Showing papers by "Angelo Schiavi published in 2013"

A Monte Carlo-based treatment planning tool for proton therapy.

Abstract: In the field of radiotherapy, Monte Carlo (MC) particle transport calculations are recognized for their superior accuracy in predicting dose and fluence distributions in patient geometries compared to analytical algorithms which are generally used for treatment planning due to their shorter execution times. In this work, a newly developed MC-based treatment planning (MCTP) tool for proton therapy is proposed to support treatment planning studies and research applications. It allows for single-field and simultaneous multiple-field optimization in realistic treatment scenarios and is based on the MC code FLUKA. Relative biological effectiveness (RBE)-weighted dose is optimized either with the common approach using a constant RBE of 1.1 or using a variable RBE according to radiobiological input tables. A validated reimplementation of the local effect model was used in this work to generate radiobiological input tables. Examples of treatment plans in water phantoms and in patient-CT geometries together with an experimental dosimetric validation of the plans are presented for clinical treatment parameters as used at the Italian National Center for Oncological Hadron Therapy. To conclude, a versatile MCTP tool for proton therapy was developed and validated for realistic patient treatment scenarios against dosimetric measurements and commercial analytical TP calculations. It is aimed to be used in future for research and to support treatment planning at state-of-the-art ion beam therapy facilities.

Comparison for non-local hydrodynamic thermal conduction models

Abstract: Inertial confinement fusion and specifically shock ignition involve temperatures and temperature gradients for which the classical Spitzer-Harm thermal conduction breaks down and a non-local operator is required. In this article, two non-local electron thermal conduction models are tested against kinetic Vlasov-Fokker-Planck simulations. Both models are shown to reproduce the main features of thermal heat front propagation at kinetic timescales. The reduction of the thermal conductivity as a function of the scalelength of the temperature gradient is also recovered. Comparisons at nanosecond timescales show that the models agree on the propagation velocity of the heat front, but major differences appear in the thermal precursor.

Recent results from experimental studies on laser-plasma coupling in a shock ignition relevant regime

Abstract: Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity Iλ 2 >10 15 Wc m −2 µm 2 at the end of the compression phase. In this intensity regime, laser–plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS). We study the laser–plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 10 13 Wc m −2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 10 15 –10 16 Wc m −2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250–850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target. Preliminary results show that the backscattered energy is in the range 3–15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers.

Energy and wavelength scaling of shock-ignited inertial fusion targets

Abstract: In inertial fusion shock ignition, separation of the stages of fuel compression and hot spot creation introduces some degree of design flexibility. A lower implosion velocity can be compensated for by a more intense ignition pulse. Flexibility increases with target (and driver) size and allows for a compromise between energy gain and risk reduction. Having designed a reference ignition target, we have developed an analytical model for (up)-scaling targets as a function of laser energy, while keeping under control parameters related to hydro- and plasma instabilities. Detailed one-dimensional simulations confirm the model and generate gain curves. Options for increasing target robustness are also discussed. The previous results apply to UV laser light (with wavelength λ = 0.35 μm). We also show that our scaling model can be used in the design of targets driven by green laser light (λ = 0.53 μm).

Stability of target irradiation for high-repetition rate direct-drive facilities

Abstract: The illumination of a spherical target for Inertial Confinement Fusion is studied in the direct-drive approach. Three main sources of laser parameter fluctuation are taken into account, and a statistical analysis of the illumination configuration has been carried out in order to assess the stability of the illumination, and the shot-to-shot fluctuation of the laser intensity pattern. The illumination uniformity of the target is one of the key issues in direct-drive Inertial Confinement Fusion (1, 2). Any departure from perfect spherical symmetry in the compression drive could have detrimental effects on the implosion of the shell target, and could lead to ignition failure (3-5). The problem becomes even more stringent when a high-repetition laser facility is envisaged, where target implosions take place several times per second in the reactor chamber. In this case the collocation of the fuel target at the centre of the compression drive might be difficult to control with accuracy and precision better than a few percent of target radius. We present a study of an illumination configuration (6), where the rms intensity non-uniformity is optimised taking into account three sources of departure from nominal parameters, i.e. the laser pointing error, the beam-to-beam power imbalance, and the target positioning. It is found that the fluctuation of the illumination asymmetry is a key parameter for assessing the stability of operation in high-repetition rate mode. Moreover the target positioning error arising from the injection system could be the dominant source of illumination asymmetry in a fusion reactor. The analysis procedure outlined in the paper is of general validity, but for the sake of clarity, the work presented is based on the study of a particular irradiation pattern, consisting of 48 different beamlines distributed on a sphere. This configuration is the one envisaged for the laser system proposed within the HiPER framework, which is aimed at the demonstration of a burst-mode fusion reactor exploiting advanced ignition schemes, such as the Shock Ignition approach or the Fast Ignitor scheme (7). The beams are grouped in 6 cones symmetrically distributed around the equator in the polar angle . Within each cone, the beams are evenly distributed around the symmetry axis with equal spacing in the azimuthal angle (8). The details of the configuration are summarised in the left panel of Figure 1. In addition to the beam centroids on the irradiation sphere, the main parameters that define the laser configuration are the time and space profile of the laser power carried by each beam. We describe the intensity spatial profile as I(r) = I0 exp(−(r/) m ), where r is the radial distance for the beam axis, I0 is the peak intensity, is the effective waist of the beam, and m is the super-Gaussian exponent. With R0 we indicate the initial outer radius of the target. The average intensity on the target is given by = I(, )d/ d, where the integration is carried out over the whole solid angle, and I(, ) is the local intensity on the target surface.

Studies on shock ignition targets for inertial fusion energy

Abstract: Shock-ignited inertial fusion targets are studied by one dimensional and two-dimensional numerical simulations. Most of the study refers to the simple all-DT HiPER baseline target (imploded mass of 0.29 mg); both the reference laser wavelength λ = 0.35 μm, and λ = 0.25 μm are considered. The target achieves 1D gain about 80 (120) with total laser energy of 260 kJ (180 kJ) at λ = 0.35 μm (0.25 μm). Operating windows for the parameters of the laser ignition spike are described. According to preliminary simulations, gain 80–100 is also obtained by a scaled target (imploded mass of 1.8 mg) driven by 1.5 MJ of green laser light (0.53 μm). Two dimensional simulations indicate robustness to irradiation nonuniformities, and high sensitivity to target mispositioning. This can however be reduced by increasing the power of the ignition spike.

Studies on shock ignition targets for inertial fusion energy

Abstract: Shock ignition [1] is an approach to direct-drive inertial confinement fusion (ICF) in which the stages of compression and hot spot formation are partly separated. The fuel is first imploded at lower velocity than in conventional ICF. Close to stagnation an additional intense laser spike drives a strong converging shock, which contributes to hot spot formation. Shock ignition shows potentials for high gain at UV laser energy below 1 MJ, and could be tested on the National Ignition Facility [2] or Laser Megajoule. Due to the lower implosion velocity, issues related to hydrodynamic instabilities are relaxed. On the other hand, the interaction of the laser spike with the plasma occurs in a regime where parametric instabilities are expected to become relevant.