David Turnbull

Focussing laser light onto the surface of a small target filled with deuterium and tritium implodes it and leads to the creation of a hot and dense plasma, in which thermonuclear fusion reactions occur. In order for the plasma to become self-sustaining, the heating of the plasma must be dominated by the energy provided by the fusion reactions—a condition known as a burning plasma. A metric for this is the generalized Lawson parameter, where values above around 0.8 imply a burning plasma. Here, we report on hydro-equivalent scaling of experimental results on the OMEGA laser system and show that these have achieved core conditions that reach a burning plasma when the central part of the plasma, the hotspot, is scaled in size by at least a factor of 3.9 ± 0.10, which would require a driver laser energy of at least 1.7 ± 0.13 MJ. In addition, we hydro-equivalently scale the results to the 2.15 MJ of laser …

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G target of 1.5. This is the first laboratory demonstration of exceeding “scientific breakeven”(or G target> 1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al.(Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Irradiating a small capsule containing deuterium and tritium fuel directly with intense laser light causes it to implode, which creates a plasma hot enough to initiate fusion reactions between the fuel nuclei. Here we report on such laser direct-drive experiments and observe that the fusion reactions produce more energy than the amount of energy in the central so-called hot-spot plasma. This condition is identified as having a hot-spot fuel gain greater than unity. A hot-spot fuel gain of around four was previously accomplished at the National Ignition Facility in indirect-drive inertial confinement fusion experiments where the capsule is irradiated by X-rays. In that case, up to 1.9 MJ of laser energy was used, but in contrast, our experiments on the OMEGA laser system require as little as 28 kJ. As the hot-spot fuel gain is predicted to grow with laser energy and target size, our work establishes the direct-drive approach to …

Recent experiments at the Omega Laser Facility have carefully studied the spraying of laser light as it propagates through underdense plasmas of relevance to inertial confinement fusion experiments. In these experiments, frequency redshits of the transmitted light were measured that exceed estimates based on the Dewandre shift caused by the plasma expansion alone. Such redshifts are a signature of forward Brillouin scattering. Here, these experiments are modeled using three-dimensional simulations that describe the necessary absorption, backscattering, filamentation, and forward scattering of the laser light. The phase plate of the experimental beam is reproduced numerically, allowing for a direct comparison with the experimentally measured transmitted beam spot and frequency. The impact of various bandwidth-based mitigation techniques is explored in anticipation of further upcoming experiments …

In cross-beam energy transfer (CBET), the interference of two laser beams ponderomotively drives an ion-acoustic wave that coherently scatters light from one beam into the other. This redirection of laser beam energy can severely inhibit the performance of direct-drive inertial confinement fusion (ICF) implosions. To assess the role of nonlinear and kinetic processes in direct-drive-relevant CBET, the energy transfer between two laser beams in the plasma conditions of an ICF implosion at the OMEGA laser facility was modeled using particle-in-cell simulations. For typical laser beam intensities, the simulations are in excellent agreement with linear kinetic theory, indicating that nonlinear processes do not play a role in direct-drive implosions. At higher intensities, CBET can be modified by pump depletion, backward stimulated Raman scattering, or ion trapping, depending on the plasma density.

. We present experimental results from the Raman amplification experimental platform at the University of Rochester's Laboratory for Laser Energetics (LLE). This platform explores Raman amplification in a unique parameter space which includes a multi-joule pump and an adjustable-energy seed with seed intensities up to 7 x 10 15 W/cm 2, which is significantly more intense than prior experiments. Initial experiments have demonstrated single-pass Raman amplification in multiple focal configurations with energy gain factors as high as 30x, energy transferred to the seed exceeding 220 mJ, and record efficiencies as high as 11.7%. The efficiency is observed to scale with seed intensity. For intense seed pulses, efficient amplification is seen well into the wavebreaking regime, as predicted by particle-in-cell simulations [1].

We show that distinct signatures in both the acceleration and stagnation phases of direct-drive cryogenic implosion experiments are consistent with 3-D modeling that induces the jetting of a peripheral mass into the nascent hot spot. The cause of such jetting remains speculative, but a candidate is the role of intrinsic target features. The signatures arise due to the injection of dense fuel and ablator material into the low-density center during early stages of hot-spot formation. First, we show that the defect modeling creates an advance of hot-spot emission during the implosion history, such as has been observed and which is not accounted for by the model with imprinting. Second, we have found a reduction of the discrepancy when the ablation front is separated from the hot spot by increased cryogenic payload, providing strong support for an origin in ablation front instability. Lastly, we show that measurements used to …

Inverse bremsstrahlung absorption was measured based on transmission through a finite-length plasma that was thoroughly characterized using spatially resolved Thomson scattering. Expected absorption was then calculated using the diagnosed plasma conditions while varying the absorption model components. To match data, it is necessary to account for (i) the Langdon effect;(ii) laser-frequency (rather than plasma-frequency) dependence in the Coulomb logarithm, as is typical of bremsstrahlung theories but not transport theories; and (iii) a correction due to ion screening. Radiation-hydrodynamic simulations of inertial confinement fusion implosions have to date used a Coulomb logarithm from the transport literature and no screening correction. We anticipate that updating the model for collisional absorption will substantially revise our understanding of laser-target coupling for such implosions.

Increasing ablation pressure to> 200 Mbar is crucial for accessing the region of design space where robust ignition would be expected for a MJ-class laser driver. Currently, crossed-beam energy transfer (CBET) severely limits the ablation pressure in directly driven implosions. Operating intensities are further constrained by the need to avoid fuel preheat by suprathermal electrons generated by the two-plasmon decay (TPD) and stimulated Raman scattering instabilities. Simulations suggest that fractional bandwidths of~ 1.5%(Δω/ω)--about 50 larger than what is available currently--would completely eliminate all laser-plasma instabilities (LPI) in OMEGA-scale implosions and thereby recover the advantages for energy coupling that were expected for direct drive (ie,> 200 Mbar ablation pressures). To test these predictions, LLE is building the first UV long-pulse prototype beamline to have that amount of bandwidth …

Broadband lasers have the potential to mitigate cross-beam energy transfer (CBET) in direct-drive inertial confinement fusion (ICF) experiments. A quantitative assessment of the bandwidth required for CBET mitigation necessitates the development of broadband ray-based CBET models that can be implemented in the radiation-hydrodynamic codes that are used to design ICF experiments. Two different approaches to broadband ray-based CBET modeling (discrete and fixed spectrum) are developed and compared to wave-based calculations. Both approaches give good agreement with wave-based calculations in ICF-relevant configurations. Full-scale 3D calculations show that the bandwidth required for adequate CBET mitigation increases with increasing scale and drive intensity.

Target preheat by superthermal electrons generated by stimulated Raman scattering and two-plasmon-decay instabilities is a potential concern for direct-drive inertial confinement fusion. The development of broadband laser technology can help suppress deleterious laser-plasma instabilities (LPI's). An experimental platform is being developed at the Omega Laser Facility to study LPI mitigation using the FLUX laser, which will be available at the beginning of 2024 and have a frequency bandwidth of~ 1.5%. FLUX beam interaction with a preformed coronal plasma, generated by nine OMEGA beams incident onto a planar or open-cone target, will be studied experimentally. Two-dimensional hydrodynamic DRACO simulations predict that coronal plasmas with a density scale length at the n c/4 surface, ranging from 150 to 400+/-5m, will be available. We present DRACO simulations and the first experiments to utilize a …

The influences of ion trapping and fluctuations of electron temperature and plasma flow on cross-beam energy transfer (CBET) are examined using two-and three-dimensional particle-in-cell simulations in parameter regimes relevant to recent CBET experiments at the OMEGA laser facility. In mid-Z plasma irradiated by an intense pump beam and weaker probe beam, ion trapping, collisional de-trapping, and plasma flow induced by thermal effects are shown to affect the CBET gain. Ion trapping can enhance or detune the CBET resonance [Nguyen et al., Phys. Plasmas 28, 082705 (2021)]. Collisional de-trapping can affect the CBET gain at low seed beam intensity near the onset threshold for ion trapping. Thermal-effects-induced flow can also detune the CBET resonance at a level comparable to that from trapping at low seed beam intensity. As a consequence, the CBET gain is sensitive to collisions and …

The primary method of diagnosing the absorption of laser light in a direct-drive implosion is the measurement of the unabsorbed light scattered from the implosion. Beam energy balance, beam mis-pointing, and polarization dependent 3-D cross-beam energy transfer (CBET) effects due to OMEGA's polarization smoothing method, all contribute to making the absorption and scattered light distributions very nonuniform over the target. A single scattered light measurement extrapolated over the entire 4π distribution can give over a 10% variation in inferred total absorption depending on the sampling location. The average of several measurements from different locations around the distribution must be used to have confidence in the measurement. On OMEGA implosions, this can be difficult to achieve due to the limited number of calibrated diagnostics, covering less than 0.5% of the distribution at most, and the lack of …

We report on the advances made under the purview of DOE award number DE-SC0019135 that was active during the period: 09/01/2018–08/31/2021. The grant investigated the application of a “flying focus” to the problem of photon acceleration—in which a dynamic refractive index gradient is used to continuously upshift a probe beam’s frequency. Codes were written to describe the creation of ionization waves of arbitrary velocity (IWAVs) for use as a photon-accelerating medium, as well as the behavior of a witness pulse residing in said medium. Experiments first verified the spatiotemporal control over laser intensity provided by a chromatic flying focus, then used that ability to produce small-diameter IWAVs in the far-field with the expected dynamics, and finally demonstrated even further flexibility by producing large-diameter IWAVs in the laser quasi-far-field that maintained the beneficial dynamics. Multiple innovative diagnostics—spectrally resolved Schlieren and spectrally resolved interferometry—were pioneered in order to diagnose the IWAVs. For IWAV production in the laboratory, however, beam quality was identified as a key limitation in the quasi-far-field. Since the original chromatic flying focus was found to result in relatively long (ps duration) intensity peaks, which could limit some applications including photon acceleration, additional techniques were invented to provide similar spatiotemporal control while also retaining ultrashort intensity peaks. While simulations have identified several interesting regimes for photon acceleration—first predicting the upshift of a counterpropagating witness pulse from the optical to the extreme …

The compression of direct-drive inertial confinement fusion (ICF) targets is strongly impacted by cross-beam energy transfer (CBET), a laser-plasma instability that limits ablation pressure by redirecting laser energy outward and that is projected to be mitigated by laser bandwidth. Here, we explore various CBET mitigation constraints to guide the design of future ICF facilities. First, we find that the flat, Gaussian, and Lorentzian spectral shapes have similar CBET mitigation properties, and a flat shape with nine spectral lines is a good surrogate for what can be obtained with other spectral shapes. Then, we conduct a comprehensive study across energy scales and ignition designs. 3D hydrodynamic simulations are used to derive an analytical model for the expected CBET mitigation as a function of laser and plasma parameters. From this model, we study the bandwidth requirements of conventional and shock ignition …

Kinetic inflation exacerbates the threat of the stimulated Raman scattering instability (SRS) to inertial confinement fusion. Continued growth of the instability requires phase matching between the incident light wave and the decay products, a scattered light wave and an electron plasma wave (EPW). In principle, a density inhomogeneity can disrupt the phase matching by changing the frequency of the EPW along the gradient. In reality, electron trapping in the EPW produces a frequency shift that can compensate this change. This autoresonance, or kinetic inflation, can substantially enhance the SRS reflectivity. Here we demonstrate that large laser bandwidth and small laser speckles can mitigate inflationary SRS and limit the reflectivity to non-inflationary levels by restricting the growth of SRS EPW. While the instantaneous reflectivity depends on the local chirp and local intensity of the incident light, the inflationary …

In the filamentation instability, the ponderomotive and thermal ejection of electrons from the high-intensity regions of a laser beam within a plasma create modulations in the plasma density and refractive index, which lead to self-focusing and filamentation of the beam. We present an experiment that utilizes the joint operation of the OMEGA 60 and OMEGA EP laser systems at the University of Rochester's Laboratory for Laser Energetics to investigate the growth rate of the filamentation instability. In our experiment, a 1ω short-pulse (1--100-ps) laser beam from OMEGA EP is coupled into a preheated plasma on the OMEGA 60 laser-plasma interaction platform. The resulting beam spray of the filamented short-pulse beam is recorded as a time-integrated 2-D image while plasma parameters are determined via Thomson scattering. By modifying the incident short-pulse beam pulse duration, we can limit the growth of the …

In indirect-drive implosions, the final core hot spot energy and pressure and, hence, neutron yield attainable in 1D increase with increasing laser peak power and, hence, radiation drive temperature at the fixed capsule and Hohlraum size. We present simple analytic scalings validated by 1D simulations that quantify the improvement in performance and use this to explain existing data and simulation trends. Extrapolating to the 500 TW National Ignition Facility peak power limit in a low gas-fill 5.4 mm diameter Hohlraum based on existing high adiabat implosion data at 400 TW, 1.3 MJ and 1× 10 16 yield, we find that a 2–3× 10 17 yield (0.5–0.7 MJ) is plausible using only 1.8 MJ of laser energy. Based on existing data varying deuterium–tritium (DT) fuel thickness and dopant areal density, further improvements should be possible by increasing DT fuel areal density, and hence confinement time and yield amplification.

• Hydrodynamic scaling underpins the extrapolation of direct-drive implosion performance from OMEGA to NIF, but not all aspects of physics scale (eg hot electron preheat)

In cross-beam energy transfer (CBET), the dynamic interference of two laser beams ponderomotively drives an ion-acoustic wave that coherently scatters light from one beam into the other. This redirection of laser energy can severely inhibit the performance of direct-drive inertial confinement fusion. Recent experiments fielded on the OMEGA laser demonstrated that CBET can saturate through a resonance detuning resulting from ion heating [1, 2]. The experiments employed one set of OMEGA laser beams to heat a gas jet plasma and a second set as pump beams for transferring energy to a frequency-detuned seed beam. In contrast, CBET in OMEGA implosions occurs between frequency-degenerate beams in a hotter, denser, inhomogeneous flowing plasma. To determine how CBET saturates in these conditions, we have conducted a series of vector particle-in-cell (VPIC) simulations using plasma profiles from a …