Fast instability caused by electron cloud in combined function magnets

Abstract

One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggests that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beams, simulating numerically the buildup of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function dipoles. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function magnets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud buildup is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multiturn accumulation allows the electron cloud to reach final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth time of about 30 revolutions and the mode frequency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the PEI code. The created instability model allows investigating the beam stability for the future intensity upgrades.

Rapid-cycling synchrotron for multi-megawatt proton facility at Fermilab

“…With or without slip-stacking, several betatron resonances would need to be completely suppressed to accommodate the increased space-charge tune-spread and chromaticity [32]. Amplification of electron cloud in the combined function magnets of the Recycler has previously lead to a severe instability that delayed commissioning of the Recycler [33]. At higher intensities the Recycler beampipe will require conditioning to mitigate the electron cloud instability, and this might constrain the duty-factor for operation at maximum intensity.…”

“…The RCS expects weaker collective instabilities despite its higher charge-density, and consequently can manage instabilities with a combination of chromaticity and bunch-by-bunch dampers. Electron cloud instabilities, such as the one that impacted the Fermilab Recycler [33], would be mitigated by a low-SEY coating beampipe coating such as titanium nitride or diamond-like carbon [69,70].…”

Accelerator Physics Center at Fermilab : History and Accomplishments (2007-2018)

“…Seminal results were obtain in studies of Integrable non-linear optics (A.Valishev, et al), coherent beam stability (A.Burov, V.Balbekov, K.Y.Ng, et al), Landau damping with electron lenses and space-charge (V.Shiltsev, Yu.Alexahin, et al), beam phase-space manipulations (P.Piot, et al), beam-beam effects (Yu.Alexahin, T.Sen, A.Valishev et al), electron lenses for beam-beam and space-charge compensation and collimation (V.Shiltsev, G.Stancari, et al). See References [4,6,7,8,15,25,29,30,37,46,47,51,54,62,96,98,121,122,132,135,145,157,171,185,194,195] in Appendix E. II.11. Beam physics considerations for far-future colliders: carried out by several APC scientists and covered selected topics of efficient wakefield acceleration, technology and cost challenges of the colliders, channeling acceleration in crystals and carbon nanotubes, etc.…”