Benjamin's DRSSTC 2 in progress

[Benjamin Lockhart]

Hey guys,
I'm really excited I just got my big DRSSTC oscillating under feedback


It uses a Phillip Slawinski UD+
0.365 uF MMC                              (Now 0.296uF)
8.375 x 33.5 inch 26AWG secondary coil
10 x 48 inch topload
about 50kHz unloaded secondary frequency
CM300 full bridge                         (Now dual paralleled full bridge)

I am  trying to set the phase lead in the driver now. I can just make out some spikes on the igbt collector-emitter signal but barely.
This is one low side IGBT, and the primary current. The bus voltage is about 120V dc, the primary current rings up to around 225A. 150us on time.
And yes this is a different scope than in the picture above.

With a slight dummy load sitting on the coil.


without the dummy load the first spike goes down a bit.


zoomed in, the spikes pretty much disappear, also some wrong triggering I couldn't get rid of.


Two different phase lead settings with the dummy load.



What do you think about these waveforms?
Thanks, Benjamin

[davekni]

I'm having a bit of trouble getting my analog scope to trigger cleanly to set the phase lead in the driver. I can just make out some spikes on the igbt collector-emitter signal but barely.

First simple step is to set trigger holdoff long enough to avoid multiple triggers within one enable pulse (one burst of oscillation).
Second, I'd try triggering on the current feedback signal (signal across 51 ohm burden resistor presuming input looks like UD2.7).  Or if you have a another CT output for scoping, use that.  Then hopefully you can trigger reliably on the first cycle that exceeds whatever current threshold you set.
Third, if your analog scope is fancy enough, use delayed trigger mode.  Set main trigger mode for current, with delay mode to enable trigger on H-bridge output.  That should clean up much of the jitter remaining with a simple current trigger.  (Or set both main and delayed trigger to H-bridge output, using delay to wait for current to build to desired level for checking phase lead.)

Good luck with your build!

Edit:  Saw above edit after posting.  Spikes near the beginning of a burst are normal, at least for UD2.7.  Phase lead does not work properly until current builds some.  Not as important early either since switching current is low.

Final scope capture definitely looks cleanest.

Are you using chop or alternate mode?  The disappearing spikes look more like an artifact of alternate mode, with trigger capturing first part of a burst when scoping current and a later part when scoping H-bridge output.  I'd suggest using chop mode.  Sufficient trigger hold-off should also fix that issue.

[Benjamin Lockhart]

Thanks, triggering from the primary current CT with DC coupling, and playing with holdoff helped a lot.
Alt vs chop didn't seem to matter very much.
Also fine tuning the startup oscillator got rid of the first spike completely! it's only set for one cycle right now and that seems to work fine.

Unfortunately I can only run the bridge up to 130V DC with isolation so I can scope it. I have 45V step down transformer.

Any tips before I set up for first light testing?

Thanks, Benjamin

[davekni]

Alt vs chop didn't seem to matter very much.

In most cases alt vs chop does not matter, especially with holdoff set correctly to get one trigger per burst.  However, occasionally alternate can produce seemingly-impossible captures, as in the earlier capture labeled:
"zoomed in, the spikes pretty much disappear, also some wrong triggering I couldn't get rid of."
Even ignoring the jitter, that capture appears to show current continuing to increase each cycle while H-bridge output has stopped.  That is not a real situation.  Capture of current was by chance occurring first while still increasing, followed by capture of H-bridge output voltage later as it ended.
For that reason I almost always avoided alt mode.  Used it only occasionally, when signal being scoped was close to chop frequency, which caused artifacts.  Even then I'd switch back and forth to make sure I wasn't getting a false display due to alternate mode.

Any tips before I set up for first light testing?

Key advice is test at low duty cycle.  Minimizes chances of hard failures if you find issues.

If your H-bridge Vbus supply is a voltage doubler from line, it may be possible to scope.  Make neutral center of doubler caps.  Connect scope probe through a capacitor (0.1uF or so) to neutral or Vbus-.  Neutral should be close to ground potential, with some ripple due to line power wire IR drop.  Capacitor provides high-frequency coupling to scope ground.  H-bridge outputs should be reasonable to view in spite of that bit of ripple.

[Benjamin Lockhart]

Interesting, the supply is a doubler. I don't have a 100:1 probe though. I'm not sure if I can run it off 120V AC and use a 10:1 probe on the bridge.

[Mads Barnkob]

Any tips before I set up for first light testing?

Key advice is test at low duty cycle.  Minimizes chances of hard failures if you find issues.

Keep DCbus above 100 VDC to avoid excessive transient ratio from IGBT output capacitance. Low voltage testing like 50VDC can fool you to think there is very large switching spikes, as the spike from the output capacitance is the same at 50VDCbus as 500VDCbus, it will look 10 times worse at the low voltage.

Keep BPS at 100BPS'ish and duty cycle low. If you also use low BPS, you can get weird intermittent errors, where the driver is not getting enough feedback.

[davekni]

Interesting, the supply is a doubler. I don't have a 100:1 probe though. I'm not sure if I can run it off 120V AC and use a 10:1 probe on the bridge.

Depends on voltage rating of your 10x probe.  Many are good to 600V.  Voltage rating does decrease as frequency increases.  At low duty cycle, frequency derating of probe is less important.

[Benjamin Lockhart]

Well I seem to have blown the driver.

The 24V input reads like there is a diode across it, and it's shorting the 24V psu.
I don't know why, perhaps a line filter going into the psu might have helped?

The only things I can think of are that I didn't use distilled water, just clean well water, in the primary cooling loop, so maybe some voltage made it's way into the 120V line through the pump, and somehow through the 24 psu and killed something in the driver?
Or maybe voltage on the ground?

All grounds are tied together, to mains ground, and a 10 foot ground rod.

It was making some small output, 8 inches or so at 140V AC input, but I think that's because I had the OCD set at around 300A. it was skipping a lot of pulses and acting kind of erratic.
It was in tune though, the scope confirmed this because the primary current was notching.

I'd appreciate any advice you have.

[Mads Barnkob]

The only things I can think of are that I didn't use distilled water, just clean well water, in the primary cooling loop, so maybe some voltage made it's way into the 120V line through the pump, and somehow through the 24 psu and killed something in the driver?

Even tap water will not carry any real current at those hose lengths. Steve Ward wrote many years ago that even distilled water is useless and he just used tap water. After a few hours of using distilled water, it has been contaminated by surrounding, ion movement from electrolysis etc.

[Benjamin Lockhart]

Ok, thanks. I need to do some testing on the driver to find out exactly what's wrong.
Could the grounding have something to do with it?

[davekni]

Ok, thanks. I need to do some testing on the driver to find out exactly what's wrong.
Could the grounding have something to do with it?

Any chance the PSU got confused and generated more than 24V?  Yes, will help to see what's wrong with driver.

AstRii just posted about his experience with FPGA resetting during ground strike rail hits when driver is grounded:
    https://highvoltageforum.net/index.php?topic=2585.msg19148#msg19148
However, I always ground everything and haven't had such an issue.

Is your driver grounded well to it's shield box?  Looks like so in your picture.

Other than PSU over-voltage, only other thought that comes to mind is driver FPGA getting confused and generating high frequency output that overheated output FETs or driver chips due to rapid switching.

[Benjamin Lockhart]

I've got an update on this build. I'm rebuilding it completely except for the primary and secondary. I've reduced the box from 20 inches to 16 inches square and I'm doubling the bus capacitance to 9900uF.

I got the UD+ fixed, it was just one dead output FET. I'm still not quite sure why it died but I'll keep an eye on temps.

I decided to use a double full bridge after seeing a split MMC used on Phillip Slawinski and Cameron Prince's tesla guns to force equal current sharing.
I'm wiring it like this unless anyone can see a reason this won't work well.

Hopefully the current will be split evenly enough to run close to twice the current of a single bridge.

I'm increasing the MMC to 0.66uF total

[Benjamin Lockhart]

The new inverter bus assembly is done!

[Mads Barnkob]

Beautiful work on that bridge, I love to see some nice handmade items

I never tried split MMC, so looking forward to hear more about your experience in that field.

I think your poor little half-bridge rectifier SKKD 100/12, is going to suffer a bit, you might need a bigger one. Surge overload current capabilities is however helped with a much lower Vrrm voltage, as you are properly at 500-600VDC?

[Benjamin Lockhart]

Thanks Mads, I thought that a 100A rectifier would be ok for a coil running off a 50A breaker?
I have a single phase 240V supply so the bus voltage should be 680V with no load.

[flyingperson23]

The breaker supplies 50A RMS. Peaks will be higher, especially with a voltage doubler.

[Mads Barnkob]

Thanks Mads, I thought that a 100A rectifier would be ok for a coil running off a 50A breaker?
I have a single phase 240V supply so the bus voltage should be 680V with no load.

There is some derating graphs in the datasheet. Here both sine wave and rectangular are mentioned, where the rectangular causes higher dissipation etc.

[Benjamin Lockhart]

Ok, that makes sense. I have a couple huge 301UR200 stud mount diodes (330A, 2000V) that I can fit in there instead. Should be plenty!

[Benjamin Lockhart]

I have a question.

I was planning to wire the extra gates with more GDT secondaries, but would it be better for switching time matching to parallel the gates directly for the doubled up IGBTs?

Since I've already made gate drive boards for each brick, it would be best if I could continue using them.
I suppose I could use one GDT secondary to feed two separate gate networks through the existing boards.

Thanks, Benjamin

[davekni]

I was planning to wire the extra gates with more GDT secondaries

Yes, I think that would be best, with a caution.
Split MMC balances current at coil frequency.  High frequency (H-bridge output transition edges) are connected more directly through MMC sections that are low impedance at high frequency.  H-bridge output transition timing needs to match.  So GDT windings and lead lengths should be matched including matched coupling to primary.
Best GDT performance (lowest leakage inductance) is using a twisted pair for each gate, half of each pair to IGBT and other half to primary.  All primary windings paralleled.  Ideally each primary winding is twisted together all the way back to driver, connected in parallel only at driver connections.
    https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949

[Benjamin Lockhart]

Thanks Dave, I'll do that

[thedoc298]

Very nice looking gear, I was wondering on your primary, it looks like your connected to the outer and the inner. How are you moving the tap, as I don't see any.

[Benjamin Lockhart]

You can see the tap point here.


I've mostly finished rebuilding the coil now. Hopefully this aluminum block will be enough heatsinking for the rectifiers. Edit: it seems to be enough.








Making things fit.


I put a few extra CTs on the bridge outputs to check current sharing and it appears to be identical on 2 separate bridge outputs!

[Benjamin Lockhart]

Hi all, Here's an update on this project.

First light was making 7 foot sparks at 100us and 240V in with minimal tuning, but a primary strike and flashover to the bottom secondary turn seems to have killed the FPGA in the UD+.

I suspect that equidrive was a bad idea and the high voltage on the inner primary turn could have caused a spike on the ground line during flashover, but I really don't know.

I've since replaced the driver with a spare for now, reconfigured the MMC for higher primary impedance (now 0.296uF instead of 0.67uF) and no split MMC.
I simply paralleled the bridge outputs.
I also raised the secondary an inch to lower coupling to about .142


Here's the new test run at only half input voltage 340V bus, and 500us on-time. It has hit 7 feet now at only 120V in and trips the 20A breaker quickly. There is 240V 50A available  for full power.
I'd like to build a better toroid with less unwanted breakouts before trying 240V in.

[flyingperson23]

.296uF seems small enough to really limit your current. That also probably makes it sound fuzzier at low bus voltages. I think it'll sound a lot better at full power, especially if you turn up the duty cycle.

[Benjamin Lockhart]

Yes it is, I might change it to 0.44uF with a larger toroid. The initial current ramp up gets to 750A and the steady state current when I turn it up to 2ms is about 500A. This is just at 340V bus.

[Benjamin Lockhart]

I added 2 more primary turns so I can experiment with a high impedance primary and larger toroid.
I'm still building the toroid at the moment.

[Benjamin Lockhart]

I've made some progress on a new 10x48 inch toroid, but if I were to do this over I'd buy aluminum tubing for a multi ring toroid instead.

It's made up of 60 cardboard strips hot glued into plywood and foam disks, and I'm going to wrap the surface with cloth soaked in glue to cover the bumps and gaps kind of like fiberglassing, then sand it down to make a smooth surface for aluminum tape.

[Mads Barnkob]

Now it's getting real, the physical form taking up all the remaining space in your workshop. The size of your monster is slowly being realized. "Where can I even put these parts..." "Where can I even run such a large coil..."

Good work and ingenuity on the topload. I made my topload too hard underneath the aluminium tape, with automobile-filler. A topload is the part most prone to transport and storage damage, so it actually needs to be able to take a bit of abuse. It's no worse, than scratches and ripped tape, is fixed with new tape and flattened out with a plastic card of short.

[Benjamin Lockhart]

Thanks Mads, Do you think I'd be better off using something softer like duct tape, rather than cloth and glue to smooth out the surface before aluminum tape?

Yes it's getting real now! It's quite big. Thankfully I have space to run it right in front of the shop.

[Mads Barnkob]

Thanks Mads, Do you think I'd be better off using something softer like duct tape, rather than cloth and glue to smooth out the surface before aluminum tape?

Yes it's getting real now! It's quite big. Thankfully I have space to run it right in front of the shop.

I am not sure what the best approach is, but cloth and glue sounds semi-soft. Aluminium tape is not going to hide any bumps or imperfections, so it will take on the pattern of the layer underneath. It's more a appearance issue than an electrical issue. As long as you can smooth out any spikes that would cause corona, its all fine.

It's only going to look nice up until the first time you drop it I cracked my DRSSTC1 topload from a drop and bumped my large ring topload, so its not looking completely circular anymore. Noone sees this but me, so it takes a builder to know it.

Everything also looks nicer on pictures than in real life.

[Benjamin Lockhart]

I decided to use the cloth and got it glued on.
I'll coat this with either more glue or varnish to make it stronger and hard enough.

[Benjamin Lockhart]

I finished the toroid and ran a test of the system!

The cloth and wood glue coating worked well.




I also made a water tank from polycarbonate for the primary cooling loop. You can also see my unfinished tesla guitar interface in the back!



Here's the whole coil standing outside.


And the tests. This is with the 0.296uF MMC and I'm not even hitting the current limit at 940A so I may need to increase capacitance.
Also only 10% primary detuning. I think I should detune the primary way more to take full advantage of this arrangement.

I had some explosions and racing sparks, and my run was cut short by snow blowing in. Also my scope wasn't set right so I don't know the exact primary current.

The biggest issue was the outer primary turn flashing over to the strike rail which caused the power plug to flash over (dumping the tank circuit between ground and the power feed line!) and melt some steel there.

Also, the sharp ends of the strike rail were initiating racing sparks. as can be seen here. Even though coupling is only 0.142
The strike rail has more clearance from the primary than my DRSSTC3 so I thought it would be ok, but I think I need to remake the strike rail with more clearance and no sharp ends.
An extra coat of epoxy on the secondary would also be a good idea even though it survived.

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Edit: Here is a picture of corona from the sharp end of the strike rail. I definitely need to do something about this!

[Benjamin Lockhart]

I'm not sure what to do regarding the strike rail.

In any case I'm going to remove the existing one, and either make a new one with a larger diameter, maybe spaced lower down in order to be further away from the secondary.

Also maybe a large acrylic disk over the primary.
Or maybe I could get away with no strike rail (I have a 0.1uF bypass cap from the negative rail to ground.)

Edit: Made a new larger strike rail and all is well now.

[davekni]

Also, the sharp ends of the strike rail were initiating racing sparks. as can be seen here. Even though coupling is only 0.142
The strike rail has more clearance from the primary than my DRSSTC3 so I thought it would be ok, but I think I need to remake the strike rail with more clearance and no sharp ends.

Also maybe a large acrylic disk over the primary.

Or maybe I could get away with no strike rail (I have a 0.1uF bypass cap from the negative rail to ground.)

I had the same issue with strike rail arcs triggering racing sparks when I first built my DRSSTC even though my primary was covered with polycarbonate plates.  Made larger diameter strike rails using larger diameter tubing (19mm).  Solved the issue for several years.  Recently a top-load defect on bottom of toroid caused many arcs to strike rail, ionizing enough air to eventually start racing sparks.  Fixed my top load now.

Electronics would hopefully survive strike to primary coil.  Initial hit from top load is often near peak top load voltage.  Top rapidly discharges causing short high-current pulse.  Electronics survival depends on path of that short high-current pulse and its resulting inductive drop through wiring.

[Benjamin Lockhart]

Edit: I just found a video still of a single shot, 8ms, 13.5 foot streamer


I made a larger strike rail with ball ends and gave the secondary a second coat of epoxy, tuned the primary 22% low and now I've hit 4 meter sparks! just over 13 feet.

Mads I think this beats your current record, and from an 850mm tall secondary. That's 4.7 times the winding length!

The current limit is set to 940A, and peaks just over 1000A. Current ramp up is pretty slow with this 14 ohm surge impedance. My theory is that slower current ramp up is easier on the secondary volts/turn and allows more power to be pushed through the system without failure compared to low impedance coils.

I detuned the primary incrementally at 15% then 19% then 22% low and the sparks kept getting bigger. I still have enough turns to go 30% low.

I also noticed something interesting, there is a notch in the primary current and it moves further out as I tune the primary lower. I think that's the point where the secondary comes into tune and pulls some energy from the primary.

This suggests I could still detune more since it comes back out of tune again? What are your thoughts on this? 22% seems like a lot already.

Also my coupling is 0.142 with the secondary raised above the primary 3/4 inch. I don't know if I dare increase it.

Scope is set 500A/div, 100us/div.

I'd like to know what to try to push this thing harder, more coupling? higher primary current? bigger MMC?
I'm pretty happy about hitting 4 meters though.
Thanks, Benjamin

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Primary 14% low


Primary 19% low


Primary 22% low and lower pulse rate

[davekni]

That's interesting that the conical primary can have higher coupling without racing sparks.

Wow, very impressive coil!  48" top load is a big asset for making long arcs.  Increases arc length for a given amount of secondary frequency change due to arc capacitance.  Much better than my 34" top.  Freewheeling looks to be helping a lot too, though I have no personal experience with such.  (My IGBT diodes wouldn't handle that much average current.)

My theory is that slower current ramp up is easier on the secondary volts/turn and allows more power to be pushed through the system without failure compared to low impedance coils.

Expect your theory is good, especially for freewheeling control.

I also noticed something interesting, there is a notch in the primary current and it moves further out as I tune the primary lower. I think that's the point where the secondary comes into tune and pulls some energy from the primary.

Yes, appears so.  The small amplitude modulation due to energy moving primary-to-secondary and back show up separately as about one cycle per division.  Modulation amplitude is limited there due to your high impedance primary, with current ramp-up time similar to modulation period.
If you were to measure secondary bottom current, should show almost 180 degree phase shift relative to primary current, from before to after dip.  Operation switches from lower pole frequency to upper pole frequency as arc load moves secondary frequency below primary frequency.  (Frequency shift isn't obvious since pole frequencies are dropping during switch due to arc capacitance increasing.)

This suggests I could still detune more since it comes back out of tune again? What are your thoughts on this? 22% seems like a lot already.

Try more to see.  By 30% I'd guess dip won't occur, extended beyond your maximum enable pulse width.  With more detuning, initial top voltage is lower.  Eventually it will be too low to grow arc to the point where it is in tune.

Also my coupling is 0.142 with the secondary raised above the primary 3/4 inch. I don't know if I dare increase it.

I'd guess you could go at least a bit higher given your high impedance primary, at least to having secondary start at primary height.  Biggest risk to higher than that may be voltage between primary and secondary.  Of course racing sparks can quickly lead to destructive arcs if coil operation is continued for long.  Always hard to decide how much risk to take in pushing performance.  Given my recent experience with humidity (and Mads' previous comments about humidity causing racing sparks), perhaps you will want to leave some margin for different operating conditions.

[Benjamin Lockhart]

Yes, appears so.  The small amplitude modulation due to energy moving primary-to-secondary and back show up separately as about one cycle per division.  Modulation amplitude is limited there due to your high impedance primary, with current ramp-up time similar to modulation period.
If you were to measure secondary bottom current, should show almost 180 degree phase shift relative to primary current, from before to after dip.  Operation switches from lower pole frequency to upper pole frequency as arc load moves secondary frequency below primary frequency.  (Frequency shift isn't obvious since pole frequencies are dropping during switch due to arc capacitance increasing.)

That's actually the freewheeling driver skipping cycles. Here's a video with just a dummy load. Is it actually switching between upper and lower pole frequency?

[davekni]

That's actually the freewheeling driver skipping cycles.

Oh, yes, makes perfect sense.  Did look a bit fast for energy cycling primary to secondary.  Thank you for clarifying.

Is it actually switching between upper and lower pole frequency?

Very likely.  Switches across the primary current notch.  I've measured that on both my normal DRSSTC and my recent ZVS driven one, and seen it in a couple other coils based on scope plots posted on this forum.  I've also replicated that behavior in LTSpice simulation.  Haven't seen any counter-examples yet.  As I'd mentioned, primary frequency doesn't actually jump up much if at all, since pole frequencies are dropping across the switch.  Best way to see it is by scoping secondary bottom current (or top electrostatic field from probe "antenna" on opposite side from breakout point) along with primary current.  Measure relative phase before and after notch.  Your high-impedance primary should make that easy to see.  With low impedance primary and short enable times, much of the energy is oscillating between primary and secondary.  (In other words, both pole frequencies are present simultaneously.)  With your coil, upper pole frequency component has dropped to insignificant before notch.  Should make switch easy to see.

[Benjamin Lockhart]

Very interesting. Makes me wonder what would happen if I set the startup oscillator to force upper pole operation from the start? I think that's possible right?

[davekni]

Very interesting. Makes me wonder what would happen if I set the startup oscillator to force upper pole operation from the start? I think that's possible right?

Might be possible given your high impedance primary, though perhaps difficult with 22% detuning.  Upper pole frequency startup is typically used for QCW and line-voltage ramped coils.  Slow starting ramp makes it easier for self-oscillation to control startup.  Higher coupling separates the two pole frequencies further apart, which also makes it easier to favor one over the other.  A PLL based driver would have a better chance at upper pole startup/operation.
Not sure there's a benefit, but perhaps.  QCW coils (and line voltage ramped coils) usually perform better at upper pole frequency.  There's been discussion as to why.  If I'm recalling correctly, benefit of upper pole may have more to do with smooth slow arc growth as bus voltage increases rather than any more general advantage.  Simulation may provide some clue as to benefit or not.  I need to think about that possibility more.

[Benjamin Lockhart]

I'm wondering what's the exact reason that a fatter secondary coil allows higher coupling without racing sparks.
This is something that I've heard and repeated to others without knowing exactly why.
It it even true?

Does it have something to do with the number of turns vs inductance?

It seems to me that for long pulse DRSSTCs specifically, there is no reason not to use the highest coupling possible, within the secondary's limits. So I'm thinking about trying a much fatter secondary (I have some 19 inch diameter fiberglass pipe) which would allow me to get coupling to 25% with a flat primary at the bottom turn.

Here's a javatc design with the same secondary height and frequency as my current secondary, but larger diameter and less turns.
Would there be much benefit to such a colossal secondary coil? (it would be very expensive)

[davekni]

I'm wondering what's the exact reason that a fatter secondary coil allows higher coupling without racing sparks.
This is something that I've heard and repeated to others without knowing exactly why.
It it even true?

In general I think it's true, but with some caveats.  I have not personally experimented with two similar coils differing in secondary diameter.  Everything below is based on theory, not personal experience.  (I think there is perhaps some experimental data, but only be comparing many different builds on this forum, which differ in many parameters.  My DRSSTC has a relatively high aspect ratio, 110cm long and 16cm diameter.  Been problematic to get high coupling without racing sparks.  However, it's also low impedance for both primary and secondary.  Hard to draw any general conclusions.)

I know little about the transmission line effects (traveling waves down secondary) caused by sudden top load discharge.  Not sure if those contribute to racing sparks.  Rest of below ignores any such traveling waves.  I'll also ignore higher modes of secondary, such as when center is at high voltage and top and bottom are both at low voltage.  I don't think there's much energy in higher modes, especially not with high impedance primary and resulting slower current ramp rate.

Racing sparks are presumably caused by high local electric field, high volts/cm up secondary.  Anything that reduces breakdown voltage will contribute to racing sparks, such as higher humidity or locally ionized air due to adjacent ground-rail strikes.  (UV radiation from adjacent ground strikes might also be significant.  I know UV is important in triggering Marx generator spark gaps close to simultaneously.)  Most of my personal experience with racing sparks are associated with adjacent ground rail strikes.

With good wire insulation and varnish or epoxy coating, racing sparks should be along surface of coating.  However, one possible risk of using large diameter (low AWG) wire is that the same volts/cm becomes more volts/turn.  If wire insulation thickness doesn't increase with wire diameter, perhaps wire insulation could fail directly turn-to-turn underneath varnish/epoxy secondary coating rather than more normal surface racing sparks.  I don't know if that is a specific concern for your proposed 20 AWG secondary wire or not.

Secondary volts/cm comes from top load resonant voltage plus voltage induced by primary.  Resonant voltage (secondary current times secondary impedance) should be roughly evenly distributed along secondary height.  Primary-induced voltage is concentrated adjacent primary.  With lower pole frequency operation, those two voltages are roughly in phase, so add.  At upper pole frequency, voltages are roughly out of phase, so subtract.  Upper pole operation reduces volts/cm at bottom.  Though for a given top load voltage, volts/cm for upper part of coil is higher.  My experience with racing sparks have always been on lower portion of secondary, so presumably during initial part of enable before secondary frequency drops below primary frequency.

If diameters are all increased proportionately (primary and secondary and primary tube diameter and primary turn pitch), coupling increases without increasing volts/cm.  Primary induces secondary voltage farther up secondary (more coupling) without changing volts/cm (without changing coupling to each cm of secondary).  If JavaTC is accurate enough for odd shapes, you could check coupling for short secondaries (say 5cm) in sections from starting at primary to 5cm above, 10cm above, etc.  If coupling to bottom 5cm is higher than for your existing coil, then I'd be concerned about racing sparks.  (Higher induced volts/cm presuming same primary and secondary impedances.)  That may be an issue with your proposed design since primary diameters didn't increase in proportion to secondary.

Hope above makes some sense.  Everyone is of course welcome to question my thinking.  There could be some flaw in my logic and/or some critical factor I'm missing.

I have some 19 inch diameter fiberglass pipe

I think fiberglass is likely to have higher dielectric loss than most other pipe materials.  Dielectric loss may not be a significant factor, however.

You already have a very impressive coil!  Further limit pushing is always fun.  But you do need to be prepared for the possibility of an expensive failure.  (I spent well over 1000 hours, perhaps closer to 2000, with my failed attempt to add MMC capacitance as arc grew in order to keep primary in tune with secondary.  Probably close to $1000 of parts in addition to the time.)

[Benjamin Lockhart]

If diameters are all increased proportionately (primary and secondary and primary tube diameter and primary turn pitch), coupling increases without increasing volts/cm.  Primary induces secondary voltage farther up secondary (more coupling) without changing volts/cm (without changing coupling to each cm of secondary).  If JavaTC is accurate enough for odd shapes, you could check coupling for short secondaries (say 5cm) in sections from starting at primary to 5cm above, 10cm above, etc.  If coupling to bottom 5cm is higher than for your existing coil, then I'd be concerned about racing sparks.  (Higher induced volts/cm presuming same primary and secondary impedances.)  That may be an issue with your proposed design since primary diameters didn't increase in proportion to secondary.

Yes what you're saying makes sense, thanks.

I looked at this in javatc and you're right, the coupling to the lowest 5cm section is higher with this design because the primary is not proportionately wider as well. Making the primary wider to match results in about 0.22k instead of 0.25k.
That still a lot more but I don't think the expense and hassle of setting up such a large secondary would be worth it just for better energy transfer probably without very much spark length improvement.
Freewheeling control and further primary detuning accomplishes a similar thing. The major downside of this is that midi suffers because shorter on-times make almost no spark.

I guess the main benefit to higher coupling with a fatter coil would be reduced V/turn stress when heavily detuned because the spark load brings the top voltage down sooner.

My experience so far is that specifically for good music playback, big toploads and high coupling are always better. Both for low and high impedance primaries.
A very nice side effect of the shielding provided by the large toroid is almost no strike rail hits.

[Anders Mikkelsen]

I have some 19 inch diameter fiberglass pipe

I think fiberglass is likely to have higher dielectric loss than most other pipe materials.  Dielectric loss may not be a significant factor, however.

It depends on the resin used in the FG pipe and what it is compared to. I think epoxy resin is worse than polyester for example, but good data is hard to find. Both of them are worse than PP and PE, but PVC is not amazing in terms of dielectric loss tangent either. The clear advantage of FG pipe is the temperature rating and lower thermal expansion coefficient, but this is more of a factor for QCW type coils and higher duty cycle opetation.

It would be interesting to do a deeper investigation into how the secondary voltage distribution is affected by transmission line effects and higher order modes, these should be two sides of the same coin, one induced by the harmonic content of the drive waveform and the other one induced by the rapid change of topload voltage during breakout and ground arcing. A good starting point could be to gather info on the conditions under which it happens and where the breakdown happens. I've personally witnessed and confirmed primary-secondary flashovers from higher order mode excitation in QCW type coils, and recently had this confirmed by another experimenter who observed the effect I documented. For coils with lower coupling, transmission line whiplash is likely a more dominant effect.

[Mads Barnkob]

I went through a lot of plastic/varnish manufactures datasheets and different testing papers to gather the tangent of loss information for plastic types and varnish / epoxy. I did unfortunately not write down any sources at that time, so it is what it is, but can all be found here: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/ scroll down to "secondary coil form material" and "secondary coil with varnish"