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The Center for Bright Beams, A National Science Foundation Science and Technology Center

New metrics to compute the beam brightness

C. M. Pierce, I.V. Bazarov, C.M. Gulliford, J.M. Maxson, S. Baturin, M.A. Gordon, Y.K. Kim


Recently, CBB has applied its previous injector optimization experience to assess the impact of exceptionally bright sources on injectors for ultrafast electron diffraction (UED), free electron lasers (FEL), and energy recovery linacs (ERL). These bright sources, now in development by CBB, use very high electric fields and photoemission with low (or zero) mean transverse energy, MTE. To assess the impact, it quantified the RMS emittance at the delivery point.

The studies show that for many bunch sizes and charges, the RMS emittance is preserved as the beam moves from source to delivery point, while for the most compact bunches and largest charge, emittance grows during delivery. But there's hope even for these beams. CBB studies show that the emittance of their core is preserved, and new figures of merit that capture this feature are now used to guide the design of improved photoinjectors providing higher brightness beams.

Graphs demonstrating the effect of bunch charge on RMS emittance for both large and small bunch sizes. The left graph is for large, and the right graph is for small. More in caption.

Two graphs. On the left a graph of bunch sizes at production. The graph shows lines on an upward slope. On the right is a graph showing charge on the x-axis and emittance growth on the y-axis. orange charge dots start low on the y-axis and increase quickly, blue charge dots stay at a similar emittance level as the charge changes.

For large (mm) bunch sizes at production, the RMS emittance is preserved even for large bunch charges. For micron size bunches, emittance is preserved for small bunch charge, but there is is a threshold bunch charge above which the RMS emittance rapidly grows due to space charge.

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