Powerful laser sheds light on fast ignition and high energy density physics

Nov 02, 2009
The OMEGA Extended Performance (EP) Laser Facility at the University of Rochester's Laboratory for Laser Energetics conducts ultrahigh-intensity laser-matter interactions research and integrated advanced-ignition experiments. Credit: P.M. Nilson, University of Rochester

A new generation of high-energy (>kJ) petawatt (HEPW) lasers is being constructed worldwide to study high intensity laser matter interactions, including fast ignition. Fast ignition is a laser-based technique for heating and igniting deuterium and tritium fuel to fusion temperatures in a two-step process. In the first phase, laser beams vaporize a fuel pellet and compress it to a thousand times its original density, while in the second phase, electrons accelerated by an intense-laser pulse deposit energy within the fuel assembly, causing rapid heating. This is akin to the way a gasoline engine works with a spark plug.

The OMEGA EP (Extended Performance) laser at the University of Rochester's Laboratory for Laser Energetics is the first of this new generation of HEPW-class lasers to be completed. OMEGA EP delivers multi-kilojoule laser energies in picosecond pulsewidths. Such high energies and short pulse durations allow significant laser powers to be produced with laser-pulse energy approaching the petawatt level - or up to a quadrillion watts.

In work that sheds new light on how to generate the powerful electron source required for fast ignition, LLE researchers have used OMEGA EP to produce intense electrical currents lasting just a few trillionths of a second. In a series of unprecedented experiments, laser energies of up to 2.1kJ were delivered to solid-density targets. These experiments represent a four-fold increase in the on-target laser energy compared to any previous petawatt-class laser system.

The LLE team used an electrostatic technique with thin-foil targets to diagnose the hot-electron source. This technique relies on foils no thicker than a human hair. During laser irradiation, the foils rapidly charge, similar to a capacitor plate. Within picoseconds, the majority of the laser-accelerated electrons transfer their energy to the target material. This occurs during multiple electron transits through the target - a process known as refluxing. Using this mechanism, electrons interact with the target material and generate high-energy x-rays. By diagnosing the x-ray source brightness, the conversion efficiency of laser light into is determined.

Using the OMEGA EP laser, constant laser-energy conversion efficiencies of around 20% and higher into energetic electrons has been demonstrated. These measurements identify what minimum fraction of intense-laser energy is available to heat the target - a critical parameter in determining the energetic and technological feasibility of full-scale fast ignition. These latest observations show that strong laser-energy coupling scales from small university-class laser systems over three orders of magnitude in laser energy to large multi-kilojoule class systems needed for fast ignition.

Achieving high conversion efficiencies with 10 to 20ps pulse durations is also paramount. The laser intensity governs the mean-electron energy. Too intense, and the energetic electrons will pass through the dense fuel and not deposit their energy in the imploded-target core. Too long, and the dense assembled fuel will decompress before it ignites. The key is to generate electrons with optimum energies while maintaining multi kilojoule laser energies to cause strong heating prior to target disassembly. Efficient production of energetic electrons with intense, 15ps long pulses at multi kilojoule laser energies as demonstrated in these experiments will allow an understanding of the requirements for full-scale fast-ignition experiments and will help define the requirements for future fast-ignition facilities.

Source: American Physical Society

Explore further: New microscope collects dynamic images of the molecules that animate life

add to favorites email to friend print save as pdf

Related Stories

Laser Fusion and Exawatt Lasers

Oct 01, 2009

(PhysOrg.com) -- In the recent past, producing lasers with terawatt (a trillion watts) beams was impressive. Now petawatt (a thousand trillion watts, or 10^15 watts) lasers are the forefront of laser research. Some labs are ...

The little beam that could

Feb 01, 2006

Scientists at Los Alamos National Laboratory, in collaboration with researchers from the University of Nevada, Reno, Ludwig-Maximilian-University in Germany, and the Max-Planck-Institute for Quantum Optics in Germany, have ...

Most powerful laser in the world fires up

Apr 08, 2008

The Texas Petawatt laser reached greater than one petawatt of laser power on Monday morning, March 31, making it the highest powered laser in the world, Todd Ditmire, a physicist at The University of Texas ...

First demonstration of new laser-driven accelerator technology

Sep 30, 2004

A team of UK scientists has used, for the first time, an extremely short-pulse laser to accelerate high-energy electrons over an incredibly short distance. Current accelerators can be hundreds of metres long, this is just a millimetre long. ...

Recommended for you

Cooling with molecules

Oct 22, 2014

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

User comments : 0