Re: Possible method for calibrating tension of FDC cathode plane

[Gerard Visser <gvisser@indiana.edu>]

Hi guys,
	To be more specific (at Simon's particular request) - how about 
something like this:

1. There's a fixture which consists of two aluminum plates and probably 
some insulating spacers. The one plate is sitting at the nominal (5mm?) 
distance from the cathode to the ground plane. The other plate is rather 
farther away (20mm?) on the other side, and is mainly there to provide a 
consistent electric environment on that side. It could be omitted but 
then there would be a sensitivity to, e.g., people walking around the 
setup during measurements.

2. The air space on both sides is vented to atmosphere, so the cathode 
plane is free to deflect. But vents should be small so extraneous air 
currents don't cause some extra deflection.

3. Both aluminum plates are grounded, all the cathode plane is connected 
together (all strips and ground areas); probably this means making some 
dummy "preamp boards" to connect to the strips.

4. The cathode plane is connected through a coupling capacitor (of order 
few thousand pF) to a capacitance meter. Which meter is using the 
technique of applying an AC voltage at few hundred kHz and measuring the 
current.

5. The cathode plane is also connected through a large resistor to a 
suitable bias voltage supply (2-3 kV variable).

6. Measure the capactance at no bias, confirm it is the correct value 
(for the 5,20 mm gap).

7. Bias the thing and measure the capacitance. Gap is now 5-x,20+x. 
Capacitance value tells you x. After some input from electrostatics 
theory and mechanics theory (deflection of a thin membrane under uniform 
loading).

	Gerard

p.s. I have _not_ done this before, so I am _not_ suggesting this based 
on experience. Could be it sounds nice but has a problem to work in 
practice. But, I think it should work ok. For an order of magnitude 
estimate, for a _uniform_ (unrealistic of course!) 100 um deflection of 
the cathode, the capacitance in the setup described above changes from 
1854.4 pF to 1830.2 pF. Should be possible to measure it to a few pF 
accuracy I think, so <25 um accuracy. Is it enough?

Tim Whitlatch wrote:
> Thanks for the idea Gerard. It sounds good. I will discuss it with the 
> others.
> 
> Tim
> 
> Gerard Visser wrote:
>> Hi Tim,
>>     Why not just do it electrostatically...? The force can be applied 
>> and deflection measured quite accurately (from a capacitance 
>> measurement). And it has a direct connection with what we are trying 
>> to control (the deflection under electrostatic force field).
>>     Just a thought - but ignore me if this sounds crazy!
>>
>>     Gerard
>>
>> Tim Whitlatch wrote:
>>> Hi Dan,
>>>
>>> Attached is my initial thoughts on calibrating the tension of the 
>>> Kapton circuit board on the FDC cathode.
>>>
>>> Cheers,
>>>
>>>       Tim
> 
> 
>