Speaker
Philip Martin
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O4.202.pdf
Staged ion acceleration from ultrathin foils with sub-ps, near-PW pulses
P. Martin1, H. Ahmed1, D. Doria1, A. McIlvenny1, S. Ferguson1, S. Zhai1, J. Jarrett2, J.
Green3, P. McKenna2, M. Borghesi1 and S. Kar1
1
Centre for Plasma Physics, Queen’s University Belfast, Belfast, United Kingdom
2
SUPA, Department of Physics, University of Strathclyde, Glasgow, United Kingdom
3
Central Laser Facility, Rutherford Appleton Laboratory, Oxfordshire, United Kingdom
Acceleration of ions using ultrashort intense laser pulses is an ongoing area of research with a
wealth of possible applications. Ions are typically accelerated via the target normal sheath
acceleration (TNSA) mechanism [1], whereby the laser generates a hot electron sheath on the
rear surface of the target, creating a very large electric field, which accelerates protons and
heavier ions present on the target rear surface to MeV energies over much shorter distances
compared to conventional RF accelerators.
Beyond TNSA, as the laser ramps up in intensity, radiation pressure acceleration (RPA)
progressively acquires more importance. If the target is sufficiently thin, the bulk of the target
is accelerated as a whole, in the so-called light sail (LS) mechanism. However, in order for LS
to work, the plasma must remain opaque to the laser radiation, meaning that effects such as
relativistic transparency must be suppressed. This has been achieved previously by using
circularly polarized laser pulses, as opposed to linearly polarized, to minimize the J×B heating
of electrons [2]. Another method to delay the onset of transparency and enhance the effect of
RPA is to split the main pulse into multiple pulses at lower intensities in a staged acceleration
scenario – subsequent pulses will irradiate the target at later times and focussed deeper into the
target. This scheme works because in the LS regime, the ion energy scales with the laser
fluence, rather than the intensity [3].
Presented here are the latest results from an experimental campaign undertaken on the Vulcan
Petawatt laser system at the Central laser facility in the UK. In this experiment ultrathin gold,
plastic, and deuterated plastic foils were irradiated with picosecond pulses in both single and
double stage scenarios. Deuterated foils were chosen because, during the LS acceleration
phase, D-D reactions in the compressed target produces a beam of high energy neutrons,
allowing the ability to diagnose the efficiency of the LS mechanism without the influence of
any TNSA field post-RPA. Examining the proton, ion and neutron spectra together allows to
identify the optimal conditions under which the LS regime is most dominant.
[1] A. Macchi, M.Borghesi, M.Passoni, Rev. Mod. Phys., 85, 751 (2013)
[2] C. Scullion et al., Phys. Rev. Lett. 119, 054801 (2017)
[3] S. Kar et al., Phys. Rev. Lett., 109, 185006 (2012)
