Abstract
Keywords
1 Introduction
New high-intensity laser facilities around the world[1–4] based on chirped pulse amplification (CPA)[5] have revolutionized both our understanding and use of plasma physics. Recent advancements in laser technology have stimulated the development of petawatt (PW) systems up to 3.3 Hz[6]. The ‘ELI-Beamlines facility’[7,8] in Dolní Břežany, close to Prague in the Czech Republic, is based on the European Strategy Forum on Research Infrastructures (ESFRI) process[9]. The project is executed in close partnership with Lawrence Livermore National Laboratory and a European–US consortium from Ekspla (Lithuania) and National Energetics. The international user facility will provide access to laser technology that is beyond the current state of the art. The 1 PW at a repetition rate of 10 Hz of HAPLS (High-repetition-rate Advanced Petawatt Laser System) and the 10 PW at 1.5 kJ in 150 fs at a shot rate of one pulse per minute will allow the generation of ultra-high focused laser intensities approaching the ultra-relativistic regime (
This paper focuses on the beam transport (BT) system of the HAPLS to P3 only as it is the first, which became fully operational in the E3 experimental hall at the end of 2019. The branch to the E3 hall with the P3 experimental infrastructure serves as a testbed for qualifying the engineering approach. The HAPLS laser BT system of ELI-Beamlines will guide in the future the 30 J, 30 fs compressed pulses under vacuum also to the other three experimental halls E2, E4 and E5 over distances of up to 100 m and via three switchyards. The commissioning was performed with a maximum pulse energy of 110 mJ.
First light from the HAPLS in P3 was obtained in December 2019. Figure 1 shows the evolution of the experimental hall E3 over less than 2 years. By November 2019, E3 had been operational and ready to receive the HAPLS beam. The P3 chamber was a pure in-house project. The design was initiated in summer 2014 and the chamber was delivered and installed in December 2018.
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Figure 1.Left: The state of the experimental hall E3 in January 2018. Note that the P3 chamber is not yet fully assembled. Right: The same location in November 2019 with a fully functional BT system and experimental chamber.
Major upgrades over the following years will include the optical switchyard chamber (MOB) as well as the L4f (10 PW, 1.5 kJ, 150 fs), L4p (sub-aperture L4f beam with adjustable pulse length up to the picosecond regime) and L4n (1.9 kJ in a few nanoseconds) BT systems. By the end of 2022, the experimental hall is planned to look as displayed in Figure 2.
Figure 2.Layout of the experimental hall by the end of 2022. With respect to the present state, the BT for L4f and L4n is missing, as is the MOB chamber.
The remainder of the paper is organized as follows. Section 2 presents an overview of the entire BT system from the compressor to the experimental chamber. The support structures and the most important optomechanical subsystems are described in Sections 3 and 4. The subsequent section, Section 5, discusses the wavefront and damage threshold of the transport mirrors. All aspects related to the cleanliness requirements are addressed in Sections 6. The alignment system and procedure are presented in Section 7, followed by the free-space laser beam propagation in Section 8. Section 9 presents the results of the beam diagnostic. The experimental chamber P3 is presented in Section 10. Section 11 presents the results of a proof-of-principle experiment generating X-rays. Finally, in Section 12, a conclusion and an outlook are given.
2 Overview of the HAPLS BT system
Figure 3 shows a bird’s eye view of the HAPLS vacuum BT, which guides the
Figure 3.Top view of the HAPLS vacuum BT system located in the basement of the laser building below the laser floor. The P3 is located in the experimental hall E3.
Figure 4.3D CAD visualization of the HAPLS BT from the injector chamber to the P3 chamber. This figure shows the actual installation in E3.
3 Optimized dynamic design of the mirror towers and breadboards
The main components in the BT system are the
Figure 5.Typical random vibration PSD plot for vibrations in the horizontal direction measured in the experimental halls with no supplies running[19]. Note that NIF limits the PSD to a maximum of at higher vibration frequencies[20].
The FEA model predictions of the first four eigenfrequencies of the second tower in E3, L3-E3-F020 (see Figure 4) and its mode shapes are shown in Figure 6. The mode shapes were optimized for the lowest angular/rotational movements, which should be according to the model
Figure 6.Model predictions of the L3-E3-F020 tower’s first four eigenfrequencies and mode shapes. The next four eigenfrequencies are at 121.9, 128.5, 136.9 and 148.2 Hz[21].
Figure 7.Measured PSD spectrum with eigenfrequencies of tower L3-E3-F020.
Figure 8.Measured acceleration PSD of granite block L3-E3-F040 and the tower L3-E3-F020. The PSD on the granite is /Hz and on the tower /Hz for the frequency band from 50 to 300 Hz. The PSD of the granite for frequencies between 1 and 50 Hz is 10–100 times lower than that of the tower.
The optical breadboards, which are used for connecting the mounted BT turn mirrors via the aluminium chamber base plate (see Figures 9 and 10) and the stainless-steel tower top plate, were vibration optimized together with the towers. The 100 mm thick aluminium breadboard with its four monolithic stainless steel support legs is depicted in Figure 9. The calculated first eigenfrequency of the breadboard is 146.5 Hz.
Figure 9.Photo of the 100 mm thick aluminium breadboard with its four monolithic stainless-steel legs.
Figure 10.Mounted HAPLS BT mirror installed in its vacuum chamber and both, chamber and breadboard legs/feet bolted onto the aluminium chamber base plate in ISO 5 class cleanroom.
It is important to note that the vacuum chamber is mounted onto the chamber base plate without a direct spng connection to the breadboard and its legs to minimize the coupling of chamber vibrations and movements to the breadboard and, subsequently, the mounting mirror (see Figure 10). This ‘decoupling’ concept is well established in the high-power laser community.
4 Design and performance of the ultra-stable mirror mounts and switchyards
To achieve an optical pointing stability of 200 nrad for a measured input vibration PSD of
Figure 11 shows a typical response of the mounted mirror to the excitation with a step function (impact hammer) in the frequency domain with a first eigenfrequency at 109 Hz. For this measurement with an Attocube IDS 3010 sensor, the mirror mount was clamped onto its breadboard, which was mounted onto the chamber base plate bolted onto the granite block L3-E3-F040 (see Figures 4 and 12). The IDS 3010 was mounted directly onto the wall of the E3 hall and detected with 10 MHz acquisition rate the vibration-induced distance changes of a 1 inch diameter retroreflecting metal mirror glued onto the top left corner of the mounted aluminium mirror dummy (Figure 12). The IDS 3010 can measure 1 m distance with a relative resolution of
Figure 11.Displacement PSD with the first eigenfrequencies of the mirror mount measured with an Attocube IDS 3010 sensor. The first eigenfrequency is at 109 Hz, and the second is at 125 Hz. The mount was clamped onto the breadboard, which was bolted onto the chamber base plate, which was bolted onto the granite block L3-E3-F040; see
Figure 12.Wall-mounted Attocube IDS 3010 sensor with mirror mount bolted onto its breadboard installed on the chamber base plate and granite block L3-E3-F040.
The RMS displacement change of the retro mirror on the upper left corner of the mirror dummy measured at a PSD input of the granite block L3-E3-F040 depicted in Figure 8 (purple curve) is 12.7 nm. The distance of the upper left corner of the mirror dummy to the centre of both rotation axes of the mount is 230 mm. If we assume that the main angular pointing stability originates from the rotation axis of the mount, we calculate 55 nrad RMS mechanical pointing stability, i.e., 110 nrad RMS optical on the granite block. This value is within a factor of two in agreement with a measurement performed with three IDS 3010 sensor heads operated with fixed phase and using precision triangulation.
To direct the HAPLS beam either to hall E2 or towards E3, E4 and E5, a 180° rotation unit is required for the lower periscope mirror of the injector (see Figure 13). In addition, a periscope mirror translation switchyard guides the beam, which travels towards E4, down to the E3 hall, whereas a vertical mirror mount translation switchyard redirects the beam to E5.
Figure 13.Overview of the HAPLS BT switchyards in the E3 experimental hall and the E5 switchyard in the L4c hall.
It is important to note that all switchyards are mounted on towers, which have a measured 10–100 times higher vibration excitation PSD compared with the granite blocks (Figure 8). This makes usage of even pre-loaded translation or rotation stages prohibitive, owing to their relatively low spngness, resulting from their ability to move in a
5 The mounted wavefront and laser damage threshold of the high-power BT mirrors and their effect on focus quality
The single-pass peak-to-valley (PV) reflected wavefront error (RWE) of the 440 mm × 290 mm × 75 mm dielectrically coated AOI
Figure 14.Measured RWE at AOI . Left: After subtracting piston, tilt, power and astigmatism. Right: After subtracting piston and tilt only.
The modelled gravity sag of the periscope mounts (
Despite the high-quality reflected wavefront, we took the effort to orient all five mirrors of the E3 BT for an optimum wavefront error cancellation when power and astigmatism are subtracted (parabola alignment). The result of this optimization is shown in Figure 15. The PV error is 97.6 nm and the RMS value amounts 15.8 nm yielding a Strehl ratio (SR) of 99%. MetroPro calculates the SR from the point spread function (PSF) according to Goodman[22]. Adding the RWEs of the
Figure 15.Summation of all five RWEs of the E3 BT mirrors guiding HAPLS to P3, after optimization of the orientation to cancel wavefront deformations and subtracting power and astigmatism. The SR is 99%.
Given these data, the only optic of the BT up to the P3 target, which affects the focus quality of HAPLS is the target parabola. For the commissioning parabola, a decent cost protected gold-coated
Figure 16.Measured surface of the mm 30° off-axis protected gold commissioning parabola, yielding an SR of 96% for the central mm aperture.
Two high-quality dielectric-coated parabolas with an SR of 99% are currently being manufactured.
In addition to the wavefront quality of the transport mirrors, their laser damage threshold (LDT) is critically important. HAPLS will have full energy 30 J in 30 fs at the exit of the pulse compressor with a nominal fluence for an ideal 20th-order super-Gaussian beam of 67 mJ/cm2. Phase errors within the laser and the compressor lead to phase-to-amplitude modulations. These intensity modulations grow, whereas the laser freely propagates to P3 (see Section 8). To keep the risk of catastrophic damage by intensity spikes of the HAPLS beam as low as possible, we chose the coating with the highest LDT that we could find on the market and from a vendor who coated similar mirrors for other high-repetition-rate Ti:Sa PW systems. Figure 17 shows the LDT of a 2 inch diameter coating witness sample evaluated in Garching with the ATLAS laser under
Figure 17.LDT measurement of 2 inch diameter turn mirror coating witness sample with an approximately diameter beam spot of the ATLAS laser on the sample: 5 Hz repetition rate, fs pulse length, p-polarization with respect to the sample, 1 mm raster scan (LEX Photonics, LMU Munich).
6 Cleaning and clean installation
6.1 Cleanliness levels
Cleanliness of the BT vacuum system that houses the multimillion-Euro coated optics, is important for ELI Beamlines because particulate and, most importantly, chemical cleanliness levels affect the LDT and consequently the lifetime of the coatings[24,25]. This damage risk is minimized by a strict cleanroom ISO 5 assembly protocol. In addition, chemical surface contamination as well as the outgassing of all vacuum components is determined prior to their installation. Organic thin-film contamination is of particular concern if the contaminated surface is close to the coated optics, specifically the compressor gratings. Particulate cleanliness levels are defined according to the IEST-STD-CC-1246D norm (equivalent to MIL-STD-1246C). Level 50 is requested for optics and level 100 as best effort for all vacuum components. This level is lowered during the final installation to level 130. Chemical cleanliness is ensured by requiring less than
6.2 Laser cleaning
The P3 chamber was gross cleaned by manual wiping upon delivery and installation, and then pumped down. Figure 18 shows an RGA spectrum after the first pumping cycles. It has several peaks above the LIGO criterion (spectrum denoted as ‘before laser cleaning’ in Figure 18). To reach the required chemical cleanliness levels, we cleaned all dry vacuum surfaces by rastering them with a high average power pulsed fibre laser system. This cleaning method relies on several phenomena. Most importantly, it is the rapid thermal expansion of contaminants (or the surface they are attached to) owing to the absorption of laser light. Heat transfer from incoming laser pulses causes internal mechanical stresses of contaminants resulting in shock waves that help to remove the adhered materials. The main forces responsible for the contamination adhesion are van der Waals forces, capillary forces and electrostatic interactions. The cleaning efficiency is linked to the laser parameters, mainly to the wavelength, fluence and repetition rate. The removal efficiency also depends on the physical properties of the contaminants and the surface. Cleaning can occur when forces induced by laser irradiation overcome the adhesion forces of the particles to the surface. In wet laser cleaning (in the presence of a thin liquid layer), the cleaning action is enhanced by rapid expansion and evaporation of the liquid around the contaminants. With high fluence laser pulses, the cleaning action can be enhanced by the plasma ablation of contaminants and the adhered layer’s emitted shrapnel. High-repetition-rate pulses turn it into a plasma and follow with decomposition to the volatile phase when gases and fumes are extracted to avoid re-deposition.
Figure 18.Comparison of P3 RGA measurements before and after P3 laser cleaning. The levels of contaminants were decreased by two to three orders of magnitude.
The laser cleaning can be partially used for the cleaning of particulates and fibres[26–30]. Cleaning efficiency is limited by absorption rates, especially for contaminants such as plastic fibres, and is relatively low. The requested particulate cleanliness levels could not have been achieved with the laser cleaning alone and required high-pressure spray wash and swiping to remove the particulates.
The laser parameters are summarized in Table 1. The P3 laser cleaning process is illustrated in Figure 19. After 4 days, the entire interior of the P3 vessel was cleaned, and an RGA spectrum was measured again; see Figure 18. The partial pressures of heavy molecules were improved by two to three orders of magnitude, reaching the required cleanliness levels with a decent margin.
Laser parameter | Value |
---|---|
Spot size | 1.9 mm × 1.9 mm |
Power | 475 W |
Wavelength | 1064.7 nm |
Pulse length | 100 ns |
Repetition rate | 5–10 kHz |
Table 1. Summary of the laser parameters of the precision cleaning device used to clean the P3 chamber. It is based on a 1064.7 nm fibre laser with a flat-top profile
Figure 19.Cleaning of the P3 chamber floor shows a clear visual difference between the cleaned surface area and the non-cleaned area[31].
6.3 Cleanliness validation
The cleanliness levels were validated with established procedures. We performed for the large size vacuum components (e.g., chambers, pipes, breadboards) liquid surface rinsing and for small area components swipe tests. The particulate contamination was determined with an automated particle counting microscope HFD4 from Jomesa with which we counted for level 100 all particles larger than 5 μm on a filter membrane, through which we poured a one litre representative particle rinse sample. The NVR was measured using evaporation and precision weighting with 1 μg accuracy. The NVR of individual components was precisely measured by swiping a defined area with a qualified ultra-clean wipe, soaked in a high strength (similar to chloroform) solvent followed by Fourier-transform infrared spectroscopy (FTIR) of the solvent. The wipes were prepared and analysed by Fraunhofer IPA, Stuttgart, with a detection limit better than a few nanograms per millilitre of solvent.
In addition, RGA was performed with a quadrupole mass spectrometer for a few subsystems as a cross-check of the NVR, and finally for each installed BT section and also for the entire system. Figure 20 shows a typical RGA spectrum with the partial pressures of all masses up to 200 AMU at a total pressure of
Figure 20.Measured RGA spectrum of a BT vacuum vessel with the partial pressures of all masses up to 200 AMU at a total pressure of 10−6 mbar.
6.4 Assembly and installation
To meet our tight schedule, the entire HAPLS BT vacuum system and the mirror mounts were tendered cleaned to the above particle- and NVR-level requirements. Because the system is extremely difficult to clean once fully installed, utmost care was taken to minimize contamination during the ISO 5 subsystem assembly and the final installation and commissioning. We ensured that the cleanliness level of the delivered components was validated in our cleanroom ISO 5 facility whenever possible (see Figure 21).
Figure 21.Assembly of HAPLS BT vacuum vessels and optomechanics in cleanroom class ISO 5.
Careful selection of gloves and wipes as well as of all other laboratory equipment, such as mops and tools was performed in a very early stage of the BT project and turned out to be crucial. All products that were causing contamination due to leaving particulates, fibres or other residues were discarded. The cleanliness levels were continuously monitored with particle counters and visual inspection with high intensity LED and UV lamps. All experimental halls are ISO 7 cleanrooms. While this cleanroom class is sufficient for the installation of BT support structures as well as sealed vacuum components, it is not adequate for connecting vacuum subsystems or accessing optics in the vacuum chambers. To be able to access mirror chambers loaded with optics, local clean tents with a few flow boxes on top were constructed and built inside the E3 experimental hall to enable clean connection of vacuum subsystems (Figure 22). This, along with establishing early on stringent ISO 5 laboratory practices and getting advice from other laser facilities, as well as training dedicated personnel, enabled a smooth clean installation with very minimal cleanliness level degradation from the installation process itself. Close attention was also paid to maintain the high cleanliness levels when connecting to the central vacuum and to the venting system. This included a series of filters for venting. We kept also the system sealed and under vacuum and with pumps running whenever possible.
Figure 22.Clean installation in experimental hall E3. Local cleanroom tents were constructed and built to allow clean access to the mirror chambers and to connect them via bellows to the adjacent DN500 pipes.
As shown in Figure 23, P3 is at present operated in an ISO 5 environment.
Figure 23.Working inside P3. The interior of the P3 experimental chamber is treated as a cleanliness class ISO 5 cleanroom, whereas the experimental hall E3 is an ISO 7 cleanroom.
7 The alignment system
There are two complementary ways to align the optics from the injector up to the target inside the P3 chamber: with blue pilot lasers (see Figure 24–26), and with the sub-aperture alignment mode of the HAPLS.
Figure 24.The blue alignment laser setup in E3.
Figure 25.3D CAD picture of the injection of the 417.5 nm blue alignment laser beam into the HAPLS BT system at the lower injector periscope in chamber E3-CH010.
Figure 26.Top: Last BT mirror in chamber E3-CH050 in front of P3 with the alignment module (left) and the second 417.5 nm ‘iBeam smart’ being injected straight to P3. Bottom: Camera image with measured iBeam smart leak beam and shadow of the mirror mount’s rear surface alignment cross (left) and mirror chamber with alignment module (black) installed at the exit of the flange to monitor the pilot laser leak beam position with respect to the alignment cross (right).
7.1 Blue alignment laser beam
A single-mode diode laser emitting at 417.5 nm (iBeam smart from Toptica) is used as a pilot laser for the alignment of the HAPLS to P3 BT system after magnifying the beam to 1 inch diameter. The blue alignment laser is mounted with its beam expander onto a breadboard that is monolithically connected to the wall behind the injector chamber E3-CH010. The setup together with the injection of the pilot beam into the HAPLS BT system is shown in Figure 25. Two 2 inch diameter motorized mirrors are used to make the blue alignment laser collinear with the nominal optical axis of the HAPLS BT system, which is defined by laser-tracker precision-positioned targets.
As depicted in Figure 26, each mirror mount has a rear surface alignment cross, which is centred at the nominal position on the optical axis of the leak beam of the pilot laser, transmitted and refracted by the 75 mm thick
When the pilot laser leak beam is centred with respect to the alignment cross, the mirror front surface reflection is centred on the nominal optical axis of the HAPLS BT system. The 0.5 mm accuracy of the position of the alignment cross is guaranteed by the laser tracker-based installation of the mirror mount onto the breadboard of the mirror chamber and the subsequent laser tracker-based installation of the entire chamber assembly onto its support. The leak beam with the centred shadow of the alignment cross (Figure 26) is demagnified and relay imaged onto a camera in the alignment module as shown in Figure 27.
Figure 27.Schematics of the alignment module in which the leak beam is demagnified and relay imaged together with the shadow of the alignment cross onto a camera.
Figure 28 shows how the front and rear surfaces of the injector bottom periscope mirror in chamber E3-CH010 reflect and transmit the alignment beam. The coated front surface of the injector bottom periscope mirror reflects the blue beam via a special design alignment cross (Figure 25) downwards and through a 100 mm diameter viewport window onto a breadboard located on the E3 floor. On this breadboard the near field (NF) of the beam is measured with respect to the alignment cross and also its nominal position inscribed into the breadboard as shown in the lower right of Figure 28. The nominal position on the E3 floor is determined with a laser tracker and allows setting the tip/tilt of the injector mirror after centring the blue alignment laser beam with the help of the second 2 inch diameter blue injection mirror on the alignment cross of the mirror mount located in the E3 translation switchyard chamber (see Figures 13 and 24).
Figure 28.Front (coated) and rear surface reflections of injector bottom periscope mirror for measuring the position of the blue beam with respect to the alignment cross and the nominal position inscribed into the breadboard on the E3 floor to also adjust the tip/tilt angle of this mirror. See also
After centring all blue leak beams with respect to their alignment crosses in all turn mirror chambers, the HAPLS BT system is aligned to its nominal optical axis up to P3. The transmission losses of the blue laser at each BT mirror are between 40% and 70%, depending on the polarization (orientation of the mirror) and the coating run. Although the beam is still visible behind the P3 focusing parabola, it was too weak for an accurate visual alignment of the AOI
Figure 29.Arrival of the full-size HAPLS beam in P3, the TM-45, the TM-17.5 turn mirror both with their rear surface alignment cross and the off-axis gold coated parabola with a focal length of . All components are pre-aligned with a laser tracker position and angle wise.
7.2 HAPLS laser alignment mode
After the completion of the alignment with the blue alignment laser beam, a 1 inch sub-aperture HAPLS beam is centrally cut out of the full-size HAPLS beam and is injected into the fully aligned BT system. Once the NF of the sub-aperture HAPLS alignment beam is centred at the alignment cross of the injector mirror similarly to the blue pilot beam and once the far field (FF) of HAPLS and the blue beam overlap in the target diagnostics, HAPLS is collinearly aligned to the nominal optical axis of the BT system and the operation of HAPLS at full aperture and energy may be started. The blue alignment laser beam allows to monitor potential misalignments and drifts of the BT system while HAPLS is in operation at any energy level by using a cheap IR blocking colour glass in front of the camera of the alignment module, which transmits the 417.5 nm pilot beam. In addition to avoiding filter wheels to attenuate HAPLS depending on its pulse energy (100 μJ to 30 J) one major advantage of the blue pilot beam alignment system is the fact that it may be automated easily and that the beam may be actively locked to the building with an additional pointing and centring diagnostics at the injector. It is important to note that the weak HAPLS sub-aperture alignment beam may not be detected behind a mirror for s-polarization due to the low transmission (
8 Laser beam propagation
8.1 Phase-to-amplitude modulations of 20th-order super-Gaussian HAPLS beam with measured phase errors
Fusion type lasers such as the Nova laser and the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory use imaging relay telescopes with spatial filtering for keeping intensity modulations as low as possible and for preventing optical damage owing to intensity spikes along the beam path[32,33]. These modulations originate from the Fresnel propagation of the phase errors of non-ideal optics, which are accumulated by the laser beam during its amplification and during the propagation through the BT system up to the target.
For the few nanosecond and narrow bandwidth pulses of fusion laser, the imaging relay telescopes may consist of two lenses. For 80 nm bandwidth, 30 J, 30 fs HAPLS pulses, the relay telescopes need to employ off-axis parabolas or other complex reflective freeform optics. These expensive optics need in addition to tip/tilt also rotation and longitudinal translation to collimate the beam and to avoid aberrations. The additional degrees of freedom result in larger pointing instabilities and significantly higher complexity for achieving optimum alignment. An additional complication arises from the locations of the switchyards (see Figure 13) and their large stay-out zones for any short pulse focus. Furthermore, even high-quality off-axis parabolas and freeform optics have relatively high mid spatial frequency (MSF) and high spatial frequency (HSF) phase errors due to the required small size sub-aperture polishing tools[34]. Owing to the switchyards, the HAPLS BT system would require for each experimental hall three reflective relay telescopes to fully image the beam up to the target focusing parabola. To assess whether the 10 times higher LDT shown in Figure 17 is a sufficiently high margin to avoid catastrophic damage of the BT and focusing optics when building a non-relay imaged BT system, we have developed in close cooperation with LightTrans International in Jena, Germany, a VirtualLab Fusion[35,36] model to calculate the full-bandwidth diffraction propagation of HAPLS to each experimental hall. Because a solid understanding and potential control of the phase-to-amplitude modulations is of vital importance for a below optical damage threshold operation of the BT system, we have benchmarked this code against the physical optics propagation of Zemax OpticStudio and against an in-house developed Efficient Matrix Approach (EMA)-based code, as described by Shakir et al.[37]. While VirtualLab Fusion may model the full-bandwidth diffraction propagation and has the capability to also include spatiotemporal coupling, Zemax and our in-house code may currently only propagate a monochromatic beam. Prior to designing and building the HAPLS BT system, without a relay imaging system, but the capability to retrofit one if need be, we propagated a phase map taken from another laser facility’s compressor grating interferograms, which we had scaled to an expected worst-case scenario. Figure 30 shows how well all three codes agree for the monochromatic case at 810 nm when an ideal flat intensity 20th-order super-Gaussian beam is propagated over 56.3 m to P3 with the first and preliminary phase error measurements of HAPLS.
Figure 30.Calculated phase-to-amplitude modulations for the free space propagation of a 20th-order super-Gaussian beam, which has at 30 J a fluence of (first row and second row left) with first and preliminary measured phase errors (second row right) over 53.6 m to P3. The VirtualLab Fusion, Zemax and in-house EMA-based code are in full agreement (third row). The peak intensity of these - and -lineouts is and the modulation depth is up to 28% with respect to the ideal super-Gaussian beam. Fourth row: Calculated 2D beam profile after 53.6 m of propagation, left with VirtualLab Fusion and right with Zemax. Fifth row: Same as the fourth row, but calculated with EMA-based code. The peak fluence of the propagated ideal super-Gaussian 2D beam is .
Figure 31 shows the horizontal
Figure 31.X-lineout of EMA model prediction for the phase-to-amplitude modulations of an ideal flat fluence 20th-order super-Gaussian beam with the measured phase errors of the HAPLS beam for different propagation distances.
The depth of the intensity modulations and their increase over the propagation distance depend critically on the MSF and HSF content of the phase errors as we have shown previously[12]. According to Goodman[22] the E-field
with
In general, there is no analytic solution and these operations need to be performed numerically, but the quadratic phase term of
The highest peak fluence of the HAPLS alignment beam measured in P3 amounts to
Figure 32.HAPLS low-power alignment beam measured in P3 after 53.6 m of propagation. When scaled to 30 J operation, the peak fluence of the 2D beam is 200 mJ/cm2.
8.2 Simplified model of phase-to-amplitude modulations
To gain more insight into the scaling of the intensity modulations with the amplitudes of the phase errors and their spatial periods, the effect has been studied with a simplified model of a monomode sinusoidal phase modulation of depth
The laser beam at the laser exit is assumed to have a Gaussian intensity distribution of radius
where the phase modulation is characterized by a wavelength
where
Consequently, the propagation length of interest is comparable to the effective Rayleigh length,
This analysis defines the domain of parameters to consider. Figure 33 shows the dependence of the relative amplitude modulations on the propagation distance for modulations with wavelength
Figure 33.Depth of amplitude modulation, , as a function of propagation distance from the incidence plane for different modulation amplitudes: (a) mm, (b)
The depth of the amplitude modulations is defined as the ratio of the difference of the maximum and the minimum laser intensity near the beam centre to the average intensity,
By contrast, for small-scale modulations
In order to be on a safe side, the small-wavelength modulations with
8.3 Pointing stability on target
A comprehensive analysis of the laser beam pointing contributions from all subsystems is a complicated task. In this section, if not mentioned otherwise, the radial RMS pointing is presented.
The subsystems of the HAPLS BT to target can be, generally, divided into systems located on the laser floor in the L3 hall, and systems located on the experimental floor in the E3 hall.
The HAPLS alignment beam as well as the two blue alignment lasers, located in the E3 hall, was used to measure the on-target pointing. An advantage of the blue alignment lasers is their capability to measure the pointing stability contribution of the P3 setup alone (second blue laser) and the contributions of all BT subsystems in the E3 hall behind the injector (first blue laser).
The focal spot plane was imaged with an infinity corrected 20× magnification microscope objective and an
Figure 34.Top: Schematic for monitoring the beam pointing stability in the P3 target chamber. Bottom: Focal spots images of full-aperture HAPLS beam (left) and 80 mm circular HAPLS sub-aperture (middle). The effect of switching off the P3 vacuum pumps is demonstrated with the blue alignment laser (right); see
The results are summarized in Table 2. The error of the pointing stability was estimated as the standard error of the sample standard deviation using the following equation:
Pointing RMS [ | |||
---|---|---|---|
Configuration/Sample | Radial | x | y |
HAPLS low-power beam full aperture | 2.444 ± 0.169 | 1.524 ± 0.105 | 1.911 ± 0.132 |
HAPLS low-power beam 80 mm aperture | 2.514 ± 0.177 | 1.597 ± 0.113 | 1.942 ± 0.137 |
Blue laser 1 | 1.372 ± 0.068 | 0.999 ± 0.049 | 0.940 ± 0.046 |
Blue laser 2 | 1.492 ± 0.113 | 0.974 ± 0.074 | 1.131 ± 0.085 |
Blue laser 2 | 0.286 ± 0.020 | 0.218 ± 0.016 | 0.185 ± 0.013 |
HAPLS uncompressed (low power) | |||
HAPLS uncompressed (high power) |
Table 2. A summary of low-power laser pointing measurements in the P3 chamber compared with HAPLS pointing, measured in the L3 hall before the compressor. The number of focal spot positions measured to determine the pointing stability was 100 except for the blue laser 1 where we took 200 images. The camera was synchronized with HAPLS, i.e., had an acquisition rate of 3.3 Hz.
where
8.4 Influence of P3 vacuum pumps on pointing stability
The blue alignment laser 2, which is mounted on the BT chamber CH055, as shown in Figure 26 (top), is used to measure the pointing contribution due to the P3 chamber alone. This laser’s beam pointing at the parabola is, to the first order, not affected by the BT[38] and, therefore, corresponds directly to the vibrations of the optical components in P3. Figures 29 and 36 show the two flat vertical mirrors, the OAP and the focal spot diagnostics in P3. All components are mounted on the optical table of P3.
There are four vacuum pumps installed on the P3 chamber: two turbo pumps Edwards STP3202 (3200 litres per second each) and two cryo-pumps Coolvac 1000BL (10,000 litres per second each).
It is expected that the cryo-pumps cause most of the chamber vibrations. When all pumps are in operation, the measured radial pointing with the blue laser 2 is approximately 1.5
9 Beam diagnostic in the spatial domain
To characterize the NF intensity pattern and the focal spot image (FF pattern) of the HAPLS beam in the P3 target chamber, two image-monitoring systems (I and II), which are based on high-quality microscope objectives are implemented in the chamber as shown in Figure 35. The system I (top) collimates the laser beam after the focus to record the NF pattern and the wavefront of the laser beam. For this purpose, a 10× microscope objective lens (Mitutoyo, Infinity-corrected, NA = 0.26) is used to provide a large Fresnel number for the collimated beam. The Fresnel number (
Figure 35.Photos of the image-monitoring system. The monitoring system 1 is for obtaining the NF intensity pattern and the wavefront of the HAPLS beam. The monitoring system 2 is for obtaining the focal spot image by the OAP.
The optical layout for the complete beam diagnostics in spatial and temporal domains inside and outside the chamber is displayed in Figure 36.
Figure 36.Diagnostic setup for the short focal length commissioning.
9.1 NF and FF patterns of the laser beam
Figure 37 shows the NF (shown in Figures 37(a) and 37(c)) patterns and focal spots (shown in Figures 37(b) and 37(d)) measured with the monitoring system. Despite the long propagation distance (
Figure 37.NF and FF (focal spot) intensity patterns measured with the monitoring system: (a) NF pattern and (b) the focal spot image obtained with the 80 mm sub-aperture beam; (c) NF pattern and (d) the focal spot image with the full-aperture beam.
9.2 Wavefront analysis
The wavefront aberration of the laser beam was measured with a Shack–Hartmann (SH) wavefront sensor (Optocraft). The focal length of the microlens of the SH sensor is 2.539 mm, and the size of a microlens is
9.2.1 80 mm diameter sub-aperture beam
First, we measured the aberrations of an 80 mm sub-aperture HAPLS beam, which will be used for the LUIS commissioning experiment. Figure 38 shows the measured wavefront of the 80 mm sub-aperture laser beam. Five spot images were taken and averaged. Figure 38(a) shows the Zernike coefficients for each Zernike mode up to the fifth radial order. The defocus (
Figure 38.(a) Zernike coefficients for the 80 mm sub-aperture beam. The error bar for each Zernike mode is its standard deviation. The absolute value () for the defocus term is not shown. (b) The wavefront map reconstructed from the Zernike coefficients (a). (c) PSF calculated from (b). (d) PSF calculated from (b) after subtracting defocus and astigmatism. HO denotes higher order than astigmatism modes.
9.2.2 Full-aperture HAPLS beam
Figure 39 shows only the central
Figure 39.Wavefront maps for the size laser beam in the target chamber.
10 The P3 infrastructure
10.1 P3 installation
The P3 target chamber has 4.5 m diameter and was manufactured by AWS in Spain from aluminium EN AW-5083 to minimize activation by ionizing radiation. It weighs 14 tons and is anchored directly to the 80 cm thick monolithic concrete floor of the E3 hall. The optical tables are not mechanically decoupled from the chamber, but directly bolted onto the chamber floor. This unusual design for laser-matter interaction experiments provides maximum usable space inside the chamber, as the chamber floor can be relatively thin, only 220 mm, without compromising the vibration stability of the optical breadboards. The high vibration stability is only achievable by using multiple anchoring points to the hall floor, which are precisely levelled via small Spinelli WSP2 Fixing Levelers between the chamber and the hall floor. A total of 25 anchors go from the inside of the chamber via levelling feet into the hall floor, and additional 20 anchor clamps fix the outside of the chamber bottom onto the floor. Figure 40 shows the anchoring of the chamber, where threaded rods are anchored 400 mm into the hall floor. As a result, the 80 cm thick reinforced concrete floor underneath the chamber can be considered as a part of the chamber structure resisting the vacuum forces and minimizing vibrations.
Figure 40.Top: The anchors used, going 400 mm deep into the concrete. Bottom: The distribution of the anchoring points and the individual levelling feet.
It is important to note that the massive 80 cm thick reinforced hall floor is pre-tensioned by the underground water pressure. The experimental hall is located 7 m below the groundwater level, which generates a significant water buoyancy load of 70 kN/m2 acting on the floor. Consequently, the floor is bent up and pre-tensioned.
Owing to the innovative anchoring of the chamber and the monolithic reinforced hall floor design, only minimal deformations occur on the chamber floor even when the enormous vacuum forces act on the chamber. Although the P3 chamber floor has a surface of 13.594 m2, which generates a vacuum force of
Figure 41.FEM simulation results of P3 chamber floor deformation in
From the practical point of view, the chamber’s direct anchoring vastly simplified the construction and installation of the P3 chamber and allowed to design simple and modular breadboard structures, as described in the next section. With decoupling, it would have been very hard to provide as much space and modularity. Before the installation of the chamber, the anchors were installed and the levelling feet precision aligned to the nominal height with the laser tracker. The chamber was positioned with the help of the laser tracker and lowered onto its final position with sub-millimetre precision. The anchors were pre-tensioned to 65 kN to minimize load variation owing to cyclic vacuum loading. The resulting load variation on the anchors is less than 5 kN. The internal anchors can be accessed from inside through internal vacuum ports.
10.2 P3 breadboard structure
Figure 42 shows the unique breadboard structure in the P3 chamber. The individual elements can be arranged depending on the requirements of the specific experiment. The system consists of a central target table with two levels and two rings with ten wedges. Each wedge is bolted to the base plate of the vacuum chamber. The top surface of the breadboard is located 350 mm below the target chamber centre (TCC). The central table has also a second level 250 mm in depth, which can be used for bigger target manipulators and accessed via the removable central breadboard covers.
Figure 42.Left: The central target table and one wedge from the inner and outer ring. Right: All 21 elements of the breadboard structure.
Figure 43 shows the general layout of the breadboard structure inside the P3 experimental chamber used for the initial commissioning period.
Figure 43.Panorama of the present P3 breadboard configuration.
11 First X-ray measurement in P3
One of the well-known techniques to characterize the interaction of high-power lasers with matter consists of an analysis of emitted X-rays through high-resolution spectroscopy[39]. When focused onto a target with sufficient intensity, the laser pulse is absorbed through different processes inside its skin depth[40,41]. Some of these processes lead to an acceleration of suprathermal electrons with energies ranging from several kiloelectronvolts to hundreds of megaelectronvolts depending on the laser parameters. Such electron energies exceed the core–shell ionization threshold of any atom. Consequently, during an interaction between hot electrons and matter, one electron is ejected from the K-shell. This hole is filled in by the transition of another electron from a higher-energy shell. Consequently, a so-called K photon is generated[42,43]. The easiest transition to detect is the
In the context of the BT commissioning, two X-ray diagnostics were set up in the P3 interaction chamber. First, an imaging crystal was installed to monitor the generation of
Figure 44 shows a typical X-ray Cu
Figure 44.Copper emission recorded during an HAPLS 110 mJ single shot. Photons are emitted relatively homogeneously from the focal spot. The white dashed circle of diameter represents the emission area.
The second diagnostic benefits from the application of a high-spectral resolution spherical crystal spectrometer monitoring all plasma emission between
Figure 45.(a) Vanadium target patterned in a series of laser bursts indicates a good reproducibility inside the same bursts. (b) Vanadium X-ray spectrum recorded by an accumulation of 165 shots.
Different bursts are visible. For each of them, the target position compared to the optimum focal spot was slightly different, which explains the different shapes of the holes. However, one can see a very good shot-to-shot stability within single bursts.
Figure 45(b) shows the first X-ray spectrum recorded in the P3 interaction chamber. The emission is spatially integrated over the full plasma expansion. Owing to having just an energy of 85 mJ delivered in individual laser shots, only the low plasma temperature
12 Conclusions and outlook
A unique high-performance BT system was carefully engineered and successfully commissioned, ready to guide the HAPLS beam to the versatile P3 target chamber of the ELI-Beamlines plasma physics experimental infrastructure. Despite the very long propagation distances reaching 100 m for the E5 hall it was demonstrated that the BT system contributes with less than
The preliminary experimental data obtained proves that the entire chain from pulse compressor, via BT, focusing optics, targeting and data acquisition is functional and ready for the energy ramp-up of HAPLS. ELI-Beamlines is conceived as a user facility in the framework of an ERIC (European Infrastructure Consortium[44,45]) and will start first user operation with the HAPLS soon.
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