Making a Magnetoplasmadynamic (Plasma) Thruster?

[gustafangus]

Hello everybody,
I was looking for ways to amplify magnetic fields for some project, but I happened to stuble upon articles about plasma thrusters. And seeing how a magnetoplasmadynamic thruster works, I thought: either this is really easy to make or I'm missing something.

Now bare with me, most of my scientific experience lies with magnetic fields and electrical engineering basically. So I may oversimplify something or say something stupid, but just know I still have some blank spots when it comes to this stuff.

As far as I know, a plasma thruster works this way:

1. a gas is propelled into a cylinder with great amounts of pressure. The cylinder is basically a conductive metal (ex. copper) that is the Anode, and there is an inner, smaller copper cylinder that acts as a Cathode.

2. The cathode and anode are connected to a high current power source.

3. when the gas enters the cylinder, it is ionized in some way. By ionizing it, we make it conductive. Lorentz' law then accelerates the ionized gas through the length of the cylinder, thus producing thrust.

Is this is? Am I missing something? Because if this is all it takes, I would try and build this contraption. But there are a few questions I have.

1. What current are we looking at when powering the cathode/anode to produce any significant thrust? 10A, 100A, 10000A?

2. How can one ionize the gas that enters? For example, I know that sparks and the electricity jumping between wires is actually air being ionized, correct? So would it be possible to attach a few spark plugs around the outer cylinder where the gas enters? So we would force all the gas that comes into go through these sparks, thus ionizing it? (this would probably mean that the gas would catch fire, correct?)

I know that this is a shot in the dark, but I thought I'd ask here, since I am unable to find any real world scenarios of this, because well... most people probably don't spend their free time building plasma thrusters and other such greatness. Also, if you'd like a sketch of what I had in mind, do let me know.

Thanks!

 

[mfb]

The electrodes are "spark plugs".

Often it is easy to make some toy model. Getting it efficient, reliable, with the right scale and for a reasonable price is the challenging thing.
The first photovoltaic cell is from 1838, the first rockets were developed hundreds of years ago and you can buy toy rockets for a few Euros/Dollars - that doesn't mean you can launch a satellite to orbit with such a toy rocket or power it with a solar cell from 1838.

 

[gustafangus]

Thank you for your reply!

The electrodes are "spark plugs".

But that would mean that I would not only require high current on the electrodes, but also high voltage to create the ark between them, correct? And should the the ark be momentous or continuous? Because when reading NASA's description, I had problems understanding the following:

Just as in an arc welder, a high-current electric arc is struck between the anode and cathode. As the cathode heats up, it emits electrons, which collide with and ionize a propellant gas to create plasma.

It says the ark is STRUCK (so only once, for like a milisecond?), but then it says the cathode heats up. That would mean that the ark should last for a long time in order to carry the current required for the actual cathode to heat up. This is the only part I do not understand.
Also, could the cathode and anode be connected to a high current power supply and at the same time to a high-voltage power supply? I could probably make the high voltage power supply required for the ark from a car ignition coil, and would use a car battery or two for the high current source. Is this a viable solution?

And don't worry, I'm not trying to send a rocket to the moon, I'm just making a small model, a showpiece if you will ;) Maybe if it works out I might build a bigger one.
Thanks again for taking the time!

 

[mfb]

All those are engineering questions that have to be figured out.

Also, could the cathode and anode be connected to a high current power supply and at the same time to a high-voltage power supply? I could probably make the high voltage power supply required for the ark from a car ignition coil, and would use a car battery or two for the high current source.

Two power sources with different voltages at the same time? No, that does not work. It will probably kill at least one of the power sources.

 

[gustafangus]

All those are engineering questions that have to be figured out.Two power sources with different voltages at the same time? No, that does not work. It will probably kill at least one of the power sources.

That's why I came up with the spark plug concept. Because having a high current and high voltage source is nearly impossible for an average Joe. Do you believe it would work with the spark plugs? Or can I connect a high voltage source to the cathodes and put some coils on the outside to run current through to act as a powerful magnetic field? As I've said, this is the only problem I have trying to make a blueprint for this thing.
Thanks!

 

[mfb]

I suggest to look up existing concepts. And do not build one yourself if you do not know how to operate with high voltages, vacuum and ionizing radiation safely. And even then: the thrust of all those devices is tiny, you would need extremely precise measurements to note it here on Earth. An acceleration giving 200 eV to each proton leads to a thrust of just a few microNewtons per watt of power - or even less if you take the efficiency into account.

 

[gustafangus]

I suggest to look up existing concepts. And do not build one yourself if you do not know how to operate with high voltages, vacuum and ionizing radiation safely. And even then: the thrust of all those devices is tiny, you would need extremely precise measurements to note it here on Earth. An acceleration giving 200 eV to each proton leads to a thrust of just a few microNewtons per watt of power - or even less if you take the efficiency into account.

Thank you for all of your input, you gave a much clearer question than what I had before. I know how to work with high voltage (atleast to a point to not get myself killed). As for ionizing radiation, do you think that there would be a significant amount of ionizing radiation that could hurt me? Of course, I won't be using this for hours everyday, like I said, I'm just trying to build to say "hey, I've built this"-like I do with many of my projects. Thank you again!

 

[mfb]

High voltage plus accelerated charges... I can't rule it out.

 

[gustafangus]

High voltage plus accelerated charges... I can't rule it out.

Fair enough.

As for looking up existing concepts, have you seen any? I tend to not ask a question in forums unless I can't find the answer beforehand, and trust me, I have searched for this. Do you know of any forum that deals with this type of stuff in more detail?
Thank you for your help!

 

[mfb]

The wikipedia article refers to something like 10 different research projects, google scholar has 22000 hits (2800 with a narrower search term).

 

[gustafangus]

The wikipedia article refers to something like 10 different research projects, google scholar has 22000 hits (2800 with a narrower search term).

One last question though, does the thruster have to be in a vacuum? I was re-reading your comments and you said something about a vacuum. And thank you for all your help.

 

[mfb]

The engine is "open" - you cannot keep it under reasonable pressure in space. The thrust is too low to use it in an atmosphere. You have a small pressure from the plasma itself, but apart from that: sure, vacuum.

 

[CWatters]

Perhaps see...

http://myelectricengine.com/projects/mpdthruster/mpdthruster.html

Although an Ion thruster might be easier...

http://makezine.com/projects/ionic-thruster/

 

[gustafangus]

Perhaps see...

http://myelectricengine.com/projects/mpdthruster/mpdthruster.html

Although an Ion thruster might be easier...

http://makezine.com/projects/ionic-thruster/

Thank you for that! I know about myelectricengine, I've even tried contacting the man who wrote it by the mail he provided, but the mail doesn't exist anymore, LOL. Anyway, one thing I want to ask though, is when ionizing air/gas, must the current be DC or AC or does it even matter? High voltage DC power supplies are hard to build, I only have a 600V one, but I could probably make a 12kV+ AC one in no time. So does it matter really or..?

 

[Astronuc]

MPD thrusters are powered with DC. It mostly depends on current, I, and F ~ I x B. The current generates the B, so F ~ I².

One would be concerned with cathode erosion, or electrode erosion in general.

http://www.nasa.gov/centers/glenn/about/fs22grc_prt.htm

One also may wish to investigate the DS4G thruster
http://www.esa.int/gsp/ACT/pro/projects/ds4g_overview.html
https://physics.anu.edu.au/cpf/sp3/ds4g/

See other concepts - http://www.esa.int/gsp/ACT/pro/index.html

 

[gustafangus]

Well yes, they use DC because usually MPD thrusters use a single power supply. One that has enough current to create a powerful magnetic field and at the same time have enough voltage to create the ark between the cathode and anode. What I would do is use a high voltage AC power supply to ionize the entering gas (make an ark) and then make a giant coil and connect it to a high current power supply to accelerate the ions, thus creating thrust. Any ideas?