Periodic Reporting for period 3 - GENESIS (GEnerating extreme NEutrons for achieving controlled r-process nucleosyntheSIS)
Periodo di rendicontazione: 2022-01-01 al 2023-06-30
The overarching goal of the GENESIS project is to test, through laboratory experiments, the models of nucleosynthesis of heavy elements in the Universe, in particular the so-called “r-process” of nucleosynthesis, which proceeds through the multiple absorption of neutrons in heavy elements in order to increase their mass. The project was thus laid out to work in two complementary directions: (1) the production of extreme brightness neutron beams, such that we can beat the very low cross-section of neutron absorption in nuclei, and realize multiple absorption at once, and (2) the generation of laboratory plasmas that can be quantitatively scaled-down to astrophysical ones, in particular mature stars and supernovae, which are thought to be the site of heavy element nucleosynthesis, once exposed to extreme fluxes of neutrons (as resulting e.g. from a merger event, or a supernova explosion). For these aspects of the project, high-power lasers have been used as the driver, since the underlying idea of the project is that: (i) our estimates showed that lasers, and in particular the upcoming generation developing the unprecedented power level of 10 PW, had the potential to produce neutron beams with fluxes many orders-of-magnitude higher than the existing ones (produced by conventional accelerators), and (ii) lasers have shown to be capable of producing scaled-down astrophysical plasmas of a wide variety. These two directions have been actively pursued since the start of the project, on 01/01/2019, and major progress has been obtained in both.
The overall strategy of the project is to combine (1) extreme neutron fluxes, with (2) astrophysically-relevant scaled plasmas, both produced by high-power lasers [S. N Chen et al., Extreme brightness laser-based neutron pulses as a pathway for investigating nucleosynthesis in the laboratory, Matter Radiat. Extremes 4, 054402 (2019); https://doi.org/10.1063/1.5081666]. To implement this strategy, we worked in parallel in two directions: (i) simulating how to optimize the generation of neutrons using multi-PW lasers, in preparation for experimental implementation, and (ii) prepare and start the experiments planned at that level of power. Indeed, 10 PW lasers are not available as of yet, but should be available within a year or two (as of yet), i.e. within the time frame of the project. An exciting milestone we have crossed is that in May 2021, we were able to perform the commissioning of the new “Apollon” laser facility at the level of 1 PW (see below). The increase to the level of 4 PW is expected to take place in March 2022. We have also had already good progress in the production of astrophysical-relevant plasmas using high-power lasers, which is promising in order to achieve injection of high-brightness neutron beams in relevant environments. Overall, the GENESIS project proceeds well, with important milestones achieved and we have good perspectives to achieve the goals within the time frame of the project.
For the first axis of the project, we tested in simulations, in order to generate extreme neutron fluxes (in the range rendering heavy elements nucleosynthesis possible), how to: (i) use multi-PW lasers to produce high-charge, high-energy protons, and then (ii) convert these protons into neutrons in a high-Z converter, using the spallation mechanism which is well know in nuclear physics and commonly employed in conventional accelerators.
Within the project, we examined the neutron yield from spallation as a function of the converter target material and the projectile proton energy, in a range within the reach of present or near-future laser systems. For this purpose, we have used the FLUKA 3D Monte Carlo code to simulate the nuclear reactions induced during irradiation of a convertor target by a mono-kinetic and mono-directional proton beam. With this, we know what optimal material to use for the neutron converter. The next steps was to see how to optimize the proton generation with multi-PW lasers in order to obtain the highest neutron fluxes. Again, this is done through simulations, in waiting for the facilities to be available. We thus researched, utilising large scale particle-in-cell and Monte Carlo simulations, a neutron source which relies on the laser-driven proton acceleration on the primary target and conversion of these protons into the neutrons in the secondary high-Z neutron converter. Inspired by the trends in the community, we investigated the use of the double layer primary targets for proton acceleration where the laser pulse is first focused in the near-critical front layer before hitting its rear solid density layer with a significantly higher intensity but a smaller spot size. Indeed, we observe the laser intensity increase by a factor of up to 3.7 in comparison with an initial intensity at the focus. Practically, we found that with the 20 fs, 1 PW laser pulse focused to the spot size of 8 µm at the 115 nm thick plastic target and with 100 µm thick secondary lead converter, the neutron flux can achieve the peak value in the order of 1e23 neutrons.cm-2.s-1 while the flux more intense than 1e20 neutrons.cm-2.s-1 necessary for the r-process to occur lasts for about 270 ps. Such a result is very promising for realizing double neutron capture in the laboratory, as the required neutron flux over a long duration is met.
As mentioned above, the nominal power level of 10 PW at which we plan to work for producing the extreme neutron beam simulated above is not yet available, but planned during 2022. Thus, as a first step, we have performed the commissioning of the "short focal length" area (SFA) of the new Apollon laser facility (Saclay, France) with the first available laser beam, at a level of 0.4 PW. The presently available laser beam at a nominal power of one petawatt delivered indeed pulses of 10 J average on-target energy with pulse durations of 25 fs. It was the first time this experimental area was used for high-power shots on targets. The commissioning went well, as detailed below, which allows us to envision ramping up the power in mid-2022, in order to reach the necessary power for the project. A range of diagnostics was in place in order to qualify the performance of the facility at this level of laser power. An imager, working in X-ray range, of the zone heated by the laser on the target demonstrated a good focusing of the laser at full power, with a heated spot of 4.5 µm in diameter, corresponding to the laser spot aligned at low flux. Solid targets as thin as 2 µm in thickness were irradiated by intense laser pulse without damaging them by the pedestal preceding the main pulse, which corresponds to a good temporal contrast characteristics. Emissions of electrons, ions and high energy electromagnetic radiation were recorded, showing good laser-target coupling and an overall performance that is very consistent with what have been reported by similar international facilities.
For the second axis of the project, we have worked actively on the generation of laboratory plasmas that can be quantitatively scaled-down to astrophysical ones. This was pursued in two main axes: (1) the use of strong magnetic fields to both (i) make laboratory plasmas more astrophysically-relevant plasmas, as well as (ii) confine them, with in view the better ability to have them be well coupled to the neutron beams we produce - these are star-scaled plasmas, and (2) the production of supernovae-scaled plasmas, so that we could explore the two possible sites were nucleosynthesis is hypothesized to take place, star mergers or supernovae. We have already in the recent past devoted quite some effort to show that we could produce star-scaled plasmas in the laboratory. We pursued this effort by investigating how high-strength magnetic fields, produced by Helmholtz-type coils surrounding the laser-driven plasmas, could be used to improve the variety of star systems we could have the laboratory plasmas scaled to, as well as to confine the plasmas. In parallel to investigate how to produce star-scaled plasmas, we have also investigated how to generate quantitatively scalable supernovae plasmas in the laboratory. For this, we have performed experiments where the aim was to directly produce a fast-propagating shock enveloppe that could be scaled to that of a supernova. Such collisionless shock waves form when energy release from supernovae encounters the tenuous magnetized space environment. An important milestone was that we could show how we could generate in practice, in the the laboratory, astrophysically relevant super-critical quasi-perpendicular magnetized collisionless shocks.
Within the project, we examined the neutron yield from spallation as a function of the converter target material and the projectile proton energy, in a range within the reach of present or near-future laser systems. For this purpose, we have used the FLUKA 3D Monte Carlo code to simulate the nuclear reactions induced during irradiation of a convertor target by a mono-kinetic and mono-directional proton beam. With this, we know what optimal material to use for the neutron converter. The next steps was to see how to optimize the proton generation with multi-PW lasers in order to obtain the highest neutron fluxes. Again, this is done through simulations, in waiting for the facilities to be available. We thus researched, utilising large scale particle-in-cell and Monte Carlo simulations, a neutron source which relies on the laser-driven proton acceleration on the primary target and conversion of these protons into the neutrons in the secondary high-Z neutron converter. Inspired by the trends in the community, we investigated the use of the double layer primary targets for proton acceleration where the laser pulse is first focused in the near-critical front layer before hitting its rear solid density layer with a significantly higher intensity but a smaller spot size. Indeed, we observe the laser intensity increase by a factor of up to 3.7 in comparison with an initial intensity at the focus. Practically, we found that with the 20 fs, 1 PW laser pulse focused to the spot size of 8 µm at the 115 nm thick plastic target and with 100 µm thick secondary lead converter, the neutron flux can achieve the peak value in the order of 1e23 neutrons.cm-2.s-1 while the flux more intense than 1e20 neutrons.cm-2.s-1 necessary for the r-process to occur lasts for about 270 ps. Such a result is very promising for realizing double neutron capture in the laboratory, as the required neutron flux over a long duration is met.
As mentioned above, the nominal power level of 10 PW at which we plan to work for producing the extreme neutron beam simulated above is not yet available, but planned during 2022. Thus, as a first step, we have performed the commissioning of the "short focal length" area (SFA) of the new Apollon laser facility (Saclay, France) with the first available laser beam, at a level of 0.4 PW. The presently available laser beam at a nominal power of one petawatt delivered indeed pulses of 10 J average on-target energy with pulse durations of 25 fs. It was the first time this experimental area was used for high-power shots on targets. The commissioning went well, as detailed below, which allows us to envision ramping up the power in mid-2022, in order to reach the necessary power for the project. A range of diagnostics was in place in order to qualify the performance of the facility at this level of laser power. An imager, working in X-ray range, of the zone heated by the laser on the target demonstrated a good focusing of the laser at full power, with a heated spot of 4.5 µm in diameter, corresponding to the laser spot aligned at low flux. Solid targets as thin as 2 µm in thickness were irradiated by intense laser pulse without damaging them by the pedestal preceding the main pulse, which corresponds to a good temporal contrast characteristics. Emissions of electrons, ions and high energy electromagnetic radiation were recorded, showing good laser-target coupling and an overall performance that is very consistent with what have been reported by similar international facilities.
For the second axis of the project, we have worked actively on the generation of laboratory plasmas that can be quantitatively scaled-down to astrophysical ones. This was pursued in two main axes: (1) the use of strong magnetic fields to both (i) make laboratory plasmas more astrophysically-relevant plasmas, as well as (ii) confine them, with in view the better ability to have them be well coupled to the neutron beams we produce - these are star-scaled plasmas, and (2) the production of supernovae-scaled plasmas, so that we could explore the two possible sites were nucleosynthesis is hypothesized to take place, star mergers or supernovae. We have already in the recent past devoted quite some effort to show that we could produce star-scaled plasmas in the laboratory. We pursued this effort by investigating how high-strength magnetic fields, produced by Helmholtz-type coils surrounding the laser-driven plasmas, could be used to improve the variety of star systems we could have the laboratory plasmas scaled to, as well as to confine the plasmas. In parallel to investigate how to produce star-scaled plasmas, we have also investigated how to generate quantitatively scalable supernovae plasmas in the laboratory. For this, we have performed experiments where the aim was to directly produce a fast-propagating shock enveloppe that could be scaled to that of a supernova. Such collisionless shock waves form when energy release from supernovae encounters the tenuous magnetized space environment. An important milestone was that we could show how we could generate in practice, in the the laboratory, astrophysically relevant super-critical quasi-perpendicular magnetized collisionless shocks.