Enhanced Acceleration of Injected Electrons in a Laser-Beat-Wave-Induced Plasma Channel

Enhanced energy gain of externally injected electrons by a $\sim 3\mathrm{c}\mathrm{m}$ long, high-gradient relativistic plasma wave (RPW) is demonstrated. Using a $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ laser beat wave of duration longer than the ion motion time across the laser spot size, a laser self-guiding process is initiated in a plasma channel. Guiding compensates for ionization-induced defocusing (IID) creating a longer plasma, which extends the interaction length between electrons and the RPW. In contrast to a maximum energy gain of 10 MeV when IID is dominant, the electrons gain up to 38 MeV energy in a laser-beat-wave-induced plasma channel.

Figure 1

(a) Lineouts of fluorescence images of hydrogen plasmas at 165 mTorr after subtracting the background for a two-wavelength short (curve $1$), single-wavelength long (curve $2$), and two-wavelength long (curve $3$) pulses. The arrowed line shows a part of the plasma where the difference between curves $3$ and $2$ is a factor of $2$ or larger. (b) Pressure dependence of peak plasma brightness for two-wavelength (circles) and single-wavelength (solid line) long pulses and a total number of accelerated electrons from 12.5–15 MeV (squares).

Figure 2

(a) Time dependence of the number of electrons with energy 22 MeV (diamonds) obtained using a short laser pulse (SP) and 27 MeV (squares) obtained with a long pulse (LP) and the schematic pulse profiles for a SP (dotted line) and a LP (dashed line). (b) Combined spectra of electrons accelerated by a LP driven PBWA recorded using two magnet settings: the low-energy side (12–27 MeV) at 0.4 T and the high-energy side (27–50 MeV) at 0.7 T. The black arrows indicate the saturation of the detector’s amplifier.

Figure 3

(a) Laser field and (b) ion distributions at 162 ps [short pulse (SP)] and 400 ps [long pulse (LP)] from the beginning of the laser pulse. The laser pulse propagates from the left to the right; the black vertical line marks the position of the laser beam focus in vacuum. The superimposed dashed lines show the laser envelope in vacuum.