Time-resolved electron beam phase space tomography at a soft x-ray free-electron laser

Abstract

High-gain free-electron lasers (FELs) in the ultraviolet and x-ray regime put stringent demands on the peak current, transverse emittance, and energy spread of the driving electron beam. At the soft x-ray FEL FLASH, a transverse deflecting microwave structure (TDS) has been installed to determine these parameters for the longitudinally compressed bunches, which are characterized by a narrow leading peak of high charge density and a long tail. The rapidly varying electromagnetic field in the TDS deflects the electrons vertically and transforms the time profile into a streak on an observation screen. The bunch current profile was measured single shot with an unprecedented resolution of 27 fs under FEL operating conditions. A precise single-shot measurement of the energy distribution along a bunch was accomplished by using the TDS in combination with an energy spectrometer. Variation of quadrupole strengths allowed for a determination of the horizontal emittance as a function of the longitudinal position within a bunch, the so-called slice emittance. In the bunch tail, a normalized slice emittance of about 2 m was found, in agreement with expectations. In the leading spike, however, surprisingly large emittance values were observed, in apparent contradiction with the low emittance deduced from the measured FEL gain. By applying three-dimensional phase space tomography, we were able to show that the bunch head contains a central core of low emittance and high local current density, which is presumably the lasing part of the bunch.

Reversible electron beam heating for suppression of microbunching instabilities at free-electron lasers

“…(2)], the induced transverse kick correlates linearly with the internal bunch length coordinate (z ¼ ct) and enables high-resolution temporal diagnostics (e.g., Refs. [14][15][16]), whereas the induced relative energy deviation correlates with the vertical offset inside the TDS and results in an induced relative energy spread Á ¼ K y y . Here, the symbol denotes the root mean square (rms) value, and y is the vertical rms beam size.…”

“…The vertical betatron motion of electrons passing through a TDS is described by Eq. (4), which enables a mapping from time (longitudinal coordinate) to the vertical [14][15][16], and finally a possibility to obtain temporal bunch information by means of transverse beam diagnostics. In a similar manner, the relative energy deviation is mapped to the horizontal in the presence of horizontal momentum dispersion, like in a magnetic bunch compressor chicane (see, e.g., Refs.…”

“…In the case of acceleration between the first and second TDS, also energy jitter, which is similar or smaller than TDS voltage jitter, due to this intermediate acceleration leads to deviation of the condition in Eq. (15). The choice of superconducting accelerator technology even provide much better rf stability [38,39].…”

“…[14][15][16][17]) and are proposed to use for novel beam manipulation methods (e.g., Refs. [18][19][20][21][22][23]).…”

Few-femtosecond time-resolved measurements of X-ray free-electron lasers

“…The resulting image on a downstream screen forms a 'streak' of the electron bunch, from which the pulse duration and temporal profile can be obtained. If this deflector streaks the electron beam horizontally and a dipole magnet disperses the beam vertically, we can obtain the full time-energy longitudinal phase space of the electron beam on a single shot by measuring the downstream transverse images 30,31 .…”