Plasma Waves Driven by Lasers: The New Surfing

Plasma Waves Driven by Lasers: The New Surfing

by Simon Mitton

WHAT'S HOT IN PHYSICS
Rank	Paper	Cites This Period (Mar-Apr 06)	Rank Last Period (Jan-Feb 06)
1	A.G. Riess, et al., *"Type Ia supernova discoveries at z > 1 from the Hubble Space Telescope: Evidence for past deceleration and constraints on dark energy evolution,"* Astrophys. J., 607(2): 665-87, 1 June 2004. [8 0U.S. and German institutions] *822LC	52	1
2	J. Sinova, et al., "Universal intrinsic spin Hall effect," Phys. Rev. Lett., 92(12): 126603, 26 March 2004. [Texas A&M U., College Station; U. Texas, Austin; Institute of Physics, Prague, Czech Republic] *807TU	32	6
3	S.P.D. Mangles, et al., "Monoenergetic beams of relativistic electrons from intense laser-plasma interactions," Nature, 431(7008): 535-8, 30 September 2004. [Imperial Coll. London, U.K.; Rutherford Appleton Lab., Didcot, U.K.; U. Strathclyde, Glasgow, U.K.; U. Calif., Los Angeles] *857YP	27	†
4	S.N. Ahmed, et al. (SNO Collaboration), "Measurement of the total active ⁸B solar neutrino flux at the Sudbury Neutrino Observatory with enhanced neutral current sensitivity," Phys. Rev. Lett., 92(18): 181301, 7 May 2004. [14 Canadian, U.S., and U.K. institutions] *818XN	26	†
5	Y.K. Kato, et al., "Observation of the spin Hall effect in semiconductors," Science, 306(5703): 1910-3, 10 December 2004. [U. Calif., Santa Barbara] *879DC	26	9
6	J. Faure, et al., "A laser-plasma accelerator producing monoenergetic electron beams," Nature, 431(7008): 541-4, 30 September 2004. [ENSTA, CNRS, Palaiseau, France; U. Dusseldorf, Germany; CEA/DAM Ile de France, Bruyeres-le-Chatel, France] *857YP	26	†
7	C.G.R. Geddes, et al., "High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding," Nature, 431(7008): 538-41, 30 September 2004. [5 U.S. institutions] *857YP	26	†
8	J. Wunderlich, et al., "Experimental observation of the spin Hall effect in a two-dimensional spin-orbit coupled semiconductor system," Phys. Rev. Lett., 94(4): 047204, 4 February 2005. [5 U.K., U.S., and Czech institutions] *894GN	25	†
9	G.G. Fazio, et al., *"The Infrared Array Camera (IRAC) for the Spitzer Space Telescope,"* Astrophys. J. Suppl. Ser., 154(1): 10-7, September 2004. [12 U.S. institutions] *850TB	24	†
10	M. Tegmark, et al., "The three-dimensional power spectrum of galaxies from the Sloan Digital Sky Survey," Astrophys. J., 606(2): 702-40, 10 May 2004. [23 institutions worldwide] *818RJ	23	†
SOURCE: Thomson Scientific's Hot Papers Database. Read the Legend.

n the Cavendish Laboratory at the University of Cambridge there is a small museum of the historic apparatus that Nobel Laureates used to open the secrets of the atom and the nucleus. A striking feature of these artifacts is their small scale. Chadwick’s setup to discover the nucleus, Wilson’s cloud chambers, and the first cathode ray tubes can each be lifted with one hand. Contrast that to the Large Hadron Collider (LHC) under construction in Geneva, 27 km in circumference, with $8 billion written on the price tag. Enormous accelerators have been at the heart of particle physics for half a century. The LHC is destined to test the standard model to destruction and search for the Higgs boson. But its enormous cost has led experimentalists to seek new strategies for designing compact accelerators. In this period, three Hot Papers (#3, #6, #7) point the way to returning particle acceleration to the laboratory.

High-power lasers can now reach focused intensities of 10¹⁹ W cm^-2 at high repetition rates. The awesome power of pulsed lasers is now such that they have even sparked several unsuccessful searches for extraterrestrial intelligence in which terrestrial observers look for alien nanosecond flashes that would exceed the brightness of their companion star by an order of magnitude. The research in #3, #6, and #7 is more down to earth: how to make lasers generate beams of relativistic electrons with small beam divergence and a narrow energy spread. Each paper reports remarkable improvements to plasma wake field acceleration, a relatively new concept.

Conventional accelerators are reaching design limits. Their radio-frequency electric fields are constrained by an absolute upper limit, beyond which the metal walls of the accelerator break down and may even melt. The ceiling gradient is tens of MeV m^-1. This limitation is responsible for the enormous length of GeV and TeV accelerators. Physicists have known for about a decade that by using a laser to drive a compression wave through plasma, electrons can be taken from rest to 100 MeV in 1 mm, a reduction of a factor of 5000 in size. However, the scatter-gun beams and bunches of electrons have been useless for experimental exploitation because of large energy spread and poor quality. All this changes with papers #3, #6, and #7, which show how to sharpen and control the accelerated beams.

All three experiments are conceptually similar. The accelerator cavity is filled with an ionized gas, or plasma, which is immune to electrical collapse. TeV laser pulses 30 - 55 x 10¹⁵s long generate waves in this plasma. Electrons or positrons "surf" in the wave’s wake, reaching extraordinary energies. The effect is similar to "wake surfing" in which the surfer rides the wake behind a powerboat speeding across a lake.

The great breakthrough reported in the three papers is the realization of monoenergetic beams of electrons with densities up to 10³ higher than previously achieved. Furthermore these beams are tightly collimated, being 10 times narrower than hitherto. The keys to success are that the three groups successfully increased the distance over which the electrons could be accelerated.

In high riser #3 the U.K. group directed by Karl Krushelnick (Imperial College, London) describe the production of electron beams with small divergence and an energy spread of just 3%. In their experiment the energy of the beam varied from shot to shot of the laser, so ongoing research should be directed to generating ultrashort bunches of tuneable energy.

According to #6, the Paris team of Victor Malka (Ecole Polytechnique, Palaiseau, France), who showed in 2002 that electrons could be propelled to 200 MeV in a distance of 1 mm, have successfully overcome the energy-spread problem by creating a bubble in the plasma first to trap and then to boost the electrons as a monoenergetic group.

The U.S. collaboration have dramatic news in #7, where the plasma wakefield group led by Wim Leemans (Lawrence Berkeley National Laboratory, California) shows how to guide an intense laser using a preformed plasma density channel. Their result points the way to extending the acceleration zone beyond the millimeter range, a prerequisite for approaching the frontiers of high-energy physics.

It will be decades before the technologies described in these groundbreaking papers displace the LHC. So in a sense they describe the next next generation in accelerators to probe the nature of the universe. But on the home front, table-top setups for use in medical research, materials testing, or light source for fine-scale imaging may be just around the corner.

Dr. Simon Mitton, a Fellow of St. Edmund’s College, Cambridge, U.K., writes on current issues in physics and astronomy.

Science Watch^®, September/October 2006, Vol. 17, No. 5
Citing URL: http://www.sciencewatch.com/sept-oct2006/sw_sept-oct2006_page6.htm