Modifying the national ignition facility to exawatt short pulses with existing technology

An exawatt-scale, short-pulse
amplification architecture based upon a novel pulse compressor arrangement
and amplification of long-duration chirped beam pulses is described. This architecture is capable of extracting the full, stored energy of a NIF or NIF-like beam line and in doing so produce from one beam line a near-diffraction-limited, laser pulse whose peak power would exceed 0.2 EW. The architecture is well suited to either low-f-number focusing or to multi-beam, dipole focusing concepts that in principle enable focused intensities in the range of 10^26 W/cm2 or 5 orders of magnitude beyond that possible from present PW systems based on chirped pulse amplification.

The “NIF exawatt” or “Nexawatt” architecture requires only existing fabrication technologies and optics operated below established damage limits. This architecture is fully compatible with high-efficiency diode-pumped laser architectures that have been developed for laser inertial fusion power plant concepts. Sub-scale versions these fusion energy laser concepts are currently being constructed
and demonstrated at the 100’s of J scale. Such lasers will ultimately produce from a single amplifier, diffraction-limited, over 10 kJ, less than 100-fs pulses at repetition rates of over 10 Hz and with wall plug efficiencies of over 20%. As such, the “Nexawatt” short pulse amplification architecture presents new opportunities for practical extension of laser-wakefield field accelerator schemes to well beyond 100 GeV in a single stage at high overall electrical efficiency.

Modern, inertial confinement fusion lasers based on Nd:glass have amplification bandwidths that are capable of supporting sub-picosecond pulses. With the implementation of chirped pulse amplification (CPA), it is possible for beam lines at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), the Laser Mega-Joule (LMJ) facility in Bordeaux, France, the LFEX laser at the Institute for Laser Engineering in Osaka, Japan and the Omega EP facility at the Laboratory for Laser Energetics in Rochester, New York to create petawatt peak power laser pulses of nominally 1-kJ energy. While these system are at the forefront of present high-energy, high-peak power capabilities, they utilize only a small fraction (typically less than 10%) of the potential stored energy of the underlying Nd:glass laser amplification system. A single beam-line at the NIF, for example, has a stored energy in excess of 25 kJ. Scaling of existing PW systems to higher peak power and high energy per beam is limited by intensity-dependent damage of final optics, lack of sufficient pre-amplifier bandwidth to support shorter duration pulses and insufficient stretched pulse durations required to avoid the onset of b-integral-dependent, non-linear damage of the main amplifiers and transport optics.

The NIF exawatt or “Nexawatt” concept circumvents traditional CPA limitations via a combination of chirped pulse and chirped beam amplification, a novel pulse compressor arrangement capable of compressing 20-ns duration pulses and an increase in the final beam area via splitting beams before final compression. When combined with wide-bandwidth pre-amplifiers these techniques can in principle produce near-diffraction limited pulses with peak powers in excess of 200 PW and pulse durations of order 100 fs from a single amplification beam-line.

The Nexawatt concept enables full extraction of the stored energy from large-scale, Nd:glass laser amplifiers and the generation of exawatt-scale, 100-ps pulses. It increases short pulse amplification efficiency from <10% to nearly 100%. The concept is based on demonstrated designs and damage thresholds for final optics, is compatible with existing tools for fabrication of compressor gratings and requires only two, 2-meter compressor gratings. The concept operates within the established pulsewidth dependent amplification capabilities of NIF and NIF-like amplifiers and unlike other exawatt concepts that aim to combine individually amplified beams, the Nexawatt concept phases “identical” beams after amplification and is compatible with existing beam phasing technologies. The multibeamlet final output of the Nexawatt is compatible with dipole focusing and theoretically could enable extension of ultrahigh intensity laser science by over 5 orders of magnitude.


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