Lessons Learned in LEX Photonics

Since LEX Photonics was a prototype test for CALA the most important findings that we learned are collected herein. Those ”lessons learned” are partially based on data presented within this work but also include the authors opinion and experience of daily operation.

Lesson 1. Regen prepulse: The temporal laser contrast is a limiting factor of the ion acceleration process. Significant prepulses are created inside the REGEN, preventing an efficient acceleration of ions, especially using thin targets.

Lesson 2. Back-reflection: It is essential to monitor the back-reflected light in order to keep the laser safe. An operation under non-zero degree suppresses back-reflected light.

Lesson 3. Pockels cell: A Pockels cell in the laser chain is essential in order to enable the opportunity to hit the target at normal incidence. For practical rea-sons the alignment possibilities of the Pockels cell are crucial and the height has to be adjustable as well as the angles of the cell and the polarizers to reach its best performance.

Lesson 4. Laser beam delivery: Observing the scattered light of the laser on each mirror (after compression) facilitates reproducible adjustment of the laser through the Laser beam delivery on a daily basis. An absolute mirror position or end-switches would be a beneficial advancement.

Lesson 5. Incoming laser light: Input laser pulse parameters must be known and well characterized for each shot and directly available to the operator. Characterizing the laser directly in the target chamber has the further advantage that problems in the LBD can potentially be detected directly. When the transmission mirror had to be reworked the lack of information was clearly perceivable during the experiment so a spare one would be desirable in CALA.

Lesson 6. Back-scattered light: The analysis of the back-reflected light can pro-vide useful information. In LEX Photonicsthe transmission of a mirror of the LBD was used, thereby suppressing the fundamental laser wavelength λL. A mirror with determined transmission for λL would be favorable. It was further useful that the mirror was highly transmissive to green lasers that could be used for alignment or potentially also heating of targets to remove contaminant layers.

Lesson 7. Spatial Intensity: The spatial distribution of the laser focus reveals that a significant amount of energy can be distributed to larger areas. The careful analysis, for example through the HDR imaging described in [150], yields accurate peak intensity values. Those are otherwise significantly overestimated.

Lesson 8. Debris: A high repetition rate with a large number of shots causes severe debris problems that significantly alloy the parabola and mirror surface. In LEX Photonics we had no solution to this problem.

Lesson 9. Plasma Mirror: It can be a very cost efficient and effective way to use a thin foil as a plasma mirror. The prototype version demonstrated the functionality but lacked the possibility to optimize the alignment and neighboring targets were often corrupted.

Lesson 10. WASP calibration: The energy measurement with the Radeyes placed behind an WASP worked quite well. However, the analysis of the data required pre-cise knowledge of the magnetic fields, distances and positions. Since the evaluation was very sensitive an inherent calibration would be beneficial.

Lesson 11. Shielding QP: The shielding of the entrance of the QPs is crucial and causes severe ablation of laser light transmitted through the target. The ablated matter is deposited onto the target due to the vicinity and can even destroy targets.

Lesson 12. Orientation QP: A good control of the rotation and tip/tilt angle of the QPs is important and misalignment cause the star-like shape of the ion focus.

The new procedure of pre-alignment was very useful. However, it would be beneficial to further adjust the rotation of the PMQ during the experiment, thus being able to optimize the focal shape analogously as it is done for the primary laser focus.

Lesson 13. Data monitoring: An automatized data acquisition was one of the major improvements compared to previous experiments. However, the data acquisi-tion did not include all diagnostics yet and should thus be further improved. It could also include further safety mechanisms. For example interlocks to prevent a laser shot with full power when the microscope is at focus position.

Chapter 4

TRIC: Temporally Resolved Intensity Contouring

Plasma dynamics measured in a single-shot (see Fig. 4.2).

Contents

4.1 Introduction to TRIC . . . 62 4.2 Setup and Configuration of the Experiment . . . 64 4.3 Interpretation of the Observation. . . 68 4.4 Analysis and Evaluation of the Measurement . . . 73 4.5 Summary and Discussion . . . 79 This chapter covers the method of TRIC. We developed a single-shot multi-frame probing technique, that enables a time-resolved study of the plasma during the interaction with a pump beam, in our case the laser pulse. It thereby can be as-sociated to pillar two of theILDIASconcept. As a first application of this technique we determined the spatio-temporal intensity distribution of the ATLAS-300 laser pulse at full energy directly at the focal position. This experiment has been carried out in the second vacuum chamber (see Fig.3.5). This chapter and its figures closely follow the publication ofTRIC [161].

Author contribution: This project has been conducted with all co-authors of [161], in particular together with Martin Speicher and Jianhui Bin. The experimen-tal campaign and the analysis of the results have been led by the author.