Plasma-driven ultrashort bunch diagnostics

Ultrashort electron bunches are crucial for an increasing number of applications, however, diagnosing their longitudinal phase space remains a challenge. We propose a new method that harnesses the strong electric fields present in a laser driven plasma wakefield. By transversely displacing driver laser and witness bunch, a streaking field is applied to the bunch. This field maps the time information to a transverse momentum change and, consequently, to a change of transverse position. We illustrate our method with simulations where we achieve a time resolution in the attosecond range.

Figure 1

Layout of the proposed streaking setup. A short-pulse laser drives a linear wakefield in a plasma target. The electron bunch is situated off-axis at the transverse maximum and longitudinal zero-crossing of the transverse fields. In a drift or imaging optic after the interaction, the imprinted momentum change is translated into a change of transverse position carrying the time information.

Figure 2

Bunch crossing the streaking wakefield at an angle $\alpha$ in the $x-z$-plane and at a displacement of ${\sigma }_r$ in the $y$-direction. The colors represent the amplitude of $E_y$ from a PIC simulation, and the electron beam is Gaussian, for illustration. The shown field slice is indicated by the dashed box on top. Please see text for details.

Figure 3

Top: Angular deflection $p_y/p_z$ depending on the initial longitudinal position $z_{\mathrm{ini}}$ within the bunch after the interaction with the streaking wakefield. In a drift or imaging optic, this correlation translates to a correlation of $y$ with $z_{\mathrm{ini}}$. Bottom: Screen image at 1 m drift after the interaction with the wakefield. The black line is the current profile, reconstructed by binning the screen image to the $y$ axis. The initial current profile is given in dashed red for comparison. For reference, $z_{\mathrm{ini}}$ corresponding to $y$ on screen is given at the left axis. The blue line (displaced for visibility) is the current profile reconstructed from the interaction geometry featuring a 5° angle between laser and electron beam path (see text).

Figure 4

astra simulations of a delay scan including an arrival time jitter of 10 fs rms between driver laser and witness bunch (a) or 10 fs timing jitter as well as $500\mu \mathrm{rad}$ pointing and $75\mu \mathrm{m}$ positioning jitter (b). For each delay 50 shots are simulated. Plasma, laser and bunch parameters are like in the PIC simulation from fig. 3. The gray dots show the median of each bunch profile in $y$ on the screen. The wavelength is reconstructed from a sine fit (solid red) to the median of all shots at each delay (dashed blue) to (a) $32\mu \mathrm{m}$ or (b) $35\mu \mathrm{m}$. The deflection amplitude is obtained from the minimum and maximum deflections to (a) 4.0 mm or (b) 4.8 mm.