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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..