Damage studies to multilayer mirrors by intense soft x-ray bursts

F. Le Guern*, J.L. Dutrannoy*, J.M. Andre**, D.Friart*, P. Troussel***
J.P. Le Breton***, C. Nazet*, J.Y. Boutin*, R. Barchewitz**

*Commissariat a l'Energie Atomique, CEM/MR, BP12, 91680 Bruyeres 
 le Chatel, France
**Laboratoire de Chimie Physique, Groupe Optique X, Universite Pierre 
 et Marie Curie, 75231 Paris, France
***Commissariat a l'Energie Atomique, DLPP/ED, 94195 Villeneuve 
 Saint-Georges, France


In the context of the MegaJoule project in France, the use of multilayer
mirrors are planned to select the soft part of the x-ray spectrum (below 10
keV).  The hard x-ray part (from 10 keV to 100 keV) produced by a
Bremsstrahlung process go through the mirror while the soft x-rays will be
reflected by the multilayer mirrors towards the diagnostic chamber.  The
expected fluence on the multilayer is in the order of 4 J/cm2 while the damage
thresholds reported in the litterature are well below 0.5 J/cm2.

In this work we measure the dynamical Bragg reflectivity of different x-ray
multilayer mirrors when they are submitted to intense and short soft x-ray
bursts.  We use the Heliotrope facility at the CEN.  Two laser plasma
emissions irradiate simultaneously the multilayer mirror:  the first one from
a NaCl target emitting a heliumlike emission line (1.3 keV) is used to probe
the multilayer reflectivity while a second one from a gold target is used to
heat the multilayer structure with a continuous spectrum ranging from 100 eV
to 1 keV.  The time evolution of the multilayer reflection is recorded by a
streak-camera with a time resolution of 20 ps.  We also tentatively simulate
the phenomena.  We consider the energy deposition within the bulk of the
multilayer structure; a thermomechanical code, taking into account the thermal
conductivity of the material components, calculates the mechanical behavior of
the stratified structure during the irradiation by the soft x-rays.  The
corresponding time evolution of the multilayer structure is then introduced in
a reflectivity code to give the dynamical reflectivity of the multilayer
mirror.