Collective deceleration: Toward a compact beam dump

With the increasing development of laser electron accelerators, electron energies beyond a GeV have been reached and higher values are expected in the near future. A conventional beam dump based on ionization or radiation loss mechanisms is cumbersome and costly, not to mention the radiological hazards. We revisit the stopping power theory of high-energy charged particles in matter and discuss the associated problem of beam dumping from the point of view of collective deceleration. The collective stopping length in an ionized gas can be several orders of magnitude shorter than that described by the Bethe-Bloch formulas and associated with multiple electromagnetic cascades in solids. At the same time, the tenuous density of the gas makes the radioactivation negligible. Such a compact beam dump without radioactivation works well for short and dense bunches, as they are typically generated from a laser wakefield accelerator. In addition, the nonuniform transverse wakefield can induce microbunching of the electron bunch by betatron oscillation. The microstructure could serve as a prebunched source for coherent radiation or feeding a free electron laser.

Figure 1

Collective deceleration and its saturation. The relative decrease of total bunch energy is given versus scaled propagating distance for different initial bunch energies $E_0$: 100 MeV, 1 GeV, 10 GeV, and 100 GeV. The plasma density is $n_e=n_b/5\approx 1.1\times {10}^{19}{\mathrm{cm}}^{-3}$.

Figure 2

Electron energy distribution during collective deceleration in nonstructured plasma. Energies are given versus longitudinal position ($x$ in units of plasma wavelength) at different propagating distances: (a) $x=1\mathrm{mm}$, (b) $x=1.25\mathrm{mm}$, and (c) $x=1.4\mathrm{mm}$. The plasma density is $n_e=2n_b\approx 4.4\times {10}^{19}{\mathrm{cm}}^{-3}$, and the initial energy of bunch electrons is 500 MeV.

Figure 3

Suggested beam dump. (a) The proposed structured plasma consisting of a stopping layer with thickness $L_s$ and periodic plasma slabs separated by a vacuum gap. $L_p$ is the period of the structured plasma. In each period, the plasma slab length is equal to the vacuum length. (b) Evolution of bunch energy for both uniform plasma and structured plasma with $L_P/{\lambda }_{pe}=2$, 5, and 10. (c) Electron energy vs $x$ position at $x=2\mathrm{mm}$ for the case of $L_P/{\lambda }_{pe}=2$. (d) Improvement of energy loss rate due to the structured plasma for different plasma densities plotted in terms of ${\sigma }_L/{\lambda }_{pe}$, keeping bunch sizes ${\sigma }_T=10\mu \mathrm{m}$ and ${\sigma }_L=3\mu \mathrm{m}$ constant.

Figure 4

Suggested beam dump. (a) The structured plasma with periodic thin foils inserted. $L_p$ is the separation of the neighboring foils. (b) Bunch energy evolution for both uniform plasma and the structured plasma with $L_P/{\lambda }_{pe}=1$, foil length $0.1{\lambda }_{pe}$, and density $n_{\mathrm{foil}}=100n_e$. (c) Electron energy vs $x$ position at $x=2\mathrm{mm}$.

Figure 5

Microbunching during deceleration. Snapshots of bunch density for the propagation distances (a) $x=0.5\mathrm{mm}$ and (b) $x=0.8\mathrm{mm}$. (c) Display of bunch density distributions along the dashed lines in (a) and (b). Simulation parameters are the same as in Fig. 2.