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Table 3. Demonstrated wavefront correction of the DM/WFS closed-loop system
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 20Focusing of the short pulse laser is achieved with an off-axis parabolic mirror. In table 4 below, OAP3 represents the optic used in the standard beam delivery for full intensity. See the description of the beam delivery on the MEC Components page of the LCLS website (until the Confluence page is updated). OAP1 and OAP2 use narrow angles and are thus difficult to use with most target frames. OAP4 has a smaller clear aperture and will clip the full energy beam.
| 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. |
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Figure 4. A false color log scale focal spot image of the focus from OAP with full wavefront correction. The measurement was taken at reduced power but full power inferred intensity is shown. Lineouts are shown in linear scale. The right image depicts a false color image in linear scale.
Radiation Interlock System
To prevent the exposure of personnel to ionizing radiation generated through high-intensity laser-matter interactions, the MEC SPL employs a Radiation Interlock System as a safety control. More information can be found here: 0.3. Radiation Interlock System.
Pulse Contrast
Nearly all user experiments using the multi-TW beam require excellent contrast to be successful. To achieve highest baseline pulse contrast, the short pulse laser system employs a double-CPA front end. After initial chirped pulse amplification of the oscillator pulses at 120 Hz, the beam is recompressed and passed through a nonlinear filter (with an OPA and SFG-based pulse cleaner [1] replacing XPW starting in Run 17) and then stretched again for final amplification. This filter removes prepulses and pedestal energy that builds up primarily in the laser front end. Measurements utilizing third order autocorrelation techniques revealed a contrast better than 107 in the > 3 picosecond range using a double CPA pulse cleaning system. To further increase this contrast, MEC can provide a 2nd harmonic stage that, in combination with dichroics, should theoretically achieve better than 1014 contrast beyond 3 ps, while sacrificing pulse energy (output ~300 mJ at 400 nm).
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