000, data were taken over an expanded range of chromaticities, which increases the sensitivity to the impedance frequency distribution. Measurements of the current-dependent phase advance around the ring help localizing the most important impedance sources.

Simulation of single bunch instabilities driven by electron cloud in the SPS

“…The same explanation has been evoked for the SPS instability. It is supported by various observations [3].…”

“…The instability obviously manifests itself only in the vertical plane, where the impedance has the higher component [9]. A positive chromaticity can strongly damp this effect, as shown in Fig.4.…”

“…Further ingredients which may affect the simulated instability rise times are optionally included, such as a nonzero chromaticity, the conventional transverse broadband impedance of the SPS, which is characterized by R ⊥ ≈ 30 MΩ/m, f r ≈ 1.3 GHz and Q = 1 [9], and transverse space charge. The maximum incoherent spacecharge tune shift in the SPS at 26 GeV is quite substantial,…”

Diffusion measurement from observed transverse beam echoes

“…For the model D(J) = D 0 + D 1 (J/J 0 ), the FWHM can be found analytically from Eq. (11). We define a variable D up which depends on a upper limit to the pulse full width (∆t) up FW and other parameters as follows…”

“…Shortly after the introduction of the beam echo concept, longitudinal unbunched beam echoes were observed at the Fermilab Antiproton Accumulator [9] and then at the CERN SPS [10] The original motivation however had been to measure transverse diffusion from transverse echoes. In the year 2000, transverse bunched beam echoes were observed in the SPS with two consecutive dipole kicks [11] but no diffusion coefficients were extracted.…”