Dear Clemente and others interested,
I would just like to expand on Michaels points with some of my experience:
When I was still at the U of Michigan, we quite commonly broke the BaF2
windows with our system with a 25W CO2 laser, particularly when using the
laser with a high power. Changing from 3 to 6 mm thickness did not help us,
in fact the thicker window may brake more readily when put under thermal
stress, particularly spot-heating by the laser and/or the hot gaseous
reaction products which rise straight up to the window. This is also true
for thick glass which is readily broken by scratching and spot-heating. Also
changing to unpolished BaF2 and polishing the windows ourselves did not help
very much (except to cut down the costs).
For the new laser line here in Tuebingen, I have used the very first 3mm
thick BaF2 window for the last two years, also with a 25W CO2 laser. The
main difference to the line in Michigan is that the chamber is higher,
increasing the distance between the sample and the window and instead of
using BrF5 I am using F2. From this I conclude that the two most important
points mentioned by Michael are: i) the distance between the sample and the
window and ii) the power density of the laser as it passes through the
window (the closer the focussing lens of the laser is to the window, the
lower the power density as the beam diameter is larger). Also, I use no
clamping devices to hold down the window - vacuum does it all.
Good luck,
Torsten.
> About two years ago we switched from 3mm thick BaF2 windows (50mm
> dia) to 6mm thick windows. Our 25W CO2 laser delivers about 17W to the
> sample chamber (after all of the turning mirrors, beam expanders and
> focusing lenses. We have done more than 7000 silicate analyses
> during that time and had only one window break.
> From talking with colleagues about this all too common
> phenomena, I
> wonder how other parameters factor into this, for example:
>
> Distance from the bottom of the window to the sample (associated
> heating of
> window, spattering on window).
>
> Power density of the laser as it passes through the window (our beam
> diameter is about 5mm as it passes through the BaF2).
>
> High stress regions introduced to the window by clamping mechanisms.
>
> BaF2 crystal quality (I am told there are at least two grades of BaF2, a
> lower one for windows, a higher quality used primarily for lenses).
>
> Hope this helps,
>
> Michael J. Spicuzza
> Dept. of Geology & Geophysics
> University of Wisconsin
> Madison, WI 53706
>
> (608) 262-9498 phone
> (608) 262-0693 fax
> [log in to unmask]
---------------------------
Dr. Torsten W. Vennemann
Institute for Geochemistry
University of Tuebingen
Wilhelmstr. 56
D-72076 Germany
Tel: +49 (0)7071 297 4992
Fax: +49 (0) 7071 29 5713
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