How to generate strong variable-freq. AC magnetic field?

[OldN00b]

I am open for suggestions... As I said, I am a newbie. I think for obvious reasons, the inductance should be a multiple of what the leads and capacitor bank has as parasitic self inductance. I have not even measured those yet... I was looking for suggestions on how to get such a strong field variable-frequency - by any and all means. I am not very knowledgeable on analogue electronics to be honest.

[PartialDischarge]

I attached a photo of my self-made 2KV capacitor (150 microFarad) to show that I am very seriously working on this project

That's a bit big for the energy. I have lots of low inductance 1700uF 900V, 0.002 ohm, capacitors that allow high discharges (up to 50kA IIRC). With a few of these type of caps you would have more energy in less space and with an easier build. This kind is usually used in busbars in high power electronics

[OldN00b]

What you have in your hand is an electrolytic capacitor and I can't use those, since I need to generate AC fields and hence the cap will need to be without polarity, since when the magnetic field collapses into the coil, the coil generates a voltage, opposite to the voltage that created the field. And when that voltage can't create a current back into the cap, there will be no oscillation and thus no AC field and the field will merely collapse, its energy lost. Electrolytics are very, very unsuitable also for many other factors such as frequency response, longevity, overvoltage sensitivity, internal resistance and Imax.

[PartialDischarge]

What you have in your hand is an electrolytic capacitor and I can't use those, since I need to generate AC fields and hence the cap will need to be without polarity, since when the magnetic field collapses into the coil, the coil generates a voltage, opposite to the voltage that created the field. And when that voltage can't create a current back into the cap, there will be no oscillation and thus no AC field and the field will merely collapse, its energy lost. Electrolytics are very, very unsuitable also for many other factors such as frequency response, longevity, overvoltage sensitivity, internal resistance and Imax.

It's a very stable film polypropylene dry type capacitor. As mentioned it has very low internal resistance, very high Ipk and an inductance in the range of tens on nH.  Don't let the aluminum can (which is actually a plus) confuse you.

[daqq]

I am open for suggestions... As I said, I am a newbie. I think for obvious reasons, the inductance should be a multiple of what the leads and capacitor bank has as parasitic self inductance. I have not even measured those yet... I was looking for suggestions on how to get such a strong field variable-frequency - by any and all means..

Well, in a previous post you have mentioned that the volume of the field should be in the range of a cubic centimeter. Based on that you will have a relatively physically small coil. With a high frequency (1-2MHz) resonance, you will not have a problem with:

...is for a very low capacitance.

A 1uH inductor and a 1nF capacitor based LC resonator will have a ~5MHz resonance frequency.

I am not very knowledgeable on analogue electronics to be honest

I do not mean to discourage you, but, if this is the case be very careful, you may get killed with stuff like this. When most people learn to juggle, they do not start out with chainsaws.

[PartialDischarge]

I am not very knowledgeable on analogue electronics to be honest

I do not mean to discourage you, but, if this is the case be very careful, you may get killed with stuff like this. When most people learn to juggle, they do not start out with chainsaws.

Maybe he should consult with Meredith Perry, she will come up in a few hours with a workable, out-of-the-box solution that experts and engineers couldn't find.

[OldN00b]

Oops, you're right, MasterTech, but the reason that cap is so small for its capacitance is the fact that its current is so low. My cap's theoretical permissible Irms is half a million amps and its actually achievable current due to all the additional metal I added to it, adding to its internal resistance is 40 KA Irms. Your cap is about 100 A Irms, which is 400 times lower. A cap that can handle the enormous currents required for a multi-Tesla field achieved by simply discharging the cap over a coil needs to be able to deliver enormous peak currents. You are correct in saying there exist high power versions, but they use much thicker metal foil and are subsequently much larger.

In any case, those are details... The problem is how to get variable frequency.

Datasheet for your caps:

http://datasheets.avx.com/ffli.pdf

[PartialDischarge]

500000 Amps Irms Sorry but your confusing Irms with Ipk. You should learn more about caps and I2t ratings

[BrianHG]

Pulsed fusion attempts.  High energy physics experiment?

[OldN00b]

Thank you, you are right, again.

My cap has an Irms of "only" 7700 A, still nearly two orders of magnitude higher than yours:

http://www.mouser.com/ds/2/88/942C-22274.pdf 

(1000 caps in parallel).

In any case, my point is that my caps are not physically too large. Any smaller, and the Ipk as well as Irms will be lower. But the most important issue is: How to get variable frequency...

[OldN00b]

@BrianHG:

It's just for a tiny prototype, a proof of concept. The eventual machine will have to generate such fields in a much larger volume. Thank you all for your input. I will think about everything.

[BrianHG]

Your gonna want some of that high temperature superconductor 5T REBCO Coated Conductor tape which is under development...

[tpowell1830]

Hmmmm.. rail gun?

[Wolfram]

100 periods is about what you will get with a high quality (low loss) capacitor, like a pulse rated polypropylene capacitor and a well designed layout. Dumping a capacitor into the coil using a spark gap will give you an exponentially decaying waveform, where the amplitude will be roughly 1/e (37%) times the initial value after Q cycles, where Q is the quality factor of your resonant circuit, or the ratio of energy lost per cycle to the total stored energy. If the capacitor and the wiring is done well, the Q factor will mainly be limited by the coil itself, and it will typically be a few hundred in this frequency range.

The physical size of the coil gives the inductance, as you can not practically have more than one turn (with the required voltage, it would be hard to insulate the coil properly), in this case about 36 nanohenries. Since the frequency is given, you can calculate the needed capacitance, which will be around 180 nF to get 2 MHz. The di/dt is higher than most available thyristors can handle, and the circuit impedance is low enough that any practical MOSFET will introduce too much damping, so a triggered spark gap or the vacuum tube equivalent is the only practical way of switching it, as far as I know.

To change the frequency, you can switch parts of the capacitor bank in and out using high voltage relays. You can do binary scaling of the switched capacitor values to minimize the number of relays. If you have N relays and N banks of capacitors with values, C, C/2, C/4, C/8 and so on, you can get any capacitance from C/2^N to 2C, with a step size of C/2^N. For example with C = 1 uF and N = 8, you would get any capacitance from 4 nF to 2 uF, with a step size of 4 nF.

Note that you need to make a capacitor bank with very little parasitic inductance, otherwise most of the magnetic field will end up inside the capacitor bank itself, induction heating the capacitors and anything around them. Consider the fact that the work coil cross sectional area is a few square centimeters, the loop area of your capacitor bank ideally needs to be less than this. If you're using a bunch of cylindrical PP capacitors, consider putting each series string into reasonably tight-fitting copper pipes to minimize the loop area, and put a bunch of these copper pipes in parallel, then it might be doable.

[The Soulman]

It's just for a tiny prototype, a proof of concept.

Next thing I was going to suggest is to build a tiny prototype, a proof of concept.
If this is that then: 
New EMP gun for the star ship Enterprise?

[OldN00b]

It's a big headache. I've been looking into a relaxation oscillator based on a Hydrogen Thyratron. The best would be a voltage-controlled oscillator. Sinewave or sawtooth.  The 3C45 Thyratron has a 0.6 uS deionization time, which would theoretically yield 1.666 MHz, nicely around my max. freq.  But it's a puny little tube. I have LOTS to learn. I unfortunately have no talent at all for either mathematics or analog electronics. But I must do this project. So I have to learn.

[OldN00b]

That is a genius idea with the digital capacitor bank, Wolfram. I am just not sure I'm visualising the topology correctly in your final sentence (with the copper pipe). I had initially wanted to make my large fixed capacitor bank by soldering the caps together in a star form and their other sides emerging from a copper pipe to minimize internal resistance.

[OldN00b]

Wolfram, I feared previously that the contacts of the high-V relais would burn shut at the high currents though.

[OldN00b]

Wolfram, I also do not understand why you first say I can get 100 periods, theoretically, but then you go on to say that with a spark gap (which conducts nearly ideally, both ways, right?) I can get only about 5 to 7 periods (you're implying, before complete decay)?

[OldN00b]

OK so we have Wolfram's idea: Binary capacitor bank + Mercury Thyratron or pseudospark switch. A triggered spark gap is too loud. I can work from there and see what problems I encounter with the self induction of the capacitor bank's construction and issues with the relais. In principle, the relais do not need to be high voltage, but high current, since they can be all closed when I apply the high voltage and the high voltage can be off when I open them again. What's required is contacts that can handle many KA. I could manufacture those relais myself.

[OldN00b]

One big doubt I have is that with the tiny capacitance, I will never reach the Tesla-range. I did some calculations and I seem to need orders of magnitude greater Coulombs/Joules stored. But I must now sleep and then work with these new ideas. Thank you all, esp. Wolfram.

[T3sl4co1l]

Your gonna want some of that high temperature superconductor 5T REBCO Coated Conductor tape which is under development...

No, that stuff will explode.  Too lossy.  LHe cryostat and type 1 (wrought Nb or alloys) are required here.  And hard vacuum (or 100% potting).

Vacuum capacitors (also made of superconductor) would seem to be ideal here, but getting enough value together to resonate a usefully sized coil will be challenging.

Tim

[Molenaar]

OK so we have Wolfram's idea: Binary capacitor bank + Mercury Thyratron or pseudospark switch. A triggered spark gap is too loud. I can work from there and see what problems I encounter with the self induction of the capacitor bank's construction and issues with the relais. In principle, the relais do not need to be high voltage, but high current, since they can be all closed when I apply the high voltage and the high voltage can be off when I open them again. What's required is contacts that can handle many KA. I could manufacture those relais myself.

A mercury thyratron has a long deionization time, so stick with hydrogen or even deuterium.

[fcb]

As someone pointed out earlier the inductance means you are going to need a pretty high voltage to get the current through the coil. And the losses will be huge, the cooling requirements extreme.

Some ideas:
1. Superconductors? Is the state of the art any good for your application?
2. Make your coil using the Litz wire idea, except with lots of fine bore pure copper tubing. Cool with liquid hydrogen (much higher heat absorption) at some very high flow rate. Possibly you could use the wires in different configurations (series/parallel) to switch inductance with frequency?
3. Examine rail gun technologies, they have some similar problems.
4. Start with a 100th scale prototype, then build a 10th scale prototype, etc...

[OldN00b]

@molenaar: A mercury thyratron would be best suitable due to the long deionization time, since what I'm looking for is full conductivity in both directions for 100 periods @ 200 KHz.