Transcript A.Lobko, Laser acceleration of electrons and ions: principles, issues

Laser acceleration
of electrons and ions: principles,
issues, and applications
Alexander Lobko
Institute for Nuclear Problems, BSU
Minsk Belarus
Motivation
• Way to table-top accelerators with relatively
low energy for various applications in
nuclear medicine, material science, biology,
chemistry, and industry (e.g. sterilization of
food)
• Basic research in laser and plasma physics,
physics of radiations, nanoscience, high
density energy physics, detector
development, etc
• Seeking collaborations of Belarus physicists
doing research in above fields, both
theoretically and experimentally
Classical acceleration process
Evaluation of electric field
at plasma wave
where
- plasma wave amplitude
Classical and plasma acceleration
Laser-induced ionization
For the hydrogen atom, the binding electric field is given (in SI units) by
This is the intensity at which any target material will
be ionized solely by the laser electric field
In fact, with laser intensities exceeding
the photon density is high enough so that
enables the multi-photon ionization process
Relativistic laser intensity
where
normalized vector amplitude
The maximum kinetic energy of the
electron can be written as
Focused laser with a normalized amplitude of
is commonly referred to as relativistic.
Motion of an electron in the laser field
Chirped pulse amplification (CPA)
Before the invention of the CPA in 1985, laser pulses could be focused only in the two
transverse dimensions by corresponding sets of lenses. The CPA technology has
allowed the compression of the laser pulses in the third, longitudinal dimension, and
this technological breakthrough has immediately led to a jump in the achievable
powers and focused intensities.
The Petawatt shots, where an adaptive mirror has been employed, have resulted in
the focal intensity
Now, three classes of laser amplifiers – CPA based on Ti-Sa, CPA based
on Neodymium glass, and Optical Parametric CPA based on DKDP
crystals can deliver approximately the same level of power,
about 1.0 PW.
Chirped pulse amplification (CPA)
Examples of lasers at operation
Laser pulse and plasma wave
N. Matlis et al // Nat. Phys. 2 (2006) 749
The maximum amplitude of the plasma wave was measured to be in the range 20%–60%
[C. E. Clayton at al // Phys. Rev. Lett. 81 (1998) 100)].
External injection
Supersonic jet target schematics
One of the solutions to control electron injection is injection at
downward density ramp with a density gradient scale length greater
than the plasma wavelength.
Beam quality
Comparison of selfinjection (top) and
injection at a density
transition (bottom)
Bubble regime-transverse injection
The laser pulse that propagates from left to right, expels electrons on his
path, forming a positively charged cavity. The radially expelled electrons
flow along the cavity boundary and collide at the bubble base, before
being accelerated behind the laser pulse. The fact that electrons are
trapped behind the laser, where they no more interact with the laser
field, contributes to improving of quality of the electron beam.
Transition between acceleration regimes
Electron beam
distribution for
different plasma
densities showing the
transition from the self
modulated laser
wakefield and the
forced laser wakefield
to the bubble/blow-out
regime.
Colliding laser pulses
Two laser pulses propagate in opposite directions
Colliding laser pulses
During the collision, some electrons get enough longitudinal
momentum to be trapped by the relativistic plasma wave
driven by the pump beam
Colliding laser pulses
Trapped electrons are accelerated in the wake of the pump
laser pulse
Experimental setup
Beam energy tunability
Beam energy spread
Beam properties
Ion acceleration
Target Normal Sheath Acceleration (TNSA)
The obliquely incident laser heats electrons on the front side of the
target. The electrons penetrate the target, that is several micron thick
and opaque to the laser light. A charge separation field is created by
these hot electrons on the back side of the target due to subsequent
field ionization of the target surface. Protons (and ions) are accelerated
in this virtual cathode target normal to the back surface
TNSA mechanism features
Ion acceleration
Break-out afterburner (BOA)
The laser starts to ionize the surface of the initially opaque target and successively
heats more and more electrons to relativistic energies. Provided, the target is thin
enough and has not blown apart under the irradiation of the laser pedestal, the laser
will eventually promote all electrons within the focal volume to hot electrons and turn
the target relativistically transparent.
At this time, which is referred to as t1, strong acceleration of the plasma ions over the
whole volume occurs, where the accelerating electric field co-moves with the ions and
the laser continuously replenishes the energy, the electrons transferred to the ions.
The acceleration ceases, when the plasma turns classically underdense.
BOA regime features
BOA regime demands
In order to enter the BOA regime with the laser systems
available today (i.e. 100 TW to a PW) targets of 5-500 nm are
necessary; with thicker targets the relativistic transparency
will not be reached and acceleration will be dominated by the
TNSA mechanism.
The use of ultra-thin nm-scaled targets also demands ultrahigh laser contrast, so that premature expansion does not
destroy the target prior to arrival of the peak pulse.
Particle spectra
Particle spectra measured by ion spectrometers at different
angles for shots that yielded peak energies (a) for carbon
ions and (b) for protons.
Maximal energies for
carbon ions and protons
For protons maximum energies are emitted on-axis regardless
of the target thickness, whereas maximum carbon ion
energies are emitted off-axis in the BOA regime ( <<1 μm) and
on-axis in the TNSA regime (> 1 μm)
Average ion energy
Thickness dependency of the average particle energy for
carbon (a) and protons (b) for thicknesses ranging from 30 nm
up to 25 μm.
Particle numbers
Thickness dependency of particle numbers for carbon ions
(a) and protons (b) for thicknesses ranging from 30 nm up to
25 μm. The peak at 200 nm corresponds to the optimum
BOA target thickness
CONCLUSION
FUTURE OF THE LASER PLASMA
ACCELERATORS
Century of Progress
Literature
1. V. Malka Electron and X-ray beams with Laser Plasma Accelerators //
Presentation at Channeling-2012 Conference
2. В.Е. Фортов Экстремальные состояния вещества М.:-2009, - 304 с.
3. А.В. Коржиманов и др. // УФН 181 (2011) 9
4. D. Jung Ion acceleration from relativistic laser nano-target interaction
// PhD thesis / Munchen 2012
5. V. Malka // PoP 19, 055501 (2012)
6. A. Pukhov // Rep. Prog. Phys. 66 (2003) 47
7. Ч. Джоши // В мире науки №5 2006
Thank you for attention