Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment

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)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. 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.

Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency

Optical parametric chirped-pulse amplification (OPCPA) using high-energy Nd:glass lasers has the potential to produce ultra-intense pulses (>1023 W/cm2). We report on the performance of the final high-efficiency amplifier in an OPCPA system based on large-aperture (63 × 63-mm2) partially deuterated potassium dihydrogen phosphate (DKDP) crystals. The seed beam (180-nm bandwidth, 110 mJ) was provided by the preceding OPCPA stages. A maximum pump-to-signal conversion efficiency of 41% and signal energy up to 13 J were achieved with a 52-mm-long DKDP crystal due to the flattop super-Gaussian pump beam profile and flat-in-time pulse shape.

Design and characterization of "flow-cell" integrated-flow active cooling for high-average-power ceramic optics

We used COMSOL Multiphysics to design a prototype actively cooled "flow-cell" substrate targeted at high-average-power applications, fabricated the prototype from cordierite ceramic, and tested the substrate under load in our thermal loading test stand. Sub-aperture testing revealed average-power handling up to 3.88-W/cm² absorbed power density, in excellent agreement with model predictions. Gratings fabricated on 2-in.-diam cordierite coupons were subjected to laser-damage testing and showed a damage threshold of 250 mJ/cm².

Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion.

Advanced laser development and plasma-physics studies on the multiterawatt laser

The multiterawatt (MTW) laser, built initially as the prototype front end for a petawatt laser system, is a 1053 nm hybrid system with gain from optical parametric chirped-pulse amplification (OPCPA) and Nd:glass. Compressors and target chambers were added, making MTW a complete laser facility (output energy up to 120 J, pulse duration from 20 fs to 2.8 ns) for studying high-energy-density physics and developing short-pulse laser technologies and target diagnostics. Further extensions of the laser support ultrahigh-intensity laser development of an all-OPCPA system and a Raman plasma amplifier. A short summary of the variety of scientific experiments conducted on MTW is also presented.

Direct-drive laser fusion: status, plans and future

Laser-direct drive (LDD), along with laser indirect (X-ray) drive (LID) and magnetic drive with pulsed power, is one of the three viable inertial confinement fusion approaches to achieving fusion ignition and gain in the laboratory. The LDD programme is primarily being executed at both the Omega Laser Facility at the Laboratory for Laser Energetics and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. LDD research at Omega includes cryogenic implosions, fundamental physics including material properties, hydrodynamics and laser-plasma interaction physics. LDD research on the NIF is focused on energy coupling and laser-plasma interactions physics at ignition-scale plasmas. Limited implosions on the NIF in the 'polar-drive' configuration, where the irradiation geometry is configured for LID, are also a feature of LDD research. The ability to conduct research over a large range of energy, power and scale size using both Omega and the NIF is a major positive aspect of LDD research that reduces the risk in scaling from OMEGA to megajoule-class lasers. The paper will summarize the present status of LDD research and plans for the future with the goal of ultimately achieving a burning plasma in the laboratory. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

High-efficiency, fifth-harmonic generation of a joule-level neodymium laser in a large-aperture ammonium dihydrogen phosphate crystal

High-energy deep ultraviolet (UV) sources are required for high-density plasma diagnostics. The fifth-harmonic generation of large-aperture neodymium lasers in ammonium dihydrogen phosphate (ADP) can significantly increase UV energies due to the availability of large ADP crystals. Noncritical phase matching in ADP for (ω + 4ω) was achieved by cooling a 65 × 65-mm crystal in a two-chamber cryostat to 200 K. The crystal chamber used helium as the thermally conductive medium between the crystal and the crystal chamber, which was surrounded by a high-vacuum chamber with a liquid nitrogen reservoir. A temperature variation of 0.2 K across the crystal aperture was obtained. The total conversion efficiency from the fundamental to the fifth harmonic at 211 nm was 26%.

High-energy parametric amplification of spectrally incoherent broadband pulses

We study and demonstrate the efficient parametric amplification of spectrally incoherent broadband nanosecond pulses to high energies. Signals composed of mutually incoherent monochromatic lines or amplified spontaneous emission are amplified in a sequence of optical parametric amplifiers pumped at 526.5 nm, with the last amplifier set in a collinear geometry. This configuration results in 70% conversion efficiency from the pump to the combined signal and idler, with a combined energy reaching 400 mJ and an optical spectrum extending over 60 nm around 1053 nm. The spatial, spectral, and temporal properties of the amplified waves are investigated. The demonstrated high conversion efficiency, spectral incoherence, and large bandwidth open the way to a new generation of high-energy, solid-state laser drivers that mitigate laser-plasma instabilities and laser-beam imprint via enhanced spectral bandwidth.

The Dynamic Compression Sector laser: A 100-J UV laser for dynamic compression research

The Dynamic Compression Sector (DCS) laser is a 100-J ultraviolet Nd:glass system designed and built by the Laboratory for Laser Energetics for experimental research at the DCS located at the Advanced Photon Source (Argonne National Laboratory). Its purpose is to serve as a shock driver to study materials under extreme dynamic pressures. It was designed to deposit energy within a uniformly illuminated 500-μm spot on target, with additional optics provided to implement spot sizes of 250 and 1000 μm. Designed after larger-scale glass lasers such as OMEGA and the National Ignition Facility, the laser consists of a fiber front end with interferometer-based pulse shaping, a Nd:glass regenerative amplifier, a four-pass rod amplifier, and a 15-cm glass disk amplifier, through which six passes are made in a bowtie geometry. The output is frequency tripled from 1053 to 351 nm by using a pair of type-II phase-matched KDP crystals, with a third to increase conversion bandwidth. The super-Gaussian spot in the far field is achieved with a distributed phase plate and a 1-m aspherical focusing lens. Beam smoothing is achieved by smoothing by spectral dispersion and polarization smoothing, resulting in a root-mean-square variation in intensity on target of ±8.7%.

Tripled yield in direct-drive laser fusion through statistical modelling

Focusing laser light onto a very small target can produce the conditions for laboratory-scale nuclear fusion of hydrogen isotopes. The lack of accurate predictive models, which are essential for the design of high-performance laser-fusion experiments, is a major obstacle to achieving thermonuclear ignition. Here we report a statistical approach that was used to design and quantitatively predict the results of implosions of solid deuterium-tritium targets carried out with the 30-kilojoule OMEGA laser system, leading to tripling of the fusion yield to its highest value so far for direct-drive laser fusion. When scaled to the laser energies of the National Ignition Facility (1.9 megajoules), these targets are predicted to produce a fusion energy output of about 500 kilojoules-several times larger than the fusion yields currently achieved at that facility. This approach could guide the exploration of the vast parameter space of thermonuclear ignition conditions and enhance our understanding of laser-fusion physics.

Chirped-pulse-amplification seed source through direct phase modulation

This work presents integration of a directly chirped laser source (DCLS) into a high-energy optical parametric chirped-pulse-amplification (OPCPA) system. DCLS is an all-fiber, chirped laser source that produces nanosecond, linearly chirped laser pulses at 1053 nm for seeding high-energy chirped-pulse-amplification systems. DCLS produces a frequency chirp on an optical pulse through direct temporal phase modulation. A 1-ns, linearly chirped pulse with a 3-nm bandwidth is produced by applying an ~1000-rad (300π) quadratic temporal phase. The chirped pulse is amplified to 76 mJ in an OPCPA system and compressed to close to its Fourier transform limit, producing an intensity autocorrelation trace with a 1.5-ps width.

Record fifth-harmonic-generation efficiency producing 211 nm, joule-level pulses using cesium lithium borate

The fifth harmonic of a pulsed Nd:YLF laser has been realized in a cascade of nonlinear crystals with a record efficiency of 30%. Cesium lithium borate is used in a Type-I configuration for sum-frequency mixing of 1053 and 266 nm, producing 211 nm pulses. Flat-topped beam profiles and pulse shapes optimize efficiency. The energies of the fifth harmonic up to 335 mJ in 2.4 ns pulses were demonstrated.

Thermal effects in an ultrafast BiB3O6 optical parametric oscillator at high average powers

An ultrafast, high-average-power, extended-cavity, femtosecond BiB₃O₆ optical parametric oscillator was constructed as a test bed for investigating the scalability of infrared parametric devices. Despite the high pulse energies achieved by this system, the reduction in slope efficiency near the maximum-available pump power prompted the investigation of thermal effects in the crystal during operation. The local heating effects in the crystal were used to determine the impact on both phase matching and thermal lensing to understand limitations that must be overcome to achieve microjoule-level pulse energies at high repetition rates.

High-average-power, 2-μm femtosecond optical parametric oscillator synchronously pumped by a thin-disk, mode-locked laser

A high-energy, extended-cavity femtosecond BiBO optical parametric oscillator synchronously pumped by a 1.0-ps, 1030-nm Yb:YAG, thin-disk pump laser is presented. The oscillator operated near degeneracy in a noncollinear interaction geometry, producing signal wavelength tunability from 1.99 to 2.20 μm. The signal pulses have an average power exceeding 2 W, producing 455-fs pulses at 7.08 MHz with energies up to 350 nJ, showing increased potential for tunable sources of scalable ultrafast pulses in the infrared.

Demonstration of Fuel Hot-Spot Pressure in Excess of 50 Gbar for Direct-Drive, Layered Deuterium-Tritium Implosions on OMEGA

A record fuel hot-spot pressure P_{hs}=56±7  Gbar was inferred from x-ray and nuclear diagnostics for direct-drive inertial confinement fusion cryogenic, layered deuterium-tritium implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System. When hydrodynamically scaled to the energy of the National Ignition Facility, these implosions achieved a Lawson parameter ∼60% of the value required for ignition [A. Bose et al., Phys. Rev. E 93, 011201(R) (2016)], similar to indirect-drive implosions [R. Betti et al., Phys. Rev. Lett. 114, 255003 (2015)], and nearly half of the direct-drive ignition-threshold pressure. Relative to symmetric, one-dimensional simulations, the inferred hot-spot pressure is approximately 40% lower. Three-dimensional simulations suggest that low-mode distortion of the hot spot seeded by laser-drive nonuniformity and target-positioning error reduces target performance.

Self-phase modulation compensation in a regenerative amplifier using cascaded second-order nonlinearities

Self-phase modulation limits the amplification of short optical pulses because of spatial self-focusing and spectral broadening. Cascaded nonlinearities are theoretically and experimentally investigated for intracavity nonlinearity compensation in a neodymium-doped yttrium-lithium-fluoride (Nd:YLF) regenerative amplifier. Experimental results are in good agreement with simulations. Spectral broadening is significantly reduced, allowing for efficient amplification in a Nd:YLF power amplifier.

Time-resolved measurements of hot-electron equilibration dynamics in high-intensity laser interactions with thin-foil solid targets

Time-resolved K(α) spectroscopy has been used to infer the hot-electron equilibration dynamics in high-intensity laser interactions with picosecond pulses and thin-foil solid targets. The measured K(α)-emission pulse width increases from ~3 to 6 ps for laser intensities from ~10(18) to 10(19) W/cm(2). Collisional energy-transfer model calculations suggest that hot electrons with mean energies from ~0.8 to 2 MeV are contained inside the target. The inferred mean hot-electron energies are broadly consistent with ponderomotive scaling over the relevant intensity range.

Analysis and suppression of parasitic processes in noncollinear optical parametric amplifiers

The influence of parasitic processes on the performance of ultra-broadband noncollinear optical parametric amplifiers (NOPA's) is investigated for walk-off and non-walk-off compensating configurations. Experimental results with a white-light-seeded NOPA agree well with numerical simulations. The same model shows that 10% of the output energy of an amplified signal can be transferred into a parasitic second harmonic of the signal. These findings are supported by quantitative measurements on a few-cycle NOPA, where a few percent of the signal energy is converted to its second harmonic in the walk-off compensating case. This effect is reduced by an order of magnitude in the non-walk-off compensating configuration. A detailed study of the phase-matching conditions of the most common nonlinear crystals provides guidelines for designing NOPA systems.

Amplifying nanosecond optical pulses at 1053 nm with an all-fiber regenerative amplifier

Nanosecond, 1053 nm optical pulses are amplified from 15 pJ to 240 nJ by a Yb-doped all-fiber regenerative amplifier (AFRA), achieving an overall gain of 42 dB. To the best of our knowledge, this is the highest output-pulse energy from an AFRA ever reported. Techniques for suppressing amplified spontaneous emission are employed to favor the signal gain 23 nm off the gain peak of the Yb-doped fiber. Numerical simulations show that increasing the number of round trips and operating the AFRA at saturation will increase the output level and improve the output stability. This is currently limited by the onset of bifurcation instability, which can be avoided at low repetition rates.

Scaling hot-electron generation to high-power, kilojoule-class laser-solid interactions

Thin-foil targets were irradiated with high-power (1 ≤ P(L) ≤ 210 TW), 10-ps pulses focused to intensities of I>10(18) W/cm(2) and studied with K-photon spectroscopy. Comparing the energy emitted in K photons to target-heating calculations shows a laser-energy-coupling efficiency to hot electrons of η(L-e) = 20 ± 10%. Time-resolved x-ray emission measurements suggest that laser energy is coupled to hot electrons over the entire duration of the incident laser drive. Comparison of the K-photon emission data to previous data at similar laser intensities shows that η(L-e) is independent of laser-pulse duration from 1 ≤ τ(p) ≤ 10 ps.

All-fiber optical isolator based on Faraday rotation in highly terbium-doped fiber

An all-fiber isolator with 17 dB optical isolation is demonstrated. The fiber Faraday rotator uses 56 wt. % terbium (Tb)-doped silicate fiber, and the fiber polarizers are Corning SP1060 single-polarization fiber. The effective Verdet constant of the Tb-doped fiber is measured to be -24.5+/-1.0 rad/(Tm) at 1053 nm, which is 20 times larger than silica fiber and 22% larger than previously reported results.

Effective Verdet constant in a terbium-doped-core phosphate fiber

The concept of an effective Verdet constant is proposed and experimentally validated. The effective Verdet constant of light propagation in a fiber includes contributions from the materials in both the core and the cladding. It is measured in a 25 wt.% terbium-doped-core phosphate fiber to be -6.2+/-0.4 rad/(Tm) at 1,053 nm, which is six times larger than silica fiber. The result agrees well with Faraday rotation theory in optical fiber.

Bulk heating of solid-density plasmas during high-intensity-laser plasma interactions

K -shell x-ray spectroscopy is used to study the interaction of small-mass copper foil targets (>20x20x2microm;{3}) with a high-intensity (>10;{19}Wcm;{2}) laser pulse. Efficient bulk heating to greater than 200eV is demonstrated using collisional-energy transfer from recirculating fast electrons. K -photon yields and bulk-electron temperatures calculated by three-dimensional numerical target-heating simulations are in good agreement with the experimental measurements.

A focal-spot diagnostic for on-shot characterization of high-energy petawatt lasers

An on-shot focal-spot diagnostic for characterizing high-energy, petawatt-class laser systems is presented. Accurate measurements at full energy are demonstrated using high-resolution wavefront sensing in combination with techniques to calibrate on-shot measurements with low-power sample beams. Results are shown for full-energy activation shots of the OMEGA EP Laser System.

A high-resolution coherent transition radiation diagnostic for laser-produced electron transport studies (invited)

High-resolution images of the rear-surface optical emission from high-intensity (I approximately 10(19) W/cm(2)) laser illuminated metal foils have been recorded using coherent transition radiation (CTR). CTR is generated as relativistic electrons, generated in high-intensity laser-plasma interactions, exit the target's rear surface and move into vacuum. A transition radiation diagnostic (TRD) records time-integrated images in a 24 nm bandwidth window around lambda=529 nm. The optical transmission at lambda=1053 nm, the laser wavelength, is 15 orders of magnitude lower than the transmission at the wavelength of interest, lambda=527 nm. The detector is a scientific grade charge-coupled device (CCD) camera that operates with a signal-to-noise ratio of 10(3) and has a dynamic range of 10(4). The TRD has demonstrated a spatial resolution of 1.4 microm over a 1 mm field of view, limited only by the CCD pixel size.

High-dynamic-range single-shot cross-correlator based on an optical pulse replicator

The operation of a single-shot cross-correlator based on a pulse replicator is described. The correlator uses a discrete sequence of sampling pulses that are nonlinearly mixed with the pulse under test. The combination of a high reflector and partial reflector replicates an optical pulse by multiple internal reflections and generates a sequence of spatially displaced and temporally delayed sampling pulses. This principle is used in a cross-correlator characterizing optical pulses at 1053 nm. A dynamic range higher than 60 dB is obtained over a temporal range larger than 200 ps.

Optical parametric chirped-pulse-amplification contrast enhancement by regenerative pump spectral filtering

We demonstrate an approach to fundamentally improve the contrast of optical parametric chirped-pulse amplifiers (OPCPA). The instantaneous parametric gain couples the temporal variations of the pump-pulse intensity to spectral variations of the intensity of the stretched signal pulse being amplified, which significantly degrade the temporal contrast of the amplified pulse after recompression. Simple and efficient pump-intensity noise reduction in an OPCPA system using a volume Bragg grating in a regenerative amplifier demonstrates contrast improvements up to 30 dB.

High-contrast optical-parametric amplifier as a front end of high-power laser systems

A high-contrast preamplifier based on optical-parametric amplification with a short pump pulse is demonstrated. A gain larger than 10(5) and measurement-limited contrast higher than 10(11) are obtained over a large temporal range extending within less than 10 ps of the peak of the pulse, because of the high instantaneous parametric gain provided by a short pump pulse in a nonlinear crystal. The energy gain and high contrast of this preamplifier make it a good seed source for high-power laser systems.

Optical testing using the transport-of-intensity equation

The transport-of-intensity equation links the intensity and phase of an optical source to the longitudinal variation of its intensity in the presence of Fresnel diffraction. This equation can be used to provide a simple, accurate spatial-phase measurement for optical testing of flat surfaces. The properties of this approach are derived. The experimental demonstration is performed by quantifying the surface variations induced by the magnetorheological finishing process on laser rods.

High-dynamic-range temporal measurements ofshort pulses amplified by OPCPA

We report on the first experimental measurement of high-dynamic-range pulse contrast of compressed optical parametric chirped-pulse-amplification (OPCPA) pulses on the picosecond scale. The measured -80-dB OPCPA contrast at 1054 nm agrees well with theoretical predictions and exceeds the estimated and measured levels for comparable amplification in a Ti:sapphire regenerative amplifier by approximately 10 dB. A key to achieving better contrast with OPCPA is the simpler experimental setup that promotes more-efficient coupling of seed pulse energy into the amplification system.

Highly sensitive, single-shot characterization for pulse widths from 0.4 to 85 ps using electro-optic shearing interferometry

Single-shot characterization using electro-optic shearing interferometry (EOSI) is shown for pulse widths ranging from their transform limit (0.4 ps) to 200x their limit (85 ps). In EOSI, the spectral phase is reconstructed by interfering two spectrally sheared replicas of the pulse under test, where the shear is produced by applying linear temporal-phase modulation. We present a new reconstruction algorithm for accurately characterizing chirped pulses, even if the pulse extends beyond the linear region of the phase modulation. Furthermore, since EOSI does not rely on nonlinear optical processes, it requires only 1 nJ pulse energies for all pulse widths, corresponding to a single-shot sensitivity 1000x higher than previously demonstrated.

High-gain, polarization-preserving, Yb-doped fiber amplifier for low-duty-cycle pulse amplification

Amplified spontaneous emission (ASE) suppression techniques were utilized to fabricate a double-pass, Yb-doped amplifier with the noise properties of a single-pass amplifier. Simulations based on a rate equation model were used to analyze the ASE and the effectiveness of the suppression techniques. These techniques were implemented in an alignment-free, double-pass, Yb-doped fiber amplifier with a 26 dB gain at a wavelength 23 nm off the gain peak and a -48 dB noise floor while amplifying linearly polarized optical pulses with a low-duty cycle.

High-conversion-efficiency optical parametric chirped-pulse amplification system using spatiotemporally shaped pump pulses

High-conversion-efficiency, high-stability optical parametric chirped-pulse amplification is demonstrated with a spatiotemporally shaped pump laser system. Broadband 5-mJ pulses are produced at a 5-Hz repetition rate with a pump-to-signal conversion efficiency of 29% and energy stability better than 2% rms. To our knowledge this is the highest conversion efficiency and stability achieved in an optical parametric chirped-pulse amplification system.

Upconversion and reduced 4F(3/2) upper-state lifetime in intensely pumped Nd:YLF

Nonexponential decay of the 4F(3/2) upper laser state in Nd:YLF was observed with time-resolved fluorescence spectroscopy and small-signal gain probes in low-doped (1.16-at. %) samples when intensely pumped to high population inversions. A rapid initial decay is observed after Q-switched laser pumping, followed by a longer nonexponential decay that is clearly identified as a phase of migration-assisted energy-transfer upconversion (ETU) in the hopping regime. During this phase, a rate-equation treatment is valid and the macroscopic ETU coefficient, alpha2 = (0.98 +/- 0.03) x 10(-16) cm3/s, is directly evaluated from the decay kinetics. The ETU and the migration microparameters are also estimated to be C(da)* approximately 200 x 10(-40) cm6/s and C(dd) approximately 1000 x 10(-40) cm6/s, respectively, and are compared with theoretical values found in the literature. On the basis of these values, rate-equation treatments of intensely pumped Nd:YLF are not strictly valid.