[Sneap] Acceleration Tube Damage

[William H. Hathaway]

Hi,
I read this as there exists a clear epoxy with the dielectric strength as
mentioned.   I'm curious - how do you measure/test the dielectric
strength of this or other materials in general?  (A reference book or
article that I can access is fine, although if the answer is simple, a
few words might be sufficient.)    Second, I'd like to try a sample
(say a standard 2" x 2" x 3/4" block) to see how the breakdown pattern
changes.   I use PMMA extensively for this, but we've got samples
in clear PVC, polycarb, glass (too brittle - they fracture either at
the time of breakdown or within minutes or at best a few weeks)
and one or two other clear polymers or epoxies.    PMMA and glass
are the *only* materials which give patterns (cracks, fractures,
discharge paths ...) which are "clear."   All others seem to "carbonize"
giving black patterns.   Interesting comparisons, but they don't
have the 'sparkle' of PMMA which is appealing to collectors of
scientific/technology items.      It would be nice to try some new
formulas in our testing.

Regards,
Wm. Hathaway  (write me at hathaway at stsci.edu if anyone wants
to see pictures of our testing results - and especially if anyone has
the time and inclination to test a few samples of this material)



Arthur Haberl wrote:

> 
> ------------------------------------------------------------------------
> 
> I'm just back on the scene and noticed the discussion about beam tube 
> sealing.  Our standard beam tube epoxy formula for many years is Emerson 
> & Cuming Eccobond 45 Clear with Catalyst 15.  This epoxy can be adjusted 
> for hardness by changing ratios.  100 parts Eccobond 45 to 50 parts 
> Catalyst 15 produces a rigid material.  equal parts of each produces a 
> slightly flexible material.  100 parts Eccobond 45 to 150 parts catalyst 
> 15 produces a distinctly soft material.  We find the 1/1 ratio just 
> right for beam tubes.  It is flexible enough to withstand temperature 
> cycling without cracking the joint.  This probably isn't important if 
> the contact is only with the glass.  While we tried to apply with care, 
> sometimes excess epoxy ran down across the glass.  There was never any 
> indication that this hurt the breakdown strength of the surface (at 12 
> kv/cm).  Room temperature curing time is about one day minimum.  
> We usually mixed the resin and catalyst using volume ratios instead of 
> the recommended weight ratios, but as pointed out by Klaus Brandt, be 
> careful to get the clear versions, not the black version which has a 
> different density (and was not tested for breakdown properties).
>  
>     Art Haberl
>