Mark J. Schmitt

Progress and prospects of ion-driven fast ignition

Fusion fast ignition (FI) initiated by laser-driven ion beams is a promising concept examined in this paper. FI based on a beam of quasi-monoenergetic ions (protons or heavier ions) has the advantage of a more localized energy deposition, which minimizes the required total beam energy, bringing it close to the ≈10 kJ minimum required for fuel densities ∼ 500 gc m −3 . High-current, laser-driven ion beams are most promising for this purpose. Because they are born neutralized in picosecond timescales, these beams may deliver the power density required to ignite the compressed DT fuel, ∼10 kJ/10 ps into a spot 20 µm in diameter. Our modelling of ion-based FI include high fusion gain targets and a proof of principle experiment. That modelling indicates the concept is feasible, and provides confirmation of our understanding of the operative physics, a firmer foundation for the requirements, and a better understanding of the optimization trade space. An important benefit of the scheme is that such a high-energy, quasi-monoenergetic ignitor beam could be generated far from the capsule (1 cm away), eliminating the need for a reentrant cone in the capsule to protect the ion-generation laser target, a tremendous practical benefit. This paper summarizes the ion-based FI concept, the integrated ion-driven FI modelling, the requirements on the ignitor beam derived from that modelling, and the progress in developing a suitable laser-driven ignitor ion beam.

Characterization and focusing of light ion beams generated by ultra-intensely irradiated thin foils at the kilojoule scale a)

We present the first observations of focused multi-MeV carbon ion beams generated using ultra-intense shortpulse laser interactions with thin hemispherical (400μm radius) targets. The experiments were performed at the Trident laser facility (80 J, 0.6 ps, 2×1020W/cm2) at Los Alamos National Laboratory and at the Omega EP (extended performance) facility (1 kJ, 10 ps, 5×1018W/cm2) at the Laboratory for Laser Energetics. The targets were chemical vapor deposition diamond, hemi-shells and were heated to remove contaminants. The ion beam focusing was characterized by tracing the projection of a witness mesh in the ion beam on a lithium fluoride nuclear activation detector. From the data, we infer that the divergence of the beam changes as a function of time. We present a 2-D isothermal model to explain the dynamics. We also present discrepancies in the peak proton and carbon ion energies from the two facilities. The implication of which is a fundamental difference in the temporal evolution of the beams from th...

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO 2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.

Initial results from the Los Alamos photoinjector-driven free-electron laser☆

Abstract We report initial results on the APEX (APLE prototype experiment) photoinjector-driven infrared free-electron laser (FEL). The APEX FEL is operating in support of a Boeing Aerospace and Electronics/Los Alamos National Laboratory collaboration to build the average power laser experiment (APLE). Our system uses a high quantum efficiency (3–7%) multi-alkali photocathode, illuminated with a frequency-doubled Nd:YLF mode locked laser at 21.7 MHz. The photocathode is located in this first cell of a six-cell 1.3 GHz, 6 MeV photoinjector that feeds a linac with a final energy up to 40 MeV. Because the illuminating laser pulse on our photocathode is short (10 ps), no pulse compression is required in the linac. Emittance measurements made after the second linac tank at 15 MeV have shown that a normalized emittance (for 90% of the particles) of less than 50π mm mrad can be achieved at a peak micropulse current of 300 A. Our initial lasing has been at a wavelength of 3.6 μm over a 30 μs macropulse with an electron beam energy of 35 MeV and a 2.7 cm period permanent magnet wiggler. We are continuing to characterize and optimize our system, with particular emphasis on understanding and minimizing electron beam emittance-growth mechanisms, and subsequently improving the quality of the beam delivered to the wiggler.

Measurement of integrated speckle statistics for CO2 lidar returns from a moving, nonuniform, hard target.

A pulsed, dual CO(2) laser lidar was used to measure return signal statistics as a function of the number of speckles integrated by the lidar receiver per laser pulse. A rotating target generated statistically independent speckle patterns on each laser pulse. Data were collected for a wide range of receiver aperture sizes. A statistical model is developed that predicts the probability density of the return lidar pulse energy, which includes speckle, depolarization by the target, and albedo sampling. The predictions of this model are compared with the measured probability density function of the return pulse energies. Very good agreement is found between the geometrically calculated number of integrated speckles and the number predicted by the model.

Development of a polar direct-drive platform for studying inertial confinement fusion implosion mix on the National Ignition Facilitya)

Experiments were performed to develop a platform for the simultaneous measurement of mix and its effects on fusion burn. Two polar direct drive implosions of all-plastic capsules were conducted for the first time on the National Ignition Facility (NIF). To measure implosion trajectory and symmetry, area image backlighting of these capsules was also employed for the first time on NIF, an advance over previous 1-D slit imaging experiments, providing detailed symmetry data of the capsules as they imploded. The implosion trajectory and low-mode asymmetry seen in the resultant radiographs agreed with pre-shot predictions even though the 700 kJ drive energy produced laser beam intensities exceeding laser-plasma instability thresholds. Post-shot simulations indicate that the capsule yield was reduced by a factor of two compared to pre-shot predictions owing to as-shot laser drive asymmetries. The pre-shot predictions of bang time agreed within 200 ps with the experimental results. The second shot incorporated a narrow groove encircling the equator of the capsule. A predicted yield reduction factor of three was not observed.

Omega EP, Laser Scalings and the 60 MeV Barrier: First Observations of Ion Acceleration Performance in the 10 Picosecond Kilojoule Short-Pulse Regime

Omega EP is capable of producing 1000 J in 10 ps and is currently the most energetic short-pulse laser in the world. The performance of EP in terms of proton beam energies is compared with other laser systems worldwide at similar intensities. Omega EP results are discussed in the context of these lasers and the empirical ~ 60 MeV barrier, which has existed since the discovery of forward laser-accelerated protons in 2000 [1–2].

Progress on ion based fast ignition

Research at Los Alamos on fusion fast ignition (FI) [1] initiated by laser-driven ion beams heavier than protons has produced encouraging results. The minimum requirements for FI are relatively well understood [2]. Based on simple considerations and on those requirements, it is shown that FI of the compressed DT fuel using laser-driven heavy ion beams has advantages compared to laser-driven proton or electron beams, along with different risks compared to those approaches. Using a technologically convenient light-ion species such as Carbon, ~ 100-fold fewer ions may deliver the energy necessary to ignite, simplifying target fabrication. Key requirements for success include the generation of a monoenergetic beam (energy spread ≤ ~ 10%), a sufficiently high ion kinetic energy (~ 450 MeV for C), and a sufficiently high conversion efficiency of laser to beam energy. An important benefit of this scheme is that such a high-energy, quasi-monoenergetic beam may be generated far from the capsule (~ 1 cm away), eliminating the need for a reentrant cone in the capsule, a tremendous practical benefit. This paper summarizes our progress in meeting those requirements, and the results of an integrated 2D design for a proof of principle FI experiment based on this concept.

Role of shocks and mix caused by capsule defects

An Eulerian code with a turbulent mix model is used to model a set of plastic (CH) ablator capsules with and without equatorial grooves. The “perfect” capsule results were used to calibrate simulations of capsules with equatorial grooves of different depths that provided information on increasingly perturbed implosions. Simulations with a turbulence model were able to calculate the same yield over mix (YOM) ratio (experiment/mix simulation) of 0.2 to 0.3 for thin (8-μm thick) and thick shell (15-μm thick) capsules with no grooves and thin capsules with shallow grooves. When the capsules have deep grooves, the YOM ratio increases to greater than unity, probably because the deformed shocks focus too strongly on the symmetry axis in our two-dimensional simulations. This is supported by a comparison of simulated and experimental x-ray images.

Observation of early shell-dopant mix in OMEGA direct-drive implosions and comparisons with radiation-hydrodynamic simulations

Temporally, spatially, and spectrally resolved x-ray image data from direct-drive implosions on OMEGA were interpreted with the aid of radiation-hydrodynamic simulations. Neither clean calculations nor those using a turbulent mix model can explain fully the observed migration of shell-dopant material (titanium) into the core. Shell-dopant migration was observed via time-dependent, spatially integrated spectra, and spatially and spectrally resolved x-ray images of capsule implosions and resultant dopant emissions. The titanium emission was centrally peaked in narrowband x-ray images. In post-processed clean simulations, the peak titanium emission forms in a ring in self-emission images as the capsule implodes. Post-processed simulations with mix reproduce trends in time-dependent, spatially integrated spectra, as well having centrally peaked Ti emission in synthetic multiple monochromatic imager. However, mix simulations still do not transport Ti to the core as is observed in the experiment. This suggests that phenomena in addition to the turbulent mix must be responsible for the transport of Ti. Simple diffusion estimates are unable to explain the early Ti mix into the core. Mechanisms suggested for further study are capsule surface roughness, illumination non-uniformity, and shock entrainment.