Vertical Emittance Tuning



The vertical emittance of electrons extracted from the NDR is measured on the NRTL wire with a quadrupole scan. The improvements to the emittance were made in conjunction with orbit studies aimed at reducing the coupling in the ring.

Beam Conditions

Single bunch of ~2E10 electrons, stored for 16.6 ms (long store).

Emittance Measurement

The vertical beam size is measured on the RTL wire DR13 164. The strength of the first upstream quadrupole is varied. Only a drift separates this quadrupole and the wire, so we avoid any modeling uncertainties in the emittance calculation. Uncertainties can arise if poorly modelled quadrupoles are in the transport line between the wire scanner and the quadrupole that is varied. This is particularly true for the strong RTL matching quadrupoles whose magnetic properties are only approximately known. Hence our choice of QUAD DR13 124 for these measurements in order to avoid these problems.
The results show a parabola fitted to the measured points even though the thick-lens calculation for the emittance does not rely on this fit. A parabola is only used in the thin-lens approximation. The thick lens calculation has the potential to be more accurate than the 4-wire emittance scans done in sector 2, for example, since many more points in phase space can be measured.


The extracted beam emittance is very sensitive to the vertical orbit through the extraction septum. The septum has a limited good-field region and can introduce skew on the beam if it is mis-steered. Observing the spot on PROF DR13 78 shows quite clearly how the spot is skewed in one direction and then in the other direction as the vertical Y position bump knob is scanned through the good field region of the extraction septum. The Y angle bump has little effect on the skew but is used in conjuntion with the position bump knob to keep the stored orbit reasonably flat while also achieving a good launch into the RTL.
The RTL launch correctors are then adjusted to minimize the beam offset in the matching quadrupoles (since this generates dispersion in the matching quadrupoles and leads to emittance growth).
A measured quadrupole emittance scan is shown, together with the thick-lens emittance calculation that shows a record

 vertical extracted emittance = 0.088 E-5 m-r.

The stored orbit in the ring is also shown, and particular note should be given to the bump in the vertical orbit in the extraction region.


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If you have comments or suggestions, email Patrick Krejcik at