ERL 07 Workshop Notes - Session 1

Michael Borland - Touschek Scattering

Beam-loss mechanisms fall into two classes:
1. Low-energy, unpredictable - collimate at low energy
2. Calculable, e.g. Touschek and Nonlinear Optics

Touschek dominates CW beam loss in low emittance rings. Probably then significant in ERL light source. Touschek is well-studied in rings, but not studied in single-pass accelerators.

Assume gas scattering is smaller than Touschek - at about 1 nTorr.

ERL electron cooler - 5 nC per bunch, 4 um emittance (100 ps bunch length). ERL beam cools both directions. 54 MeV bunches travel 100 m down ERL cooling line - some space charge acting. Will compensate space charge using weak solenoids every 10 m.

3rd-harmonic cavity is used to flatten energy gain of the very long (100 ps) bunch so that energy spread is small enough (4.10^-4 rather than few 10^-3 without 3rd-harmonic)

BBU threshold is high with the small number of cavities and modelled HOMs.

Have a dog-leg which moves the beam H and V at the same time (5 quads for dispersion matching). No perceived issue with space charge at the centre of the dog-leg quad.
300 m turnaround for the beam path.
Accumulation of ions will be only in the turnaround - the cooling region has a big ion beam which will repel the ions in that 100m section.
Path length correction is mechanical - move the arc.
Amplitude stability is 10^-4 in RF - should be good enough.