In-situ real-time Langmuir probe and target current investigations of fs-laser irradiated optical components and targets (LPTC)

Programul: PNCDI III, Programul 5. Subprogramul 5.1 ELI-RO
Tipul proiectului : ELI-RO
Nr. contract: 04/16.10.2020
Durata proiectului: 16.10.2020-15.10.2023
Director Proiect: Dr. Valentin Craciun
Colaboratori: ELI-NP, grup Dr. Daniel Ursescu

ELI-NP, one of the most important research infrastructure existing in Romania must be run at the designed parameters with minimum cost. The optical beam transport system, which is at the center the facility must work flawlessly and any problems should be detected as early as possible to allow for rectifying measures to be swiftly taken before any catastrophic events would occur disrupting the operation of the facility. One of the most worrisome problem for the optical beam transport system is that of fs-laser induced damage of optical components (mirrors, beam splitters, gratings, etc.). The fluence and number of pulses for the laser induced damage threshold (LIDT) value provided by the manufacturers are affected by errors due to stochastic nature of the involved mechanisms. During multipulse fs-laser irradiation various phenomena are occurring in the optical coatings that are not completely understood; thus, a generic term, the “incubation effect” is employed to account for all these effects [Appl Surf.Sci.233, 1-4, 2004]. The appearance of the incubation effect lowers the LIDT value and leads to a catastrophic damage of the coatings. Besides the high cost of the destroyed optical component used in ELI-NP facility, a catastrophic damage could create a plasma mirror on the surface that might deflect the extremely powerful laser pulse in an unpredictable direction and cause additional extensive damage to other expensive equipment nearby. There is a complex monitoring control system using detectors, CCD cameras, etc, ready to detect any malfunction.

Figure 1. Mobile reaction vacuum chamber for LP characterization of laser induced plasmas.
Unfortunately, the main limitation of all implemented monitoring techniques is their inability to detect the appearance of the incubation effect: in other words, since the optical characteristics of a mirror, gratings or beam splitter are marginally affected during the multipulse fs-laser irradiation at fluences below the LIDT value, these techniques could not estimate when the incubation phase started and the optical component is close to reaching the threshold for irreversible damage and be destroyed by the next laser pulse. Based on our previous results that showed that the leakage current in an optical dielectric coating increases by orders of magnitude when the incubation stage is reached we hypothesize that such an increase could be detected by a Langmuir probe (LP) installed in the proximity of the optical component to measure the emitted current after fs-laser irradiation. Therefore, the LP is an in situ and in real time sensor monitoring the correct working of optical components, being dielectrics or metals, but also the beginning of the incubation stage and warning when that component is close to its end of life. Such a LP system is easy to be installed, since all components are under vacuum, either in the transport beam system or inside reaction chambers. In addition, we also propose a complementary technique, the measurement of the current through the target, which must “mirror” the emitted charge and showcase small modification in the target’s structure and properties. This will allow us to develop clear methodologies for determination of incubation effect or any changes that might be induced during irradiation. Another major advantage of the proposed LP technique is its ability to detect the emission of adatoms from optical components or targets that were exposed to the ambient. These adatoms, when present, might interfere with the optical characteristics of the component or target.

Figure 2. Our research on Langmuir probe characterization of laser induced plasma is on the cover of PLASMA PROCESSES & POLYMERS 17, 11e2000136, 2020

The third major advantage is that LP, as many articles showed, could be used as a plasma diagnostic tool to estimate the velocity and energy of emitted electron and ion beams during fs laser irradiation of various targets. Our approach has a major advantage over a Thomson parabola: the LP position with respect to the irradiated target could be easily adjusted without breaking the vacuum with a mechanical x-y-z manipulator to allow for a spatially and radially resolved diagnostic of the plasma and emitted beams. By performing such experiments with ns, ps and fs lasers, over 6 orders of magnitude, we will also gain a better understanding of the LP working and the physics of laser-matter interaction. The project team synergistically assembles theoreticians and experimentalists with great expertise in the area of fs-laser, laser-mater interaction, vacuum science and plasma diagnostics.