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Abstract
The latest advances in petawatt laser technology within the ELI Beamlines project have stimulated the development of large surface area dielectrically coated mirrors meeting all demanding requirements for guiding the compressed 30 J, 25 fs HAPLS laser beam at 10 Hz repetition rate and a center wavelength of 810 nm entirely in vacuum. We describe the production and evaluation of Ta2O5/HfO2/SiO2 ion beam sputtered coated (440 × 290 × 75) mm3 beam transport mirrors. No crazing was observed after thirty vacuum-air cycles. A laser induced damage threshold of 0.76 J/cm2 (fluence on mirror surface) was achieved and maintained at high shot rates.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Since Mike Perry’s et al. milestone achievement of breaking the petawatt (PW, $10^{15}$ watts) peak power barrier in 1996 at the Lawrence Livermore National Laboratory [1], new high-intensity laser facilities around the world [2–4] have put enormous effort to advance PW laser technology and its applications in high energy density science, plasma physics [5,6] as well as in societal applications for instance in the medical field [7]. While chirped pulse amplification (CPA) [8] avoids nonlinear effects and laser induced damage in the amplifiers by stretching the short-pulse by typically $\sim 30,000\,{\rm x}$ and consequently reduces the peak intensity by the same amount, it cannot reduce the ultra-high peak intensities on optics behind the pulse compressor. The final grating of the pulse compressor and all PW beam transport and focusing optics need to withstand pulses with duration of $20\,$fs up to few ps and energies ranging from a few $10\,$J for fs pulses to a few kJ for ps pulses. To avoid catastrophic damage of the compressor gratings and the downstream mirrors, the laser beam is magnified in front of the compressor to not exceed $\sim 0.3\,{\rm J/cm^{2}}$ for fs pulses ($< 0.5\,$meter size optics) and a few ${\rm J/cm^{2}}$ in the case of $10\,$ps pulses (meter size optics) [9]. The gain medium of choice for few $10\,$fs pulses is Ti:Sapphire (Ti:Sa) with a center wavelength at $800\,$nm and a typical bandwidth of $60$ to $80\,$nm. PW lasers with $150\,$fs to few ps pulse duration employ Nd:glass often combined with optical parametric chirped pulse amplification (OPCPA) in the front-end and centered at $1053\,$nm with few nm to $11\,$nm bandwidth [3]. Being designed for $10\,$Hz repetition rate, which is a major advancement over existing petawatt laser technology, the high-repetition-rate advanced petawatt laser system (HAPLS) is a Ti:Sa system. HAPLS was built at Lawrence Livermore National Laboratory (LLNL). It is the world’s first diode-pumped and with $300\,$W the highest average power petawatt laser system ever built with a beam normal fluence of roughly 70 mJ/cm$^{2}$, if an ideal flat intensity top-hat beam is assumed. The real beam has intensity modulations originating from the propagation of phase errors of non-ideal optics of the amplifier and compressor system as well as the beam transport system up to the target [5]. As a consequence, the primary challenge of the HAPLS beam transport mirror coating design and manufacturing described in this study, was to obtain a highest possible laser induced damage threshold (LIDT) while meeting the reflectivity (${\rm Rp\ge 99.8\%}$), the group delay dispersion (${\rm GDD\leq 50\ fs^{2}}$) and bandwidth ($80\,$nm for p-polarization) requirements at an angle of incidence (AOI) of $45^{\circ }$. The LIDT of the large sized optics is expected to be decreased by target debris and local intensity spikes. Thus the dielectric mirrors require the highest possible LIDT margin.
This demand illustrates a basic conflict in thin film technology as there is just a limited number of coating materials available for the production of high-end laser mirrors. In general, dielectric materials with highest index of refraction are characterized by a lower optical band gap while dielectric materials with a lower index of refraction show a higher optical band gap. In the femtosecond regime the LIDT is mainly driven by electronic processes via multiphoton and avalanche ionization [10,11]. As a consequence, the LIDT of dielectric layers deposited from high refractive index materials is worse than for low refractive index materials [12]. In the last years several approaches successfully demonstrated that the LIDT can be improved by the formation of new layer structures as the deposition of ternary composites or quantized nanolaminates [13–15]. Both concepts, as well as numerical design approaches, can be applied to optimize the electric field distribution inside the layer stack and thus improve the LIDT of optics required for high power PW applications [16–18]. Such sensitive approaches require stable coating processes with highest thickness precision mostly provided by sputtering techniques.
However, in PW applications the beam transport requires large size optics. In general, such mirrors are manufactured applying classical electron beam evaporation processes (EBE) [19]. This technique can provide large coated areas with high deposition rates in contrast to low rate sputtering techniques, but EBE coatings have the disadvantage to be more porous [20,21]. The fundamental characteristic of porous layers results in changing properties of EBE coatings depending on the surrounding media. Well-known effects are a spectral shift of the transmission curve (wet-dry-shift), and a change of mechanical coating stress. Such a change of tensile stress leads to an enhanced risk of coating crazing in vacuum on low thermal expansion substrate materials, e.g. fused silica [22]. Furthermore the roughness of EBE coatings is worse compared to IBS coatings which can locally lead to higher absorption and thus decrease the LIDT [23]. In the past large sized mirrors up to a diameter of 550 mm were fabricated by IBS. Such mirrors were optimized for lowest losses at 1064 nm for the Laser Interferometer Gravitational wave Observatory (LIGO) and VIRGO interferometers [24,25].
In our study we show that coatings fabricated by ion beam sputtering (IBS) on large size optics can be superior to the state-of-the art electron beam evaporation coatings. We describe the IBS coating machine, that was developed at Laseroptik for large size optics. Further the LIDT, reflectivity, GDD, wavefront and vacuum compatibility of the $(440 \times 290 \times 75)\,$mm$^{3}$ mirrors are characterized.
These mirrors were installed in the HAPLS beam transport [26] system at ELI beamlines, first to the Gammatron beamline [27]. The Gammatron beamline is a user-oriented source of bright, ultrafast hard X-ray radiation with a photon flux beyond $10^{11}$ photons per pulse with optimized laser and plasma conditions [28–30] and operates at the repetition rate of 10 Hz. In addition these mirrors will be used to guide HAPLS to the experimental hall E5 for electron acceleration, that requires high LIDT turning mirrors to avoid possible damage caused by the phase-amplitude modulation during ca. 100 meter of propagation.
2. Materials and methods
2.1 Deposition of large optics applying ion beam sputtering
An IBS coating machine has been developed and built for the production of the large-aperture petawatt mirrors for the HAPLS beam transport. It comprises three major components: commercially available ion sources, a large size vacuum chamber and a planetary gear system (Fig. 1). The latter is an in-house design capable for loading it with two substrates of up to a maximum diameter of $550\,$mm and $50\,$kg weight.
Fig. 1. Schematics of the developed IBS-coating machine: two planets can be equipped with round or rectangular substrates up to a diameter/diagonal of 550 mm.
In detail, an Argon plasma is generated in a RF ion source from which ions are extracted and accelerated with $1.5\,$kV by a grid system onto a target dedicated to the desired coating material. The target tower can be equipped with three different sputter materials. A secondary assisting ion source is emitting oxygen ions and can be used to pre-condition the optical surfaces. This configuration enables sputtering of dielectric materials with a well-balanced stoichiometry. The sputtered atoms are directed towards the planetary gear system. The resulting dielectric layer stack is formed by the condensation of the sputtered atoms on the substrate/layer surface. The process requires a base pressure of $10^{-6} - 10^{-7}$ mbar. For the process control, the layer thickness is measured during coating with an in-situ high-precision optical broad band monitor [31]. The rotation of the substrate is required to achieve optimum uniformity of the dielectric layers across the deposition area. An additional planetary gear system is further optimizing the radial layer uniformity. In total the layer gradient is less than $0.5$ % over the full diameter of 550 mm on every planet. Mirror blanks are cleaned in an ultra-sonic cleaning stage and handled with automatic lifting tools before and after coating.
2.2 LIDT setup
Maximizing the LIDT of dielectric mirrors is one of the key tasks for the development of petawatt lasers. The damage test setup at ELI Beamlines is shown in Fig. 2. The femtosecond laser (805 nm, 42 fs, 1 kHz) propagates through a beam shutter, which controls the number of applied laser pulses with a closing activation time of less than 4.1 ms. An energy attenuator is used to control the pulse energy. It consists of an achromatic half-wave plate to continually rotate the polarization of the incident beam with respect to a thin film polarizer (TFP). The TFP reflects the s-polarized component into a beam dump while the one with p-polarization is transmitted and guided by a leaky mirror into the vacuum chamber. Beam leak is usually less than 1$\,\%$ of energy. The beam is then focused with a spherical silver mirror (f = 800 mm) down to 150 $\mathrm{\mu}$m diameter onto the surface of the test sample.
Fig. 2. LIDT setup: The femtosecond laser propagates through an ultrafast shutter, half-wave plate, thin film polarizer (TFP), and is reflected by the leaky mirrors and the focusing mirror onto the sample. Behind the first and second leaky mirror there is an online diagnostic of the laser energy and the beam profile. The damage detection camera monitors online the scattered light of a 530 nm laser diode by the damage sites on the surface. The laser beam profile is shown on the bottom right. The effective diameter of the focused laser beam on the sample surface is $150\,{\mathrm{\mu} m}$.
Precise beam size and energy measurements are crucial for LIDT measurements. Before and after each measurement, energy calibration and beam size measurements are performed. The laser energy calibration is performed according to Velpula et al. [32]. The beam profile is determined by a vacuum compatible CMOS camera which measures the leak from a dielectric mirror after the focusing mirror. The beam profile measurement from the leak of the focused beam is cross-checked twice before and after the LIDT test with the direct measurement of the beam profile at the location of the sample surface. The damage detection is based on the online measurement of the scattered light from the sample surface, which is illuminated with a 530 nm laser diode. In addition, the reflectivity of the sample at the test wavelength of 805 nm is measured during the tests.
Prior to the LIDT tests the samples are kept 72 hours in the vacuum chamber at a pressure of approximately $1 \times 10^{-6}$ mbar. Subsequently the samples are irradiated with multiple pulses according to the test protocols R-on-1 and S-on-1 of the ISO standard 21254 [33]. The R-on-1 (ramp) is a non-standard test [34]. The initial R-on-1 tests are used for choosing the maximum fluence for the S-on-1 measurements, because the ramped LIDT is higher than in the S-on-1 case [35]. This helps minimizing the debris. After determining the fluence level for catastrophic damage with the R-on-1 measurements, the S-on-1 measurements with S = 100,000 shots are performed. The sample under test is irradiated with the same pulse energy at five different sites. Then the energy is decreased and five different sites are exposed. After completion of the laser irradiation an ex-situ damage site evaluation is performed with a Nomarski differential interference contrast (DIC) microscope (Olympus BX50 and OLS5100) equipped with a 50$\times$ magnification objective.
3. Results and discussion
The coating design is optimized to meet the requirements of the turning mirrors $(440 \times 290 \times 75)$ mm$^{3}$ for the HAPLS beam transport (Table 1).
Table 1. Minimum requirements of the dielectric mirrors for the HAPLS beam transport.
According to the mirror design the reflectivity reaches ${\rm Rp\ >\ 99.8}$ % and is centered at 810 nm with a group delay dispersion (GDD) of ${\rm |GDD(Rp)|\leq 50\,fs^{2}}$ over a spectral width of $\Delta \lambda _p = 95$ nm. For s-polarized light the corresponding design yields a reflectivity of ${\rm Rs\ >\ 99.9}\,\%$ and a ${\rm |GDD(Rs)|\leq 50\,fs^{2}}$ over a spectral width of $\Delta \lambda _s = 170$ nm (Fig. 3 a)). In addition the reflectivity of 50.8 mm diamter witness samples is measured with a spectrophotometer (Photon RT by Essent Optics). The calculated and measured reflectivity for both, s- and p- polarized light agree well within the measurement noise as shown in Fig. 3 b).
Fig. 3. Reflectivity for s- and p-polarized light of the applied coating design for an AOI of $45^{\circ }$. a) calculated reflectivity curves and b) comparison of theory and spectrometer measurement.
The GDD measurement (performed with a commercial white light interferometer "Chromatis${\rm ^{TM}}$") matches well with the calculated design (Fig. 4). Measurements were performed for several samples of different coating runs and show no significant deviations.
Fig. 4. Group delay dispersion (GDD) of the mirror design compared with the measurement applying white light interferometery at an AOI of $45^{\circ }$. a) s-polarization and b) p-polarization.
The spectral width of the GDD and the reflectivity covers fully the requirement for the HAPLS beam. Such a large bandwidth for a high LIDT design is a major advantage of the optimized coating design and the high precision and reproducible IBS process.
Before manufacturing the turning mirrors, the uniformity of the mirror coating was verified. Therefore, numerous 25 mm diameter fused silica substrates were positioned on different radii on the planet in the IBS machine. The dimension of this test mount is equal to the size of $(440 \times 290 \times 75)\,{\rm mm^{3}}$ of the turning mirrors (Fig. 5 a) - upper right). Both, the measurement of the transmission with a spectrophotometer (Perkin Elmer Lambda1050) (Fig. 5 a) , and the measurement of the dispersion (GDD) (Fig. 5 b), show that the design requirements are fulfilled over the full aperture of the turning mirrors on both planets. The deviation of the shift of the designs of these witness samples is smaller than $0.25\,\%$. This value was determined by finding the central wavelength of every measured transmittance spectrum of the HR810nm/45$^{\circ }$ mirror and calculating the respective relative ratio. This minimal shift is fully covered by the spectral width of the design and does not impact the spectral requirements of the turning mirror.
Fig. 5. Measurement performed on witness samples at ${\rm AOI\ =\ 45^{\circ }}$ with p-polarized light. The witness samples ($\varnothing 25\,{\rm mm}$) are located at different positions on the substrate mount and cover the entire aperture (440 x 290) ${\rm mm^{2}}$ of a turning mirror. The colored positions of the mount are related to the three measured curves at radius r$=0\,, 120\,\, {\rm and}\, 220\,$mm. a) GDD-measurement applying a white light interferometer and b) transmission measurement applying a spectrometer.
Sputtered layers are very dense and thus characterized by a negligible spectral shift from vacuum to air. It is verified by measuring the transmission of a coating witness sample at the same position in vacuum after 2 h at $2 \times 10^{-7}$ mbar and subsequently at ambient air by using the spectrometer of the broad band monitor (BBM). A vacuum-air shift of less than 0.004 % (40 ppm) was determined by evaluating the shift of the relative central wavelength. Such a small shift can be caused by the coating itself or by a slightly changed AOI of the measurement due the mechanical forces during pump-down of the coating chamber.
The primary challenge for the design and fabrication of the dielectric coating is the required high laser induced damage threshold for 25 fs pulses. For the design selection some well-placed HfO$_2$ layers are added to a typical $\lambda$/4 stack of alternating Ta$_2$O$_5$ and SiO$_2$ layers to manipulate the electric field distribution inside the coating stack similar to an approach presented by Patel et al. [36]. The LIDT is expected to increase significantly with the added HfO$_2$ layers, which is a major benefit of Laseroptik’s proprietary coating design.
Coating witness samples with a diameter of 50.8 mm and the same surface finish were placed next to the large size mirror blanks during every coating run. These witness samples were used for testing the LIDT. A R-on-1 measurement was performed before and after an S-on-1 measurement to check the stability of the laser system. In the R-on-1 test the energy is ramped up until catastrophic damage occurs (Fig. 6 b). The R-on-1 LIDT amounts to $(1.3 \pm 0.05)$ J/cm$^{2}$ on the surface for s-polarized light at an AOI of $45^{\circ }$ and 42 fs in vacuum. A damage threshold of $(0.9 \pm 0.09)$ J/cm$^{2}$ on the surface (target plane) is determined for S-on-1 with 100,000 pulses. Figure 6 c) and d) shows the details results of the LIDT tests. The LIDT results of two typical samples from two different coating runs are displayed for R-on-1 and S-on-1 (Fig. 6a). The measured LIDT is identical within the error bars for both samples. This underlines the repeatability of the IBS process and the stablility of the LIDT test bench at ELI Beamlines.
Fig. 6. LIDT testing protocols. a) Summary of S-on-1 and R-on-1 tests of two typical samples with the same coating design but from two different coating runs. b) Typical R-on-1 ramp up for sample B. Black curve indicates the increasing fluence values over time. The red curve is the damage indicator, which rises abruptly when damage occurrs. The damage indicator shows the scattering from the sample registered with the damage detection camera (see Fig. 2 for LIDT setup). c) and d) LIDT S-on-1 test with 100,000 shots shown for sample A and B. The fluence values are calculated on the sample surface for s-polarized light.
Two failure modes were observed during the LIDT tests. Before catastrophic damage occured, color changes were detected with a Nomarski type DIC microscope (Fig. 7 a) and c)), which do not generate scattered light for the damage detector. Consequently the irradiation is performed to the full 100,000 shot cycle. The ablation of the coating occured at the highest fluence and always triggered the beam abort due to the scatter diagnostics (Fig. 7 b) and d)). The color change visible under the DIC microscope can be a change of the refractive index n caused by the laser exposure [37,38].
Fig. 7. Coating damage sites: Images were taken with a laser confocal microscope a) surface fluence $0.97\,{\rm J/cm^{2}}$, 100k pulses, b) surface fluence $1.08\,{\rm J/cm^{2}}$, 578 pulses, c) DIC color changes and catastrophic damage overview, DIC images were converted to the gray scale to increase the contrast of the damages, d) surface fluence $1.35\,{\rm J/cm^{2}}$, 393 pulses.
Within this LIDT study we also evaluated the impact of different substrate cleaning processes on the LIDT. We compared manually and ultrasonically cleaned substrates before coating. While the difference of the LIDT for two different cleaning processes was negligible in the case of s-polarization, there was a significant difference for p-polarization. The LIDT of the sample with the manually cleaned substrate before coating showed more than 50 % difference between of s- and p-polarized light. For the coating on the ultrasonically cleaned substrate the difference in LIDT between s- and p-polarization was only 30 %. Lower LIDT using p- versus s-polarization has been reported in literature [39]. For the final coating run it was decided to clean the substrates automatically in different ultrasonic cleaning stages, which are highly reproducible and reliable.
By applying the scaling law of Mero et al. [40] the measured LIDT of $0.9\,$J/cm$^{2}$ (surface fluence/ target plane) at 42 fs and 1 kHz can be scaled to other pulse lengths. At the pulse length of $25\,$fs (HAPLS) a S-on-1 (with 100,000 shots) LIDT of 0.76 J/cm$^{2}$ is calculated for s-polarized light.
However, a precise forecast for the LIDT behavior of the turning mirrors during operation with a full size HAPLS beam is difficult. The measured LIDT might be conservative due to the 100 times lower repetition rate of the HAPLS beam [41,42]. In contrast, a beam size dependence of the LIDT and a possible influence of nodules could make the threshold too optimistic [43,44]. In addition intensity modulations within the ($210 \times 210$) mm$^{2}$ HAPLS beam could limit the maximum possible average operation fluence. Based on scaled low intensity beam profile measurements after 54 m of propagation a peak fluence of 200 mJ/cm$^{2}$ is expected at 30 J operation [26]. Compared to the presented LIDT measurement with 150 $\mu$m test diameter, the 4$\times$ safety factor in LIDT should allow a save operation with the full size 30 J HAPLS beam.
An additional major source of laser induced damage in optical materials are particles and chemical contamination from the assembly and handling [45]. Sputtered IBS layers are very compact and tend to absorb less chemical contamination. One turning mirror was contaminated with water stains during handling. This contamination could be removed by manual and automatic ultrasonic cleaning without damaging the layer surface. Ultrasonic cleaning is prohibitive for porous e-beam (EBE) coatings.
The introduced mechanical stress of IBS coatings is independent of the surrounding media (vacuum or air) and thus the mirrors show a negligible wavefront change and spectral shift during vacuum cycles. The $(440 \times 290 \times 75)$ mm$^{3}$ IBS coated turning mirrors for HAPLS beam transport were tested with 30 vacuum cycles at a pressure of $10^{-6}$ mbar and for 10 hours per cycle. Visual inspection afterwards showed no crazing of the coating which is often seen for conventional mirrors manufactured by EBE [46], when exposed to vacuum or dry atmosphere. The prevention of crazing and insignificant wavefront changes as well as spectral shifts when exposed to vacuum are the key benefits of sputtered coatings for PW applications.
As mentioned above, plane substrates will become slightly convex after applying an IBS coating due to the compressive coating stress. The radius of curvature (ROC) of the manufactured turning mirror is estimated to be 101 km for the 75 mm thick substrates using measured stress data of the dielectric materials as input for the Stoney formalism [47]. The resulting flatness of the coated mirror depends strongly on the surface of the substrate. In the case of the described HAPLS beam transport mirrors all substrates had a specified peak-to-valley (PV) value $\leq 125$ nm. Both the polish of the substrate and the induced stress of the coating cause a reflected wavefront error (RWE), which was evaluated and analyzed with a Zygo Verifire interferometer operating at 1053 nm and at an AOI of $64^{\circ }$. This high AOI is required to cover the full clear aperture of the turning mirror ($396 \times 261$) mm$^{2}$ with the 12"- interferometer at Laseroptik. In total 10 large size turn mirrors were characterized before and after coating.
Figure 8 shows the measured surface (${\rm AOI} = 0^{\circ }$) of a typical substrate before and after coating with tilt and piston removed. The resulting flatness of ten analyzed uncoated substrates varies from about +120 nm PV (convex) up to -120 nm PV (concave shape). After coating the PV ranges from -385 nm up to -214 nm. If required, the RWE can be decreased by a well calculated stress compensation rear side coating optimized for the specific front side coating.
Fig. 8. Measured surface deformation using a Zygo Verifire interferometer. Power and astigmatism are not removed. The CA is set to (395 x 261) ${\rm mm^{2}}$. a) uncoated turn mirror substrate surface with a peak-to-valley (PV) of 63 nm, 12.6 nm RMS. b) Coated turn mirror substrate surface showing a convex shape with a PV of 335 nm, 65 nm RMS.
Figure 9 shows the calculated single pass RWE of the turn mirror depicted in Fig. 8 b) at use angle, i.e. ${\rm AOI\ =\ 45^{o}}$ and masked to ($230 \times 230$) mm$^{2}$. This leaves more than sufficient margin for the alignment of the ($210 \times 210$) mm$^{2}$ super-Gaussian HAPLS beam. It is important to note that power and astigmatism of the RWE may be compensated to a large extend by an adequate alignment of the focusing parabola in the target chamber. In Fig. 10, we show a coated HAPLS beam transport mirror during its installation in the ISO5 (class 100) cleanroom facility of ELI Beamlines.
Fig. 9. Single pass RWE of a coated turn mirror at ${\rm AOI\ =\ 45^{o}}$. Both plots are masked to (230 x 230) ${\rm mm^{2}}$ and piston and tilt are removed. a) with power removed and b) power and astigmatism removed.
Fig. 10. Photo of a coated (440 x 290 x 75) mm$^{3}$ HAPLS beam transport mirror during its cleanroom ISO 5 installation at ELI Beamlines.
4. Conclusion
The ELI Beamlines user facility [48,49] currently commissioned in Dolní Břežany close to Prague in the Czech Republic, provides access to beyond the current state-of-the-art high average power petawatt laser technology for basic science and multidisciplinary applications. For the reliable operation of the complex HAPLS beam transport system large size broadband mirrors are key optical components. These mirrors require optimization for highest laser induced damage threshold and vacuum compatibility [26].
This paper reports the successful coating design and deposition process development as well as the construction and operation of a dedicated IBS coating machine to manufacture these large size dielectric petawatt mirrors. The new coating machine is capable to deposit up to two substrates with a respective diameter of up to $550\,$mm and a weight of 50 kg. We have deposited with this machine the $(440 \times 290 \times 75)\,{\rm mm^{3}}$ turning mirrors for the HAPLS beam transport system to the Gammatron x-ray source of ELI Beamlines and validated that they meet the highly demanding requirements on LIDT, bandwidth and wavefront. We have developed a coating design which fulfills the $\ge 76\,{\rm nm}$ bandwidth requirements and provides a very high LIDT. The LIDT of the design was tested at ELI Beamlines with s-polarized light on $50.8\,$mm diameter witness samples and has a measured multi-pulse damage threshold (S-on-1) of 0.9 ${\rm J/cm^{2}}$ (in target plane) at 42 fs, 1 kHz, 805 nm, S = 100,000 in vacuum. The R-on-1 LIDT is about 1.3 ${\rm J/cm^{2}}$. The uniformity over the entire clear aperture of the turning mirrors is better than 0.25 %. We have measured a negligible spectral vacuum-air shift of less than $< 40\,{\rm ppm}$. The comparison of the measured reflected wavefronts of both, the uncoated substrates and the manufactured turning mirrors show negligible degradation of the Strehl ratio [50] of the focused beam, when the target parabola is aligned to compensate for the coating stress introduced power and astigmatism. The power term is the dominant surface error. The manufactured turning mirrors showed no crazing after 30 vacuum cycles. Another key benefit of the dense IBS coatings is the possibility to clean them fully automatically and reliably with ultrasonic cleaning stages. This is a major benefit when compared to conventional porous e-beam coatings, which have a high risk to craze in vacuum and may be cleaned only manually. We expect that this will enlarge the lifetime of IBS coated petawatt mirrors significantly and consequently increases the up-time and lowers the operation costs of PW laser beamlines.
Funding
Horizon 2020 Framework Programme (871161); European Regional Development Fund (CZ.02.1.01/0.0/0.0/15_003/0000449, CZ.02.1.01/0.0/0.0/16_019/0000789).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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