July 07, 2011

Laser Particle Acceleration: Status and Perspectives for Nuclear Physics

Laser Particle Acceleration: Status and Perspectives for Nuclear Physics (12 pages)

High power short-pulse lasers with peak powers presently reaching Terawatts and even Petawatt levels routinely reach focal intensities of 10^18–10^21 W/cm2. These lasers are able to produce a variety of secondary radiation, from relativistic electrons and multi-MeV/nucleon ions to high energetic X-rays and gamma-rays. In many laboratories world-wide large resources are presently devoted to a rapid development of this novel tool of particle acceleration, targeting nuclear, fundamental and high-field physics studies as well as various applications e.g. in medical technology for diagnostics and tumor therapy. Within the next 5 years a new EU-funded large-scale research infrastructure (ELI: Extreme Light Infrastructure) will be constructed, with one of its four pillars exclusively devoted to nuclear physics based on high intensity lasers (ELI-Nuclear Physics, to be built in Magurele/Bucharest). There the limits of laser intensity will be pushed by three orders of magnitude to yet unprecedented 10^24 W/cm2.

The unprecedented density of laser-driven ion beams will allow for a novel reaction scheme that holds promise to generate much more neutron-rich isotopes than accessible with conventional techniques, especially targeting the region of the r-process waiting point at N = 126, where presently the last known isotope is 15 neutrons away from the r-process path. This region is crucial for understanding the nucleosynthesis of the heaviest elements. In the ‘fission–fusion’ reaction scenario we propose to produce exotic nuclides in this mass range by fissioning a dense laser-accelerated thorium ion bunch in a second thorium target, where the light fission fragments of the beam will fuse with the light fission fragments of the target.

Based on a laser energy of 300 J and a pulse width of 32 fs (as envisaged for the APOLLON laser planned for the ELi-Nuclear Physics project in Bucharest), an order-of-magnitude estimate gives a yield of neutron-rich fusion products in the mass range of A = 180–190 of about 10^3 per laser pulse in the scenario with collectively reduced electronic stopping. While presently the laser repetition rate is limited to about 1 pulse/min., extensive efforts are ongoing to reach about 100 Hz within the next few years.

A recent laser enabled megawatt class fusion propulsion system was recently proposed.

In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10^18 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

The focal intensities of 2 X 10^18 watts per square centimeter is not a problem.

There are lower power lasers that can be pulsed at 100 megahertz (100 million times per second.)

There was a proposal in 2007 for a 100-MHz multi-terawatt femtosecond Ti:sapphire laser with a regenerative amplifier

We suggest and demonstrate an approach to generate a femtosecond multi-terawatt pulse train at repetition rate of 100 MHz in a 10-Hz chirped pulse amplification Ti:sapphire laser system. With an electro-optic Q-switch regenerative amplifier, we can obtain an adjustable repetition-rate chirped pulse train. After amplification and compression, the 100-MHz pulse train with 46-fs pulse width and 1-TW peak power per pulse is obtained in our 10-TW-class Ti:sapphire laser system.

At the SPIE conference - ELI: Ultrarelativistic Laser-Matter Interactions and Petawatt Photonics there was a paper on Scaling the technology of the Texas petawatt laser to exawatt peak powers.

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