Historical research guide · 0920 · 2015
Coherent control of plasma dynamics by feedback-optimized wavefront manipulation
Z.-H. He · B. Hou · G. Gao · V. Lebailly · J. A. Nees · R. Clarke · K. Krushelnick · A. G. R. Thomas
In one minute
Shine a short, intense laser pulse into a gas and it strips electrons loose and drives a wave through the plasma behind it — a wave whose electric field can accelerate electrons far harder than any metal cavity. The trouble is that nobody can calculate in advance what shape the incoming light should have, because the plasma reshapes the beam as the beam makes the plasma. So the Michigan group of Z.-H. He and colleagues stopped calculating and started searching. They put a deformable mirror in the beam, let an evolutionary algorithm bend it, and used the experiment’s own output — the electron beam itself — as the score to be improved. The mirror finds the wavefront no one could have written down. Applied to two experiments, electron acceleration in a plasma wave and self-compression of the pulse by the gas it is ionising, the search improved the beam’s charge, its divergence and its emittance by orders of magnitude. Steering a plasma by choosing the phase of the light is coherent control, now demonstrated on plasma.
Connection
Chapter 9 is about plasma that organises itself into structures, and this is the paper that shows those structures can be steered from outside by shaping the light that makes them, rather than merely observed. It is also chapter 10’s principle in a working laboratory: what is being adjusted is the phase front, and phase is the handle.
From the source
Plasmas generated by an intense laser pulse support coherent structures, such as a large-amplitude wakefield, and those structures can determine the outcome of an experiment.Abstract, sentence 1; Physics of Plasmas 22, 056704 (2015)
The method is coherent control by feedback-optimised wavefront manipulation: a deformable mirror shapes the phase front of the driving laser pulse, and the experimental outcome is used directly as the feedback signal in an evolutionary algorithm that searches for the optimum.Abstract, sentences 2 and 3
The same method was applied to two different experiments — acceleration of electrons in laser-driven plasma waves, and self-compression of optical pulses induced by the nonlinearity of ionisation — so the control technique is not specific to one target.Abstract, sentence 4
Manipulating the laser wavefront produced orders of magnitude of improvement in the electron beam properties: the peak charge, the beam divergence and the transverse emittance.Abstract, sentence 5
What to watch: the authors state that the demonstration of coherent control for plasmas opens new possibilities for future laser-based accelerators and their applications, and the measurement that turns that into engineering is whether a feedback-shaped wavefront holds a laser-plasma accelerator stable shot after shot at high repetition rate, on beam charge, divergence and emittance together, rather than optimising a single campaign.Abstract, final sentence
The improvement is not simply a matter of making the sharpest focus: in the group’s companion paper the highest-quality vacuum focal spot produced a greatly inferior electron beam, and the winning wavefronts were instead the particular laser phases that steer the plasma wave into a final state with optimal accelerating fields — which is what makes this coherent control rather than alignment.Companion paper, He, Hou, Lebailly, Nees, Krushelnick and Thomas, Coherent control of plasma dynamics, Nature Communications 6, 7156 (2015), abstract; preprint arXiv 1501.04117
The way in
https://doi.org/10.1063/1.4921159How to cite it
Z.-H. He, B. Hou, G. Gao, V. Lebailly, J. A. Nees, R. Clarke, K. Krushelnick, A. G. R. Thomas (2015) Coherent control of plasma dynamics by feedback-optimized wavefront manipulation doi:10.1063/1.4921159