Laser accelerationof 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 fieldat plasma wave

where - plasma wave amplitude

Classical and plasma acceleration

Laser-induced ionizationFor 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 fieldIn fact, with laser intensities exceedingthe photon density is high enough so that enables the multi-photon ionization process

Relativistic laser intensity

normalized vector amplitudewhere

Focused laser with a normalized amplitude ofis commonly referred to as relativistic.

The maximum kinetic energy of the electron can be written as

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 waveN. 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 self-injection (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 wakefieldto 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 accelerationTarget 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 accelerationBreak-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 ultra-high 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 forcarbon 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

CONCLUSIONFUTURE OF THE LASER PLASMA

ACCELERATORS

Century of Progress

Literature1. V. Malka Electron and X-ray beams with Laser Plasma Accelerators //

Presentation at Channeling-2012 Conference2. В.Е. Фортов Экстремальные состояния вещества М.:-2009, - 304 с.3. А.В. Коржиманов и др. // УФН 181 (2011) 94. D. Jung Ion acceleration from relativistic laser nano-target

interaction // PhD thesis / Munchen 20125. V. Malka // PoP 19, 055501 (2012)6. A. Pukhov // Rep. Prog. Phys. 66 (2003) 477. Ч. Джоши // В мире науки №5 2006

Thank you for attention