Longitudinal profile diagnostic scheme with subfemtosecond resolution for high-brightness electron beams

High-resolution measurement of the longitudinal profile of a relativistic electron beam is of utmost importance for linac based free-electron lasers and other advanced accelerator facilities that employ ultrashort bunches. In this paper, we investigate a novel scheme to measure ultrashort bunches (subpicosecond) with exceptional temporal resolution (hundreds of attoseconds) and dynamic range. The scheme employs two orthogonally oriented deflecting sections. The first imparts a short-wavelength (fast temporal resolution) horizontal angular modulation on the beam, while the second imparts a long-wavelength (slow) angular kick in the vertical dimension. Both modulations are observable on a standard downstream screen in the form of a streaked sinusoidal beam structure. We demonstrate, using scaled variables in a quasi-1D approximation, an expression for the temporal resolution of the scheme and apply it to a proof-of-concept experiment at the UCLA Neptune high-brightness injector facility. The scheme is also investigated for application at the SLAC NLCTA facility, where we show that the subfemtosecond resolution is sufficient to resolve the temporal structure of the beam used in the echo-enabled free-electron laser. We employ beam simulations to verify the effect for typical Neptune and NLCTA parameter sets and demonstrate the feasibility of the concept.

Figure 1

Conceptual scheme for longitudinal profile diagnostic with sub-fs resolution. The undulator provides coupling between the laser mode and ultrashort electron bunch to impart an angular modulation. The deflecting cavity provides a vertical sweep in the orthogonal direction. The pattern is swept on a distant screen. The beam distributions at each element along the beam line are given by $X_i$, where $i=0,1,2,3$ corresponds to the 6D beam distribution (0) initially, (1) after the modulator, (2) after the deflector, and (3) at the screen, respectively.

Figure 2

Energy modulation of the beam (top) and horizontal angular modulation (bottom) after the laser interaction in the undulator calculated using Eq. (7). The longitudinal coordinates are normalized to the drive laser wavelength.

Figure 3

Depiction of the transverse distribution on the screen calculated from the analytical expressions of Eq. (10). The axes are normalized to ${\sigma }_{x_D}$, the unperturbed beam size at the detector due only to the drift (i.e. with the laser and deflector turned off).

Figure 4

Elegant simulation of the transverse profile at the end of the diagnostic line using Neptune parameters. The horizontal modulation and vertical streak are provided by the laser modulator and the deflecting cavity, respectively.

Figure 5

Beam longitudinal phase space at the exit of the second chicane for the case when beam density modulation is generated by the 1590 nm laser alone (left) and that by the interplay of the two lasers (right).

Figure 6

Transverse beam profile at the screen for the case when the beam density modulation is generated by the 1590 nm laser individually (left) and that when the density modulation is generated by the interplay of the two lasers (right). The top figures show these cases when the deflector is powered on but the laser modulator (undulator) is turned off. The bottom figures illustrate the enhanced resolution when the laser modulator is turned on.