Collimated Multi-MeV Ion Beams from High-Intensity Laser Interactions with Underdense Plasma L. Willingale, 1 S. P. D. Mangles, 1 P. M. Nilson, 1 R. J. Clarke, 2 A. E. Dangor, 1 M. C. Kaluza, 1 S. Karsch, 3 K. L. Lancaster, 2 W. B. Mori, 4 Z. Najmudin, 1 J. Schreiber, 3,5 A. G. R. Thomas, 1 M. S. Wei, 1, * and K. Krushelnick 1 1 Blackett Laboratory, Imperial College London, London SW7 2BZ, United Kingdom 2 Central Laser Facility, Rutherford-Appleton Laboratory, Chilton, Oxon, OX11 0QX, United Kingdom 3 Max-Planck Institut fu ¨r Quantenoptik, Hans-Kopfermann-Straße 1, D-85748 Garching, Germany 4 Department of Physics and Astronomy & Department of Electrical Engineering, UCLA, Los Angeles, California 90095, USA 5 Ludwig-Maximilians-Universita ¨t Mu ¨nchen, Am Coulombwall 1, D-85748 Garching, Germany (Received 14 December 2005; published 22 June 2006) A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high- intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He 2 of 40 3 8  MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10  . 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space- charge fields normal to its boundary. DOI: 10.1103/PhysRevLett.96.245002 PACS numbers: 52.38.Kd, 52.38.Hb, 52.65.Rr With the development of short-pulse high-intensity la- sers it has become possible to generate energetic particles such as relativistic electrons [1],  rays [2], and MeV ions [3–5] from laser-plasma interactions. The acceleration of ions has received particular attention due to the high qual- ity of the energetic beams produced. The ion beams pro- duced are charge neutralized and are found to have high energy, large total charge, small emittance [6], and small virtual source size [7]. This makes them particularly valu- able for many proposed applications such as proton radi- ography [7], isochoric heating [8], medical isotope preparation [9], cancer therapy [10], and even as possible injectors for higher energy accelerators [11]. Laser-plasma interactions can be broadly categorized into either underdense or overdense interactions depending on whether or not the laser can propagate through the plasma. Most recent investigations of ion acceleration from a laser-plasma interaction have concentrated on over- dense plasmas, and, in particular, on the beam of ions emitted from the rear surface of laser irradiated solid targets. The ions are accelerated by the large quasistation- ary space-charge fields set up at both front and rear sur- faces of the target or by electrostatic shocks propagating through the target. The space-charge separation at the front surface is due to the ponderomotive potential of the laser pulse [12,13]. At the rear surface, the accelerating field is set up by the expulsion into vacuum of the beam of hot electrons which is generated at the front surface and trans- ported through the target [14]. The balance between the front and rear acceleration fields is determined by the laser conditions, in particular, the prepulse level, laser intensity, and target geometry [15]. The ion beam primarily consists of protons which are found as water and organic contami- nants on most solid targets. Thus to accelerate higher Z ions efficiently, significant effort must be taken to prepare and clean the targets before irradiation [13,16]. Multi-MeV energy ions have also been observed from underdense plasma targets [17]. In this case the ions were found to be emitted transverse to the direction of laser beam propagation. A Coulomb explosion [18] accelerates these ions through the large space-charge field generated by the ponderomotive expulsion of electrons from the central channel by the high-intensity laser. Recently, it has been observed that the maximum energy of the trans- verse ions is increased by collisionless shock acceleration [19]. This underdense ion acceleration mechanism allows ions from gas of any required Z to be efficiently acceler- ated. However, the lack of collimation of the ions limits the use of this mechanism for most applications. In this Letter we present the first observation of a colli- mated beam of MeV ions generated in the forward direc- tion from underdense laser-plasma interactions. The bal- ance between transverse and longitudinal acceleration is found to be primarily dependent on plasma density. At the highest density investigated, the forward going ion beam was found to have a maximum energy much higher than the transverse ion beam from the same interaction. Two- dimensional particle-in-cell (PIC) simulations were used to investigate the effect of the plasma density on the accel- eration of the ions in the forward direction. It is found that the longitudinally accelerated ions are generated by a sheath acceleration mechanism, similar to those generated in solid target interactions. The laser energy is efficiently absorbed mainly by direct laser acceleration of the plasma electrons to energies up to hundreds of MeV [20]. As the fast electrons leave the plasma, they generate a large PRL 96, 245002 (2006) PHYSICAL REVIEW LETTERS week ending 23 JUNE 2006 0031-9007= 06=96(24)=245002(4) 245002-1  2006 The American Physical Society