ELSEVIER Journal of Nuclear Materials 246 (1997) 256-259 Letter to the Editors Photothermal/photoacoustic method for in situ evaluation of radiation-hardened polyimide films Randy Logan a, A.A. Maznev a, Keith A. Nelson a, *, Janez Megusar b a Department of Chemistry, Massachusetts Institute of Technology, Room 6-232, Cambridge, MA 02139, USA b Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 4 March 1997; accepted 3 April 1997 Abstract A non-contact, non-destructive method is used to determine viscoelastic and thermal transport properties of a DuPont Kapton film before and after neutron irradiation to determine the changes that would be expected to occur in a nuclear reactor environment. The results demonstrate general capabilities for in situ, non-destructive evaluation (NDE) of radiation-induced material degradation. The specific application demonstrated is evaluation of insulation materials to be used in a superconducting magnet design in the International Thermonuclear Experimental Reactor (ITER). © 1997 Elsevier Science B.V. 1. Introduction Magnets used to confine plasma in fusion reactors will experience extreme operating conditions. The magnets are operated at 4.2 K and are exposed to high fluences of fast-neutron and gamma irradiation as well as shear and compressive forces. The sensitivity of the magnet insula- tion material to these operating conditions is a critical feature of the magnet performance. In particular, degrada- tion of the mechanical properties of magnet insulation upon fast-neutron and gamma irradiation must be assessed throughout the projected 25 year lifetime of the magnet. One such magnet insulation is a hybrid consisting of a barrier or coating in combination with vacuum-pressure impregnation and pre-preg primary insulation [l]. The barrier layer in this hybrid system can be a polyimide film. In this study, a novel characterization method was used to assess the degradation of the mechanical properties of the polyimide barrier material upon irradiation. Furthermore, it is shown that this characterization tool could be used as a non-contact, non-destructive monitoring device providing continuous feedback of the insulation material perfor- mance over the lifetime of the magnet. Impulsive stimulated thermal scattering, or ISTS, is an all-optical method for noncontact, non-destructive, real- time measurement of mechanical and physical properties of thin film materials [2-4]. Two subnanosecond excitation laser pulses are spatially and temporally crossed at the surface of an absorbing sample to form an optical interfer- ence or 'grating' pattern. Optical absorption and sudden spatially periodic heating give rise to thermal expansion and acoustic responses at the grating wavevector. The wavevector is a function of the crossing angle and the wavelength of the excitation light pulses. The thermal and acoustic responses of the material cause a spatially peri- odic modulation or 'ripple' of the sample surface and can be monitored through time-dependent diffraction of a quasi-cw probe beam. 2. Experimental Corresponding author. Tel.: + 1-617 253 1423; fax: + 1-617 253 7030; e-mail: kanelson@mit.edu. The ISTS experimental setup has been described else- where [2] and is shown in Fig. 1. Frequency-tripling of the output of a Q-switched, mode-locked and cavity-dumped Nd:YAG laser yields 355 nm excitation pulses of 100 ps 0022-3115/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0022-3115(97)00090- 1