In a message dated 3/29/00 7:22:09 AM Pacific Standard Time,
[EMAIL PROTECTED] writes:
<< I would add
that if this phenomenon is indeed responsible for the problems were having
with the profiles, then the tube manufacturers should challenged to address
the matter. It would appear that a simple shield, with a slot in it,
between the filament and the anode would solve this (most unpleasant)
problem. >>
If you crack open a sealed X-ray tube, you will see that " a simple shield,
with a slot in it, between the filament and the anode" is indeed in place and
serves as the bias cup for electron focussing. Unlike for open tube systems,
this bias cannot be regulated independently and is at filament heater
potential. As the filament ages, it will sag and lose its intended position
with respect to the cup opening. Defocussing sets in and it seems you cannot
do much about it with current W-filament technology.
L. Keller
