LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

7.8.2 Bunch Length Diagnostics
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
The peak current delivered to the LCLS undulator is a critical parameter. It is determined by
both the charge and the final bunch length. To setup the compression, the bunch length needs to
be measured before and after BC1, and after BC2. In addition, once the bunch compressors are
set up, a bunch length feedback system will be required for stabilization of the compression.
These feedback systems have not yet been fully designed.
The bunch lengths of interest are approximately 1000, 200, and 20 µm rms (10, 2, and
0.2 psec full width, respectively). Measuring 10-psec accurately using a streak camera is fairly
standard. The 2-psec measurement is more challenging and probably not reliable. Direct
measurement of the final 0.2-psec bunch is quite a different issue. Bunch length monitors [42]
designed to use coherent synchrotron radiation (CSR) have demonstrated fast, non-invasive
measurements in the LCLS regime. They, however, provide a relative bunch length measurement.
Absolute bunch length requires an understanding of the frequency spectrum of the radiation, the
various component attenuation functions, and the CSR process.
7.8.2.1 Transverse RF Deflector
A very promising technique to measure the micro-bunch after BC2 is to use a transverse rf
deflecting cavity. This idea has been used in the past [43], [44] and has been suggested again
recently [45]. The high frequency time variation of the deflecting field is used to ‘pitch’ or ‘yaw’
the electron bunch, while the resulting transverse beam width is measured on a simple profile
monitor (OTR). This is a reliable, single-shot measure of the absolute bunch length. The
technique is completely analogous to a streak camera, but with much better potential resolution.
Detailed studies have been made of this technique [46], including wakefield and chromatic
effects, and recent beam measurements have also been made [47]. As an additional benefit, 2.44-
meter long S-band rf deflecting structures are immediately available at SLAC, where they were
fabricated and tested in the early 1960’s [48]. A cut-away view of the S-band traveling-wave rf-
deflector is shown in Figure 7.63.
The bunch length, σz, can be calculated from knowledge of the deflecting voltage, V0, the rf
wavelength, λrf, and the beam energy at the screen, Es.
2 2 ( σy −σy0)
λrf
E
σ
s
z ≈
2π eV0
sin∆ψ cosϕ
βdβs (7.27)
Included here is the product of (βdβs) 1/2 sin(∆ψ), which is the (measurable) vertical transfer
matrix element from angle-to-position and deflector-to-screen. Finally, ϕ is the rf phase of the
deflector (ϕ = 0 at zero-crossing) and σy and σy0 are the measured vertical beam sizes with rf-on
and rf-off, respectively. The voltage of the deflector is easily calibrated using simple BPM
measurements as a function of RF phase. Table 7.23 lists the parameter values associated with
this bunch length measurement after BC2. The rf deflecting structure will be placed downstream
of the BC2 chicane at 5.4 GeV at the 25-5a location (at S ≈ 475 m in Figure 7.18) where an
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