The experimental scheme of the backward Raman amplifier described in Refs. 13 and 14. The seed pulse after splitter S1 goes through the Raman shifter, compressor C2 and enters the vacuum chamber from below. The pump pulse after splitter S1 goes through compressor C1 and enters the chamber from above.
The plasma density profile in the plasma capillary. The upper plot shows the 2D density profile in the capillary assuming axial symmetry. Also shown is the path of the pump propagation in vacuum in the experiments with tilted alignment of the laser beams. The pump pulse enters the channel from the left and the seed enters the plasma from the right. The lower plot shows the plasma density profile along the path of the pump beam in axial and tilted experiments (Refs. 13 and 14).
Scaling of the amplified seed duration with its energy. The solid line represents the theoretical fit with . A least mean square error fit gives .
Amplified pulse (red solid line) and original seed pulse (blue dashed line) autocorrelation measurements in tilted experiments. The original seed pulse is amplified by pump and reaches the energy of . The autocorrelation measurements indicate that the output pulse has an intense spike and either the tail of the secondary spikes or a precursor. The duration of the main spike is about and it contains about 50% of the total pulse energy (Refs. 13 and 14).
The upper plot shows the output energy of the amplified pulse in experiments with tilted geometry of the laser beams. Pump energy , pump duration , input seed energy mkJ, plasma channel length . The tilt of the laser beams (both the pump and the seed pulses) is done with respect to the axis of the plasma channel. Lack of symmetry in the plane (tilt in horizontal and vertical direction across the channel) is not understood. We speculate that it is caused by misalignment of the laser beams due to refraction in the plasma channel. The lower plot shows the output energy in experiments with flipped sign of the pump chirp (negative chirp). Pump energy, pump duration, seed energy, and plasma channel length were the same as for the upper plot. The study of the negative pump chirp (lower plot) set up of the experiment is less complete than positive pump chirp set up. However, we observe the reduced efficiency in the tilted experiments compared to the axial experiments.
Simulated output amplified pulse intensity profile. The upper plot: The initial Gaussian seed of duration and is amplified in the uniform pump of intensity in plasma. Pump chirp . The overall efficiency of amplification . The lower plot: The seed pulse is the same as above but the tails of the Gaussian seed are truncated at the 20% level. The output pulse is shorter and more intense than if the tails of the Gaussian seed are not suppressed.
The upper plot shows a theoretically estimated efficiency of BRA as described above. It has well defined maximum at a certain pump intensity. The lower plot shows experimentally measured dependence of the BRA efficiency vs the pump energy (Ref. 14). The intensity of the pump is proportional to the pump energy since the other parameters of the pump are fixed. The solid line is the least square parabolic fit of the experimental data points.
Article metrics loading...
Full text loading...