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The short pulse laser at MEC is a double-CPA laser designed to operate at >25 TW peak power, with < 40 fs pulse duration and > 1 J per pulse. A schematic of the laser system is shown in Figure 1. A commercial CPA (Legend) system, seeded by an oscillator that is RF locked to LCLS, generates mJ-level compressed pulses, which are passed through an OPA pulse cleaning system before being stretched again for further amplification. The first bowtie amplifier operates at 120 Hz, matching the LCLS x-ray pulse rate, outputting >10 mJ per pulse. A second bowtie stage amplifies to >1.5 J at 5 Hz repetition rate. After compression, this yields > 1 J in 40 fs, for 25 TW peak power.
Figure 1. Diagram of the MEC short pulse laser system.
Pulse Duration
The final output has a pulse duration of approximately 35-55 fs, with a beam diameter of 65 mm, and an energy of 1 J. The compressor grating spacing can be adjusted to produce between 35 fs and 8 ps pulses. For pulses of longer duration, stretched pulses with 150 ps duration have been provided successfully.
Figure 2. (legacy information: please contact) Wizzler measurement of the pulse shape of the MEC short pulse laser indicating compression to nearly transform-limited pulse at 23 nm bandwidth.
Wavefront Correction
After amplification, a 90 mm clear aperture deformable mirror (DM; cf. Figure 3), using feedback from an online wave front sensor (WFS), corrects the distortions produced in the laser chain. Focal spot and wave front measurements in the chamber allow for pre-correction of wave front distortion from the compressor and focusing optics. This system is necessary for producing the highest intensity at focus.
Figure 3. A photo of the deformable mirror of the MEC short pulse laser is shown.
| Before Correction | Before Correction | After Correction |
|---|---|---|
| RMS | 0.6 µm | 22 nm |
| P-V | 2.5 µm | 100 nm |
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Focusing
Short pulse experiments use non-standard beam delivery, so users should contact MEC scientists to discuss the best focusing options and expected performance for their experiment. An inventory of off-axis parabolic mirrors (OAPs) is given in table 4. Note that the standard full-power (1 J, <50 fs) beam size is roughly 65 cm, and so OAP 4 is incompatible with this mode. Recent example measurement of the focus from OAP 3 is given in figure 4.
MEC OAP Inventory for Run 20
| Name | CA (mm) | Effective focal length(mm) | Off-axis angle (deg) | coating | notes |
|---|---|---|---|---|---|
| OAP1 | 99 | 311.66 | 17.55 | Dielectric | Incompatible with standard target mount |
| OAP2 | 99 | 311.66 | 17.55 | Dielectric | Incompatible with standard target mount |
| OAP3 | 99 | 330.4 | 35.31 | Silver | Primary OAP for full intensity |
| OAP4 | 50 | 500.58 | 34.41 | Silver | For long focal length and low power. Only compatible with reduced beam size and power. |
Table 4. The OIPs owned by MEC. OAP 3 is standard for full intensity experiments. OAP 1 and 2 have too tight of an angle to use with the standard target frame, but may be used in certain customized experiments. OAP 4 is intended for frequency doubled or uncompressed experiment in which the beam size is smaller than standard. Typical use is to produce an f/10 focus.