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Aireal
2006-Mar-29, 07:03 PM
Could Ion Drive technology be the key to radiation shielding, while at the same time improving ion drive efficiency?

The standard ion drive has a stage of operation where it collects excess electrons and injects them into the ion beam to prevent the spacecraft from accumulating a large negative potential.

What if that stage could be eliminated, and the negative potential be used to help shield the crew and equipment. First we must look at the problems of radiation shielding for crews in outer space. The three main schools of thought on these are: Material Shielding, which has the drawback of mass. A major factor for a ship with ion drive. Magnetic Shielding, which is weak at some points, much like the Earth is. It also requires a strong magnetic field of about 20 teslas, which might have its own ill effects, we don't know yet. Electrostatic Shielding, by giving the hull a positive charge of 2 billion volts or so , you can repel cosmic-ray protons. But it creates a bombardment of electrons in the range of its field effect.

The key might be to combine the strengths of each of these, while reducing their drawbacks, with a unique hull concept.

Picture the hull as a giant capacitor, and the ion drive giving it a charge with the electrons that would normally be collected and ejected into the positive ion stream. Each charged layer of the hull would be separated by an insulator material that is also a good radiation shielding material. The most likely bet would be a plastic, or maybe a ceramic. The skin of the hull should have a neutral charge, where as one end of the ship would have a positive charge, and the other a negative, powered by the hull capacitor system. This will generate a magnetic shield around the ship. Now all three of the main shielding systems are in place, let's look at how they interact.

At this time a nuclear electric propulsion (N.E.P.) can deliver power levels of several hundred kilo watts to an ion drive, and may reach the megawatt category in the near future. This means an ion drive could generate a large charge in the hull to create a magnetic shield around the ship. I am not sure what the max. field strength would come to, but most likely not large enough to stop the radiation on its own. The good new is, it does not have to stop it, just redirect a few particles toward the poles of the ship, and slow down the rest.

Once the charged cosmic particles have been slowed down by the magnetic field, they will encounter the hull. Passing through the skin of the outer hull, they will next encounter the material which serves as insulator and shielding. This will slow the particles down even more. Then it will encounter the first charged layer of the hull capacitor system. These charged layers will act as electrostatic shielding within the hull. Then the cosmic particles must repeat the process of going through material and charged layers till it is hopefully stopped. Because our hull has both positive and negative layers in it, it will protect against a greater range of charged particles than the standard electrostatic field design. To a charged particle, this process will act as if the hull was a material shield with electrostatic shields within it, combining these two shielding concepts. As this setup is used to create the magnetic field, all three shielding concepts are now one system. So we now have all three of the main radiation shielding systems in place, all working together. So what are the advantages of this approach, let us look.

The first advantage is an overall reduction in mass needed to protect the crew. Every gram of weight saved in material shielding would be worth its weight in gold for an ion drive ship. With just material shielding alone, it would take about one kilogram of material per square centimeter to protect a crew, that's a lot of mass. Where as magnetic and electrostatic systems have huge energy requirements. By combining the magnetic and electrostatic systems into one, we have already cut the power needed by half. With the system using the potential difference created by the ion drive, the need for its own power supply is reduced or eliminated, another reduction in weight. Because the load of protecting the ship is split between three systems, we do not have to use as much power for magnetic and electrostatic systems as if the were run alone. So the overall energy requirements for the ship is reduced also. Yet every design has some drawbacks, so let's look at how these can be reduced in this system.

Magnetic fields provides little shielding at the poles. As our magnetic field is created by the charged areas at each end of the ship, this problem will be increased from particle bombardment in these regions. The simplest way to solve this is to place the crew area in the center of the ship, away from the poles. Antenna or other extensions from the ship could move the poles even farther away from the crew, for an added measure of safety. As the crew would also need shielding from the reactor, the ship design may look something like this. A long cylinder with a antenna projecting from the front of it, the reactor and ion drive at the rear, with the crew area in the center. Of course other designs are possible with this concept. The standard electrostatic shielding concept has the drawback of causing particle bombardment due to its field radius in space around the ship. By locking these charged areas in the hull with a capacitive design, we limit the range that this field expands into space, thereby reducing the problem of particle bombardment. Next we must pick the shielding/insulator material. As it must fill two jobs, it may take some research to determine the best choices. Plastics like polyethylene might be a good starting point. If a lightweight material for the charged layers could be found, this would help in mass reduction, and make the charged layers better as material shielding. Work is already being done in this area. Adding carbon, graphite or nanotubes to plastics is one approach, polycarbonate might be a good chice for this, as it would add some impact protection also. Interpenetrating Polymer Network ( I.P.N.), and similar work by companies like the Eeonyx Corporation are promising also. Thus we can reduce the disadvantages of each of these systems while taking advantage of their strong points. The next question is, are there any other aspects we need to address with this design concept?

What if the capacitive charge in the hull started climbing too high for some reason? There would need to be a system in place to bleed off excess power and divert it to the ion drive or some other use. In the event of a solar storm, primary power could be diverted to increase the field strength. After the danger has passed, the extra power could be sent to the ion drive. This approach may allow for less than max. protection in the normal state, and protection increased when needed, resulting in an even lighter ship design. Perhaps a Faraday Cage (RF shielding) could be constructed around the crew area to help protect them from the magnetic field effects also. Another area for improvements would be the outer skin of the hull. Could it be made to generate usable power from the constant bombardment of particles. A recent discovery by researchers of the Material Sciences Division of Berkeley and partners show that alloys of indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight, from the near infrared to the far ultraviolet, to electrical current. So even the outer hull of the ship may have more than one use in the future. The hull should be modular in construction and design. This will lessen the chance of system failure if a hull section gets damaged, allow repairs to be conducted in flight, and make construction quicker and more cost effective.

An increase in ion drive efficiency might be obtained by correctly locating the positive pole at the rear of the ship in relation to the ion drive. As the positive ion stream left the ion drive it would encounter the positively charged field at the rear of the ship. The ion stream would be repelled even faster away from the ship, while imparting a slight increase in thrust to the ship. While it may not be much of an increase, every little bit helps over long interstellar trips.

Hopefully this approach would reduce the weight and energy requirement of our spaceship by 2/3 over standard designs, while still protecting the crew and equipment.

antoniseb
2006-Mar-29, 07:12 PM
Thanks Aireal, and welcome to the BAUT forum.

It looks like you've given this a lot of thought. I will sift through it and make sme suggestions if anything stands out as wrong.

Can you model it mathematically for a specific plausible spacecraft, and give some numbers for both th efficiency and the radiation protection? I'm guessing that it will only protect against relatively easy radiation to stop.

TinFoilHat
2006-Mar-29, 09:12 PM
Picture the hull as a giant capacitor, and the ion drive giving it a charge with the electrons that would normally be collected and ejected into the positive ion stream. Each charged layer of the hull would be separated by an insulator material that is also a good radiation shielding material. The most likely bet would be a plastic, or maybe a ceramic. The skin of the hull should have a neutral charge, where as one end of the ship would have a positive charge, and the other a negative, powered by the hull capacitor system. This will generate a magnetic shield around the ship.
Unless I'm missing something here, this setup won't generate a magnetic field. Magnetic fields are only generated by moving electric charges. The magnetic field setups proposed for cosmic ray shielding usually involve a veyr large loop of superconductor with a large amount of current circling through it constantly. Your capacitor stores large amounts of unmoving charge, and once it reaches a steady charge level it won't generate any magnetic field at all.

Ara Pacis
2006-Mar-30, 05:44 AM
I recall a story on cold plasma being used for shielding here (http://www.space.com/businesstechnology/technology/cold_plasma_000724.html). By the way, excess electrical generation could be used to spin up large flywheels

Aireal
2006-Mar-31, 08:32 AM
Antoniseb: Yes I have gave it a lot of thought, 5-6 years worth, on and off. Yes this is part of a specific plausible spacecraft. The early part of my post listed the requirments for 100% protection, except for material shielding, which depends of the material used. If water was used, it would take 10 meters to stop 100% of the radiation. using all 3 systems, I cut the max needed for each by 2/3 but 100% is to much to hope for. At 50%, it is still more protection than current protection at the I.S.S. in most areas. So I would be happy with 25% for a starting point. A lot also depends on the amount of electrons the ion drive produces. One electron for every postive ion made. The more fuel used, the higher the output of electrons. With ion drive technology on the move, this is changing quickly, and will vary during operation.
Max field strength would be related to ship length. Past a certain point, there would be a discharge. With this design, you don't want that to happen. I may make a post about my spacecraft concept at a later date.

TinFoilHat: I am a retired stage hand with a lot of time on his hands. I have taken courses on and off all my life, but only have a 2 year degree. Thus there are gaps in my knowledge that I might not have with a more formal education. However I was under the impression that magnetic fields are generated around dipoles and magnets. Guess I will have to look into that point, never questioned it till now. This ship is in a large dipole object, a positive and negative pole, with a netural area inbetween. Even if I am wrong, it is an easy fix, just more wireing to make an electo-magnet. The system can never reach a steady charge state in space, unless it was left adrift for many years. Plus the charged layers of the capacitor system act as electrostatic shield layers, even at a steady state, unless that state is 0, no charge.
Current magnetic field designs are trying to do the job all alone, so they are massive systems. Other current shielding concepts were not any better, being stand alone concepts also.

Ara Pacis, Thank you very much for that link. Gases like hydrogen and helium are great shielding material for their mass. If this is incressed by being made into a plasma, it could be very usefull. Thanks again.

Aireal
2006-Jul-27, 06:23 AM
Here is a simplifed version of my first post, minus the ion drive as a power source.


Radiation Shielding

The three main schools of thought on these are: Material Shielding, which has the drawback of mass. A major factor for a ship design. Magnetic Shielding, which is weak points at the poles, much like the Earth does. It also requires a strong magnetic field, which might have its own ill effects, we don't know yet. Electrostatic Shielding, by giving the hull a large positive charge, you can repel cosmic-ray protons. But it creates a bombardment of electrons in the range of its field effect.

The key might be to combine the strengths of each of these methods, while reducing their drawbacks. This can give us a reduction in mass and energy requirements over any stand alone system. Lets start with electrostatic shielding and how it can be improved.

As stated earlier, a section of a ship can be given a charge to repel like charges, but it attracts like charges. Both positive and negative particles are a problem in space. Instead of charging the outside of the hull around the crew where bombardment would be a problem, move the charged areas to the ends of the ship, away from the crew. Use both a positively and a negatively charged region at opposite ends of the ship, perhaps on extensions to increase distance from the crew area. This will draw charged particles away from the area between the poles, reducing the number headed for the crew area. We have in effect turned the ship into a large dipole.

As cosmic radiation bombards the poles of the ship, the charged particles will cause a change in the potential of the dipole we have created. This change in potential will caused the dipole to generate a magnetic field. A magnetic field will also help to deflect cosmic radiation away from the crew area and towards the poles of the ship. As we are using the electrostatic system to generate the magnetic field, there is little to no increase in mass and energy requirements for the added protection. The dipole system could be operated from within the ship to generate a magnetic field instead of depending on the effects of cosmic radiation. I have yet to determine with method would be the most efficient.

Material shielding is last on our list. Its use should be limited to vital areas due to mass. Plastics like polyethylene fit the bill of low mass and good protection. Recent work that allows plastics to carry a charge however can make material shielding even more effective. Charged layers of plastic within the plastic shielding, whose charge does not radiate into space, would help to stop charged particles that penetrate the hulls skin. This will increase the effectiveness of material shielding, allowing less to be used for the same degree of protection. An overall reduction in mass.

So by combining all three shielding concepts into a single design concept, an overall reduction in mass and energy requirements from protection from cosmic radiation can be achieved.

Damburger
2006-Jul-27, 10:58 AM
A bit of an aside... you mention that a 20 tesla field would be required for the magnetic method of shielding. Would such a field be large enough to have a diamagnetic effect on the crew? (http://www.hfml.ru.nl/froglev.html) A 16 Tesla field was enough to counter the effect of Earth gravity on a frog, so would a 20 Tesla field be able to provide a noticable force on a human inside a space capsule?

If the same system can shield you from radiation AND give you enough 'gravity' to avoid the health effects of weightlessness, might it not be worth looking into further?

Of course, there would be large power requirements and you'ld have to control the environment.

selden
2006-Jul-27, 11:36 AM
Don't forget that you also have to shield against high energy photons, X and gamma rays, as well as high energy neutral particles like neutrons. Lead normally is used for this.

Aireal
2006-Jul-30, 07:40 AM
Damburger

I greatly enjoyed the link you gave. Science with a sense of humor, great stuff. I read it all, and all the links.

On a serious note. I was worried that 20 Tesla fields might have an adverse effect on humans. By combining magnitic shielding with other shielding methods, I had hoped to reduce the field into the 7 - 10 Tesla range for the same degree of protection. However if the fields could produce a replacement for gravity, that might be a good thing. So far my research has turned up no work on the effects of magnitic fields in this range on humans. As always, more research is needed, a never ending quest it seems.

Selden

The cosmic radiation I spoke of is composed of the items you listed in your post. Of them, neutrons are the hardest to deal with, as they lack a charge, by which they can be controlled. Luckally they are one of the smallest componants of cosmic radiation, which is comprised mostly of high energy protons. On earth lead is often used as a shielding material. However its weight makes it a costly item to lift into space. Also the mass of lead would be an added burden to the ships engines for travel and waste fuel. Thus we a forced to look for lighter materials to use for shielding in space.

selden
2006-Jul-31, 12:22 AM
Aireal,

The total mass in the shielding (actually the total "radiation length") is what matters in blocking neutral particles and photons. If you use a lower density material, you have to use more of it. You can't reduce the required launch weight if you're going to provide the same amount of protection.

Aireal
2006-Jul-31, 06:40 AM
selden

True, thats why some lighter materials make better shielding materials, especialy in space. Better "radiation length" to mass ratio, due to secondary reactions.

selden
2006-Jul-31, 11:47 AM
Please provide examples and calculations that show this.

loglo
2006-Jul-31, 02:56 PM
Aireal,
Very interesting post. Anything that can help with the shielding problem is good because I think it is a major hurdle. I wonder if you have looked at the Helicon Doube Layer Thruster ion engine design that has been under development for the last couple of years at the ANU.
(http://prl.anu.edu.au/020research/130SP3/020research/HDLT).

The advantage of this design is that the electrons don't need to be funnelled back into the ion beam, they are accelerated backwards down the thrust tube. This might present a handy way to separate out the electrons for use in your shieldng mechanism.

Anyway just thought I'd bring it to your attention, don't think it has had much press outside of Australia. Good luck with your design.