For static bias, look up "electret" for ideas on some other possible
options.
I would recommend against your option 2 capacitor - that's a dangerous
amount of energy to store in something that may be fooled around with
experimentally. Also, even though it's a lot of C, being electrolytic,
the charge will eventually leak off anyway - probably faster than any
charge loss from using the machine.
The option 2 (2 nF at 4.2 kV) seems more appropriate for this use,
because of the much higher sensitivity attainable. It's charge will leak
off too, but since it's likely a plastic or oil capacitor, the retention
time will hopefully be OK overall.
I wouldn't want to take a jolt from either one. In the ultimate design,
be sure to use some sort of series current limiting resistance to
isolate the capacitor from the outside world. The R can be quite high
(megohms, and of course suitable for the maximum voltage) since not much
current is needed for operation, so the contact/fault hazard would be
reduced from dangerous to a tingle. It would be good to also have a safe
discharging method - another R - that can be switched or jammed in, to
quickly clear the charge for safe keeping when not in use, or during
design.
In the old days, optical methods were used for "gain," as in a mirror
galvanometer, for instance. Putting some simple magnification and
illumination (sun light if electricity is a no-no) in the system can
increase the visibility of any deflection.
Lastly, regarding capacitors, a good option if available, is to use the
nice HV oil caps that can be salvaged from older-era (before they went
to switching supplies) microwave ovens. These are typically rated around
1 uF, 2 kV AC. Two in series would do for up to 4-5 kV service. Since
you don't want bleeder/balancing Rs in this application, it would be
best to use identical caps, or slightly more complicated charging
circuitry. They can bought new, but may be pretty spendy, depending on
the project budget. I have dozens of them - saved from every microwave
oven I've junked out over the years.
At 1 uF, these would have much better retention time, with hazard energy
between the original options.
Ed
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