January 28, 2013

Table Top Neutral-Atom Accelerators - Scaling to Million Electron Volt Energies

Accelerating neutral atoms, contrary to laser-based as well as conventional particle accelerators, is a formidable feat, given the inert, ‘neutral’ response of these atoms to accelerating fields. Our recent studies provide a crucial breakthrough in the generation of accelerated neutral atoms, with energies as large as an MeV, as a result of the interaction of intense lasers with nanoclusters.

Laser-based plasma accelerators, on the contrary, follow radically different acceleration schemes and can produce GeV electron bunches in a ‘wakefield-accelerator’ as well as proton energies of 60 mega-electron-volts (MeV) in a so-called ‘target-normal sheath acceleration’ (TNSA).

How do we accelerate neutral particles- i.e. particles that cannot be energized by electrical voltages? And do it over millimeters rather than hundreds of meters and moreover using lasers? Research at Ultra Short Pulse High Intensity Lab in TIFR has now found a novel scheme that can do precisely this. The concept uses the ability of powerful lasers to strip nearly 8 electrons per atom in a nano sized, cooled aggregate of argon atoms- a nano piece of ice. A 40,000 atom cluster of argon is charged to 320,000 by a laser that lasts only a 100 billionth of a millionth of a second. Such a super highly charged ice piece explodes soon after, accelerating the charged atoms (Ions) to a million electron volts of energy. The TIFR research now found that all the expelled electrons can be put back into the charged ion that has been accelerated so that it now reverts to being a neutral atom but at high energies. To top it all, this process is nearly 100% efficient at neutralizing the speeding ions and converting them to fast atoms!

Accelerated neutral atoms are very important for many applications. Unaffected by electric or magnetic fields, they penetrate deeper in solids than electrons/ions and thereby create high finesse microstructures for novel electronics and optical devices. Fast atoms are used both as diagnostics and heating sources in Tokomak machines like the ITER (International Thermonuclear Experimental Reactor) in France, that are being developed to create sustained thermo-nuclear fusion. The TIFR scheme can produce a point source of fast neutral atoms close to the location of an intended application.

Caption: Highly charged Argon ions (orange) exploding from a nanocluster are reduced to neutrals (blue) in a mm accelerator due to dense excited clusters (green). Credit: Dr. Rajeev Rajendran, TIFR

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