Direct Observation of Spatiotemporal Dynamics of Short Electron Bunches in Storage Rings

In recent synchrotron radiation facilities, the use of short (picosecond) electron bunches is a powerful method for producing giant pulses of terahertz coherent synchrotron radiation. Here we report on the first direct observation of these pulse shapes with a few picoseconds resolution, and of their dynamics over a long time. We thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses (a broadband Terahertz comb) stemming from the shortness of the electron bunches. Above this threshold, a large part of the emission is dominated by drifting structures, which appear through spontaneous self-organization. These challenging single-shot THz recordings are made possible by using a recently developed photonic time stretch detector with a high sensitivity. The experiment has been realized at the SOLEIL storage ring.

Figure 1

(a) Global experimental arrangement. The SOLEIL storage ring is operated with many short electron bunches (209 in the experiment: 208 successive bunches and one isolated bunch). The THz coherent synchrotron radiation pulses emitted in a bending magnet are recorded in single shot with picosecond resolution at the THz/IR AILES beam line, using high-sensitivity photonic time-stretch electro-optics sampling. (b) Distribution of the electron bunch charges over one turn for this experiment. The three dots indicate the electron bunches whose CSR pulse evolutions are displayed in Figs. 2 and 3. (c) Photonic time stretch electro-optical sampling setup. Blue lines are polarization-maintaining (PM) fibers (PM980). Green lines are single mode fibers (HI1060). Fibered polarizing beam splitter (PBS), off-axis parabolic mirror (OAPM), polarizer (P), 5 mm long, [110]-cut gallium phosphide crystal (GaP), quarter-wave plate (QWP), variable optical attenuator (VOA), and adjustable delay line (delay). The balanced detector is an DCS-R412 from discovery semiconductors. The use of the four ZnSe plates enables us to enhance the setup detectivity (see text and Ref. [15]). The polarizations of the laser and THz field are parallel and oriented along the $[-110]$ axis of the GaP crystal (and peremptory to the figure plane). The QWP is adjusted at 45° with respect to the laser polarization.

Figure 2

Typical single-shot electro-optic sampling traces $V_n^{\mathrm{EOS}}(t)$ (corresponding to the THz CSR electric field emitted by one electron bunch, at four successive turns in the storage ring). The electron charge is 75 pC [second dot, i.e., bunch no. 65 on Fig. 1]. For clarity, the signals are shifted vertically by 0.1 V.

Figure 3

(a)–(c) Terahertz pulses shapes [electro-optic sampling signal $V_n^{\mathrm{EOS}}(t)$] vs round-trips $n$ in the storage ring. Below and above the microbunching instability threshold. Panels (a)–(c) correspond to bunch charges of $Q=42$, 75, and 118 pC, respectively [i.e., bunch numbers 61, 65, and 92 in Fig. 1]. Each horizontal cut corresponds to the electric field shape (as in Fig. 2). (d)–(f) Pulse energy versus turn number (integration over the time of the square of the EOS signal, and low-pass filtered at 50 kHz). Horizontal black lines and dots placed for visual guide. (g)–(i) Average electro-optic signal $V_{\mathrm{avg}}^{\mathrm{EOS}}(t)$. Note that the data (a) and (b) are recorded simultaneously (i.e., they correspond to a single oscilloscope recording).

Figure 4

Numerical simulations of the bunch dynamics, with a charge $Q$ of 42, 95, and 140 pC, left, middle, and right column, respectively. (a)–(c) Electron distribution in the phase space (at the turn number $n=0$). (d)–(f) Evolution of the electric field pulses (inside the chamber) $E_{wf}(t,n)$ vs turn number, with $t=-q{\sigma }_z/c$, $n=\theta /(2\pi f_s{T}_0)$, $c$ the light velocity. (g)–(i) average over the turns of the emitted pulses. Simulation parameters: $E_0=2.75\mathrm{GeV}$, $f_s=928\mathrm{Hz}$, ${\sigma }_E=E_0\times 1.017\times {10}^{-3}$, ${\sigma }_z=0.92\mathrm{mm}$, ${\tau }_s=3.27\mathrm{ms}$, $R_c=5.36\mathrm{m}$, $T_0=1.181\mu \mathrm{s}$, $h=12.5\mathrm{mm}$. Measured average bunch length (rms) from numerical data: 3.25, 3.5, 3.9 ps for $Q=42$, 95, and 140 pC, respectively.