The PS in the LEP injector chain - 50th anniversary of the CERN ...

flat-top was attained. After the lepton beam reached the required energy spread along the flat-top a
first batch of four bunches was ejected. The second batch of four bunches waited for the reloading of
the kicker magnets before it was ejected. During the kicker reloading time, the blow-up of the bunch
dimension due to the quantum excitation was less than 5%.
The ‘bunch compression’ shaping method was tested. However, it was discarded as a result of
the overheating of the ferrites at this 7.6 MHz frequency and due to the relative complexity of the
method compared to other bunch shaping techniques.
Table 1: ‘Bunch compression’ method – parameters at injection and on the 3.5 GeV flat-top before and after
compression (1983 status)
N b E (GeV) σ s (m) σ e /E J e V RF(1) (kV) f RF(1) (MHz) V RF(2) (kV) f RF(2) (MHz)
Injection 10 10 0.6 0.5 0.6 × 10 -3 3.1 120 7.6 (h = 16) – –
Before compression 10 10 3.5 0.5 0.3 × 10 -3 3.1 220 7.6 (h = 16) 5 114 (h = 240)
After compression 10 10 3.5 0.5 10 -3 3.1 220 7.6 (h = 16) 1000 114 (h = 240)
8.2.2 Bunch expansion
In the ‘bunch expansion’ method, the same 114 MHz RF system of two cavities (h = 240) accelerated
the beam from 500 MeV at injection to 3.5 GeV, the acceleration rate being the same as for ‘bunch
compression’. Unlike the ‘bunch compression’ mode only a single RF system was necessary. The
required energy spread σ e /E = 10 −3 was obtained at the end of the acceleration by adjusting the
longitudinal damping partition number from its natural value J e = 4 down to J e = 0.2 with two
Robinson wiggler magnets, each making changes ΔJ e = −1.9.
The required bunch length of σ s = 0.16 m was achieved by setting at about 950 kV the total
accelerating voltage of the two 114 MHz cavities on the 3.5 GeV flat-top. At this energy the
longitudinal damping time was 200 ms. As the bunch energy spread at the beginning of the flat-top
was smaller than the equilibrium energy spread, σ e /E and σ s slowly expanded to the required values
until the proper longitudinal emittance was reached (see Table 2 and Figs. 8 and 9).
Table 2: ‘Bunch expansion’ method – parameters at injection and at 3.5 GeV before ejection (1983 status)
N b E (GeV) σ s (m) σ e /E J e V RF(1)
(kV)
f RF(1) (MHz)
V RF(2)
(kV)
Injection 10 10 0.5 0.2 0.6× 10 −3 0.2 40 114 (h = 240) – –
Ejection 10 10 3.5 0.16 10 −3 0.2 950 114 (h = 240) – –
f RF(2)
(MHz)
Fig. 8: Electron cycle with the ‘bunch expansion’ mode (time-scale 100 ms/div):
a) circulating beam current, b) detected wideband pick-up electrode, c) dB/dt
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