How to generate strong variable-freq. AC magnetic field? - Page 1
Forum section: electronics / projects

author: OldN00b
author_rank: Contributor
timestamp: June 22, 2017, 08:57:44 pm
content: I need to generate rather strong EM AC fields over quite a frequency range. What I have in mind is a coil at most the size of an induction heater, but at a much greater field strength. There are various oscillator designs for such a L-C oscillating setup, but none of them are variable-frequency. The difficulty lies in due to the impracticality of having such a large C variable over such a long range.My coil would ideally just be a single winding with a diameter of 1 to 3 cm (about 1 inch max.). This to be able to maximise the current and hence the field strength.The goal is to have a high as possible field strength (2 Tesla or higher would be ideal) and to have an as wide as possible frequency range, starting at 200 kHz and at least to 2 MHz but up to 20 MHz would be good too.I'm looking forward to your suggestions. Please suggest any outlandish idea, as long as it works, because money is no object, this device may cost 50,000 USD in parts.I am fully aware of the craziness of building such a device, I know full well that the coil for ex. would need water cooling and that a high voltage, high power power source will be required. And that shielding will be required to stay within the RF emission laws. I'm willing to construct my own variable capacitor. Full workshop available. Thank you for your suggestions.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 23, 2017, 04:59:10 am
content: 2T is within the range of what a MRI scanner works at, but static rather than oscillating. MRI scanners do also have oscillating electromagnets to do the scan, maybe one of those drivers modified can achieve a bigger field strength in a smaller volume?

author: BrianHG
author_rank: Super Contributor
timestamp: June 23, 2017, 05:47:04 am
content: 2T at 200KHz-2MHzThis sounds like a super high powered radio transmitter...Without shielding, you'll kill any AM radio broadcasts in the neighborhood.

author: jbb
author_rank: Super Contributor
timestamp: June 23, 2017, 06:24:32 am
content: Holy #### thats a high T.Hz product. This thing will be outright dangerous so don't skimp on the shielding & safety. You can expect significant heating effects for anything conductive or ferromagnetic. The field gradient (i.e. dT / dt) will probably be directly dangerous to your health too (for example, Peripheral Nerve Stimulation).I suggest you get someone versed in the art of magnetics to help you model your output coil with 3D FEM. Effects such as the proximity and skin effect could make this nearly impossible. High L * dI/dt rates could lead to sufficient electric fields that the coil experiences corona discharge.I'mguessinghere, but I suspect that some kind of resonant LC circuit will be required (a la induction heater) to get enough alternating current. The trick will getting a widely-variable capacitance value. For a resonant stage, the basic equation is:f_r = 1 / (2 pi sqrt(L . C) )To get a frequency tuning range of 10:1 with a single value inductor, you would need a C range of 100:1, which is quite extreme but probably possible. Getting a frequency range of 100:1 would require a C range of 10000:1, which is almost certainly impractical.Some questions which might help you think about the project:How do you feel about interchangeable heads?Or building a couple of different machines to handle the results?Additionally, what kind of tuning do you need to do?Do you need continuously variable frequency?Or are steps OK? What size steps?Do you need to change the frequency while the field is on, or can you turn it off?Is it OK if the frequency undershoots/overshoots for a while before settling to the target value?Something like aThyristor Switched Capacitor (TSC)and an electronically switched capacitor bank could be adapted to provide the variable capacitance. (The thyristors will need to be updated to some more modern switch. Zero Current Switching (ZCS) could be used to help with switching losses.)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2017, 07:23:14 am
content: I know of one setup, used to make a 0.1 T field at 55 MHz. So somewhat in the direction you are looking for. They used an low loss LC resonator and tube based amplifier to drive it. Already this was quite a crazy thing. One had to be careful with the choice of materials: everything needed to be low dielectric loss (e.g. PTFE) - normal plastics would melt from dielectric loss due to the electric field that comes with the magnetic one.One might have a chance short time - thus just a single pulse of a high magnetic field. This also contains higher frequency components. Though usually this is more like fractions of a ms and not going to such high frequencies. Even with just a single turn the voltage is too high. Maybe the explosive compression could be that fast - but nothing for an easy experiment and with a lot of paperwork / permits.

author: jbb
author_rank: Super Contributor
timestamp: June 23, 2017, 09:00:29 am
content: Mmmm, high power vacuum tubes. Still the best when your specs go to crazy-town.50 MHz 0.1 T makes a product of 5 T MHz.2 MHz 2 T makes a product of 4 T MHz.So, probably possible, but very difficult. You can look forward to machined PTFE and other such joys. Maybe getting the heating coil plated with hard gold (so it's smooth and stable to manage skin effect) will be required as well as the near-certainty of water cooling.For the power amplifier, modern silicon carbide(SiC) or Gallium Nitride (GaN) devices might well suffice rather than exotic vacuum tube kit.This will need extensive modelling...

author: Fungus
author_rank: Super Contributor
timestamp: June 23, 2017, 09:04:07 am
content: Another wireless phone charger?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2017, 09:07:00 am
content: PM me if you'd like some consultation on this.Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 23, 2017, 09:21:39 am
content: For some rough approximations, to get a 2 Tesla RMS AC field at 2 MHz in a 30 mm single turn air cored coil:If the coil is wound with 5 mm copper pipe, the current needed to get a 2 tesla field is approximately 8000 amperes. The inductance of the coil will be around 36 nH. This gives the coil an inductive reactance of 0.45 ohms at 2 MHz, so you need about 3.6 kV at 2 MHz to get 8 kA in the coil. Air coils like this typically have a Q factor of a few hundred, so the coil losses will be at least 100 kW, likely more. A suitable capacitor bank for this would be something like 12 parallel Celem CRPI300 100 nF capacitors. The sheer physical size of such a capacitor bank would inevitably add more parasitic inductance, which means that you need more capacitors to compensate it, quickly leading to diminishing returns.I have no idea if it is even realistic to have 100s kW of dissipation in such a small coil, for sure it needs to be cooled from the outside with water at a high flow rate, but even then it might not be practically doable.If you reduce your field strength requirement and go for a single frequency, it might be possible to do it within your budget. You could probably buy an induction heater commercially that could do half a tesla at 400 kHz with the budget you have.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 23, 2017, 12:04:23 pm
content: Quote from: OldN00b on June 22, 2017, 08:57:44 pmA 1-3cm diameter coil is going to be too small physically just to handle the level of current that will be necessary here (in the realm of thousands of amp*turns), and if you could somehow wind more than one turn to reduce the current required that would just result in a massive voltage gradient between turns, leading to incessant arcing.The only practical circuit topology is a parallel resonant tank with a superconducting coil and vacuum dielectric variable capacitors switched in and out of circuit with hydrogen thyratrons (e.g. - Excelitas). With this arrangement you can probably get to 2T at 2MHz, but I simply don't think 2T at 20MHz will be possible except maybe as a pulse-excited, naturally damped oscillation; definitely not CW operation.

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 02:17:55 pm
content: Thank you all for your valuable suggestions and highly interesting information, and for taking me seriously. I appreciate this brainstorming. I attached a photo of my self-made 2KV capacitor (150 microFarad) to show that I am very seriously working on this project - that Cornell Dubilier cap cost me 5,000 USD in parts and quite a few days of CNC-drilling and soldering. Its internal resistance is 0.05 Ohm. I did not measure its self induction yet. Please note that I am a NOOB. I did not want to buy a Chinese cap bec. my cap is superior in terms of being able to handle much larger and for a much a longer time, ability to self-repair at over-voltage as well as gaining experience with such projects.I will carefully consider all your comments and suggestions and will base my decisions on them, as well as on further discussion/research/experimentation/calculation/simulation. And I may PM those who offered consultancy.I will address your points now. Minimum specs in more detail:- The device MUST be variable-frequency and it must be settable to any arbitrary frequency between 0.5 and 2 MHz (20 MHz was a "nice to have" feature but let's stick to the essentials first) by an analogue dial and the device should be able to operate for at least 10 minutes at a time before cooldown is required. The freq. should be changeable steplessly while the field is on. The field strength should be between 0 and 2T, 2T being nice but 0.5 would already be acceptable. The volume of the field should be at least approx. 1 cubic centrimetre.- IMPORTANT: It's permissible to have a short duty-cycle. For ex. the device operates for 100 ms and then is off for 900 ms. That would reduce heating by an order of magnitude.- The budget is 50,000 USD - 100,000 USD.- Thanks for suggesting tubes. I started to think about that already.- Thanks for talking about various physics-related issues that are complicating design factors. I tried machining Teflon - indeed difficult. I could freeze it (even in liquid nitrogen) to make that easier. The comment about gold-plating: I actually have several KG of gold at my disposal so I could even make the coil out of solid gold, but isn't silver a better electrical as well as thermal conductor? I will perhaps make the coil out of thick litz, to help with the higher frequencies. A disadvantage is that you can't run water through litz.- I have been thinking of constructing a variable-induction coil but I think that's less practical than a variable C.- This is a private project with perhaps commercial potential in the far future.- I am aware of potential adverse health effects and will strive to provide adequate shielding with mu-metal (crazy expensive stuff...) etc.- I will resist the urge to try how quickly I can wirelessly charge my smartphone with it. If I break down and can't resist that urge anymore, I will try it first with my wife's phone and get back to you with the results.- I am aware of potential harmful radio interference and will work to avoid getting into trouble with the law. I have RF probes for my spectrum analyzer and successfully done FCC pre-compliance testing on one of my products so I should be able to detect such stray radiation.- The attached cap I made was for the purpose of generating a multi-T AC field for just a few microsecs. Using a spark gap as a switch and a MOT-based PSU.- Interchangable heads is an interesting idea but there still needs to be a very significant freq. range, so perhaps not practical.- Different machines: Too expensive/impractical. I rather spent even 100 grand on one machine and have it weigh a tonne.- It is NOT OK if the frequency undershoots/overshoots significantly for a while before settling to the target. The freq. should be stable to a reasonable degree but can vary due to thermal issues etc. There are no requirements as to maintaining a set freq. stably. Any significant deviations of the freq, even for one microsecond, are NOT permissible.- The field should look like a sine wave and not have harmonics or other freq. components.- The coil, absent heroic measures, will come apart due to the enormous forces generated inside of it, so it will have to be heavily reinforced by composite materials.- Any fellow aspies noticing that those bolts are ugly-long: I replaced them with shorter ones :-)

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 23, 2017, 02:34:57 pm
content: Quote from: MagicSmoker on June 23, 2017, 12:04:23 pmThe problem here is, for the given coil size, you need a few hundred nanofarads of capacitance to get a resonant frequency of 2 MHz. This would be very difficult to do with a vacuum capacitor, without also adding more stray inductance than the work coil itself.Otherwise, your suggestion is probably the closest it is possible to get to the target performance, but even this is a (multi) million dollar implementation, far from the 50 000 suggested parts budget in the original post.Also, I just noticed that I was off by an order of magnitude in my previous calculation, the losses in a copper coil would be in the megawatt range for 2 T at 2 MHz, a further nail in the coffin for any non-superconducting coil.

author: The Soulman
author_rank: Super Contributor
timestamp: June 23, 2017, 03:02:56 pm
content: Quote from: Wolfram on June 23, 2017, 02:34:57 pmSomeone did mention liquid nitrogen...I'm curious, what is the application for this device?

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 03:51:24 pm
content: I prefer not to disclose that.

author: The Soulman
author_rank: Super Contributor
timestamp: June 23, 2017, 03:56:57 pm
content: Quote from: OldN00b on June 23, 2017, 03:51:24 pmFair enough, we'll file it under "personal pleasure".

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 04:04:26 pm
content: Since the feasibility of such a device seems to be low, let's concentrate on the duty cycle.I said previously that a 10% duty-cycle would be acceptable, but in fact, a duty cycle of 1% or 0.1% or even 0.01% will also be acceptable.Meaning, every 10 seconds a pulse of 1 ms with that variable frequency.Would that make the device more feasible? Certainly in terms of mechanical stress and cooling, as well as the power needed for continuous operation.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 23, 2017, 04:47:38 pm
content: That makes a huge difference. If it is pulsed at a low duty cycle, it does not neccesarily have to be that difficult. A 0.1 % duty cycle at 2 T and 2 MHz brings down the coil losses to the single digit kW level, roughly approximated, which can be managed with internal water cooling at a modest flow rate.For the pulsing, what is the minimum acceptable pulse length? Is it acceptable if the pulse amplitude changes during the pulse? If so, it could be as simple as dumping a capacitor into the coil at regular intervals.The very loosely defined requirements and the secrecy does not give me a good feeling about the project, but in any case it is an interesting thought exercise.

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2017, 05:46:53 pm
content: Interesting requirementsMaybe you could make a high voltage push pull DAC out of Behlke switches, with a low pass filter on the output?

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 05:50:39 pm
content: Thanks Wolfram. The requirements, I'm constraining them to the bare minimum, based on your feedback.I realize that I do not need more than say 100 periods of the "signal" per experimental iteration taking say 10 secs at minimum, and since it's for ex. a 1 MHz signal, 100 periods would be a mere 0.1 ms.Dumping a capacitor is exactly what I want to do, but then you get 7 periods (due to the decay) and of course it's fixed-frequency. I'd like the amplitude to be roughly constant over those 100 periods.Basically, what I need is to have a variable-frequency oscillator with ridiculously high power. 100 periods generated at 1.21 MHz, ten seconds later 100 periods generated at 1.22 MHz, etc.It would be NICE to have more features, as I initially mentioned up to 20 MHz, continuous operation etc. but none of it is a hard requirement, as I am forced, by your comments, to examine more harshly what is absolutely required for an initial setup.The problem remains: How to make this variable freq...

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 05:53:49 pm
content: About the secrecy: It's a very interesting application that I wish to remain secret for the time being, but later I will vlog about it and you guys will be the first to know.

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 05:58:44 pm
content: As to any type of solid-state switching: I examined that option but I ended up with the need for Thyristor switches costing 15 grand. They are very fickle (break easily) and the manufacturer said they could be used to trigger the explosive lens around a nuclear bomb, so they refused to sell it to my company, not even with the proper forms filled out (no re-export to Iran etc.) So if it comes to dumping a cap into a coil, a (triggered) spark gap seems the way to go. I'll construct those myself.

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2017, 06:04:31 pm
content: QuoteNote that I'm not experienced in the art of high powered oscillators, so what I say might be rubbish.Since you have a fixed coil, then you can only change the capacitance in an LC resonator. Variable caps do exist, even for very high powers. For example see:http://www.comet-pct.com/products/vacuum-capacitors/variable-capacitorsThe naive approach would be to set your capacitance in your LC circuit, then dump your capacitor bank (different from the capacitor in the resonator) into the LC resonator and let it resonate. Sort of like a tesla coil with a spark gap. See:http://www.electron-tubes.cz/en/spark-gapfor a local supplierQuoteWell, thyratrons are still manufactured :-)http://www.electron-tubes.cz/en/51-tr-50

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 06:13:25 pm
content: I think that a motorized variable cap with the specs I need does not exist and a custom-made one may cost 1 million dollars... The one here:http://www.comet-pct.com/products/vacuum-capacitors/motorized-capacitors...is for a very low capacitance.And the switches are not my problem - a triggered spark gap works fine (when sound-insulated and cooled - in fact I'd design my own, since a regular spark gap does not last longer than a few hours of operation in my case).I start to think that my best option would be to construct a motorized variable capacitor. When you build it yourself, 50 grand goes a long way.

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 06:29:30 pm
content: What I could do is have many fixed-value caps that I can put in parallel, using thick copper cables and manual switches (easier than plugs) that can handle many dozens of KA and then one variable cap of the same range as one of those fixed caps. Then make some kind of Colpitts-type oscillator, some kind of circuit used in induction cooking to generate the field. But the issue then becomes semiconductors not liking to conduct 10 KA to 50 KA. And them either not existing or being very, very expensive and constantly breaking with these kind of experiments. And such an elaborate setup will have a significant self induction and resistance.

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2017, 06:34:52 pm
content: What kind of an inductance are you hoping to use? As in how many uH?

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 06:44:21 pm
content: 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.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 23, 2017, 07:01:08 pm
content: Quote from: OldN00b on June 23, 2017, 02:17:55 pmThat'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

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 07:31:08 pm
content: 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.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 23, 2017, 07:40:25 pm
content: Quote from: OldN00b on June 23, 2017, 07:31:08 pmIt'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.

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2017, 08:23:20 pm
content: QuoteWell, 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:QuoteA 1uH inductor and a 1nF capacitor based LC resonator will have a ~5MHz resonance frequency.QuoteI do not mean to discourage you, but, if this is the casebe very careful, you may get killed with stuff like this. When most people learn to juggle, they do not start out with chainsaws.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 23, 2017, 08:41:21 pm
content: Quote from: daqq on June 23, 2017, 08:23:20 pmMaybe 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.

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 09:29:58 pm
content: 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

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 23, 2017, 09:59:48 pm
content: 500000 Amps IrmsSorry but your confusing Irms with Ipk. You should learn more about caps and I2t ratings

author: BrianHG
author_rank: Super Contributor
timestamp: June 23, 2017, 10:04:54 pm
content: Pulsed fusion attempts. High energy physics experiment?

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 10:06:23 pm
content: 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...

author: OldN00b
author_rank: Contributor
timestamp: June 23, 2017, 10:09:59 pm
content: @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.

author: BrianHG
author_rank: Super Contributor
timestamp: June 23, 2017, 11:57:08 pm
content: Your gonna want some of that high temperature superconductor 5T REBCO Coated Conductor tape which is under development...

author: tpowell1830
author_rank: Frequent Contributor
timestamp: June 24, 2017, 12:18:08 am
content: Hmmmm.. rail gun?

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 24, 2017, 12:25:52 am
content: 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.

author: The Soulman
author_rank: Super Contributor
timestamp: June 24, 2017, 12:26:57 am
content: Quote from: OldN00b on June 23, 2017, 10:09:59 pmNext 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?

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:38:42 am
content: 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.

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:47:05 am
content: 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.

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:49:27 am
content: Wolfram, I feared previously that the contacts of the high-V relais would burn shut at the high currents though.

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:51:42 am
content: 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)?

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:56:34 am
content: 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.

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:58:26 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2017, 01:42:35 am
content: Quote from: BrianHG on June 23, 2017, 11:57:08 pmNo, 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

author: Molenaar
author_rank: Regular Contributor
timestamp: June 24, 2017, 07:47:25 am
content: Quote from: OldN00b on June 24, 2017, 12:56:34 amA mercury thyratron has a long deionization time, so stick with hydrogen or even deuterium.

author: fcb
author_rank: Super Contributor
timestamp: June 24, 2017, 09:05:02 am
content: 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...

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 10:48:53 am
content: @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.

author: Fungus
author_rank: Super Contributor
timestamp: June 24, 2017, 10:50:32 am
content: Quote from: OldN00b on June 23, 2017, 05:53:49 pmI knew it. It's another phone charger.

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 10:54:22 am
content: @fcb:Superconductors is not an option. Litz wire would be an option for a commercial embodiment, fine copper/silver tubing with water cooling will be the experimental prototype's embodiment due to cooling requirements. Due to the low required duty cycle, I may even be able to get away with Litz. It needs experimentation. A commercial embodiment can be fixed-frequency so many issues disappear.My experimental prototype is already the absolute smallest I can make it, due to an absolute requirement of 1 CM3 minimal volume. I can go to 20 KV though, to have a small value cap and a Helmholtz coil with just a couple windings and still achieve a high resonance freq. A commercial embodiment would better use a lower voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2017, 11:03:25 am
content: Quote from: OldN00b on June 24, 2017, 10:54:22 amYou never said it was pulsed. You implied it was CW.Tim

author: OldN00b
author_rank: Contributor
timestamp: June 24, 2017, 12:59:19 pm
content: I said that It's OK to have a 0.01% duty cycle.