Commissioning & Day 1 Experiments

Commissioning Experiments with High Power Laser Systems

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.

Commissioning experiments at E1

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 [1]. 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 [2]. 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 [2]. 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) [3]. 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.

References

[1] I.C.E. Turcu, et al., Romanian Reports in Physics, Vol. 68, Supplement, P. S145–S231, 2016
[2] K. V. Lezhnin, et al., Physics of Plasmas 25, 123105 (2018)
[3] T. Nakamura et al., Phys. Rev. Lett. 108, 195001 (2012)
[4] T.Esirkepov, et al., PRL., 92, 175003 (2004)
[5] B.Qiao et at., PRL 102, 145002 (2009)
[6] D. Del Sorbo, et al., NJP 20 (2018)

Commissioning experiments at E6

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 [8]. 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 [9]. 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

[8] A. J. Gonsalves, et al., Physical Review Letters 122, 084801 (2019)
[9] Xiaoming Wang et al., Nature Communications 4, 1988 (2013)

Commissioning experiments of E5

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) [10]. 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 [14]. 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 [18]. 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) [19].
In particular, we intend to benchmark results related to the nonlinear bubble regime with electron self-injection employing a single stage acceleration scheme.

References

[10] A. Macchi, et al., Rev. Mod. Physics, 85, 751 (2013)
[11] S. Kar, et al., Phys. Rev. Lett. 109, 185006 (2012)
[12] C. Scullion, et al., Phys. Rev. Lett. 119, 054801 (2017)
[13] A. Higginson, et al., Nat. Comm. 9, 724 (2018)
[14] Tajima, Dawson, Physical Review Letters 43, 267 (1979)
[15] S.P.D. Mangles et al, Nature (2004)
[16] C.G.R. Geddes et al, Nature (2004)
[17] J. Faure et al, Nature (2004)
[18] A. J. Gonsalves, et al., Physical Review Letters 122, 084801 (2019)
[19] S.P.D. Mangles, Proceedings of the CERN Accelerator School (2014)

Commissioning experiments can start in 2020 at 100 TW laser output at ELI-NP. The main goal of the 100 TW commissioning experiments is to search for the dark matter via. the laser four wave mixing experiments. This experiment can be performed in an extremely good vacuum condition in order to detect the scattering laser signals from Axion.

Commissioning experiments at the E4

The searches for new particles which can be candidates of Dark Matter (DM) or/and Dark Energy are of great importance to understand the properties of the universe. Especially dark matter attracts a strong interest by many scientists across several scientific fields, due to the fact that several independent experiments with completely different approaches suggested the existence of dark matter. Any experimental attempts until now, however, could not reach discovery of dark matter.
Direct production of dark matter by a collider-type experiment is expected as a direct/active approach to discover dark-matter particle. Development of high-energy accelerator can create high-energy frontier to access extremely "heavy" dark matter beyond the mass scale of 100 GeV. In contrast, high-intensity laser facilities such as ELI-NP has a big potential to access "light" dark matter below the mass scale of 1 eV and to enhance the experimental sensitivity to overcome against small coupling constant of dark matter production. High-intensity laser is an excellent source to produce photon-photon collisions at extremely high-photon density in tiny volume.
The commissioning experiment with 100 TW laser system at E4 [1] was designed to search Axion-Like Particles (ALPs) [1], which is also regarded as a candidate of "light" dark matter. Several theoretical models predict the existence of ALPs but have difficulty in evaluating the exact physical mass. Therefore, the systematic survey over a wide area of parameter space composed by the effective coupling constant and the physical mass are required for the experiment. In the commissioning experiment, two color lasers combine and focus with spatial and temporal synchronization. A laser is used for ALPs (DM) creation, and the other is used as an inducing field to enhance scattering amplitude as well as to produce wavelength shift of final-state scattered photon. Such a scattering process is interpreted as four-wave mixing process in vacuum [3,4]. Therefore key apparatus of the commissioning experiment are 100 TW Ti:Sapphire laser for creation field, and a 100 MW-class Nd:YAG laser or another high-intensity light source with different wavelength, and an ultra-high vacuum system to suppress the background generation from the atomic four-wave mixing process. The figure shows a schematic diagram of four-wave mixing process with an axion-like resonance state.
Another aspect of the commissioning experiment is that photon-photon collision is carried out in Quasi-Parallel colliding System (QPS), which is opposite with respect to the head-on configuration of the collider-type experiments. Axion-like resonance state can be produced at the mass range of M_ ≤2ω sin⁡θ by high-intensity laser with proper design of the focusing system, where M_ is the mass of ALPs (DM), ω is the energy of a photon, and θ is the incident angle of initial-state photon

References

[1] K. Homma et al., Rom. Rep. Phys., 68, S233 (2016)
[2] R. D. Peccei and H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977)
[3] K. Homma, D. Habs, T. Tajima, Appl. Phys. B 106:229-240 (2012)
[4] T. Tajima and K. Homma, Int. J. Mod. Phys. A vol. 27, No. 25, 1230027 (2012)

Commissioning experiments at the E7

The commissioning experiment in the experimental area E7 is isomer production.
The current techniques for the production of isomeric targets are based on photon activation techniques (see, e.g., [1] and [2]). The main sequence of these techniques consists on the extraction of electron bunches from linear accelerators, production of bremsstrahlung photons from an electron-photon converter (typically consisting of tungsten targets of few mm) and consequent irradiation of the secondary target for the population of the isomers. In ELI-NP, the 1 PW laser output at 1 Hz repetition rate which will be delivered in E7 will be used for the production of electron beams through the Laser Wakefield Acceleration mechanism [3], thus providing an electron source alternative to the linear accelerators. This is one of the topics for an R&D effort that will be carried out at E7, in order to maximize the electron beam charge and decrease the divergence. Recent experiments using 100 TW laser have shown the possibility to produce tens of nC per laser pulse [4].
The conceptual scheme of the PPEx experiment has been described in the literature [5]. The narrow energy bandwidth of the gamma beam photons makes this probe beam well suited for precise measurements of cross sections of photonuclear reactions induced on nuclear metastable states, also called ‘isomers’. These measurements will make possible the study of nuclear processes that occur in astrophysical environments, in particular the photonuclear processes responsible of the production of p-nuclei, which require temperatures in the order of billion Kelvin, and therefore nuclear excited levels are in these environments thermally populated [6]. The proposal has been updated, according to the results of numerical simulations performed for the feasibility study of the experiment.
An example of the output of our simulations can be found in [7], in which it is shown that high power laser pulses are able to produce thousands of isomers via the (γ, γ') channel. The bremsstrahlung radiation has been estimated by importing the LWFA electron distribution from the PIC simulation performed with EPOCH, in a GEANT4 code.
Apart from the (γ, γ') channel, other channels can be investigated.
By combining the bremsstrahlung photon spectrum with the cross-section calculated with TALYS, we obtain that the number of isomers which can be produced per laser pulse via (γ, γ') is 1.5 × 104 , while with the (γ, n) channel the number of isomers per laser pulse is 2 × 105 . Therefore both channels are feasible for the first stage of the experiment, from the point of view of subsequent detection of the isomer states.

Demonstration of LWFA of electrons to multi-GeV energy

References

[1] T.D. Thiep et al., Phys. Part. Nuclei Lett. 6, 126–133 (2009)
[2] V.M. Mazur et al., Phys. Rev. C 87, 044604 (2013)
[3] T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979)
[4] B. Shen et al., Physics of Plasmas 19, 033106 (2012)
[5] K. Homma et al., Rom. Rep. Phys., 68, S233 (2016)
[6] H. Utsunomiya et al., Nucl. Phys. A, 777, 459 (2006)
[7] S. Ataman et al., AIP Conf. Proc., 1852, 070002 (2017)

Commissioning Experiments with Gamma Beam

Commissioning experiment at E8: Photoactivation of 180Ta and decay study

The proposed commissioning experiment is the photoactivation of 180Ta nucleus, in an aim to study of intermediate states (IS) or doorway states through which the Jπ = 9- isomeric state (t1/2 > 1.2 x 1015 year) de-excites to the ground state Jπ = 1+ (t1/2 = 8.1 h), as shown in Fig. 1. Nine such doorway states in the energy interval between 1 MeV and 3 MeV were suggested from Bremsstrahlung experiment [1]. The detailed knowledge of the doorway states and the flux that passes through them provide a sensitive thermometer for studying the star conditions during the nucleosynthesis. Recently, the de-excitation cross-section of the isomeric-state of 180Ta has been measured [2] by the HIγS facility and a novel work done on search for dark matter induced de-excitation of 180Tam [3].

Fig. 1: Low lying decay scheme of 180Ta (taken from Ref.[1]).

References

[1] D. Belic et al., Physical Review C 65, 035801 (2002)
[2] W. Tornow et al., Nuclear Inst. and Methods in Physics Research A 928, 79 (2019)
[3] B. Lehnert et al., arXiv:1911.07865

Commissioning experiment at E9: Nuclear structure of the giant dipole resonance in 208Pb

One of the key cases to be explored with ELIGANT-GN is the study of the giant dipole resonance (GDR) in 208Pb. This nucleus was recently measured at the Research Center for Nuclear Physics (RCNP) in Osaka [1] where a fine structure was observed. It is a very suitable reaction for an in-beam commissioning of the instrument as the absolute population cross-section is already known from Reference [1]. In addition it will allow us to study the properties and origin of this fine structure in detail. In the proposed commissioning experiment we will measure the decay cross-section as well as the neutron-gamma competition in this decay. In general the gamma branch is expected to be on the order of 1% of the total decay and important nuclear properties can be obtained from the two-step gamma-decay of these resonant states. We will also investigate the two-step neutron-gamma decay into 207Pb and from this data we will extract the composition of the wave function depending on the population of excited states in the daughter nucleus. With the high-resolution ELI-NP gamma beam we will be able to scan the resonance with high precision over the entire energy range and decompose the different structures observed with respect to the nuclear structure of the excitation.

Spectrum of the 208Pb (p,p') reaction at Ep=295 MeV with the spectrometer placed at 0°. (taken from Ref.[1]).

References

[1] A. Tamii et al., Physical Review Letters 107, 062502 (2011)