The source has been developed by Thales, which commenced delivery and installation of what is said to be the world’s most powerful laser system in late 2016. Since the start of 2019 the project has initialized and progressively scaled-up the system’s output to increase its pulse energy and peak power level.
Nearly six years after the start of construction, the Extreme Light Infrastructure for Nuclear Physics (ELI-NP) facility at Magurele, Romania, has hit a milestone figure in the output of its ultra-high intensity laser system.
According to the ELI-NP roadmap, 10 PW beams are intended to be available for use in the project’s research program in the first quarter of 2020.
The Magurele facility is one plank of the Extreme Light Infrastructure project, a European initiative intended to collectively deploy the most intense laser beamline system available worldwide.
HPLS & LBTS
The High Power Laser System (HPLS) and the Laser Beam Transport System LBTS are key components of the ELI-NP facility. These systems together allow the creation and delivery of the required laser pulses to the experimental areas where the experiments take place.
The HPLS consists of two Laser arms, capable of delivering laser pulses at six outputs: 2x 0.1PW@10Hz, 2x 1PW@1Hz and 2x 10PW @1/min repetition rate.
The LBTS is made of meter-large aperture adjustable mirrors installed in a vacuum system of pipes and enclosures that are able to direct the Laser pulses to the required experimental setups with micrometric accuracy.
The systems are operated and monitored using specialized hardware and software interfaces. The operation of HPLS and LBTS is done under controlled environmental conditions in terms of air quality and vibration that have to meet specified parameters.
Alignment beams generations
The alignment system is supported by a continuous laser emitting at 785nm. The alignment laser will be installed in the HPLS optical table, after the deformable mirror. In the picture is shown the 10PW beam path, where the beam passes through the deformable mirror, a crossed periscope and a beam expander, before entering in the compressor chamber.
Figure 1 : 3D representation of the table before compressor
Each PW arm has its own alignment laser system, independent from each other.
Full Aperture alignment Laser (FAL)
This beam is shaped in order to obtain a 550mm ±10mm diameter, super-Gaussian beam that will propagates in the LBTS. The alignment beam insertion will be made in the optical table containing the beam expander BEX-452-130, between the upper mirror of the periscope and the first mirror of the beam expander.
The first step is to create references of HPLS beam to be used to align the alignment laser.
Figure 2 : HPLS references recording
The HPLS is first aligned at low energy following Thales procedure. Then two MIRE are placed in the HPLS beam path and aligned to be a reference for centering of the alignment system. Before the first mirror of the beam expander, a mirror is inserted in the beam path and a far field camera is used to image the far field of HPLS. The HPLS far field is now set as the far field reference for the alignment laser.
In order to have a pointing resolution of 10mrad, an appropriate choice of the camera and lenses has to be done. If we consider a lens with a focal length of F=1m, Drad=1mrad pointing deviation corresponds to Dxy=1mm focal spot displacement. If we consider a commercial camera with a pixel size of Spixel=6,5mm x 6,5mm, we can discriminate a focal spot displacement higher than 6,5mm.
Adding to the camera a microscope objective with a magnification factor of M, it increases the focal spot displacement by a factor of M, hence the resolution. In this case, 1mrad deviation corresponds to a Mx1mm focal spot displacement, obtaining a new resolution of R=6,5mm/(MxF).
|Dxy (Drad=1mrad)||R (M=1)||R (M=4)|
Table 1: Pointing resolution for different focal lengths and magnification factors
As we can see from table 1, the solution with F=0,5 and no microscope objective doesn’t meet the specification, as the smallest displacement that can be seen by the camera is 13mm.
The focal length chosen is then F=1m as it fulfills the specification. In case a higher resolution is needed, there’s the possibility to add a microscope objective.
Once the MIRE are aligned and the far field recorded, we can now inject the alignment laser into the HPLS beam path.
Figure 3 : Centering and pointing of the alignment laser
The FAL is generated through the use of a diode laser with a fiber at the output, directly connected to a beam expander. The laser diode is a Lambda Beam 785 250 from RGB Photonics Gmbh. It’s stabilized in temperature, coupled into a monomode fiber, delivering up to 200mW @785nm. The beam expander ABEX is a high resolution telecentric lens with a magnification factor of M=0.089, delivering a beam of 160mm of diameter.
Both the laser and the ABEX are fixed on a support able to perform the following operations:
- Translating in and out the beam path the ABEX
- Translations for ABEX on the X and Y directions (orthogonal to the propagation direction)
- Tip/tilt for ABEX
- Creation of an additional beam for pointing monitoring
Here below, two images shows the ABEX support.
Figure 4 : ABEX support
Once the ABEX has been inserted in the beam path, the two translations in X and Y are adjusted in order to center the beam into the two MIREs. Now that the FAL is centered and roughly aligned in pointing, the MIREs are removed and the tip/tilt are adjusted in order to center the FAL focused beam in the far field camera, where the HPLS pointing position was previously recorded.
This procedure needs the recording of HPLS pointing position, hence the presence of the HPLS beam every time we want to insert the ABEX in the HPLS beam path. In order to overcome this need, an additional mask is fixed to the ABEX, providing a 5mm diameter beam, which is used to create a new reference of pointing. This small beam is collected through the use of a retroreflector, which can be removed during operations, and other fixed optics on HPLS table, including a far field measurement. In this way, every time we want to insert the ABEX in the HPLS beam path, the following operations have to be performed:
- the retroreflector is put in place together with the mask with 5mm hole
- a far field measurement is taken
- the ABEX is aligned in tip/tilt if needed
- the retroreflector and the mask are removed to avoid clipping of the FAL
Figure 5 : Far field reference for ABEX using additional mask and retroreflector
Figure 6 : Removing the additional mask and the retroreflector, there is no FAL clipping
The selected solution for the visualization system is based on a direct observation of the mirror with the system inside each vessel. After several tests performed on the different solutions explored, it turns out that this is the solution that minimizes the risks to detect the entire field of view of the mirror.
The vessels equipped with this visualization system are as follows:
– for 1PW mirrors: FMR1 (1 syst.), FMR4 (1 syst.) and FMR5 (1 syst.)
– for 10PW mirrors: FMR2 (1 syst.), FMR3 (1 syst.) and FMB1 (2 syst.)
This corresponds to 7 visualization systems installed in the LBTS.
The spherical mirror PME6 (10 PW) is not taken into considerations as regards to the provision of the alignment and visualization systems. Only the DN160 aperture and the flange will be provided (out of ARDOP INDUSTRIE Scope of Work). Also, vessels including 1PW mirrors were also out of the initial Scope of Work. ARDOP has adapted the design of 10PW mirrors visualization and alignments systems to the 1PW mirrors, but only on the vessels which will be equipped with mirrors for commissioning (i.e. FMRx mirrors).
The visualization system are then scaled from 10PW mirror to 1PW mirrors to fit in the mounts. This document will present only the 10PW mirrors visualization systems.
The visualization system consists in three sub-systems:
- The edge illumination system
- The mechanical ferrule
- The acquisition system
Figure 7 : Mirror visualization system layout (see enlighten parts)
Edge illumination system
To be able to observe an event at the surface of the mirror, it is necessary to provide a light source close to the surface of the mirror. It can be spot lights or integrated systems with fiber stack. As regards to the existing bibliography, it has been demonstrated that the most commonly used solution is an edge lightning using LED strips.
Figure 8 : Principle of edge illumination system of the mirror
To illuminate the mirror uniformly, it is necessary to place the LEDs strips on each edge of the mirror. The mirrors are expected to be 100mm thick and inserted into a frame of 120mm of depth. And due to the configuration of the vessel and the space allocated inside, there is no choice but to place the strips in the same volume than the frame (to avoid any laser beam clipping). The solution consists in making a notch in the frame so as to allocate a free area where to place the LED strips as represented on Figure 3 .
Due to vacuum compatibility requirements, the strips must be made of low-outgassing material or be placed into a sealed “capsule”. The capsule must be thick enough to resist to pressure difference (variation) between atm. pressure and vacuum levels reachable in the vessels. The capsule will be made of aluminum. One side of the capsule will be covered with a vacuum window (6mm thick) so as to let the light pass. We plan a lateral adjustment (about 30mm) of the capsule in the frame depth so as to optimize the edge illumination conditions and uniformity, if necessary. This is ensured through a groove made into the frame. The frame manufacturer has confirmed the size and position of the groove so as to guarantee the resistance of the frame.
The mechanical ferrule consists of an aluminum welded tubes assembly of diam. 120mm, terminated at each side with CF flanges. One side will be screwed on the top panel of the vessel with half clamps on a DN160 hole. The other extremity will be sealed with a 3’’ vacuum window. The two tubes are welded at 135 deg. so as to let the tube pass through the top panel of the vessel.
The ferrules are supposed to be clamped at a distance of 440mm (in X and Y) from the center
Figure 9 : Visualization ferrule position (FMB 1 vessel)
This top ferrule extremity is covered by a connector flange. This flange contains the RJ45 connector to allow the connection with the visualization camera. And it also contains a SUB-D15 connector to power supply the camera (if not provided over Ethernet, through the RJ45 connector). This flange is screwed on the vacuum flange and easily removable to access to the camera for its adjustments.
Figure 10 : Ferrule with the connectors flange
EMP and EMI protection
The materials inside the chambers, including mirrors are potentially exposed to strong EMP (Electromagnetic Pulses) of hundreds of KV/m generated within the target chambers. This is due to the action of the high intensity laser on targets. Electronic devices are sensitive to EMP which can damage them (partially or not) and consequently have serious impacts on their nominal functioning.
As regards to the bibliography, some studies indicate that EMP evolves like I², where I is the laser intensity (several … Furthermore EMP levels and frequencies ranges will strongly depend on target geometries and materials and is not taken into account in this scaling law
According to our proposal, ARDOP INDUSTRIE was not in charge of the EMP aspects for the visualization system. Nevertheless, we have taken into considerations this topic and developed our system in a way to make it able to address EMP issues. The sub-systems we considered sensitive to EMP shielding are the LEDs strip and the camera.
Concerning the LEDs bars, the capsule is made of aluminum and the window is a sandwich of two sheet of glass enclose a fine metal cloth which is attached along the edges by the cloth or an a appropriate conductive gasket. This impact a little bit the transparency (~20%) but this guarantees a attenuation as follows:
Freq 100 KHz 1 MHz 100 MHz 400 MHz 1 GHz
Attenuation 10 dB 20 dB 55 dB 60 dB 55 dB
Concerning the camera, it is integrated inside an aluminum ferrule terminated by a 3’’ optical window on one side and an aluminum flange on the other side. Connectors integrated on the latter one are shielded, but the optical window is not.