The very first experiments that will be carried at ELI-NP are commissioning experiments meant to verify the functioning and performance of the experimental setups. The main goal of the 10 PW commissioning experiments is to validate through physics experiments the achievement of the key parameter for nuclear physics studies, extreme light intensity in the focus of the laser beam. This is a parameter which cannot be directly measured at full power, but must be inferred from the physical output of the experiment.
The E1 experimental area will provide a laser beam with the unprecedented power of 10 PW for the investigation
of laser solid matter interaction. The objectives delineated by the ISAB (International Scientific Advisory Board)
are the following:
- Demonstrate extreme focal intensity through laser-γ conversion
- Demonstrate over 200 MeV proton acceleration (neutron generation add-on)
- Dense, heavy ion beams for nuclear physics (time permitting)
Demonstration of extreme focal intensity
The laser characteristics are of crucial importance in the laser-matter interaction, and the features of the particles produced are the signature of this interaction. Direct measurement of the laser intensity in the focal spot at full power is virtually impossible to perform. Therefore, a reasonable way to estimate such intensity is by looking at the generated secondary particle sources, like, for instance, X-rays. The 10 PW laser beam of E1 can be focused to an unprecedented intensity of up to sub-1023 Wcm-2. At this laser intensity, theoretical PIC (particle-in-cell) simulations predict a high conversion of laser power into a flash of gamma rays via nonlinear Thomson scattering. This is in contraposition with the radiation generate at laser intensities below 1021 Wcm-2, which are fundamentally bremsstrahlung dominated and strongly dependent on the target material . Therefore, the observation of the features of the gamma radiation, i.e., angular distribution, polarization, energy spectrum, will unequivocally identify the mechanism underlying the emission, then confirming the laser intensity indirectly. The nonlinear Thomson scattering is expected to dominate the emission at intensities ranging from 1022 Wcm-2 to a few 1023 Wcm-2 with a gamma photon energy peaked around a few MeV, but also gamma photons with energy up to a few 100s MeV generated via Compton scattering are predicted with much lower intensity . For a given target material, the gamma yield seems to be nearly independent on the target thickness if more than a few micrometers, while it is strongly affected by the presence of a preplasma [2, 3]. 2D PIC simulations predict that the laser interaction with a plane foil target in the absence of preplasma gives a conversion efficiency of the order of a few percent, while under appropriate preplasma conditions the conversion efficiency can reach up to tens of percent . In the light of this, the commissioning experiment will be accomplished by shooting plastic targets of several thicknesses larger than 2µm at the maximum intensity allowed and collecting data about the solid angle distribution of the photon emission and the photon energy spectrum. Those two data set will give an estimation of the laser-to-gamma conversion efficiency and the mechanism underlying the emission. Further, according to theory and 2D PIC simulations, the laser power (PL) converted into gamma power (Pγ) should scale to the second power, i.e. Pγ ∝ PL2 for laser power below one petawatt and going asymptotically towards a linear behavior for higher laser power (see Fig.1) . Although, the power conversion strongly depends on the target properties (e.g. a preplasma may significantly enhance the gamma yield), the scaling is preserved, and therefore, a 10 PW laser power should give an increase in gamma emitted power between one and two orders of magnitudes in comparison with 1 PW laser. So, another goal of the commissioning of E1 will be to demonstrate the occurrence of such scaling law and identify the actual scaling exponent.
Demonstration proton acceleration to energy exciding 200 MeV and ultradense heavy ion beam generation
By thinning the plastic target, from 10s of micrometers down to 10s of nanometers we will then optimize the ion acceleration fulfilling another experiment outlined by the ISAB. The goal of this laser-driven acceleration experiment will be to achieve proton beams with cut-off energy greater than 200 MeV. Lowering the target thickness, we will go across different acceleration regimes, from TNSA to RPA. This, along with laser intensity tuning, will allow investigating the scaling law of those mechanisms up to laser intensity close to 1023 Wcm-2, where we should see some QED effects coming into play [5-7]. Furthermore, time permitting, neutron generation by using a pitcher-catcher configuration will be explored; and high intensity laser shots will also be performed on to metal foils of 100s of nanometer thickness to generate ultradense heavy ion, carbon, and proton. This will demonstrate the accessibility to nuclear physics experiments. An important implication of commissioning experiments is that they will be performed in a single experimental setup in which only target thickness (and eventually its material) will be changed.
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The E6 experimental area will provide a laser beam with the unprecedented power of 10 PW for
the investigation of LWFA and QED. Although, the objectives delineated by the ISAB
(International Scientific Advisory Board) for the commissioning phase is the following:
- LWFA of electrons to multi-GeV energy
Demonstration of LWFA of electrons to multi-GeV energy
Accelerators based on LWFA have shown to be able to achieve a large accelerating field, typically of tens of GV/m, which is several orders of magnitude
larger than conventional radiofrequency technology. This could allow for compact accelerators and then lead to several possible applications.
This is, therefore, an important subject of research which has led the scientific community to establish worldwide a number of
projects (e.g., EuPRAXIA) committed to the pursuit of multi-GeV high-quality electron beams and high reproducibility. Recently,
at The Berkeley Lab Laser Accelerator (BELLA) Center has been demonstrated a quasi-monoenergetic 7.8 GeV electron using a sub-PW
laser . The ELI-NP facility, with the 10 PW laser system, has, therefore, an enormous potential to become soon a leading research
institute in this field soon. The goal of the commissioning experiment will be the generation of multi-GeV electron beams with good
characteristics. For that to achieve, a gas cell up to 50 mm long and a gas with an electron density, ne < 1018 cm-3 will be used.
We will exploit the non-linear regime employing a single stage acceleration scheme. Experimental results and simulations indicate
that by this acceleration scheme, we should be able to reach multi-GeV electron . Simulations were performed with turboWAVE's
ponderomotive guiding center (PGC) algorithm by using the ELI-NP parameters and the commissioning setup, show that we can reach up to
7 GeV electron with a peak around 5.6 GeV, as reported in Fig. 2.
Therefore, we will do a parameter scan to achieve the ISAB goal. The electron energy spectrum will be measured by dispersing the electron beam with a dipole magnet 800 mm long, having a gap of 30 mm, and a B-field of about 1 Tesla. The detection of the electrons will be performed by using Lanex screens and an imaging system located outside the interaction chamber. In the next future also a second parabolic mirror with short focal length (f-number ~ 3) will be available inside the interaction chamber for QED related experiments.
References A. J. Gonsalves, et al., Physical Review Letters 122, 084801 (2019)
 Xiaoming Wang et al., Nature Communications 4, 1988 (2013)
The E5 experimental area will provide 2 x 1 PW laser beams for the investigation
of laser-solid and laser-gas interaction. The objectives delineated by the ISAB
(International Scientific Advisory Board) are the following:
- Benchmark of proton acceleration via TNSA
- Benchmark of LWFA electron acceleration
Benchmark of proton acceleration via TNSA
The commissioning of the petawatt laser at E5 will be accomplished through the benchmark of well-established results obtained worldwide in precedent experiments by using petawatt laser systems with similar characteristics. The laser beam characteristics play a significant role in laser-matter interaction. Therefore, by using conventional foil targets, a few metal types and plastic, the intensity and other characteristics of the laser beam will be indirectly examined by investigating the laser-driven ion acceleration. The laser intensity will be increased gradually until the maximum intensity achievable (~5 x 1021 Wcm-2), and also a range of target thickness will be employed during the experiments. This will allow exploring different acceleration mechanisms expected at different laser intensity. The presence of a deformable mirror will allow for laser focal spot optimization and a quarter waveplate will allow changing the laser polarization from linear to circular. Initially, the TNSA (Target Normal Sheath Acceleration) mechanism will be investigated, because more robust and reliable for a comparative study (see Fig. 3) . To do so, we will use thick targets foils of the order of a few micrometers to tens of micrometers. Once the expected results from TNSA are confirmed, the target thickness will be gradually reduced down to hundreds and then tens of nanometers, in an attempt to observe the signature of the radiation pressure acceleration (RPA) mechanism [11-13]. To diagnose the secondary products of the interaction (i.e. protons, ions, and electrons) we will use usual diagnostics, both passive and active ones.
Benchmark of LWFA electron acceleration
For over a decade, LWFA has been harvested for laser-based electron acceleration to GeV energy as large accelerating
ields (typically of tens of GV/m) are generated by laser-plasma interaction, as firstly foreseen by Tajima and Dawson
in the 1979 . This is a very active field of research due to the perspective of creating compact accelerators in
the near future, giving rise to several possible applications. Three landmark papers demonstrated in 2004 that non-thermal
electron spectra were achievable [15-17], and from that time a long and fruitful study has been accomplished.
At the moment, the electron beam with the highest energy (7.8 GeV) has been generated at The Berkeley Lab Laser Accelerator
(BELLA) Center in 2018 . This latest result is coming from the effort of many years of worldwide research and improvement
of the LWFA techniques.
Therefore, there is plenty of literature, from theory to experiment, on this subject, and the commissioning of E5 will be fundamentally dedicated to reproducing the well-established results in accordance with the experimental parameters available at E5 (see Fig. 4) .
In particular, we intend to benchmark results related to the nonlinear bubble regime with electron self-injection employing a single stage acceleration scheme.
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