free electron lasers 159
Since the repetition rates of the typical FELs is still below
that of storage rings, the average brilliance of FELs exceeds
that of third-generation light sources typically by about 2–4
orders of magnitude.
The FELs are also unsurpassed in temporal resolution,
exceeding the third-generation by 2–4 orders of magnitude,
able to reach femtosecond resolutions in special configurations of low-charge sliced beams (the methods of ultra-short
bunch generation are discussed in more detail in Chapter 10).
8.6.4 Typical FEL design and accelerator challenges
Modern high-brightness FELs aimed at hard X-ray ranges are
sophisticated machines. A typical layout of such an FEL is
shown in Fig. 5.39. A high-brightness laser-driven photo RF
gun, operating from a kHz to potentially MHz range, can
power a linear accelerator, a normal conducting (NC) or a super conducting (SC). Typically, two bunch compressors, at intermediate and final energies, are used to achieve the shortest
possible beam length. The accelerated and compressed electron beam is sent into an undulator, or switched between different undulators.
Practical constraints (size, cost) split the FEL designs into
two families — NC and SC FELs, which operate under two
distinct sets of parameters. Normal conducting FELs can have
the highest electron beam energy and therefore the shortest
X-ray wavelength; however, they are pulsed and usually have
rather low (sub-kHz) repetition rates.
Superconducting FELs, on the other hand, can be either
pulsed (but with long pulses and many bunches in a single pulse) or are continuously operating (CW) with repetition rates of bunches up to MHz. They, however, are generally limited in electron energy and cannot reach the shortest
wavelength (see Table 8.2).
TABLE
NC and SC FEL, typical parameters
Characteristics
NC-FEL
SC-FEL
Linac energy
6–15 GeV
2–4 GeV
Linac frequency
S, C, X band L band
Repetition frequency 50–200 Hz kHz–MHz
Operation mode
pulsed
long pulse to CW
Minimum wavelength 0.1 nm
0.3–1 nm
8.2
Accelerator physics challenges in FELs start from the
source — a very low emittance gun is needed, seeing as the
normalized emittance cannot be improved in the linac. Acceleration and compression through the linac needs to be performed to keep the emittance low while managing the collective effects of high-brightness beams (which is not trivial,
Since the repetition rates of the typical FELs is still below
that of storage rings, the average brilliance of FELs exceeds
that of third-generation light sources typically by about 2–4
orders of magnitude.
The FELs are also unsurpassed in temporal resolution,
exceeding the third-generation by 2–4 orders of magnitude,
able to reach femtosecond resolutions in special configurations of low-charge sliced beams (the methods of ultra-short
bunch generation are discussed in more detail in Chapter 10).
8.6.4 Typical FEL design and accelerator challenges
Modern high-brightness FELs aimed at hard X-ray ranges are
sophisticated machines. A typical layout of such an FEL is
shown in Fig. 5.39. A high-brightness laser-driven photo RF
gun, operating from a kHz to potentially MHz range, can
power a linear accelerator, a normal conducting (NC) or a super conducting (SC). Typically, two bunch compressors, at intermediate and final energies, are used to achieve the shortest
possible beam length. The accelerated and compressed electron beam is sent into an undulator, or switched between different undulators.
Practical constraints (size, cost) split the FEL designs into
two families — NC and SC FELs, which operate under two
distinct sets of parameters. Normal conducting FELs can have
the highest electron beam energy and therefore the shortest
X-ray wavelength; however, they are pulsed and usually have
rather low (sub-kHz) repetition rates.
Superconducting FELs, on the other hand, can be either
pulsed (but with long pulses and many bunches in a single pulse) or are continuously operating (CW) with repetition rates of bunches up to MHz. They, however, are generally limited in electron energy and cannot reach the shortest
wavelength (see Table 8.2).
TABLE
NC and SC FEL, typical parameters
Characteristics
NC-FEL
SC-FEL
Linac energy
6–15 GeV
2–4 GeV
Linac frequency
S, C, X band L band
Repetition frequency 50–200 Hz kHz–MHz
Operation mode
pulsed
long pulse to CW
Minimum wavelength 0.1 nm
0.3–1 nm
8.2
Accelerator physics challenges in FELs start from the
source — a very low emittance gun is needed, seeing as the
normalized emittance cannot be improved in the linac. Acceleration and compression through the linac needs to be performed to keep the emittance low while managing the collective effects of high-brightness beams (which is not trivial,