G. Tietbohl

Petawatt laser pulses

We have developed a hybrid Ti:sapphire-Nd:glass laser system that produces more than 1500 TW (1.5 PW) of peak power. The system produces 660 J of power in a compressed 440+/-20 fs pulse by use of 94-cm master diffraction gratings. Focusing to an irradiance of >7x10(20) W/cm (2) is achieved by use of a Cassegrainian focusing system employing a plasma mirror.

25 ps neutron detector for measuring ICF‐target burn history

We have developed a fast, sensitive neutron detector for recording the fusion reaction‐rate history of inertial‐confinement fusion (ICF) experiments. The detector is based on the fast rise time of a commercial plastic scintillator (BC‐422) and has a response <25 ps FWHM. A thin piece of scintillator material acts as a neutron‐to‐light converter. A zoom lens images scintillator light to a high‐speed (15 ps) optical streak camera for recording. A retractable nose cone positions the scintillator between 1 and 50 cm from a target. A simultaneously recorded optical fiducial pulse allows the streak camera time base to be calibrated relative to the incident laser power. Burn histories have been measured for deuterium‐tritium filled targets with yields ranging between 108 and 2×1013 neutrons.

An overview of LLNL high-energy short-pulse technology for advanced radiography of laser fusion experiments

The technical challenges and motivations for high-energy, short-pulse generation with the National Ignition Facility (NIF) and possibly other large-scale Nd : glass lasers are reviewed. High-energy short-pulse generation (multi-kilojoule, picosecond pulses) will be possible via the adaptation of chirped pulse amplification laser techniques on NIF. Development of metre-scale, high-efficiency, high-damage-threshold final optics is a key technical challenge. In addition, deployment of high energy petawatt (HEPW) pulses on NIF is constrained by existing laser infrastructure and requires new, compact compressor designs and short-pulse, fibre-based, seed-laser systems. The key motivations for HEPW pulses on NIF is briefly outlined and includes high-energy, x-ray radiography, proton beam radiography, proton isochoric heating and tests of the fast ignitor concept for inertial confinement fusion.

125-TW Ti:sapphire/Nd:glass laser system

We have demonstrated a Ti:sapphire/Nd:glass laser system that produces up to 51 J of energy in 395-fs pulses (125TW). Focusing at f/3 to a 2.5-times diffraction-limited spot results in a peak irradiance greater than 10(20) W/cm(2) . Our 40-cm-diameter gold diffraction gratings have a damage threshold of 0.42 J/cm(2) for 320-fs pulses.

Petawatt laser system

We recently demonstrated the production of over a petawatt of peak power in the Nova/Petawatt Laser Facility, generating > 600 J in approximately 440 fs. The Petawatt Laser Project was initiated to develop the capability to test the fast ignitor concept for inertial confinement fusion, and to provide a unique capability in high energy density physics. The laser was designed to produce near kJ pulses with a pulse duration adjustable between 0.5 and 20 ps. At the shortest pulse lengths, this laser is expected to surpass 1021 W/cm2 when focused later this year. Currently, this system is limited to 600 J pulses in a 46.3- cm beam. Expansion of the beam to 58 cm, with the installation of 94-cm gratings, will enable 1 kJ operation. Target experiments with petawatt pulses will be possible either integrated with Nova in the 10 beam target chamber or as a stand alone system in an independent, dedicated chamber. Focusing the beam onto a target will be accomplished using an on axis parabolic mirror. The design of a novel targeting system enabling the production of ultrahigh contrast pulses and an easily variable effective focal length is also described.

Diagnostic systems for the National Ignition Facility (NIF) (invited)

A tentative schedule of experiments for the ignition campaign on the National Ignition Facility (NIF) has been developed. These experiments will be used to validate beam pointing and balance, to tune time history and symmetry of drive of NIF hohlraums, and to implode subignition and igniting targets. The initial target diagnostics are designed to validate beam pointing and to demonstrate the properties of the hohlraums.

The commissioning of the advanced radiographic capability laser system: experimental and modeling results at the main laser output

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the first of a kind megajoule-class laser with 192 beams capable of delivering over 1.8 MJ and 500TW of 351nm light [1], [2]. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In order to advance our understanding, and enable short-pulse multi-frame radiographic experiments of dense cores of cold material, the generation of very hard x-rays above 50 keV is necessary. X-rays with such characteristics can be efficiently generated with high intensity laser pulses above 1017 W/cm² [3]. The Advanced Radiographic Capability (ARC) [4] which is currently being commissioned on the NIF will provide eight, 1 ps to 50 ps, adjustable pulses with up to 1.7 kJ each to create x-ray point sources enabling dynamic, multi-frame x-ray backlighting. This paper will provide an overview of the ARC system and report on the laser performance tests conducted with a stretched-pulse up to the main laser output and their comparison with the results of our laser propagation codes.

Neutron detector for fusion reaction-rate measurements

We have developed a fast, sensitive neutron detector for recording the fusion reaction-rate history of inertial-confinement fusion (ICF) experiments. The detector is based on the fast rise-time of a commercial plastic scintillator (BC-422) and has a response < 25 ps FWHM. A thin piece of scintillator material acts as a neutron-to-light converter. A zoom lens images light from the scintillator surface to a high-speed (15 ps) optical streak camera for recording. The zoom lens allows the scintillator to be positioned between 1 and 50 cm from a target. The camera simultaneously records an optical fiducial pulse which allows the camera time base to be calibrated relative to the incident laser power. Bursts of x rays formed by focusing 20 ps, 2.5 TW laser pulses onto gold disk targets demonstrate the detector resolution to be < 25 ps. We have recorded burn histories for deuterium/tritium-filled targets producing as few as 3 X 107 neutrons.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Production of Petawatt Laser Pulses

Chirped-pulse amplification applied to broad-bandwidth solid-state lasers has created a revolution in the production and use of terawatt and now petawatt class lasers.1,2 The concepts and technology contributing to this revolution have evolved continuously since the early 1970’s. Following the grating compressor work of Treacy3, Bischell4 and others described the application of chirped-pulse amplification to Nd:Glass lasers. This was followed by a large amount of work on fiber-grating pulse compression for communication research.5 In 1985, Strickland and Mourou combined many of these ideas into the first practical demonstration of chirped-pulse amplification with a solid-state laser.6 Following this initial demonstration, rapid developments in technology such as the stretcher design of Martinez7 led to small scale systems capable of terawatt8 and multiterawatt pulses.9–11 Occurring in parallel with the development of chirped-pulse amplification technology using Nd:Glass lasers, was the development of the new laser material, titanium-doped sapphire. The commercial availability of this unique laser material dramatically propelled the revolution in CPA based solid-state lasers. An overwhelming majority of CPA lasers now employ Ti:sapphire either throughout the entire laser system or at least as the oscillator material.13 These early developments and the large amount of effort that has gone into the laser technology in recent years have culminated in high pulse energy systems producing pulses with a peak power of 125 TW14 and very short-pulse systems producing multiterawatt pulses which only contain a few optical cycles.15–18 Here, we describe the limits of CPA technology in the context of a large scale system producing pulses with a peak power exceeding 1.25 petawatts (1250 TW).

Production of high-intensity laser pulses with adaptive optic wavefront correction

We have developed a Ti:sapphire/Nd:glass laser system which produces > 1.25 PW peak power. An irradiance of 1020 - 1021 W/cm2 is achieved utilizing an on-axis parabolic mirror, with adaptive optic wavefront correction. Experimental results will be described.