Hi Gail,

Gail F. Eaton wrote the following about Re: blown filaments...:

> I had this problem on my VG PRISM last year.  I had run on one filament for
> more than two years, so I was very surprised when I suddenly went through
> the last few filaments I had in just a few days. As it turns out, my trap
> current had increased to 1000 microamps, even though the computer  readout
> said it was at 800 uamps.  I don't understand what had caused it to creep
> up, and the only explanation I could get out of Micromass was a known
> problem with the electronics.  I now run at 600 uamps (on computer) which
> corresponds to about ~700 uamps on the gauge---and I am no longer blowing
> filaments. However, I now monitor the trap current and emission current daily.

There is very little difference in filament current between, say,
600uA and 1000uA.  Unless the filament is old, and tends to sag at
different temperatures, you should never have a problem.  It's worth
while flicking your source monitor onto Filament Current(A), just to
see what is really going through it.  A sagging filament will tend to
run at a higher current, regardless of what trap current you are
trying to achieve.  I've had no problems with either of our PRISMs or
our SIRAs running at 1000uA for months.  It's much more likely that
you had a couple of duff filaments, perhaps from a dodgy batch.  It's
always worth visually inspecting the thoria (or Yttria) coating for
signs of inconsistency and flaking before putting a new filament in,
and also visually checking that the wire has no kinks in it.  Either
of these conditions will result in short filament lifetime.

Andrew