Abstract
Rapid strides in the technology of laser plasma-based
acceleration of charged particles leading to high
brightness, tunable, monochromatic energetic beams of
electrons and ions has been driven by potential
multidisciplinary applications in cancer therapy, isotope
preparation, radiography and thermonuclear fusion.
Hitherto laser plasma acceleration schemes were
confined to large-scale facilities generating a few tens of
terawatt to petawatt laser pulses at repetition rates of 10
Hz or less. However, the need to make viable systems
using high-repetition-rate femtosecond lasers has
impelled recent research into novel targetry [1,2].
Of contemporary importance is the generation of supra
thermal electrons, beyond those predicted by the scaling
relation, reflected in both theoretical and computational
work [3,4]. In this work we present evidence of
generation of relativistic electrons (temperature >200
keV, maximum energies >1 MeV) at intensities that are
two orders of magnitude lower than the relativistic
intensity threshold. The novel targets [6] are 15 micron
sized crystals suspended as aerosols in a gas interacting
with a kHz, few-mJ femtosecond laser focussed to
intensities of 10 PW/cm2. A pre-pulse with 1-5% of the
intensity of the main pulse, arriving 4 ns early, is critical
for hot electron generation. In addition to this
unprecedented energy enhancement, we also characterize
the dependence of X-ray spectra on the background gas
of the aerosol. Intriguingly, easier the gas is to ionise,
greater is the number of hot electrons observed, while the
electron temperature remains the same.
2-D Radiation hydrodynamics and Particle-in-cell (PIC)
simulations explain both the experimentally observed
electron emission and the role of the low-density plasma
in yield enhancement. We observe a two-temperature
electron spectrum with about 50 and 240 keV
temperatures consistent with the measurements made in
the experiments. The simulations show that the
following features contribute to the high-energy electron
emission. The pre-pulse generates a hemispherical
plasma-profile that enhances the coupling of the laser
light. Overdense plasma is generated about the
hemispherical cavity on the particle due to the main
pulse interaction. The gradient in the plasma density in
and around the cavity serves as a reservoir of low energy
electrons to be injected into the particle potential and
enables the hot electron generation observed in the
experiments. Higher energy electron emission is
dominantly from the edges of the hemispherical
cavitation. The increase in total X-ray yield observed in
the experiments scales with the number of electrons
generated in the low density neighborhood surrounding
the particle. In a simple-man picture, the laser interacts
with the particle and ejects electrons from the particle.
The particle acquires a strong positive potential that can
only be brought down by ion expansion that occurs over
10's of picoseconds. The particle with strong positive
potential acts as an 'accelerating electrode' for the
electrons ionized in the low-density gas neighborhood.
These results assume importance in the context of
applications such as fast fuel ignition [6] or in medical
applications of laser plasmas [7] where high irradiance of
energetic electrons is of consequence.
1. D. Gustas et al., Phys. Rev. Accel. Beams, 21,
013401 (2018).
2. S. Feister et al , Opt. Express, 25, 18736 (2017).
3. B. S. Paradkar, S. I. Krasheninnikov, and F. N.
Beg, Physics of Plasmas, 19, 060703 (2012).
4. A. P. L. Robinson, A. V. Areev, and D. Neely,
Phys. Rev. Lett., 111, 065002 (2013).
5. R. Gopal, et al., Review of Scientific
Instruments, 88, 023301 (2017).
6. M. Tabak et al., Physics of Plasmas, 1, 1626
(1994).
7. A. Sjogren, M. Harbst, C.-G. Wahlstrom, S.
Svanberg, and C. Olsson, Review of Scientific
Instruments, 74, 2300 (2003).
| Original language | English |
|---|---|
| Number of pages | 1 |
| Publication status | Published - 1 Nov 2020 |
| Event | 4th Asia-Pacific Conference on Plasma Physics - Duration: 26 Oct 2020 → 31 Oct 2020 |
Conference
| Conference | 4th Asia-Pacific Conference on Plasma Physics |
|---|---|
| Period | 26/10/20 → 31/10/20 |