Multispecies ion acceleration from intense laser interactions with thin foils

Student thesis: Doctoral ThesisDoctor of Philosophy


The understanding and demonstration of novel laser driven ion acceleration mechanisms is key to overcoming the limitations of both well understood laser based processes and becoming competitive with conventional particle accelerators. This thesis presents two methods of enhancing the ion energies achievable in thin solid targets irradiated by intense laser pulses. In the first approach, the transition to the Light-Sail Radiation Pressure Acceleration mechanism was explored with the use of ultra-thin foils, and a contrast enhanced circular polarised pulse with a peak intensity of 5.5×10 Wcm-2 . The experimental data indicates the presence of an (intensity dependent) optimum thickness of 15 nm for C6+ (33 MeV/nucleon) where a local minimum of 18 MeV is instead observed for H+. At this thickness, each species’ maximum energy is shown to scale differently with laser intensity; C6+ scales favourably in the radiation pressure scheme (I1.2) with protons scaling much slower, indicating acceleration by plasma heating. This species dependence is attributed to the precursor laser intensity impinging onto the target. This is shown to result in a multi-species expansion which allows for the preferential acceleration of bulk C6+ ions over sheath accelerated H+ which are displaced from the target prior to the arrival of the main intensity peak. This is shown for the first time and is of major significance to the development of laser based ion accelerators at currently available, and future, high power lasers. The second approach indicates the enhancement of radiation pressure accelerated ions in temporally stretched pulses, with a lower peak intensity, by measuring both higher ion energies and a greater red-shift in the back reflected light than for a fully compressed, higher intensity pulse. This is against what would be expected in a typical interaction. It is shown that proton energies are increased from 30 MeV to 50 MeV by stretching the pulse by over a factor of 2 with the maximum red-shift of the 2nd harmonic increasing from 55 nm to 75 nm, indicating an enhanced hole boring velocity from 0.1c to 0.14c. Numerical simulations indicate that this effect is due to the development of a hot electron current, and subsequently equalising return currents, in the overdense bulk plasma, which result in the growth of significant (10’s kT) magnetic fields within the target. These currents repel and cause a significant drop in target density reducing the internal plasma pressure and allowing radiation pressure to dominate on the front surface over what is observed with the higher intensity pulses. The experimental trends are qualitatively reproduced and the enhancement of front surface acceleration is demonstrated.
Date of AwardJul 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsThe Royal Society/Engineering and Physical Sciences Research Council
SupervisorMarco Borghesi (Supervisor) & Satyabrata Kar (Supervisor)


  • laser ion acceleration
  • ion acceleration
  • light sail
  • hole boring
  • target normal sheath acceleration
  • relativistic transparency
  • multispecies
  • radiation pressure acceleration
  • polarisation
  • carbon
  • protons

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