PETAWATT ION ACCELERATION FROM NANOMETER SCALE TARGETS

1
PETAWATT ION ACCELERATION FROM NANOMETER SCALE
TARGETS
C. Bellei1, A.E. Dangor1, N. Dover1, S. Kneip1,
S.P.D. Mangles1, S.R. Nagel1, Z. Najmudin1, A.
Rehman1, J. Schreiber1, M. Yeung2, M. Zepf2, S.
Hassan3, M. Tatarakis3, S. Bott4, F. Beg4 and A.
Henig5. 1Imperial College London, London,
2Queens University Belfast, Belfast, 3Laboratory
of Optoelectronics, Lasers and Plasma Technology,
TEI Crete, 4Centre for Energy Research,
University of California San Diego and
5Max-Planck-Institut für Quanten Optik, Garching.

ABSTRACT Ion beam profiles of beams and modulated
ion spectra resulting from the irradiation of
nanometer scale diamond-like carbon (DLC) foils
will be presented. The experiment was performed
using the linearly polarised VULCAN Petawatt
laser at the Rutherford Appleton Laboratory,
reaching intensities of 1021 W/cm2. 
The generated ion beams are seen to be highly
structured and may indicate the presence
of Radiation Pressure Acceleration (RPA). 
LASER DRIVEN ION ACCELERATION

• Laser energy transferred to ions via electrons.
• TNSA
• Hot e- form a sheath at rear surface, which
  ionises surface atoms and accelerates ions normal
  to the surface.
• Ep (I?2)½
• RPA
• Strong ponderomotive force compresses e- in
  target leaving depletion layer of ions, which is
  then accelerated by strong E-field.
• Ep (It/s)2

• VULCAN Petawatt
• Ndglass ?1054nm
• Contrast ratio 10-11
• Pulse energy 300J
• Pulse length 660fs
• I 5x1020W/cm2
• a0 0.89(I?2)½ 19.6

Plasma mirror
49 reflectivity
Figure 3 (above) Simulation snapshots taken from
T. Esirkepov et al. PRL 92 17 (2004) to show the
acceleration of the foil as a whole in the RPA
regime.

• TARGETS
• 5nm, 20nm, 30nm and 50nm thick DLC targets.

t 0 fs
t 0 fs

• Rayleigh-Taylor Instability
• occurs when a light fluid (photons of laser)
  accelerates into a heavy fluid (target).
• perturbations at the interface rapidly grow into
  characteristic interpenetrating fingers.

Figure 1 Schematic of ion diagnostic set up
showing the positions of three stacks of
radiochromic film, to measure the ion beam
profile, through which holes had been cut to
allow a line of sight for Thomson parabola
spectrometers and other diagnostics.
t 45 fs
t 80 fs
Figure 4 (left) Ion density plots, at different
delays, taken from OSIRIS PIC simulations of a
linearly polarised pulse, incident from the left,
onto a 200nm thick overdense target showing the
development of the instability.
RADIOCHROMIC FILM RESULTS
5.6 MeV
7.2 MeV
2.7 MeV
4.4 MeV
Modulations in the spectra of high charge ions
peaks superimposed on TNSA maxwellian spectrum.
Figure 5 (Above) Schematic of Thomson Parabola
(TP) Spectrometer and (Right) results from the
laser TP for the 5 nm DLC shot .
t 80 fs
t 45 fs

• FUTURE WORK
• Repeat the experiment using a circularly
  polarised pulse to reduce target heating by
  suppression of jxB heating and therefore TNSA.

REFERENCES A.P.L. Robinson et al., New Journal of
Physics 10 013021 (2008) T. Esirkepov et al.,
Phys. Rev. Lett. 92 17 (2004) S. Nagel,
Simulations of nm-thick carbon foils
unpublished (2009) N. Dover et al., HEDP Summer
School Poster Session (2009)
ACKNOWLEDGEMENTS The authors would like to
acknowledge the vital assistance of the VULCAN
operations team throughout this experiment.