CornellVideo Talk. Dec page 1 John Byrd Experimental studies of the fast- beam ion instability in the ALS- The Sequel John Byrd Lawrence Berkeley National Laboratory

CornellVideo Talk. Dec page 2 John Byrd Motivation FBII predicted in First experimentally observed in dedicated tests at ALS and PLS in Initial tests mostly done as an existence proof. Since observed in several other rings. Predicted to be severe in ILC DR and serious for ultralow emittance operation of 3.5 generation light sources. Simulations indicate mitigating effects on growth rates (Landau damping, ion frequency spread, etc.) Further experimental evidence needed to inform ILC DR design. –direct measurement of growth rates via grow/damp technique to compare with theory and simulations –effectiveness of feedback to control instability –evaluation of incoherent and coherent growth

CornellVideo Talk. Dec page 3 John Byrd Indirect Growth Rate Measurement at a given pressure, we can vary the instability growth rate by varying the length of the bunch train (using a fixed current/bunch) for the experimental conditions, the theory predicts a growth rate of ~1/msec at about 8 bunches. Our FB damping rate for 0.5 mA/bunch is ~1.2 msec.

CornellVideo Talk. Dec page 4 John Byrd FBII Revisited Several things have changed in the ALS since the original expts. low vertical emittance mode (C. Steier) Achieved 5 pm vertical emittance (6.75 nm ALS motivated to understand effects limiting brightness HOM damping of main and harmonic cavities nearly eliminates effects of conventional instabilities. improved instrumentation digital bunch-to-bunch recording of BPM difference signal new pinhole array synch light monitor; possibility of using gated camera (F. Sannibale) optical streak camera mature understanding of transverse feedback system Try to observe and characterize FBII under conditions approaching ILC DR!

CornellVideo Talk. Dec page 5 John Byrd ALS Experimental Plan Demonstrate grow/damp technique under nominal conditions evaluate resolution of turn-by-turn vertical motion measure growth rates of conventional instabilities and optimize damping rate of TFB Re-establish low-emittance mode record vertical beam size vs. emittance, bunch number, etc. observe FBII via vertical spectrum and beam size establish conditions where TFB can control FBII; increase pressure if necessary measure FBII growth rates via grow/damp

CornellVideo Talk. Dec page 6 John Byrd Summary Potential exists to characterize FBII in a situation which approaches ILC DR conditions –low vertical emittance –< 1 ntorr vacuum Several experimental techniques available –beam size; time resolved via streak camera or gated CCD –turn-by-turn data analysis sideband spectra amplitude/phase shift along train –direct growth rate measurement via grow/damp to compare with simulation/theory –variable fill patterns Please let us know if you want to help!