Achievement of ultralow emittance coupling in the Australian Synchrotron storage ring

Investigations into producing an electron beam with ultralow vertical emittance have been conducted using the Australian Synchrotron 3 GeV storage ring. A method of tuning the emittance coupling (${ϵ}_y/{ϵ}_x$) has been developed using a machine model calibrated through the linear optics from closed orbits method. Direct measurements of the beam emittance have not been possible due to diagnostic limitations, however two independent indirect measurements both indicate a vertical emittance of 1.2–1.3 pm rad (${ϵ}_y/{ϵ}_x=0.01\%$). Other indirect measurements support the validity of these results. This result is the smallest vertical emittance currently achieved in a storage ring.

Figure 1

Storage ring magnets and lattice functions. Dipoles (yellow), quadrupoles (red/blue), and sextupoles (green/purple) are shown. Skew quadrupoles are located in the 2nd and 6th sextupole. Betatron tunes are ${\nu }_x=13.29$, ${\nu }_y=5.216$.

Figure 2

Layout of magnets and support girders showing adjustment points. The top image shows a top view of the arc sector magnets. The bottom image illustrates the three types of support girder and adjustable supports used in the storage ring.

Figure 3

Plots of LOCO fitted BPM vertical coupling. Plot (a) shows the results over several fits of varying machine condition, Plot (b) shows the results for multiple fits to the same machine configuration.

Figure 4

Examples of LOCO fitted skew quadrupole components for two of the quadrupole families in the storage ring. Plot (a) shows the focusing quadrupoles in the inner, long girder; plot (b) shows the focusing quadrupoles on the outer, short girders.

Figure 5

Result of a typical dispersion measurement. Effects of BPM coupling can clearly be seen as the peaks of the horizontal dispersion (top) appear in the vertical dispersion plot (bottom).

Figure 6

Dispersion measurement of uncorrected lattice (blue) and dispersion minimized lattice (red). The top plot shows the raw dispersion data, while the bottom plot has the BPM coupling effect removed. Both show a clear reduction in vertical dispersion.

Figure 7

Tune measurements of horizontal (blue) and vertical (red) tunes as they are brought to resonance. The upper plot shows the 0.9% coupling measurement and the lower plot shows the 0.07% measurement.

Figure 8

Comparison of XDB images for each coupling setting. Minimal coupling is the top left image, and 1% coupling is in the bottom right. The coupling increases left to right, top to bottom.

Figure 9

Measured (blue) and model predicted (red) beam tilt angles at XDB source point. An offset of 1.06 degrees has been subtracted from the measurements to account for the relative tilt of the CCD.

Figure 10

Plot of Touschek lifetime vs $\sqrt{\kappa }$. Line of best fit shows a clear linear relation. The figure was taken from 14.

Figure 11

Example of the beam current decay fit to extract the Touschek component. Data is shown in green; the fit is in blue.

Figure 12

Measured Touschek parameter at different rf voltages for three coupling settings, 0.1%, uncorrected (0.06%) and minimal (0.01%). Colored lines are best fit coupling values based on the Touschek equation. Error bars indicate uncertainty in the Touschek parameter from the decay curve fits. Fitted emittance ratio values are 0.0916% (red), 0.043% (green), and 0.0116% (blue).