The generation of a quasi-monoenergetic electron beam in laser-driven plasma acceleration is reported. A monoenergetic electron beam with an energy of 7 MeV was emitted from a high-density plasma ͑electron density Ͼ10 20 cm −3 ͒ produced by a 2 TW 50 fs laser pulse. The divergence of the monoenergetic beam was ±1.2°. The first Stokes satellite peak of stimulated forward Raman scattering was observed in the spectrum of the light transmitted through the plasma. The plasma wave was excited in the region of which electron density was around 1.3ϫ 10 20 cm −3. The acceleration length was estimated to be 500 m from the length of the side-scattered light image. It is considered that the monoenergetic beam generation is due to the matching of the acceleration length to the dephasing length determined by the velocity difference between the accelerated electrons and the plasma wave.

We have demonstrated the acceleration of a monoenergetic electron beam by a laser-produced wakefield. Experiments were performed by focusing 2-TW laser pulses of 50 fs on supersonic gas-jet targets. The focused intensity was 5 ϫ 10 18 W0cm 2~a 0 ϭ 1.5!. At an electron density of 1.5 ϫ 10 20 cm Ϫ3 , the clear monoenergetic electron beam from the plasma was obtained at 7 to 15 MeV. The Stokes satellite peak in the forward scattering explained the energy spectra of electrons at various plasma densities well. Although the wakefield propagated 500 microns, which was far beyond the dephasing length, monoenergetic electron beams were obtained.

Monoenergetic electron beams were generated in the self-modulated laser wakefield acceleration regime using a 2-6 TW, 50 fs Ti:sapphire laser system. The monoenergetic electron beams of energies up to 15 MeV and 30 MeV, with a plasma density around 1.5ϫ 10 20 cm −3 and 3.5 ϫ 10 19 cm −3 , respectively, were observed. The monoenergetic energy was found to be inversely proportional to the plasma density. The monoenergetic electron beam was generated at only specific plasma densities for each experimental condition. The plasma density dependence of the electron energy spectrum, the forward scattered light spectrum, and the side scattered light image of the laser pulse was studied in detail. The conditions for monoenergetic electron beam generation are discussed based on the results of the plasma diagnostics.

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