Energy slicing analysis for time-resolved measurement of electron-beam properties

Abstract

We present a new diagnostic method allowing one to reconstruct certain properties of a relativistic electron beam and of a laser pulse by looking at the result of their interaction. The latter takes place within an undulator and results in energy modulation of electrons. By taking advantage of the correlation between time and electron-beam energy, measurements of the electron-beam length, current and energy spread are possible simultaneously. Moreover, the scheme allows an accurate measurement of the timing jitter between the electron beam and the external laser and also of the laser’s actual pulse length at the interaction point. Experimental demonstration of the last two measurements is reported in this paper. Finally, this diagnostic method provides an independent measure of the energy transferred from the electron beam to free-electron laser light produced when seeded electrons are injected into a long undulator. Experimental results obtained at the FERMI free-electron laser are presented after a description of the proposed technique

Online single-shot characterization of ultrafast pulses from high-gain free-electron lasers

“…By ‘slicing’ Fig. 2 a along the time dimension, we can obtain the time-dependent electron beam parameters after lasing [32] . The magenta, red and green lines represent the electron beam current profile , the central energy and the FEL-induced energy spread , respectively.…”

“…There are also indirect methods to acquire the FEL temporal profiles by measuring electron bunches after lasing. One of them is based on the measurement of the electron beam longitudinal phase space with a transverse deflecting cavity [29] , [30] , [31] , [32] . The reconstruction of the temporal profile of a single photon pulse becomes possible when comparing the phase space distributions between “lasing-on” and pre-recorded “lasing-off” shots.…”

Laser heater commissioning at an externally seeded free-electron laser

“…Data shown in this section were acquired for a 2.8 ps (rms) long, 500 pC charge electron beam that is shortened by about a CF 5.5 in BC1 (R 56 ¼ −41 mm). The X-band cavity installed upstream of BC1 was switched off during these measurements so that the magnetic compression acted upon a nonlinear energy profile and produced a ramped current distribution close to that shown in [35], with a peak current ∼350 A. In this case, both the current and the energy spectrum of the beam appeared temporally modulated at the end of the linac.…”

How the optical timing system, the longitudinal diagnostics and the associated feedback systems provide femtosecond stable operation at the FERMI free electron laser

“…In order to guarantee that the seed overlaps with the region of the electron bunch with the required properties, the relative temporal stability between both (jitter and drift) should be better than 100 fs (rms) [14] ; therefore, the seeding laser needs to be phase locked to the machine phase reference signal to achieve such a goal [15] . Also, in the transverse plane, the orbit of the electron bunch and the launch path of the seeding laser have to be co-planar for at least the length of the modulator undulator (≈3 m) within <20 µm.…”