LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
7.8.1 Transverse Emittance Diagnostics
There are five different transverse emittance diagnostic stations distributed along the LCLS
accelerator, of which four are new installations (the existing sector-28 station [41] will be slightly
modified). In three of these cases the emittance measurement is accomplished with four
consecutive profile monitors placed along the beamline with appropriate phase advance between
monitors to optimize resolution. (Only three monitors are necessary, with four used to improve
resolution and provide redundancy.) The two low-energy stations use three consecutive profile
monitors over a drift section. These allow non-invasive emittance measurements to be made
during normal machine operation, or can also be made using a ‘quadrupole-scan’ technique,
taking advantage of the nominal beam waist on the center profile monitor. The emittance
measurement stations and their parameters are summarized below in Table 7.22.
Table 7.22 Transverse emittance measurement stations along the LCLS (γεx , y = 1 µm).
Location Station
Name
Energy
(GeV)
σ x
(µm)
σ y
(µm)
No. of Prof.
Monitors
Existing
Following L0 ED0 0.150 65-130 65-130 3 No
Following BC1 ED1 0.250 40-80 40-80 3 No
At the end of Linac-2 L2-ED 4.1-4.5 41-72 42-70 4 No
Sector-28 in Linac-3 L3-ED 9.1-10.9 40-55 39-57 4 Yes
Prior to undulator ED2 14.35 16 16 4 No
The energy range of each diagnostic station listed in Table 7.22 (e.g., L2-ED) indicates
accelerator sections separate the monitors. A range of beam sizes in the table represents the
minimum and maximum over the several profile monitors. The ED0, ED1 and ED2 stations listed
in Table 7.22 are dedicated, non-accelerating emittance diagnostic stations designed to produce
reasonable sizes in x and y at all monitors. The betatron phase advance between profile monitors
(PR) is set to the optimal value for a three or four-monitor station (60˚ or 45°, respectively).
These three sections are shown schematically in Figure 7.33 (ED0), Figure 7.19 (ED1), and
Figure 7.36 (ED2), with small circles indicating profile monitor locations on the beamline
schematics. In the case of ED0 and ED1, drift sections separate the monitors. In ED2, quadrupole
doublets separate the monitors. In all cases there exists an upstream variable matching section
which can be tuned in order to empirically match the beam. For three (four) monitors, a 60˚ (45°)
separation minimizes emittance resolution sensitivity to incoming beta mismatch errors. Figure
7.62 shows the emittance measurement resolution for the four-PR systems ED2 (solid) versus the
phase of an incoming beta-mismatch with a large amplitude of ζ = 1.5, as defined in Eq. (7.26).
In this case, the subscripted parameters in Eq. (7.26) represent the ideal matched beam and the
non-subscripted parameters are the perturbed beam.
A C C E L E R A T O R ♦ 7-89