Abstract
Without specific counter measures, the LHC type beam in the SPS suffers from longitudinal coupled bunch instabilities. To prevent them, the SPS impedance has been decreased over the last few years and the operation of a high frequency Landau damping system has been established. In the absence of this Landau damping system one may alternatively introduce an rf amplitude modulation to stabilize the beam. We present results obtained by this method in the SPS and considerations for a potential increase of the longitudinal beam stability in the LHC.
Highlights
In the SPS, LHC type beams are accelerated from 26 to 450 GeV
SPS on cycle timing off manual adjustment of gain switch amplifier 200 MHz band pass experimental setup for amplitude modulation RF system reference frequency remote control of amplitude and phase manual phase adjustment phase control loop other cavities & test beam one turn delay feedback load 200 MHz traveling wave cavity longitudinal damper
For the measurements presented here we set up the rf amplitude (rf AM) in the following way: The rf frequency changes as the beam velocity increases with increasing particle energy
Summary
In the SPS, LHC type beams are accelerated from 26 to 450 GeV Without specific countermeasures they suffer from both longitudinal single and coupled bunch instabilities. The bunches show an increased tail population which could lead to losses at injection into the LHC due to uncaptured particles To overcome these problems, the SPS impedance has been decreased and unused cavities have been removed [1,2,3]. Microwave instabilities are no longer observed at low energy, but coupled bunch instabilities still appear starting at energies between 200 and 280 GeV and continuing up to high energy To suppress these latter instabilities a Landau damping rf system is used [2 – 4]. A Landau damping system increases the incoherent synchrotron frequency spread, the spread of single particles within a bunch. In [10] the rf amplitude was modulated sinusoidal whereas we applied a linear rf amplitude change
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