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HV MMCs was Re: Magnifier conversion
Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>
At 01:50 PM 4/15/2003 -0600, you wrote:
>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>
>The C2 MMC (High Voltage => (HV-MMC)) is easy to make. It is far easier
>than I thought it would be, so I don't think these "small" magnifiers
>mentioned by Antonio are hard to make at all. With a large toriod, C2's
>value is pretty high which lowers the secondary voltage (<80kV peak)
>helping to prevent primary to secondary breakdown and putting the voltage
>easily into the HV-MMC range. At first, I forgot about the other stray
>capacitances so my HV-MMC probably should have had smaller value
>caps. But I can go down to 100pF right now which should still be fine (it
>can be tapped at 99 points ;-)). The HV-MMC seems like it will be perfect
>and it can even self heal if it takes streamer hits ;-)) I am also
>putting in the bleeder resistors since they will work just as well in this
>case. A 150pF cap at 150kV could pack a pretty big punch especially
>considering it could arc 3 inches!!
While getting the right capacitance for a 200kV MMC isn't all that tough,
getting low corona losses might be more of a challenge. Those caps have
wire leads.. You might need some sort of grading rings or such to keep the
local field around the lead junctions controlled.
Given that this is a short pulse kind of application, DI water as a
dielectric might be way to go, or, use the C of a transmission line
connecting secondary to tertiary as your capacitor.
here's some calculations on a coaxial capacitor made of metal tubing.
For an OD of 6" and an ID of 4", the capacitance is 137 pF/meter
For an applied voltage of 1 kV, the field is .485 kV/cm (200kV is about 96
kV/cm.. well over breakdown for air)
For 6" ID and 12" OD you get 37.8 kV/cm (almost within reason) and 80 pF/meter
10/16 >> 118pF/meter, 33.5 kV/cm Starting to be reasonable, and the OD is
getting reasonable to reduce corona off of it, if it's the HV side.
C = 2*pi*8.85E-12*epsilon*ln(router/rinner)
E = V/(rinner*ln(router/rinner))
For the smaller tubes, pressurizing the air is an option.. 15 or 30psig is
easy to get, not too stressful, and increases the breakdown by a factor of
2 or 3.
Water dielectric has an epsilon of 80..