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Advantages of a "Coaxial" HV Air-Core Transformer

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jpsmith123

Thu Feb 25 2016, 03:37PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

A few years ago I came across this paper

(C. Fortgang et al.) wherein the authors describe a novel air-core HV pulse transformer design with a coaxial primary "winding" and a toroidal secondary coil.

This "coaxial" transformer design has some apparent advantages over other designs and the authors indicate that they intend to continue studying these transformers to "explore their voltage and energy limits".

I looked for some kind of follow-up paper on this type of transformer design, but I never found anything (from these authors at least). I did recently come across a paper published in 2014,

(Novac et al.) where a HV coaxial transformer was designed and built.

In the recent paper, Novac et al. built a coaxial transformer capable of 600 kv on the secondary (with massive energy throughput). They note several advantages of the coaxial design:

(1) high coupling coefficient possible (> 0.9)
(2) doesn't require oil or pressurized gas
(3) can be scaled to multi-MV operation
(4) can carry a high current in the primary circuit
(5) relatively easy to manufacture
(6) extremely robust, able to handle extremely large forces
(7) coaxial input
(8) very low secondary winding capacitance

They also note two disadvantages:

(1) the output is not coaxial
(2) cannot drive large currents in the secondary winding

I'm having trouble imagining scaling such a transformer to a multi-megavolt output; especially without using oil or pressurized SF6 or something. And how would you bring a "wire" (say @ 2 MV) out through a grounded conductor? It seems you'd have to use a carefully designed bushing, which would take away from the "easy to manufacture" feature. Apparently some of the advantages would be mutually exclusive as you approach limits.

Oh well, it may still be something interesting to think about.

Sulaiman

Thu Feb 25 2016, 04:50PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

you want multi-megavolt pulses !

before you start
calculate Ipk = C.dV/dt for the output winding, initially assume an impractically low 10pF winding capacitance.

jpsmith123

Thu Feb 25 2016, 06:38PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Not only do I want multi-MV pulses, but I want them easy and cheap; am I asking too much :)? (But seriously your point is well taken).

I never gave up on my idea for a multi-MV pulsed electron accelerator.

One approach I was looking at was to use a coaxial resonant cavity as a pulse transformer. The idea was to use an inductively coupled fast marx generator, blumlein, stacked blumlein, transmission line, etc., to load the cavity up with magnetic energy, which would then be available as electric field energy to accelerate particles 90 degrees later.

Another idea I had is to fast-charge a transmission line or blumlein (as an energy store) which would then discharge quickly through an over-volted (and therefore hopefully fast and multi-channel) spark gap switch, which would drive a tapered transmission line transformer.

Maybe something similar to the idea depicted in the attached image.

Sulaiman wrote ...

you want multi-megavolt pulses !

before you start
calculate Ipk = C.dV/dt for the output winding, initially assume an impractically low 10pF winding capacitance.

Patrick

Thu Feb 25 2016, 07:15PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

is this not the domain of a tesla coil ?

Sulaiman

Thu Feb 25 2016, 10:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I remember having a tiny part in commissioning a 55 MeV LINAC for Harwell in the '70's
electron acceleration by 'surfing' on microwaves,
power from 4x 20 MW peak klystrons (if my memory is working ok)
I suspect some clever fellow has a microwave magnetron pumped cavity
overall it may be easier than actual high voltage.

OTOH there was a two-stage vandergraph generator for >1 MeV but it did have it's own tall slim building ;)

jpsmith123

Thu Feb 25 2016, 11:44PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

In the literature I've seen some pulsed "Tesla coil" based electron accelerators (e.g. US patent #3450996) but they seem big and bulky at the multi-megavolt level with big primary caps and lots of stored energy.

I wanted to do something with short pulses and/or high frequency, so as to reduce the size, the stored energy, and the cost, so I was looking at using a moderate voltage Tesla transformer as a first stage to charge a blumlein or transmission line and then step up the voltage with a resonant cavity or transmission line transformer.

I think 2 MV may be within reach with the hardware I have right now, but I'd like to get more than that if possible.

Patrick wrote ...

is this not the domain of a tesla coil ?

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