Plasma mirrors generate high harmonics

For many researchers working with high-intensity lasers, plasma formation is often bad news. For others, it’s an enabling step for putting their laser systems to good use, in the form of so-called plasma mirrors. Now, collaborators at CEN Saclay (Gif-sur-Yvette, France), École Polytechnique (Palaiseau, France), and the University of Toronto (Toronto, Ont., Canada) have used a chain of plasma mirrors to create extreme-ultraviolet (EUV) pulses that they predict to be on the attosecond timescale, all with a tabletop laser.

In the highest intensity femtosecond-laser systems, a nanosecond-scale prepulse can be enough to ablate the surface of samples before most of the pulse energy arrives. Enter the plasma mirror, a simple but effective solution to increase the contrast between the prepulse and the main pulse. By placing a highly polished mirror blank in the beam at Brewster’s angle, the nanosecond-scale prepulse passes unchanged. The rising edge of the main pulse, however, converts the surface of the mirror into a highly reflective plasma, specularly reflecting only the main pulse. Once the prepulse is cleaned up by two plasma mirrors, a third forms the basis for high-order harmonic generation (HHG).

C. Thaury et al.,Nature Phys. doi:10.1038/nphys595 (2007).

Laser vision fuels energy future

Photonics.com reported:

The proposed European High Power laser Energy Research (HiPER) facility -- a device intended to demonstrate the feasibility of laser-driven fusion as an energy source -- is entering the preparation phase after completion of a two-year study by an international team of scientists.

Their conclusions have allowed the HiPER project to be selected as part of the European roadmap for future large-scale science facilities. The preparatory phase of the HiPER facility is expected to begin in January and to last for three years.

Achieving nuclear fusion using lasers is the goal of the National Ignition Facility, the latest in a series of high-power laser facilities used for research in inertial confinement fusion. Now under construction at the Lawrence Livermore National Laboratory, in Livermore, Calif., NIF is being built by the US Department of Energy as part of its Stockpile Stewardship program, and as such has a strong defense mission. "This is largely due to the fact that NIF converts its laser light to x-rays, and those x-rays are then used to implode the pellet (or perform other, classified experiments)," Dunne said. "This is meant to be analogous to the use of x-rays in thermonuclear weapons."

HiPER removes this link to defense science, Dunne said. "It uses the optical laser light directly to drive the implosion and initiate fusion. The physics associated with the interactions of lasers with matter have no relevance whatsoever to nuclear weapons, so we see this as very much a 'swords into ploughshares' undertaking."

Laser Fusion Facilities
LASER FACILITY	LOCATION	COMPRESSION ENERGY	IGNITION POWER	ESTIMATED START
National Ignition Facility	United States	1.8 MJ	NA	2009
Laser Mégajoule	France	2.0 MJ	NA	2011
FIREX-I + Gekko XII	Japan	10 kJ	1 PW (10 kJ)	2007
OMEGA EP	United States	30 kJ	2 PW (5 kJ)	2007
HiPER	Europe	200 kJ	10 PW (70 kJ)	proposed

For more information, visit: www.hiper-laser.org

Relativistic tennis with photons

Science daily news reported that a team from Advanced Photon Research Center at the Japan Atomic Energy Agency has demonstrated to generate an ultrashort and ultraintense x-ray pulse using the ordinary laser.

Sergei Bulanov of the Advanced Photon Research Center at the Japan Atomic Energy Agency in Kyoto and colleagues say they have a prototype that can generate pulses of x-ray laser light on the cheap. The researchers call their technique "relativistic tennis with photons," but a more violent analogy may better convey how it works. Suppose you throw a golf ball at a locomotive that is speeding toward you. The golf ball will bounce off it and come flying back at you with tremendous energy--just before you get run over.

The golf ball is a pulse of ordinary low-energy photons. With a tabletop setup, Bulanov and colleagues create the equivalent of a locomotive by firing a different laser into a cloud of plasma, where it creates a wake that travels at near-light speed. When the photons hit the wake, their energy increases 56-fold. They are also focused into an ultrashort, ultraintense blast by the wake, which is shaped like a miniature radar dish.

Improvement of the multipass amplifier

In order to protect the crystal, we tried many ways including decreasing the pump energy, walking away the focal point from crystal, reflecting back the pump beam, however the crystal still was burned sometimes. So this time we try to using the second beam to pump from the other side. The second was sent to dump before, now we added three mirrors to steer the beam into the crystal.

A good overview of plasma wakefield

Dr. Chan Joshi, the professor of University of California, Los Angeles, reviews recent progress in the development of plasma-based particle accelerators and considers the challenges still to be overcome to turn this concept into a practical technology for high-energy physics. This overview was published on recent CERN Courier Vol 47, No.5(2007).

(a) A simple 1D schematic of how wakefields are excited by a short-laser (top) or particle-beam (bottom) driver in a plasma. (b) 3D computer simulation of an extremely nonlinear wakefield excited by the drive beam in the "bubble" regime. The wakefield can accelerate an appropriately phased trailing beam at ultra-high gradients.

Who first observed the ponderomotive force?

In research field of high intensity laser, the ponderomotive force is the important concept. Most of us got the description from Kruer's book, in fact this phenomena was obtained at first time in 1957. From web site of Nature Physics looking back, it tells us:

In 1957, Boot and Harvie reported the observation of a force on charged particles in an inhomogeneous electric field, which originated from second-order terms of the equation for the Lorentz force on the particles. Almost immediately it was realized that this 'ponderomotive force' could be used to trap and control electrons. But the force is weak: only with the development of modern laser technology is the ponderomotive force being exploited in new particle-acceleration techniques and inertial confinement fusion.

Nature 180, 1187 (1957)