Science on LMJ-PETAL

The coupling of a long pulse, large energy, multi-beams facility (LMJ) with a high power, short duration, auxiliary laser (PETAL), offers new research possibilities. Four priority research themes were identified and four international scientific working groups were established to prepare the Scientific Case of LMJ-PETAL:

The brief description given below is a summary of this scientific case for the four topics :

  • Materials and High Energy Density Physics
  • Laboratory Astrophysics
  • Inertial Confinement Fusion by Direct Drive for Energy Production
  • High Energy physics

Materials and High Energy Density Physics

The irradiation of a sample by a high intensity laser pulse (LMJ) creates a plasma cloud, which expands toward the laser source and drives by reaction a compression wave into the sample. This ablation process brings the matter to almost unexplored conditions.

An ultra-high intensity laser pulse (PETAL) can generate short X-rays pulse from a backlighter, which can probe the evolution of the sample structure during shock or ramp compression.

LMJ/PETAL, coupling long and short pulses, will offer a unique opportunity to reach and study matter at ultra-high pressures, allowing investigating completely new materials and phenomena relevant to planetology and physical-chemistry of materials.

The figure below shows a plot in temperature-density space indicating regions encompassed by different physical processes and conditions. Regions that will be accessible with LMJ are indicated in the figure and 1Mbar contour is shown.

The materials structures at these densities and temperatures along compression path are poorly known and will reveal entirely new phenomena and new phases. At high temperatures, direct investigations of stellar interior conditions by a combination of nanosecond pulses to create the conditions and short pulses to probe the matter should open new dialogs with stellar astrophysics.

A challenge of special interest is the high density low temperature regime (T < 104K, pressure exceeding 10 Mbar = 1 terapascal (TPa)). This is so far almost unexplored by solid state physics and materials science and is beyond reach for plasma physics.

At the border of presently accessible pressure-temperature conditions, a new paradigm is emerging: extreme pressures seem to turn simplicity into complexity. For example, simple metals that are expected to end up in the simple body cubic centered (bcc) structure at high pressures and temperatures adopt open and complex structures at TPa pressures, complexifying considerably the phase diagrams.

Superionic phases are emerging in some molecular systems, which is a key to the modeling of transport properties of icy giant planets.

Pressure-induced chemical complexity (e.g. immiscibilities) is of prime importance in solid state physics, materials science and planetology.

The LMJ/PETAL setup will open up a unique possibility, using the LMJ (“pump”) to reach this pressure-temperature regime and using PETAL (“probe”) to produce short pulses of X-rays to characterize this matter.

Due to the limited access and high cost of a single shot on such a facility, the LMJ/PETAL shots should be dedicated to reach the few TPa range in close coordination with the studies of materials that will in parallel be investigated up to the TPa range in front of synchrotron, XFEL and KJ laser facilities. This complementarity should motivate numerous scientists involved in extreme conditions at synchrotron and XFEL to get interested in this pressure extension of their normal investigation domain.

Laboratory Astrophysics

The means of astronomical observations have been developed considerably over the last decade, often bringing a new vision of our universe. However, it remains challenges for astrophysical objects or situations that spatial measurements cannot fully meet. Physicists have therefore developed a “local” way to satisfy dedicated studies of these topics: Laboratory Astrophysics.

Examples of physical processes that can be addressed experimentally include strong shocks, hydrodynamic instabilities, ejection processes, accretion processes, complex opacities, magnetic field generation or particle acceleration. All these processes are involved in a wide range of astrophysical phenomena such as supernovae, young stars jets, planet formation, evolution and structure of stars, etc.

Indeed, laboratory astrophysics consists in recreating in the laboratory the conditions existing throughout the universe, reproducing complex objects… To this end, three categories of experiments have been distinguished:

  • identical (exact conditions of objects are created),
  • similar (astrophysical situation can be scaled exactly into the laboratory)
  • or resemble (only part of processes involved are relevant).

Over 90 % of the visible baryonic matter in the universe is in the form of plasma. The effects and the transport properties of radiation in various astrophysical environments, together with a deep understanding of the interplay between hydrodynamics and radiation, remain a central issue in astrophysics. In stellar interiors, plasmas cover a very wide thermodynamical domain, that only high energy and high power lasers enable to explore. They pave the way for the experimental determination of fundamental physical data, such as the radiative properties – the opacities – of hot plasmas in stellar envelopes and interiors, or, relevant for stellar evolution and Big Bang nucleosynthesis, the direct determination of nuclear reaction rates among low Z elements, retrieving genuinely complex plasma screening effects.

Moreover, one advantage of the unsteady nature of the laser experiments is their ability to simulate a large panel of violent phenomena occurring in the universe, e.g. associated with supernova explosions, star formation, accretion and ejection of matter from young stars or compact objects, or, else, radiation dominated photoionized plasmas, near strong XUV sources. LMJ and PETAL will be able to produce and diagnose high velocity shocks, moving at a several hundreds kilometers per second, evolving well in the so-called radiative shock regime, affected by various dynamical and thermal instabilities, pertinent for studies of supernovae remnants and accretion/ejection processes.

Moreover, important domains pertinent for high energy astrophysics will be experimentally addressed, thanks to the ability of PETAL to produce copious proton beams, in the 10-100 MeV energy range. Among them, the physics of collisionless shocks will be experimentally explored for the first time, together with Weibel instability, cosmic ray acceleration by shocks and diffusion, kinetic dynamo from turbulence. In this respect, due to large experimental scales and high temperatures, LMJ will enable scaled experiments with magnetic Reynolds numbers significantly larger than presently achieved.

The PETAL-LMJ facility will provide new capabilities to this field both by generating macroscopic volumes of matter under astrophysical conditions (LMJ) and by probing it through the production of secondary sources of protons, X rays and gamma rays (PETAL). PETAL will offer direct measurements, with high spatial and temporal resolution, of the density and the velocity field in the plasma, as well as the measurement, using proton beams, of intense magnetic fields.

Inertial Confinement Fusion by Direct Drive for Energy Production

Inertial Fusion for Energy (IFE) relies on the underlying concept of Inertial Confinement Fusion (ICF) where a spherical shell containing a few milligrams of hydrogen isotopes is first compressed then ignited from a hot spot. A major milestone is the achievement of nuclear gain by means of ICF using existing laser facilities. Different schemes have been proposed for the implosion and ignition of ICF targets. One of these schemes is direct laser illumination (direct drive) for the compression stage and shock or fast ignition to trigger the nuclear burn.

Shock Ignition (SI), as well as the conventional central ignition scheme (CCI), gets ignition from a central hot spot driven to high pressures (500 -1000 Gbar) by hydrodynamic motion.  In the conventional scheme, this pressure results from the conversion of the kinetic energy imparted to the imploding shell into internal energy. CCI requires a threshold implosion velocity in the 350-400 km/s range. Conversely, SI targets are driven at sub ignition velocities (250-300 km/s), thus enhancing the hydrodynamic stability and reducing deleterious effects related to laser-plasma interaction. A converging shock wave launched at the end of the implosion raises the central pressure up to ignition. This shock wave is driven by a specific temporal profile of the laser pulse (ignition spike). Since the energy of the spike is about 200-300 kJ for a duration of a few hundreds of ps, it can be obtained from lasers having the same technology than the compression lasers, and even from the same beam lines.

In the Fast Ignition concept (FI), a beam of energetic particles, (low Z ions or relativistic electrons) delivers its energy to the hot spot. These particles are accelerated by means of a short but powerful auxiliary laser pulse. Typical energy is close to 100 kJ, for a 10-20 ps duration. Although the energy-power diagram of Petal falls well below these requirements, the LMJ-PETAL system offers a unique opportunity to optimize the production of the ignition beam and evidence hot spot heating in realistic ICF conditions.

The long term goal is nuclear gain in the LMJ target chamber. Its success depends on progresses expected in the fields of physics, applied mathematics and technology. The following key elements have been identified:

  • Fuel compression. This issue is shared by all schemes considered here. It will have to deal with the specific polar arrangement of LMJ beams. Achieving a polar direct drive (PDD) implosion platform on LMJ is a major milestone on the path towards ignition.
  • Hot spot heating. This involves interaction and transport physics in very different regimes from one another in conventional ignition, SI and FI.
  • Diagnostics and metrology are required in order to control all stages of the process with a resolution sufficient to settle between theoretical assumptions and/or fulfill design requirements.
  • Targetry. This involves the production and characterization of warm or cryogenic shells within given specifications of roughness, sphericity and homogeneity.
  • High performance computing. The hydro code is the essential companion of this program. Its validation and the demonstration of its predictive capabilities are central issues. It is interfaced with databases for material properties, post processors for diagnostics and output analysis. It must also be linked to codes devoted to detailed physics as plasma kinetics or nuclear combustion.

High Energy physics

This topic is related to the generation of energetic particles and radiation during the interaction of high power laser beams with matter. The use of LMJ/PETAL is proposed for specific cases taking advantage of the large energy delivered by the PETAL beam in a relatively short duration, in order to investigate:

  • the acceleration of electrons to ultra-relativistic energies by laser wakefield in one stage, in a regime of low density and long plasmas; alternative methods to accelerate electrons to a few GeV, such as direct laser acceleration in vacuum;
  • the generation of intense relativistic ions beams in the high intensity and high energy regime and their interaction;
  • the relativistic electron-positron pair jets and plasmas and the consequent gamma-ray burst due to pair annihilation;
  • the generation of radiation and its amplification in plasmas;
  • the nonlinear and dispersive properties of quantum vacuum in strong fields.

Particle accelerators are used to probe and understand the structure of matter. The accelerating gradients of modern accelerators are limited to 20-100 MV/m range, which has led to large devices, culminating in the 27 km circumference, 7 TeV LHC at CERN. These huge accelerators are used to answer questions on the structure of matter beyond the standard model, the nature of mass (the Higgs mechanism), supersymmetry etc. Microwave cavities of conventional accelerators however break down above field gradients of 100 MV/m due to plasma formation, which makes even modest energy linear accelerators long and expensive devices.

One of the big challenges of accelerator development is to design an electron-positron collider at the energy frontier (> TeV) with an affordable cost. Validating laser wakefield acceleration mechanism for energies of the order of 100 GeV would exceed the current state-of-the-art energy using plasma as an acceleration medium by two orders of magnitude.

Two big challenges are envisioned for ion acceleration: the demonstration of the production of intense GeV proton beams and the control of the characteristics of such relativistic beams (energy spread, divergence, number of energetic ions…). These challenges will have strong implications for studies on laboratory astrophysics, Ion Fast Ignition concept for ICF and on laser hadrontherapy.

Relativistic electron-positron pair plasmas and jets are believed to exist in many astrophysical objects and are invoked to explain energetic phenomena related to Gamma Ray Bursts and black holes. On earth, positrons from radioactive isotopes or accelerators are used extensively at low energies (sub-MeV) in areas related to surface science positron emission tomography, basic antimatter science such as antihydrogen experiments, Bose-Einstein condensed positronium, and basic plasma physics.

Creating dense relativistic electron-positron (antimatter) plasmas has been elusive, due to the difficulties to produce the pairs in high density and their highly relativistic energies. With the advent of new, much more powerful lasers, we expect to create extreme relativistic plasmas that display conditions never before encountered. Comprehending observed phenomena under those conditions, whether predicted by existing theoretical and computational capabilities or not, will be a challenge.