The recent hysteria, deservedly so, on the landmark historic discovery of a new Higgs-boson-like particle of mass 125 giga-electron-volts (GeV) at the Large Hadron Collider (LHC) in CERN probably highlights the gargantuan heights scaled by conventional particle accelerators in recent times, routinely accelerating particles to even tera-electron-volt (TeV) energies. Laser-based plasma accelerators, on the contrary, follow radically different acceleration schemes and can produce GeV electron bunches (W. P. Leemans et al., Nature Phys. 2, 696 (2006)) in a wakefield-accelerator as well as proton energies of 60 mega-electron-volts (MeV) in a so-called target-normal sheath acceleration (TNSA) scheme (R. A. Snavely et al., Phys. Rev. Lett. 85, 2945 (2000)) energies which sound mundane in comparison with conventionally accelerated particle energies, until one realizes that they have been achieved on compact inexpensive table-top accelerators, as opposed to kilometer-long tunnels across nations.
The basic underlying physics of accelerating a particle, however, hinges on an accelerating electric field, both in conventional as well as in laser-based particle accelerators, occasionally coupled with magnetic fields that steer the particle beam. Thus, while it is the electric field in a radiofrequency (RF) cavity in a conventional accelerator, in TNSA it is the electrostatic sheath field produced at the target rear, whereas in an electron accelerator, it is the wakefield in the plasma wave travelling in the wake of the laser pulse. However diverse the acceleration scheme, they are all based on accelerating a charged particle an electron, proton or an ion by an electric field. Consequently, all the aforesaid acceleration schemes are rendered completely ineffective in the face of the challenging prospect of accelerating a neutral particle, which does not respond to electric or magnetic fields precisely the reason why neutral particles can often penetrate deeper, into regions which are otherwise inaccessible to charged particles.
Previous experiments (U. Eichmann et al., Nature 461, 1261 (2009) and C. Maher-McWilliams et al., Nature Photonics 6, 386 (2012)) have reported milli-electron-volt (meV) neutral atoms by various laser-induced acceleration mechanisms.
Our recent studies ( R. Rajeev et al., Nature Phys. DOI: 10.1038/NPHYS2526) provide a crucial breakthrough in the generation of accelerated neutral atoms, with energies as large as a mega-electron volt (MeV), nearly six orders of magnitude higher compared to previous results.
Inert gases like argon can conglomerate to clusters, each cluster being an aggregate of a few tens of thousands of atoms. The interaction of an intense (~1016 W/cm2) laser pulse with an argon cluster can remove as many as eight electrons from each atom in the cluster, which typically comprises about 40,000 atoms on an average. The swarm of electrons released from this laser-excited region loosely attach themselves to the clusters in the vicinity, forming a halo around them, and giving rise to so-called Rydberg-excited clusters.
On the other hand, the clusters which have been ionized by the laser (thereby generating the swarm of electrons) are reduced to assemblages of ions, bursting with their nascent charge, and exploding under their own self-charge Coulomb repulsion. This spews out mega-electron-volt ions, which then traverse through the sheath of Rydberg-excited clusters surrounding the laser-focus. A highly effective electron transfer happens from the Rydberg-excited cluster to the energetic ion, thereby engendering highly energetic neutral atoms.
Hence, in summary, the modus operandi of the acceleration mechanism may be envisaged in a scheme comprising laser-ionization, followed by acceleration of the ions and their subsequent neutralization via electron-recapture. Our experiments show that under optimum conditions, the conversion of ions to neutral atoms can be nearly 100%, thereby resulting in the first compact table-top laser-based MeV neutral atom source.
With new-age technologies foraying into probing extremes of matter, accelerated mega-electron-volt neutral atoms have a distinct advantage over their charged counterparts. Unaffected by electric or magnetic fields, these neutral atoms penetrate deeper in solids than electrons or ions and thereby create high-finesse microstructures for novel electronics and optical devices. Fast atoms are also used both as diagnostics and heating sources in magnetic fusion devices called tokomaks, the most notable example being the International Thermonuclear Experimental Reactor (ITER) in France, the world s most expensive scientific venture, aimed at alternative eco-friendly schemes of harnessing sustained nuclear power
Figure - Energy spectrum of neutral atoms generated with argon clusters. The spectrum for ions and neutrals is not too different from that for neutrals alone, thereby indicating a near-100% conversion efficiency even at energies of mega-electron-volts