Eur. Phys. J. D (2015) 69: 12
https://doi.org/10.1140/epjd/e2014-50695-y

Regular Article

Overlapping laser profiles used to mitigate the negative effects of beam uncertainties in direct-drive LMJ configurations

¹ Centre de Mathématiques et de Leurs Applications, ENS Cachan and CNRS, 61 Av. du President Wilson, 94235 Cachan Cedex, France
² CEA, DIF, 91297 Arpajon Cedex, France
³ AWE plc, Aldermaston, Reading, Berkshire, RG7 4PR, UK
⁴ ETSI Aeronáuticos, Universidad Politécnica de Madrid, 28040 Madrid, Spain

^a e-mail: mauro.temporal@hotmail.com

Received: 22 September 2014
Received in final form: 31 October 2014
Published online: 13 January 2015

Abstract

A direct-drive shock ignition scheme in the context of the Laser MegaJoule facility has been considered. The irradiation uniformity provided by two laser beam configurations using a total of 10 or 20 quads to drive the first compression phase has been analyzed. Firstly, a numerical method is used to optimize the laser intensity profiles in the context of the illumination approximation model; then these profiles are used to calculate the irradiation non-uniformity of a spherical target of radius r₀ = 1000 μm assuming the beam uncertainties: power imbalance 5%, pointing error 50 μm and target positioning 20 μm. These uncertainties deteriorate the quality of the irradiation increasing considerably the irradiation non-uniformity; moreover, it is found that the pointing error provides the major contribution to the degradation of the irradiation. A strategy to mitigate the negative effect induced by the beam uncertainties is proposed. It consists in using a composite profile in each beam: a first large and flat intensity profile provides a background that reduces pointing error and target positioning effects, whilst a second overlapping profile optimizes the illumination irradiation. It is found that the introduction of the flat background with an intensity of 55% with respect to the maximum intensity reduces by about 40% the non-uniformity of the irradiation due to beam uncertainties.