A C

C E

LE R

A T

O R

A N

D S

O U

R C

E

1 8 1 I H I G H L I G H T S 2 0 2 3

Energy [GeV] 6

Maximum current [mA] 200 Horizontal emittance (Gaps opened) [pm] 140 Horizontal emittance (Gaps closed) [pm] 120 to 130 Vertical emittance [pm] 10 Revolution frequency [kHz] 355 Number of bunches 1 to 992 Min. Time between bunches [ns] 2.82

Beam parameters of the storage ring

Table 2 shows a summary of the characteristics of the storage ring electron beam in optimal conditions. The horizontal emittance varies in USM depending on the gap settings. Table 3 gives the main optical functions, electron beam sizes and divergences at various source points. For insertion device source points, the beta functions, dispersion, sizes and divergences are calculated in the middle of the straight section. The bending magnet beamlines are separated by types of short bend (SB), 2-pole wiggler (2PW), and 3-pole wiggler (3PW). Values are averaged over all beam lines.

Electron beam profiles are Gaussian, and the sizes and divergences are presented in terms of rms values. The associated full width at half-maximum sizes and divergences are 2.35 times larger. Horizontal electron beam sizes and divergences are given for the multi-bunch filling modes and apply to almost all filling patterns,

except when the current per bunch is larger than 1.5 mA, for which a slightly larger size and divergence are attained because of the increased energy spread of the electron beam.

Vertical electron beam sizes and divergences are given for a vertical emittance of 10 pm, which is presently the optimal condition for multi-bunch modes and a horizontal emittance of 125 pm.

The lifetime, bunch length and energy spread mainly depend on the filling pattern. These are given in Table 4 for a few representative patterns. Note that in 16-bunch and 4-bunch filling patterns, the energy spread and bunch length decay with the current (the value indicated in the table corresponds to the maximum current). The bunch lengths are given for the usual radiofrequency accelerating voltage of 6 MV and are extrapolated from single bunch measurements. The current is still limited in few bunch modes by over-heating of the ceramic chambers, despite the fact that all the kicker ceramic chambers were re- coated for a thicker Ti coating.

Table 2: Main beam parameters of the EBS storage ring.

ID BM (SB) BM (2PW) BM (3PW)

Horiz. beta function [m] 6.74 1.89 1.84 1.84 Horiz. dispersion [mm] 1.28 17.17 18.01 17.94 Horiz. rms e- beam size [µm] 29.1 23.04 23.54 23.5 Horiz. rms e- divergence [µrad] 4.31 16.2 16.94 17.56 Vert. beta function [m] 2.66 2.54 2.55 2.53 Vert. rms e- beam size [µm] 5.16 5.04 5.05 5.03 Vert. rms e- divergence [µrad] 1.94 1.98 1.98 1.99

Filling pattern Unit Uniform 7/8 + 1 Hybrid 16-bunch 4-bunch

Number of bunches 992 868+1 28x12+1 16 4 Maximum current [mA] 200 200 200 75 40 Lifetime [hr] 25 24 16.6 5.5 5 Rms energy spread [%] 0.094 0.094 0.111 0.158 0.213 Rms bunch length* [ps] 12.5±1 12.5±1 18.8±1 33.4±2 42.8±2

Table 3: Beta functions, dispersion, rms beam size and divergence at the various source points.

Table 4: Measured current, lifetime, bunch length and energy spread for a selection of filling patterns. * The error bar corresponds to the difference between the model and measured values.